Analytical Chiral Separation of the Stereoisomers of a Novel Carbonic Anhydrase Inhibitor and Its Deethylated Metabolite,and the Assignment of Absolute Configuration of the Human Metabolite and Chiral Degradation Products

Several approaches to the separation of four stereoisomers, 1–4, of a novel, topically active, carbonic anhydrase inhibitor, 1, with two chiral centers in the molecule and four isomers, 5–8, of its chiral metabolite, 5, were evaluated. These methods include nonchiral derivatization followed by separation on chiral stationary phases (CSPs) and chiral derivatization and separation on nonchiral columns and on CSPs. Baseline separation of stereoisomers 1–4 was achieved in less than 15 min after chiral derivatization with (S)-(+)-l-(l-naphthyl)ethyl isocyanate (NEIC) and chiral chromatography on a (R)-N-(3,5-dinitrobenzoyl)phenyl glycine (DNBPG) column under normal phase (NP) conditions. Similarly, isomers 5-8 were baseline separated in less than 20 min after derivatization with NEIC and chromatography on nonchiral (nitrophenyl) and chiral [(S)-(3,5-dinitrobenzoyl)leucine; DNBL] columns in series under the same NP chromatographic conditions. Only partial separation of the diastereomeric derivatives was observed on a variety of nonchiral columns. In addition, all other direct and indirect chiral separation approaches gave only partial separation of at least two stereoisomers within the group of 1–4 or 5–8. The details of chiral separations using various methods and separation () and capacity factors (k) of the derivatized isomers 1–8 on a series of chiral and nonchiral columns are presented. Using these methods, the absolute configuration of the human metabolite of 1 was established as S ₁ S ₂ (5), and the heat (HD) and light (LD) degradation products of 1 as R ₁ S ₂ (3) and S₁ S ₂ (5), respectively.     

Pharmacokinetics of aprepitant after single and multiple oral doses in healthy volunteers

Aprepitant is the first NK1 receptor antagonist approved for use with corticosteroids and 5HT3 receptor antagonists to prevent chemotherapy-induced nausea and vomiting (CINV). The effective dose to prevent CINV is a 125-mg capsule on day 1 followed by an 80-mg capsule on days 2 and 3. Study 1 evaluated the bioavailability of the capsules and estimated the effect of food. The mean (95% confidence interval [CI]) bioavailabilities of 125-mg and 80-mg final market composition (FMC) capsules, as assessed by simultaneous administration of stable isotope-labeled intravenous (i.v.) aprepitant (2 mg) and FMC capsules, were 0.59 (0.53, 0.65) and 0.67 (0.62, 0.73), respectively. The geometric mean (90% CI) area under the plasma concentration time curve (AUC) ratios (fed/fasted) were 1.2 (1.10, 1.30) and 1.09 (1.00, 1.18) for the 125-mg and 80-mg capsule, respectively, demonstrating that aprepitant can be administered independently of food. Study 2 defined the pharmacokinetics of aprepitant administered following the 3-day regimen recommended to prevent CINV (125 mg/80 mg/80 mg). Consistent daily plasma exposures of aprepitant were obtained following this regimen, which was generally well tolerated.     

Simultaneous determination of Aprepitant and two metabolites in human plasma by high-performance liquid chromatography with tandem mass spectrometric detection

A method for the simultaneous determination of Aprepitant, I (5-[[2(R)-[1(R)-(3,5-bistrifluoromethylphenyl)ethoxy]-3(S)-(4-fluorophenyl) morpholin-4-yl]methyl]-2,4-dihydro-[1,2,4]triazol-3-one) and two active metabolites (II and III) in human plasma has been developed. The method was based on high-performance liquid chromatography (HPLC) with atmospheric pressure chemical ionization tandem mass spectrometric (APCI-MS-MS) detection in positive ionization mode using a heated nebulizer interface. The analytes and internal standard (IV) (Fig. 1) were isolated from basified plasma using liquid–liquid extraction. The organic extracts were dried, reconstituted in mobile phase and injected into the HPLC-MS/MS system.

The analytes were chromatographed on a narrow bore (50 mm×2.0 mm, 3 μm) Keystone Scientific’s Prism R.P. analytical column, with mobile phase consisting of acetonitrile (ACN):water containing trifluoroacetic acid with pH adjusted to 3 (40:60, v/v) pumped at a flow rate of 0.5 ml/min. The MS-MS detection was performed on a Sciex API 3000 tandem mass spectrometer operated in selected reaction monitoring mode. The precursor→product ion combinations of m/z 535→277, 438→180, 452→223 and 503→259 were used to quantify I, II, III, and IV, respectively, after chromatographic separation of the analytes. The assay was validated in the concentration range of 10–5000 ng/ml for I and II and 25–5000 ng/ml for III when 1 ml of plasma was processed. The precision of the assay (expressed as coefficient of variation, CV) was less than 10% at all concentrations within the standard curve range, with adequate assay accuracy. Matrix effect experiments were performed to demonstrate the absence of any significant change in ionization of the analytes when comparing neat standards to analytes in the presence of plasma matrix. This assay was utilized to support a clinical study where multiple oral doses of I were administered to healthy subjects to investigate the pharmacokinetics, safety, and tolerability of Aprepitant. Concentrations of the two most active metabolites, which if present in high concentrations would increase the neurokinin-1 (NK₁) receptor occupancy level and therefore potentially contribute to the antiemetic action of Aprepitant, were determined.     

