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.

Evaluation of the safety and pharmacokinetics of the multi-targeted receptor tyrosine kinase inhibitor sunitinib during embryo-fetal development in rats and rabbits.

BACKGROUND: Angiogenesis plays a key role in embryo-fetal development and, based on nonclinical safety data, the majority of vascular endothelial growth factor (VEGF)-targeted antiangiogenic agents used in cancer therapy are not recommended during pregnancy. We investigated the effects of sunitinib (an oral inhibitor of multiple receptor tyrosine kinases [RTKs] including VEGF-receptors) on embryo-fetal development. METHODS: Presumed-pregnant Sprague-Dawley rats and New Zealand White rabbits received repeated daily oral doses of sunitinib (0-30 mg/kg/day), during the major period of organogenesis. Clinical/physical examinations were performed throughout the gestation phase, and blood samples were collected to determine systemic exposure. Necropsy (including uterine examination) was performed on all animals and fetal morphology was examined. RESULTS: The no-observed-adverse-effect level was 1-5 mg/kg/day for maternal toxicity and 3 mg/kg/day for developmental toxicity in rats; 1 and 0.5 mg/kg/day, respectively, in rabbits. Embryo-fetal toxicity included decreases in the number of live fetuses and increases in the numbers of resorptions and post-implantation/complete litter losses; these were observed at doses of > or =5 mg/kg/day in rats and 5 mg/kg/day in rabbits. Malformations included fetal skeletal malformations (generally thoracic/lumbar vertebral alterations) in rats and cleft lip/palate in rabbits. These developmental effects were observed at approximately 5.5- (rats) and approximately 0.3-times (rabbits) the human systemic exposure at the approved sunitinib dose (50 mg/day). CONCLUSIONS: Similar effects have been reported with the prototype monoclonal antibody bevacizumab. As is typically observed for potent inhibitors of RTKs involved in angiogenesis, sunitinib was associated with embryo-fetal developmental toxicity in rats and rabbits at clinically relevant dose levels.

Ibrutinib Dosing Strategies Based on Interaction Potential of CYP3A4 Perpetrators Using Physiologically Based Pharmacokinetic Modeling.

Based on ibrutinib pharmacokinetics and potential sensitivity towards CYP3A4-mediated drug-drug interactions (DDIs), a physiologically based pharmacokinetic approach was developed to mechanistically describe DDI with various CYP3A4 perpetrators in healthy men under fasting conditions. These models were verified using clinical data for ketoconazole (strong CYP3A4 inhibitor) and used to prospectively predict and confirm the inducing effect of rifampin (strong CYP3A4 inducer); DDIs with mild (fluvoxamine, azithromycin) and moderate inhibitors (diltiazem, voriconazole, clarithromycin, itraconazole, erythromycin), and moderate (efavirenz) and strong CYP3A4 inducers (carbamazepine), were also predicted. Ketoconazole increased ibrutinib area under the curve (AUC) by 24-fold, while rifampin decreased ibrutinib AUC by 10-fold; coadministration of ibrutinib with strong inhibitors or inducers should be avoided. The ibrutinib dose should be reduced to 140 mg (quarter of maximal prescribed dose) when coadministered with moderate CYP3A4 inhibitors so that exposures remain within observed ranges at therapeutic doses. Thus, dose recommendations for CYP3A4 perpetrator use during ibrutinib treatment were developed and approved for labeling.

Simple apparatus for serial blood sampling in rodents permitting simultaneous measurement of locomotor activity as illustrated with cocaine.

A simple device has been developed for serial venous blood sampling which permits the simultaneous measurement of locomotor activity in the freely moving rat. The device can be easily constructed from routine laboratory material and it does not interfere with the light beams used to measure locomotor activity. The device, in conjunction with an activity cage, has been applied to the combined pharmacokinetic and pharmacodynamic modeling of cocaine. The relationship between the locomotor activity following a single short iv infusion of cocaine (5 mg/kg) and cocaine plasma concentrations can be adequately described by the Sigmoid-E(max) model. Further, the relationship between activity and time can be described by the same model coupled with an effect compartment. These results suggest the applicability of the device in facilitating pharmacokinetic/pharmacodynamic modeling of drugs that affect locomotor activity.

