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Effects of PhXA41, A New Prostaglandin F2α Analog, on Aqueous Humor Dynamics in Human Eyes
Effects of PhXA41, A New Prostaglandin F2a Analog, on Aqueous Humor Dynamics in Human Eyes Carol B. Toris, PhD, Carl B. Camras, MD, Michael E. Yablonski, MD, PhD Purpose: PhXA41, a new phenyl-substituted analog of a prostaglandin F2 " (PGF2 ,,) prodrug (13, 14-dihydro-17-phenyl-18, 19,20-trinor-prostaglandin F2,,-1-isopropyl ester), is an effective ocular hypotensive agent in patients with glaucoma. To understand its mechanism of action, various components of aqueous humor dynamics were examined after topical application to human eyes. Methods: In a randomized, double-masked, placebo-controlled study, PhXA41 (0.006%) was given topically twice daily for 1 week to one eye each of 22 volunteers with normotension or ocular hypertension. The other eye was similarly treated with vehicle. Intraocular pressure (lOP) was measured by pneumatonometry and tonographic outflow facility by pneumatonography. Aqueous flow and outflow facility were determined either directly or indirectly by a fluorophotometric technique, and uveoscleral outflow was calculated secondarily. Comparison of values obtained in treated versus contralateral control eyes and on baseline versus day 8 of treatment were made. Results: Compared with baseline measurements, PhXA41 significantly (P < 0.001) reduced lOP by 5.5 ± 0.6 mmHg (mean ± standard error of the mean) as measured 3 hours after the last dose on the eighth day of treatment. Aqueous flow, tonographic outflow facility, and fluorophotometric outflow facility were not chanqed by PhXA41. However, uveoscleral outflow was significantly greater in the PhXA41-treated eyes (0.87 ± 0.22 JLI/minute) compared with either the contralateral vehicle-treated eyes (0.14 ± 0.30; P < 0.02) or baseline measurements (0.39 ± 0.20 JLI/minute; P < 0.05). Conclusions: PhXA41 decreases lOP in humans by increasing uveoscleral outflow without significantly affecting other parameters of aqueous humor dynamics. Ophthalmology 1993; 100: 1297-1304
Originally received: November 8, 1992. Revision accepted: March 8, 1993. From the Department of Ophthalmology, University of Nebraska Medical Center, Omaha. Presented in part at the Prostaglandin and Related Compounds meeting, Montreal, July 1992, and at the American Academy of Ophthalmology Annual Meeting, Dallas, November 1992. Supported in part by grant EY07836 and EY07865 from the National Eye Institute, Bethesda, Maryland, and a contribution from Kabi Pharmacia Ophthalmics, Uppsala, Sweden. Drs. Camras and Yablonski are consultants for Kabi Pharmacia Ophthalmics, Uppsala, Sweden, and Allergan Pharmaceuticals, Irvine, California, respectively. Reprint requests to Carol B. Toris, PhD, Department of Ophthalmology, University of Nebraska Medical Center, 600 South 42nd Street, Omaha, NE 68198-5540.
When applied topically to normotensive or glaucomatous eyes of experimental animals'< and humans.v' prostaglandin F 2" (PGF2,, ) and its analogs are potent ocular hypotensive agents. In the cynomolgus monkey, PGF2,,-Iisopropyl ester decreased intraocular pressure (lOP) by increasing uveoscleral outflow'-' and total outflow facility4.6,7 but with no effect on trabecular outflow facility." In humans, PGF2,,-isopropyl ester caused a decrease in lOP when applied at both higher (0.002%) and lower (0.00 1%) concentrations but resulted in a slight increase in tonographic outflow facility, which was measured with an electronic Schiotz-type indentation tonography unit, at only the higher concentration." When measured by pneumatonography, no change in tonographic outflow facility was found after PGF2,,-isopropyl ester treatment."
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In all clinical and experimental studies, a change in neither tonographic outflow facility8.9 nor aqueous flow9 •10 has been sufficient to account for the observed lOP drop . The most likely mechanism of the PGF 2a hypotensive effect in humans is an increase in uveoscleral outflow. To decrease ocular side effects, other PGF 2a analogs have been evaluated. PhXA34 (l3,14-dihydro-15[R,S]17-phenyl-I 8,19,20-trinor-PGF2a-l -isopropyl ester) is an epimeric mixture of a new 17-phenyl substituted analog of PGF 2a-isopropyl ester. In normotensive or ocular hypertensive human eyes, it effectively reduced lOP with few side effects compared with other prostaglandin anaIOgs. II-1 3 In humans, it did not alter aqueous flow but it did increase tonographic outflow facility.'? Like PGF 2a isopropyl ester, PhXA34 reduced lOP (Justin et aI, unpublished data; presented at the 1991 ARVO Annual Meeting) by increasing uveoscleral outflow (Selen et ai, unpublished data; presented at the 1991 ARVO Annual Meeting) in monkeys. PhXA41 (Iatanoprost) is the more active R-epimer of PhXA34, and therefore is a more potent ocular hypotensive agent in normotensive and ocular hypertensive humans (Gharagozloo et aI, AIm et aI, unpublished data; presented at the 1992 ARVO Annual Meeting). To determine the mechanism by which PhXA4I reduces lOP , we used a modified fluorophotometric method developed for human study.I4-16 Of particular interest was the effect of this drug on fluorophotometric outflow facility and uveoscleral outflow.
Materials and Methods Subjects Twent y-two volunteers were studied, including 6 subjects with normotension and 16 patients with ocular hypertension. The project was approved by the University ofNebraska Medical Center Institutional Review Board, and each subject gave written informed consent following federal guidelines. Subjects were excluded if they had visual field defects, active ocular infection, unstable cardiovascular or lung disease, previous intraocular surgery, laser therapy or ocular trauma, history of narrow angles, or acute angle-closure glaucoma . In addition , subjects younger than 21 years old, pregnant, unreliable, or noncompliant were excluded. Before commencing the study, a medical history and complete ophthalmologic examination, including a Snellen visual acuity, automated visual field test (Humphrey perimetry), slit-lamp biomicroscopic examination, lOP by Goldmann applanation tonometry, gonioscopy, and direct and indirect ophthalmoscopy, were performed on each patient . Anyone wearing contact lenses was asked to refrain from doing so for the weekof the study. If treated for their ocular hypertension, patients taking beta-adrenergic blockers were asked to discontinue these medications at least 3 weeks before the start of the study. Measurements The night before the baseline measurements, a solution of 2% fluorescein was instilled in each eye by the subject
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at home starting at 11:00 PM . One drop was placed on the cornea every 5 minutes for a total of six drops per eye. The volunteers were then instructed to.go to sleep after the six sets of drops so that their eyes remained closed. This ensured that the corneal stroma was uniformly stained and that there was sufficient time for the ratio of corneal-to-cameral fluorescence to reach a steady state. I ? In the clinic at 8:30 the following morning (baseline day), the corneal thickness and anterior chamber depth were measured with pachymetry, and anterior chamber volume was calculated as previously described." Four sets of fluorophotometry measurements with a fluorophotometer (Fluorotron Master, Coherent, Palo Alto , CA) were made at 45-minute intervals beginning at approximately 8:30 AM. At each fluorophotometry reading, two to three scans were made in quick succession in each eye, and the values were averaged. After the fourth set of scans, at approximately II :30 AM , lOP measurements were made with a pneumatonometer (Digilab Modular One, Bio-Rad, Cambridge, MA), which was tested on a daily basis with a calibration verifier as specified by the manufacturer. Accuracy also was checked periodically by measuring the same eye of a volunteer with both the pneumatonometer and a Goldmann applanation tonometer. Tonometry was performed three times in each eye and averaged. Afterward , acetazolamide (250 mg) was given orally and a drop of 0.5% timolol was instilled in each eye to reduce the lOP by decreasing aqueous flow, thus enabling calculation of outflow facility (see equation I, below in the Calculations section). The time of drug administration was recorded and was designated, "INTERVENTION." F1uorophotometry and lOP measurements were repeated at 1.0 to 1.5, 1.75 to 2.25, and 2.5 to 3.25 hours after INTERVENTION. A total of seven sets (4 before and 3 after INTERVENTION) of fluorophotometry measurements and four sets (1 before and 3 after INTERVENTION) of lOP measurements were collected for each eye. The time periods between scans 4 to 5, 5 to 6, and 6 to 7 were designated intervals A, B, and C, respectively (Fig 1). After the last lOP measurement, a 2-minute tonography test was performed with the pneumatonography unit (Digilab Modular One, Bio-Rad). Tonographic outflow facility was determined as previously described. 18 The patient was instructed to topically administer one drop of a solution of 0.006 % PhXA4I to one eye and its vehicle to the other in a randomized, doublemasked fashion. Bottles were coded and randomly assigned by the manufacturer (Kabi Pharmacia Ophthalmics, Uppsala, Sweden). Volunteers were asked to record, on specially designed forms , the time of drop administration and an y subjective side effects. Drops were self-administered every 12 hours starting at 8 PM on the baseline day (day I) until 8 PM on day 7. The final drops ofPhXA41 and its vehicle were given by the investigator at 8:30 AM on day 8 for a total of 14 consecutive doses. At 11 PM on day 7, subjects again instilled six drops of 2% fluorescein as before. All measurements were repeated on day 8 exactly as on day 1.
Toris et al . Prostaglandins and Aqueous Humor Dynamics in Humans 3.2
Cfl
INTERVEIImON 3.0
Z
2.6
l;l::::-
2.4
wE O~
a:-
(Cfl;
+ CflD + Cfld/3.
(2)
If acetazolamide and timolol administration did not decrease lOP by more than 1 mm Hg from baseline , the Cfl value was not used. The pressure-independent outflow, presumed to primaril y reflect uveoscleral outflow (Fu),2owas calculated 15 using equation 5 which was derived from 3 and 4.
