Evaluation of receptor mechanisms mediating fentanyl analgesia and toxicity

Eur~~eu~ .Iournal of Ph#~ac~Io~, 197 (1991) 135-141 0 1991 Elsevier Science Publishers B.V. ~14.2~9/91/$03.50

135

ADONIS lIO14299991QO3495

E!.JP51861

Young Jang and Byron C. Yoburn

Received 16 July 1990. revised MS received 18 December 1990, accepted 26 February 1991

In the present study the antago~sm of fent~yl p~~rna~~~cs was studied in the mouse and the receptor population m~iating the analgesic and lethal effects of fentanyl were examined. Both 1 and 8 days fo~o~ng impl~t~tion (s.c) of a 15 mg nahrexone pellet there was a significant shift to the right of the fentanyl dose-response curves for analgesia and lethality. The analgesia dose-response curves were shifted significantly more (l?O-to 264.fold) than the lethality curves (13- to 16-fold) in the presence of naltrexone. In addition, acute naloxone (0.1 mg,%g s.c.), antagonixed fentanyl analgesia more than lethality. ~ons~uently, the relative safety ratio of fentanyl (LD5s/EDs0) was decreased in the presence of opioid antagonists. Fretr~tment with naloxon~~e (35 mg/kg se.) 24 h prior to testing effectively i~bited f~ntanyl-educe analgesia, but not fent~yl-~du~d legality. However, pretreatment with ~-funaltrexa~ne (&FNA) (20 mg/kg, se.) 24 h prior to testing inhibited both fentanyl-induced analgesia and lethality. Implantation (SC.) of a 75 mg morphine pellet for 72 h resulted in cross-tolerance to both fentanyl analgesia and lethality. However, the degree of the cross-tolerance was I.&fold for analgesia and 4.5fold for lethality. Displacement studies of [3HJnaltrexone by fentanyl in mouse brain homogenate indicated two populations of binding sites. Taken together, the ph~macod~~c studies and the binding studies suggest that fentanyl exerts its analgesic and lethal effects through different receptor ~op~ations. Analgesia; Toxicity; Fentanyl; Morphine; Tolerance; Opioid receptors; &Funaltrexamine; Naloxonazine

1. In~uc~on Naltrexone is currently a treatment option for opioid abusers. As the clinical use of naltrexone increases (Ginzberg, 1984; O’Brien et al., 1983; Greenstein et al., 1984), an important question is how to deal with the blockade of opioid receptors if opioid analgesic medication is required. Since the degree of antagonism of the analgesic and toxic effects of opioids for such patients has not been systemically studied, it is important to develop strategies using an animal model for treating the naltrexone-maint~ned patient with opioid analgesic medication. In the present studies, we have examined whether chronic blockade of opioid receptors with naltrexone and acute blockade with naloxone altered to the same degree the analgesic and lethal effects of the opioid agonist fentanyl. The S.C. naltrexone pellet impl~ta~on technique was employed to produce chronic blockade of opioid receptors (Yobum et al., 1985). Fentanyl has

Correspondence to: B.C. Yoburn, Depa~ment of Pharmaceuti~l Sciences, College of Ph~macy and Allied Health P~fessions, St. John’s University, Queens, NY 11439, U.S.A.

been employed as an ago~st since the plasma level produced by S.C.naltrexone pellets in these expe~ments made it difficult to overcome the blockade of opioid receptors with a drug which has lower potency and solubility such as morphine (Yobum et al., 1989). In order to assess the receptor populations that mediate analgesia and lethality, we have examined the effects of naloxonazine and ~-f~altrexa~ne (BFNA), irreversible opioid antagonists-selective for p, (Pastemak, 1982; Ling et al., 1985; Hahn et al., 1982) and p sites (Portoghese et al., 1980; Takemori et al., 1981; Corbett et al., 1985; Hayes et al., 1985; Ward et al., 1982a,b), resp~tively. iodine and fentanyl cross-tolerance studies were conducted to further evahtate opioid receptor mechanisms mediating analgesia and lethality. Finally, the in vivo pharmacology studies using naltrexone and fentanyl were modeled by in vitro studies in which the displacement of ~3H~naltrexone ~rndi~ by fentanyl in mouse brain was determined. These in vivo pharmacological and in vitro binding studies have enabled us to assess the receptor mechanisms of opioid analgesia and toxicity. In addition, the results of these studies may be useful clinically, by providing some information concerning the use of opioid analgesic medication in naltrexone-maint~ned patients.

