Further studies on the pharmacokinetics of perhexiline maleate in humans

Study of the roles of cytochrome P450 (CYPs) in the metabolism and cytotoxicity of perhexiline

Perhexiline is a prophylactic antianginal agent developed in the 1970s. Although, therapeutically, it remained a success, the concerns of its severe adverse effects including hepatotoxicity caused the restricted use of the drug, and eventually its withdrawal from the market in multiple countries. In the clinical setting, cytochrome P450 (CYP) 2D6 is considered as a possible risk factor for the adverse effects of perhexiline. However, the role of CYP-mediated metabolism in the toxicity of perhexiline, particularly in the intact cells, remains unclear. Using our previously established HepG2 cell lines that individually express 14 CYPs (1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C18, 2C19, 2D6, 2E1, 3A4, 3A5, and 3A7) and human liver microsomes, we identified that CYP2D6 plays a major role in the hydroxylation of perhexiline. We also determined that CYP1A2, 2C19, and 3A4 contribute to the metabolism of perhexiline. The toxic effect of perhexiline was reduced significantly in CYP2D6-overexpressing HepG2 cells, in comparison to the control cells. In contrast, overexpression of CYP1A2, 2C19, and 3A4 did not show a significant protective effect against the toxicity of perhexiline. Pre-incubation with quinidine, a well-recognized CYP2D6 inhibitor, significantly attenuated the protective effect in CYP2D6-overexpressing HepG2 cells. Furthermore, perhexiline-induced mitochondrial damage, apoptosis, and ER stress were also attenuated in CYP2D6-overexpressing HepG2 cells. These findings suggest that CYP2D6-mediated metabolism protects the cells from perhexiline-induced cytotoxicity and support the clinical observation that CYP2D6 poor metabolizers may have higher risk for perhexiline-induced hepatotoxicity.

Pleiotropic mechanisms of action of perhexiline in heart failure

INTRODUCTION

The re-purposing of the anti-anginal drug perhexiline (PHX) has resulted in symptomatic improvements in heart failure (HF) patients. The inhibition of carnitine palmitoyltransferase-1 (CPT-1) has been proposed as the primary mechanism underlying the therapeutic benefit of PHX. This hypothesis is contentious.

AREAS COVERED

We reviewed the primary literature and patent landscape of PHX from its initial development in the 1960s through to its emergence as a drug beneficial for HF. We focused on its physico-chemistry, molecular targets, tissue accumulation and clinical dosing.

EXPERT OPINION

Dogma that the beneficial effects of PHX are due primarily to potent myocardial CPT-1 inhibition is not supported by the literature and all available evidence point to it being extremely unlikely that the major effects of PHX occur via this mechanism. In vivo PHX is much more likely to be an inhibitor of surface membrane ion channels and also to have effects on other components of cellular metabolism and reactive oxygen species (ROS) generation across the cardiovascular system. However, the possibility that minor effects of PHX on CPT-1 underpin disproportionately large effects on myocardial function cannot be entirely excluded, especially given the massive accumulation of the drug in heart tissue.

A multicenter assessment of single-cell models aligned to standard measures of cell health for prediction of acute hepatotoxicity

Assessing the potential of a new drug to cause drug-induced liver injury (DILI) is a challenge for the pharmaceutical industry. We therefore determined whether cell models currently used in safety assessment (HepG2, HepaRG, Upcyte and primary human hepatocytes in conjunction with basic but commonly used endpoints) are actually able to distinguish between novel chemical entities (NCEs) with respect to their potential to cause DILI. A panel of thirteen compounds (nine DILI implicated and four non-DILI implicated in man) were selected for our study, which was conducted, for the first time, across multiple laboratories. None of the cell models could distinguish faithfully between DILI and non-DILI compounds. Only when nominal in vitro concentrations were adjusted for in vivo exposure levels were primary human hepatocytes (PHH) found to be the most accurate cell model, closely followed by HepG2. From a practical perspective, this study revealed significant inter-laboratory variation in the response of PHH, HepG2 and Upcyte cells, but not HepaRG cells. This variation was also observed to be compound dependent. Interestingly, differences between donors (hepatocytes), clones (HepG2) and the effect of cryopreservation (HepaRG and hepatocytes) were less important than differences between the cell models per se. In summary, these results demonstrate that basic cell health endpoints will not predict hepatotoxic risk in simple hepatic cells in the absence of pharmacokinetic data and that a multicenter assessment of more sophisticated signals of molecular initiating events is required to determine whether these cells can be incorporated in early safety assessment.

