Metabolites of phencyclidine

Bioactivation of Phencyclidine in Rat and Human Liver Microsomes and Recombinant P450 2B Enzymes: Evidence for the Formation of a Novel Quinone Methide Intermediate

The hypothesis that the psychological side effects associated with the anesthetic phencyclidine (PCP) may be caused by irreversible binding of PCP or its reactive metabolite(s) to critical macromolecules in the brain has resulted in numerous in vitro studies aimed at characterizing pathways of PCP bioactivation. The studies described herein extend the current knowledge of PCP metabolism and provide details on a previously unknown metabolic activation pathway of PCP. Following incubations with NADPH- and GSH-supplemented human and rat liver microsomes and recombinant P450 2B enzymes, two sulfhydryl conjugates with MH+ ions at 547 and 482 Da, respectively, were detected by LC/MS/MS. Shebley et al. [(2006) Drug Metab. Dispos. 34, 375-383] have also observed the GSH conjugate 1 with MH+ at 547 Da in PCP incubations with rat P450 2B1 and rabbit P450 2B4 isoforms fortified with NADPH and GSH. The molecular weight of 1 is consistent with a bioactivation pathway involving Michael addition of the sulfhydryl nucleophile to the putative 2,3-dihydropyridinium metabolite of PCP obtained via a four-electron oxidation of the piperidine ring in the parent compound. The mass spectrum of the novel GSH adduct 2 with an MH+ ion at 482 Da was suggestive of a unique PCP bioactivation pathway involving initial ortho- or para-hydroxylation of the phenyl ring in PCP followed by spontaneous decomposition to piperidine and an electrophilic quinone methide intermediate, which upon reaction with GSH yielded adduct 2. The LC retention times and mass spectral properties of enzymatically generated 2 were identical to those of a reference standard obtained via reaction of GSH with synthetic p-hydroxyPCP in phosphate buffer (pH 7.4, 37 degrees C). 1H NMR and 13C-distortionless enhancement by polarization transfer (DEPT) NMR spectral studies on synthetically generated 2 suggested that the structural integrity of the p-hydroxyphenyl and cyclohexyl rings likely was preserved and that the site of GSH addition was the benzylic carbon joining the two scaffolds. The formation of 2 in human microsomes was reduced upon addition of the dual P450 2C19/P450 2B6 inhibitor (+)- N-3-benzylnirvanol. Consistent with this finding, both recombinant P450 2B6 and P450 2C19 catalyzed PCP bioactivation to 2. In the absence of GSH, synthetic p-hydroxyPCP underwent rapid decomposition (t1/2 approximately 5.2 min) to afford p-hydroxyphenylcyclohexanol and p-hydroxyphenylcyclohexene, presumably via the quinone methide intermediate. Overall, our findings on the facile degradation of synthetic p-hydroxyPCP to yield an electrophilic quinone methide intermediate capable of reacting with nucleophiles, including GSH and water, suggest an inherent instability of the putative phenolic PCP metabolite. Thus, if formed enzymatically in vivo, p-hydroxyPCP may not require further metabolism to liberate the quinone methide, which can then react with macromolecules. To our knowledge, this is the first report of a quinone methide reactive intermediate obtained in human-liver microsomal metabolism of PCP.

Determination of lysergide (LSD) and phencyclidine in biosamples

Lysergic acid diethylamide (LSD) is difficult to detect and to quantify in biosamples because of its very low active dose. Although there are a number of tests available, routine analysis of LSD is rarely performed. Immunoassays largely vary in their specificity and cross-reactivities with other molecules often make these tests unreliable. Because of its low concentration and the instability of the derivatives (e.g. trimethylsilyl-LSD), routine gas chromatography-mass spectrometry (GC-MS) detection and quantitation of LSD remains a difficult task. The most promising procedures for LSD determination seems to be liquid chromatography-MS analysis using electrospray ionisation and selected ion monitoring (SIM). Extraction, derivatization, GC or high-performance liquid chromatography conditions and the different detection modes will be summarised. Phencyclidine (PCP) is an abused drug seldom found outside the United States. Well established detection and quantitation procedures include radioisotopic and nonradioisotopic immunoassays and GC-MS analysis using SIM mode with deuterated PCP as internal standard. Alternatively, GC with nitrogen-phosphorus detection or capillary electrophoresis has been used. Recent progress in PCP analysis will be summarised.

