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Diffusion of Sulfonamides from Plasma Into Prostatic Fluid
Vol. 104, Oct. Printed in U.S.A.
THE JOURNAL OF UROLOGY
Copyright © 1970 by The Williams & Wilkins Co.
DIFFUSION OF SULFONAJVIIDES FROM PLASMA INTO PROSTATIC FLUID D. G. WINNINGHAM
.\.ND
T. A. STAMEY
From the Division of Urology, Stanford University School of }vleclicine, Stanford, California
The failure of most antimicrobial agents to diffuse from plasma (P) to prostatic fluid (PF) has been shown in both clinical1 and experimental studies. 2 Of 9 antimicrobial drugs infused intravenously in the dog, only the basic macrolides erythromycin and oleandomycin could be detected in prostatic fluid.' These data together with other studies on drug partition between stomach and plasma suggested that several physical characteristics of antimicrobial agents might be important in determining diffusion into prostatic fluid, among them ion trapping during non-ionic diffusion, dissociation constant of the drug (pKa), lipid solubility and protein binding.a, 4 Because of the many different sulfonamides available for study, offering a wide range of dissociation constants, variable lipid solubilities and protein binding, and because of the urgent clinical need for an effective chemotherapeutic agent, a more detailed examination of the sulfonamides seemed warranted. METHODS
l\!Iature male mongrel dogs weighing 20 to 40 kg. were anesthetized with pentobarbital, 25 mg. per kg. body weight, followed by maintenance doses as needed. Through a lower abdominal incision, a double ligature was placed around the bladder neck and the bladder was Accepted for publication October 24, 1969. Read at annual meeting of Western Section, American Urological Association, Seattle, Washington, July 27-August 1, 1969. Supported by United States Public Health Service Training Grant number AM 05513 and by I-Ioffmann-LaRoche Laboratories, Inc. 1 Meares, E. M. and Stamey, T. A.: Bacteriologic localization patterns in bacterial prostatitis and urethritis. Invest. Urol., 5: 492, 1968. 2 Winningham, D. G., Nemoy, N. J. and Stamey T. A.: Diffusion of antibiotics from plasma int~ prostatic fluid. Nature, 219: 139, 1968. 3 Brodie, B. B. and Hogben, C. A. M.: Some physico-chernical factors in drug action. J. Pharrn. Pharrnacol., 9: 345, 1957. 4 Shore, P. A., Brodie, B. B. and Hogben, C. A. M.: The gastric secretion of drugs: a pH part1t10n hypothesis. J. Pharrnacol. Exper. Therap., 119: 361, 1957.
emptied by needle aspiration. Polyethylene catheters were passed to each kidney through bilateral ureterotomy. The vasa deferentia were ligated and the abdominal inci:;ion was closed. The penis was circumcised and the dog was placed on the ventral surface for direct collection of prostatic fluid. N" onnal saline was started at a rate of 10 to 20 ml. per minute to assure a brisk urine flow rate. About 150 ml. blood was taken from the femoral vein to provide normal plasma for use as a diluent of subsequent plasma control urine was also collected for similar purposes. Sulfonamide was rapidly injected intravenously and 5 minutes later a plasma was taken from the femoral vein to estimate the peak plasma level. Approximately 30 minutes later the plasma clearance of the sulfonamide was measured by obtaining a timed urine collection with appropriate plasma samples. Endogenous creatinine clearance was measured in about half of the dogs. After completion of the clearance study, 25 mg. pilocarpine was given intravenously to stimulate prostatic secretion; simultaneous plasma, urine and prostatic fluid samples were collected. Prostatic fluid was collected in consecutive 2 to 5 ml. aliquots. The Bratton-:\farnhall method for measuring free sulfonamide was used on all samples. 5 The range of sensitivity varied with different sulfonamides, but was generally between 0.25 and 10 mg. per 100 ml. Plasma and urine samples were diluted with control specimens previously obtained before sulfonamide administration. Prostatic fluid samples were diluted with control prostatic fluid when necessary. RESULTS
The general data are presented in table 1. The different sulfonamides are arranged in descending order of their approximate molecular complexity. In general the plasma concentration at the time of pilocarpine stimulation in table l had been established for at least 60 minutes, 5 Bratton, A. C. and Marshall, E. K., Jr.: New coupling component for sulfanilarnide determination. J. Biol. Chern., 128: 537, 1939.
