Organic anion transporting polypeptide-mediated transport of, and inhibition by, asunaprevir, an inhibitor of hepatitis C virus NS3 protease.

Asunaprevir (ASV), an investigational, highly protein-bound inhibitor of hepatitis C virus NS3 protease, shows considerable hepatic compartmentalization in animal models. Preclinical data showed ASV inhibition of human OATP1B1 (IC50 = 0.3 µM), OATP2B1 (0.27 µM), and, to a lesser extent OATP1B3 (3.0 µM), confirmed by modest (<2-fold) clinical elevations in rosuvastatin exposure with concomitant ASV. Although no significant OATP transport of ASV was observed in vitro at standard micromolar assay concentrations, clinical coadministration of ASV with a single dose of the OATP inhibitor rifampin gave large, variable increases in ASV plasma Cmax (21-fold mean) and AUCinf (15-fold mean), consistent with reduced hepatic uptake. In vitro reevaluation at therapeutically relevant low-nanomolar concentrations of unbound ASV showed active, saturable human hepatocyte uptake (Km = 0.685 µM) and rifampin-reversible transport by OATP1B1 and OATP2B1, but not OATP1B3. At therapeutically relevant concentrations, ASV is therefore a sensitive substrate for, and weak inhibitor of, human OATP1B1, 1B3 and 2B1.

The effect of atazanavir and atazanavir/ritonavir on UDP-glucuronosyltransferase using lamotrigine as a phenotypic probe.

Atazanavir (ATV) is known to inhibit UGT1A1-mediated glucuronidation. Here we report the effect of ATV and ATV/ritonavir (RTV) on another UGT1A isoenzyme, UGT1A4. Twenty-one healthy volunteers received a single dose of 100 mg of oral lamotrigine on days 1, 13, and 27; on each occasion blood was sampled before the dose was administered and through 120 h after ingestion of the drug. On days 8-17 the subjects received oral ATV 400 mg q.d. On days 18-30 the subjects received oral ATV 300 mg plus oral RTV 100 mg q.d. Seventeen subjects were evaluable for pharmacokinetic analysis. Geometric mean ratios (+90% confidence intervals (CIs)) of lamotrigine area under the plasma concentration-time curve (AUC)(0-inf) and peak plasma concentration (C(max)) for ATV + lamotrigine and for lamotrigine alone were 0.88 (0.86-0.91) and 0.99 (0.95-1.02), respectively; the corresponding ratios for ATV/RTV and for lamotrigine were 0.68 (0.65-0.70) and 0.94 (0.90-0.97), respectively. The mean ratio of lamotrigine-2N-glucuronide to lamotrigine AUC(0-inf) increased from 0.45 for lamotrigine to 0.71 for ATV/RTV + lamotrigine. ATV alone does not significantly influence glucuronidation of lamotrigine. In contrast, ATV/RTV results in moderately decreased exposure to lamotrigine.

The effect of ritonavir on the pharmacokinetics of meperidine and normeperidine.

STUDY OBJECTIVE: To determine the effects of ritonavir on the pharmacokinetics of meperidine and normeperidine. DESIGN: Open-label, crossover, pharmacokinetic study. SETTING: United States government research hospital. SUBJECTS: Eight healthy volunteers who tested negative for the human immunodeficiency virus. INTERVENTION: Subjects received oral meperidine 50 mg and had serial blood samples collected for 48 hours. They then received ritonavir 500 mg twice/day for 10 days, followed by administration of a second 50-mg meperidine dose and collection of serial samples. MEASUREMENTS AND MAIN RESULTS: Plasma samples were assayed for meperidine, normeperidine, and ritonavir. Meperidine's area under the curve (AUC) decreased in all subjects by a mean of 67+/-4% in the presence of ritonavir (p<0.005). Mean +/- SD maximum concentration was decreased from 126+/-47 to 51+/-21 ng/ml. Normeperidine's mean AUC was increased 47%, suggesting induction of hepatic metabolism. CONCLUSION: Meperidine's AUC is significantly reduced, not increased, by concomitant ritonavir. Based on these findings, the risk of narcotic-related adverse effects from this combination appears to be minimal. However, increased concentrations of normeperidine suggest a potential for toxicity with increased dosages or long-term therapy.

Pulmonary delivery of a dopamine D-1 agonist, ABT-431, in dogs and humans.

