BIOPHARMACEUTICS & DRUG DISPOSITION, VOL. 12, 189-199 (1991)

EFFECTS OF ETHANOL AND A9-TETRAHYDROCANNABINOL ON

PHENCYCLIDINE DISPOSITION IN DOGS

PAUL J . GODLEY, EMORY S. MOORE, JAMES R. WOODWORTH*t AND JERRY FINEG

The University of Texas at Austin, College of Pharmacy. Austin. Texas 78712

ABSTRACT A three-way crossover study was performed to determine the influence of A'-tetrahydro- cannabinol (THC) and ethanol (EtOH) separately upon phencyclidine (PCP) disposition in dogs. Seven dogs were given three single dose treatments: 1.5 mg PCP kg-' i.v., 1.5mg PCP kg-' i.v. with 0.4mgkg-' THC i.v., and 1.5mg PCP kg-' i.v. with 1.25g EtOH kg-' i.v. PCP was measured in plasma samples collected for 24 h after adminis- tration of each treatment, with several pharmacokinetic parameters calculated from the plasma concentration vs time data. The PCP serum C1, values were significantly lower when administered with THC than when administered alone, with no significant change in Vs or tOh. EtOH did not induce significant changes in any PCP pharmacokinetic parameter, although mean C1, and Va were increased. These results confirm the observed THC inhibition of PCP metabolism, and suggest that the enhanced pharmacologic action of PCP by THC may result from higher serum PCP concentrations. These results further suggest that enhanced PCP actions by acute EtOH administration may result from increased PCP distribution to the CNS.

KEY WORDS Drug interaction Ethanol Pharmacokinetics Phencyclidine (PCP) A9-Tetrahydrocannabinol (THC)

INTRODUCTION

Phencyclidine (PCP, angel dust) is a drug of abuse which has, in recent years, experienced a resurgence in popularity, and has become the most widespread drug of abuse among children and young adults (ages 6 1 9 years) in some parts of the USA.' Noted pharmacological effects, including tachycardia, in- coherence, and hallucinations, with resulting aggravated behavioral effects including excessive violence and a schizophrenic-like mental status, make use of this drug a major concern. PCP intoxication is responsible for more hospital- izations and emergency department visits in some areas than any other drug of

It is common for drug abusers to ingest more than one drug at once to enhance their euphoria. Ethanol (EtOH) is most often used as a concomitantly

* Present address: Lilly Laboratory for Clinical Research, Eli Lilly and Company, Wishard Memorial Hospital, 1001 W. 10th Street, Indianopolis, Indiana 46202. t Addressee for correspodence.

0 142-2782/9 1/030 189-1 1$05.50 0 1991 by John Wiley & Sons, Ltd.

Received 10 March 1990 Revised 25 July 1990

190 PAUL J . GODLEY ET A L .

administered drug because of its availability and low cost. PCP users reportedly take other drugs 42 per cent of the time PCP is inge~ted;~ 98 per cent of PCP users admit using EtOH at least once with PCP.4 In addition to concomitant use with EtOH, PCP is often used with A9-tetrahydrocannabinol (THC) in marijuana cigarettes.

Studies performed in mice show PCP and EtOH act synergistically, even after a single dose of EtOH.S Both EtOH and PCP are metabolized extensively, although by different routes. Nonetheless, EtOH has inhibitory and stimulatory effects on microsomal enzymes, which vary with the time of EtOH exposure and which is species and substrate dependent.6 A study of rat microsomal preparations indicated PCP metabolism was slightly enhanced after chronic EtOH administration.’ However, another study indicated higher and prolonged concentrations of 3H-PCP in adipose, brain, and plasma after chronic EtOH administration to rats,8 possibly due to an inhibition of PCP metabolism by EtOH. The results of these two studies conflict and provide no conclusive results of EtOH’s chronic effects on PCP disposition. EtOH administered acutely did not affect brain or plasma PCP concentrations up to 2 h after dosing in rats, but the data were limited to three sample collections during a 2 h test period.8

The behavioral and pharmacologic effects of PCP are reportedly enhanced by marijuana constituents, most notably THC.9-” In addition, concomitant administration of both THC and PCP has produced increased concentrations of 3H-PCP in various tissues.I2 Although the total radioactivity increased, no differentiation was made between PCP or its metabolites.