559
1--0-11
H H-N
0
-
SULFONAMIDE
Sulfanilamide
H
R=
-H
cr, 0
GENERAL DATA
I S-N-R II 0
1/ ~
,:.n
TABLE I
FORMULA
CLEARANCE DATA**
SIMULTANEOUS CONCENTRATIONS
Creatinine
Sulfonamide Dog no. and Wt. (Lbs)
LY.Dose (gms)
Peak [Pl
(mg%)
I - 55
2.0
II.I
2- 60
2.0
14.4
[PF] (mg%)
[Pl (mg%)
[PFJ/[PJ
[UJ (mg%)
UV/P (ml/min)
6.20
6.31
.98
49
55
3.20
.08
80
115
[UJ (mg')[)
UV/P
~!.:minl.
0 Sulfacetamide
Sulfapyridine
Sulfadiazine•
II
-C-CH3 -
-0 -{:)
.24
~ H 3 - 51
2.0
10.2
4.70
6.45
.73
19.5
41
2.9
84
z z H
'L; Q
~
'-' H
4- 49 5 -44
2.0 2.0
19.8 13.3
1.68 1.89
IO.IO 8.06
.17 .23
47 38.5
100 61
2.9 3.9
122 99
z>"'"
t:I [fl
Sulfamerazine$
Sulfamethazine• (sulfadimidine)
-{'ca,
-<1:
Sulfameter (Sulla)
{)-ocH3
Sulfamethoxypyridazine (Kynex)
-(:}-ocH3 N-N
t-l
6- 46 7 - 44
2.0 2.0
14.5 10.3
2.70 2.10
10.30 5.70
.26 .37
34.5 31.1
>"'"
91
8 - 57 9-60
2.0 2.0
21.3 19.9
4.57 4.37
8.46 7.25
.54 .60
31. l
30
10- 54
2.0
13.8
1.50
10.70
.14
70.5
9.6
II - SS
2.0
36.1
7.68
21.20
.36
81
4.7
5.4 3.4
4.9
::::: 127
72
M t-<1
Sulfisomidine (Elkosin)
-OCH; -OOCH3
12- SI
2.0
22.S
3.72
12.60
.30
69
5.1
SS
59
CH3
Sulfodimethoxine (Madribon)
N
OCH3 Sulfonnethoxine
~ s \)
H5CO Sulfathiazole
Sulfamethizole (Thiosulfil)
NOCH3
ti
H
13-45 14-65
2.0 2.0
17.9 16.1
.89 .56
9.45 12.00
.09 .OS
12.3 6.7
11.2 10.5
4.4 3.9
63 62
>tj >tj
q
[/) H
0
z
IS - 57 16- 60
1.0 2.0
11.4 18.9
.49 .90
7.00 11.60
.07 .08
30.7 71
0
29.5 31.6
>tj [/)
q
ts
>tj
0 17-42
l\SYCH3
2.0
20.3
.85
8.33
.10
43
60
2.9
57
z
"'
;:;: H
ti l".l [/)
18- 52
4.0
7S.O
.94
32.SO
.03
>tj ~
0
N-N
;:;: '"d
Sulfaethidole (Sul-Spantab)
~syC2Hs
19-69
2.0
18.7
.25
15.30
.02
92.5
123
5.9
106
N-N
ts
"';:;: "'z
[/)
H
>-3
Sulfisoxazole (Gantrisin)
uH~
2.5
22.S
.28
IS.10
0
.02
'"d
~
0
CH 3
N Sulfamethoxazole (Gantanol)
20- 58
UCH3 0 CH3
[/)
>-3
21 -48
2.0
22.8
1.28
12.60
.10
30.8
13.2
6.4
60
"' >-3
H
Cl >tj
ts
q H
ti
CGHs I Sulfaphenazole (Sulfa bid)
-(j
22 - 48
2.0
34.8
1.20
22.SO
.05
22.3
9.5
l
•Usual mixture of triple sulfa combinations.