Use of 'dilute-and-shoot' liquid chromatography-high resolution mass spectrometry in preclinical research: application to a DMPK study of perhexiline in mouse plasma

This work describes a simple, sensitive and rapid liquid chromatography-high resolution mass spectrometry method for the quantitation of perhexiline and the simultaneous detection of perhexiline metabolites in C57bl/6 mice plasma. Only 5 μL of plasma was used for analysis. Pretreatment was limited to a 100-fold dilution ('dilute-and-shoot'). The analyte was detected by high resolution mass spectrometry (Orbitrap™ technology). Three scan events were performed over the entire chromatogram. Targeted single ion monitoring with data dependent acquisition was employed for perhexiline quantitation and confirmation, while full scan was used to perform untargeted detection of perhexiline phase I and phase II circulating metabolites. The calibration curve was linear (r(2)=0.990) ranging from 0.305 ng/mL (LLOQ) to 10000 ng/mL. Matrix effect was limited to 6.1%. The method was applied to a pharmacokinetic study of perhexiline in mouse plasma and the results obtained were compared to a standard sample preparation method based on protein precipitation and liquid chromatography-tandem mass spectrometry (MRM mode) detection. The new approach provided comparable results in terms of pharmacokinetics parameters estimate with a high sensitivity, additional information on perhexiline circulating metabolites and a low consumption of biological sample. The combination of the 'dilute-and-shoot' approach together with HRMS targeted and untargeted detection represents a suitable alternative to classic bioanalytical approaches in preclinical research.

Comparison of CYP2D metabolism and hepatotoxicity of the myocardial metabolic agent perhexiline in Sprague-Dawley and Dark Agouti rats

1. Perhexiline, a chiral anti-anginal agent, may be useful to develop new cardiovascular therapies, despite its potential hepatotoxicity. 2. This study compared Dark Agouti (DA) and Sprague-Dawley (SD) rats, as models of perhexiline's metabolism and hepatotoxicity in humans. Rats (n = 4/group) received vehicle or 200 mg/kg/d of racemic perhexiline maleate for 8 weeks. Plasma and liver samples were collected to determine concentrations of perhexiline and its metabolites, hepatic function and histology. 3. Median (range) plasma and liver perhexiline concentrations in SD rats were 0.09 (0.04-0.13) mg/L and 5.42 (0.92-8.22) ng/mg, respectively. In comparison, DA rats showed higher (p < 0.05) plasma 0.50 (0.16-1.13) mg/L and liver 24.5 (9.40-54.7) ng/mg perhexiline concentrations, respectively, 2.5- and 3.7-fold higher cis-OH-perhexiline concentrations, respectively (p < 0.05), and lower plasma metabolic ratio (0.89 versus 1.55, p < 0.05). In both strains, the (+):(-) enantiomer ratio was 2:1. Perhexiline increased plasma LDH concentrations in DA rats (p < 0.05), but had no effect on plasma biochemistry in SD rats. Liver histology revealed lower glycogen content in perhexiline-treated SD rats (p < 0.05), but no effects on lipid content in either strain. 4. DA rats appeared more similar to humans with respect to plasma perhexiline concentrations, metabolic ratio, enantioselective disposition and biochemical changes suggestive of perhexiline-induced toxicity.

Effects of aging, renal dysfunction, left ventricular systolic impairment, and weight on steady state pharmacokinetics of perhexiline

MATERIALS AND METHODS

Two hundred patients at steady-state on long-term perhexiline were identified retrospectively. The ratio of maintenance dose to steady-state plasma concentration (dose:[Px]) was correlated with the following putative determinants via simple and multiple linear regression analyses: age, weight, left ventricular ejection fraction (LVEF), and creatinine clearance (CrCl, Cockroft-Gault formula). A Mann-Whitney U test was performed to determine if severe left ventricular systolic impairment affected maintenance dose.

RESULTS

Advanced age, left ventricular systolic impairment, and renal impairment were frequently encountered. Using simple linear regression, age was a negative correlate of dose:[P] (R = 0.23, P = 0.001), whereas weight (R = 0.27, P = 0.0001) and CrCl (R = 0.30, P < 0.0001) were positive correlates. Mann-Whitney U analysis showed no difference between dose: [Px] among patients with LVEF of less than 30% versus 30% or greater. Advancing age was strongly associated with decreasing weight (R = -0.45, P < 0.00001) and calculated CrCl varied directly with weight, as expected (R = 0.66, P < 0.0001). Stepwise multiple linear regression using age, LVEF, CrCl, and weight as potential predictors of dose:[P] yielded only weight as a significant determinant.