(0.01-0.06 mg/kg, N = 6-12/dose) mg/kg, N = S-lo/dose).

or lethality (7.5-60

2.5. Brain opiaid binding M&,

Saks-W&SK%

tice

(22-24 & Taconic

Farms, Ge~anto~~~~. NY) were utilized in a!1 studies. Mice were ~~~t~~~ 5-18 per cage with free access to food and water. Animals were housed for at least 24 h prior to experimentation and used only once.

Mice were impl~ted S.C.with a single pellet containing 15.0 mg naltrexone. Controls were implanted with a placebo pellet. All implantations were conducted while the mice were lightly anesthetized with halothane (4% ba~otb~e : 96% oxygen). The pellets were not removed. One or 8 days following pellet implantation, mice were we&bed and baseline nociceptive responses {tail~ck, see below) were reermined. Animals were then tested for analgesia 15 min following fentanyl (0.01-10.0 mg/kg. N = 4-‘LG/dose). Lethahty was deterrmned 24 h folIowin fentanyi (4-375 mg/kg, N = 5-16/dose). Separate groups of mice were weighed and baseline tailflick determined. Mice were injected with naloxone (0.1 mg/kg s.c.) or saline (IO ml/kg s.c.) and 5 min later with fentanyl. Mice were tested for analgesia 15 min fo~lo~~~ing fent~yI(O.O2-0.3 mg/kg, N = S-8/dose). Lethality was determined 24 h following fentanyl(2.0-60 mg/kg. N = 8-2O/dose). 2.3. Effect of nala_xonazine and j3-FNA plrar~?~aco~~~~ramics

an fentanyl

Mice were pretreated with naloxonazine (35 mg/kg s.c.). naloxone HCl (35 mg/kg s.c.) or vehicle (0.01% acetic acid 10 ml/kg s.c.). Twenty-four hours later, animals were weighed and baseline tailflick determined, and then examined for fentanyl analgesia (0.06 mg/kg s.c.. N = l~I3/pretreatmeut) or lethality (15 mg or 23 mg,‘kg se., N = 5-lO/pretreatment). Separate groups of mice were pretreated with &FNA (20 mg/kg s.c.), naloxone (20 mg/kg s.c.) or saline (10 ml/kg s.c.). Twenty-four hours later, animals were weighed and baseline tailflick deter~ned. Mice were then tested for fentanyl analgesia (0.06 mg/kg s.c., N = 7~pretreatment) or lethality (23 mg/kg s.c., N = IO,/pretreatment). 2.4. Effect of morphine tolerance an fentanyi pharmacody~~~~~~c~

Mice were implanted S.C. with either a 75 mg morphine or a placebo pellet. Seventy-two hours following implantation, mice were weighed and baseline tailflick determined and then tested for fentanyl analgesia

The assay is a modification of the procedure of Pasternak et a!. (1975) as described previously (Yobum et al., 1989). Briefly, mice were killed and whole brain rapidly removed and homogenized in 20 volumes of 50 mmol potassium phosphate buffer (pH = 7.2). Homogenates were centrifuged for 15 min; the supernat~t discarded and the pellet resuspended and centrifuged again. The pellet was resuspended and incubated for 30 min at 25*C. Following incubation, homogenates were centrifuged a third time and resuspended in 20 volumes of buffer. An aliquot of the final homogenate was assayed in triplicate. Displacement of [ 3H]naltrexone (1 nM) by fentanyl (o-2000 nM) was determined in the presence or absence of cold naloxone ~5000 nM). Specific binding was the difference between total binding determined in the absence and presence of cold naloxone. 2.6. A ~afgesi~ assay