Incorporation of phencyclidine and its hydroxylated metabolites into hair

The incorporation of phencyclidine(PCP) and its three major hydroxylated metabolites, 1-(1-phenylcyclohexyl)-4-hydroxypiperidine(PCHP), trans-4-phenyl-4-piperidinocyclohexanol(t-PPC) and trans-1-phenyl-1-(4'-hydroxypiperidino)-4-cyclohexanol(t-PCPdiol) into rat hair was studied. Three Dark Agouti male rats were intraperitoneally administered with PCP x HCl at a dose of 0.5 or 1.0 mg/kg once a day for 10 successive days. The plasma samples were collected from 5 min to 360 min after injection of each drug. The hair samples were collected 28 days after the first administration. The hair samples were extracted with methanol-5N hydrochloric acid(20:1) for 1 h under sonication. The plasma and hair extracts were extracted or purified with Bond Elut Certify and the extracts were silylated for the determination of PCP and its metabolites by GC/MS. The plasma AUCs were as follows; PCP(2.03 microg x min/ml) > t-PCPdiol(0.60 microg x min/ml) > PCHP(0.11 microg x min/ml) > t-PPC (0.065 microg x min/ml), while the hair concentrations were as follows; PCP(7.51 ng/mg) > PCHP (1.22 ng/mg) > t-PPC(0.10 ng/mg) > t-PCPdiol (0.05 ng/mg). In view of their AUCs, the hair concentration of t-PCPdiol was quite low, whereas that of PCP was so high. PCHP, t-PPC or t-PCPdiol was separately administered as the parent drug to the rats, and then the plasma and hair samples were analyzed in the same manner as PCP experiments. The incorporation rates ([hair concentration]/[AUC]) of PCP and its hydroxylated metabolites were as follows; PCP(2.29) > PCHP(0.79) > t-PPC(0.36) > t-PCPdiol(0.32). These data suggest that the decrease in lipophilicity caused by the hydroxylation of PCP suppresses the incorporation of the metabolites from blood into hair and the hydroxylation on cyclohexane ring(t-PPC) induces the decrease of the drug incorporation into hair more than that on piperidine ring(PCHP).

Effects of the major metabolite of phencyclidine, the trans isomer of 4-phenyl-4-(1-piperidinyl)cyclohexanol, on [3H]N-(1-[2-thienyl] cyclohexyl)-3,4-piperidine ([3H]TCP) binding and [3H]dopamine uptake in the rat brain

The major metabolite of phencyclidine (PCP), the trans isomer of 4-phenyl-4-(1-piperidinyl)cyclohexanol [(trans)-4-PPC], inhibited [3H]N-(1-(2-thienyl)cyclohexyl)-3,4-piperidine ([3H]TCP) binding to well-washed rat cortical membranes with much less activity than PCP itself. In contrast, it inhibited [3H]dopamine ([3H]DA) uptake in rat striatal synaptosomes to a similar extent as PCP. Considering our previous observations that intraperitoneally administered (trans)-4-PPC elicits dose-related increases in locomotor activity and rearing in mice, (trans)-4-PPC as well as PCP may be involved in psychotomimetic effects of PCP due to its inhibitory effect on DA uptake.

