Inter- and intra-subject variability in gabapentin absorption and absolute bioavailability.

UNLABELLED: Gabapentin (GBP) is a non-metabolized, non-plasma protein bound, renally excreted antiepileptic drug that is actively absorbed via the system L amino acid transporter. Previous studies have demonstrated that gabapentin displays dose-dependent absorption. OBJECTIVES: These studies were conducted to determine inter- and intra-subject variability of gabapentin absorption. Two prospective clinical studies in healthy adult volunteers were conducted. Coefficient of variation (CV) was used to express variability of gabapentin absorption. METHODS: Study A: 400-mg single dose, randomized, cross-over study to assess bioavailability of four different gabapentin formulations (n=20, 9 males, 11 females; mean age and weight 41 years, 75.1 kg). Plasma was serially collected up to 48 h and bioavailability (F) calculated post-dose to determine concentration-time curves (AUC). All four formulations were bioequivalent, thus repeated measures analysis was performed to assess inter-and intra-subject variability. Study B: 600-mg single dose study (n=50, 15 males, 35 females; mean age and weight 31.1 years, 72.7 kg) was conducted to determine inter-subject variability in gabapentin F. Urine was collected over 48 h and bioavailability (F) calculated. Urine and plasma gabapentin concentrations were measured by HPLC-UV. RESULTS: Study A: Overall mean (CV) of GBP AUC values was 34.1+/-24 ug/h per ml. Inter-subject CV for AUC was 22.5% and intra-subject CV was 12.1%. Study B: Overall mean (SD) GBP F was 49.3+/-13.6%. Inter-subject CV of F was 27.6%. DISCUSSION: The inter-subject variability in gabapentin absorption is substantially less than that of the inter-subject variability. This indicates that one would expect a wide range in gabapentin absorption between subjects; however, a much smaller variability within a subject. The within subject variability of gabapentin is small enough that plasma drug monitoring may be used to assess gabapentin absorption for a given subject and the benefit of dose individualization.

Contrasting nutrient effects on the plasma levels of an amino acid-like antiepileptic agent from jejunal administration in dogs.

The absorption of gabapentin was investigated by monitoring drug plasma levels as a function of time following midjejunal administration in mongrel dogs. From previous work, dose-dependent absorption had been postulated to be a consequence of carrier-mediated transport and a paracellular pathway had been postulated to contribute to the passive absorption component in mammalian small intestine. The potential for amino acid inhibition of the carrier-mediated absorption component was investigated by drug coinfusion with leucine and phenylalanine. The potential for monosaccharide-enhanced increases in drug absorption was studied by drug coinfusion with D-glucose and 3-O-methylglucose. While lower drug plasma levels were observed with amino acid coinfusion versus controls in each of the dogs studied, mean area under the plasma level time curves (AUC) were not statistically significantly different (p < or = 0.07). Monosaccharide coinfusion significantly increased gabapentin AUC over control studies (p < or = 0.014) and over coinfusion with L-system amino acids (p < or = 0.0025). Implications for the mechanisms of intestinal absorption of this amino acid-like antiepileptic drug in this canine model are discussed.

Tolerance and pharmacokinetics of single-dose atorvastatin, a potent inhibitor of HMG-CoA reductase, in healthy subjects.

Tolerance and pharmacokinetics after single-dose administration of atorvastatin, an investigational inhibitor of HMG-CoA reductase, were examined in 22 healthy volunteers in a three-period, partially-blinded study. Participants received capsule and solution doses of atorvastatin (0.5 to 120 mg) and placebo at weekly intervals. Atorvastatin was well tolerated at doses as high as 80 mg. The adverse event profile was similar after administration of atorvastatin capsules and placebo. Atorvastatin solution was slightly less well tolerated. The most common side effect after administration of capsules and solution was headache, followed by sporadic reports of diarrhea, flatulence, and nausea. At the 120-mg solution dose, one participant experienced mild, transient restlessness, euphoria, and mental confusion that were considered to be dose-limiting side effects. Mean concentrations of atorvastatin, maximum concentration (Cmax), and area under the concentration-time curve from time 0 to the time of the last detectable concentration (AUCo-tldc) increased with increasing dose. Plasma elimination half-life (t1/2) ranged from 14.7 to 57.6 hours. The bioavailability of atorvastatin capsules was similar to that of solution. These results suggest that atorvastatin is well tolerated after single doses as high as 80 mg, and may require administration only once daily.

Measurement of a new anticonvulsant, (S)-3-(aminomethyl)-5-methylhexanoic acid, in plasma and milk by high-performance liquid chromatography.