Analgesia was determined using the tailflick assay in which a beam of light was focused on the dorsal tail surface approximately 2.5 cm from the tip of the tail. The tailflick apparatus was adjusted so that baseline tailflicks prior to drug ad~~stration were typically between 2-4 s. Following fentanyl administration if a mouse failed to flick by 10 s the trial was terminated and a latency of 10 s recorded and mouse was defined as analgesic. Based on preliminary studies mice were tested for analgesia at the time of peak effect of fentanyl (15 mm). All tests were conducted in a blind manner. 2.7. Drugs and drug Qdministr~~ion Naltrexone pellets (30 mg naltrexone base), morphine pellets (75 mg morphine base), placebo pellets, fentanyl HCl, /3-FNA, and t3H]naltrexone (13.66 Ci/mmol) were obtained from Research Triangle Institute (Research Triangle Park, NC) throu~ the Research T~~olo~ Branch of the National Institute on Drug Abuse (NIDA) (Rockville, MD). Fenta~yl citrate was obtained from Sigma (St. Louis, M(9). Naloxone HCI was generously supplied by DuPont (Wilmington, DE). Naloxonazine was generously supplied by Dr. G.W. Pasternak (Memorial Sloan-Kettering Cancer Center, New York, NY), Prior to implantation naltrexone pellets were cut in half yielding an average of 15 mg + 1.86 (SD.) naltrexone per pellet. Morphine pellets were implanted uncut. All pellets were wrapped in nylon mesh prior to implantation.

137

0.01

0.1

Fentanyl

1

10

0.01 0.1

100

(mg/kg)

Fentanyl

1

10

100

(mg/kg)

Fig. 1. Mice were implanted with a 15 mg naltrexone (NTX) or placebo (PLA) pelkt and 1 day later were injected S.C.with fentanyl. Animals were tested for anaig~ia (ED) 15 min following fentanyl (0.015-10.0 mg/kg, N = 6-g/dose). Lethality (LD) was determined 24 h following fentanyl(4-375 mg/kg, N = S-lo/dose) (see table 1).

Fig. 2. Mice were implants with a 15 mg naltrexone (NTX) or piacebo (PLA) pellet and 8 days later were injected S.C.with femanyl. Animals were tested for analgesia (ED) 15 min following fentanyl (0.01-2.5 mg/kg, N = 4-16/dose). Lethality (LD) was determined 24 h following fentanyl(4.0-150 mg/kg, N = 5-16/dose) (see table 1).

Fe~tanyl and naloxone were dissolved in 0.9% saline and administered S.C. In the studies using naloxonazine, both naloxonazine and naloxone were dissolved in 0.01% acetic acid just prior to injection and administered S.C. &FNA was dissolved in 0.01% acetic acid just prior to injection and a~~stered S.C. All doses are expressed as the free base.

1 and 8 days (table 1). The relative safety of fe~tanyl (LD,0/ED5,,) for placebo-treated mice was 291 and 249 on day i and 8 of impl~tation; whereas, it was 14 and 51 for naltrexone-treated mice on day 1 end 8. respectively. This represents = 20-fold decrease in the safety ratio following 1 day naltrexone treatment and = S-fold decrease in the safety ratio following 8 day naltrexone treatment. The results from studies of antagonism of fentanyl pharmacodyna~cs by acute S.C. naloxone administration were similar to the results with chronic antagonism by S.C. implanted naltrexone pellets. The analgesia dose-response curve was shifted more than the lethality curve in the presence of naloxone (table 2).

2.8. Data analysis Dose-response functions were analyzed using probit analysis (Finney, 1971) which estimated EIS,,s, LD,,s, 95% confidence limits, and relative potency. Statistical significance was determined using ANOVA, the Fisher exact probability test and the Z-test based on the normal distribution. Displacement studies were analyzed using non-liner regression (G~PHPAD, ver. 3.0; San Diego, CA). The K, was calcuiated according to Cheng and Prusoff (1973): Ki = I&/(1 -t- tV&Kdk where the IC,, is the concentration of fentanyl required to inhibit radioligand binding by 5058, L is the concentration of ~3H]naltrexone, and K, is the dissociation constant of [3H]naltrexone which was set at 1 nM.

3. Results

TABLE 1 Antagonism of fentany~-indu~ analgesia and lethality by S.C. implanted naltrexone pellets. Mice were implanted with a I5 mg naltrexone or placebo pellet and 1 or 8 days later were injected with fentanyl (0.01-375 mg/kg SC, N = 4.16fdose). Mice were tested for analgesia (15 min) or for lethality (24 h). ED&, LD+ and 95% confidence limits were estimated by probit analysis. Potency ratios were calculated as the EDso or LI&, in the presence of naitrexone. divided by the EDs, or LDso in the absence (placebo) of nahrexone. Treatment 1 day Placebo