The purpose of this study was to evaluate the feasibility of intrapulmonary delivery of ABT-431, a selective D1 receptor agonist. Following intratracheal instillation of the drug solution, the lung bioavailability was found to be approximately 75% in dogs. An aerosol suspension formulation was then developed by dispersing the drug in tetrafluoroethane, HFC-134a, with the aid of poloxamer 124 and vitamin E. This ABT-431 MDI aerosol formulation showed about 40% of the particles emitted from the valve and actuator system to be under 5 microm in diameter. Also, the primary package (15 mL aluminum container, DF10/ACT-150 valve, and Micron-4-actuator with the orifice 0.4 mm) was satisfactory for accurate and reproducible dosimetry. Using tracheostomized beagle dogs, the C(max) following tracheal administration of 5 mg aerosolized ABT-431 was found to be 13.3+/-0.9 ng ml(-1) and the AUC(0-24) was estimated at 33.2+/-10.6 h ng ml(-1). The lung bioavailability of the aerosolized drug was 34% compared to intravenous injection in dogs. In humans, results from a single rising dose study demonstrated that rapid absorption of ABT-431 following oral inhalation administration resulted in a dose-dependent increase in the area under the plasma-time curve at dosage levels between 3.3 and 13.2 mg. There is a possibility of up to 25% absorption of the drug from human lung. Thus, pulmonary bioavailability of ABT-431 is significantly greater than that of oral administration. Also, these findings suggest that small and lipophilic compounds, especially with hepatic first pass effect, may be effectively delivered systemically using oral inhalation aerosols.

Alprazolam increases dehydroepiandrosterone concentrations.

The gamma-aminobutyric acid (GABA) agonist alprazolam is known to decrease adrenocorticotropic hormone and cortisol concentrations. Dehydroepiandrosterone (DHEA) is secreted synchronously with cortisol by the adrenal glands and demonstrates diurnal variation. The major objective of this study was to determine whether alprazolam affects concentrations of DHEA and DHEA-S, the sulfated metabolite. In vitro studies have demonstrated that DHEA-S, and perhaps DHEA, have GABA antagonistic activity. Another objective was to determine whether DHEA-S and/or DHEA concentrations are related to psychomotor impairment after alprazolam. Thirty-eight healthy volunteers (25 young men, aged 22-35, and 13 elderly men, aged 65-75) received a single intravenous dose of alprazolam 2 mg/2 min (part 1). Fifteen young and 13 elderly men responded to alprazolam and agreed to participate in part 2 of the study, which was a crossover of placebo and alprazolam infusion to plateau for 9 hours. Plasma samples at 0, 1, 4, and 7 hours were assayed for steroid concentrations. Alprazolam produced (1) significant increases in DHEA concentrations at 7 hours in both young and elderly men; (2) significant decreases in cortisol concentrations; and (3) no change in DHEA-S concentrations. The relationship between psychomotor decrement and DHEA concentrations at 7 hours after alprazolam 2 mg/2 min was described by a u-shaped curve (p < 0.0047). Both the linear and quadratic components of the equations for the tests were significant (p < 0.002). These results suggest that alprazolam modulates peripheral concentrations of DHEA and that DHEA and/or DHEA-S may have an in vivo role in modulating GABA receptor-mediated responses.

Ritonavir. Clinical pharmacokinetics and interactions with other anti-HIV agents.

Ritonavir is 1 of the 4 potent synthetic HIV protease inhibitors, approved by the US Food and Drug Administration (FDA) between 1995 and 1997, that have revolutionised HIV therapy. The extent of oral absorption is high and is not affected by food. Within the clinical concentration range, ritonavir is approximately 98 to 99% bound to plasma proteins, including albumin and alpha 1-acid glycoprotein. Cerebrospinal fluid (CSF) drug concentrations are low in relation to total plasma concentration. However, parallel decreases in the viral burden have been observed in the plasma, CSF and other tissues. Ritonavir is primarily metabolised by cytochrome P450 (CYP) 3A isozymes and, to a lesser extent, by CYP2D6. Four major oxidative metabolites have been identified in humans, but are unlikely to contribute to the antiviral effect. About 34% and 3.5% of a 600 mg dose is excreted as unchanged drug in the faeces and urine, respectively. The clinically relevant t1/2 beta is about 3 to 5 hours. Because of autoinduction, plasma concentrations generally reach steady state 2 weeks after the start of administration. The pharmacokinetics of ritonavir are relatively linear after multiple doses, with apparent oral clearance averaging 7 to 9 L/h. In vitro, ritonavir is a potent inhibitor of CYP3A. In vivo, ritonavir significantly increases the AUC of drugs primarily eliminated by CYP3A metabolism (e.g. clarithromycin, ketoconazole, rifabutin, and other HIV protease inhibitors, including indinavir, saquinavir and nelfinavir) with effects ranging from an increase of 77% to 20-fold in humans. It also inhibits CYP2D6-mediated metabolism, but to a significantly lesser extent (145% increase in desipramine AUC). Since ritonavir is also an inducer of several metabolising enzymes [CYP1A4, glucuronosyl transferase (GT), and possibly CYP2C9 and CYP2C19], the magnitude of drug interactions is difficult to predict, particularly for drugs that are metabolised by multiple enzymes or have low intrinsic clearance by CYP3A. For example, the AUC of CYP3A substrate methadone was slightly decreased and alprazolam was unaffected. Ritonavir is minimally affected by other CYP3A inhibitors, including ketoconazole. Rifampicin (rifampin), a potent CYP3A inducer, decreased the AUC of ritonavir by only 35%. The degree and duration of suppression of HIV replication is significantly correlated with the plasma concentrations. Thus, the large increase in the plasma concentrations of other protease inhibitors when coadministered with ritonavir forms the basis of rational dual protease inhibitor regimens, providing patients with 2 potent drugs at significantly reduced doses and less frequent dosage intervals. Combination treatment of ritonavir with saquinavir and indinavir results in potent and sustained clinical activity. Other important factors with combination regimens include reduced interpatient variability for high clearance agents, and elimination of the food effect on the bioavailibility of indinavir.

