Indravadan H. Patel

Valproic acid dosage and plasma protein binding and clearance

Valproic acid clearance was determined in six normal subjects during a single‐dose (250‐mg) study and multiple‐dose experiments of 500, 1,000, and 1,500 mg/day. Eight consecutive oral doses were taken at 12‐hr intervals at each dosing level. Valproate levels and protein binding were determined at steady state. Clearance declined 20% from 8.33 ± 2.44 to 6.67 ± 1.25 ml/hr/kg between the single‐dose and the 500‐mg/day steps (p = 0.05). Clearance was unchanged between the 500‐ and 1,000‐mg/day steps despite a 44% increase in mean free fraction (0.0703 ± 0.0381 vs 0.1011 ± 0.0438, p < 0.05), implying a balanced opposing decline in intrinsic clearance (from 89.2 ± 71.0 to 72.0 ± 20.8 ml/hr/kg; p = 0.025). In four subjects completing the 1,500‐mg/day step, clearance increased from 6.76 ± 1.48 ml/hr/kg (1,000 mg/day) to 8.20 ± 1.62 ml/hr/kg, corresponding to a further increase in free fraction. Free fraction varied within a single dosing interval (%SD = 11% to 49%). The apparent dose‐related decline in intrinsic clearance suggests autoinhibition or saturation of metabolism.

Pharmacokinetics of plasma enfuvirtide after subcutaneous administration to patientswith human immunodeficiency virus: Inverse Gaussian density absorption and 2-compartment disposition*

Enfuvirtide (T‐20) is the first of a novel class of human immunodeficiency virus (HIV) drugs that block gp41‐mediated viral fusion to host cells. The objectives of this study were to develop a structural pharmacokinetic model that would adequately characterize the absorption and disposition of enfuvirtide pharmacokinetics after both intravenous and subcutaneous administration and to evaluate the dose proportionality of enfuvirtide pharmacokinetic parameters at a subcutaneous dose higher than that currently used in phase III studies.

Simultaneous modeling of the pharmacokinetics and pharmacodynamics of midazolam and diazepam.

The pharmacokinetics and pharmacodynamics of midazolam and diazepam were compared after intravenous infusions of 0.03 and 0.07 mg/kg midazolam and 0.1 and 0.2 mg/kg diazepam on four separate occasions in 12 healthy male subjects in a randomized four‐way crossover design. The Digit Symbol Substitution Test (DSST) was used as a measure of drug effect. Subjects performed three practice tests before dosing to account for any effects caused by familiarization (“learning curve”) with the testing procedure. Pharmacokinetic and pharmacodynamic data were simultaneously fitted to a semiparametric model. In this model, a pharmacokinetic model related dose to plasma concentrations, a link model related plasma concentrations to the concentration at the effect site, and a pharmacodynamic model related the effect site concentration to the observed effect. The plasma—effect site equilibrium half‐life was approximately 2½ times longer for midazolam than for diazepam, which is in good agreement with previously published data. Based on the estimated effect site concentration at which half of the maximal effect was reached, midazolam had approximately a sixfold greater intrinsic potency than diazepam. This difference in potency was also observed in a previous study that used transformed electroencephalographic (EEG) data to assess pharmacodynamic activity. The findings reported here with a clinically relevant pharmacodynamic marker (DSST) confirm the utility of surrogate drug effect measures such as EEG. This work also shows the feasibility of conducting pharmacokinetic pharmacodynamic analysis during the drug development process.

Phenobarbital‐valproic acid interaction

The effect of subchronic valproate treatment on the single‐dose kinetics of phenobarbital was investigated in 6 normal subjects. The study consisted of 2 drug treatments assigned through randomized crossover design. In one treatment subjects received a 60‐mg dose of phenobarbital orally. In the other, subjects received 250 mg valproic acid orally twice daily for 14 consecutive days and a 60‐mg dose of phenobarbital orally on day 4. Nineteen plasma samples (over 12 days) and two 48‐hr urine samples were collected during each treatment. Plasma and urine phenobarbital levels were determined by gas chromatograph interfaced with mass spectrometer in a chemical ionization mode (GLC/CI/MS) and plasma valproic acid levels by GLC. Valproic acid induced several changes in the elimination parameters of phenobarbital: (1) phenobarbital half‐life rose from 96 to 142 hr (p = 0.006); (2) plasma clearance fell from 4.2 to 3.0 ml/hr/kg (p = 0.009); (3) renal clearance was unchanged, and metabolic clearance fell from 3.3 to 2.0 mg/hr/kg (p = 0.006); and (4) the fraction of dose excreted unchanged rose from 0.22 to 0.33 (p = 0.015), and the fraction of dose metabolized fell from 0.78 to 0.67 (p = 0.015). The findings indicate that valproic acid inhibits phenobarbital metabolism.

Protein Binding of Valproic Acid in the Presence of Elevated Free Fatty Acids in Patient and Normal Human Serum

Summary: Two recent reports indicate that free fatty acids (FFA) can decrease the binding of valproic acid (VPA) to plasma proteins. Since FFA can increase in several physiological and clinical situations, it becomes necessary to define the relationship between elevated FFA and VPA free fraction. This study was conducted in two phases. In the first phase, serum was obtained from nine patients who were receiving Intralipid®. A base‐line sample was taken when therapy was absent. A second sample was taken during or immediately after Intralipid® therapy. Protein binding VPA (equilibrium dialysis) and FFA levels were determined on each sample. FFA increased in eight patients; the median increase was 80.1%. VPA free fractions increased in six of the eight patients showing an increase in FFA; the median increase in free fraction was 18.7%. Rank correlation between FFA differences and free fraction differences indicated a significant positive linear correlation (r= 0.800). In the second phase, oleic acid was added to normal serum in five different concentrations (500‐5,000 μEq/liter) with VPA at 100 μg/ml. Free fraction of VPA increased continuously from a blank value of 9.6% (±1.4) to 44.1% (±2.5) with increasing concentrations of oleic acid.

