A phase I, single-dose study of the disposition of 14C-radiolabeled gabapentin enacarbil in healthy male volunteers.

UNLABELLED: Gabapentin enacarbil (GEn) is a prodrug of gabapentin that is effective in restless legs syndrome (RLS) and has dose-proportional gabapentin exposure. OBJECTIVE: This Phase I, open-label, non-randomized, single-center study of 14C-GEn in healthy male volunteers (XenoPort, Inc. protocol: XP065) characterized the mass balance, absorption, metabolism, and elimination pathways of GEn after oral administration of 14C-GEn. METHODS: Subjects received GEn 600 mg as two gelatin capsules containing 300 mg immediate release GEn solution each with approximately 50 µCi of 14C-GEn. Pharmacokinetic assessments included total radioactivity excreted in urine (Aeu(0-t)) and feces (Aef(0-t)), mean maximum concentration (Cmax), 14C-GEn-derived radioactivity in plasma and whole blood, and gabapentin area under the concentration-time curve extrapolated to infinity (AUC0-inf). Tolerability was assessed using adverse events (AEs), vital signs, clinical laboratory tests, and ECGs. Six male subjects aged 24 - 46 years were recruited to the study. RESULTS: Mean total recovery of 14C-GEn-derived radioactivity was 99.3% (94.1% in urine and 5.2% in feces). Mean Cmax and AUC for GEn-derived total radioactivity were similar in whole blood and plasma; the blood to plasma ratio for 14C-GEn-derived total radioactivity was 0.91. 14C-gabapentin was the only radioactive species present in blood. More than 85% of the radioactive dose was recovered in urine within 24 h of dosing. Eight treatment-emergent AEs were reported by 3 subjects; all were mild in intensity. There were no clinically relevant changes in vital signs, laboratory values, or ECGs. CONCLUSIONS: GEn was extensively absorbed and rapidly eliminated from plasma and whole blood.

The effect of food with varying fat content on the clinical pharmacokinetics of gabapentin after oral administration of gabapentin enacarbil.

UNLABELLED: Gabapentin enacarbil, an actively transported prodrug of gabapentin, provides sustained and dose-proportional exposure to gabapentin. OBJECTIVE: To evaluate the effect of food of varying fat content on the pharmacokinetics and tolerability of gabapentin enacarbil. METHODS, MATERIALS AND SUBJECTS: A randomized, open-label, crossover study of 1,200 mg gabapentin enacarbil was conducted in 12 healthy adults, under four conditions: fasted, or following low-fat (200 - 300 kcal total, approximately 6% from fat), moderate-fat (500 - 600 kcal total, approximately 30% from fat) or high-fat meals (1,000 kcal total, approximately 50% from fat), separated by a washout period of >or= 5 days. RESULTS: Ten subjects completed treatment under all four conditions. Data from all subjects were used for pharmacokinetic and safety analyses unless stated otherwise. Mean (standard deviation) bioavailability (based on urinary recovery) of gabapentin from gabapentin enacarbil was 42.0 (6.1)% (fasted), 64.3 (13.2)% (low-fat meal), 64.9 (16.9)% (moderate-fat meal), and 76.1 (14.4)% (high-fat meal). Gabapentin exposures (AUC(inf)) in fed conditions were 23% (low-fat meal), 31% (moderate-fat meal), and 40% (high-fat meal) greater than the exposure under fasted condition. Fed conditions did not significantly delay median t(max), but a trend for delayed gabapentin enacarbil absorption was seen in t(max) ranges following moderate- and high-fat meals compared with the fasted state or low-fat meal. The most commonly reported treatment-emergent adverse events (TEAEs) were dizziness (4 subjects), balance disorder (4 subjects) and somnolence (3 subjects). All TEAEs were rated as mild in intensity. CONCLUSION: Administration of gabapentin enacarbil with food enhanced gabapentin exposure compared with fasted conditions, regardless of the fat or caloric content, and gabapentin enacarbil was generally well tolerated.