2.8
iii
==
~..J
2.2
Fu == Fa - Ftr
(3)
DI
2.0
Ftr
==
Cfl (lOP - Pev)
(4)
1.8
Fu
==
Fa - Cfl (lOP - Pev)
(5)
!!:. .!!
1.6 1.4 9:00
10:00
11:00
12:00
13:00
14:00
15:00
TIME
Figure 1. An example of the washout of fluorescein from the cornea and ant erior chamber before and after administration of 0.5% topical timolol and 250 mg oral acetazolamide at the time indicated by the solid vertical line labeled INTERVENTION. Intra ocular pressure measurements were taken at the times marked by the one solid and three dotted vert ical lines. Fluorophotometrlc scans of th e cornea and anterior chamber are marked by an open box or solid circle, respectively.
Calculations Before and after INTERVENTION, anterior chamber aqueous humor flow rates were calculated as previously described. 19 Equation 1 was used to calculate fluorophotometric outflow facility, which is the inverse of the resistance to trabecular outflow. This equation is the ratio of a change in flow to the corresponding change in pressure. Timolol and acetazolamide were administered to produce these needed changes in flow and pressure. Fluo rophotometric outflow facility was calculated for the three time intervals after intervention (see Fig 1) with equation 1: Fa - Fa, Cfl, == lOP - lOP,
(1)
where Cfl, == fluorophotometric outflow facility during interval A, B, orC; Fa == aqueous flow rate, determined from the four scans preceding INTERVENTION; Fax == aqueous flow rate during int erval A, B, or C; lOP == lOP just before INTERVENTION; and lOP, == average ofIOP values between the beginning and the end of interval A, B, or C, respecti vely. Because each flow after INTERVENTION is the flow rate during each time interval, an average lOP for each time interval is used in the outflow facility calculation. With th is formula, three separate outflow facility values were determined, and the reported Cfl was an average of these three, i.e.,
Equation 3 is derived from the fact that flow into the anterior chamber (Fa) must be balanced by flow out (Fu and trabecular outflow [Ftr]). Determined from equation 4, Ftr is the prod uct of facility (inverse of resistance) and the pressure change across the outflow tissue (lOP - Pev). The episcleral venous pressure (Pev) is assumed to be 9 mmHg. After measurements from all 22 volunteers were collected, the codes were broken by the manufacturer, and the data were analyzed using the Student's two-tailed t test for paired samples. Comparison of results obtained in treated versus contralateral control eyes and on baseline versus day 8 of treatment were made , with P < 0.05 con sidered to be statistically significant. If values were not obtained for either eye before and after treatment, the parameter was omitted for that person. Of the 22 subjects enrolled in this study, 14 were women and 8 were men , and one was black, two were Asian , and the remainder was white. Their ages ranged from 31 to 75 years (mean of 50 ± 3 years). Six had normotension, ten had previously untreated ocular hypertension, four had been treated with 0.5% timolol, and two had been treated with 0.25% betaxolol suspension, which were discontinued 3 weeks before the study.
Results The mean lOP was 21.5 ± 0.9 mmHg (± standard error of the mean) in all eyes before drug treatment (range , 1432 mmHg). Compared with baseline measurements, lOP decreased significantly (P < 0.00 I) in both PhXA4ltreated and contralateral control eyes, but the treated eye had a significantly (P < 0.00 1) greater drop than the control eye (-5.5 ± 0.6 versus -1.8 ± 0.5 mmHg, respectively) (Table 1). PhXA4l-treated eyes with higher baseline lOP exhibited a greater lOP drop after treatment than thos e with lower baseline lOP (r == 0.71 ; P < 0.0003, Pearson 's correlation). The reduction of lOP after PhXA4l ranged from 2 (12%) to 11 (36%) mmHg, the latter representing a 51% decrease in outflow pressure. PhXA41 did not significantl y alter aqueous flow when compared with either baseline or contralateral control values. However, baseline aqueous flow was significantly (P < 0.03) different between the two eyes (Table I). Adequate tonographic tracings were obtained in both eyes of 16 of the 22 subjects before and after treatment
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before
after
Volume 100, Number 9, September 1993
before
after
TREATED EYE
CONTROL EYE
etazolamide and timolol adequately reduced lOP and aqueous flow (see Fig 2). In two subjects, there was less than a l-mmHg decrease in lOP after acetazolamide and timolol administration, thereby preventing assessment of fluorophotometric outflow facility. In the remaining subjects, the average fluorophotometric outflow facility did not change significantly in either eye after treatment. Because the calculation ofuveoscleral outflow requires a fluorophotometric outflow facility value, uveoscleral outflow was obtained in 20 of the 22 subjects (Table 3). Uveoscleral outflow was significantly (P < 0.05) increased in the treated eye compared with either baseline or contralateral control values. Of the 22 subjects, minor ocular side effects, such as redness and burning, were reported in the PhXA41-treated eye of 8, in the vehicle-treated eye of 1, and in both eyes of2 subjects at least once during the course of treatment.
Discussion
B 3.7
• 3.2
~ 0
-'
0
BASELINE
11II II
interval A
~
interval C
interval B
2.7
~ ? :> E
0::'-
w ..:; :> 0
-0:
2.2
1.7
1.2 before
after
CONTROL EYE
before
after
TREATED EYE
Figure 2. The effects of 250 mg oral acetazolamide and 0.5% topical timolol on intraocular pressure (A) and aqueous flow (B) before and after topical application of 0.006% PhXA41 to one eye (treated) and vehicle to the contralateral control eye in each of 22 human subjects. Each bar represents a mean ± standard error of the mean during the time interval either before (baseline) or during intervals A, B, or C after acetazolamide and timolol administration (INTERVENTION). Intervals A, B, and C are measured 1 to 1.5, 1.75 to 2.25, and 2.5 to 3.25 hours after INTERVENTION, respectively. Each asterisk indicates a significant (P < 0.0001) difference between values during baseline compared with those during intervals A, B, and C, using a two-tailed paired Student's t test.
(Table 2). Six subjects were not included because at least one of the four tracings for each was not acceptable for adequate interpretation. After treatment, tonographic outflow facility was slightly higher in treated eyes compared with contralateral control eyes, but not compared with baseline measurements. Fluorophotometric outflow facility was successfully determined in 20 of the 22 subjects (Table 2), since ac-
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PhXA4l (0.006%) applied twice daily for 1 week reduced lOP by an average of 5.5 mmHg, without altering aqueous flow. A recent study reported similar findings with PhXA4l after 5 days of treatment in healthy volunteers or in patients with ocular hypertension (Gharagozloo et al, unpublished data; presented at the 1992 ARVO Annual Meeting). Similarly, treatment with PhXA34, the epimeric mixture of PhXA4l, 12 or with PGF2,,-isopropyl ester9 , 10 showed an lOP reduction with no change in aqueous flow in healthy volunteers. The statistically significant difference in aqueous flow on the baseline day between control and treated eyes appears to be an example of the 5% chance of finding a significant difference in the means of the two data sets when a difference does not really exist. This is a type I error in statistics. Nevertheless, because aqueous flow in the treated eye was not significantly different compared with either baseline or contralateral control values, we can conclude with confidence that PhXA4l does not affect aqueous flow. In the current study, neither fluorophotometric outflow facility nor tonographic outflow facility were significantly affected by PhXA4l treatment compared with baseline measurements. Although several clinical studies have demonstrated an increase in tonographic outflow facility after topical application of PGF2,,-isopropyl ester, 8 PhXA34 12 or PhXA4l (Gharagozloo et al, unpublished data; presented at the 1992 ARVO Annual Meeting), the change in tonographic outflow facility could not completely account for the magnitude of the lOP reduction in any of these studies. Other studies have reported either increased or unaltered outflow facility after topical prostaglandin administration in rabbits,21,22 cats,7,23 and monkeys4,6,7,24,25 as determined by Schiotz's indentation tonographic, constant rate infusion, isotope accumulation or two-level constant pressure perfusion techniques. Based on a comparison of the PhXA4l-treated eye with baseline and contralateral control values, we found that uveoscleral outflow is increased sufficiently to account for the entire ocular hypotensive effectofPhXA4l in humans.
Toris et al . Prostaglandins and Aqueous Humor Dynamics in Humans Table 1. Intraocular Pressure and Aqueous Flow before and after Topical Application of 0.006% PhXA41 Applied Twice Daily for 8 Days to One Eye Each of 22 Human Subjects" lOP (mmHg)
Baseline Day 8 lOP
=
Aqueous Flow (JLl/min)
Contralateral Control Eye
Treated Eye
Contralateral Control Eye
Treated Eye
21.3 ± 0.9 19.5 ± 0.7t
21.6 ± 1.0 16.1 ± 0.7§
3.2 ± 0.2 2.9 ± 0.2
2.8 ± 0.2t 2.7 ± 0.1
intraocular pressure.
• Values represent mean ± standard error of the mean. P values are determined with the paired, two-tailed Student's t test.
t t
P < 0.03 compared with the contralateral vehicle-treated (control) eyes.
P < 0.001 compared with baseline values. § P < 0.0001 compared with either baseline or contralateral vehicle-treated (control) eyes.