Determination of a thermally labile metabolite of a novel growth hormone secretagogue in human and dog plasma by liquid chromatography with ion spray tandem mass spectrometric detection

A sensitive and selective assay for the determination of N-[1(R)-[(1,2-dihydro-1-methylsulfonylspiro[3H-indole-3,4'-piperidin]-1'-yl)carbonyl]-2-(phenylmethoxy)-ethyl]-2-hydroxyamino-2-methylpropanamide (I), a hydroxyl amine metabolite of a novel growth hormone secretagouge (II) has been developed utilizing high-performance liquid chromatography with ion spray tandem mass spectrometric detection (HPLC-MS-MS). The analyte and an internal standard (III) were isolated from the basified biological matrix using a liquid-liquid extraction with methyl tert.-butyl ether (MTBE). The organic extract was evaporated to dryness at room temperature. The residue was reconstituted in the mobile phase and injected into the HPLC-MS-MS system. Multiple reaction monitoring using the precursor-->product ion combinations of m/z 545-->267 and 543-->267 was used to quantify I and III, respectively, after chromatographic separation under isocratic conditions. The assay was validated in the concentration range of 0.5 to 500 ng/0.1 ml in both human and dog plasma. The precision of the assay, expressed as relative standard deviation, was less than 10% over the entire concentration range with the exception of the low concentration of 0.5 ng/0.1 ml which was 14.0% for human plasma. The HPLC-MS-MS method provided sufficient sensitivity to completely map the pharmacokinetic time course of I following a single 5 mg dose of II to human subjects and a 0.5 mg/kg dose to beagle dogs.     

Determination of a cyclic hexapeptide, a novel antifungal agent, in human plasma by high-performance liquid chromatography with ion spray and turbo ion spray tandem mass spectrometric detection

Methods for the determination of a semi-synthetic cyclic hexapeptide (I, MK-0991) in human plasma based on high-performance liquid chromatography (HPLC) with tandem mass spectrometric (MS-MS) detection using pneumatically assisted electrospray (ion spray, ISP) and turbo ion spray (TISP) interfaces were developed. Drug and internal standard (II, an isostere of I) were isolated from plasma by solid-phase extraction (SPE). The eluent from SPE was evaporated to dryness, the residue was reconstituted in mobile phase and injected into the HPLC system. The use of ISP, TISP and heated nebulizer (HN) interfaces as sample introduction systems were evaluated and showed that the heated nebulizer was not adequate for analysis due to thermal instability and/or adsorption of I and II to glass surfaces of the interface. Compounds I and II were chromatographed on a wide pore (300 A), 150x4.6 mm C8 analytical column, and the HPLC flow-rate of 1.2 ml/min was split 1:20 prior to introduction to the ISP or TISP interface of the mass spectrometric system. The MS-MS detection was performed on a PE Sciex API III Plus tandem mass spectrometer operated in selected reaction monitoring mode (SRM). The precursor-->product ion combinations of m/z 1093.7-->1033.6 and 1094.7-->1033.6 were used to quantify I and II, respectively, after chromatographic separation of the analytes. The assay was validated in the concentration range of 10-1000 ng/ml using ISP, and 2.5-500 ng/ml of plasma using TISP with good precision and adequate accuracy. The effects of HPLC mobile-phase components on the ionization efficiency and sensitivity of detection in the positive ionization mode, the evaluation of the matrix effect, and limitations in sensitivity of detection of I due to the formation of multiply charged species are presented.     

Evidence for route dependent biotransformation of cyclobenzaprine hydrochloride

Cyclobenzaprine hydrochloride was administered to healthy volunteers as a 10mg oral tablet, 10mg and 20mg intramuscular injections, and a 10mg intravenous injection. Urinary excretion and plasma level data for cyclobenzaprine together provide evidence for route dependent biotransformation. Urinary excretion of total cyclobenzaprine (unchanged plus the glucuronide conjugate) was greater for the oral treatment than for the parenteral treatments (i.m. and i.v.). Area under the plasma concentration-time curve for unchanged cyclobenzaprine, however, was less for the oral treatment than for the parenteral treatments. Based on area calculations, the bioavailability of the 10 mg oral tablet, 10 mg i.m. and 20 mg i.m. injection was 0.33, 0.76, and 0.56, respectively, when compared to the 10mg i.v. injection of cyclobenzaprine hydrochloride. The four treatments were well tolerated and no clinically adverse effects were observed.     