Behavioral effects of phencyclidine and its major metabolite, (trans)4-phenyl-4-(1-piperidinyl)cyclohexanol, in mice

To elucidate the biological activity of natural metabolites of phencyclidine (PCP), we examined the behavioral effects of a major metabolite, the trans isomer of 4-phenyl-4-(1-piperidinyl)cyclohexanol [(trans)PPC], in mice, (Trans)PPC caused dose-related increase in locomotor activity and rearing in mice when injected intraperitoneally at the doses ranging from 10 to 30 mg/kg. (Trans)PPC at any dose tested failed to produce swaying and falling. On the other hand, PCP at the doses ranging from 1 to 10 mg/kg caused swaying and falling as well as hyperlocomotion in a dose-related manner. These indicate that unlike PCP, hyperlocomotion and rearing may be the predominant behavioral responses to (trans)PPC in the 10-30 mg/kg dose range. Furthermore, it is feasible to surmise that not only PCP but also its major metabolite (trans)PPC is involved in psychotic reactions produced by PCP.

"Designer drugs"--a current perspective

Since the late 1970s, in an effort to quench the ever burgeoning appetite for pharmacological substances of abuse and to satiate their own need for profit, unscrupulous chemists have set up clandestine laboratories to produce and market new drugs for street sale. Using fairly common industrial chemicals, they have altered or modified preexisting controlled substances such as fentanyl, meperidine, mescaline, amphetamine, and phencyclidine, producing derivatives of these parent compounds that, up until 1986, were able to temporarily elude the guidelines of the Federal Controlled Substances Act due to their new and unique chemical structures. Unsuspecting users continue to use the drugs recreationally. This article will present a comprehensive review of these "Designer Drugs" looking at historical data, pharmacokinetics, treatment, abuse trends, and some of the more recent additions to the social pharmacopoeia.

Behavioral effects of phencyclidine and ketamine alone and in combination with other drugs

The behavioral effects of phencyclidine (PCP) and ketamine administered alone and in combination with naloxone, atropine, methyl atropine, chlorpromazine and d-amphetamine were studied in squirrel monkeys trained to press a response lever under a fixed-ratio 30 schedule maintained by the termination of a stimulus associated with electric shock presentation. Under non-drug conditions, a period of high-rate responding in the presence of the stimulus associated with shock presentation was followed by a period of no responding during a 40-s timeout scheduled between fixed-ratio components. Mean rates of responding during fixed-ratio components decreased monotonically as PCP dose increased from 0.1 to 0.56 mg/kg, and doses of 3.0 and 5.6 mg/kg ketamine produced decreases in mean response rate comparable to doses of 0.3 and 0.56 mg/kg PCP. The dose-effect functions revealed that ketamine was approximately one-tenth as potent as PCP. The present data also characterized the time-course effects of PCP and ketamine, with the former having effects that were slower in onset yet more persistent in time. None of the drugs studied in combination with PCP and ketamine provided evidence of a pharmacological antagonism of the behavioral effects of the latter two drugs. Rather, the data indicated an enhancement of behavioral effects when certain drug combinations were studied.

Phenylcyclohexylamine: effect of a metabolite of phencyclidine on the efflux of dopamine in the rat

The effect of phenylcyclohexylamine (PCA) on the efflux of dopamine (DA) in the neostriatum was examined using in vivo electrochemical techniques. Phenylcyclohexylamine produced a long-lasting dose-dependent biphasic effect on the efflux of DA in the rat. This response, to one of the major metabolites of phencyclidine, was similar in duration to but less potent than that seen with phencyclidine.

Metabolic interactions of phencyclidine (PCP) and delta 9-tetrahydrocannabinol (THC) in the rat

The in vitro effects of THC on the metabolism of PCP by rat liver were determined. Samples containing 1 mM PCP were incubated for 1 hr at 37 degrees C with an NADPH-generating system containing 10,000 X g supernatant or Ca++-precipitated rat liver microsomes. These incubations were carried out in the presence or absence of THC and at the end of 1 hr, PCP metabolites were determined by gas chromatography. In the presence of 0.1, 0.05, 0.025 and 0.0125 mM THC, the production of 1-(1-phenyl-4-hydroxycyclohexyl)piperidine (metabolite I) by the 10,000 X g supernatant was decreased by 46, 29, 23 and 16% respectively. Similarly, production of 1-(1-phenylcyclohexyl)-4-hydroxypiperidine (metabolite II) was reduced significantly by 58, 44, 34 and 23% with the respective concentrations of THC. However, the production of 1-phenylcyclohexylamine (metabolite III) was increased by 18, 32, 30 and 22% with 0.1, 0.05, 0.025 and 0.0125 mM THC. Incubations with Ca++-precipitated liver microsomes revealed similar trends in PCP metabolism in the presence or absence of THC. Metabolites I and II were reduced by 62 and 67% by 0.1 mM THC. Another concentration of THC (0.025 mM) caused a 50 and 62% decrease in I and II. These observations suggest that THC alters the in vitro microsomal metabolism of PCP.