Quantitative determination of cocaine, cocaethylene (ethylcocaine), and metabolites in plasma and urine by high-performance liquid chromatography.

A sensitive HPLC assay was developed to quantitate the relatively nonpolar compounds cocaine, cocaethylene (ethylcocaine), norcocaine, and norcocaethylene, as well as the relatively polar metabolites benzoylecgonine and benzoylnorecgonine, in rat plasma and urine. The assay in plasma employed two successive liquid-liquid extractions and separate injections onto two different columns (C8 and C18) to separate and quantitate the relatively polar and nonpolar compounds. In urine, the procedure employed a liquid-liquid extraction followed by solid-phase extraction (C18 light-load cartridges) and two separate injections as for plasma. The UV absorbance of the effluent was monitored at 235 nm. Linear standard curves were obtained over the concentration ranges of 25 to 1000 ng/mL and 5 to 250 ng/mL in plasma and urine, respectively. The inter- and intraday coefficients of variation for the assay of all compounds in plasma and urine were < 18% at low concentrations (12.5-100 ng/mL) and < 12% at high concentrations (125-250 ng/mL). There was no degradation of these compounds during the extraction procedure or during 2 months of storage at -20 degrees C. The quantitation limits for the assay of the relatively nonpolar and polar compounds in plasma were 25 (2.5 ng in 0.1 mL) and 50 ng/mL (5 ng in 0.1 mL), respectively. For the assay in urine, the quantitation limits were 5 (2.5 ng in 0.5 mL) and 12.5 ng/mL (6.25 ng in 0.5 mL) for the assay of the relatively nonpolar and polar compounds, respectively. The methods have been applied to quantitate those compounds in rat plasma.

Characterization of the properties of cocaine in blood: blood clearance, blood to plasma ratio, and plasma protein binding.

As part of a study to examine cocaine disposition and interaction with ethanol, it was necessary to characterize various properties of cocaine in the blood of the experimental animal. All studies were conducted using blood from healthy adult male Sprague-Dawley rats. Cocaine was incubated in whole blood at 37 degrees C at concentrations of 500-4000 ng/mL. The apparent first-order rate constant for cocaine loss was independent of concentration. Blood clearance, calculated assuming blood volume to be 64 mL/kg, was estimated to be 0.056 +/- 0.003 mL/(min.kg); a value considerably smaller than estimates of systemic clearance. The addition of NaF increased the rate of loss to form benzoylecgonine, as a result of increased chemical degradation and as a consequence of increased pH (to pH 8.0 over 30 h). This NaF-enhanced degradation was abolished when NaF was added to blood buffered to pH 7.4. Ethanol had no influence on cocaine degradation, and there was no evidence of cocaethylene (ethylcocaine) formation. Blood to plasma ratios determined in spiked and authentic samples were constant (0.94-1.05 and 0.99-1.03, respectively) and independent of concentration (100-1500 ng/mL) and pH (7.2-7.6). This ratio was not influenced by NaF or ethanol. The unbound fraction (fu) of cocaine determined in spiked plasma varied from 0.62 to 0.63 over the concentration range (75-2025 ng/mL). Ethanol had no effect on binding. The values for fu determined from authentic blood samples taken from rats dosed intravenously with cocaine (10 mg/kg) ranged from 0.67 to 0.69 (over the concentration range 300-1500 ng/mL). Cocaine plasma protein binding was independent of concentration but depended upon plasma pH (fu, 0.765 and 0.486, at pHs 7.0 and 7.8, respectively.

Pharmacokinetics and interspecies scaling of a novel VEGF receptor inhibitor, SU5416.