c.n
• • ll lh."(lrfl't.'ll'd
2:
562
WINNINGHA:W AND STAMEY
2. Relationship of pKa, PF /P concentration ratio, and lipid solubility for acid sulfonamides
TABLE
Sulfonamide
pKa
Experimental PF/P
Sulfanilamide Sulfa pyridine Sulfamethazine Sulfamethoxypyridazine Sulfamerazine Sulfisornidine Sulfadiazine Sulfameter Sulfathiazole Su\famethoxazole Sulfacetamide Sulformethoxine Sulfadimethoxine Sulfaphenazole Sulfamethizo\e Sulfisoxazole S.1Ifaethidole
10.43 8.43 7. 70 7 .20 6. 98 7 .57 6.52 7 .02 7 .25 6.05 5. 78 6.10 6.32 6.09 5. 45 5.00 5, 65
.98 .73 .57 . 36 .32 .30 .20 .14 .10 .10 .08 .07 .07 .05 .03 .02 .02
Lipid Solubility•
1.00~-~-~-~-~--~-~..~ ,90 .50
2.6 75.8 68. 6 48,6 17. 8 11. 9 51.1 6.8 9.6 0, 7 74.1 53.6
•
Suli'.'ona- .7 0 midG
•
[PFJ/lP] .60
•
.50
....
.40 .30
•
. 20 3,8 3.6
* Percent~ge distribution of drug in lipid layer of a mixture of chloroform and sodium phosphate buffer in a 1 to 1 ratio.
since prostatic fluid stimulation occurred only after estimating the plasma clearance of the sulfonamide.* The prostatic fluid concentrations vary from complete equilibration with plasma (PF /P ratio of 0.98) in the case of sulfanilamide to virtual exclusion of sulfisoxazole and sulfamethizole from prostatic fluid (PF /P ratios of 0.02 and 0.03). In table 2 the PF /P ratios correspond closely to the dissociation constants (pKa) for each drug. This linear relationship is more readily seen in figure 1. The relative lipid solubilities of the sulfonamides in a 1 to 1 chloroform to sodium phosphate buffer at pH 7.4 (table 2) are taken from the data of Rieder. 6 DISCUSSION
These studies (table 2 and figure 1) establish that the diffusion of sulfonamides from plasma to prostatic fluid is determined by the pKa of the particular sulfonamide. Killman and Thaysen * The plasma clearances of the sulfonamides represent only approximations because no attempt was made to obtain a steady state or to insure reproducibility of consecutive clearance periods. Nevertheless, they serve as gross estimates; the long-acting sulfonamides like sulfamethoxypyridazine had low clearances, while the short.acting drugs had high clearances (sulfadiazine). 6 Rieder, J.: Physicochemical and biological studies on sulfonamides. 1. Pharmacologically interesting physicochemical characteristics of 21 sulfonamides and 6 sulfonamide metabolites. Arzneimittelforschung., 13: 81, 1963.
•
.
• . .\• •
10 ID
Ge
98
I
.oo4L·--5~-..Lo----'7---'8~-9L--1..Lo-~1I
pKa Fro. 1. Relationship between pKa and PF /P ratio of 17 different sulfonamides in 22 dogs. also showed that the pKa determined the salivary fluid per plasma ratio in patients receiving sulfonamides with different dissociation constants.7 We have recently presented a detailed discussion of non-ionic diffusion and ion trapping as the likely explanation of why erythromycin concentration in prostatic fluid exceeded the plasma level and why acid antimicrobial agents are ion trapped on the plasma side and can only approach the concentration in prostatic fluid in proportion to the uncharged fraction of the drug. 8 When we began these studies we were not aware that all sulfonamides dissociate as acids, that is they donate a proton in solution (fig. 2). Strong acid sulfonamides like sulfamethizole (pKa 5.4) will have about 100 charged ions in plasma of pH 7.4 for every uncharged one, while weak acids like sulfanilamide (pKa 10.4) will have only 1 charged ion for every 1,000 uncharged. Since only the uncharged ion can cross the lipid layer of cells, sulfanilamide in prostatic fluid approaches the plasma concentration; sulfamethizole, with only a fraction of 7 Killman, S. A. and Thaysen, J. H.: Permeability of human parotid gland to series of sulfonamide compounds, paraaminohippurate and inulin. Scand. J. Clin. Lab. Invest., 7: 86, 1955. 8 Stamey, T. A., Meares, E. M. and Winningham, D. G.: Chronic bacterial pros ta ti tis and the diffusion of drugs into prostatic fluid. J. Urol.,
103: 187, 1970.