The baseline t~l~ick latencies prior to fe~tanyl administration for naltrexone- and placebo pellet-treated mice did not differ (P > 0,05). Fentanyl dose-response curves for analgesia and lethality were signifi~tly shifted to the right by naltrexone (figs. 1 and 2; table 1). Analgesia curves were shifted more than lethality curves in the presence of naltrexone at both 1 and 8 days following pellet impiant~tion. The potency ratios for fentanyl analgesia (ED,,ntx/ED,,pla) and lethality ~LD~~ntx/LD~~pla) were significantly different at both

Naitrexone 8 days Placebo Naltrexone

EDSO fme/kal 0.026 (O.Ol&- 0.045) 6.87 ’ (4.93 -10.45) 0.020 @.016- 0.024) L55 ’ 11.23 - 1.871

Potency ratio

264.2

77.5 b

LDW fmg/kgf

Potency ratio

7.57 f4.64- 11.14) 13.1 C 39.35 a (53.07-144.87) 4.97 (3.9% 6.11) 79.52 a ~69.~7-103.27)

16.0 c

a Significantly different from corresponding placebo group (P c 0.05). ’ Significantly different from 1 day analgesia potency ratio (P < 0.05). ’ Significantly different from corresponding analgesia potency ratio (P < 0.05).

3.3. Effect of rno$p~t~netolerance on fent~~tyl pk~rrn~codynamics

.-\wt.+mtsm of fentan~~-indict analgesia and lethality by acute na1o~one administration. hliLz were injected S.C. with naloxone (0.1 mg.zkg) or dine (10 ml/!@ 5 ntin prior to kntanyl (0.02-60 mg/kg. S = S-X/dose. s.c.) admim .tration and tested for analgesia (15 min) op 1ethstit?_ (24 b). ED+. L&,s and 95% confidence limits were probit analysis. P&cncy ratios were calculated as the in the presence of nalosone, divided by the EDju or bsenrr of naioxone (saline). Trestment

I?&” (mg/k,e)

Saline

OXI25 ~~.~2~-0.029~

Naloxonc

0.261’ (0.224-0.298)

Potency ratio

Potency ratio

LDj, (mg/kg)

-

Implantation of a morphine pellet for 72 h caused a rightward shift in the analgesic and lethal dose-response functions for fentanyl (cross-tolerance). Chronic morphine treatnlent was associated with 1.8-fold decrease in the analgesic potency of S.C. fentanyl and 4.5fold decrease in the lethal potency (table 3). 3.4. &z&z opioid binding

7.64 (5.29-10.97) 19.65 a (13.2 -28.4)

10.4

The displacement curve of 1 nM [3Hjnaltrexone binding by fentanyl (o-2000 nM) exhibited a relatively shallow slope suggesting two binding sites (fig. 4). Nonlinear regression confirmed that a Zsite model described the displacement significantly better than a l-site model (F = 12.2, P -z 0.001) (table 4). Further, the Hill coefficient determined in seven independent experi-

2.6 b

z Significantly different from corresponding saline group (P < 0.05). ’ Significantly different from analgesia potency ratio (P < 0.05).

TABLE 3 Effect of morphine tolerance on fentauyl pharmacodynamics. Mice were impl~ted S.C. with either a 75 mg morphine or placebo pellet. Seventy-two hours following implantation, fentanyl (0.01-M) mg/kg, N = 5-lffdose) was administered and analgesia (15 mitt) and lethality (24 h) determined. EDs,s, LD5,-,s and 95% confidence limits were estimated by probit analysis. Potency ratios were calculated as EDs, or LD*, in the morphine-tolerant group, divided by EDs0 or LDs, in the control group.

There were no significant differences in the baseline tailflick latencies prior to fentanyl administration among vehicle. naloxone, naloxonazine and P-FNA pretreatments (P > 0.05). Naloxone and vehicle iretreated groups showed 100% analgesia following 0.W mg/kg fentanyl S.C. injection (fig. 3A and B), whereas naloxonazine and &FNA pretreatment effectively limited the analgesic effects of fentanyl (fig. 3A and B) (P < 0.05). In contrast, naloxonazine pretreatment did not alter the lethal effect of two different doses of fentanyl (15 or 23 mg/kg) (P > 0.05) (fig. 3C and D). However, pretreatment with fl-FNA effectively attenuated fentanyl-induced (23 mg/kg) lethality (P < 0.05) (fig. 3E).