Pharmacokinetics of cidofovir in renal insufficiency and in continuous ambulatory peritoneal dialysis or high-flux hemodialysis.

BACKGROUND: Cidofovir is an antiviral agent used for the treatment of cytomegalovirus infection in patients with acquired immunodeficiency syndrome. Because cidofovir is primarily eliminated by the kidneys and because its main adverse effect is nephrotoxicity, an understanding of the pharmacokinetic disposition of cidofovir in patients with renal insufficiency is necessary. METHODS: Twenty-four subjects were enrolled into this study and were divided into 6 groups depending on their degree of renal dysfunction, including subjects receiving maintenance continuous ambulatory peritoneal dialysis and high-flux hemodialysis. The creatinine clearance (CLCR) for subjects not receiving dialysis ranged from 12 to 164 mL/min. Each subject received a single 0.5 mg/kg intravenous dose of cidofovir over 1 hour. Subjects not receiving dialysis were given intravenous hydration with 1 L normal saline solution and concomitant oral probenecid. Serial serum and urine samples were collected to determine pharmacokinetic parameters with use of noncompartmental methods. RESULTS: Mean +/- SD cidofovir clearance (CL) in control subjects (normal renal function; n = 5) was 1.7 +/- 0.1 mL/min/kg, which decreased with declining renal function as indicated by the regression equation: CL (mL/min/kg) = 0.94 x CLCR (mL/min/kg) + 0.064 (r2 = 0.91). Mean volume of distribution at steady state did not change significantly in subjects with kidney disease and cidofovir serum elimination half-life was significantly increased in subjects with severe renal impairment. Cidofovir was not significantly cleared during continuous ambulatory peritoneal dialysis, but high-flux hemodialysis resulted in the removal of 52% +/- 11% of the dose administered. CONCLUSION: The significant (P < .001) correlation observed between CLCR and CL in subjects with varying degrees of renal insufficiency indicates that aggressive dosage reduction of cidofovir would be necessary in subjects with kidney disease to ensure comparable drug exposure based on serum levels.

In vivo oxidative DNA damage: measurement of 8-hydroxy-2'-deoxyguanosine in DNA and urine by high-performance liquid chromatography with electrochemical detection.

HPLC with electrochemical detection is a highly sensitive and selective method for detecting the oxidatively modified DNA residue oh8dG. By this method, the detection of oh8dG from DNA and urine offers a powerful approach for assessing in vivo oxidative damage. Application of this technique to the detection of oh8dG from DNA permits the quantitation of the steady-state levels of this oxidatively modified deoxynucleoside and overcomes the detection problems associated with the extremely low levels present in DNA. In addition, the selectivity gained by this detection method eliminates the problem of separating the signal for oh8dG from normal deoxynucleosides. The quantitation of oh8dG in urine complements the measurement of oh8dG in DNA by estimating cumulative oxidative DNA damage in the body. In addition, the urinary assay provides a noninvasive means of measuring this type of damage in laboratory animals and human populations. Thus, an individual animal or human subject may be monitored over time, possibly under various prooxidant conditions, using oh8dG as a sensitive marker for oxidative DNA damage. This analytical approach may allow one to estimate the exposure of an individual to prooxidant conditions associated with lifestyle, genetic predisposition, degenerative diseases, and environmental toxins.

Clinical pharmacokinetics of the antiviral nucleotide analogues cidofovir and adefovir.