DIFFUSION OF SULFONAMIDES FROM PLASMA INTO PROSTATIC FLUID
Ionization d' a Sulfonamide (H-1
1--1
,_,,
1-Q-~ I I/ ~ S--N-R
H-N
-
II
Jr : 1-0-''
H 1-1-N
r
'
-
II _ S-N-R II
+
0 FIG. 2. Para-amino nitrogen group, with pKa between 2 and 3 (sulfanilamide), accepts proton
(acts as base) only in highly acid solutions. Its ionization as base, therefore, can be ignored in physiologic solutions.
the drug in plasma in the freely diffusible uncharged form, can barely be detected in prostatic fluid. Because none of these sulfonamides dissociate as bases at the pH of plasma (fig. 2), they cannot be concentrated in the more acidic prostatic fluid. The relative lipid solubilities of the sulfonamides in table 2 do not seem to play a major role in determining diffusion into prostatic fluid. For example sulfanilamide with a PF /P ratio of 0.98 has about one-thirtieth the relative lipid solubility of sulfadimethoxine, which has a PF /P ratio of only 0.07. It is possible that the relative lipid solubilities of the different sulfonamides might have influenced the rate of PF to P equilibration if we had stimulated prostatic fluid earlier than 60 minutes after intravenous injection. But since clinical usage allows several hours for potential equilibration between oral doses, there would seem to be no practical reason to pursue further the importance of lipid solubility of the sulfonamides. We also lack data to compare the lipid solubilities of the sulfonamides with other antimicrobial agents we have studied. 2 For example kanamycin and polymyxin B are both bases with a favorable pKa but they cannot be detected in prostatic fluid, presumably because they are lipid insoluble. Thus, it will be important to study the relative differences between snlfanilamide and kanamycin or polymyxin B in their lipid solubility in order to understand better the limitations of drug diffusion across prostatic epithelium.
563
It should be emphasized that these data only establish which sulfonamides can diffuse into prostatic fluid. The degree of diffusion (PF /P ratio) is determined and, therefore, can be predicted by the pKa of the drug. Antibacterial activity of the drug, once the sulfonamide is present in prostatic fluid, is a separate question which we have not studied. For example, prostatic fluid is acidic to plasma by at least 1 unit. of our dogs consistently excreted prostatic fluid with a pH of 6.0. With sulfanilamide, all of the drug in prostatic fluid will be in the uncharged fraction. Studies of sulfonamide bacteriostasis in vitro at pH 7.0 have shown that the most effective antibacterial activity occurs when dissociation is about 50 per cent, that is half in the charged form and half uncharged. 9 Thus, the possibility exists that a sulfonamide like sulfadiazine (pKa 6.5), with a PF /P ratio of 0.20, might be more antibacterial than sulfanilarnide with a PF/P ratio of 0.98. Finally, we have had no clinical experience with any of these sulfonamides except sulfisoxazole and sulfamethizole. These drugs have not been helpful in patients with chronic bacterial prostatitis and the studies reported here show the reason. However, it is noteworthy that sulfamethazine (sulfadimidine) with a PF /P ratio of 0.57 is the most commonly used sulfonamide for both urinary infection and meningitis in Britain. 10 If other sulfonamides are used clinically in an attempt to take advantage of diffusion into prostatic fluid, the clinician must be cautious for side affects such as precipitation of sulfonamide crystals in the renal tubules. SUMMARY
The diffusion of 17 different sulfonamides from plasma to prostatic fluid was studied in 22 The PF /P ratios varied from 0.02 to 0.98. A linear relationship was observed between the pKa of the sulfonamide and the PF /P ratio. The commonly used sulfonamides, sulfisoxazole and sulfamethizole were virtually excluded from pros-· tatic fluid. 9 Bell, P.H. and Roblin, R. 0., Jr.: Studies in chemotherapy; theory of relation of structure to activity of sulfanilamide type compounds. J. Amer. Chem. Soc., 64: 2905, 1942. 10 Garrod, L. P. and O'Grady, F.: Antibiotics and Chemotherapy. London: E. & S. Livingstone Ltd., pp. 12-47, 1968.