These results are consistent with studies performed with several different prostaglandins in cats or monkeys, all of which conclude that prostaglandins act, primarily or solely, by increasing uveoscleral outflow (Selen et al, unpublished data; presented at the 1991 ARVO Annual Meeting) (Toris et al, unpublished data; presented at the 1992 ARVO Annual Meetingj.f' In the past, to determine uveoscleral outflow in humans, tonographic outflow facility has been used for C in the formula Fu = Fa - C(lOP - Pev).14,26 The determination of tonographic outflow facility involves assumptions about ocular rigidity and pseudofacility. The twolevel constant pressure perfusion technique is independent of ocular rigidity but still includes pseudo facility in its outflow facility measurement. Fluorophotometric outflow facility is determined by directly measuring the change in flow of aqueous humor through the anterior chamber without corneal indentation, volumetric changes, or artificiallyraising lOP, which may alter aqueous production. This eliminates the influence of ocular rigidity and pseudofacility from the measurement. Therefore, using fluo-
rophotometric outflow facility instead of tonographic outflow facility for C in the above formula should give a more accurate estimate of uveoscleral outflow. Tonographic outflow facility represents the sum oftrabecular outflow facility, pseudofacility, and uveoscleral facility. It is possible that the observed increase in tonographic outflow facility after prostaglandin treatment (Gharagozloo et al, unpublished data; 1992 ARVO Annual Meetingj'v'? may actually reflect an increase in pseudofacility or uveoscleral facility, without altering trabecular outflow facility at all.?" In support of this, monkeys treated with PGF 2a exhibited an increase in tonographic outflow facility, yet there was no change in trabecular outflow facility as determined by an isotope accumulation method." The fluorophotometric method to determine uveoscleral outflow is based on an assumption that uveoscleral outflow is pressure-independent aqueous outflow which is not changed by the lOP decrease caused by the administration of timolol or acetazolamide. In support of this, uveoscleral outflow has been found to be relatively in-
Table 2. Outflow Facility before and after Topical Application of 0.006% PhXA41 Applied Twice Daily for 8 Days to One Eye Each of Human Subjects· Tonographic Outflow Facility (JLl/min/mmHg) (n = 16)
Baseline Day 8
F1uorophotometric Outflow Facility (ul/min/rnmblg) (n = 20)
Contralateral Control Eye
Treated Eye
Contralateral Control Eye
Treated Eye
0.25 ± 0.03 0.18 ± 0.03t
0.25 ± 0.03 0.25 ± O.03t
0.24 ± 0.04 0.27 ± 0.04
0.22 ± 0.03 0.30 ± 0.04
• Values represent mean ± standard error of the mean. P values are determined with the paired, two-tailed Student's t test.
t t
P < 0.04 compared with baseline values. P < 0.009 compared with values in the contralateral vehicle-treated (control) eyes.
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Table 3. Uveoscleral Outflow before and after Topical Application of 0.006% PhXA41 Applied Twice Daily for 8 Days to One Eye Each of Human Subjects" Uveoscleral Outflow (n = 20) Baseline Day 8
(~ljmin)
Contralateral Control Eye
Treated Eye
0.44 ± 0.27 0.14 ± 0.30
0.39 ± 0.20 0.87 ± O.22t
• Values represent mean ± stand ard error of the mean .
t
p < 0.05 compared with baseline values, an d P < 0.02 compared with values in the contralateral veh icle-treated control eye using the two tailed , paired Student's t test.
dependent of lOP in the normal to high range in nonhuman primates. 2o,28,29 Therefore, facility of uveoscleral outflow should be negligible at the lOPs evaluated in this study. However; some studies suggest that prostaglandin treatment may actuall y increase facility of uveoscleral outflow.v" The proposed mechanisms include the relaxation of the ciliary muscle" or the alteration of its interstitial matrix." allowing a greater pressure-dependent flow through the uvea. Facility measured by the fluorophotometric technique (Cfl) represents the sum of trabecular (Ctr) and uveoscleral (Cfu) facility (Cfl = Ctr + Cfu). The absence of a change in fluoro photometric outflow facility after PhXA41 treatment may indicate that neither trabecular nor uveoscleral outflow facility was appreciably altered by the drug treatment in our subjects. The effectoftopical prostaglandins on episcleral venous pressure is not clear. Episcleral venous pressure was not changed in cats after PGF2,,-isopropyl ester." was slightly elevated in rabbits after PGE l ,31 and may have decreased in monkeys after PGF2,,-isopropyl ester," despite increased local blood flow in the anterior segment." Because the effect of prostaglandins on episcleral venous pressure appears to be minimal, we chose to use the same value for episcleral venous pressure in all calculations of uveoscleral outflow . A value of 9 mmHg was selected based on venomanometer measurements ofepiscleral venous pressure in 12 eyes of 6 humans with normotension in another study (Toris et ai, unpublished data ; presented at the 1991 ARVO Annual Meeting). Values ranging from 933 to 10 mrnl-lg" also have been reported in humans. If the value of 10 mmHg were used in the formula instead of 9, the calculated uveoscleral outflow values would be larger (1.17 ± 0.24 and 0.41 ± 0.28 in treated and contralateral control eyes on day 8, respectively), and the difference between the two eyes would be more significant (P < 0.006). Outflow facility determined with fluorophotometry is measured as the ratio of pharmacologically induced changes in aqueous flow and in lOP . This requires the use of agents that decrease lOP solely by decreasing aqueous flow without altering trabecular outflow facility, uveoscleral outflow or episcleral venous pressure. In addition, these agents must induce their effect when admin-
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istered along with the test drug. Timolol and acetazolamide work well for this PUrpose.1 4,34 In our stud y, the administration oftimolol and acetazolamide further decreased lOP and aqueous flow in all groups of eyes both before or after PhXA41 treatment. This decrease was maintained for at least 3 hours , which was sufficient time to adequately determine fluorophotometric outflow facility. Since a consistent reduction of lOP is maintained for at least 12 hours after each prostaglandin dose,2,8,9.12,13 it was assumed that any PhXA41-induced effect on fluorophotometric outflow facility would ha ve reached a new steady state which was not affected by acetazolamide and timolol on da y 8 of treatment. This assumption appears to be true because the means of CflA , CflB , and Cflc were not different by paired Student's t test analysis. A similar method to determine fluorophotometric outflow facility and uveoscleral outflow has been used in the clinical studies of argon laser trabeculoplasty" or pilocarpine." Each study found an increase in fluorophotometric outflow facility after treatment. The pilocarpine study confirmed previous reports in monkeys," thus demonstrating the feasibility of using the noninvasive fluorophotometric method to evaluate the effectsof drugs and surgery on aqueous humor dynamics. Previously reported uveoscleral outflow rates in humans calculated by using fluorophotometric outflow facility as C included 0.6 Ill/minute in patients with ocular hypertension " and 0.5 Ill/minute in patients with openangle glaucoma.P Using tonographic outflow facility as C, uveoscleral outflow was calculated to be 0.8 Ill/minute in young health y humans." Measured more directly with an intracamerally infused radioactive tracer, a uveoscleral outflow of 0.3 Ill/minute was reported in one older human with malignant melanoma." In the current study , the mean uveoscleral outflow from 40 eyes of20 subjects was 0.42 Ill/minute, which is consistent with the previous studies. Unlike other clinical reports with prostaglandins, we found a slight, but significant, lOP decrease in the contralateral vehicle-treated eyes of patients. Before concluding that this represents a contralateral effect, other possibilities must be ruled out , including inadvertent application of PhXA41 to the control eye by the patient. Because other parameters of aqueous humor dynamics were not significantly altered , the mechanism of this contralateral reduction of lOP is not clear. However, very high doses of prostaglandins applied in monkey eyes have demonstrated a definite contralateral effect.25,37,38 Similar to findings with PGF2,,-isopropyl ester 8.9,39 and with PhXA34,11-13 the current study demonstrates that PhXA41 has a greater hypotensive effect in patients with ocular hypertension than in volunteers with normotension. In conclusion, PhXA41 is a potent and well-tolerated ocular hypotensi ve agent in humans. It does not alter aqueous flow or trabecular outflow facility but does significantl y increase uveoscleral outflow, its primary mechanism of action. This mechanism suggests that excellent additivity ofthe hypotensi ve effectcould be achieved when used in combination with aqueous humor suppressants,
Toris et al . Prostaglandins and Aqueous Humor Dynamics in Humans such as beta-adrenergic blockers, as already demonstrated for PGF2a-isopropyl ester. 40 ,4 1
References I. Bito LZ, Camras CB, Gum CG , Resul B. The ocular hypotensive effects and side effects of prostaglandins on the eyes of experimental animals. In: Bito LZ, Stjernschantz J, eds. The Ocular Effects of Prostaglandins and Other Eicosanoids. New York: Alan R. Liss, 1989;349-68. (Prog Clin Bioi Res;312). 2. Camras CB, Podos SM. Reduction of intraocular pressure by exogenous and endogenous prostaglandins in monkeys and humans. In: Orance SM, Van Buskirk EM, Neufeld AH, eds. Pharmacology of Glaucoma. Baltimore: Williams and Wilkins, 1992; 175-83. 3. Aim A, Villumsen J. Effects of topically applied PGF2a and its isopropylester on normal and glaucomatous human eyes. In: Bito LZ, Stjernschantz J, eds. The Ocular Effects of Prostaglandins and Other Eicosanoids. New York: Alan R. Liss, 1989;447-58. (Prog Clin Bioi Res;312). 4. Gabelt BT, Kaufman PL. Prostaglandin F 2a increases uveoscleral outflow in the cynomolgus monkey. Exp Eye Res 1989;49:389-402. 5. Nilsson SFE, Samuelson M, Bill A, Stjernschantz J. Increased uveoscleral outflow as a possible mechanism of ocular hypotension caused by prostaglandin F 2a-I-isopropylester in the cynomolgus monkey. Exp Eye Res 1989;48: 707-16 . 6. Gabelt BT, Kaufman PL. The effect of prostaglandin F2a on trabecular outflow facilit y in cynomolgus monkeys. Exp Eye Res 1990;51 :87-91. 7. Lee P-Y, Pod os SM, Severin C. Effect of prostaglandin F2a on aqueous humor dynamics of rabbit, cat , and monkey. Invest Ophthalmol Vis Sci 1984;25: 1087-93. 8. Camras CB, Siebold EC, Lustgarten JS, et aJ. Maintained reduction of intraocular pressure by prostaglandin F2a-lisopropyl ester applied in multiple doses in ocular hypertensive and glaucoma patients. Ophthalmology 1989;96: 1329-37. 9. Villumsen J, AIm A. Prostaglandin F2a-isopropylester eye drops: effects in normal human eyes. Br J Ophthalmol 1989;73:419-26. 10. Kerstetter JR, Brubaker RF, Wilson SE, Kullerstrand U. Prostaglandin F2a-I-isopropylester lowers intraocular pressure without decreasing aqueous humor flow. Am J Ophthalmol 1988; I05:30-4. II. Villumsen J , Aim A. PhXA34-a prostaglandin F2a analog. Effect on intraocular pressure in patients with ocular hypertension, Sr J OphthalmoI1992;76:214-27. 12. Aim A, Villumsen J. PhXA34, a new potent ocular hypotensive drug. A study on dose-response relationship and on aqueous humor dynamics in healthy volunteers. Arch Ophthalmol 1991;109: 1564-8. 13. Camras CB, Schumer RA, Marsk A, et aJ. Intraocular pressure reduction in ocular hypertensive patients with PhXA34, a new prostaglandin analog. Arch Ophthalmol 1992; 110: 1733-8. 14. Schenker HI , Yablonski ME, Podos SM, Linder L. Fluorophotometric study of epinephrine and timolol in human subjects. Arch Ophthalmol 1981;99: 1212-16. 15. Yablonski ME, Cook OJ , Gray J. A fluorophotometric study of the effect ofargon laser trabeculoplasty on aqueous humor dynamics. Am J Ophthalmol 1985;99:579-82.
