Pharmacokinetics of rifapentine in subjects seropositive for the human immunodeficiency virus: a phase I study.

Rifapentine is undergoing development for the treatment of pulmonary tuberculosis. This study was conducted to characterize the single-dose pharmacokinetics of rifapentine and its 25-desacetyl metabolite and to assess the effect of food on the rate and extent of absorption in participants infected with human immunodeficiency virus (HIV). Twelve men and four women, mean age, 38.6 +/- 6.9 years, received a single 600-mg oral dose of rifapentine in an open-label, randomized two-way, complete crossover study. Each volunteer received rifapentine following a high-fat breakfast or during a fasting period. Serial blood samples were collected for 72 h and both rifapentine and its metabolite were assayed by a validated high-performance liquid chromatography method. Pharmacokinetics of rifapentine and 25-desacetylrifapentine were determined by noncompartmental methods. Mean (+/- the standard deviation) maximum concentrations of rifapentine in serum and areas under the curve from time zero to infinity following a high-fat breakfast were 14.09 +/- 2.81 and 373.63 +/- 78.19 micrograms/ml, respectively, and following a fasting period they were 9.42 +/- 2.67 and 256.10 +/- 86.39 micrograms. h/ml, respectively. Pharmacokinetic data from a previously published healthy volunteer study were used for comparison. Administration of rifapentine with a high-fat breakfast resulted in a 51% increase in rifapentine bioavailability, an effect also observed in healthy volunteers. Although food increased the exposure of these patients to rifapentine, the infrequent dosing schedule for the treatment of tuberculosis (e.g., once- or twice-weekly dosing) would be unlikely to lead to accumulation. Additionally, autoinduction has been previously studied and has not been demonstrated with this compound, unlike with rifabutin and rifampin. Rifapentine was well tolerated by HIV-infected study participants. The results of our study suggest that no dosage adjustments may be required for rifapentine in HIV-infected patients (Centers for Disease Control and Prevention classification A1, A2, B1, or B2) undergoing treatment for tuberculosis.

Single-dose pharmacokinetics of rifapentine in women.

Gender can be an important variable in the absorption and disposition of some drugs. In this open-label study, 15 healthy, nonsmoking women received a single 600-mg oral dose of rifapentine. Plasma samples were obtained at frequent intervals for up to 72 hr after the dose to determine the pharmacokinetic (PK) parameters of rifapentine and its active metabolite, 25-desacetyl-rifapentine. Peak plasma rifapentine concentrations (Cmax) were observed 5.9 hr after ingestion of the single dose. The mean area under the rifapentine plasma concentration-time curve [AUC(0-->infinity)] was 325 micrograms.hr ml and the mean elimination half-life (t1/2) was 16.3 hr. Plasma concentrations for the 25-desacetyl metabolite peaked at 15.4 hr after the rifapentine dose and declined with a terminal half-life of 17.3 hr. These rifapentine and 25-desacetyl-rifapentine PK data in women were compared to data generated previously in healthy men. Striking similarities in the PK profiles of parent drug and metabolite were found in the two populations. Mean differences in rifapentine CL/F (12%) and t1/2 (2%) were small. The only adverse event reported in the female subjects was discoloration of the urine. Based on these PK and safety data, no dosage adjustments for rifapentine based on gender are recommended.

Single-dose pharmacokinetics of rifapentine in elderly men.

PURPOSE: This study was undertaken to characterize the pharmacokinetic profiles of rifapentine and its active metabolite, 25-desacetlyl rifapentine, in elderly men. METHODS: Fourteen healthy, nonsmoking male volunteers between the ages of 65 and 82 years received a single oral 600 mg dose of rifapentine. Plasma samples were collected at frequent intervals for up to 72 hours postdose. The control group consisted of 20 healthy, young (18-45 years) males volunteers from a previous, single-dose (600 mg) rifapentine pharmacokinetic study. RESULTS: Plasma rifapentine concentrations above the minimum inhibitory concentration for M. tuberculosis were observed at 2 hours after dosing. Disposition of rifapentine was monophasic with a mean terminal half-life of 19.6 hours. The peak plasma concentration of 25 desacetyl-rifapentine was found 21.7 hours, on average, after the rifapentine dose; the mean 25-desacetyl-rifapentine t1/2 was 22.9 hours. Compared to the younger subjects, apparent oral clearance of rifapentine (24%) was lower in the elderly male (p < 0.05), and Cmax (28%) was higher. The only adverse event reported in both the older and younger subjects in these single-dose studies was discoloration of the urine. CONCLUSIONS: Because the aged-related changes in the pharmacokinetic profile of rifapentine observed in this study were modest and unlikely to be associated with toxicity, no dosage adjustments for this antibiotic are recommended in elderly patients.

Pharmacokinetics of rifapentine in patients with varying degrees of hepatic dysfunction.