These documented increases in PCP’s pharmacologic action may be due at least in part to an effect of EtOH and THC upon the disposition of PCP. Current and proposed treatments of PCP detoxification rely upon enhancing the drug’s elimination from the body.I3J4 How EtOH and THC affect the distri- bution and elimination of PCP may affect not only the pharmacologic actions of PCP but also its detoxification. This study was undertaken to determine the influence of THC and EtOH upon PCP disposition in dogs.

METHODS

The protocol for this study was reviewed and approved by the Animal Resources Committee of the University of Texas at Austin.

Drugs

PCP.HC1 (as a powder) and THC (as an ethanolic solution) were obtained from the National Institute on Drug Abuse (NIDA; Research Triangle Park, NC). EtOH was purchased from Abbott Pharmaceuticals (N. Chicago, 111.). THC for injection was diluted to a 1 mg ml-l ethanolic solution. A 5 mg PCP

PHENCYCLIDINE INTERACTIONS 191

base ml-' solution was prepared for injection by dissolving PCP.HC1 in sterile water. EtOH was used as supplied. All preparations were made within 1 h of use.

Analytical PCP. A radioimmunoassay (RIA) specific for PCP was used for quantifica-

tion of PCP in serum. Radiogland (1251-labelled PCP), rabbit anti-PCP sera, and polymer bound goat antirabbit immunoglobulins in phosphate-buffered saline were kindly provided by Roche Diagnostics. Analysis of samples was performed using procedures published previously.ls Radioactivity from samples was quantified by an LKB Model 1282 gamma counter. Standard curves between 1 and l00ngml-l were prepared from blank dog serum obtained at least 1 week before experimentation. The lower limit of sensitivity determined from a 0.1 ml serum sample was 1 ng ml-l.

EtOH. EtOH was quantified using a head space gas chromatographic method from a 0.05 ml whole blood sample. A Perkin-Elmer Sigma 2000 gas chromato- graph was used with a flame ionization detector. The unit was outfitted with a 1-8 x 3 mm aluminium column packed with 80-100 mesh Porapak QS; the carrier gas was nitrogen (30 ml min-I). Temperature settings: column, 190°C; injector and detector, 240°C. Lower limit of sensitivity was 5mgdl-I. This method has been used previously.16

Animal experiments Dogs were selected as the experimental animal because of the similarity of

PCP metabolism between dogs and Seven mongrel dogs (16 to 32 kg) were used in this study. All animals were housed and dosed in the Animal Resources Center of the University of Texas at Austin. A three-way crossover study was performed with the following treatments:

1. a reference treatment of 1 *5 mg kg-' dose of PCP given intravenously (i.v.); 2. a 1.5mgkg-' i.v. dose of PCP given concomitantly with an 0.4mgkg-I

i.v. dose of THC; 3. a 1.5 mg kg-I i.v.dose of PCP given concomitantly with an i.v. 1.25 gm EtOH/

kg infusion. The EtOH was diluted in 250 ml normal saline and allowed to flow freely

over a 15-min interval. When THC or EtOH were administered, the dose of PCP was given 30min after the administration of THC or the end of the EtOH infusion. At least 1 week separated each treatment.

All dogs were fasted overnight and remained fasted for at least 4 h after dosing. Doses of PCP, THC, and EtOH were administered via the brachial vein. Blood samples for the analysis of PCP were obtained from the right or left jugular or from the brachial vein not used for dosing. Blood samples

192 PAUL J . GODLEY ET A L .

were collected at various times until 24 h after dosing. Additional blood samples were obtained during the 24 h collection period for the determination of EtOH concentrations.