Cidofovir and adefovir are members of a new class of antiviral compounds. They are acyclic phosphonate analogues of deoxynucleoside monophosphates. Both compounds undergo intracellular activation to form diphosphates that are potent inhibitors of viral DNA polymerases. Cidofovir has broad spectrum antiviral activity against herpesviruses, papillomaviruses and poxviruses, whereas adefovir has potent activity against retroviruses and certain DNA viruses, including herpesviruses and hepadnaviruses. Intravenous cidofovir is approved for treatment of cytomegalovirus retinitis in patients with AIDS. Cidofovir and adefovir are dianionic at physiological pH and have low oral bioavailability in animals and humans. After intravenous administration to HIV-infected patients, the pharmacokinetics of both drugs are independent of dose and are consistent with preclinical data. Systemic exposure is proportional to the intravenous dose and both drugs are cleared by the kidney and excreted extensively as unchanged drug in the urine. Intracellular activation of a small fraction (< 10%) of the dose by cellular kinases leads to prolonged antiviral effects that are not easily predicted from conventional pharmacokinetic studies. The observed rate of elimination of cidofovir and adefovir from serum may not reflect the true duration of action of these drugs, since the antiviral effect is dependent on concentrations of the active phosphorylated metabolites that are present within cells. For both drugs, > 90% of an intravenous dose is recovered unchanged in the urine over 24 hours. Metabolism does not contribute significantly to the total clearance of either drug. Concomitant oral probenecid decreases both the renal clearance of cidofovir and the incidence of nephrotoxicity, presumably by blocking its active tubular secretion. This is the basis of the clinical use of concomitant probenecid as a nephroprotectant during cidofovir therapy. Subcutaneous administration produces exposure equivalent to that following intravenous administration. Drug interaction studies with cidofovir are ongoing, but there is no evidence of an interaction between zidovudine and either cidofovir or adefovir. Clearance of cidofovir in patients with renal impairment showed a linear relationship to creatinine clearance. The low oral bioavailability of adefovir has led to the development of an oral prodrug, adefovir dipivoxil, currently in development for the treatment of HIV and hepatitis B infections.

Administration of 9-[2-(phosphonomethoxy)propyl]adenine (PMPA) for prevention of perinatal simian immunodeficiency virus infection in rhesus macaques.

Simian immunodeficiency virus (SIV) infection of newborn macaques is a useful animal model to explore novel strategies to reduce perinatal human immunodeficiency virus (HIV) infection. The availability of two easily distinguishable virus isolates, SIVmac251 and the simian/human immunodeficiency virus chimera SHIV-SF33, allows tracing the source of infection following inoculation with both viruses by different routes. In the present study, we evaluated the efficacy of pre- and postinoculation treatment regimens with 9-[2-(phosphonomethoxy)propyl]adenine (PMPA) to protect newborn macaques against simultaneous oral SIVmac251 and intravenous SHIV-SF33 inoculation. Untreated newborns became persistently infected following virus inoculation. When three pregnant macaques were given a single subcutaneous dose of PMPA 2 hr before cesarean section, their newborns became SIV-infected following SIV and SHIV inoculation shortly after birth. In contrast, when four newborn macaques were inoculated simultaneously with SIV and SHIV, and started immediately on PMPA treatment for 2 weeks, only one animal became persistently SIV-infected; the remaining three PMPA-treated newborns, however, had some evidence of an initial transient virus infection but were seronegative and healthy at 8 months of age. Our data demonstrate that PMPA treatment can reduce perinatal SIV infection and suggest that similar strategies may also be effective against HIV.

Bioavailability and metabolism of cidofovir following topical administration to rabbits.