THE JOURNAL OF UROLOGY
Copyright © 1970 by The Williams & Wilkins Co.
DIFFUSION OF SULFONAJVIIDES FROM PLASMA INTO PROSTATIC FLUID D. G. WINNINGHAM
.\.ND
T. A. STAMEY
From the Division of Urology, Stanford University School of }vleclicine, Stanford, California
The failure of most antimicrobial agents to diffuse from plasma (P) to prostatic fluid (PF) has been shown in both clinical1 and experimental studies. 2 Of 9 antimicrobial drugs infused intravenously in the dog, only the basic macrolides erythromycin and oleandomycin could be detected in prostatic fluid.' These data together with other studies on drug partition between stomach and plasma suggested that several physical characteristics of antimicrobial agents might be important in determining diffusion into prostatic fluid, among them ion trapping during non-ionic diffusion, dissociation constant of the drug (pKa), lipid solubility and protein binding.a, 4 Because of the many different sulfonamides available for study, offering a wide range of dissociation constants, variable lipid solubilities and protein binding, and because of the urgent clinical need for an effective chemotherapeutic agent, a more detailed examination of the sulfonamides seemed warranted. METHODS
l\!Iature male mongrel dogs weighing 20 to 40 kg. were anesthetized with pentobarbital, 25 mg. per kg. body weight, followed by maintenance doses as needed. Through a lower abdominal incision, a double ligature was placed around the bladder neck and the bladder was Accepted for publication October 24, 1969. Read at annual meeting of Western Section, American Urological Association, Seattle, Washington, July 27-August 1, 1969. Supported by United States Public Health Service Training Grant number AM 05513 and by I-Ioffmann-LaRoche Laboratories, Inc. 1 Meares, E. M. and Stamey, T. A.: Bacteriologic localization patterns in bacterial prostatitis and urethritis. Invest. Urol., 5: 492, 1968. 2 Winningham, D. G., Nemoy, N. J. and Stamey T. A.: Diffusion of antibiotics from plasma int~ prostatic fluid. Nature, 219: 139, 1968. 3 Brodie, B. B. and Hogben, C. A. M.: Some physico-chernical factors in drug action. J. Pharrn. Pharrnacol., 9: 345, 1957. 4 Shore, P. A., Brodie, B. B. and Hogben, C. A. M.: The gastric secretion of drugs: a pH part1t10n hypothesis. J. Pharrnacol. Exper. Therap., 119: 361, 1957.
emptied by needle aspiration. Polyethylene catheters were passed to each kidney through bilateral ureterotomy. The vasa deferentia were ligated and the abdominal inci:;ion was closed. The penis was circumcised and the dog was placed on the ventral surface for direct collection of prostatic fluid. N" onnal saline was started at a rate of 10 to 20 ml. per minute to assure a brisk urine flow rate. About 150 ml. blood was taken from the femoral vein to provide normal plasma for use as a diluent of subsequent plasma control urine was also collected for similar purposes. Sulfonamide was rapidly injected intravenously and 5 minutes later a plasma was taken from the femoral vein to estimate the peak plasma level. Approximately 30 minutes later the plasma clearance of the sulfonamide was measured by obtaining a timed urine collection with appropriate plasma samples. Endogenous creatinine clearance was measured in about half of the dogs. After completion of the clearance study, 25 mg. pilocarpine was given intravenously to stimulate prostatic secretion; simultaneous plasma, urine and prostatic fluid samples were collected. Prostatic fluid was collected in consecutive 2 to 5 ml. aliquots. The Bratton-:\farnhall method for measuring free sulfonamide was used on all samples. 5 The range of sensitivity varied with different sulfonamides, but was generally between 0.25 and 10 mg. per 100 ml. Plasma and urine samples were diluted with control specimens previously obtained before sulfonamide administration. Prostatic fluid samples were diluted with control prostatic fluid when necessary. RESULTS
The general data are presented in table 1. The different sulfonamides are arranged in descending order of their approximate molecular complexity. In general the plasma concentration at the time of pilocarpine stimulation in table l had been established for at least 60 minutes, 5 Bratton, A. C. and Marshall, E. K., Jr.: New coupling component for sulfanilarnide determination. J. Biol. Chern., 128: 537, 1939.
