Biopharmaceutical factors in seizure control and drug toxicity

Problems related to bioequivalence and bioavailability for four antiepileptic drugs (AEDs) are reviewed. Bioequivalence and bioavailability of AEDs can be affected by many factors, including physicochemical characteristics of the agent, the dosage form, and physiological condition of the patient. In 1988, breakthrough seizures prompted an FDA investigation of one company's generic carbamazepine tablets. Results indicated that the manufacturer had changed its source of carbamazepine, which led to a wide range of dissolution characteristics for different lots of tablets. In two separate studies, clonazepam was shown to be more rapidly absorbed in patients with a normal gastric pH than in those with a higher-than-normal gastric pH. With phenytoin, which exhibits nonlinear pharmacokinetics, differences in the rate and extent of absorption can adversely affect the bioavailability of this agent. Finally, the bioequivalence of generic primidone was contested in an adolescent girl who appeared to experience more frequent seizures with a generic product than with a trade formulation. The effectiveness of a drug depends on complex interactions involving the drug, the drug product formulation, and the patient. Minimizing variability in the absorption process is particularly important with AEDs, because of their narrow therapeutic range.

Characteristics of vascular smooth muscle in the maternal resistance circulation during pregnancy in the rat

OBJECTIVE

Our purpose was to determine if pregnancy results in a decrease in arterial sensitivity to receptor-independent stimuli and a change in vascular smooth muscle membrane potential.

STUDY DESIGN

Mesenteric resistance arteries from late pregnant (n = 19) and age-matched virgin control (n = 20) Sprague-Dawley rats were studied in a pressurized arteriograph system or isometric myograph.

RESULTS

Arteries from pregnant rats were less sensitive to membrane depolarization by K+ than were those from nonpregnant rats (mean effective concentration that produced a 50% response 49 vs 39 mmol/L, pregnant vs nonpregnant, p < 0.05). Arterial basal tone and the myogenic response to increasing pressure steps were also reduced in arteries from pregnant rats compared with nonpregnant controls. The vascular smooth muscle membrane of the arteries from the pregnant rats was hyperpolarized compared with that from the control rats (-64 mV from pregnant rats vs -57 mV from nonpregnant rats, p < 0.01). This was associated with a reduction in vasomotion in the arteries from the pregnant rats (10% for pregnant rats vs 45% from nonpregnant rats, p < 0.01).

CONCLUSION

Pregnancy results in alterations of the vascular smooth muscle, including changes in the regulation of membrane potential and a reduced sensitivity to receptor-independent stimuli.

The effect of gastric pH on the absorption of controlled-release theophylline dosage forms in humans

The bioavailability of three marketed controlled-release dosage forms and a reference solution of theophylline was studied in eight subjects with normal gastric fluid acidity and seven subjects who were achlorhydric. Gastric pH was monitored with a Heidelberg capsule. One of the controlled-release dosage forms dissolved more rapidly in vitro when exposed to acid conditions, one dissolved more rapidly in pH 7.5 media, and the third dissolved at a rate independent of pH. Using a crossover design, each subject received each dosage form twice. Blood was sampled for up to 47 hr after each dose, and serum was assayed for theophylline by HPLC. The product which dissolved more rapidly under acid conditions in vitro exhibited a 3 hr longer Tmax in the achlorhydrics compared to the normal subjects. The product which dissolved more rapidly in the pH 7.5 media exhibited a relatively higher AUC(0-infinity) in the achlorhydric subjects than in normal subjects after the AUC data were normalized for clearance differences between the two subject groups. The in vivo bioavailability of these dosage forms could be related to the in vitro dissolution characteristics for some parameters. However, with the exception of the mean Tmax values, the mean bioavailability parameters differed by less than 20% between the two subjects groups.

Hepatitis B surface antigen screening in a nonindigent population

The recommendation for universal screening of all pregnant women for hepatitis B surface antigen (HBsAg) is based on data from publicly funded hospitals. We retrospectively reviewed screening results of 2,696 mothers who delivered between May 1989 and April 1990. Our population was 85% privately funded. Screening for HBsAg was positive in 0.07%, negative in 80.4% and not done in 19.6%. All positive screens would have been identified by screening only patients with recognized risk factors. We conclude that the sensitivity of identifiable risk factors to detect HBsAg carriers may be high in some populations. We speculate that universal screening in these populations is not cost efficient.