The bioavailability and metabolism of the antiviral nucleotide analog cidofovir (HPMPC) were examined in New Zealand white rabbits following topical administration to normal and abraded skin. Male rabbits (four per group) received 14C-cidofovir (100 microCi/kg) intravenously (1 mg/kg) as a solution or topically (2 mg/animal) as a 1% w/w gel containing hydroxyethylcellulose (HEC) with or without propylene glycol (PG). The same PG/HEC formulation was applied topically to an abraded skin site in a fourth group of animals. All radioactivity detected in plasma and skin was accounted for by cidofovir. Plasma concentrations of radioactivity declined multiexponentially following intravenous administration, with a terminal half-life of 5.4 h. For intact skin, the absolute bioavailabilities of the HEC and PG/HEC formulations were 0.2 and 2.1%, respectively. For abraded skin, the bioavailability for the PG/HEC gel was 41%. Radioactivity in kidneys was attributed to cidofovir ( > 95%) and cyclic HPMPC. Concentrations in kidney following topical administration of cidofovir to normal skin were < 4% of those following intravenous dosing. Topical application of cidofovir to intact skin led to negligible systemic exposure to the drug. The topical bioavailability and hence the flux of cidofovir through intact skin was enhanced by the presence of PG in the formulation. Abrasion of the skin removed the principal barrier to absorption and led to significant systemic exposure to cidofovir.

Pharmacokinetics, safety and bioavailability of HPMPC (cidofovir) in human immunodeficiency virus-infected subjects.

We conducted a single-center, double-blind, placebo-controlled phase I study in HIV-positive subjects to ascertain the safety, tolerance, bioavailability, pharmacokinetics, and maximum tolerated dose of HPMPC (cidofovir). Five subjects were randomized to receive drug and two to receive placebo at each of three dosage tier (1, 3, and 10 mg/kg) with a 2-week washout period doses. Subjects at 1 and 3 mg/kg received single doses of HPMPC by subcutaneous (s.c.) intravenous (i.v.), and oral (p.o.) routes, while subjects at 10 mg/kg received only i.v. and p.o. doses. For subjects already taking zidovudine, zidovudine AUC values are determined before and then with HPMPC administration for each route. The AUC values of HPMPC were dose-proportional. Subcutaneous bioavailability was essentially equivalent to that of the intravenous route, but the development of transient local fibrosis ad the volumes needed for subcutaneous dosing precluded higher subcutaneous dosing than 3 mg/kg. Oral bioavailability was poor, estimated to be less than 5%. Drug elimination was predominantly renal. Nephrotoxicity in one subject was the only serious adverse event observed. This subject had a significant lag period prior to oral absorption and also had the highest AUC values for both HPMPC and zidovudine. We found no consistent effect on zidovudine AUC concomitant HPMPC.

Effect of oral probenecid coadministration on the chronic toxicity and pharmacokinetics of intravenous cidofovir in cynomolgus monkeys.

In animals and humans, intravenous administration of the antiviral nucleotide analogue cidofovir results in a dose-limiting nephrotoxicity characterized by damage to the proximal tubular epithelial cells. Probenecid, a competitive inhibitor of organic anion transport in the proximal tubular epithelial cells, was evaluated for its effect on the chronic toxicity and pharmacokinetics of cidofovir. Cynomolgus monkeys (5/sex/group) received cidofovir for 52 consecutive weeks as a once weekly intravenous bolus injection at 0 (saline), 0.1, 0.5, or 2.5 mg/kg/dose alone or at 2.5 mg/kg/dose in combination with probenecid (30 mg/kg/dose via oral gavage 1 h prior to cidofovir administration). Cidofovir-associated histopathological changes were seen only in the kidneys, testes, and epididymides. Nephrotoxicity (mild to moderate cortical tubular epithelial cell karyomegaly, tubular dilation, basement membrane thickening) was present only in monkeys receiving 2.5 mg/kg/dose cidofovir without probenecid. The incidence and severity of testicular (hypo- and aspermatogenesis) and epididymal (severe oligo- and aspermia) changes were increased in monkeys administered cidofovir at 2.5 mg/kg/dose, either alone or in combination with oral probenecid. Renal drug clearance was decreased between Weeks 1 and 52 in the 2.5 mg/kg/dose groups and resulted in an increased systemic exposure to cidofovir (as measured by AUC) that was significantly greater in monkeys administered cidofovir alone (312% increase in males, 98% in females) than in those coadministered probenecid (32% increase in males, 3% in females). These results demonstrate that oral probenecid coadministration protects against the morphological evidence of nephrotoxicity and the accompanying decrease in renal clearance in monkeys receiving chronic intravenous cidofovir treatment.

