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)
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 EtOHs 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 PCPs 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 drugs 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.
The protocol for this study was reviewed and approved by the Animal Resources Committee of the University of Texas at Austin.
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, 190C; injector and detector, 240C. 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.
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.
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