The bioinequivalence of carbamazepine tablets with a history of clinical failures

The bioavailability of three lots of a generic 200-mg carbamazepine tablet, which had been withdrawn from the market, was compared to the bioavailability of one lot of the innovator product in 24 healthy volunteers. Fifty-three lots of the generic product had been recalled by the manufacturer because of concerns over reports of clinical failures for several of the lots. The three generic lots tested in this study exhibited a wide range of bioavailability, as well as large differences in the in vitro dissolution rates. The mean maximum carbamazepine plasma concentrations for two of the generic lots were only 61-74% that of the innovator product, while the third lot was 142% of the innovator. The mean areas under the plasma concentration-time curve for the three generic lots ranged from 60 to 113% that of the innovator product. The results clearly indicate a significant difference in the rate and extent of absorption of the generic products compared to the innovator, as well as among the generic lots. A good relationship was found between the in vivo parameters and the in vitro dissolution results for the four dosage forms.

Composition and mechanics of mesenteric resistance arteries from pregnant rats

We tested the hypothesis that the systemic resistance vasculature of the rat is remodeled during pregnancy as evidenced by significant alterations in the passive mechanical properties and extracellular matrix proteins in mesenteric arteries. Mechanical characteristics were determined for arteries from 20-day pregnant rats (n = 6) and age-matched controls (n = 5). Lumen diameter and wall thickness were measured in pressurized arteries (250-microns diameter) using a dimension analyzing system. Distensibility (the relative change in diameter per unit change in pressure) was less in the arteries from the pregnant rats (P less than 0.01). The calculated stress-strain relationships and elastic moduli indicated that the arteries were less stiff by late gestation (P less than 0.05). Ultramicro amino acid analysis and radioimmunoassay were used to measure hydroxyproline, desmosine, and leucine as indicators of collagen, elastin, and total protein, respectively, in similar-sized arteries. Hydroxyproline/leucine (index of collagen) and desmosine/leucine (elastin concentration) decreased 19 and 15% by late gestation (P less than 0.05). The significant alterations in passive mechanics and in extracellular protein content support the concept that arterial wall remodeling in the peripheral vasculature may be one component of the cardiovascular adaptations during pregnancy.

The effect of raising gastric pH with ranitidine on the absorption and elimination of theophylline from a sustained-release theophylline tablet

Prior to evaluating the effect of ranitidine on theophylline absorption from a sustained-release theophylline tablet, the effect of ranitidine on the time course of gastric pH in 12 healthy subjects was evaluated with an encapsulated radio-telemetry device (Heidelberg capsule). Gastric pH was measured hourly from 7 AM to 1 PM prior to beginning ranitidine treatment at 2 PM (150 mg every 4 hr for eight doses). The next day, pH was again measured hourly from 7 AM to 7 PM. Subjects fasted overnight and remained fasted until lunch at 11 AM. Prior to ranitidine treatment, the mean morning gastric pH remained between 1.5 and 2.2. After lunch, the pH increased to 2.2-2.3. During ranitidine treatment the mean morning gastric pH measurements were 5.5 to 5.8, decreasing after lunch to 3.1 by 4 PM and increasing to 3.9 at 7 PM. One week later the subjects participated in a three-way crossover theophylline bioavailability study receiving at weekly intervals, single doses at 7 AM of (a) 5 x 100-mg immediate-release tablets, (b) 2 x 300-mg sustained-release theophylline tablets, and (c) 2 x 300-mg sustained-release theophylline tablets after ranitidine pretreatment of 150 mg every 4 hr beginning at 2 PM the previous day. The increase in gastric pH with ranitidine had no effect (P greater than 0.05) on the rate and extent of absorption or on the elimination rate of theophylline.

Circadian rhythms in theophylline disposition: simulations and observations in the dog

A constant-rate iv infusion of aminophylline approaching steady state in four dogs was used to investigate a circadian rhythm in theophylline disposition. A significant circadian rhythm (p less than 0.05, group cosinor analysis) was observed for serum theophylline concentrations and for the urinary excretion rates of theophylline and 3-methylxanthine. The phase relationship of the observed circadian rhythms in urinary pH to circadian changes in serum theophylline concentrations were supportive of a circadian rhythm in renal clearance of theophylline. Computer simulations indicated that circadian changes in serum theophylline concentration and urinary excretion of theophylline and its metabolites could be accounted for by a circadian rhythm in theophylline renal clearance and metabolism to 3-methylxanthine. A circadian rhythm in theophylline volume of distribution could not be totally dismissed as potentially having some effect on disposition. The timing of food intake is hypothesized to play a role in synchronizing the observed rhythm in serum theophylline concentrations.