Data analysis

Standard model independent analyses were used for the determination of several pharmacokinetic parameters, including systemic clearance (Cl,), volume of distribution ( Vp), volume of distribution at steady state (V,,), half-life ( t H ) , and mean residence time (MRT).20,21 Parameter estimations for non-linear sys- tems (e.g., EtOH) were calculated using statistical moment theory with adjust- ments for MRT.22 Comparison of parameters between treatments was performed by an analysis of variance. Differences were considered significant for p < 0.05 and were differentiated into appropriate groups using a Student- Newman-Keuhls test.

RESULTS

One of the seven dogs was unable to complete the entire crossover treatments; thus, the treatment blocks had an uneven distribution. All dogs were given the control treatment (PCP only) and the PCP + THC, with all but dog 3 completing PCP + EtOH. Dog 4 was administered a 1.0 g kg-l dose of EtOH rather than a 1.25 g kg-I dose for the PCP + EtOH treatment.

Samples for EtOH analysis were not collected from one dog in the PCP + EtOH trial. EtOH concentrations were targeted for 150mgdl-I; the first time samples were collected after infusion had ended (approximately 15 min) pro- duced whole blood EtOH concentrations ranging from 13 1 to 182 mg dl-I. Mean ethanol whole blood concentrations remained above 150mgdl-I for 1 h after the end of infusion.

THC, EtOH interaction with PCP

Table 1 lists the mean PCP serum concentrations for all treatments; Figure 1 shows the serum concentration vs time curves for all treatments given to two dogs. The mean calculated PCP pharmacokinetic parameters from all treat- ments are given in Table 2. PCP when given alone exhibited large Vs, V,, and C1, values.

Changes were apparent for some of the pharmacokinetic parameters when compared between treatments. Figure 2 compares the individual C1, values and Figure 3 .V, and terminal rate constant values for all treatments. Although the values of C1, and Vs remained large for the two test treatments, C1, was significantly smaller for the PCP + THC when compared to the control treat- ment. C1, for the PCP + EtOH treatment was larger than the control treatment,

PHENCYCLIDINE INTERACTIONS 193

Table 1. Mean ( f SD) PCP serum concentrations, all treatments

PCP PCP + THC PCP + EtOH (n = 7) (n = 7) (n = 6)

Time(h) Conc. (ngml-I) Time(h) Conc. (ngml-I) Time(h) Conc. (ngml-I)

0.25 235 f 112 0.25 299 f 7 7 t 0.51 143 f 38 0.52 182 f 55 0.78 114 f 25 0.65 146 f 30t 1.0 100 f 34t 1 .o 129 f 367 1.5 73 f 28 1.5 119 f 2 1 2.0 56 f 18 2.0 79 f 18 3.1 29 f 9.4 3.0 48 f 20 4.0 23 f 6.8 4.0 40 f 26 6.0 15 f 6.4 6.0 26 f 20 9.1 9.1 f 2.8 9.0 18 f 14

14.5 5.8 f 2.2 12.0 12 f 5.9 23.5 2.5 * 0.6 22.9 5.4 f 3.2

0.27 175 f 6 6 0.54 113 f 6 2 0.77 86 f 4 0 1 .o 69 f 2 6 1.5 51 f 2 4 2.0 37 *10 3.0 22 f 6.8 4.1 16 f 3.8 6.1 11 f 4.7 9.1 9.2 f 4.9

12.0 6.5 f 1.0$ 24.1 3.3 * 0.8$

t n = 6. $ n = 4.

Table 2. Mean (f SD) PCP pharmacokinetic parameters, all treatments

Dog PCP Vp V,, t s t MRT

h weight dose CIS

Treatment n kg mg mlminkg-l Ikg-' lkg-l h

PCP 7 20.1 32.1 57.7 40.1 22.3 7.77 6.37 (5.35) (7.73) (16.1) (18.5) (8.51) (0.97)

PCP + THC 7 20.7 31.2 35.2* 37.6 24.8 7.94 13.8 (5.65) (8.46) (9.90) (35.9) (25.3) (1 7.9)

PCP+EtOH 6 22.5 33.7 70.6 59.3 37.3 9.31 9.00 (5.48) (8.21) (21.9) (16.7) (14.0) (2.89)

* p < 0.05 when compared to reference. t Harmonic mean.

but was not statistically significant. VB also changed. The mean did not change for the PCP + THC treatment when compared to the control treatment, but the variability of values for this test treatment was quite large. The PCP + EtOH treatment produced larger Vs values, although they were not statistically differ- ent from the control. The changes in C1, and Vs produced no net changes in the terminal elimination rate constants for either of the two test treatments.