Bioavailability of alendronate and vitamin D(3) in an alendronate/vitamin D(3) combination tablet

These studies were designed to demonstrate that the alendronate (ALN) component of an ALN/vitamin D(3) combination tablet was bioequivalent to the 70-mg ALN tablet and that the pharmacokinetic parameters of vitamin D(3) were similar with or without ALN. These were open-label, randomized, 2-part, 2-period, crossover studies. In part I, participants received either a single combination tablet or ALN 70 mg. In part II, participants received either a single combination tablet or vitamin D(3) alone. Results from part I showed that the geometric mean ratio (GMR) for total urinary excretion of ALN for both studies fell within the prespecified bioequivalence bounds. Results from part II showed that the pharmacokinetic profiles of vitamin D(3) with or without ALN were also similar. The combination tablets are bioequivalent to the ALN 70-mg tablet with respect to ALN bioavailability. The bioavailability of vitamin D(3) is similar in the combination tablets and when administered alone. No serious adverse experiences were reported.     

Stereoselective disposition of the geometric isomers of a novel lipoxygenase cyclo-oxygenase inhibitor in dog and photochemical interconversion of its isomers

A sensitive (10 ng/mL) and specific high-performance liquid chromatographic (HPLC) assay, with electrochemical (EC) detection, for the geometric isomers of 3-hydroxy-N-(2-phenyl-2-(2-thienyl)ethenyl-5-(trifluoromethyl)benzo(b) thiophene-2-carboxamide in dog and human plasma has been developed. Both isomers strongly absorb light, leading to an efficient E in equilibrium Z photoisomerization. After iv administration of a single isomer (Z) to a dog, only the Zisomer was detected in plasma; no in vivo conversion to the E isomer was observed. However, when a mixture of the E and Z isomers (58.6:41.4) was administered in the same manner to the same dog, the E:Z ratio decreased significantly to 47.5:52.5 six hours after drug administration, indicating stereoselective disposition of the isomers. The elimination of the E isomer was found to be faster than that of the Z isomer.     

Cyclobenzaprine pharmacokinetics, including the effects of age, gender, and hepatic insufficiency.

The pharmacokinetics and bioavailability of cyclobenzaprine, a widely used muscle relaxant, were investigated in four clinical studies, and the effects of age, gender, and hepatic insufficiency were characterized. Cyclobenzaprine plasma clearance was 689 ml/min, and the bioavailability of a 5 mg oral dose was 0.55. Following oral doses of 2.5 to 10 mg tid in healthy young subjects, cyclobenzaprine pharmacokinetics were linear, and plasma concentrations generally increased proportional to dose. There was about a fourfold accumulation of the drug in plasma on multiple dosing, corresponding to an effective half-life of 18 hours. Steady-state plasma concentrations of cyclobenzaprine in elderly subjects were twice as high as in young subjects following oral doses of 5 mg tid. Steady-state plasma concentration also appeared to be up to twofold higher in subjects with mild hepatic insufficiency compared to healthy controls. The magnitude of any difference in steady-state plasma concentration between males and females appears to be small relative to intersubject variability. A reduction in dose or dosing frequency should be considered in the elderly and in patients with liver disease.     

Determination of efavirenz,a selective non-nucleoside reverse transcriptase inhibitor,in human plasma using HPLC with post-column photochemical derivatization and fluorescence detection

Methods for the quantitative determination of efavirenz in human plasma and the qualitative assessment of the stereochemical integrity of efavirenz in post-dose human plasma samples are described. After the addition of an internal standard, plasma samples were extracted with hexane-methylene chloride (65/35, v/v%). The extracts were evaporated to dryness and reconstituted in mobile phase. Upon exposure to UV light, the analyte was found to form fluorescent products; the major fluorescent product was isolated and identified as a substituted quinoline. Thus, the plasma extracts were analyzed via HPLC with post-column photochemical derivatization and fluorescence detection. Reverse phase chromatography was used for the quantitative assay, whereas chromatography with a column containing a chiral stationary phase (dinitrobenzoyl leucine) was used for the stereochemical assessment. The quantitative assay has been validated in the concentration range of 50-1000 ng/ml using 0.5 ml samples. Analyte recovery was better than 89% at all points on the standard curve. Intra-day precision was better than 5% C.V., while accuracy was between 95 and 104% of nominal over the range of the assay. The selective detection method reduces the likelihood of interference by co-administered medications or endogenous species. The stereochemical configuration of efavirenz was confirmed to remain intact in post-dose human plasma samples. The quantitative method has been successfully utilized to support a study in which a possible drug interaction between co-administered HIV protease inhibitors and efavirenz was evaluated.