16. Hayashi M, Yablonski ME, Novack GO. Trabecular outflow facility determined by fluorophotometry in human subjects. Exp Eye Res 1989;48:621-5. 17. Topper JE, Mcl.aren J, Brubaker RF . Measurement of aqueous humor flow with scanning ocular fluorophotometers. CUrT Eye Res 1984;3: 1391-5. 18. Langham ME, Leydhecker W, Krieglstein G , Waller W. Pneumatonographic studies on normal and glaucomatous eyes. Adv Ophthalmol 1976;32:108-33. 19. Yablonski ME, Hayashi M, Cook OJ, et aJ. Fluorophotometric study of intravenous carbonic anhydrase inhibitors in rabbits. Invest Ophthalmol Vis Sci 1987;28:2076-82. 20. Bill A. Conventional and uveo-scleral drainage of aqueous humour in the cynomolgus monkey (Macaca iruss at normal and high intraocular pressures. Exp Eye Res 1966;5:45-54. 21. Camras CB, Bito LZ, Eakins KE. Reduction of intraocular pressure by prostaglandins applied topically to the eyes of conscious rabbits. Invest Ophthalmol Vis Sci 1977;16:112534. 22. Poyer JF, Gabelt B, Kaufman PL. The effect of topical PGF 2a on uveoscleral outflow and outflow facility in the rabbit eye. Exp Eye Res 1992;54:277-83. 23. Hayashi M, Yablonski ME, Bito LZ. Eicosanoids as a new class of ocular hypotensive agents. 2. Comparison of the apparent mechanism of the ocular hypotensi ve effects of A and F type prostaglandins. Invest Ophthalmol Vis Sci 1987;28: 1639-43. 24 . Kaufman PL. Effects of intracamerally infused prostaglandins on outflow facility in cynomolgus monkey eyes with intact or retrodisplaced ciliary muscle . Exp Eye Res 1986;43: 819-27. 25. Crawford K, Kaufman PL, Gabelt BT. Effects of topical PGF2a on aqueous humor dynamics in cynomolgus monkeys. CUrT Eye Res 1987;6: 1035-44. 26. Townsend OJ, Brubaker RF. Immediate effect of epinephrine on aqueous formation in the normal human eye as measured by fluorophotometry. Invest Ophthalmol Vis Sci 1980; 19:256-66. 27. Kaufman PL. PhXA34-induced ocular hypotension [letter]. Arch Ophthalmol 1992; 110: 1042. 28. Bill A. Further studies on the influence of the intraocular pressure on aqueous humor dynamics in cynomolgus monkeys. Invest Ophthalmol 1967;6:364-72. 29 . Toris CR, Pederson JE. Effect of intraocular pressure on uveoscleral outflow following cyclodialysis in the monkey eye. Invest Ophthalmol Vis Sci 1985;26:1745-9. 30. Ltitjen-Drecoll E, Tamm E. Morphological study of the an terior segment of cynomolgus monkey eyes following treatment with prostaglandin F 2a • Exp Eye Res 1988;47 :761-9. 31. Kass MA, Podos SM , Moses RA, Becker B. Prostaglandin E 1 and aqueous humor dynamics. Invest Ophthalmol 1972;11: 1022-7. 32. Stjernschantz J, Nilsson SFE, Astin M. Vasodynamic and angiogenic effects of eicosanoids in the eye. In: Bito LZ, Stjernschantz J, eds. The Ocular Effects of Prostaglandins and Other Eicosanoids. New York: Alan R. Liss, 1989; 15570. (Prog Clin Bioi Res;312). 33. Kupfer C, Ross K. Studies of aqueous humor d ynamics in man. I. Measurements in young normal subjects. Invest Ophthalmol Vis Sci 1971;10 :518-22. 34. Kupfer C. Clinical significance of pseudofacility. Stanford R. Gifford Memorial Lecture. Am J Ophthalmol 1973;75: 193-204. 35. Bill A, Walinder P'E, The effects of pilocarpine on the dynamics of aqueous humor in a primate iMacaca irus). Invest Ophthalmol 1966;5:170-5.
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Volume 100, Number 9, September 1993 39. Villumsen J, AIm A, Soderstrom M. Prostaglandin F2,,-isopropylester eye drops: effect on intraocular pressure in openangle glaucoma. Br J Ophthalmol 1989;73:915-9. 40. Villumsen J, AIm A. The effect of adding prostaglandin F 2,,isopropylester to timolol in patients with open angle glaucoma. Arch Ophthalmol 1990;108:1102-5. 41. Lee P-Y, Shao H, Camras CB, Podos SM. Additivity of prostaglandin F2,,-I-isopropyl ester to timolol in glaucoma patients. Ophthalmology 1991;98: 1079-82.
36. Bill A, Phillips CI. Uveoscleral drainage of aqueous humour in human eyes. Exp Eye Res 1971;12:275-81. 37. Camras CB,BitoLZ. Reduction of intraocularpressurein normal and glaucomatous primate (Aotus trivirgatus) eyes by topically applied prostaglandin F2a• Curr Eye Res 1981;1:205-9. 38. Camras CB, Podos SM, Rosenthal JS, et al. Multiple dosing of prostaglandin F2"or epinephrine on cynomolgus monkey eyes. I. Aqueous humor dynamics. Invest Ophthalmol Vis Sci 1987;28:463-9.
Discussion
by PaulL.Kaufrnan,~D
Although the posterior uveoscleral drainage pathway has long been recognized, I techniques for its quantitation have been complex, invasive, imprecise, terminal, or, at best, marginally tolerated in experimental animals, and unsuited to humans.P Toris et al present a noninvasive means of estimating it clinically from measurement of the other parameters of the modified Goldmann equation. Tonographic facility represents the arithmetic sum of trabecular, uveoscleral, and aqueous inflow, or pseudo facility;, compromising the evaluation of surgical or pharmacologic manipulations that affect the latter two parameters. The authors' fluorophotometric technique purports to measure primarily trabecular facility. Unfortunately, this is not strictly so. Uveoscleral facility also will affect the relationship between a known magnitude of aqueous flow suppression and the magnitude of the intraocular pressure fall it produces. Normally, uveoscleral and inflow facility are negligible, and assumptions regarding diffusional loss of fluorescein from the anterior segment are accurate, so that the authors' technique will give reasonable estimates oftrabecular facility and consequently of uveoscleral outflow. However, so will conventional tonography, as illustrated by the authors' own data. Regarding the PhXA41 data, the first limitation of the study is that the authors did not measure episcleral venous pressure. Whether the episcleral venous pressure was truly the assumed 9 mmHg would not affect their qualitative conclusion about the drug's effect on uveoscleral outflow, as long as the prostanoid did not itself affect the episcleral venous pressure. However, no data in support of such constancy are provided, and the conjunctival/episcleral hyperemia might suggestan episcleral venous pressure effect. Second, there is evidence from monkey studies that prostaglandin F2" (PGF2,,) may increase uveoscleral facility,2,4 and from both human' and monkey" studies it is shown that it may increase iridociliary vascular and/or epithelial perFrom the Department of Ophthalmology, University of Wisconsin, Clinical Science Center, Madison. Supported by NIH grant EY02698, Bethesda, Maryland.
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meability to fluorescein, again contravening assumptions underlying the authors' calculations. Such factors, along with inherent limitations in the precision of the measurements themselves, may help explain some of the variability in the authors' data. These caveats notwithstanding, the authors' conclusion that the intraocular pressure fall induced by PhXA41 in the human is primarily and perhaps completely consequent to increased uveoscleral outflow is almost certainly correct and consistent with prior humarr':" and monkey2,3,5 studies using other PGF2" congeners. References I. Kaufman, PL. Aqueous humor dynamics. In: Tasman W,
2. 3.
4. 5.
6. 7.