559
1--0-11
H H-N
0
-
SULFONAMIDE
Sulfanilamide
H
R=
-H
cr, 0
GENERAL DATA
I S-N-R II 0
1/ ~
,:.n
TABLE I
FORMULA
CLEARANCE DATA**
SIMULTANEOUS CONCENTRATIONS
Creatinine
Sulfonamide Dog no. and Wt. (Lbs)
LY.Dose (gms)
Peak [Pl
(mg%)
I - 55
2.0
II.I
2- 60
2.0
14.4
[PF] (mg%)
[Pl (mg%)
[PFJ/[PJ
[UJ (mg%)
UV/P (ml/min)
6.20
6.31
.98
49
55
3.20
.08
80
115
[UJ (mg')[)
UV/P
~!.:minl.
0 Sulfacetamide
Sulfapyridine
Sulfadiazine•
II
-C-CH3 -
-0 -{:)
.24
~ H 3 - 51
2.0
10.2
4.70
6.45
.73
19.5
41
2.9
84
z z H
'L; Q
~
'-' H
4- 49 5 -44
2.0 2.0
19.8 13.3
1.68 1.89
IO.IO 8.06
.17 .23
47 38.5
100 61
2.9 3.9
122 99
z>"'"
t:I [fl
Sulfamerazine$
Sulfamethazine• (sulfadimidine)
-{'ca,
-<1:
Sulfameter (Sulla)
{)-ocH3
Sulfamethoxypyridazine (Kynex)
-(:}-ocH3 N-N
t-l
6- 46 7 - 44
2.0 2.0
14.5 10.3
2.70 2.10
10.30 5.70
.26 .37
34.5 31.1
>"'"
91
8 - 57 9-60
2.0 2.0
21.3 19.9
4.57 4.37
8.46 7.25
.54 .60
31. l
30
10- 54
2.0
13.8
1.50
10.70
.14
70.5
9.6
II - SS
2.0
36.1
7.68
21.20
.36
81
4.7
5.4 3.4
4.9
::::: 127
72
M t-<1
Sulfisomidine (Elkosin)
-OCH; -OOCH3
12- SI
2.0
22.S
3.72
12.60
.30
69
5.1
SS
59
CH3
Sulfodimethoxine (Madribon)
N
OCH3 Sulfonnethoxine
~ s \)
H5CO Sulfathiazole
Sulfamethizole (Thiosulfil)
NOCH3
ti
H
13-45 14-65
2.0 2.0
17.9 16.1
.89 .56
9.45 12.00
.09 .OS
12.3 6.7
11.2 10.5
4.4 3.9
63 62
>tj >tj
q
[/) H
0
z
IS - 57 16- 60
1.0 2.0
11.4 18.9
.49 .90
7.00 11.60
.07 .08
30.7 71
0
29.5 31.6
>tj [/)
q
ts
>tj
0 17-42
l\SYCH3
2.0
20.3
.85
8.33
.10
43
60
2.9
57
z
"'
;:;: H
ti l".l [/)
18- 52
4.0
7S.O
.94
32.SO
.03
>tj ~
0
N-N
;:;: '"d
Sulfaethidole (Sul-Spantab)
~syC2Hs
19-69
2.0
18.7
.25
15.30
.02
92.5
123
5.9
106
N-N
ts
"';:;: "'z
[/)
H
>-3
Sulfisoxazole (Gantrisin)
uH~
2.5
22.S
.28
IS.10
0
.02
'"d
~
0
CH 3
N Sulfamethoxazole (Gantanol)
20- 58
UCH3 0 CH3
[/)
>-3
21 -48
2.0
22.8
1.28
12.60
.10
30.8
13.2
6.4
60
"' >-3
H
Cl >tj
ts
q H
ti
CGHs I Sulfaphenazole (Sulfa bid)
-(j
22 - 48
2.0
34.8
1.20
22.SO
.05
22.3
9.5
l
•Usual mixture of triple sulfa combinations.