Pharmacokinetic measurement of drugs in lung epithelial lining fluid by microdialysis: aminoglycoside antibiotics in rat bronchi.

A novel technique for in vivo intrabronchial pharmacokinetic measurements using microdialysis is described. The technique was applied to the measurement of tobramycin and gentamicin in the anesthetized rat. The concentration versus time profiles of the drugs in the lung epithelial lining fluid (ELF) following intravenous bolus administration were determined. The mean penetration ratios into the ELF were determined and found to be 0.36 +/- 0.06 (mean +/- SEM, n = 5) for gentamicin and 0.56 +/- 0.09 (mean +/- SEM, n = 5) for tobramycin. The findings suggest that the technique can be used for intrabronchial pharmacokinetic measurement of drugs in the lung, either as an extension of the bronchoalveolar lavage technique or as a practical alternative to it.

Oral formulations of adefovir dipivoxil: in vitro dissolution and in vivo bioavailability in dogs.

The effect of formulation on oral bioavailability of the antiviral nucleotide analogue adefovir from the prodrug adefovir dipivoxil was examined in beagle dogs. A suspension formulation of adefovir dipivoxil granules was administered to five fasted male beagle dogs (250 mg prodrug per dog; 135.7 mg-equiv of adefovir per dog). Tablets prepared from the same granulation (batch B94) were administered at 2 x 125 mg prodrug per dog. In addition, the same tablets were administered to dogs in the fed state or following pentagastrin pretreatment. Two further tablet batches (H94 and D501) with slight formulation changes were also evaluated in pentagastrin pretreated dogs (n = 5). Concentrations of adefovir in plasma were determined by HPLC following fluorescence derivatization. Tablet dissolution was examined at pH 2.0. One batch of adefovir dipivoxil tablets showed a 5-fold slower dissolution rate in vitro (B94 = H94 >> D501). Adefovir dipivoxil was completely converted to adefovir following oral absorption in dogs. The oral bioavailability of adefovir from the suspension was 35.0 +/- 8.9%. The oral bioavailability of adefovir from the tablet formulation was 34.7 +/- 10.3%, 37.2 +/- 4.5%, and 44.9 +/- 5.9% in fasted dogs, fed dogs and fasted dogs pretreated with pentagastrin, respectively. All three tablet batches had equivalent bioavailability in dogs. Oral bioavailability of adefovir from the prodrug in dogs (35-46%) was unaffected by formulation, food, or the acidic pH of the gastrointestinal tract. In vitro dissolution of adefovir dipivoxil tablets did not correlate with oral bioavailability. Oral bioavailability of adefovir dipivoxil appears to be limited by low permeability and biological conversion of the prodrug to adefovir.

Distribution and metabolism of intravitreal cidofovir and cyclic HPMPC in rabbits.

PURPOSE: This study was designed to evaluate the intraocular distribution and metabolism of the antiviral nucleotide analogs cidofovir and cyclic 1-[(S)-3-hydroxy-2-(phosphonomethoxy) propyl]cytosine (HPMPC) in New Zealand white rabbits following intravitreal administration. METHODS: Male rabbits received either 14C-cidofovir or 14C-cyclic HPMPC by intravitreal injection into both eyes (50 micrograms/eye, 11 microCi/eye). Two animals per group were sacrificed at 24, 48, 72 or 240 h post-dose. Ocular tissues, kidney and liver were oxidized to determine total radioactivity and metabolites were determined by HPLC. RESULTS: At 24 h post-dose, total radioactivity was 9.96 and 5.18 micrograms-equiv/g for cidofovir and cyclic HPMPC, respectively, in vitreous and 20.9 and 3.54 micrograms-equiv/g, respectively, in retina. Although the initial vitreal clearance was 2-fold faster for the cyclic analog, the estimated terminal elimination half-lives in vitreous (42 hr) and in retina (66-77 hr) were similar for both drugs. By 240 h post-dose, radioactivity in all ocular tissues was approximately ten-fold higher for cidofovir. Radioactivity in vitreous at 240 h after intravitreal dosing with either drug contained cidofovir, cyclic HPMPC and cidofovir-phosphocholine. CONCLUSIONS: The long retinal half-life observed presumably reflects formation of phosphorylated cidofovir within retinal cells. Cidofovir achieved a ten-fold higher level of phosphorylated drug in retina than cyclic HPMPC: Therefore, intravitreal cidofovir may be expected to suppress progression of retinitis for a longer period than an equivalent intravitreal dose of cyclic HPMPC: The intravitreal half-life of cidofovir was 20-fold longer than that of ganciclovir in the same animal model.