A specific and sensitive isocratic method for the measurement of a new anticonvulsant, (S)-3-(aminomethyl)-5-methylhexanoic acid, in rat plasma and milk is described. Following deproteinization, the compound and internal standard [1-(aminomethyl)cycloheptaneacetic acid] were derivatized utilizing 2,4,6-trinitrobenzene sulfonic acid and extracted with cyclohexane. Analytes were resolved on a 5 microns Spherisorb ODSII column (250 mm x 4.6 mm) using a mobile phase of 57% acetonitrile in 0.1 M ammonium acetate, pH 4.0. Absorbance was monitored at 350 nm. Limit of quantitation was 1.00 microgram/ml for a 100-microliters aliquot of plasma or milk.

Effect of food on the bioavailability of atorvastatin, an HMG-CoA reductase inhibitor.

To determine whether atorvastatin, a new HMG-CoA reductase inhibitor, could be administered with food in Phase II and III clinical trials, a nonblind, randomized, two-way crossover study was conducted to assess the effect of food on rate and extent of atorvastatin absorption. Sixteen healthy volunteers received single 80-mg atorvastatin capsule doses on two occasions one week apart: once after an 8-hour overnight fast and once with a medium-fat breakfast. The single 80-mg atorvastatin capsule doses were well-tolerated. Mean maximum plasma atorvastatin equivalent concentration (Cmax) and area under the concentration-time curve (AUC) values with food were 47.9% and 12.7% lower, respectively, than without food. Mean time of maximum observed concentration (tmax) and elimination half-life (t1/2) values were 5.9 and 32.0 hours, respectively, with food and 2.6 and 35.7 hours, respectively, without food. A medium-fat breakfast decreased the rate of atorvastatin absorption significantly, but had little impact on extent of drug absorption. Changes in rate of atorvastatin absorption are not expected to have a clinically significant effect, as subsequent multiple-dose clinical studies have shown that dose but not plasma atorvastatin concentration profiles correlates with lipid-lowering effects.

Disposition of gabapentin (neurontin) in mice, rats, dogs, and monkeys.

Gabapentin, an analog of gamma-aminobutyric acid, exhibits anticonvulsant properties in both animal models and humans. Gabapentin pharmacokinetics was studied in laboratory animals using HPLC and radiometry. Oral bioavailability was 40% in monkeys administered 25 mg/kg, 79% in mice and rats receiving 50 mg/kg, and 80% in dogs administered 50 mg/kg. Binding to plasma proteins was < 3%. Maximum blood or plasma concentrations generally occurred within 2 hr of an oral dose. In rats and monkeys, increases in maximum plasma concentrations and/or areas under the curve were less than dose-proportional following oral administration, most likely because of saturable absorption. However, intravenous pharmacokinetics in rats were linear over the dosage range of 4-500 mg/kg. Mean intravenous elimination half-life was 1.7 hr in rats, 2.9 hr (14C only) in dogs, and 3.0 hr in monkeys. In rats and dogs, repeated administration did not alter gabapentin or 14C pharmacokinetics. Additionally, gabapentin did not induce hepatic cytochrome P450 monooxygenases in rats. There were no age- (rats only) or gender-associated changes in pharmacokinetic parameters. [14C]Gabapentin was extensively distributed to tissues. In the dog, gabapentin was metabolized to N-methylgabapentin (approximately 34% of dose); whereas metabolism in mouse, rat, and monkey was minimal (< 5%). The principal route of excretion was via urine. In summary, as an antiepileptic drug, gabapentin exhibited desirable pharmacokinetic properties, such as linear elimination kinetics, not highly bound to plasma proteins, not extensively metabolized, and not an inducer of hepatic cytochrome P450.

Lack of interaction of gabapentin with carbamazepine or valproate.

Gabapentin (GBP) studies were conducted in patients with epilepsy receiving carbamazepine (CBZ, n = 12) or valproate (VPA, n = 14) monotherapy. The effects of GBP coadministration on steady-state CBZ or VPA concentrations and of these antiepileptic drugs (AEDs) on GBP pharmacokinetics were investigated. GBP (400 mg) was coadministered every 8 h for 3 1/3 days with CBZ or for 5 1/3 days with VPA. GBP was well tolerated. Mean steady-state plasma CBZ/CBZ-10,11-epoxide (CBZ-E) and serum VPA concentrations before, during, and after GBP administration were not significantly different. Mean steady-state GBP pharmacokinetic parameters during CBZ or VPA coadministration were similar to steady-state parameters reported in healthy subjects. Thus, no pharmacokinetic interaction exists between CBZ or VPA and GBP. No dosage adjustment is necessary when GBP and CBZ or VPA are coadministered.

Pharmacokinetics, mass balance, and induction potential of a novel GABA uptake inhibitor, CI-966 HCl, in laboratory animals.

