Pharmacokinetics of recombinant human interferon-alpha 2a combined with 5-fluorouracil in patients with advanced colorectal carcinoma

We evaluated the pharmacokinetics of 5-fluorouracil (5-FU) combined with recombinant human interferon (IFN)-alpha 2a in 10 previously untreated patients with advanced colorectal carcinoma. 5-FU was administered as a continuous i.v. infusion, 750 mg/m2/day for 5 days during week 1. One s.c. injection of IFN-alpha 2a, 9 x 10(6) IU, was administered during week 2. Beginning with week 3, a continuous i.v. infusion of 5-FU 750 mg/m2/day for 5 days was administered in combination with IFN-alpha 2a, 9 x 10(6) IU s.c. three times per week. The combination of 5-FU and IFN-alpha 2a was continued every other week until either 3 months after complete remission or tumor progression. No grade 4 toxicity was observed. Granulocytopenia (two patients), leukopenia (one patient), thrombocytopenia (one patient), stomatitis (two patients), fatigue (one patient) and hand-foot syndrome (one patient) were the major (grade 3) toxic reactions encountered. Overall, one complete and six partial responses were noted. The results of the paired t-test showed no statistically significant differences between the means of the two treatments, 5-FU and 5-FU plus IFN-alpha 2a, with respect to the steady-state plasma concentration, area under the concentration-time curve, total body clearance, or steady-state volume of distribution of 5-FU, or the serum concentration of IFN. We conclude that 5-FU and IFN-alpha 2a do not interact pharmacokinetically at the doses and schedules in the regimen studied.

An evaluation of the integration of pharmacokinetic and pharmacodynamic principles in clinical drug development. Experience within Hoffmann La Roche

The integration of pharmacokinetic and pharmacodynamic principles into drug development has been proposed as a way of making it more rational and efficient. The use of these principles in drug development to make scientific and strategic decisions is defined as the 'pharmacokinetic-pharmacodynamic guided approach to drug development'. The objectives of this survey were: (i) to assess the extent the pharmacokinetic-pharmacodynamic guided approach to drug development has been used in a large multinational pharmaceutical company: (ii) to evaluate the impact of pharmacokinetic and/or pharmacodynamic results on clinical drug development; and (iii) to identify factors which prevented the full application of the pharmacokinetic-pharmacodynamic guided approach. This was done by looking at 18 projects in the current development portfolio at Hoffman La Roche and evaluating the use of this approach by interviewing the responsible clinical pharmacologist using a standardised questionnaire. (i) Benefits from using the pharmacokinetic-pharmacodynamic guided approach were reported in every project, independent of development phase and therapeutic area. This approach was more extensively used in the recent projects. The selection of dosages in clinical studies was found to be the most important application of pharmacokinetic-pharmacodynamic results in terms of an impact on drug development. (ii) Time savings, up to several months, could be quantified in 8 projects during the entry-into-man studies and in 6 projects during the phase II or III studies. In 4 projects, 1 clinical study was avoided. (iii) The most important scientific factor preventing the full application of the approach was the lack of knowledge on the predictive value of the pharmacodynamic or surrogate marker for effect (6 projects). The results of the survey have shown that the use of the pharmacokinetic-pharmacodynamic guided approach has contributed to making clinical drug development more rational and more efficient. Opportunities to apply the pharmacokinetic-pharmacodynamic approach should be identified in each project and a project specific strategy for the pharmacokinetic-pharmacodynamic guided approach should be defined during phase 0 of drug development.

Negligible systemic absorption of topical isotretinoin cream: implications for teratogenicity

The objective of the study was to assess the extent of systemic exposure of retinoic acid metabolites after excessive application of 0.1% isotretinoin cream in patients with photodamaged skin. This was a single-center, open-label, noncomparative, multiple-dose study of isotretinoin cream. Eighteen female patients with photodamaged skin received a 10 g topical application of isotretinoin cream once daily to a surface area of approximately 2,300 cm2 for 42 days. The patients were not allowed to have high vitamin A-containing foods, vitamin A supplements, or concomitant medications during the entire study period. Plasma levels of four retinoic acids (isotretinoin, tretinoin, 4-oxo-isotretinoin, and 4-oxo-tretinoin) were evaluated after 42 days of isotretinoin application and compared with baseline (pretreatment) levels. The mean area under the curve (AUC) in plasma increased by 48% (+/-SE 9.2) and 77% (+/-13) from the 24-hour pretreatment baseline level for isotretinoin and 4-oxo-isotretinoin, respectively, after treatment with excessive amounts of isotretinoin cream, suggesting systemic absorption of isotretinoin cream. This increase in systemic exposure of retinoic acids was less than that reported earlier after the U.S. recommended daily allowance of 5,000 i.u. of vitamin A supplementation (isotretinoin 141 +/- 19% and 4-oxo-isotretinoin 171 +/- 27%). The minimal systemic availability of isotretinoin cream compared with the U.S. recommended daily allowance for vitamin A supplements provides reasonable evidence for lack of its potential teratogenic risk.

Antimalarial activity in crude extracts of Malawian medicinal plants

Aqueous and organic fractions from Cassia abbreviata, Senna petersiana (both Caesalpiniaceae) and Azanza garckeana (Malvaceae) were tested for in-vitro antimalarial activity against the multi-drug-resistant, Vietnam-Smith strain of Plasmodium falciparum; VI/S. Both roots and leaves from these Malawian medicinal plants were investigated. High activity, with a median inhibitory concentration < 3 micrograms/ml, was seen in the organic fractions of C. abbreviata and S. petersiana, the two species most commonly cited by traditional healers in an ethnobotanical investigation of Malawian antimalarials. Extracts of A. garckeana showed weaker activity. Biologically active compounds have thus been detected within species of the family Caesalpiniaceae. Ethnobotanical investigation appears to be useful in identifying plants with antimalarial activity.