In this open-label investigation, the pharmacokinetics of rifapentine and its active metabolite, 25-desacetyl-rifapentine, were characterized in patients with varying degrees of hepatic dysfunction. Eight patients with mild-to-moderate chronic, stable hepatic dysfunction and seven patients with moderate-to-severe hepatic dysfunction received single oral 600-mg doses of rifapentine. Maximum plasma concentration of rifapentine was lower, time to maximum plasma concentration (tmax) was greater, and elimination half-life (t 1/2) was longer in the patients with moderate-to-severe hepatic dysfunction than in those with mild-to-moderate dysfunction. However, mean area under the concentration-time curve extrapolated to infinity (AUC0-infinity) for the two groups was similar. AUC0-infinity values in patients with hepatic dysfunction were 19% to 25% higher than values previously reported for healthy volunteers. The 25-desacetyl metabolite appeared in plasma slowly after the single oral dose of rifapentine. Similar to findings for the parent drug, comparable plasma exposures of 25-desacetyl-rifapentine based on AUC0-infinity were found in the two groups of patients with mild-to-moderate and moderate-to-severe hepatic dysfunction. Rifapentine was well tolerated in this patient population, irrespective of the etiology or severity of hepatic dysfunction. These safety and pharmacokinetic results suggest that no dosage adjustments for rifapentine are needed in patients with hepatic impairment.

Pharmacokinetics and safety of single intravenous and oral doses of dolasetron mesylate in healthy women.

Twenty-four healthy women received 2.4 mg kg-1 dolasetron mesylate (1.8 mg kg-1 dolasetron base) by a 10 min intravenous administration and by oral administration. Pharmacokinetics of dolasetron and of its active reduced metabolite MDL 74156 were monitored for 48 h in plasma. Urine was collected from 0 to 48 h, blood pressure and heart rate were measured at 0, 0.08, 1, 2, 12, 24, and 36 h, and ECGs were measured at 0, 0.08 (intravenous only), 1, 2, and 36 h after dosing. Dolasetron was widely distributed and rapidly reduced (mean t1/2 = 0.23 h) to MDL 74156 (mean t1/2 = 8.05 and 9.12 h after intravenous and oral administration respectively). MDL 74156 was extensively distributed; between 27 (oral route) and 33% (intravenous route) was eliminated unchanged in urine. Safety assessment showed mild to moderate headache, dizziness, and hot flushes after the intravenous administration and headache, abdominal cramps or pain, and constipation after oral administration. Small and clinically non-significant changes in PR, QRS, and QTc intervals were observed. We conclude that there is no obvious difference in dolasetron pharmacokinetics between healthy women and men and that dolasetron can be used as safely in women as in men.

Population pharmacokinetic modeling of isoniazid, rifampin, and pyrazinamide.

Isoniazid (INH), rifampin (RIF), and pyrazinamide (PZA) are the most important drugs for the treatment of tuberculosis (TB). The pharmacokinetics of all three drugs in the plasma of 24 healthy males were studied as part of a randomized cross-over phase I study of two dosage forms. Subjects ingested single doses of INH at 250 mg, RIF at 600 mg, and PZA at 1,500 mg. Plasma was collected for 36 h and was assayed by high-performance liquid chromatography. The data were analyzed by noncompartmental, iterative two-stage maximum a posteriori probability Bayesian (IT2B) and nonparametric expectation maximization (NPEM) population modeling methods. Fast and slow acetylators of INH had median peak concentrations in plasma (C[max]) of 2.44 and 3.64 microg/ml, respectively, both of which occurred at 1.0 h postdose (time of maximum concentrations of drugs in plasma [T(max)]), with median elimination half-lives (t1/2) of 1.2 and 3.3 h, respectively (by the NPEM method). RIF produced a median C(max) of 11.80 microg/ml, a T(max) of 1.0 h, and a t1/2 of 3.4 h. PZA produced a median C(max) of 28.80 microg/ml, a T(max) of 1.0 h, and a t1/2 of 10.0 h. The pharmacokinetic behaviors of INH, RIF, and PZA were well described by the three methods used. These models can serve as benchmarks for comparison with models for other populations, such as patients with TB or TB with AIDS.

Pharmacokinetics of the active metabolite (MDL 74,156) of dolasetron mesylate after oral or intravenous administration to anesthetized children.

BACKGROUND: Dolasetron mesylate is a selective 5-HT3 receptor antagonist under investigation as an antiemetic in children. Published studies indicate that its antiemetic activity results from the active metabolite (MDL 74,156), which is produced within 10 minutes of administration of dolasetron mesylate. METHODS: The pharmacokinetics of MDL 74,156 and the safety and tolerability of dolasetron mesylate were studied after a single oral or intravenous dose of 1.2 mg.kg-1 dolasetron mesylate to healthy children from 2 to 12 years of age. Oral dolasetron was administered to 12 children 1 to 2 hours before anesthesia. Intravenous dolasteron was administered to 18 children at induction of anesthesia. Serial blood samples were collected for 24 hours after dosing to measure the plasma concentration of MDL 74,156. Indexes of liver and kidney function were determined, and electrocardiograms and adverse events were recorded.