CI-966 exhibits anticonvulsant properties in various animal models. The drug acts by inhibiting synaptic uptake of gamma-aminobutyric acid (GABA). Oral absorption of CI-966 in dogs given 1.39 mg/kg is rapid with a tmax of 0.7 hr. In rats given 5 mg/kg oral, a mean tmax of 4.0 hr was observed. Following iv administration of the same respective doses, elimination t1/2 in dogs and rats averaged 1.2 and 4.5 hr. Absolute oral bioavailability of CI-966 was 100% in both species. Following oral dosing of [14C]CI-966 HCl to dogs, fecal, and urinary excretion accounted for 89% and 2.3% of the 14C dose, respectively. In bile-duct cannulated rats, biliary excretion is the major elimination pathway of radioactivity (75%). Urinary and fecal excretion accounted for 4.1 and 12%, respectively. CI-966 does not induce or inhibit mouse hepatic mixed function oxidases, as determined by hexobarbital sleeping time.

Measurement of CI-979 (a candidate drug for the treatment of age-related disorders of cognition) in human plasma by capillary gas chromatography with nitrogen-selective detection.

A specific and highly sensitive method for the measurement of CI-979 in human plasma is described. The compound and internal standard were extracted from alkalinized plasma with methyl tert.-butyl ether and analyzed by capillary gas chromatography with nitrogen-selective detection. The method was demonstrated to be accurate and precise. Since the limit of quantitation was 0.10 ng/ml, this method was suitable for clinical pharmacokinetic studies in which subjects received repeated administration of 0.5-2.5 mg CI-979 every 6 h.

Effect of cimetidine administration on the pharmacokinetics of pirmenol.

The potential for a drug-drug interaction between pirmenol, an extensively metabolized antiarrhythmic agent, and cimetidine, an inhibitor of hepatic drug-metabolizing enzymes, was evaluated in eight healthy adults. A single 150-mg oral dose of pirmenol was administered on study days 1 and 8 and oral cimetidine, 300-mg QID, was administered on study days 4 through 11. Plasma and urine samples were collected after each pirmenol dose for determination of pirmenol concentration. Mean pirmenol concentration-time curves and pharmacokinetic parameters, including elimination rate constant, were not significantly altered by concomitant administration of cimetidine.

Biotransformation of methyl parathion by human foetal liver glutathione S-transferases: an in vitro study.

The role of human foetal liver glutathione S-transferases in the detoxification of methyl parathion was investigated. Glutathione S-transferases were partially purified by affinity chromatography utilizing reduced glutathione as the ligand coupled to epoxy-activated Sepharose 4B. This resulted in the isolation of material with an average activity (mean +/- S.E.) of 58.90 +/- 4.83 mumol 1-chloro-2,4-dinitrobenzene conjugate formed/min per mg, representing a purification of 70-fold. These partially purified foetal liver transferases catalysed the metabolism of methyl parathion exclusively to desmethyl parathion via O-dealkylation. High-performance liquid chromatography, radiometric analysis of the enzymic reaction, and co-chromatography with reference standard on thin-layer chromatography confirmed the sole metabolite as desmethyl parathion. The range of foetal liver activity towards methyl parathion was from 30 to 122 nmol desmethyl parathion formed/min per mg. Analysis of the kinetic parameters of three partially purified foetal liver preparations with gestational ages of 14, 16 and 21 weeks resulted in Km values for methyl parathion of 0.24, 0.38 and 0.86 mM, respectively; whereas, the Km values assessed for glutathione were 0.20, 0.10 and 0.18 mM. The ability of human foetal liver glutathione S-transferases to catalyse the metabolism of methyl parathion exclusively to desmethyl parathion via O-dealkylation represents a major qualitative biochemical difference from the rat-liver isozymes.

Partial purification and some biochemical properties of neonatal rat cutaneous glutathione S-transferases.

1. Previous studies have demonstrated the presence of glutathione S-transferases in the skin of rodents and humans. This study represents the first attempt to purify cytosolic glutathione S-transferases from skin of 3-day-old rats. 2. A partial purification of the enzyme was achieved by a two-step procedure: affinity chromatography followed by HPLC. Two peaks, one major (P-1) and one minor (P-2), were resolved by HPLC containing about 82% and 10% of the recovered activity, respectively. 3. The major form exhibited an overall purification of about 2270-fold with a specific activity of about 73 mumoles/min/mg protein towards 1-chloro-2,4-dinitrobenzene. 4. The kinetic data for P-1 yielded mean Km values of 2.39 mM for 1-chloro-2,4-dinitrobenzene and 0.72 mM for reduced glutathione, while the respective average Vmax values were found to be 212 and 101 mumoles/min/mg protein. 5. Significantly inhibition of enzyme activity was noted in the presence of 0.2 mM HgCl2, 0.63 microM 1.2-naphthoquinone, 1.0 microM triphenyltin chloride, and 12.5 microM 17 beta-estradiol-3-sulfate.