Use of an indirect pharmacodynamic stimulation model of MX protein induction to compare in vivo activity of interferon alfa-2a and a polyethylene glycol-modified derivative in healthy subjects

Interferon alfa-2a was chemically modified by the covalent attachment of a polyethylene glycol (PEG) moiety to enhance its circulating half-life and to reduce its immunogenicity. A comparative evaluation of the pharmacokinetics of the PEG-modified interferon alfa-2a showed a greater than twofold increase in the circulating half-life as a result of this chemical modification. An indirect physiologic response model was developed to characterize the time course of the MX protein response after subcutaneous administration of single ascending doses of either interferon alfa-2a or PEG-interferon alfa-2a in healthy volunteers. Analysis of the pharmacokinetic-pharmacodynamic relationship suggested that the PEG-modified interferon alfa-2a could not be administered less than twice weekly and therefore offered little therapeutic advantage over its unmodified counterpart, which is administered three times weekly. These results were consistent with findings in phase II trials. This study substantiates the usefulness of pharmacodynamic modeling as a tool for the development of dose recommendations and for the early selection of drug candidates in the drug development process.

The effect of orlistat, an inhibitor of dietary fat absorption, on the pharmacokinetics of beta-carotene in healthy volunteers

To assess the influence of orlistat, a lipase inhibitor, on the absorption of beta-carotene, an open-label, parallel, placebo-controlled, randomized, two-way crossover study was performed in 48 healthy volunteers between the ages of 19 and 58 years. Each subject received a single oral dose of 0, 30, 60, or 120 mg beta-carotene (12 subjects per dose level) on the fourth day of treatment with orlistat (120 mg) or placebo 3 times a day for 6 days. The treatments were separated by a washout period of at least 5 weeks. Serial blood samples were collected before and at appropriate intervals after administration of beta-carotene to determine plasma concentrations of unchanged beta-carotene. Short-term (3 to 6 days) treatment with orlistat did not alter endogenous profiles of beta-carotene in plasma. When beta-carotene was given during orlistat treatment, its absorption was reduced by approximately one-third. This reduction was consistent for all three dose levels of beta-carotene studied; however, the results for the 30-mg dose level were subject to greater variability, particularly for area under the concentration-time curve (AUC). It was concluded that two thirds of a supplemental dose of beta-carotene will be absorbed during orlistat treatment; this may be sufficient to achieve physiologic levels of beta-carotene with an appropriate dose of beta-carotene, should supplementation be needed in obese patients who have developed beta-carotene deficiency during therapy with orlistat.

Review of limited systemic absorption of orlistat, a lipase inhibitor, in healthy human volunteers

Orlistat, a lipase inhibitor, acts locally in the gastrointestinal tract. Systemic absorption is not required for its efficacy, but knowledge of the extent of its systemic absorption is important for its safe use in obese patients, the intended target population. Pharmacokinetic screening was carried out by monitoring plasma concentrations of unchanged orlistat in 25 phase 1 studies (including two mass balance studies) in normal and obese healthy volunteers. The results of these studies indicate an extremely low degree of systemic absorption for orlistat when administered with a hypocaloric, well-balanced diet with 20% to 30% of calories derived from fat (50-80 gm). To further characterize the pharmacokinetics and excretion pathways of orlistat, two mass balance studies using 14C-labeled orlistat were conducted. After oral dosing of radiolabeled orlistat with a fatty meal (28-30 gm fat), almost the entire dose was recovered from fecal samples; little was found in plasma and urine. It is concluded that systemic absorption of orlistat is negligible; at a clinically efficacious dose level, orlistat is unlikely to produce systemic lipase inhibition.

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 2 1/2 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.

The influence of orlistat on the pharmacokinetics and pharmacodynamics of glyburide in healthy volunteers

To assess the influence of orlistat on the pharmacokinetics and pharmacodynamics (the blood glucose-lowering effect) of glyburide, an open-label, placebo-controlled, randomized, two-way crossover study was done in 12 healthy male volunteers. Each subject received single 5-mg oral doses of glyburide (Micronase; The Upjohn Company, Kalamazoo, MI) on the fifth day of treatment with placebo (treatment A) and 80-mg orlistat (treatment B) three times a day for 4 1/3 days; the two treatments were separated by a five-day washout period. Serial blood samples were collected before and at appropriate intervals after each glyburide dose to determine plasma concentrations and blood glucose levels. Values of Cmax and AUC of glyburide showed an equality of the two treatments by the analysis of variance. There was an apparent correlation between blood glucose level and the logarithm of plasma glyburide concentration; this relationship appeared to not be altered when glyburide was administered with orlistat. In conclusion, orlistat administered at doses of 80-mg three times daily does not significantly alter the pharmacokinetics and blood glucose-lowering effect of a single 5-mg oral dose of glyburide in healthy volunteers.

The pharmacokinetic-pharmacodynamic (Digit Symbol Substitution Test) relationship of flumazenil in a midazolam steady-state model in healthy volunteers

To characterize the plasma concentration-effect relationship of flumazenil in the presence of a predefined midazolam level, a double-blind, placebo-controlled, randomized two-way crossover study was conducted in nine healthy male subjects. After reaching a criterion level of midazolam-induced depression of the Digit Symbol Substitution Test (DSST), volunteers received a dose of flumazenil (1.0 mg) or placebo over 1 minute, with the infusion of midazolam continued. Blood samples were collected, simultaneously with the DSST assessment, at predetermined intervals and were assayed for flumazenil and/or midazolam plasma concentrations. Pharmacokinetic-pharmacodynamic modeling techniques were used to estimate the equilibration rate constant (keo) between plasma concentration and effect for flumazenil; a sigmoidal maximum-effect model was used to relate the DSST score to the flumazenil plasma concentration. Flumazenil exhibited a rapid onset (the half-life of equilibration between drug concentration in the blood and drug effect was 3.3 minutes) and short duration of action (the flumazenil plasma concentration causing half-maximal effect was 7.4 ng/ml, which was reached about 1 hour after dosing). The results of this study also show the competitive nature of flumazenil as a midazolam antagonist.