Effects of cigarette smoke and 3-methylcholanthrene on the disposition of phencyclidine and its N-ethylamine analogue in the isolated perfused lung of rats

The isolated perfused lung (IPL) of rats were used to examine the pulmonary disposition and metabolism of radiolabeled phencyclidine (PCP) and N-ethyl-1-phenylcyclohexylamine (PCE). The IPL removed PCP and PCE from the perfusate and converted them to free and conjugated metabolites. At the conclusion of a 1-h perfusion, the lung accumulated at least 20% of the administered radioactivity and metabolized more than 30% of the added drug. Pretreatment of rats with 3-MC or cigarette smoke enhanced significantly PCP and PCE metabolism by the IPL. The concentration of conjugated PCE metabolite in the perfusate of the IPL was increased significantly by both 3-MC and cigarette smoke pretreatments whereas the concentration of conjugated PCP metabolite was not affected by cigarette smoke exposure and increased only slightly after 3-MC pretreatment. Pretreatment of rats with 3-MC or cigarette smoke also altered the amount of radioactivity accumulated by the lung tissue at the conclusion of a 1-h perfusion. Inasmuch as PCP and PCE are often abused by humans via smoke inhalation, a significant amount of these drugs may be stored or metabolized by the lung.

The use of microorganisms for the study of drug metabolism

The potential for the use of microorganisms as tools in the study of drug metabolism appears to be unlimited. The selected examples cited here are only the beginning of what could develop into a widely accepted alternative in vitro model system for studying drug metabolism in humans. As with any other in vitro model system, it is not expected that microbial systems could ever replace animals in biomedical research. The acquisition of data regarding absorption, distribution, and excretion will still require whole animal systems. However, it is clear from the examples cited that microbial systems offer a reliable, reproducible alternative to small animal models for preliminary drug metabolism studies. Due to significant species variation, small animal models may, in many cases, be less reliable than microorganisms as predictive models of human metabolism. It has been estimated that approximately 70 million animals are used each year in the U.S. for biomedical research. The development of any techniques which curtail the sacrifice of such large numbers of animals is welcomed both by animal welfare groups who wish to ensure the humane treatment of laboratory animals and by researchers who additionally appreciate the more practical and economical benefits of such alternatives.

Biotransformation of phencyclidine

PCP is metabolized extensively in the body via a variety of metabolic routes. Biotransformation is a major mechanism of PCP elimination in humans and termination of PCP action in mice. In general, PCP metabolites are less active pharmacologically than PCP itself. Primary metabolism involves hydroxylation of the alicyclic rings at several carbon atoms by cytochrome P-450-mediated monooxygenase. Hydroxylation of the aromatic ring seems to be less likely and has not been conclusively demonstrated. Hydroxylation of PCP at carbon 2 of the piperidine ring to form the unstable carbinolamine leads to formation of a series of polar, open-ring compounds. Monohydroxylated metabolites are conjugated with glucuronic or sulfuric acid, or are further hydroxylated to dihydroxy derivatives that can also be subject to conjugation. Formation of highly reactive electrophilic metabolites of PCP have been demonstrated in vitro in microsomal preparations. Covalent modification of tissue macromolecules by reactive intermediates can be responsible for suicide inactivation of cytochrome P-450 and can possibly mediate some long-term toxic effects of PCP. PCP inhaled by cigarette smoking is metabolized via similar routes. About 50% of the PCP in cigarette smoke is converted to PC, a major product of thermal degradation of PCP. PC and its hydroxylated and conjugated metabolites appear to contribute little to the pharmacology or acute toxicity of PCP.