DISCUSSION

Perhexiline has become a "last-line" agent for refractory angina as a result of complex pharmacokinetics and potential toxicity. Use has increased predictably in the aged and infirm who have exhausted standard medical and surgical therapeutic options. Beyond genotype, the effect of patient characteristics on maintenance dose has not been explored in detail. In this study, dose requirement declined with age in a frail and wasting population as a result of weight-related pharmacokinetic factors. LVEF had no apparent effect on maintenance dose and should not be considered a contraindication to use.

CONCLUSION

A weight-adjusted starting dose may facilitate the safe and effective prescription of perhexiline and is calculated by 50 + 2 × weight (kg) mg/d, rounded to the closest 50 mg/day.

Steady-state pharmacokinetics of the enantiomers of perhexiline in CYP2D6 poor and extensive metabolizers administered Rac-perhexiline

What is already known about this subject. Perhexiline (PHX) is administered as a racemic mixture and exhibits enantioselective pharmacokinetics in both poor and extensive metabolizers of CYP2D6 (PM and EM, respectively). Extensive metabolism by CYP2D6 is primarily responsible for the observed enantioselectivity in EM, but the process responsible in PM is unknown. Analysis of the steady-state plasma concentration-time profiles of the enantiomers of PHX in PM and EM was undertaken in order to elucidate the observed enantioselectivity, particularly with respect to PM. What this study adds. This is the first study to examine the steady-state plasma concentration-time profiles of the enantiomers of PHX in EM and PM over the course of an interdosing interval. The apparent oral clearance of each enantiomer was calculated from their respective AUC rather than from trough concentrations and was enantioselective in both phenotypes, with higher apparent oral clearances of (-)-than (+)-PHX. Renal clearance, calculated for EM and subsequently assumed for PM, constitutes a greater proportion of the total apparent oral clearance of each enantiomer in PM than EM, but was not enantioselective and thus unable to explain the enantioselectivity observed in PM.

AIMS

To determine the steady-state pharmacokinetics of perhexiline (PHX) enantiomers over one interdosing interval in CYP2D6 extensive and poor metabolizer (EM and PM, respectively) patients administered rac-PHX. To elucidate the processes responsible for enantioselectivity, particularly in PM patients.

METHODS

Blood samples were taken over one interdosing interval from six EM and two PM patients at steady-state with respect to rac-PHX metabolism. Complete urine collections were taken from five EM patients. PHX concentrations in plasma and urine were determined with enantioselective high-performance liquid chromatography methods.

RESULTS

EM patients had 16- and 10-fold greater median apparent oral clearances of (+)- and (-)-PHX, respectively, than PM patients (P < 0.05 for both) and required significantly larger doses of rac-PHX (69 vs. 4.2 microg kg(-1) h(-1), P < 0.05) to maintain therapeutic concentrations in plasma. Patient phenotypes were consistent with CYP2D6 genotypes. Both groups displayed enantioselective pharmacokinetics, with higher apparent oral clearances for (-)-PHX compared with (+)-PHX, although PM patients exhibited significantly greater enantioselectivity (P < 0.05). The renal clearance of PHX enantiomers was not enantioselective and accounted for <1% of the median apparent oral clearance of each enantiomer in EM patients. Assuming the same renal clearances for PM patients accounts for approximately 9 and 4% of their median apparent oral clearances of (+)- and (-)-PHX, respectively.

CONCLUSIONS

The enantioselective pharmacokinetics of PHX are primarily due to metabolism by CYP2D6 in EM patients. The mechanism responsible for the enantioselective pharmacokinetics of PHX in PM patients is unknown, but may be due to enantioselective biliary or intestinal excretion.

Effect of CYP2D6 metabolizer status on the disposition of the (+) and (−) enantiomers of perhexiline in patients with myocardial ischaemia

AIMS

This study investigated the effects of increasing doses of rac-perhexiline maleate and CYP2D6 phenotype and genotype on the pharmacokinetics of (+) and (-)-perhexiline.

METHODS

In a prospective study, steady-state plasma concentrations of (+) and (-)-perhexiline were quantified in 10 CYP2D6 genotyped patients following dosing with 100 mg/day rac-perhexiline maleate, and following a subsequent dosage increase to 150 or 200 mg/day. In a retrospective study, steady-state plasma concentrations of (+) and (-)-perhexiline were obtained from 111 CYP2D6 phenotyped patients receiving rac-perhexiline maleate.