Retrospective population-based analysis of the dose-response (fecal fat excretion) relationship of orlistat in normal and obese volunteers

Orlistat, an inhibitor of gastrointestinal lipases, limits the absorption of ingested fat and could become a potential treatment for obesity. This analysis was performed to elucidate the relationship between orlistat dose and intensity of inhibition of dietary fat absorption (assessed by measuring fecal fat excretion). In 11 phase I double-blind, placebo-controlled, parallel-group randomized studies, a total of 171 subjects received oral daily doses that ranged from 30 to 1200 mg orlistat or matching placebo three times a day for 9 to 10 days. The results of the daily mean fecal fat excretion percentage (relative to ingested fat) were correlated to the orlistat daily dose. A simple maximum-effect model that included a basal value was used to fit the dose-response relationship for all evaluable subjects. The mean maximum percentage of ingested fat excreted in the feces was approximately 32% during orlistat administration compared with 5% during placebo administration. The orlistat daily dose that produced 50% of the maximum effect was 98 mg/day. The model-fitting suggests the existence of a steep portion of the dose-response curve up to approximately 400 mg/day, with a subsequent tendency to plateau at higher doses. Such an analysis was instrumental in identifying appropriate doses to be used in therapeutic trials for weight loss in obese patients.

High clearance of (S)-warfarin in a warfarin-resistant subject

A 30 year old black male required a 60 mg daily dose of warfarin to elicit a therapeutic anticoagulant response (normal warfarin dose 2.5-10 mg day-1; maximum 15 mg day-1). Hereditary warfarin resistance was suspected after compliance, diet, concurrent medication and any gastrointestinal disorder were eliminated as contributory causes. The disposition of vitamin K and vitamin K epoxide was examined in the propositus, his two sisters and 13 control black male subjects. Each subject was given an i.v. bolus dose (5 mg) of vitamin K prior to and after 2 weeks of warfarin therapy (5 mg day-1). The oral clearances of (S)- and (R)-warfarin were also measured in each subject during the last day of warfarin therapy. The mean (+/- s.d.) systemic clearance of vitamin K was similar in all subjects before (114 +/- 35 ml min-1) and after (112 +/- 40 ml min-1) warfarin therapy. The mean (+/- s.d.) AUC value for vitamin K epoxide was increased by warfarin treatment (6.5 +/- 5.4 micrograms ml-1 min before and 139 +/- 78 micrograms ml-1 min after) in all subjects. In the propositus, the oral clearance of (S)-warfarin (14.5 ml min-1) and the clearance ratio for (S)/(R)warfarin (2.6) differed by more than 7 standard deviations from the control group (4.3 +/- 1.1 ml min-1 and 1.2 +/- 0.2, respectively). In one sister of the propositus, the stereoselective disposition of warfarin was comparable with that of her brother ((S)-warfarin clearance = 16.2 ml min-1; and (S)/(R)-warfarin clearance ratio = 2.7).

Pharmacokinetics of Ro 23-9424, a dual-action cephalosporin, in animals

Ro 23-9424 is a dual-action cephalosporin with an aminothiazolylmethoxyimino-type side chain at the 7 position and fleroxacin esterified at the 3' position. The new compound has broad and potent antibacterial activity in vitro and in vivo, reflecting contributions from both the beta-lactam moiety and the quinolone moiety. In animals, the ester bond potentially could be hydrolyzed enzymatically or nonenzymatically, to yield the active metabolites desacetylcefotaxime and fleroxacin. The extent to which Ro 23-9424 acts in vivo as a true dual-action cephalosporin, or acts as a combination of active metabolites, is therefore a function of its pharmacokinetic properties. To investigate these properties, Ro 23-9424 was administered as a single intravenous dose of 20 mg/kg of body weight to mice, rats, dogs, and baboons. Timed plasma samples were assayed by an ion-paired high-pressure liquid chromatography method that allowed detection of both intact Ro 23-9424 and fleroxacin. The pharmacokinetic parameters of Ro 23-9424 were similar to published results for cefotaxime, while concentrations of fleroxacin in plasma were low and fairly constant (about 1 to 3 micrograms/ml) in all species, suggesting that excretion of the intact molecule is a major route of elimination for Ro 23-9424, as it is for cefotaxime. For technical reasons, urinary recovery of Ro 23-9424 was not quantitated, but intact Ro 23-9424 was found in high concentrations (greater than 400 micrograms/ml) in mouse urine aspirated directly from the bladder. In all species, low concentrations of free fleroxacin in plasma persisted after the elimination of Ro 23-9424 was complete, but fleroxacin did not accumulate unduly in a 14-day multiple-dose experiment in baboons. Thus, it seems likely that the activity seen in vivo is primarily due to intact Ro 23-9424, although the low levels of free fleroxacin may also have some therapeutic significance.

Synthesis and mechanisms of decomposition of some cephalosporin prodrugs

The delta-3 and delta-2 methyl esters of cefazolin were synthesized. The kinetics and mechanisms of degradation of the methyl esters and the delta-3 and delta-2 isomers of pivaloyloxymethyl prodrug esters of the new cephalosporin ceftetrame (Ro 19-5247) were investigated in buffer systems and in human plasma in vitro. The major hydrolytic products of all the delta-3 and delta-2 esters were the inactive delta-2 cephalosporin free acids. The following reaction scheme describes the in vitro hydrolysis of these compounds: [formula: see text]. In addition, there was evidence of opening of the beta-lactam ring to form cephalosporoic acid when the methyl ester of cefazolin was studied in human plasma and in the presence of penicillinase. For the methyl esters, the processes represented by k12, k21, and k20 were operative in buffers; in human plasma, the processes represented by k12, k21, and k20 were operative in addition to cephalosporoic acid formation. For the isomers of the cephalosporin prodrug ester Ro 19-5248 only k12 and k20 were operative in buffers; in human plasma all pathways were operative and there was no evidence of cephalosporoic acid formation. In all cases, the processes represented by k12, k21, and k20 were subject to general and/or specific base catalysis.