Assessment of drug-drug interaction potential of enfuvirtide in human immunodeficiency virus type 1-infected patients

Enfuvirtide is the first drug to block human immunodeficiency virus type 1 (HIV‐1) glycoprotein 41–mediated viral fusion to host cells. This study investigated whether enfuvirtide can influence the activities of cytochrome P450 (CYP) enzymes in HIV‐1–infected patients.

Efficacy testing of valproic acid compared to ethosuximide in monkey model: II. Seizure, EEG, and diurnal variations.

The efficacy of valproic acid (VPA) compared t o ethosuximide (ESM) was tested in an alumina‐gel monkey model (N= 12) in‐ strumented with indwelling catheters (for drug infusion and blood sampling) and an EEG plug (for sleep staging and spike recording). The results indicated that VPA was effective in the plasma level range 50‐ 150μg/ml in terms of seizure frequency, duration, and severity, as well as EEG sharp activity. However, focal seizures were not attenuated until the higher plasma concentrations were reached. At the lower plasma levels there was an immediate but transitory effect of VPA on seizure frequency which lasted 2 days only, and a more permanent decrease in seizure frequency at the higher plasma levels. Even with a constant‐rate intravenous infusion of VPA, the mean diurnal fluctuations in plasma drug levels was 36% and considerably more in some animals. EEG bursts per minute tended to correlate inversely with VPA circadian, plasma concentration increases at night and positively with ESM concentration in several monkeys. ESM exacerbated seizure frequency to some extent. The changes in seizure frequency during administration of either drug appeared not to change the basic patterns of time of occurreixe of the seizures for each animal, but rather augmented o r attenuated epileptic activity within those patterns. The findings indicate the ability of this model to quantify correlative processes in efficacy evaluation which lead to a greater understanding of mechanisms of action of antiepileptic drugs.

Diurnal Oscillations in Plasma Protein Binding of Valproic Acid

Summary: In view of the observed variation of valproic acid (VPA) free fraction (fp) during a dosing interval and the competitive binding effect of free fatty acids (FFA) in vitro, this study was designed to address the existence of diurnal variations in the fp of VPA. Six subjects were hospitalized at 7 a.m. for 25 h, and plasma samples were collected every 2 h. The protocol was repeated in 4 of the 6 subjects one week later. In vitro binding of VPA (100 μg/ml) was determined by equilibrium dialysis (14C‐VPA), and FFAs were assayed col‐orimetrically. Phenytoin (PHT) binding was also determined for comparison. VPA fp ranged rom 8.10 ± 1.16 to 9.63 ± 1.54. Intrasubject variability was also measured by the ratio of maximum to minimum fp. values (fpmax/fp min) over 24 h: This ratio ranged from 1.30 to 1.68 (mean 4pL %SD = 1.51 ± 7.7%, n = 10). For PHT, fp ranged from 10.88 ± 0.50 to 12.39 ± 1.07, and fp max from 1.09 to 1.31 (1.17 ± 5.1%, n = 10). The fp max was observed between 2 and 6 a.m. in 7 out of 10 cases for VPA and 5 out of 10 cases for PHT. FFA levels, although in the normal range, varied two‐to fourfold within 24 h. A significant correlation was observed between mean FFA levels at each sampling time and the corresponding fp values for VPA (p < 0.001), but not for PHT.

The influence of free fatty acids on valproic acid plasma protein binding during fasting in normal humans

SummaryThe effect of physiologic variations of free fatty acid levels on in vivo valproic acid plasma protein binding was studied in 6 healthy adult subjects. 14 blood samples were taken during a 12-h dosing interval at steady state while in a fed condition and also during a 27 h fast. Free fraction and total valproate concentration were determined by equilibrium dialysis and GLC, respectively. Free fatty acid levels were determined from both fresh samples and samples incubated at 37°C for 12 h, the latter in order to simulate equilibrium dialysis conditions. Fasting resulted in increased serum free fatty acid levels in all subjects, ranging from 34–182% (p<0.01). Incubation also caused free fatty acid levels to rise, more so in fed samples (50–87%,p<0.01) than in fasting samples (10–50%,p<0.01). Fasting resulted in a 9% increase in the mean free fraction for all subjects combined (p<0.01). Regression analysis of 180 sets of values for free fraction, total valproate concentration and free fatty acid level suggested that valproate concentration accounts for 17% and free fatty acid level for 37% of the variation in free fraction. Mean clearance was unchanged by fasting despite an increased free fraction suggesting decreased intrinsic clearance (i.e. decreased metabolism) of valproate under these conditions.

Diurnal Variation of Valproic Acid Plasma Levels and Day‐Night Reversal in Monkey

Four normal monkeys each equipped with an EEG plug and two indwelling catheters for drug infusion and sampling, respectively, were administered valproic acid (VPA) before and after a 12‐hr light, 12‐hr dark phase shift. Before day‐night reversal, diurnal oscillations of VPA plasma levels under steady‐state intravenous constant‐rate infusions were 30–50%, with maximum concentrations during the dark phase of the cycle. After reversal, maximum VPA plasma concentrations tended to follow the dark phase shift. The correlation was not perfect, nor was the sleep cycle completely reversed since the animals slept less after the phase shift. Possible mechanisms of the diurnal plasma level fluctuations and the importance of oscillations of this magnitude to clinical drug regimens are discussed.