Gas chromatographic-mass spectrometric analysis of plasma nifedipine

A gas chromatographic-mass spectrometric assay is described for the determination of plasma nifedipine using nitrendipine as an internal standard. Chromatographic separation was performed on a dimethylsilicone capillary column, and detection was by selected ion monitoring of electron impact-generated base peak ions. The lower limit of quantifiable detection of nifedipine was 2 ng/ml. The method was applied to plasma samples obtained from a human subject who had been dosed with a 10-mg nifedipine capsule every 8 h for eight doses.

In vitro and in vivo evaluation of seven 50 mg and 100 mg nitrofurantoin tablets

Four 50 mg and three 100 mg marketed nitrofurantoin tablets were studied in 14 healthy male subjects. Urine was collected 1, 2, 3, 4, 6, 8, 12, and 23 h after each dose, and nitrofurantoin was assayed by HPLC. The in vitro dissolution of the tablets was determined using USP Apparatus 1 and 2, with 0.1 N hydrochloric acid and pH 7.2 buffer as the dissolution fluids. One of the 50 mg tablets was more rapidly and completely absorbed than the other six products. The incidence of side-effects for this product was as low or lower than the other products. It was determined that the use of the USP Apparatus 1, at 100 rev min-1, with sampling of the pH 7.2 fluid at 30 min, provided for the best overall relationship between the urinary excretion and in vitro dissolution.

The absorption of sustained-release methylphenidate formulations compared to an immediate-release formulation

A crossover study in 18 subjects evaluated the plasma concentration-time profile of two different 20 mg sustained-release (SR) methylphenidate (MPH) tablets administered before breakfast, compared to a 10 mg immediate-release (IR) tablet administered before breakfast and again 5 h later, before lunch. Plasma MPH concentrations were determined using a sensitive and precise gas chromatography-mass spectrometry method, incorporating a deuterated internal standard. The mean peak MPH concentration was 6.4 ng ml-1 for the IR product versus 4.6 ng ml-1 and 4.8 ng ml-1 for the two SR formulations. Peak concentrations occurred at 3.3 h after dosing with the SR products, compared to 1.5 h after the first dose of the IR product. The extent of absorption for the three products, as determined from areas under the plasma concentration-time curves, were within 5 per cent of each other. There was no significant difference in rate or extent of absorption between the two SR formulations.

Gas chromatographic-mass spectrometric analysis of plasma oxybutynin using a deuterated internal standard

A gas chromatographic-mass spectrometric method is described for the quantitative analysis of plasma oxybutynin. Deuterated oxybutynin served as the internal standard and its synthesis is described. Chromatographic separation on a methylsilicone capillary column avoided the thermal decomposition observed using a packed column. Electron-impact ionization and selected-ion monitoring of the alpha-cleavage fragments of drug and internal standard permitted quantitation of oxybutynin down to 0.25 ng/ml of plasma. At the 2 ng/ml level the accuracy and precision are 4 and 10%, respectively, and improved at higher drug concentrations. Application of the method to the pharmacokinetics of oral oxybutynin in man demonstrated rapid absorption and elimination of the drug.

Relative bioavailability of a controlled-release albuterol formulation for twice-daily use

A two-way-crossover bioavailability study was completed with 15 healthy male volunteers to evaluate the relative bioavailability of an orally administered controlled-release (CR) formulation of albuterol as compared to the immediate-release (IR) formulation of albuterol. The CR formulation of albuterol used in this study was based mechanistically on the elementary osmotic pump. Each subject received one 8 mg CR tablet every 12 h for 8 consecutive days and one 4 mg IR tablet every 6 h for 8 consecutive days, with 7 days separating each phase. During day 8 of dosing, hourly blood samples (0-12 h) were collected after the morning dose and three additional samples were obtained 16, 20 and 24 h following this dose. Plasma concentrations were measured using a gas chromatography-mass spectrometry procedure. No statistically significant differences were observed in mean steady-state values for Cmax, Cmin, AUC(0-12 h) and peak-to-trough fluctuations (PTF) in comparing the two dosage formulations. Mean steady state Tmax values were 2.6 and 6.0 h for the TR and CR formulations, respectively. It was concluded that twice daily administration of the controlled-release formulation of albuterol provides an alternative to four times daily administration of conventional (immediate-release) albuterol tablets.