Lack of probenecid effect on nonrenal excretion of ceftriaxone in anephric patients

Probenecid has been shown to decrease renal and biliary excretion of organic acids. In a randomized crossover study, the effect of coadministered probenecid on nonrenal excretion of ceftriaxone was studied in six functionally anephric patients in whom ceftriaxone is eliminated exclusively by nonrenal or presumably by biliary excretion. Each patient received 0.5 g IV ceftriaxone without and with probenecid (0.5 g at 10 and 2 hours prior to ceftriaxone and 0.5 g q12h X 3 doses post ceftriaxone). Serial blood samples were collected over 48 hours and plasma analyzed for ceftriaxone by high performance liquid chromatography (HPLC). Pharmacokinetic analysis was based on a model-independent approach. Probenecid did not significantly affect the disposition of ceftriaxone in this study, thus suggesting that nonrenal excretion of ceftriaxone is not inhibited by probenecid.

The pharmacokinetics of midazolam in patients with congestive heart failure

The disposition of midazolam was investigated in six patients with congestive heart failure (CHF) and six age- and weight-matched healthy subjects by administering two single doses of the drug (3.75 mg i.v. and 7.5 mg p.o.) separated by 1 week. Serial blood samples were collected for 24 h after each dose and plasma was assayed for midazolam by GC-MS. In the CHF patients, the elimination half-life was prolonged (4 to 4.5 vs less than 3 h), the systemic clearance was lowered (376 vs 551 ml min-1) and the peak plasma drug concentration after the p.o. dose was higher (76 vs 42 ng ml-1). The systemic availability (45 vs 41%), the steady state volume of distribution (111 vs 108 l) and the time of peak plasma drug concentration after the p.o. dose (0.9 vs 0.9 h) were unchanged. The predominant effect of CHF was on the clearance of midazolam which was decreased by 30%. The drug was well tolerated and did not cause any adverse effects.

Increased volume of distribution prolongs midazolam half-life

It has recently been shown by several investigators that the half-life (t1/2) of midazolam is prolonged (greater than 7 h) in a small proportion of the population. One group has inferred that this subpopulation represents a group of slow metabolizers of midazolam to alpha-OH-midazolam. Others disagree and postulate that there is an increase in the volume of distribution (V) resulting in a prolonged t1/2. This controversy led us to report experience from 90 subjects and patients where t1/2, V, and clearance (CL) were determined by both model-dependent and -independent pharmacokinetic analysis. We found a 5.6% (5 of 90) incidence of prolonged t1/2, similar to that previously reported. V was clearly increased without a decrease in CL in the five subjects with prolonged t1/2. Thus, the prolonged t1/2 is secondary to an increase in V and not a result of alterations in CL and metabolism.

Multiple intravenous dose pharmacokinetic study of carumonam in healthy subjects

Steady-state pharmacokinetics of carumonam were investigated in twelve healthy adult male volunteers after 20 min intravenous infusions of three consecutive carumonam dosage regimens: 1 g every 8 h (3 g/day) from 0-72 h, 2 g every 8 h (6 g/day) from 88-120 h and 2 g every 6 h (8 g/day) from 132-216 h. Serial plasma samples were collected after the first dose of the first regimen and the last dose of all three regimens and were analysed for carumonam by a specific HPLC method. The overall mean maximal plasma concentrations were 108 and 211 mg/l at the end of infusion of 1 and 2 g, respectively. The steady-state pharmacokinetic parameters and plasma concentration-time profiles of carumonam when adjusted for dose were similar for the three regimens. The overall terminal elimination half-life was 1.4 h (range 1.1-1.8 h), the apparent volume of distribution at steady state reached 11.5 l (range 8.7-15.5 l) and the total systemic clearance amounted to 118 ml/min (range 83-158 ml/min). Considering the frequency of dosing relative to the elimination half-life, accumulation of the drug in plasma was not expected and none was found from any of the three regimens. Carumonam was well tolerated up to 8 g/day and exhibited dose-independent pharmacokinetics which were not altered upon multiple dosing.

Potencies of ceftriaxone and cefotaxime in a single chamber pharmacokinetic system simulating their in vivo half-lives

Ceftriaxone and cefotaxime are third-generation cephalosporins with similar in vitro potencies and spectra. However, previous studies have shown that ceftriaxone had superior in vivo activity (mouse PD50 values greater than or equal to 2-fold lower) compared to cefotaxime in 23 of 46 tested enterobacteriaceae. This superior activity was thought to be due to ceftriaxone's 5- to 8-fold longer half-life. The relationship between half-life (ceftriax-one 6 h, cefotaxime 1 h) and potency was examined by following bacterial kill curves in a single chamber, open-ended perfusion model over an 8-hour period. Both antibiotics were compared for efficacy at both half-lives against four gram-negative bacteria. For two of the bacterial strains antibiotic potency differences in the perfusion model were determined largely by pharmacokinetics. For the other two strains intrinsic bacterial and antibiotic properties were of prime importance.

Effects of age, gender and oral contraceptives on intramuscular midazolam pharmacokinetics

The effects of age, gender and low-dose (50 mcg or less) oral contraceptive steroids (OCS) on the pharmacokinetics of midazolam were evaluated following a single 7.5 mg intramuscular dose to five groups (8/group) of healthy volunteers consisting of young males, young females, elderly males, elderly females, and young female users of oral contraceptives. Blood samples were collected at specified times over a 24-hour period, and plasma concentrations of midazolam and its 1-hydroxymethyl metabolite were determined by a GC-EC assay. Midazolam was rapidly absorbed following intramuscular administration to the different groups. Comparison of young men vs elderly men, young women vs elderly women, young men vs young women, elderly men vs elderly women, and young women OCS-users vs young women non-OCS users indicated no substantial differences in the pharmacokinetic profile of midazolam between groups except for the comparison between the young and elderly men groups. The rate of elimination of midazolam was significantly slower in the elderly males compared to the young men. The pharmacokinetic profile of 1-hydroxymethyl midazolam paralleled that of the parent compound. This is to be expected since this metabolite exhibits formation rate-limited kinetics. Except for one subject who reported hives and itching, considered to be remotely related to test drug, no other adverse experiences or laboratory abnormalities were reported.