RESULTS

In the prospective study, comprising one poor and nine extensive/intermediate metabolizers, the apparent oral clearance (CL/F) of both enantiomers increased with the number of functional CYP2D6 genes. In the nine extensive/intermediate metabolizers receiving the 100 mg/day dose, the median CL/F of (+)-perhexiline was lower than that of (-)-perhexiline (352.5 versus 440.6 l/day, P<0.01). Following the dosage increase, the median CL/F of both enantiomers decreased by 45.4 and 41.4%, respectively. In the retrospective study, the median (+)-/(-)-perhexiline plasma concentration ratio was lower (P<0.0001) in phenotypic extensive/intermediate (1.41) versus poor metabolizers (2.29). Median CL/F of (+) and (-)-perhexiline was 10.6 and 24.2 l/day (P<0.05), respectively, in poor metabolizers, and 184.1 and 272.0 l/day (P<0.001), respectively, in extensive/intermediate metabolizers.

CONCLUSIONS

Perhexiline's pharmacokinetics exhibit significant enantioselectivity in CYP2D6 extensive/intermediate and poor metabolizers, with both enantiomers displaying polymorphic and saturable metabolism via CYP2D6. Clinical use of rac-perhexiline may be improved by developing specific enantiomer target plasma concentration ranges.

Concentration-time profile for perhexiline and hydroxyperhexiline in patients at steady state

AIM

To define inter- and intraday variability in plasma perhexiline concentrations, time-to-maximum plasma perhexiline concentration and variability in the ratio of hydroxyperhexiline to parent perhexiline concentrations over the course of the day in patients at steady state.

METHODS

Eight blood samples were taken over a 24-h period from 12 adult patients already taking perhexiline for the treatment of angina pectoris. These patients were assumed to be at steady state, having taken the same dose of perhexiline for more than 4 weeks and having no changes made to other drug therapy that might have affected plasma perhexiline concentrations (especially drugs that interfere with CYP2D6). Perhexiline was assayed by HPLC/FL. The percentage increase over baseline concentration was determined for each patient for both perhexiline and hydroxyperhexiline.

RESULTS

Trough plasma perhexiline concentrations from two patients were below the limit of quantification of the assay (0.05 mg l-1) and thus were excluded from the analysis. The greatest mean percentage increase in plasma perhexiline concentration over the day was 21% (95%CI 9%, 33%, range -19% to 45%) which occurred 6 h postdose. The greatest mean percentage increase in plasma hydroxyperhexiline concentration was 10.8% (95%CI -5.3%, 26.9%, range -13% to 60%) which occurred 4 h postdose. However individual patients demonstrated > 60% intraday variability in perhexiline concentrations which was not related to the concomitant use of drugs that affect CYP2D6 activity. Changes in random plasma perhexiline concentration which are attributed to changes in concomitant drug therapy should be supported by additional kinetic data. Inter-day variability in plasma perhexiline concentration as determined by the ratio of C24 : C0 was small (mean 0.90, 95%CI 0.77, 1.03) which supports C0 as the best sampling time for perhexiline concentration monitoring. The variability in C24 : C0 for hydroxyperhexiline concentrations was smaller (mean 0.96, 95%CI 0.81, 1.11). Variability in the ratio of plasma concentrations of hydroxyperhexiline to perhexiline over the day was also small. The ratio of plasma hydroxyperhexiline to perhexiline concentration over the day fell within a narrow range for all subjects with 95% confidence intervals being < 15% for eight patients and < 25% for the remaining patient. This suggests that formation of the metabolite occurs rapidly and may be presystemic. It also supports the calculation of the hydroxyperhexiline : perhexiline ratio (in patients at steady state) on blood samples taken at any time during the dosing interval.

CONCLUSIONS

The within-day variability in plasma perhexiline concentrations was small. While C0 is probably the best time for therapeutic drug monitoring purposes, it is not unreasonable to use samples drawn at any time during the dosing interval. The therapeutic range used in this hospital (0.15-0.6 mg l-1) was devised from earlier work using 4 h postdose blood sampling which is close to the 'peak' concentration and a mean of 16% higher than C0 in this study. This increase is probably clinically insignificant and a different C0 range is therefore not warranted.