Jaeger EA, eds. Duane's Clinical Ophthalmology, Philadelphia: Harper & Row, 1985; v. 3, chap. 45. Gabelt BT, Kaufman PL. Prostaglandin F2" increases uveoscleral outflow in the cynomolgus monkey. Exp Eye Res 1989;49:389-402. Nilsson SFE, Samuelsson M, Bill A, Stjernschantz J. Increased uveoscleral outflow as a possible mechanism of ocular hypotension caused by prostaglandin F 2" l-isopropylester in the cynomolgus monkey. Exp Eye Res 1989;48: 707-16. Gabelt BT, Kaufman PL. The effect of prostaglandin F2" on trabecular outflow facility in cynomolgus monkeys. Exp Eye Res 1990;51:87-91. AIm A, Villumsen J. PhXA34, a new potent ocular hypotensive drug. A study on dose-response relationship and on aqueous humor dynamics in healthy volunteers. Arch Ophthalmol 1991;109:1564-8. Crawford K, Kaufman PL, Gabelt BT. Effects of topical PGF 2a on aqueous humor dynamics in cynomolgus monkeys. Curr Eye Res 1987;6:1035-44. Villumsen J, AIm A. Prostaglandin F2a-isopropylester eye drops: effects in normal human eyes. Br J Ophthalmol 1989;73:419-25.
Originally received: November 8, 1992. Revision accepted: March 8, 1993. From the Department of Ophthalmology, University of Nebraska Medical Center, Omaha. Presented in part at the Prostaglandin and Related Compounds meeting, Montreal, July 1992, and at the American Academy of Ophthalmology Annual Meeting, Dallas, November 1992. Supported in part by grant EY07836 and EY07865 from the National Eye Institute, Bethesda, Maryland, and a contribution from Kabi Pharmacia Ophthalmics, Uppsala, Sweden. Drs. Camras and Yablonski are consultants for Kabi Pharmacia Ophthalmics, Uppsala, Sweden, and Allergan Pharmaceuticals, Irvine, California, respectively. Reprint requests to Carol B. Toris, PhD, Department of Ophthalmology, University of Nebraska Medical Center, 600 South 42nd Street, Omaha, NE 68198-5540.
When applied topically to normotensive or glaucomatous eyes of experimental animals'< and humans.v' prostaglandin F 2" (PGF2,, ) and its analogs are potent ocular hypotensive agents. In the cynomolgus monkey, PGF2,,-Iisopropyl ester decreased intraocular pressure (lOP) by increasing uveoscleral outflow'-' and total outflow facility4.6,7 but with no effect on trabecular outflow facility." In humans, PGF2,,-isopropyl ester caused a decrease in lOP when applied at both higher (0.002%) and lower (0.00 1%) concentrations but resulted in a slight increase in tonographic outflow facility, which was measured with an electronic Schiotz-type indentation tonography unit, at only the higher concentration." When measured by pneumatonography, no change in tonographic outflow facility was found after PGF2,,-isopropyl ester treatment."
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In all clinical and experimental studies, a change in neither tonographic outflow facility8.9 nor aqueous flow9 •10 has been sufficient to account for the observed lOP drop . The most likely mechanism of the PGF 2a hypotensive effect in humans is an increase in uveoscleral outflow. To decrease ocular side effects, other PGF 2a analogs have been evaluated. PhXA34 (l3,14-dihydro-15[R,S]17-phenyl-I 8,19,20-trinor-PGF2a-l -isopropyl ester) is an epimeric mixture of a new 17-phenyl substituted analog of PGF 2a-isopropyl ester. In normotensive or ocular hypertensive human eyes, it effectively reduced lOP with few side effects compared with other prostaglandin anaIOgs. II-1 3 In humans, it did not alter aqueous flow but it did increase tonographic outflow facility.'? Like PGF 2a isopropyl ester, PhXA34 reduced lOP (Justin et aI, unpublished data; presented at the 1991 ARVO Annual Meeting) by increasing uveoscleral outflow (Selen et ai, unpublished data; presented at the 1991 ARVO Annual Meeting) in monkeys. PhXA41 (Iatanoprost) is the more active R-epimer of PhXA34, and therefore is a more potent ocular hypotensive agent in normotensive and ocular hypertensive humans (Gharagozloo et aI, AIm et aI, unpublished data; presented at the 1992 ARVO Annual Meeting). To determine the mechanism by which PhXA4I reduces lOP , we used a modified fluorophotometric method developed for human study.I4-16 Of particular interest was the effect of this drug on fluorophotometric outflow facility and uveoscleral outflow.
Materials and Methods Subjects Twent y-two volunteers were studied, including 6 subjects with normotension and 16 patients with ocular hypertension. The project was approved by the University ofNebraska Medical Center Institutional Review Board, and each subject gave written informed consent following federal guidelines. Subjects were excluded if they had visual field defects, active ocular infection, unstable cardiovascular or lung disease, previous intraocular surgery, laser therapy or ocular trauma, history of narrow angles, or acute angle-closure glaucoma . In addition , subjects younger than 21 years old, pregnant, unreliable, or noncompliant were excluded. Before commencing the study, a medical history and complete ophthalmologic examination, including a Snellen visual acuity, automated visual field test (Humphrey perimetry), slit-lamp biomicroscopic examination, lOP by Goldmann applanation tonometry, gonioscopy, and direct and indirect ophthalmoscopy, were performed on each patient . Anyone wearing contact lenses was asked to refrain from doing so for the weekof the study. If treated for their ocular hypertension, patients taking beta-adrenergic blockers were asked to discontinue these medications at least 3 weeks before the start of the study. Measurements The night before the baseline measurements, a solution of 2% fluorescein was instilled in each eye by the subject
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at home starting at 11:00 PM . One drop was placed on the cornea every 5 minutes for a total of six drops per eye. The volunteers were then instructed to.go to sleep after the six sets of drops so that their eyes remained closed. This ensured that the corneal stroma was uniformly stained and that there was sufficient time for the ratio of corneal-to-cameral fluorescence to reach a steady state. I ? In the clinic at 8:30 the following morning (baseline day), the corneal thickness and anterior chamber depth were measured with pachymetry, and anterior chamber volume was calculated as previously described." Four sets of fluorophotometry measurements with a fluorophotometer (Fluorotron Master, Coherent, Palo Alto , CA) were made at 45-minute intervals beginning at approximately 8:30 AM. At each fluorophotometry reading, two to three scans were made in quick succession in each eye, and the values were averaged. After the fourth set of scans, at approximately II :30 AM , lOP measurements were made with a pneumatonometer (Digilab Modular One, Bio-Rad, Cambridge, MA), which was tested on a daily basis with a calibration verifier as specified by the manufacturer. Accuracy also was checked periodically by measuring the same eye of a volunteer with both the pneumatonometer and a Goldmann applanation tonometer. Tonometry was performed three times in each eye and averaged. Afterward , acetazolamide (250 mg) was given orally and a drop of 0.5% timolol was instilled in each eye to reduce the lOP by decreasing aqueous flow, thus enabling calculation of outflow facility (see equation I, below in the Calculations section). The time of drug administration was recorded and was designated, "INTERVENTION." F1uorophotometry and lOP measurements were repeated at 1.0 to 1.5, 1.75 to 2.25, and 2.5 to 3.25 hours after INTERVENTION. A total of seven sets (4 before and 3 after INTERVENTION) of fluorophotometry measurements and four sets (1 before and 3 after INTERVENTION) of lOP measurements were collected for each eye. The time periods between scans 4 to 5, 5 to 6, and 6 to 7 were designated intervals A, B, and C, respectively (Fig 1). After the last lOP measurement, a 2-minute tonography test was performed with the pneumatonography unit (Digilab Modular One, Bio-Rad). Tonographic outflow facility was determined as previously described. 18 The patient was instructed to topically administer one drop of a solution of 0.006 % PhXA4I to one eye and its vehicle to the other in a randomized, doublemasked fashion. Bottles were coded and randomly assigned by the manufacturer (Kabi Pharmacia Ophthalmics, Uppsala, Sweden). Volunteers were asked to record, on specially designed forms , the time of drop administration and an y subjective side effects. Drops were self-administered every 12 hours starting at 8 PM on the baseline day (day I) until 8 PM on day 7. The final drops ofPhXA41 and its vehicle were given by the investigator at 8:30 AM on day 8 for a total of 14 consecutive doses. At 11 PM on day 7, subjects again instilled six drops of 2% fluorescein as before. All measurements were repeated on day 8 exactly as on day 1.
Toris et al . Prostaglandins and Aqueous Humor Dynamics in Humans 3.2
Cfl
INTERVEIImON 3.0
Z
2.6
l;l::::-
2.4
wE O~
a:-
(Cfl;
+ CflD + Cfld/3.
(2)
If acetazolamide and timolol administration did not decrease lOP by more than 1 mm Hg from baseline , the Cfl value was not used. The pressure-independent outflow, presumed to primaril y reflect uveoscleral outflow (Fu),2owas calculated 15 using equation 5 which was derived from 3 and 4.
2.8
iii
==
~..J
2.2
Fu == Fa - Ftr
(3)
DI
2.0
Ftr
==
Cfl (lOP - Pev)
(4)
1.8
Fu
==
Fa - Cfl (lOP - Pev)
(5)
!!:. .!!
1.6 1.4 9:00
10:00
11:00
12:00
13:00
14:00
15:00
TIME
Figure 1. An example of the washout of fluorescein from the cornea and ant erior chamber before and after administration of 0.5% topical timolol and 250 mg oral acetazolamide at the time indicated by the solid vertical line labeled INTERVENTION. Intra ocular pressure measurements were taken at the times marked by the one solid and three dotted vert ical lines. Fluorophotometrlc scans of th e cornea and anterior chamber are marked by an open box or solid circle, respectively.