c.n
• • ll lh."(lrfl't.'ll'd
2:
562
WINNINGHA:W AND STAMEY
2. Relationship of pKa, PF /P concentration ratio, and lipid solubility for acid sulfonamides
TABLE
Sulfonamide
pKa
Experimental PF/P
Sulfanilamide Sulfa pyridine Sulfamethazine Sulfamethoxypyridazine Sulfamerazine Sulfisornidine Sulfadiazine Sulfameter Sulfathiazole Su\famethoxazole Sulfacetamide Sulformethoxine Sulfadimethoxine Sulfaphenazole Sulfamethizo\e Sulfisoxazole S.1Ifaethidole
10.43 8.43 7. 70 7 .20 6. 98 7 .57 6.52 7 .02 7 .25 6.05 5. 78 6.10 6.32 6.09 5. 45 5.00 5, 65
.98 .73 .57 . 36 .32 .30 .20 .14 .10 .10 .08 .07 .07 .05 .03 .02 .02
Lipid Solubility•
1.00~-~-~-~-~--~-~..~ ,90 .50
2.6 75.8 68. 6 48,6 17. 8 11. 9 51.1 6.8 9.6 0, 7 74.1 53.6
•
Suli'.'ona- .7 0 midG
•
[PFJ/lP] .60
•
.50
....
.40 .30
•
. 20 3,8 3.6
* Percent~ge distribution of drug in lipid layer of a mixture of chloroform and sodium phosphate buffer in a 1 to 1 ratio.
since prostatic fluid stimulation occurred only after estimating the plasma clearance of the sulfonamide.* The prostatic fluid concentrations vary from complete equilibration with plasma (PF /P ratio of 0.98) in the case of sulfanilamide to virtual exclusion of sulfisoxazole and sulfamethizole from prostatic fluid (PF /P ratios of 0.02 and 0.03). In table 2 the PF /P ratios correspond closely to the dissociation constants (pKa) for each drug. This linear relationship is more readily seen in figure 1. The relative lipid solubilities of the sulfonamides in a 1 to 1 chloroform to sodium phosphate buffer at pH 7.4 (table 2) are taken from the data of Rieder. 6 DISCUSSION
These studies (table 2 and figure 1) establish that the diffusion of sulfonamides from plasma to prostatic fluid is determined by the pKa of the particular sulfonamide. Killman and Thaysen * The plasma clearances of the sulfonamides represent only approximations because no attempt was made to obtain a steady state or to insure reproducibility of consecutive clearance periods. Nevertheless, they serve as gross estimates; the long-acting sulfonamides like sulfamethoxypyridazine had low clearances, while the short.acting drugs had high clearances (sulfadiazine). 6 Rieder, J.: Physicochemical and biological studies on sulfonamides. 1. Pharmacologically interesting physicochemical characteristics of 21 sulfonamides and 6 sulfonamide metabolites. Arzneimittelforschung., 13: 81, 1963.
•
.
• . .\• •
10 ID
Ge
98
I
.oo4L·--5~-..Lo----'7---'8~-9L--1..Lo-~1I
pKa Fro. 1. Relationship between pKa and PF /P ratio of 17 different sulfonamides in 22 dogs. also showed that the pKa determined the salivary fluid per plasma ratio in patients receiving sulfonamides with different dissociation constants.7 We have recently presented a detailed discussion of non-ionic diffusion and ion trapping as the likely explanation of why erythromycin concentration in prostatic fluid exceeded the plasma level and why acid antimicrobial agents are ion trapped on the plasma side and can only approach the concentration in prostatic fluid in proportion to the uncharged fraction of the drug. 8 When we began these studies we were not aware that all sulfonamides dissociate as acids, that is they donate a proton in solution (fig. 2). Strong acid sulfonamides like sulfamethizole (pKa 5.4) will have about 100 charged ions in plasma of pH 7.4 for every uncharged one, while weak acids like sulfanilamide (pKa 10.4) will have only 1 charged ion for every 1,000 uncharged. Since only the uncharged ion can cross the lipid layer of cells, sulfanilamide in prostatic fluid approaches the plasma concentration; sulfamethizole, with only a fraction of 7 Killman, S. A. and Thaysen, J. H.: Permeability of human parotid gland to series of sulfonamide compounds, paraaminohippurate and inulin. Scand. J. Clin. Lab. Invest., 7: 86, 1955. 8 Stamey, T. A., Meares, E. M. and Winningham, D. G.: Chronic bacterial pros ta ti tis and the diffusion of drugs into prostatic fluid. J. Urol.,
103: 187, 1970.