Isolation and identification of a metabolite of cidofovir from rat kidney.

Cidofovir is an acyclic nucleotide analog with potent and broad-spectrum antiviral activity against adenoviruses and herpesviruses including cytomegalovirus (CMV). Cidofovir undergoes intracellular phosphorylation by host enzymes to cidofovir phosphate and cidofovir diphosphate (the active form). An unidentified metabolite has been observed previously in rat tissues and in urine of rabbits, rats and monkeys dosed with cidofovir. In the present study, this metabolite was isolated from rat kidney following an intravenous dose of 100 mg kg-1 cidofovir. The metabolite (metabolite I) was separated from cidofovir and impurities using extraction on anion-exchange resin followed by preparative normal and reversed-phase high-performance liquid chromatography (HPLC). The isolated metabolite I was subjected to proton, 13C and phosphorus nuclear magnetic resonance (NMR) and matrix-assisted laser desorption/ionization mass spectroscopy, and confirmed to be cidofovir-phosphocholine. The uptake of cidofovir by rat kidney was saturated at an intravenous dose of 100 mg kg-1, probably as a result of saturation of the renal tubular secretion pathway. However, the relative abundance of cidofovir phosphocholine was not affected by dose.

Intravitreal and plasma cidofovir concentrations after intravitreal and intravenous administration in AIDS patients with cytomegalovirus retinitis.

This study was undertaken to evaluate the intravitreal and plasma concentrations of cidofovir (HPMPC) after intravitreal and intravenous administration in AIDS patients with cytomegalovirus retinitis. Cohort series; undiluted vitreous and blood were collected from 9 patients at the time of pars plana vitrectomy. Vitreous samples from 9 eyes of 9 patients and plasma samples from 4 patients were assayed with high-performance liquid chromatography to determine cidofovir levels. The only eye that had a detectable vitreous concentration (673.7 ng/ml) was injected with 20 microg 24 hours prior to the surgery. The remaining samples including plasma were below the detection point of the assay (100 ng/ml) and were injected between 5 and 40 days prior to sampling. The intravitreal concentration of cidofovir in humans is consistent with pharmacokinetics data in laboratory animals, and suggests that the long duration of antiviral effect (1-3 months) in clinical trials is due to a prolonged intracellular half-life in retinal tissue.

Randomised clinical trial: arbaclofen placarbil in gastro-oesophageal reflux disease--insights into study design for transient lower sphincter relaxation inhibitors.