Calculations Before and after INTERVENTION, anterior chamber aqueous humor flow rates were calculated as previously described. 19 Equation 1 was used to calculate fluorophotometric outflow facility, which is the inverse of the resistance to trabecular outflow. This equation is the ratio of a change in flow to the corresponding change in pressure. Timolol and acetazolamide were administered to produce these needed changes in flow and pressure. Fluo rophotometric outflow facility was calculated for the three time intervals after intervention (see Fig 1) with equation 1: Fa - Fa, Cfl, == lOP - lOP,
(1)
where Cfl, == fluorophotometric outflow facility during interval A, B, orC; Fa == aqueous flow rate, determined from the four scans preceding INTERVENTION; Fax == aqueous flow rate during int erval A, B, or C; lOP == lOP just before INTERVENTION; and lOP, == average ofIOP values between the beginning and the end of interval A, B, or C, respecti vely. Because each flow after INTERVENTION is the flow rate during each time interval, an average lOP for each time interval is used in the outflow facility calculation. With th is formula, three separate outflow facility values were determined, and the reported Cfl was an average of these three, i.e.,
Equation 3 is derived from the fact that flow into the anterior chamber (Fa) must be balanced by flow out (Fu and trabecular outflow [Ftr]). Determined from equation 4, Ftr is the prod uct of facility (inverse of resistance) and the pressure change across the outflow tissue (lOP - Pev). The episcleral venous pressure (Pev) is assumed to be 9 mmHg. After measurements from all 22 volunteers were collected, the codes were broken by the manufacturer, and the data were analyzed using the Student's two-tailed t test for paired samples. Comparison of results obtained in treated versus contralateral control eyes and on baseline versus day 8 of treatment were made , with P < 0.05 con sidered to be statistically significant. If values were not obtained for either eye before and after treatment, the parameter was omitted for that person. Of the 22 subjects enrolled in this study, 14 were women and 8 were men , and one was black, two were Asian , and the remainder was white. Their ages ranged from 31 to 75 years (mean of 50 ± 3 years). Six had normotension, ten had previously untreated ocular hypertension, four had been treated with 0.5% timolol, and two had been treated with 0.25% betaxolol suspension, which were discontinued 3 weeks before the study.
Results The mean lOP was 21.5 ± 0.9 mmHg (± standard error of the mean) in all eyes before drug treatment (range , 1432 mmHg). Compared with baseline measurements, lOP decreased significantly (P < 0.00 I) in both PhXA4ltreated and contralateral control eyes, but the treated eye had a significantly (P < 0.00 1) greater drop than the control eye (-5.5 ± 0.6 versus -1.8 ± 0.5 mmHg, respectively) (Table 1). PhXA4l-treated eyes with higher baseline lOP exhibited a greater lOP drop after treatment than thos e with lower baseline lOP (r == 0.71 ; P < 0.0003, Pearson 's correlation). The reduction of lOP after PhXA4l ranged from 2 (12%) to 11 (36%) mmHg, the latter representing a 51% decrease in outflow pressure. PhXA41 did not significantl y alter aqueous flow when compared with either baseline or contralateral control values. However, baseline aqueous flow was significantly (P < 0.03) different between the two eyes (Table I). Adequate tonographic tracings were obtained in both eyes of 16 of the 22 subjects before and after treatment
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before
after
Volume 100, Number 9, September 1993
before
after
TREATED EYE
CONTROL EYE
etazolamide and timolol adequately reduced lOP and aqueous flow (see Fig 2). In two subjects, there was less than a l-mmHg decrease in lOP after acetazolamide and timolol administration, thereby preventing assessment of fluorophotometric outflow facility. In the remaining subjects, the average fluorophotometric outflow facility did not change significantly in either eye after treatment. Because the calculation ofuveoscleral outflow requires a fluorophotometric outflow facility value, uveoscleral outflow was obtained in 20 of the 22 subjects (Table 3). Uveoscleral outflow was significantly (P < 0.05) increased in the treated eye compared with either baseline or contralateral control values. Of the 22 subjects, minor ocular side effects, such as redness and burning, were reported in the PhXA41-treated eye of 8, in the vehicle-treated eye of 1, and in both eyes of2 subjects at least once during the course of treatment.
Discussion
B 3.7
• 3.2
~ 0
-'
0
BASELINE
11II II
interval A
~
interval C
interval B
2.7
~ ? :> E
0::'-
w ..:; :> 0
-0:
2.2
1.7
1.2 before
after
CONTROL EYE
before
after
TREATED EYE
Figure 2. The effects of 250 mg oral acetazolamide and 0.5% topical timolol on intraocular pressure (A) and aqueous flow (B) before and after topical application of 0.006% PhXA41 to one eye (treated) and vehicle to the contralateral control eye in each of 22 human subjects. Each bar represents a mean ± standard error of the mean during the time interval either before (baseline) or during intervals A, B, or C after acetazolamide and timolol administration (INTERVENTION). Intervals A, B, and C are measured 1 to 1.5, 1.75 to 2.25, and 2.5 to 3.25 hours after INTERVENTION, respectively. Each asterisk indicates a significant (P < 0.0001) difference between values during baseline compared with those during intervals A, B, and C, using a two-tailed paired Student's t test.
(Table 2). Six subjects were not included because at least one of the four tracings for each was not acceptable for adequate interpretation. After treatment, tonographic outflow facility was slightly higher in treated eyes compared with contralateral control eyes, but not compared with baseline measurements. Fluorophotometric outflow facility was successfully determined in 20 of the 22 subjects (Table 2), since ac-
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PhXA4l (0.006%) applied twice daily for 1 week reduced lOP by an average of 5.5 mmHg, without altering aqueous flow. A recent study reported similar findings with PhXA4l after 5 days of treatment in healthy volunteers or in patients with ocular hypertension (Gharagozloo et al, unpublished data; presented at the 1992 ARVO Annual Meeting). Similarly, treatment with PhXA34, the epimeric mixture of PhXA4l, 12 or with PGF2,,-isopropyl ester9 , 10 showed an lOP reduction with no change in aqueous flow in healthy volunteers. The statistically significant difference in aqueous flow on the baseline day between control and treated eyes appears to be an example of the 5% chance of finding a significant difference in the means of the two data sets when a difference does not really exist. This is a type I error in statistics. Nevertheless, because aqueous flow in the treated eye was not significantly different compared with either baseline or contralateral control values, we can conclude with confidence that PhXA4l does not affect aqueous flow. In the current study, neither fluorophotometric outflow facility nor tonographic outflow facility were significantly affected by PhXA4l treatment compared with baseline measurements. Although several clinical studies have demonstrated an increase in tonographic outflow facility after topical application of PGF2,,-isopropyl ester, 8 PhXA34 12 or PhXA4l (Gharagozloo et al, unpublished data; presented at the 1992 ARVO Annual Meeting), the change in tonographic outflow facility could not completely account for the magnitude of the lOP reduction in any of these studies. Other studies have reported either increased or unaltered outflow facility after topical prostaglandin administration in rabbits,21,22 cats,7,23 and monkeys4,6,7,24,25 as determined by Schiotz's indentation tonographic, constant rate infusion, isotope accumulation or two-level constant pressure perfusion techniques. Based on a comparison of the PhXA4l-treated eye with baseline and contralateral control values, we found that uveoscleral outflow is increased sufficiently to account for the entire ocular hypotensive effectofPhXA4l in humans.
Toris et al . Prostaglandins and Aqueous Humor Dynamics in Humans Table 1. Intraocular Pressure and Aqueous Flow before and after Topical Application of 0.006% PhXA41 Applied Twice Daily for 8 Days to One Eye Each of 22 Human Subjects" lOP (mmHg)
Baseline Day 8 lOP
=
Aqueous Flow (JLl/min)
Contralateral Control Eye
Treated Eye
Contralateral Control Eye
Treated Eye
21.3 ± 0.9 19.5 ± 0.7t
21.6 ± 1.0 16.1 ± 0.7§
3.2 ± 0.2 2.9 ± 0.2
2.8 ± 0.2t 2.7 ± 0.1
intraocular pressure.
• Values represent mean ± standard error of the mean. P values are determined with the paired, two-tailed Student's t test.
t t
P < 0.03 compared with the contralateral vehicle-treated (control) eyes.
P < 0.001 compared with baseline values. § P < 0.0001 compared with either baseline or contralateral vehicle-treated (control) eyes.
These results are consistent with studies performed with several different prostaglandins in cats or monkeys, all of which conclude that prostaglandins act, primarily or solely, by increasing uveoscleral outflow (Selen et al, unpublished data; presented at the 1991 ARVO Annual Meeting) (Toris et al, unpublished data; presented at the 1992 ARVO Annual Meetingj.f' In the past, to determine uveoscleral outflow in humans, tonographic outflow facility has been used for C in the formula Fu = Fa - C(lOP - Pev).14,26 The determination of tonographic outflow facility involves assumptions about ocular rigidity and pseudofacility. The twolevel constant pressure perfusion technique is independent of ocular rigidity but still includes pseudo facility in its outflow facility measurement. Fluorophotometric outflow facility is determined by directly measuring the change in flow of aqueous humor through the anterior chamber without corneal indentation, volumetric changes, or artificiallyraising lOP, which may alter aqueous production. This eliminates the influence of ocular rigidity and pseudofacility from the measurement. Therefore, using fluo-
rophotometric outflow facility instead of tonographic outflow facility for C in the above formula should give a more accurate estimate of uveoscleral outflow. Tonographic outflow facility represents the sum oftrabecular outflow facility, pseudofacility, and uveoscleral facility. It is possible that the observed increase in tonographic outflow facility after prostaglandin treatment (Gharagozloo et al, unpublished data; 1992 ARVO Annual Meetingj'v'? may actually reflect an increase in pseudofacility or uveoscleral facility, without altering trabecular outflow facility at all.?" In support of this, monkeys treated with PGF 2a exhibited an increase in tonographic outflow facility, yet there was no change in trabecular outflow facility as determined by an isotope accumulation method." The fluorophotometric method to determine uveoscleral outflow is based on an assumption that uveoscleral outflow is pressure-independent aqueous outflow which is not changed by the lOP decrease caused by the administration of timolol or acetazolamide. In support of this, uveoscleral outflow has been found to be relatively in-
Table 2. Outflow Facility before and after Topical Application of 0.006% PhXA41 Applied Twice Daily for 8 Days to One Eye Each of Human Subjects· Tonographic Outflow Facility (JLl/min/mmHg) (n = 16)
Baseline Day 8
F1uorophotometric Outflow Facility (ul/min/rnmblg) (n = 20)
Contralateral Control Eye
Treated Eye
Contralateral Control Eye
Treated Eye
0.25 ± 0.03 0.18 ± 0.03t
0.25 ± 0.03 0.25 ± O.03t
0.24 ± 0.04 0.27 ± 0.04
0.22 ± 0.03 0.30 ± 0.04
• Values represent mean ± standard error of the mean. P values are determined with the paired, two-tailed Student's t test.
t t
P < 0.04 compared with baseline values. P < 0.009 compared with values in the contralateral vehicle-treated (control) eyes.