Influence of food on bioavailability of amdinocillin pivoxil

The bioavailability of amdinocillin was not altered when amdinocillin pivoxil was ingested 1 h before a standard breakfast, and it increased by 20% when amdinocillin pivoxil was ingested with or 1 h after a standard breakfast. Amdinocillin pivoxil would be convenient for patients since it may be taken with or without food.

Ceftriaxone pharmacokinetics following multiple intramuscular dosing

The steady-state pharmacokinetics and tolerance of ceftriaxone after multiple i.m. doses of 0.5 and 1 g q12 h for 3.5 days were investigated in 12 healthy, adult volunteers. Ceftriaxone was rapidly absorbed after i.m. administration with mean peak times ranging from 1.3 to 1.9 h. Steady-state plasma concentrations were apparent after the third dose of both dosage regimens, with trough plasma concentrations of 24 +/- 6 and 39 +/- 8 micrograms/ml (mean +/- SD) after the 0.5 and 1 g q12 h regimens, respectively. Multiple i.m. administrations of ceftriaxone did not alter its elimination half-life; however, small increases were observed in the plasma clearance and volume of distribution at the 1-g regimen. These increases were attributed to the non-linear binding of ceftriaxone to human plasma proteins, and are therapeutically unimportant. Ceftriaxone was well tolerated and serious or lasting adverse reactions were not encountered in the study.

Influence of food on midazolam absorption

The influence of food on the absorption of midazolam, a new benzodiazepine derivative, was investigated in 18 healthy volunteers in a four-way, randomized, crossover study with a one-week washout period between treatments. Single 15-mg oral doses of midazolam were administered one hour before, with, and one hour after a standard meal as well as under fasting conditions (control). Following serial blood sampling over the next 24-hour period, midazolam plasma concentrations were determined by gas chromatography and mass spectrometry for pharmacokinetic evaluation. The maximum plasma concentration (Cmax), time of maximum concentration (tmax), lag time prior to absorption (tlag), area under the plasma concentration-time curve (AUC), and elimination rate constant of midazolam and 1-hydroxymethylmidazolam were determined. Significant changes in these parameters were not found when midazolam was taken one hour before or with a meal as compared with the control condition. Significant changes in the Cmax, tmax, and AUC parameters for both midazolam and its metabolite were seen when midazolam was ingested one hour after a meal: There was a delayed and reduced rate of absorption as well as a small reduction in the extent of absorption. Thus, ingestion of midazolam within one hour after a meal may result in a delay in the onset of the pharmacologic effect. These changes may be of some clinical significance in that they may potentially delay the onset of sleep.

Pharmacokinetics of intravenous amdinocillin in healthy subjects and patients with renal insufficiency

Five healthy volunteers and 31 patients with various degrees of renal impairment received a 10-mg/kg intravenous dose of amdinocillin by infusion over 15 min to establish the disposition profile of the drug in plasma and urine. Both clearance from plasma and elimination rate constant showed a linear relationship with creatinine clearance. It was noted that in subjects with creatinine clearances of greater than 50 ml/min, the elimination half-life remained relatively constant; however, as the creatinine clearance decreased from 50 to 5 ml/min, there was a progressive rise in the elimination half-life. Despite the removal of the drug by hemodialysis (32 to 72% of the dose), concentrations of amdinocillin in plasma remained in the therapeutic range. In patients undergoing peritoneal dialysis, less than 4.0% of the infused dose was removed by dialysis during the hourly exchanges over a 14- to 18-h period. Although the clearance from plasma and the half-life of amdinocillin were altered up to fourfold in patients with creatinine clearances of less than 15 ml/min, the amdinocillin dosage per se may not need to be reduced for these patients if the frequency of dosing is reduced from six to three or four times daily. This is based on drug accumulation estimates of 56% from a regimen of 10 mg/kg every 8 h in these patients as compared with less than 10% from a regimen of 10 mg/kg every 4 h in subjects with normal renal function. In addition, supplemental doses may not be necessary during or at the end of hemodialysis for patients undergoing hemodialysis.

Dose dependent pharmacokinetics of midazolam

The pharmacokinetics of midazolam and 1-hydroxymethylmidazolam were investigated following oral administration of 7.5, 15 and 30 mg doses of midazolam in solution to 12 healthy subjects. Compared to the 7.5 mg dose, the Cmax and AUC parameters of both midazolam and 1-hydroxymethylmidazolam increased proportionally after the 15 mg dose and more than proportionally after the 30 mg dose. The t1/2 for midazolam remained relatively constant between the 7.5 and 15 mg doses whereas it increased slightly but significantly after the 30 mg dose. These data indicated that the pharmacokinetics of midazolam and 1-hydroxymethylmidazolam were linear between the 7.5 and 15 mg oral dose range. However, after the 30 mg dose, the systemic availability of midazolam and the AUC for 1-hydroxymethylmidazolam appeared to be greater than that anticipated from the lower doses, possibly due to saturation of midazolam first-pass metabolism. This is not expected to have any clinical significance under the conditions of therapeutic use.

Pharmacokinetic profile of ceftriaxone in man

In human subjects, ceftriaxone exhibits an exceptionally long elimination half-life (5.8 to 8.7 hours) and a small degree of nonlinearity in its pharmacokinetics which can be ignored in its clinical applications. Thirty-three to 67 percent of a dose is excreted in the urine as unchanged drug, and the remainder is secreted in the bile and ultimately is found in the feces as microbiologically inactive compounds. Ceftriaxone is rapidly and completely absorbed following intramuscular administration. Multiple dosing of ceftriaxone with doses ranging from 0.5 to 2 g at 12- or 24-hour intervals by intravenous and intramuscular routes resulted in 15 to 36 percent accumulation of ceftriaxone in plasma and no change in its elimination half-life. The volume of distribution and the plasma clearance of ceftriaxone in pediatric patients were threefold greater than those in adults, and ceftriaxone penetrated the inflamed meninges of infants and children with bacterial meningitis. Small changes in the pharmacokinetics of ceftriaxone in elderly subjects or patients with renal or hepatic dysfunction are such that dose adjustments should not be necessary with a ceftriaxone dosage up to 2 g per day. Ceftriaxone was not removed to any significant extent from plasma by hemodialysis. In a small percentage of patients, on dialysis, the elimination rate of ceftriaxone was significantly reduced, suggesting that plasma concentrations of ceftriaxone should be monitored in these patients to determine if dosage adjustments are necessary.