High-performance liquid chromatographic procedures for the quantitative determination of paclitaxel (Taxol) in human urine.

A reversed-phase high-performance liquid chromatographic (RP-HPLC) method has been developed and validated for the quantitative determination of paclitaxel in human urine. A comparison is made between solid-phase extraction (SPE) and liquid-liquid extraction (LLE) as sample pretreatment. The HPLC system consists of an APEX octyl analytical column and acetonitrile-methanol-0.2 microM ammonium acetate buffer pH 5 (4:1:5, v/v) as the mobile phase. Detection is performed by UV absorbance measurement at 227 nm. The SPE procedure involves extraction on Cyano Bond Elut columns. n-Butylchloride is the organic extraction fluid used for the LLE. The recoveries of paclitaxel in human urine are 79 and 75% for SPE and LLE, respectively. The accuracy for the LLE and SPE sample pretreatment procedures is 100.4 and 104.9%, respectively, at a 5 micrograms/ml drug concentration. The lower limit of quantitation is 0.01 microgram/ml for SPE and 0.25 microgram/ml for LLE. Stability data of paclitaxel in human urine are also presented.

Pharmacokinetics of paclitaxel and metabolites in a randomized comparative study in platinum-pretreated ovarian cancer patients.

PURPOSE: To investigate the pharmacokinetics and pharmacodynamics of paclitaxel in a randomized comparative study with four different treatment arms in patients with platinum-pretreated ovarian carcinoma. PATIENTS AND METHODS: Eighteen patients were entered onto this study in which paclitaxel was administered at a high dose of 175 mg/m2 versus a low dose of 135 mg/m2 on a 3- or 24-hour infusion schedule. A solid-phase extraction technique for sample pretreatment followed by a reverse-phase high-performance liquid chromatographic (HPLC) assay was used for analysis of plasma. RESULTS: Grade 3 neutropenia occurred in all four treatment arms. However, it was more severe on the 24-hour infusion schedule. Paclitaxel concentrations as low as 0.012 mumol/L were measured with the HPLC assay. With this low quantitation threshold, we found the plasma disappearance of paclitaxel to be triphasic, with half-lives t1/2(alpha), t1/2(beta), and t1/2(gamma) mean values for the different treatment arms of 0.19 hours (range, 0.01 to 0.4), 1.9 hours (range, 0.5 to 2.8), and 20.7 hours (range, 4 to 65), respectively. Eleven possible metabolites were found, of which three were identified as taxanes by on-line HPLC-photodiode array (PDA) detection. Investigation of pharmacodynamics shows no clear relationship between the pharmacokinetic parameters area under the plasma concentration time curve (AUC), area under the plasma concentration moment curve (AUMC), maximal plasma concentration (Cmax), clearance, and toxicity. However, a relationship was found between the duration of plasma concentrations above a threshold of 0.1 mumol/L with absolute neutrophil count (ANC) and white blood cell count (WBC). CONCLUSION: Paclitaxel is metabolized, and putative metabolic products can be found in plasma of patients treated with the drug. Our results indicate that myelosuppression can be predicted by the measurement of the duration of plasma concentrations above the threshold of 0.1 mumol/L.

Alpha-1 adrenergic coupling events induced by full and partial agonists in rabbit aorta.

Differences in the ability of full vs. partial agonists to initiate alpha-1 adrenergic receptor-mediated coupling events were studied in isolated segments of rabbit aorta. Mono- and dimethoxysubstituted tolazolines produced contractile responses which, at their maximum, were 27 to 100% of the response produced by the full agonist phenylephrine. In addition to differences in maximum response, contraction kinetics varied between full and partial agonists. Responses to partial agonists displayed a slower approach to peak tension and loss of the rapid phase of tension development which is associated with release of intracellular Ca++. Among the tolazoline series 3,5 dimethoxy-, 3 methoxy-, and 2 methoxy derivatives were compared further with phenylephrine for their ability to cause phosphatidylinositol cycle turnover, intracellular Ca++ release and Ca++ influx. For each of these coupling events, a rank of phenylephrine greater than or equal to 3, 5 greater than 3 greater than 2 was observed. However, a higher percentage of Ca++ influx vs. Ca++ release was observed for the partial agonists, suggesting that their contractile responses may be more dependent upon extracellular Ca++ than intracellular Ca++. Our results indicate that partial agonists initiate the same coupling events as full agonists; however, the relative proportion of Ca++ release and influx may be different for partial agonists because of the reduced rate of second messenger production.