-p h E 4

100

z J & CL

25

.g

100

cl

pretreated

with

Placebo

0.018 (0.013-0.023)

-

Morphine

0.033 b (0.026-O.~S)

1.8

LDs, (mg/ka)

Potency ratio

11.57 (9.34-14.33) 52.23 b (42.52-64.14)

4.5 a

a Significantly different from analgesia (P c 0.05). b Significantly different from placebo (P c 0.05).

O.O6mg/kg

VEH

C.

NALOX AZINE

15mg/kg

VEH

D.

N&OX

@-Ff4A

23mQh

E.

23w/kg

75 50

25 *o

t a were

B.

Potency ratio

50

z p:

3. Mice

O.O6mg/kg

EDso (mg/kg)

75

d 4

Fig.

A.

Treatment

VEH naloxonazine

(veh) (O-01’%acetic acid (A, C, D) mg/kg s.c.. N=; 7-13/Pretreatmeut)

NALOX AZINE

VEH

NALOX AZlNE

VEH

NALOX ,9-FNA

(atine) (35 mg/kg s.c.), &FNA (20 mg/kg s.c.) naloxone (nalox) (35 or 20 mg/kg s.c.) or vehicle

or saline 10 ml/kg (B, E) s.c.). Twenty-four hours later, animals were examined for fentanyl analgesia (0.06 (A and B) or lethality (15 or 23 mg/kg s.c.. N = 5_lO/pretreatment) (C, D and E). * Significantly different from vehicle-pretreated group (P -= 0.05) by Fisher’s exact probability test.

133

0.1

7

10

FENTANYL

100 1000 [nmol]

Fig. 4. Displa~m~nt of ~3H]~t~xone (1 nM) binding in mouse wide brain homogenate by fentanyl (O-Zoo0nM). Data are presented as the percent of control binding versus fentanyl concentration. Control is specific birding in the absence of fentanyl. Data are representative results from a single experiment,

ments (mean Z!IS.E. = -0.73 f 0.04) was sig~ficantly different from - 1 (P < 0.01).

4. Discussion Fenzanyl is a very potent opioid (Jaffe and Martin, 1985) frequently used in anesthesia and classified as a pure p agonist (Villiger et al., 1983; Yeadon and Kitchen, 1988; Leysen et al., 1983). Fentanyl has been shown to have a very large safety ratio (LD50/EDs0 * 250-300~ in this and other studies (Janssen, 1985). In comparison morphine’s safety is significantly smaller (70-90) (Janssen, 1985; Yoburn et al., 1989), The results of this study indicate that both chronic naltrexone and acute naloxone antagonize the analgesic and lethal effects of fentanyl; su~estin~ that both effects are opioid receptor mediated. However, analgesia was antagonized more than lethality following 1 and 8 days of naltrexone or acute naloxone treatrne~t. This uneq~~ antago~sm appears to be due to the mediation of the analgesic and lethal effects by different receptor mechanisms. Our results suggest that analgesia is mediated by TABLE 4 Displacement of 1 nM ~3H@altrexone by fentanyl in mouse brain homogenate (see fig. 4). Data were fit individually for seven experiments to l- and S-site models using non-linear regression. Data represent the mean f S.E.M. parameters from seven experiments. 1 -site Ki

Bax

(nmol)

(fmoVmg)

27.8f3.3

13.1 f0.2

a-site ’

9.3*1.1 ’ Signi~c~tly

10.4 f 0.7

363.2 f 65.0

3.0 *to.6

better fit than l-site model, F=12.2 (P +C0.001).