Ceftriaxone pharmacokinetics in patients with various degrees of renal impairment

The effects of renal impairment on the pharmacokinetics of ceftriaxone in humans were examined after intravenous infusion of a 1-g dose over 15 min to 30 renally impaired patients. The study included 12 dialysis patients and 18 patients with severe, moderate, or mild renal impairment. Plasma and, where appropriate, urine and dialysate samples were collected at predetermined times and analyzed for ceftriaxone by high-pressure liquid chromatography. The elimination half-life (group mean ranged from 11.7 to 17.3 h) and plasma clearance (group mean ranged from 529 to 705 ml/h) did not correlate linearly with creatinine clearance. The renal clearance and fraction of dose excreted unchanged in urine were related linearly, however weakly, with creatinine clearance. Ceftriaxone was not removed from plasma to a significant extent during hemodialysis. The half-life was prolonged twofold, the plasma clearance was lowered less than 50%, and the volume of distribution was relatively unchanged in renally impaired patients compared with young or elderly healthy subjects with normal renal function at an equivalent dose. Since these changes are moderate, adjustment in the dosage regimen of ceftriaxone for patients with impaired renal function should not be necessary when ceftriaxone dosage is 2 g or less per day (2 g every 24 h or 1 g every 12 h). It was reported that the elimination half-life of ceftriaxone is substantially prolonged in a small percentage of patients with end-stage renal disease maintained on hemodialysis. Therefore, plasma concentrations of ceftriaxone should be monitored in dialysis patients to determine whether dosage adjustments are necessary.

Age and ceftriaxone kinetics

One gram ceftriaxone was injected at a constant rate in an intravenous infusion over 30 min to eight elderly subjects (mean age, 70.5 yr) and eight young subjects (mean age, 28.9 yr); the latter served as body weight-matched controls. Plasma and urine samples were collected in serial order for 48 hr and assayed for unchanged drug. Selected plasma samples were subjected to protein binding determinations by equilibrium dialysis. Statistical comparison of data for the old and young indicated no significant changes in means of (1) maximum plasma concentration (140 and 133 micrograms/ml); (2) elimination rate constant (0.078 and 0.093 hr-1) and elimination t1/2 (8.9 and 7.5 hr); (3) apparent volume of distribution (10.69 and 11.01 l); (4) plasma clearance (833 and 1023 ml/hr); (5) nonrenal clearance (515 and 606 ml/hr); and (6) percent dose excreted unchanged in urine (39.6 and 41.4). There was, however, a significant decrease in the renal clearance (318 and 416 ml/hr) and a significant increase in the plasma free fractions (0.157 and 0.136 at 100 micrograms/ml and 0.146 and 0.114 at 60 to 70 micrograms/ml) of ceftriaxone in elderly subjects. The 24% decrease in renal clearance in the elderly subjects corresponded to the 19% decrease in their creatinine clearance. Since the age-related changes in kinetics were relatively small, it is concluded that dosage adjustment is probably not necessary for elderly subjects requiring ceftriaxone.

Pharmacokinetics of ceftriaxone in pediatric patients with meningitis

Pharmacokinetics of ceftriaxone after a single dose of 50 or 75 mg/kg were determined in 30 pediatric patients with bacterial meningitis. Data for doses of 50 and 75 mg/kg, respectively, were as follows (mean +/- standard deviation): maximum plasma concentrations, 230 +/- 64 and 295 +/- 76 mug/ml; elimination rate constant, 0.14 +/- 0.06 and 0.14 +/- 0.04 h(-1); harmonic elimination half-life, 5.8 +/- 2.8 and 5.4 +/- 2.1 h; plasma clearance, 51 +/- 24 and 55 +/- 18 ml/h per kg; volume of distribution, 382 +/- 129 and 387 +/- 56 ml/kg; mean concentration in cerebrospinal fluid 1 to 6 h after infusion, 5.4 and 6.4 mug/ml. A dosage schedule of 50 mg/kg every 12 h for bacterial meningitis caused by susceptible organisms is suggested for pediatric patients over 7 days of age.

Single-dose ceftriaxone pharmacokinetics in pediatric patients with central nervous system infections

Ceftriaxone has greater in vitro and in vivo efficacy against many common bacteria than other third-generation cephalosporins. Single-dose ceftriaxone pharmacokinetics were studied in 17 patients, aged 0.6 to 52 months, with infections of the central nervous system. Patients received a randomized dose of 50 or 75 mg/kg ceftriaxone intravenously over 5 minutes on the second to fifth day of illness. Serial blood samples were collected over 24 hours in all patients, and cerebrospinal fluid (CSF) was obtained 1 to 4.5 hours after injection. Ceftriaxone mean peak plasma concentrations, determined by high-power liquid chromatography, were 267 and 184 microgram/ml for the 75 and 50 mg/kg dosage groups, respectively. The harmonic mean elimination half-life was 4.2 hours, and the mean percent drug penetrance into CSF was 4.8 +/- 3.5%. Of CSF studies evaluated, the glucose concentration was correlated most closely (inversely) with CSF penetration of ceftriaxone. Individual CSF concentrations of ceftriaxone exceeded the minimal inhibitory concentrations of the respective bacteria causing infection by 480 to 5,600 times. Ceftriaxone may be useful in the treatment of serious pediatric infections, including meningitis.