BACKGROUND: Arbaclofen placarbil is a pro-drug of the gamma-aminobutyric acid-B agonist R-baclofen that has been shown to reduce reflux episodes in patients with gastro-oesophageal reflux disease (GERD). AIM: To evaluate the efficacy and safety of arbaclofen placarbil vs. placebo as adjunctive therapy in subjects with troublesome GERD symptoms despite therapy with once-daily doses of a proton pump inhibitor (PPI) and to identify the characteristics of patients who were responders. METHODS: Patients (n = 460) with symptomatic GERD experiencing troublesome symptoms on once-daily PPI therapy were enrolled in this phase II, randomised, multicentre, double-blind, placebo-controlled, dose-ranging study. Patients were randomised to receive placebo or arbaclofen placarbil (20 or 40 mg once daily, 20 or 30 mg twice daily) with their current PPI for 6 weeks. Patients recorded heartburn and other GERD symptoms in a daily diary and rated severity of each event. The primary endpoint was percent change from baseline in heartburn events per week. RESULTS: In the primary analysis, there was no significant difference between arbaclofen placarbil and placebo. Post hoc analyses removing mild and very mild heartburn events resulted in greater percent reductions for all arbaclofen placarbil doses with nominal P values <0.05 for each dose compared with placebo. There was a dose-related increase for the most common adverse events. CONCLUSIONS: Arbaclofen placarbil was not superior to placebo in the primary analysis. Post hoc analyses suggest that subjects with more clinically relevant moderate or severe symptoms are more likely to respond to arbaclofen placarbil (clinicaltrials.gov NCT00978016).

High-performance liquid chromatographic method for the determination of N-methylated metabolites of nicotine.

An analytical method has been developed, using cation-exchange high-performance liquid chromatography, for the analysis of N-methylated metabolites of nicotine. This method has been used to detect and quantitate seven potential in vivo urinary metabolites of [2'-14C]-nicotine, including four methylated nicotine derivatives, in the guinea pig.

In vitro characteristics of guinea pig lung aromatic azaheterocycle N-methyltransferase.

The in vitro characteristics of guinea pig lung aromatic azaheterocycle N-methyltransferase have been determined, using R-(+)-nicotine as substrate. The enzyme is a cytosolic, S-adenosyl-L-methionine-dependent N-methyltransferase exhibiting apparent KM values of 14.2 and 13.2 microM for R-(+)-nicotine and S-adenosyl-L-methionine, respectively. The pH and temperature optima for enzyme activity were 7.4 and 37 degrees C, respectively. An interesting stereospecificity was demonstrated for nicotine enantiomers toward the N-methyltransferase. S-(-)-Nicotine acts as a competitive inhibitor (Ki = 6.25 X 10(-5) M) of the N-methylation of its optical antipode by the above enzyme. The enzyme also exhibits potent feedback inhibition by S-adenosyl-L-homocysteine (Ki = 3.2 X 10(-5) M). Neither S-(-)-cotinine nor nicotinamide is a substrate for the enzyme; however, the latter compound did act as a weak competitive inhibitor (Ki = 41.8 X 10(-5) M) of the N-methylation of R-(+)-nicotine.

Biotransformation of primary nicotine metabolites. II. Metabolism of [3H]-S-(-)-cotinine in the guinea pig: determination of in vivo urinary metabolites by high-performance liquid-radiochromatography.

1. The metabolism of [3H]-S-(-)-cotinine in vivo has been investigated in the guinea pig. The quantitive determination of urinary metabolites has been carried out using a high-performance liquid radiochromagraphic procedure. Metabolite analysis of C- and N-oxidation products, and N-methylcotininium ion, were carried out on two chromatographic systems: (a) Partisil-10 SCX cation-exchange chromatography, and (b) Partisil-10 ODS reverse-phase chromatography. 2. 3-Hydroxycotinine was the major urinary metabolite of cotinine in the guinea pig, accounting for 57% of the total radioactivity in the urine. Cotinine-N-oxide constituted 18% of total metabolites in the urine, whereas neither nicotine nor the N-methylcotininium ion were detected as urinary metabolites of [3H]-S-(-)cotinine. S-(-)-Cotinine was extensively metabolized in the guinea pig; total recovery of radioactivity in 24 h urine was very high (greater than 95%) and very little cotinine was detected (less than 1%) in the urine. 3. Two unidentified metabolites of [3H]-S-(-)-cotinine were detected which collectively constituted approximately 20% of total urinary tritium.