DISCUSSION

The large volume of distribution and clearance of PCP found in this study has been previously described in dogs;18 the values generated from the control treatment are in good agreement with those values reported previously.

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PAUL J. GODLEY ET AL.

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---T-T r-,->-,

0 5 10 15 20 25

Time (hours)

Figure 1 . Concentration vs time curves of two dogs on log-linear scales. In both graphs closed circles (0) represent PCP, closed squares (W) represent PCP + THC, and open circles (0) represent PCP + EtOH. PCP disposition in the presence of ethanol (PCP + EtOH) was inconsistent between the animals, producing PCP concentrations either higher or lower than the PCP alone. However, PCP disposition in the presence of THC (PCP + THC) produced consistently higher PCP concen-

trations when compared to PCP alone

The significant reduction of C1, and maintenance of Vs without a concomitant increase in t , is deceiving in the PCP + THC treated group. In some animals, the reduced C1, was accompanied by a prolongation in t lh while the Vs remained constant or increased. In others, the reduced C1, was accompanied by a reduc- tion in Va, resulting in no change or a reduction in tH . In any event, changes did occur for any one animal between the test and control treatments for Vs and t lA. However, unlike CIS, these changes were not consistent for either para- meter. Consequently, average values appeared unchanged but were accom- panied by a large standard deviation. The changes apparent in t lh and V, could only be explained by interindividual differences between the animals.

The reduction of PCP C1, by THC is not entirely surprising. Both PCP and THC form monohydroxymetabolites; THC and other cannabinoids (speci- fically, cannabidiol) have demonstrated an ability to inhibit phase I metabolism of several compounds other than PCP.23-26 Although no PCP metabolites were measured in either the urine or plasma, the potential for this interaction and the reduction of PCP C1, support this method of interaction.

PCP C1, reduction by THC would produce higher PCP plasma concentrations for longer periods of time. These effects correlate well with the observed prolon- gation of PCP's actions by THC.9-"

Comparison of our study to a similar trial performed in ratsI2 appears to

PHENCYCLIDINE INTERACTIONS 195

0 110 loo^ 90

60

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50

n n

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Figure 2. Comparison of CIS values between each treatment group. Bars represent standard devia- tion (* p < 0.05)

clarify observed differences noted in the previous study. The rat study tested the effects of acute (single dose) and subacute (daily doses for 1 week) THC administration upon PCP disposition and also measured neurologic responses. Acute coadministration of 14C-THC and 3H-PCP showed enhanced neurologic impairment, but did not cause significant changes in plasma or brain 3H-PCP concentrations when compared to a control treatment. However, no differenti- ation was performed to determine whether the measured 'H concentrations were from parent drug or metabolites. The results from our dog studies suggest that although total radioactivity does not change between the control and test treatments, the enhanced neurologic impairment observed in rats is most likely due to greater CNS concentrations of parent drug rather than metabolites.

This same study indicated that concomitant )H-PCP and 14C-THC adminis- tration produced higher brain concentrations of 14C, with no significant change in plasma 14C concentrations.'* THC concentrations were not measured in that

196

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PAUL J . GODLEY ET AL.

0.15

PCP PCP+ PCP+ THC EtOH

0.00

0

I: 0

PCP PCP+ PCP+ THC EtOH

Figure 3. Comparisons of Vs and terminal rate constant values between treatment groups. Bars represent standard deviation

study, so parent compound and metabolites were not differentiated in either brain or plasma. Nonetheless, it appears that PCP may influence the distribution or metabolism of THC or its metabolites. Our study was unable to confirm this observation since THC was not quantified.