Pharmacokinetics of the antianginal agent perhexiline: relationship between metabolic ratio and steady-state dose

AIMS

1) To develop an estimate of oral clearance (CL(Px)/F) for the antianginal agent perhexiline based on the ratio of cis-OH-perhexiline metabolite/parent perhexiline plasma concentrations at steady-state (C(OHPx,ss)/C(Px,ss)). 2) To determine whether the ratio measured in the first fortnight of treatment (C(i)(OHPx)/C(i)(Px)) may be used to guide patient dosing with perhexiline, a drug with a narrow therapeutic index, long half-life and saturable metabolism via CYP2D6.

METHODS

Two retrospective studies were conducted reviewing patient records and data obtained from routine monitoring of plasma perhexiline and cis-OH-perhexiline concentrations.

RESULTS

Study 1 (n=70). At steady-state, the frequency distributions of CL(Px)/F and C(OHPx,ss)/C(Px,ss) were consistent with CYP2D6 metabolism. Putative poor metabolizers (approximately 8%) were identified by CL(Px)/F< or =50 ml min(-1) or C(OHPx,ss)/C(Px,ss)< or =0.3. A group of patients with CL(Px)/F> or =950 ml min(-1) may have been ultra-rapid metabolizers. In this group, the high CL(Px)/F values suggest extensive first-pass metabolism and poor bioavailability. In patients with therapeutic plasma perhexiline concentrations (0.15-0.60 mg l(-1)), the variability in dose appeared directly proportional to CL(Px)/F (r2=0.741, P<0.0001). Study 2 (n=23). Using C(i)(OHPx)/C(i)(Px) patients were tentatively identified as poor, extensive and ultra-rapid metabolizers, with CL(Px)/F of 23-72, 134-868 and 947-1462 ml min(-1), respectively, requiring doses of 10-25, 100-250 and 300-500 mg day(-1), respectively.

CONCLUSIONS

The cis-OH-perhexiline/perhexiline concentration ratio may be useful for optimizing individual patient treatment with the antianginal agent perhexiline.

Population pharmacokinetics of perhexiline from very sparse, routine monitoring data

Using NONMEM, the population pharmacokinetics of perhexiline were studied in 88 patients (34 F, 54 M) who were being treated for refractory angina. Their mean +/- SD (range) age was 75 +/- 9.9 years (46-92), and the length of perhexiline treatment was 56 +/- 77 weeks (0.3-416). The sampling time after a dose was 14.1 +/- 21.4 hours (0.5-200), and the perhexiline plasma concentrations were 0.39 +/- 0.32 mg/L (0.03-1.56). A one-compartment model with first-order absorption was fitted to the data using the first-order (FO) approximation. The best model contained 2 subpopulations (obtained via the $MIXTURE subroutine) of 77 subjects (subgroup A) and 11 subjects (subgroup B) that had typical values for clearance (CL/F) of 21.8 L/h and 2.06 L/h, respectively. The volumes of distribution (V/F) were 1470 L and 260 L, respectively, which suggested a reduction in presystemic metabolism in subgroup B. The interindividual variability (CV%) was modeled logarithmically and for CL/F ranged from 69.1% (subgroup A) to 86.3% (subgroup B). The interindividual variability in V/F was 111%. The residual variability unexplained by the population model was 28.2%. These results confirm and extend the existing pharmacokinetic data on perhexiline, especially the bimodal distribution of CL/F manifested via an inherited deficiency in hepatic and extrahepatic CYP2D6 activity.

Validation of a novel molecular orbital approach (COMPACT) for the prospective safety evaluation of chemicals, by comparison with rodent carcinogenicity and Salmonella mutagenicity data evaluated by the U.S. NCI/NTP

The molecular dimensions and electronic structures of 100 chemicals of structural diversity have been determined from molecular orbital calculations and molecular mechanics. From these parameters of molecular structure, those chemicals that are likely substrates of cytochromes P4501 and P4502E have been identified by the computer-optimized molecular parametric analysis of chemical toxicity (COMPACT) programme, and their potential toxicity, mutagenicity and carcinogenicity evaluated. The degree of correlation between COMPACT prediction of toxicity and rodent two species life-span carcinogenicity data is estimated to be 92%, and between COMPACT and Salmonella mutagenicity (Ames test) data is 64%. Anomalous rodent carcinogens are rationalized on the basis of biochemical mechanisms of metabolism, genotoxicity and carcinogenicity. Correlation of the Ames test data with rodent carcinogenicity data was 64%, but correlation of COMPACT plus Ames data versus rodent carcinogenicity data provided the highest correlation of 94%.