Species differences in the in vitro metabolism of phencyclidine

The metabolism of 1-(1-phenylcyclohexyl)-piperidine (phencyclidine or PCP) by liver preparations from cat, monkey, rabbit and rat has been studied. 4-Phenyl-4-piperidinocyclohexanol (I), 1-1-phenylcyclohexyl-4-hydroxy-piperidine (II), N-(5-hydroxypentyl)-1-phenylcyclohexylamine (IX) and 5-(1-phenylcyclohexylamino)-valeric acid (X) were found in all species, but liver preparations of rat and rabbit were much more active than those of cat or monkey in metabolizing PCP. Only rabbit produced 4-(4'-hydroxypiperidino)-4-phenylcyclohexanol (III) in amounts detectable by g.l.c. Mass balance calculations of PCP, I, II, III, IX and X in the cat, monkey and rat indicate that other metabolic pathways not measured in this study are operative.

The dispositional kinetics of phencyclidine and its N-ethylamine analogue in rats

The uptake kinetics of [3H]-labelled phencyclidine (PCP) and N-ethyl-l-phenycyclohexylamine (PCE) in rats, measured in terms of decreases in the blood concentrations of the drugs after i.v. administration of a single 1.09 mumol dose, were not significantly different. Within a week of administration, the rats excreted about 93% of the [3H]-PCP and about 65% of [3H]-PCE via their urine and faeces; their urine contained nore [3H], mainly as metabolites of [3H]-PCP and of [3H]-PCE, than their faeces. Similarly, more [3H] remained in the tissues of rats treated with [3H]-PCE than in the tissues of [3H4-PCP-treated rats. The fact that PCE is metabolized and excreted more slowly than PCP may account for the higher psychotropic effects of PCE.

Guide to the analysis of phencyclidine and its metabolites in biological material

Phencyclidine was introduced as an anaesthetic in 1960; and has now become a major drug of abuse in some countries. The rapid advance in the various fields of analytical chemistry during the past decades has made it possible to measure the levels of the compounds in tissues and body fluids. These methods may also be used to study the metabolism and pharmacokinetics of PCP. The resulting publication is an updated guide to these analyses particularly with the application of these techniques in human intoxication.

Effect of acute and chronic ethanol pre-treatment on the disposition of phencyclidine (PCP) in the rat

Disposition of [H] Phencyclidine in brain, plasma and adipose tissue of rats acutely and chronically-treated with ethanol was studied using a method possessing high sensitivity and specificity for PCP. In rats acutely-treated with ethanol (5 g/kg PO dose) and PCP (10 mg/kg IP dose), dispositional factors did not play a role in the intensifies pharmacological and behavioral effects of PCP. However in rats chronically-treated with 2.5 g/kg PO dose of ethanol twice a day for 19 days, the disposition of PCP (5 mg/kg IP dose) was significantly altered and the values of PCP in brain, plasma and adipose tissue were significantly higher than those in the control group. Although inhibition of PCP metabolism and a comparatively slower rate of its elimination appear to account for the potentiation of drug effects in animals chronically-treated with ethanol, interaction of drugs at the level of the central nervous system cannot be ruled out.

Occupational intoxication and long-term persistence of phencyclidine (PCP) in law enforcement personnel

By utilizing a glass capillary gas chromatographic nitrogen detector (GC2-N) method specific for phencyclidine (PCP) and sensitive to pg/mL in blood or urine samples, we have demonstrated occupational intoxication of law enforcement personnel charged with handling confiscated illegal PCP preparations. Further, we have demonstrated persistence of PCP in blood and urine for at least 6 months after the last known occupational exposure in one officer. Some aspects of the PCP problem are outlined, and possible mechanisms of the occupational intoxication are discussed.