EtOH’s influence upon metabolism of other compounds has been well docu- mented.27 Chronic administration of EtOH may lead to tissue damage in the liver from the production of acetaldehyde. However, the direct intracellular effects of EtOH upon metabolic systems vary. EtOH has been shown to induce cytochrome P450IIE 1, but only after chronic EtOH administration. Acute administration may cause an inhibition of metabolism, by direct competition for a common microsomal metabolic pathway or indirectly by decreasing the supply of NADPH. In this fashion, glucoronidation may be influenced as well as oxidative-reduction pathways. The PCP plasma concentrations from our study suggest EtOH did not influence the oxidative metabolism of PCP after acute EtOH administration. No PCP metabolites were measured in our study, so the effects of EtOH upon total metabolite production or the glucuronidation of PCP’s monohydroxymetabolites are unknown.

Although not statistically significant, the observed increase in PCP’s volume of distribution after administration of EtOH is noteworthy. The increase in volume of distribution could partially explain the increase in 3H present in brain and adipose tissues after administration of 3H-PCP to chronically EtOH-

PHENCYCLIDINE INTERACTIONS 197

treated rats.* It would also help explain the potentiation of PCP’s effects after acute doses observed in the same study. However, it would not explain the increase in plasma 3H concentrations noted from chronic EtOH administration. The authors suggested EtOH inhibited PCP metabolism to cause an increase in plasma 3H concentrations.

In chronic EtOH treatment, acetaldehyde-induced hepatic damage may cause a net reduction of xenobiotic metabolism despite the documented EtOH P450- induction. Indeed, clearance of several compounds is generally reduced in chronic alcoholics.28 In addition, the presence of EtOH may cause a metabolic inhibition by direct competition with various substrates for microsomal enzymes. A reduction of PCP metabolism after chronic EtOH administration cannot be considered abnormal.

Table 3. Mean (f SD) EtOH pharmacokinetic parameters from PCP + EtOH treatment

5 0.515 0.1 16 4.08 2.45 0.59 (0.186) (0.118) (0.66) (0.47) (0.08)

* Maximum rate (velocity) for Michaelis-Menten systems. t Michaelis-Menten rate constant.

The concentrations of EtOH produced from a 1.25 g kg-I dose were within a range of intoxication. Indeed, the behavior of the dogs after EtOH infusion confirmed this observation. The whole blood EtOH concentrations allowed calculation of several pharmacokinetic parameters for EtOH, which are sum- marized in Table 3. The blood concentrations from our study are consistent for the dose administered when compared to studies where EtOH has been administered a l ~ n e . ~ ~ - ~ ’ EtOH mean clearance is lower than that reported in other studies, but clearance in saturable systems is dose-dependent and a reduc- tion would be expected with the higher dose administered in our study. The calculated V, for EtOH was similar to total body water volume and agreed very well with previous EtOH studies performed in dogs.29 Although these indications suggest PCP has no influence upon EtOH disposition, our studies could not verify this conclusion since no appropriate control treatment was included.

Our results confirm the in vitro inhibition of PCP metabolism by THC and indicate circulating PCP concentrations are increased upon concomitant admin- istration. The potentiation of PCP’s effects by THC appear to be directly related to increased concentrations of the parent compound. Our results further suggest that acute administration of EtOH may cause greater extravascular distribution of PCP, possibly to affected tissues. Although this increase in PCP distribution was not statistically significant, it would explain the potentiation of PCP’s

198 PAUL J . GODLEY ET AL.

pharmacologic effects when administered concomitantly with EtOH, despite the slight increase observed in PCP’s clearance values.

ACKNOWLEDGEMENTS

The authors would like to gratefully acknowledge the assistance of Dr Carl Erickson for ethanol determinations and Dr Harvey Snyder for radiogland and antibody supplies. Funding for this study was provided by a Basic Science Research Grant (BSRG) through the University of Texas.

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