Pharmacokinetic characteristics of intravenous ceftriaxone in normal adults

The multiple-dose pharmacokinetics and tolerance of intravenous ceftriaxone were investigated in 44 adults with normal renal function. Doses of 0.5, 1.0, and 2.0 g every 12 h and 2 g every 24 h were administered intravenously at a constant rate over 30 min. Plasma and urine samples were collected after the first (day 1) and last (day 4) dose and assayed for ceftriaxone by high-pressure liquid chromatography. Considering all four doses, mean peak plasma concentrations ranged from 79 to 255 micrograms/ml on day 1 and from 101 to 280 micrograms/ml on day 4. Trough concentrations at 12 h on day 1 were 15 to 45 micrograms/ml and 20 to 59 micrograms/ml on day 4. After a dose regimen of 2 g every 24 h, trough levels were still in the clinically therapeutic range (13 to 15 microgram/ml). The mean beta-phase t1/2 was markedly long (6.3 to 6.9 h) and was independent of dose. The fraction of dose excreted unchanged in the urine (0.33 to 0.44) indicated a substantial nonrenal mechanism of elimination. The plasma clearance ranged between 1,002 and 1,449 ml/h, and renal clearance ranged from 353 to 529 ml/h. The apparent volume of distribution varied from 9.2 to 13.5 liters. The dose-related increases in calculated Vd and Clp could be attributed to concentration-dependent plasma protein binding because of a larger free fraction of drug at higher concentrations. The drug was well tolerated, and no significant clinical or laboratory abnormalities were noted.

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

The 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 degrees 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 less than 0.01). Incubation also caused free fatty acid levels to rise, more so in fed samples (50-87%, p less than 0.01) than in fasting samples (10-50%, p less than 0.01). Fasting resulted in a 9% increase in the mean free fraction for all subjects combined (P less than 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.

Induction effect of phenobarbital on carbamazepine-10,11-epoxide kinetics in the rhesus monkey

A previous study showed that coadministration of phenobarbital (PB) and carbamazepine (CBZ) to the rhesus monkey resulted in an increase in the steady-state ratio of carbamazepine-10,11-epoxide (EPO) to CBZ. Several postulates were proposed suggesting in induction in the formation pathway of EPO. This study was undertaken to determine whether PB can also modify the elimination kinetics of EPO in monkey. Five rhesus monkeys received an intravenous bolus dose of EPO on day 1, day 5, and day 15. In addition, the animals received an acute loading dose (110 mg) of PB on day 5 followed by 10 daily maintenance doses (35 mg/day). Plasma samples were assayed for PB and EPO by GC/CI/MS. The mean (+/- SD) of plasma clearance (CL), volume of distribution (V), and half-life (t 1/2) during the control period for EPO were 5.57 +/- 2.15 liters/hr, 7.48 +/- 2.50 liters and 0.95 +/- 0.16 hr, respectively. An acute loading dose of PB had no significant effect on any of these pharmacokinetic parameters at p = 0.05. Subchronic administration of PB caused an increase in CL (8.23 +/- 2.03 liters/hr, p less than 0.05), a decrease in t 1/2 (0.67 +/- 0.09, p larger than 0.05 but less than 0.10), and no change in V. This study suggests that PB produces an increase in systemic and intrinsic hepatic clearance of EPO. Therefore, the reported increase in the EPO-CBZ steady-state ratio after PB administration could occur only if the formation clearance of EPO is increased to a greater extent than its elimination clearance.

Pharmacokinetics and tolerance of ceftriaxone in humans after single-dose intramuscular administration in water and lidocaine diluents

The effects of 1% lidocaine as a diluent on the pharmacokinetics and tolerance of ceftriaxone administered intramuscularly were investigated in 12 adult volunteers. Each subject received two 0.5-g doses of ceftriaxone (one in water and the other in 1% lidocaine) at least 1 week apart in a randomized crossover fashion. Plasma and urine samples were collected serially and assayed for ceftriaxone content by high-performance liquid chromatography. The mean peak plasma concentration, time to attain the peak, area under the plasma curve from time zero to infinity, and elimination half-life were 45 micrograms/ml, 2.5 h, 578 micrograms . h/ml, and 7.1 h, respectively, after intramuscular administration of ceftriaxone in water diluent. The corresponding mean values in 1% lidocaine diluent were 42 micrograms/ml, 3 h, 577 micrograms . h/ml, and 7.0 h. The pharmacokinetic data suggested that 1% lidocaine does not alter either the elimination parameters or the bioavailability of intramuscularly administered ceftriaxone. The intensity and frequency of pain at the injection site were reduced considerably by the coadministered lidocaine.

Diurnal oscillations in plasma protein binding of valproic acid

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 micrograms/ml) was determined by equilibrium dialysis (14C-VPA), and FFAs were assayed colorimetrically. Phenytoin (PHT) binding was also determined for comparison. VPA fp ranged from 8.10 +/- 1.16 to 9.63 +/- 1.54. Intrasubject variability was also measured by the ratio of maximum to minimum fp values (fp max/fp min) over 24 h: This ratio ranged from 1.30 to 1.68 (mean +/- %SD = 1.51 +/- 7.7%, n = 10). For PHT, fp ranged from 10.88 +/- 0.50 to 12.39 +/- 1.07, and fp max/fp min 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 less than 0.001), but not for PHT.