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Volume 100, Number 9, September 1993
Table 3. Uveoscleral Outflow before and after Topical Application of 0.006% PhXA41 Applied Twice Daily for 8 Days to One Eye Each of Human Subjects" Uveoscleral Outflow (n = 20) Baseline Day 8
(~ljmin)
Contralateral Control Eye
Treated Eye
0.44 ± 0.27 0.14 ± 0.30
0.39 ± 0.20 0.87 ± O.22t
• Values represent mean ± stand ard error of the mean .
t
p < 0.05 compared with baseline values, an d P < 0.02 compared with values in the contralateral veh icle-treated control eye using the two tailed , paired Student's t test.
dependent of lOP in the normal to high range in nonhuman primates. 2o,28,29 Therefore, facility of uveoscleral outflow should be negligible at the lOPs evaluated in this study. However; some studies suggest that prostaglandin treatment may actuall y increase facility of uveoscleral outflow.v" The proposed mechanisms include the relaxation of the ciliary muscle" or the alteration of its interstitial matrix." allowing a greater pressure-dependent flow through the uvea. Facility measured by the fluorophotometric technique (Cfl) represents the sum of trabecular (Ctr) and uveoscleral (Cfu) facility (Cfl = Ctr + Cfu). The absence of a change in fluoro photometric outflow facility after PhXA41 treatment may indicate that neither trabecular nor uveoscleral outflow facility was appreciably altered by the drug treatment in our subjects. The effectoftopical prostaglandins on episcleral venous pressure is not clear. Episcleral venous pressure was not changed in cats after PGF2,,-isopropyl ester." was slightly elevated in rabbits after PGE l ,31 and may have decreased in monkeys after PGF2,,-isopropyl ester," despite increased local blood flow in the anterior segment." Because the effect of prostaglandins on episcleral venous pressure appears to be minimal, we chose to use the same value for episcleral venous pressure in all calculations of uveoscleral outflow . A value of 9 mmHg was selected based on venomanometer measurements ofepiscleral venous pressure in 12 eyes of 6 humans with normotension in another study (Toris et ai, unpublished data ; presented at the 1991 ARVO Annual Meeting). Values ranging from 933 to 10 mrnl-lg" also have been reported in humans. If the value of 10 mmHg were used in the formula instead of 9, the calculated uveoscleral outflow values would be larger (1.17 ± 0.24 and 0.41 ± 0.28 in treated and contralateral control eyes on day 8, respectively), and the difference between the two eyes would be more significant (P < 0.006). Outflow facility determined with fluorophotometry is measured as the ratio of pharmacologically induced changes in aqueous flow and in lOP . This requires the use of agents that decrease lOP solely by decreasing aqueous flow without altering trabecular outflow facility, uveoscleral outflow or episcleral venous pressure. In addition, these agents must induce their effect when admin-
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istered along with the test drug. Timolol and acetazolamide work well for this PUrpose.1 4,34 In our stud y, the administration oftimolol and acetazolamide further decreased lOP and aqueous flow in all groups of eyes both before or after PhXA41 treatment. This decrease was maintained for at least 3 hours , which was sufficient time to adequately determine fluorophotometric outflow facility. Since a consistent reduction of lOP is maintained for at least 12 hours after each prostaglandin dose,2,8,9.12,13 it was assumed that any PhXA41-induced effect on fluorophotometric outflow facility would ha ve reached a new steady state which was not affected by acetazolamide and timolol on da y 8 of treatment. This assumption appears to be true because the means of CflA , CflB , and Cflc were not different by paired Student's t test analysis. A similar method to determine fluorophotometric outflow facility and uveoscleral outflow has been used in the clinical studies of argon laser trabeculoplasty" or pilocarpine." Each study found an increase in fluorophotometric outflow facility after treatment. The pilocarpine study confirmed previous reports in monkeys," thus demonstrating the feasibility of using the noninvasive fluorophotometric method to evaluate the effectsof drugs and surgery on aqueous humor dynamics. Previously reported uveoscleral outflow rates in humans calculated by using fluorophotometric outflow facility as C included 0.6 Ill/minute in patients with ocular hypertension " and 0.5 Ill/minute in patients with openangle glaucoma.P Using tonographic outflow facility as C, uveoscleral outflow was calculated to be 0.8 Ill/minute in young health y humans." Measured more directly with an intracamerally infused radioactive tracer, a uveoscleral outflow of 0.3 Ill/minute was reported in one older human with malignant melanoma." In the current study , the mean uveoscleral outflow from 40 eyes of20 subjects was 0.42 Ill/minute, which is consistent with the previous studies. Unlike other clinical reports with prostaglandins, we found a slight, but significant, lOP decrease in the contralateral vehicle-treated eyes of patients. Before concluding that this represents a contralateral effect, other possibilities must be ruled out , including inadvertent application of PhXA41 to the control eye by the patient. Because other parameters of aqueous humor dynamics were not significantly altered , the mechanism of this contralateral reduction of lOP is not clear. However, very high doses of prostaglandins applied in monkey eyes have demonstrated a definite contralateral effect.25,37,38 Similar to findings with PGF2,,-isopropyl ester 8.9,39 and with PhXA34,11-13 the current study demonstrates that PhXA41 has a greater hypotensive effect in patients with ocular hypertension than in volunteers with normotension. In conclusion, PhXA41 is a potent and well-tolerated ocular hypotensi ve agent in humans. It does not alter aqueous flow or trabecular outflow facility but does significantl y increase uveoscleral outflow, its primary mechanism of action. This mechanism suggests that excellent additivity ofthe hypotensi ve effectcould be achieved when used in combination with aqueous humor suppressants,
Toris et al . Prostaglandins and Aqueous Humor Dynamics in Humans such as beta-adrenergic blockers, as already demonstrated for PGF2a-isopropyl ester. 40 ,4 1
References I. Bito LZ, Camras CB, Gum CG , Resul B. The ocular hypotensive effects and side effects of prostaglandins on the eyes of experimental animals. In: Bito LZ, Stjernschantz J, eds. The Ocular Effects of Prostaglandins and Other Eicosanoids. New York: Alan R. Liss, 1989;349-68. (Prog Clin Bioi Res;312). 2. Camras CB, Podos SM. Reduction of intraocular pressure by exogenous and endogenous prostaglandins in monkeys and humans. In: Orance SM, Van Buskirk EM, Neufeld AH, eds. Pharmacology of Glaucoma. Baltimore: Williams and Wilkins, 1992; 175-83. 3. Aim A, Villumsen J. Effects of topically applied PGF2a and its isopropylester on normal and glaucomatous human eyes. In: Bito LZ, Stjernschantz J, eds. The Ocular Effects of Prostaglandins and Other Eicosanoids. New York: Alan R. Liss, 1989;447-58. (Prog Clin Bioi Res;312). 4. Gabelt BT, Kaufman PL. Prostaglandin F 2a increases uveoscleral outflow in the cynomolgus monkey. Exp Eye Res 1989;49:389-402. 5. Nilsson SFE, Samuelson M, Bill A, Stjernschantz J. Increased uveoscleral outflow as a possible mechanism of ocular hypotension caused by prostaglandin F 2a-I-isopropylester in the cynomolgus monkey. Exp Eye Res 1989;48: 707-16 . 6. Gabelt BT, Kaufman PL. The effect of prostaglandin F2a on trabecular outflow facilit y in cynomolgus monkeys. Exp Eye Res 1990;51 :87-91. 7. Lee P-Y, Pod os SM, Severin C. Effect of prostaglandin F2a on aqueous humor dynamics of rabbit, cat , and monkey. Invest Ophthalmol Vis Sci 1984;25: 1087-93. 8. Camras CB, Siebold EC, Lustgarten JS, et aJ. Maintained reduction of intraocular pressure by prostaglandin F2a-lisopropyl ester applied in multiple doses in ocular hypertensive and glaucoma patients. Ophthalmology 1989;96: 1329-37. 9. Villumsen J, AIm A. Prostaglandin F2a-isopropylester eye drops: effects in normal human eyes. Br J Ophthalmol 1989;73:419-26. 10. Kerstetter JR, Brubaker RF, Wilson SE, Kullerstrand U. Prostaglandin F2a-I-isopropylester lowers intraocular pressure without decreasing aqueous humor flow. Am J Ophthalmol 1988; I05:30-4. II. Villumsen J , Aim A. PhXA34-a prostaglandin F2a analog. Effect on intraocular pressure in patients with ocular hypertension, Sr J OphthalmoI1992;76:214-27. 12. Aim A, Villumsen J. PhXA34, a new potent ocular hypotensive drug. A study on dose-response relationship and on aqueous humor dynamics in healthy volunteers. Arch Ophthalmol 1991;109: 1564-8. 13. Camras CB, Schumer RA, Marsk A, et aJ. Intraocular pressure reduction in ocular hypertensive patients with PhXA34, a new prostaglandin analog. Arch Ophthalmol 1992; 110: 1733-8. 14. Schenker HI , Yablonski ME, Podos SM, Linder L. Fluorophotometric study of epinephrine and timolol in human subjects. Arch Ophthalmol 1981;99: 1212-16. 15. Yablonski ME, Cook OJ , Gray J. A fluorophotometric study of the effect ofargon laser trabeculoplasty on aqueous humor dynamics. Am J Ophthalmol 1985;99:579-82.