DIFFUSION OF SULFONAMIDES FROM PLASMA INTO PROSTATIC FLUID
Ionization d' a Sulfonamide (H-1
1--1
,_,,
1-Q-~ I I/ ~ S--N-R
H-N
-
II
Jr : 1-0-''
H 1-1-N
r
'
-
II _ S-N-R II
+
0 FIG. 2. Para-amino nitrogen group, with pKa between 2 and 3 (sulfanilamide), accepts proton
(acts as base) only in highly acid solutions. Its ionization as base, therefore, can be ignored in physiologic solutions.
the drug in plasma in the freely diffusible uncharged form, can barely be detected in prostatic fluid. Because none of these sulfonamides dissociate as bases at the pH of plasma (fig. 2), they cannot be concentrated in the more acidic prostatic fluid. The relative lipid solubilities of the sulfonamides in table 2 do not seem to play a major role in determining diffusion into prostatic fluid. For example sulfanilamide with a PF /P ratio of 0.98 has about one-thirtieth the relative lipid solubility of sulfadimethoxine, which has a PF /P ratio of only 0.07. It is possible that the relative lipid solubilities of the different sulfonamides might have influenced the rate of PF to P equilibration if we had stimulated prostatic fluid earlier than 60 minutes after intravenous injection. But since clinical usage allows several hours for potential equilibration between oral doses, there would seem to be no practical reason to pursue further the importance of lipid solubility of the sulfonamides. We also lack data to compare the lipid solubilities of the sulfonamides with other antimicrobial agents we have studied. 2 For example kanamycin and polymyxin B are both bases with a favorable pKa but they cannot be detected in prostatic fluid, presumably because they are lipid insoluble. Thus, it will be important to study the relative differences between snlfanilamide and kanamycin or polymyxin B in their lipid solubility in order to understand better the limitations of drug diffusion across prostatic epithelium.
563
It should be emphasized that these data only establish which sulfonamides can diffuse into prostatic fluid. The degree of diffusion (PF /P ratio) is determined and, therefore, can be predicted by the pKa of the drug. Antibacterial activity of the drug, once the sulfonamide is present in prostatic fluid, is a separate question which we have not studied. For example, prostatic fluid is acidic to plasma by at least 1 unit. of our dogs consistently excreted prostatic fluid with a pH of 6.0. With sulfanilamide, all of the drug in prostatic fluid will be in the uncharged fraction. Studies of sulfonamide bacteriostasis in vitro at pH 7.0 have shown that the most effective antibacterial activity occurs when dissociation is about 50 per cent, that is half in the charged form and half uncharged. 9 Thus, the possibility exists that a sulfonamide like sulfadiazine (pKa 6.5), with a PF /P ratio of 0.20, might be more antibacterial than sulfanilarnide with a PF/P ratio of 0.98. Finally, we have had no clinical experience with any of these sulfonamides except sulfisoxazole and sulfamethizole. These drugs have not been helpful in patients with chronic bacterial prostatitis and the studies reported here show the reason. However, it is noteworthy that sulfamethazine (sulfadimidine) with a PF /P ratio of 0.57 is the most commonly used sulfonamide for both urinary infection and meningitis in Britain. 10 If other sulfonamides are used clinically in an attempt to take advantage of diffusion into prostatic fluid, the clinician must be cautious for side affects such as precipitation of sulfonamide crystals in the renal tubules. SUMMARY
The diffusion of 17 different sulfonamides from plasma to prostatic fluid was studied in 22 The PF /P ratios varied from 0.02 to 0.98. A linear relationship was observed between the pKa of the sulfonamide and the PF /P ratio. The commonly used sulfonamides, sulfisoxazole and sulfamethizole were virtually excluded from pros-· tatic fluid. 9 Bell, P.H. and Roblin, R. 0., Jr.: Studies in chemotherapy; theory of relation of structure to activity of sulfanilamide type compounds. J. Amer. Chem. Soc., 64: 2905, 1942. 10 Garrod, L. P. and O'Grady, F.: Antibiotics and Chemotherapy. London: E. & S. Livingstone Ltd., pp. 12-47, 1968.