Effect of pH on the in vitro dissolution and in vivo absorption of controlled-release theophylline in dogs

Dogs were used to examine the effect of elevated gastric pH on the absorption of controlled-released theophylline dosage forms with pH-dependent dissolution. In vitro studies showed that a controlled-release theophylline tablet dissolved more rapidly if it was initially exposed to an acidic media. In contrast, a controlled-release theophylline beaded capsule was slightly more rapidly dissolved in the absence of an initial exposure to an acidic media. Gastric pH was increased from 0.5-2.5 to 4.5-7.0 in four dogs by using 150-mg ranitidine HCl tablets, administered every 3 h, to induce an achlorhydric condition. Gastric pH was monitored using a Heidelberg capsule. Ranitidine was shown to have no apparent effect on the absorption or clearance of theophylline administered to the dogs as an oral liquid. The mean area under the concentration-time curve to infinity (AUCinf) for the controlled-release theophylline tablet was 21% greater (p less than 0.05) when administered to the four dogs without ranitidine treatment, compared with that following dosing with ranitidine. In contrast, the controlled-release beaded capsule exhibited a 10% greater AUCinf when ranitidine was given concomitantly. In general, ranitidine-induced changes in the in vivo absorption rate parameters for both dosage forms were opposite to those predicted from the in vitro dissolution rates. The results of this study demonstrated that the extent of theophylline absorption from controlled-release dosage forms, in control dogs and dogs with ranitidine-induced achlorhydria, corresponds to the pH-dependent in vitro dissolution properties of the products.

Circadian rhythm in theophylline disposition during a constant-rate intravenous infusion of aminophylline in the dog

The disposition of theophylline in three dogs was determined during a 48-h constant-rate intravenous infusion of aminophylline. A systematic fluctuation in serum theophylline concentrations was observed over a 24-h period, which appeared to be characteristic of a circadian rhythm. Neither assay variability nor fluctuations in the infusion pump rate could account for the observed variations in the serum concentrations. It was concluded that the changes in the theophylline concentrations were the result of a circadian rhythm in theophylline disposition.

Influence of a standard meal on the absorption of a controlled release pseudoephedrine suspension

The influence of a standard meal on the extent and rate of absorption of pseudoephedrine from a liquid controlled release (CR) formulation (Pennkinetic System) was studied in 16 normal male volunteers. Equivalent single doses of an immediate release reference syrup (IR-Sudafed) and the CR suspension were each studied under both fasted and postprandial conditions. AUC results showed no significant influence of food or formulation, indicating that the CR formulation was absorbed to the same extent as the IR product under all conditions. CMAX and TMAX tabulations under fasted conditions indicated that the CR preparation peaked at a lower level at a later time. Food diminished IR CMAXs but the CR suspension's CMAXs and TMAXs were apparently unaffected by food. This study indicates that this CR pseudoephedrine suspension releases drug independently of food intake.

Dialyzability and pharmacokinetics of indomethacin in adult patients with end-stage renal disease

Indomethacin, a nonsteroidal antiinflammatory drug, has been shown to be effective for the treatment of pericarditis in chronic hemodialysis patients. Data regarding the dialyzability of indomethacin in these patients, however, is lacking. The aim of this study, therefore, was to evaluate (1) the dialyzability, and (2) the absorption and elimination kinetics of indomethacin, using six stable anephric adult patients who were maintained on chronic hemodialysis and were receiving indomethacin for the management of their uremic pericarditis. The results from this study demonstrate that indomethacin is dialyzable, but not to an appreciable extent. The mean predialysis and postdialysis indomethacin plasma levels were 3.4 and 1.6 micrograms/ml, respectively. The mean total amount of indomethacin removed by five hours of hemodialysis was 19.6 percent of the single dose of indomethacin 100 mg po. However, mean Cpmax, tmax, t1/2, and AUC0-infinity during and in the absence of hemodialysis were 5.4 and 5.4 micrograms/ml (not statistically significant [NS]), 1.9 and 2.0 h (NS), 6.1 and 5.3 h (NS), and 30.9 and 35.7 micrograms h ml-1 (NS), respectively. Based on these findings, it can be concluded that although indomethacin is dialyzable, no dosage adjustment is required in patients receiving indomethacin for the management of their uremic pericarditis when undergoing maintenance hemodialysis.

Bioavailability of seven furosemide tablets in man

A seven-way crossover study was conducted in 14 healthy male volunteers to evaluate the relative bioavailability of seven different marketed 40 mg furosemide tablets. Each dose was administered as a single tablet after an overnight fast, and blood samples were obtained for 16 hours. Plasma was assayed by HPLC. There were no statistically significant differences among the seven products for the mean peak concentration (1.01-1.29 micrograms/ml), mean time of peak (1.2-2.1 h) or mean area under the plasma concentration-time curves, which differed by less than 14 per cent. However, one product exhibited greater intersubject variability, and on this basis was considered inequivalent to the other six products. Furosemide is a potent and widely used diuretic. Currently the United States Food and Drug Administration (USFDA) has granted approval to at least twelve manufacturers of 40 mg furosemide tablets, based in part on bioavailability data obtained in human subjects. In addition, the USFDA has granted an 'AB' therapeutic equivalence evaluation to each of these products, which is understood by many to indicate the therapeutic equivalence and interchangeability of these products. The objective of the present investigation was to directly compare the bioavailability of seven 40 mg furosemide tablet products which had previously been approved by the Food and Drug Administration.