16. Hayashi M, Yablonski ME, Novack GO. Trabecular outflow facility determined by fluorophotometry in human subjects. Exp Eye Res 1989;48:621-5. 17. Topper JE, Mcl.aren J, Brubaker RF . Measurement of aqueous humor flow with scanning ocular fluorophotometers. CUrT Eye Res 1984;3: 1391-5. 18. Langham ME, Leydhecker W, Krieglstein G , Waller W. Pneumatonographic studies on normal and glaucomatous eyes. Adv Ophthalmol 1976;32:108-33. 19. Yablonski ME, Hayashi M, Cook OJ, et aJ. Fluorophotometric study of intravenous carbonic anhydrase inhibitors in rabbits. Invest Ophthalmol Vis Sci 1987;28:2076-82. 20. Bill A. Conventional and uveo-scleral drainage of aqueous humour in the cynomolgus monkey (Macaca iruss at normal and high intraocular pressures. Exp Eye Res 1966;5:45-54. 21. Camras CB, Bito LZ, Eakins KE. Reduction of intraocular pressure by prostaglandins applied topically to the eyes of conscious rabbits. Invest Ophthalmol Vis Sci 1977;16:112534. 22. Poyer JF, Gabelt B, Kaufman PL. The effect of topical PGF 2a on uveoscleral outflow and outflow facility in the rabbit eye. Exp Eye Res 1992;54:277-83. 23. Hayashi M, Yablonski ME, Bito LZ. Eicosanoids as a new class of ocular hypotensive agents. 2. Comparison of the apparent mechanism of the ocular hypotensi ve effects of A and F type prostaglandins. Invest Ophthalmol Vis Sci 1987;28: 1639-43. 24 . Kaufman PL. Effects of intracamerally infused prostaglandins on outflow facility in cynomolgus monkey eyes with intact or retrodisplaced ciliary muscle . Exp Eye Res 1986;43: 819-27. 25. Crawford K, Kaufman PL, Gabelt BT. Effects of topical PGF2a on aqueous humor dynamics in cynomolgus monkeys. CUrT Eye Res 1987;6: 1035-44. 26. Townsend OJ, Brubaker RF. Immediate effect of epinephrine on aqueous formation in the normal human eye as measured by fluorophotometry. Invest Ophthalmol Vis Sci 1980; 19:256-66. 27. Kaufman PL. PhXA34-induced ocular hypotension [letter]. Arch Ophthalmol 1992; 110: 1042. 28. Bill A. Further studies on the influence of the intraocular pressure on aqueous humor dynamics in cynomolgus monkeys. Invest Ophthalmol 1967;6:364-72. 29 . Toris CR, Pederson JE. Effect of intraocular pressure on uveoscleral outflow following cyclodialysis in the monkey eye. Invest Ophthalmol Vis Sci 1985;26:1745-9. 30. Ltitjen-Drecoll E, Tamm E. Morphological study of the an terior segment of cynomolgus monkey eyes following treatment with prostaglandin F 2a • Exp Eye Res 1988;47 :761-9. 31. Kass MA, Podos SM , Moses RA, Becker B. Prostaglandin E 1 and aqueous humor dynamics. Invest Ophthalmol 1972;11: 1022-7. 32. Stjernschantz J, Nilsson SFE, Astin M. Vasodynamic and angiogenic effects of eicosanoids in the eye. In: Bito LZ, Stjernschantz J, eds. The Ocular Effects of Prostaglandins and Other Eicosanoids. New York: Alan R. Liss, 1989; 15570. (Prog Clin Bioi Res;312). 33. Kupfer C, Ross K. Studies of aqueous humor d ynamics in man. I. Measurements in young normal subjects. Invest Ophthalmol Vis Sci 1971;10 :518-22. 34. Kupfer C. Clinical significance of pseudofacility. Stanford R. Gifford Memorial Lecture. Am J Ophthalmol 1973;75: 193-204. 35. Bill A, Walinder P'E, The effects of pilocarpine on the dynamics of aqueous humor in a primate iMacaca irus). Invest Ophthalmol 1966;5:170-5.
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Volume 100, Number 9, September 1993 39. Villumsen J, AIm A, Soderstrom M. Prostaglandin F2,,-isopropylester eye drops: effect on intraocular pressure in openangle glaucoma. Br J Ophthalmol 1989;73:915-9. 40. Villumsen J, AIm A. The effect of adding prostaglandin F 2,,isopropylester to timolol in patients with open angle glaucoma. Arch Ophthalmol 1990;108:1102-5. 41. Lee P-Y, Shao H, Camras CB, Podos SM. Additivity of prostaglandin F2,,-I-isopropyl ester to timolol in glaucoma patients. Ophthalmology 1991;98: 1079-82.
36. Bill A, Phillips CI. Uveoscleral drainage of aqueous humour in human eyes. Exp Eye Res 1971;12:275-81. 37. Camras CB,BitoLZ. Reduction of intraocularpressurein normal and glaucomatous primate (Aotus trivirgatus) eyes by topically applied prostaglandin F2a• Curr Eye Res 1981;1:205-9. 38. Camras CB, Podos SM, Rosenthal JS, et al. Multiple dosing of prostaglandin F2"or epinephrine on cynomolgus monkey eyes. I. Aqueous humor dynamics. Invest Ophthalmol Vis Sci 1987;28:463-9.
Discussion
by PaulL.Kaufrnan,~D
Although the posterior uveoscleral drainage pathway has long been recognized, I techniques for its quantitation have been complex, invasive, imprecise, terminal, or, at best, marginally tolerated in experimental animals, and unsuited to humans.P Toris et al present a noninvasive means of estimating it clinically from measurement of the other parameters of the modified Goldmann equation. Tonographic facility represents the arithmetic sum of trabecular, uveoscleral, and aqueous inflow, or pseudo facility;, compromising the evaluation of surgical or pharmacologic manipulations that affect the latter two parameters. The authors' fluorophotometric technique purports to measure primarily trabecular facility. Unfortunately, this is not strictly so. Uveoscleral facility also will affect the relationship between a known magnitude of aqueous flow suppression and the magnitude of the intraocular pressure fall it produces. Normally, uveoscleral and inflow facility are negligible, and assumptions regarding diffusional loss of fluorescein from the anterior segment are accurate, so that the authors' technique will give reasonable estimates oftrabecular facility and consequently of uveoscleral outflow. However, so will conventional tonography, as illustrated by the authors' own data. Regarding the PhXA41 data, the first limitation of the study is that the authors did not measure episcleral venous pressure. Whether the episcleral venous pressure was truly the assumed 9 mmHg would not affect their qualitative conclusion about the drug's effect on uveoscleral outflow, as long as the prostanoid did not itself affect the episcleral venous pressure. However, no data in support of such constancy are provided, and the conjunctival/episcleral hyperemia might suggestan episcleral venous pressure effect. Second, there is evidence from monkey studies that prostaglandin F2" (PGF2,,) may increase uveoscleral facility,2,4 and from both human' and monkey" studies it is shown that it may increase iridociliary vascular and/or epithelial perFrom the Department of Ophthalmology, University of Wisconsin, Clinical Science Center, Madison. Supported by NIH grant EY02698, Bethesda, Maryland.
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meability to fluorescein, again contravening assumptions underlying the authors' calculations. Such factors, along with inherent limitations in the precision of the measurements themselves, may help explain some of the variability in the authors' data. These caveats notwithstanding, the authors' conclusion that the intraocular pressure fall induced by PhXA41 in the human is primarily and perhaps completely consequent to increased uveoscleral outflow is almost certainly correct and consistent with prior humarr':" and monkey2,3,5 studies using other PGF2" congeners. References I. Kaufman, PL. Aqueous humor dynamics. In: Tasman W,
2. 3.
4. 5.
6. 7.
Jaeger EA, eds. Duane's Clinical Ophthalmology, Philadelphia: Harper & Row, 1985; v. 3, chap. 45. Gabelt BT, Kaufman PL. Prostaglandin F2" increases uveoscleral outflow in the cynomolgus monkey. Exp Eye Res 1989;49:389-402. Nilsson SFE, Samuelsson M, Bill A, Stjernschantz J. Increased uveoscleral outflow as a possible mechanism of ocular hypotension caused by prostaglandin F 2" l-isopropylester in the cynomolgus monkey. Exp Eye Res 1989;48: 707-16. Gabelt BT, Kaufman PL. The effect of prostaglandin F2" on trabecular outflow facility in cynomolgus monkeys. Exp Eye Res 1990;51:87-91. AIm A, Villumsen J. PhXA34, a new potent ocular hypotensive drug. A study on dose-response relationship and on aqueous humor dynamics in healthy volunteers. Arch Ophthalmol 1991;109:1564-8. Crawford K, Kaufman PL, Gabelt BT. Effects of topical PGF 2a on aqueous humor dynamics in cynomolgus monkeys. Curr Eye Res 1987;6:1035-44. Villumsen J, AIm A. Prostaglandin F2a-isopropylester eye drops: effects in normal human eyes. Br J Ophthalmol 1989;73:419-25.