The pharmacokinetics and allometric relationships of SU5416, a novel small anti-angiogenesis agent, were studied. The pharmacokinetics of SU5416 were examined in mice, rats, dogs, and cancer patients. The in-vitro intrinsic clearance (CLint) was estimated from the in-vitro metabolism study in mouse, rat, dog, monkey and human liver microsomes. The parameters of interest were correlated across species as a function of bodyweight using an allometric approach. The steady-state volume of distribution (Vd(ss)), plasma clearance (CLs), and CLint of SU5416 were well correlated across species. The exponent of the allometric relationship (b) of the corresponding parameters was 0.92, 0.80 and 0.66, respectively. The elimination half-life (t1/2) was consistent across species and independent of bodyweight. The prediction of CLs, Vd(ss), CLint, and t1/2 in humans using the data from mouse, rat, and dog, and monkey (for CLint) was reasonably good (within 4-fold of the observed values). However, an improved prediction (within 2-fold of the observed values) of the corresponding parameters in humans was obtained when extrapolation from only the rodent data was performed, suggesting that the rodent data are sufficient for the scale-up of SU5416 pharmacokinetic parameters in humans. Using allometry, it was possible to achieve reasonable predictions of the pharmacokinetic parameters of SU5416 in cancer patients with various solid tumours.

Separation and quantitation of the enantiomers of methamphetamine and its metabolites in urine by HPLC: precolumn derivatization and fluorescence detection.

To study the disposition kinetics of methamphetamine (MAP), we have developed a sensitive high-performance liquid chromatographic (HPLC) assay to quantitate the enantiomers of MAP and its major metabolites, amphetamine (AP), p-hydroxymethamphetamine (p-OH-MAP), and p-hydroxyamphetamine (p-OH-AP), the latter two of which are hydroxylated metabolites, in rat urine. To determine conjugated hydroxylated metabolites, urine samples were treated with beta-glucuronidase. Both hydrolyzed and nonhydrolyzed p-OH-MAP and p-OH-AP were extracted into ethyl acetate and back extracted with 0.05M HCl. To determine MAP and AP, urine samples were extracted with benzene, followed by back extraction into 0.05M HCl. The acid layer was collected, and to it was added (-)-1-(9-fluorenyl)ethyl chloroformate (FLEC) for the derivatization of MAP and its metabolites. Derivatization was allowed to proceed for 24 h at room temperature. The derivatized products were separated on a C18 column with a mobile phase consisting of acetate buffer (pH 3.6)-acetonitrile-tetrahydrofuran. Quantitation was achieved using a fluorescence detector at an excitation wavelength of 265 nm and an emission wavelength of 330 nm. Linear standard curves were obtained over the concentration range of 5-100 ng/mL. The interday and intraday coefficients of variation for the assay for all eight enantiomers at 10 and 75 ng/mL were less than 13%. The detection limit was 5 ng/mL or 0.5 ng on-column.(ABSTRACT TRUNCATED AT 250 WORDS)

Disposition kinetics of d- and l-amphetamine following intravenous administration of racemic amphetamine to rats.

Amphetamine (AP), a chiral drug, displays stereoselective differences in biological action. The effect of stereochemistry on the disposition kinetics of the enantiomers has not been thoroughly studied. We examined the disposition kinetics of AP in rats using a sensitive precolumn derivatization HPLC method that can separate the enantiomers of AP and its metabolites. Male Sprague-Dawley rats were given a short intravenous infusion of racemic AP (15 mg/kg). The systemic and renal clearances, steady-state volume of distribution, and terminal half-life for l-AP were (mean +/- SD), respectively: 65.6 +/- 9.25 ml/min.kg; 15.4 +/- 2.55 ml/min.kg; 4.33 +/- 0.71 liters/kg; and 0.96 +/- 0.13 hr. The corresponding values for d-AP were: 50.8 +/- 6.88 ml/min.kg; 12.5 +/- 2.02 ml/min.kg; 3.84 +/- 0.55 liters/kg; and 1.12 +/- 0.09 hr. There are statistically significant differences between the enantiomers in all pharmacokinetic parameters except half-life. About 40% of the l-AP dose was excreted in urine as l-p-hydroxyamphetamine and 24% as intact drug. The corresponding values for d-AP were 32% and 26%, respectively. p-Hydroxyamphetamine was primarily excreted into urine as the conjugated form. These data indicate stereoselective differences in the pharmacokinetics of the enantiomers of AP after administration of racemic drug in the rat.