Stimulation of synaptosomal tyrosine hydroxylation by phencyclidine in vitro

Phencyclidine (PCP), a potent psychoactive drug, produces some animal behaviors that are believed to be mediated by dopaminergic and/or cholinergic neurons in the basal ganglia. In this study, we have monitored the effects of PCP in vitro on the synthesis, uptake, and release of dopamine (DA) in rat striatal synaptosomes. Using tyrosine hydroxylation as an index of DA synthesis, we observed a concentration-dependent stimulation of DA synthesis by PCP. The stimulatory effect was antagonized by reserpine (1 micro M) and was observed only when synaptosomes were preincubated under conditions which prevented the spontaneous release of [3H]DA. Two hydroxylated metabolites of PCP were also tested and found to have little effect on tyrosine hydroxylation. Like PCP these metabolites are potent inhibitors of synaptosomal [3H]DA uptake, but they apparently lack PCP's ability to release synaptosomal DA. Taken together, these results support our hypothesis that PCP stimulates synaptosomal DA synthesis by releasing DA from an inhibitory pool.

Metabolism of phencyclidine by microorganisms

A number of microorganisms were screened for their ability to metabolize phencyclidine. Two microorganisms, Beauveria sulfurescens and Cunninghamella echinulata, produced hydroxylated metabolites, which were identified as 1-(1-phenylcyclohexyl)-4-hydroxypiperidine and 4-phenyl-4-piperidinocyclohexanol by high-pressure liquid chromatographic analysis.

The effects of phencyclidine on amphetamine stereotypy in rats

In two separate experiments a 9 point rating scale was used to assess the effects of various doses of phencyclidine on the behavioral stereotypy produced by d-amphetamine in rats. A dose of phencyclidine (2.5 mg/kg) which had no effect when given alone, enhanced the behavioral effects of 1 and 3 mg/kg of d-amphetamine. Higher dises (5 and 10 mg/kg) of phencyclidine produced some stereotypy when given alone but they also produced ataxia which confounded the rating of their other behavioral effects. These higher doses did not enhance the effects of d-amphetamine. This study provides further evidence that phencyclidine may have dopaminergic activity similar to amphetamine.

Plasma phencyclidine pharmacokinetics in dog and monkey using a gas chromatography selected ion monitoring assay

Phencyclidine was determined by gas chromatography selected ion monitoring in six dogs and seven monkeys. Aliquots of venous blood were taken over 4 h in the monkey after 1.1 mg kg-1 and over 24 h in the dog after 1.0 mg kg-1 of phencyclidine i.v. Pentadeuterated phencyclidine was used as the internal standard. In the electron impact mode the most abundant fragments in the mass spectrum of phencyclidine were m/e 91 and 200, and 96 and 205 in the [2H5]phencyclidine spectrum. These fragments were used to quantitate the amount of phencyclidine present. In both species, a complex exponential decline of plasma phencyclidine was found in most animals that fit a two compartment open model. In monkeys, the mean half-life (beta phase) was 2.36 h and in the dog it was 2.86 h. Compared with the monkey, the dog considerable emergence delirium. The two species had rather different pharmacokinetics which may be relevant to the observed differences in degree of anesthesia and recovery.

Anticholinesterase and antiacetylcholine activity of 1-phenylcyclohexylamine derivatives

The antiacetylcholine and anticholinesterase potencies of four 1-phenylcyclohexylamine derivatives were estimated by measuring their antagonism to the contractile response of smooth and striated muscles and their inhibition of cholinesterase activity. In addition, their affinities towards the central muscarinic receptor from mouse brain homogenate were determined by competition experiments in vitro. Relative to atropine, these drugs exerted mild antimuscarinic activity in both isolated smooth muscle and in the competition experiments. On the other hand, they were found to exert antinicotinic potencies equal to that of d-tubocurarine in the striated muscle. The concentration of (3H)-phencyclidine taken up by mouse brain in vivo could be correlated with its dissociation constants from the central muscarinic binding sites, as well as with the Ki values for acetylcholinesterase inhibition, both determined in vitro. Since these drugs have a similar rigid spatial molecular structure, it is proposed that the variations in the potency of their cholinergic interactions stemmed mainly from the structural changes in the region of the 'cationic head'.