H.p.l.c. separation and study of the charge isomers of human placental glutathione transferase.

Glutathione transferase (GST) from human placenta was purified by affinity chromatography and anion-exchange h.p.l.c. The enzyme exhibited different chromatographic and electrophoretic behaviours according to the concentration of GSH, suggesting a possible change in the net charge of the molecule and a concomitant conformational change due to ligand binding. Two interconvertible forms were quantitatively separated into distinct catalytically active states by h.p.l.c. Depending upon the GSH concentration, polyacrylamide-gel electrophoresis revealed the presence of one or two bands. A Kd of 0.42 mM for GSH was determined fluorimetrically. The loss in intrinsic fluorescence also suggested a conformational change in the enzyme. Kinetic studies using ethacrynic acid were conducted to determine whether the presumed conformational change could effect the catalytic capability of placental GST. A biphasic response in initial velocities was observed with increasing concentrations of GSH. Two apparent Km values of 0.38 and 50.27 mM were obtained for GSH, whereas Vmax. values showed a 46-fold difference. It was concluded that the enzyme assumes a highly anionic form in the presence of a low GSH concentration, whereas it is converted into relatively weaker anionic form when its immediate environment contains a high GSH concentration. Since the average tissue concentration of total GSH was estimated at 0.11 mM for term placenta, the results suggest that the high-affinity-low-activity conformer would predominate in vivo.

Human placental glutathione S-transferase-mediated metabolism of methyl parathion.

The ability of human placental glutathione S-transferase (GSHTr) to metabolize methyl parathion (MeP) was examined. MeP was found to be a substrate for both partially purified pre-term and highly purified term placental GSHTr. The characterization of the reaction by high performance liquid chromatography revealed the presence of desmethyl parathion (DesMeP) as the sole metabolite. Term placental GSHTr activity towards MeP ranged from 2.22 to 3.53 nmoles DesMeP formed X min-1 X mg-1 while an activity of 0.60 to 1.12 nmoles DesMeP formed X min-1 X mg-1 was observed with the pre-term placental enzyme. The absence of the O-dearylation reaction by pre-term and term placental GSHTr represents a major species- and/or tissue-specific difference.

A rapid, novel high performance liquid chromatography method for the purification of glutathione S-transferase: an application to the human placental enzyme.

A simple High Performance Liquid Chromatography procedure is detailed for the purification of Glutathione S-transferase. The human placental transferase was used to assess its potential. Unlike conventional methods of purification, the procedure is rapid and resolution of the various forms is achieved in less than 20 min. Since recovery is essentially complete, it is possible to isolate different minor forms. Three forms, one major and two minor, were separated. The major form represented about 97% of the total recovered activity and exhibited a specific activity of 254.94 mumoles/min/mg protein with a purification of 1342-fold. Electrophoresis of the major form revealed the presence of a single band, suggesting homogeneity.

Comparative theobromine metabolism in five mammalian species.

Biotransformation of theobromine (TBR) was compared in rats, mice, hamsters, rabbits, and dogs by assaying urinary metabolites using HPLC after oral administration of a 5 mg/kg dose containing 8-14C-TBR. Recovery of radioactivity ranged from 60-89% of the dose in urine, and from 2-38% of the dose in feces, with most material being excreted during the first 48 hr after dosing. TBR was most extensively metabolized by rabbits and male mice. The primary metabolite excreted by rats and mice was 6-amino-5-[N-methylformylamino]-1-methyluracil (6-AMMU); male mice converted TBR to this metabolite more extensively than did female mice. Rabbits and dogs metabolized TBR primarily to 7-methylxanthine (7-MX) and 3-methylxanthine (3-MX), respectively; the major metabolites excreted by hamsters were 6-AMMU and 7-MX. Overall N-demethylase activity yielding monomethyl metabolites was greatest in rabbits and lowest in rats. Ring N-demethylation at position 3 predominated over 7-N-demethylation in all species except the rat and dog. In dogs, TBR was N-demethylated primarily at position 7, while N-demethylase activity in rats was without apparent positional specificity. Oxidation of methylated xanthines to the corresponding uric acids was a relatively minor metabolic pathway in all species, but had greatest activity in mice. Oxidation of TBR to 3,7-dimethyluric acid was significantly greater in female rats than in male rats. In summary, excretion patterns of TBR and its metabolites were qualitatively similar among species, indicating that TBR is metabolized along similar pathways. Except for the excretion of small quantities of an unidentified but apparently unique metabolite by dogs, only quantitative species- and sex-related differences were observed in the metabolic disposition of TBR.