Determination of free disopyramide plasma concentrations using ultrafiltration and enzyme multiplied immunoassay

Disopyramide is an antiarrhythmic drug that exhibits nonlinear binding to plasma proteins. As a result, the total body clearance increases with increasing total drug plasma concentration. A rapid and sensitive method for the determination of free (unbound) disopyramide plasma concentrations is described. The procedure employs an ultrafiltration system (Centrifree), which can be used for basic drugs, along with an enzyme multiplied immunoassay system (EMIT) for the measurement of free disopyramide concentrations in plasma water filtrate. The EMIT method was adapted to permit measurement of disopyramide in plasma over a concentration range of 0.02-1.2 micrograms/ml. Plasma storage at -20 degrees C, filtration volume, or the presence of buffer and mono-N-dealkylated metabolite in plasma did not affect the binding determinations. There was no loss of drug during the filtration process. A good correspondence was found between the EMIT assay and a high performance liquid chromatography method, when applied to plasma samples obtained from a human subject who had ingested disopyramide. Furthermore, the extent of protein binding determined by the ultrafiltration system and by equilibrium dialysis were in good agreement. The binding of disopyramide in fortified human plasma decreased from 64 to 52% over a total drug concentration range of 1-5 micrograms/ml.

Bioavailability of dyphylline and dyphylline-guaifenesin tablets in humans

A six-way-crossover bioavailability study was conducted with twelve healthy male volunteers to evaluate the relative bioavailability of three tablet formulations containing dyphylline and three tablet formulations containing dyphylline-guaifenesin. Each subject was administered two tablets of each product with greater than or equal to 3 d separating each dose. Blood samples were obtained just prior to each dose and at 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, 4.0, 6.0, 8.0, and 10.0 h following each dose. An HPLC method was used to assay dyphylline in the serum. The mean tmax ranged from 0.6 to 1.0 h for the six products. The mean values for Cmax differed by 29%, and the AUC values differed by less than 8%. It was noted that the dyphylline-guaifenesin products exhibited a lower bioavailability than the products which only contained dyphylline. It was concluded that the three combination products were bioequivalent, as were the three dyphylline products.

Bioequivalence, dose-proportionality, and pharmacokinetics of naltrexone after oral administration

Healthy male volunteers (N = 24) participated in a four-way crossover study to compare the rate and extent of absorption of naltrexone after administration of 50 mg tablets as 50, 100, and 200 mg doses and a 10 mg/ml reference syrup. A high-performance liquid chromatographic method was employed to measure naltrexone and 6-beta-naltrexol in plasma and urine. Compared to the syrup, the 50 mg tablets were absorbed more slowly but equally well. There was excellent linearity between the administered dose and the area under the plasma concentration-time profile, as well as total urinary recovery of both drug and metabolite. The mean half-lives for naltrexone and beta-naltrexol were approximately 4 and 12 hours, respectively. The fraction of drug reaching the systemic circulation was estimated to be 5% of the administered dose because of extensive first-pass metabolism. Less than 1% of the dose was excreted in the urine as naltrexone after 48 hours, while 25% was recovered as unconjugated beta-naltrexol. The renal clearance of naltrexone and beta-naltrexol was approximately 127 ml/min and 283 ml/min, respectively. The total systemic clearance for naltrexone was approximately 94 L/hr.

Absorption of phenobarbital from tablets and elixir

A three-way crossover study was conducted with 24 healthy male volunteers to determine the relative bioavailability of four different 100-mg phenobarbital tablets compared with a reference elixir. Each subject received two of the tablets and the elixir at 30-d intervals. Blood samples were collected daily for 19 d after each dose. Plasma phenobarbital concentrations achieved with the five dosage forms differed by less than 20% within 2-3 h after dosing. The extent of absorption for all dosage forms, as determined from area under the plasma concentration-time profiles, were within 10% of each other. The peak plasma concentration was the greatest and the time to peak concentration was the shortest for the elixir. One of the tablets exhibited a time to peak concentration of 8.6 h, which was significantly longer than any of the other dosage forms. The time to peak concentration correlated with the percent of drug dissolved in 60 min, as determined in 0.1 M HCl, using the USP XX paddle method at 50 rpm.