The importance of pharmacokinetic and receptor studies in drug safety evaluation

The importance of pharmacokinetic and receptor studies in the preclinical and clinical safety evaluation of candidate drugs is reviewed with reference to a number of recently developed drugs. Different aspects of the relationships between pathways of metabolism, pharmacokinetics, receptor interactions, and drug toxicity are illustrated. The failure of animal toxicity studies to predict drug toxicity in humans, due to species differences in metabolism and pharmacokinetics, is illustrated by reference to the anti-inflammatory antiviral terpenoid carbenoxolone, the antiasthmatic candidate drug FPL 52757, and the cardiotonic drug amrinone. The false prediction of adverse effects in man from toxicity manifested in experimental animals, due to species differences in pharmacokinetics or receptor activities, is exemplified with reference to the antiepileptic valproic acid, the hypolipidemic drug ciprofibrate, the antipeptic ulcer drug, omeprazole, and the progestogen lynestrenol. Finally, the importance of adequate, repeat-dose, clinical pharmacokinetic studies in patients as distinct from healthy volunteers to evaluate any effect of the disease state, in the elderly and the young to examine the effects of age, and in sufficiently large populations to detect genetic anomalies and idiosyncrasies is illustrated by reference to the anti-rheumatoid drug benoxaprofen, the antiangina drug perhexiline, and the diuretic tienilic acid.

The enterohepatic circulation of perhexiline metabolites in the male Wistar rat

1. The biliary excretion of some perhexiline metabolites has been assessed in male Wistar rats with biliary cannulation. 2. After intragastric administration of perhexiline maleate (2 mg/kg body weight) multiple perhexiline metabolites were detected in bile. 3. When aliquots of this metabolite-laden bile were administered intraduoduodenally to further 'recipient' rats with biliary cannulation, similar metabolites were detected in the bile of these rats, but at reduced concentrations equivalent to 30-35% of those present in the bile of 'donor' rats. 4. These findings indicate that in the male Wistar rat, there may be substantial enterohepatic circulation of some perhexiline metabolites.

Studies on the metabolism of perhexiline in man

We have studied the disposition of perhexiline and its two major metabolites, M1 and M3, in healthy volunteers and in patients with biliary T-tube drains after cholecystectomy. In healthy volunteers the genetic control for impaired hepatic oxidation is identical for debrisoquine, sparteine, and perhexiline. Poor metabolizers demonstrate markedly reduced production and excretion of the major metabolite, M1. Their production of M3 is also reduced, but to a lesser degree than for M1, confirming substrate stereoselectivity by hepatic oxidases. Biphasic urinary elimination of M1 and M3 is seen in intact extensive oxidizers, whereas only the first phase is apparent in patients with biliary T-tube drainage. This suggests the possibility of enterohepatic recycling of these compounds, which may account for their prolonged elimination. More than 90% of an ingested dose of perhexiline maleate remains unaccounted for at 24 h after ingestion, even in extensive metabolizers. A careful, radiolabelled tissue-distribution study is warranted to elucidate the complicated metabolic fate of perhexiline.

Stereoselective pharmacokinetics of perhexiline

Blood plasma and urine excretion pharmacokinetics of the (+) and (-) enantiomers of perhexiline have been determined in oral single-dose studies in eight human volunteers, and compared with the pharmacokinetics of the racemate drug in the same subjects. The (-) enantiomer is more rapidly metabolized and eliminated, and is stereoselectively hydroxylated to the cis-monohydroxy-perhexiline. The peak plasma concn of unchanged perhexiline is greater, while that of the cis-monohydroxy-perhexiline metabolite is lower, after administration of the (+) enantiomer than after the (-) enantiomer or the racemate. Similarly, the AUC values for unchanged perhexiline and for the trans-monohydroxy-perhexiline metabolite are greatest and the AUC value for the cis-monohydroxy-perhexiline metabolite is lowest for the (+) enantiomer. The three stereoisomeric forms of perhexiline all had the same times to peak plasma concn of the unchanged drug or of the cis-metabolite, and all three forms had a similar plasma elimination half-life for unchanged perhexiline. Metabolism of racemic perhexiline to the cis-monohydroxy metabolite is the major mechanism of elimination of the drug in man and has been shown to be polymorphic in human populations. The (-) enantiomer which shows stereoselective metabolism to the cis metabolite might therefore show a greater polymorphic effect. Studies with rat-liver microsomal preparations in vitro showed that, in contrast to the human studies in vivo, hydroxylation of perhexiline yields mostly the trans-monohydroxy metabolite. The DA strain of rats exhibited slower rates of hydroxylation in vitro than Wistar or Lewis strains of rats.