mechanisms that are sensitive to antagonism by naloxonazine and &-FNA. However, lethality was antagon&d only by P-FNA. This confirms the findings of others (Pasternak et al., 1980; Pastemak and Hahn, 1980; Hahn et al., 1982; Ling et al., 1983; 3985; 1986) in which p~~eatrnent with naloxonazine selectiveiy antagonized analgesia without affecting respiratory depression or lethality; indicating different receptor mechanisms for the analgesic and toxic effects of opioids. Although &FNA can antagonize morphine analgesia but not respiratory depression under certain circumst~~ {Ward and Takemori, 1983). taken Eogether, the naloxonazine and /?-FNA studies suggest different receptor mechanisms for analgesia and lethality. This su~estion is consistent with an early study (Goldste~ and Sheehan, 1969) in which it is proposed that opioid drugs produce motor activation and analgesia via a mecha~sm very different from that responsible for death. pA2 studies (McGilliard and Takemori, 1978a) have dernonstr~t~ that the iffier of ~~loxone for respiratory depression receptors is ~~~~~~1~ different from that for analgesia; also suggesting distinct mechanisms for ~~lgesia and toxic effects. Chronk morphine treatment produced cross-tolerance to both the analgesic and lethal effects of fentanyl. The magnitude of cross-tolerance was 1.8 for analgesia and 4.5 for lethality suggesting independent mechanisms for fentanyl analgesia and lethality. Previous studies have shown that rno~~ne treatment produced greater tolerance to morphine-induced analgesia (3.65.8) (Yoburn et al., 1990; ~cGil~~d and Takemori, 1978b) than to rno~~n~i~duced respiratory depression (2.6) (McGilliard and Takemori, 1978b). A possible explanation for the difference in the morphine tolerance and fentanyl cross-tolerance studies is that morphine may produce its analgesic effects through a as well as non-p sites {e.g. Takemori and Portoghese, 19871, whereas fentanyl appears to be a highly p-selective opioid agonist (Villiger et al., 1983; Yeadon and Kitchen, 1988) with higher intrinsic efficacy than morphine. The receptor selectivity of fentanyl combined with its high intrinsic efficacy may result in the smaller shift in the analgesia dose-response curve for fentanyl (1.8) compared to morphine (3.6-5.8) in morphine tolerant animals. In the case of respiratory depression or toxicity, perhaps fentanyl has fairly low intrinsic efficacy at these binding sites, and this confers more cross-tolerance than is observed for analgesia. Thus, the spare receptor population for fe~~tanyl-induced analgesia may exceed that of the spare receptor population for letha~ty. This suggestion is consistent with proposals that agents with high int~nsi~ efficacy will produce less tolerance than those with low efficacy (Stevens and Yaksh, 1989a,b). In any case, numerous studies have shown incomplete cross-tolerance in opioid tolerant preparations {e.g. Brase, 1986; Moulin et al”, 1988%

r&?;sis of the binding data indicated both affinity ~~~~~~~ sites for fe~ta~yi. The ~ff~~~t~sites appeared to be greater than that of ~ow a~f~~~~t~ sites. Since analgesia is produced at :p ~owcr dose than lethality. we presume that the high ~~~~~~~~t~ site mediates analgesia and the low affinity site ity. Taken together, the pharmacodymediates t ing data suggest that naltrexone occupanamic sncf tion of the high affinity fentanyl site requires a greater ase in fentanyl dose to produce an effect (analgesia) that observed for the low affinity site (lethality). Naltresone treatment has been shown to increase brain opioid birding sites (receptor upregulation) (Lahti and Collins, 1978; Bardo et al., 1983; Tempel et al.. 19S5; Ynburn et al.. 1985). One day following naltrexone treatment or acute naloxone treatment. the number of opioid receptors is not increased: however, the ~~~~~r of ~~~~~d receptors is increased 8 days foilowing ~a~tre~o~e treatment (Yoburn et al., 1985; 1986). In the present study. the difference in receptor numbers for 1 and 8 day naltrexone-treated groups did not alter the greater antagor.ism of fentanyl analgesia compared to antagonism of ~eth~~ty. This indicates that regardless of the ~~~~ber of receptors. the relative safety of fentanyl is decreased in the presence of antagonists. In conclusion, fentanyl induces analgesia and Iethaiity by two different receptor mechanisms, Firstly, chronic naltrexone and acute naloxone differentially ~tago~ed fenta@ analgesia and lethality. Secondly, ~a~oxon~ne and /3-FNA inhibited the analgesic effects of fenta~y~” but the lethal effects are attenuated by B-FNA, only. Thirdly, different degrees of cross-tolerance were observed for the two effects. Finally, the shallow slope observed in displacement studies suggest two different binding sites. While it is difficult to extrapolate from our animal studies to man, these results suggest that opioid agonist-induced toxicity and analgesia may not be equally antagonized in the naltrexone-maintained patients who requires opioids for pain relief. Thus, some caution may need to be exercised in providing opioid analgesic medication in the ~altrexo~e-m~nt~ned patient. w

Our thanks to Mr. Victor Sierra and Mr. Kabir Lutfy for helpful suggestions and comments. Dr. M.T. Turnock provided support and critical discussion during the course of these studies. The studies reported here comprise a portion of a thesis submitted by the first author in partial fulfillment of the requirements for a Masters degree in pharmaceutical sciences. These studies were supported by NIDA Grant DAO4185.

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