Pharmacokinetics of ceftriaxone in humans

Pharmacokinetics of the investigational cephalosporin ceftriaxone were studied after 30-min intravenous infusions of three ascending single doses of 0.5, 1, and 2 g crossed over in 12 normal subjects. Serially collected plasma and urine samples were analyzed for ceftriaxone by high-performance liquid chromatography. Plasma concentration-time profiles were characterized by a linear two-compartment open model with the following respective mean (+/- standard deviation) parameters at 0.5-, 1-, and 2-g dose levels: elimination half-life, 6.5 +/- 0.7, 6.2 +/- 0.8, and 5.9 +/- 0.7 h; apparent volume of distribution, 8.5 +/- 1.1, 9.0 +/- 1.1, and 10.1 +/- 1.0 liters; and plasma clearance, 929 +/- 150, 1,007 +/- 130, and 1,190 +/- 150 ml/h. The respective renal excretion parameters were as follows: renal clearance, 373 +/- 60, 399 +/- 50, and 533 +/- 128 ml/h; and percentage of dose excreted unchanged in the 48-h urine samples, 41 +/- 8, 39 +/- 5, and 43 +/- 10. The 6-h elimination half-life of ceftriaxone was 2- to 10-fold longer than those reported for marketed and other known investigational cephalosporins. The small dose-related increases in the apparent volume of distribution and clearance parameters can be explainhe 48-h urine samples, 41 +/- 8, 39 +/- 5, and 43 +/- 10. The 6-h elimination half-life of ceftriaxone was 2- to 10-fold longer than those reported for marketed and other known investigational cephalosporins. The small dose-related increases in the apparent volume of distribution and clearance parameters can be explainhe 48-h urine samples, 41 +/- 8, 39 +/- 5, and 43 +/- 10. The 6-h elimination half-life of ceftriaxone was 2- to 10-fold longer than those reported for marketed and other known investigational cephalosporins. The small dose-related increases in the apparent volume of distribution and clearance parameters can be explained by the concentration-dependent plasma protein binding of ceftriaxone in humans. The impact of the small dose-dependent changes in the pharmacokinetics of ceftriaxone is anticipated to be of negligible clinical significance.

Induction effect of phenobarbital on the carbamazepine to carbamazepine-10, 11-epoxide pathway in rhesus monkeys

The induction effect of phenobarbital on the carbamazepine to carbamazepine-10, 11-epoxide pathway was investigated in seven rhesus monkeys. Each animal received an 8-hr infusion of carbamazepine on days 1, 5 and 15. In addition, animals received an acute dose (130 mg) of phenobarbital on day 5 followed by 10 daily maintenance doses (40 mg). Plasma samples were assayed for phenobarbital, carbamazepine and epoxide by gas chromatography interfaced with a mass spectrometer in chemical ionization mode. The control mean +/- S.D. values of plasma clearance, volume of distribution and T1/2 for carbamazepine were 7.3 +/- 2.37 liters/hr, 9.61 +/- 2.29 liters and 0.96 +/- 0.02 hr, respectively. The control steady-state ratio of epoxide to carbamazepine was 0.065 +/- 0.023. An acute loading dose of phenobarbital caused an apparent decrease in clearance (6.05 +/- 1.80 liters/hr, P = .06) with no apparent changes in other parameters. Subchronic administration of phenobarbital resulted in increases in clearance (14.42 +/- 4.91 liters/hr, P = .006) and volume distribution (12.00 +/- 2.06 liters, P = .01), a decrease in T1/2 (0.62 +/- 0.15 hr, P = .006) and an apparent increase in the epoxide to carbamazepine steady-state ratio (0.085 +/- 0.013, P = .06). The increase in the epoxide to carbamazepine ratio after subchronic administration of phenobarbital suggests that phenobarbital induces the formation of epoxide more than its elimination. This is in direct contrast to the lowering of the epoxide to carbamazepine ratio by carbamazepine autoinduction.

Protein binding of valproic acid in the presence of elevated free fatty acids in patient and normal human serum

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 microEq/liter) with VPA at 100 microgram/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.

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/kd 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 +/0 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.

Simultaneous analysis of phenobarbital and p-hydroxyphenobarbital in biological fluids by GLC-chemical-ionization mass spectrometry

A sensitive and specific GLC-chemical-ionization mass spectrometric method was developed for the simultaneous assay of phenobarbital (I) and p-hydroxyphenobarbital (II) in biological fluids (urine and plasma) using stable isotope analogs of the compounds as internal standards. After extraction, the compounds were methylated with diazomethane and quantitated by GLC-chemical-ionization mass spectrometry. The detection limit of the method was 0.1 micrograms/ml for both compounds. The intraday precision (RSD) for 0.4-2.4 micrograms/ml was < 2% for I and < 4% for II. The interday precision for 0.55 and 2.11 micrograms/ml of each compound was 5.5 and 2.9% for I and 7.3 and 5.0% for II, respectively. This method has been applied in several pharmacokinetic studies.

Phenobarbital--valporic 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/hg/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 metablized fell from 0.78 to 0.67 (p = 0.015). The findings indicate that valproic acid inhibits phenobarbital metabolism.

Intramuscular absorption of carbamazepine in rhesus monkeys

The intramuscular absorption characteristics of carbamazepine were investigated in a group of six chair-adapted rhesus monkeys from three parenteral formulations [A:100 mg/ml of carbamazepine in PEG-400; B:50 mg/ml of carbamazepine in PEG-400; and C:50 mg/ml of carbamazepine in a PEG-400-Tween-80 mixture (9:1)]. The absolute bioavailability was determined by administering formulations A or B intravenously. The kinetic profiles obtained after intramuscular administration suggested biphasic absorption in the majority of animals: an initial rapid absorption phase yielding peak concentrations in less than 1 hr followed by a slower phase where absorption was probably rate limiting. The absolute bioavailability was 38% from formulation A, 81% from formulation B and 82% from formulation C. In two of four cases, Tween-80 eliminated the rate-limiting absorption phase. The data suggest that an intramuscular formulation of carbamazepine with acceptable bioavailability may be feasible.

Carbamazepine revisited in a monkey model

In a previous study on carbamazepine (Lockard et al., 1974), the problem of its low bioavailability in solid form and its short half-life in monkey were addressed. The present research was designed to evaluate carbamazepine under constant-rate intravenous infusion in our alumina-gel monkey model. Since carbamazepine is insoluble in an aqueous solution, polyethylene glycol 400 was used as the vehicle for administration of this drug to a group of 8 epileptic monkeys. The attenuation of seizures by carbamazepine was not statistically significant since the serum levels of carbamazepine after enzyme induction were less than 2.0 micrograms/ml. This study (a) illustrates that some problems in drug evaluation may be insoluble with our present technology even though we are cognizant of them; (b) makes explicit the fact that the efficacy of carbamazepine is a function of adequate serum levels; (c) demonstrates endogenous oscillations of carbamazepine serum concentrations; and (d) reports simultaneous serum levels of carbamazepine and its 10--11 epoxide in the monkey model.