Alprazolam in young and elderly men: sensitivity and tolerance to psychomotor, sedative and memory effects.

This study was designed to determine whether age influences sensitivity to alprazolam and/or rate of acute tolerance development to the effects of alprazolam. Three treatments were each separated by 4 weeks. Twenty-five young (ages 22-35) and 13 elderly (ages 65-75) men received 2 mg of alprazolam/2 min i.v. Blood samples were obtained over 48 hr, and sedative, psychomotor and memory effects were assessed serially for 12 hr. Clearance was lower (P = .05) and elimination t[1/2] was longer (P = .005) in the elderly, but area under the concentration curve to 12 hr and maximum concentration did not differ by age group. Maximum impairment was greater in the elderly for all assessments. Mean EC50 values differed between the elderly (25.3 and 25.0 ng/ml) and the young (39.8 and 36.5 ng/ml) on card sorting and digit symbol substitution, respectively (P < .001). Bolus treatment data were used to individualize doses for the crossover of placebo and alprazolam; infusions were designed to maintain a plateau alprazolam concentration between 1 and 9 hr. Alprazolam concentrations through 12 hr did not differ between the young and elderly. Median t[1/2] for offset of effect for digit symbol substitution was 2.8 hr in the young and 4.9 hr in the elderly (P = .05). Therefore, aging decreases alprazolam clearance and increases sensitivity to effects of alprazolam through a mechanism other than pharmacokinetics; aging also decreases the rate of offset of effect of alprazolam. In addition, the data provide insight into the intensity of initial effect as a determinant of rate of tolerance development.

Use of in vitro and in vivo data to estimate the likelihood of metabolic pharmacokinetic interactions.

This article reviews the information available to assist pharmacokineticists in the prediction of metabolic drug interactions. Significant advances in this area have been made in the last decade, permitting the identification in early drug development of dominant cytochrome P450 (CYP) isoform(s) metabolising a particular drug as well as the ability of a drug to inhibit a specific CYP isoform. The major isoforms involved in human drug metabolism are CYP3A, CYP2D6, CYP2C, CYP1A2 and CYP2E1. Often patients are taking multiple concurrent medications, and thus an assessment of potential drug-drug interactions is imperative. A database containing information about the clearance routes for over 300 drugs from multiple therapeutic classes, including analgesics, anti-infectives, psychotropics, anticonvulsants, cancer chemotherapeutics, gastrointestinal agents, cardiovascular agents and others, was constructed to assist in the semiquantitative prediction of the magnitude of potential interactions with drugs under development. With knowledge of the in vitro inhibition constant of a drug (Ki) for a particular CYP isoform, it is theoretically possible to assess the likelihood of interactions for a drug cleared through CYP-mediated metabolism. For many agents, the CYP isoform involved in metabolism has not been identified and there is substantial uncertainty given the current knowledge base. The mathematical concepts for prediction based on competitive enzyme inhibition are reviewed in this article. These relationships become more complex if the inhibition is of a mixed competitive/noncompetitive nature. Sources of uncertainty and inaccuracy in predicting the magnitude of in vivo inhibition includes the nature and design of in vitro experiments to determine Ki, inhibitor concentration in the hepatic cytosol compared with that in plasma, prehepatic metabolism, presence of active metabolites and enzyme induction. The accurate prospective prediction of drug interactions requires rigorous attention to the details of the in vitro results, and detailed information about the pharmacokinetics and metabolism of the inhibitor and inhibited drug. With the discussion of principles and accompanying tabulation of literature data concerning the clearance of various drugs, a framework for reasonable semiquantitative predictions is offered in this article.

Effect of neuroactive steroids on [3H]flumazenil binding to the GABAA receptor complex in vitro.

Modulation of benzodiazepine receptor ligand binding to the GABAA receptor complex by the neuroactive steroids 3 alpha-hydroxy-dihydroprogesterone (3 alpha-OH-DHP) and 3 alpha-hydroxycorticosterone (3 alpha- THDOC) was assessed in an in vitro binding assay with the benzodiazepine antagonist [3H]flumazenil using rat cortical membranes. Neuroactive steroids, pentobarbital, GABA and bicuculline did not significantly affect flumazenil binding. However, the addition of neuroactive steroids significantly decreased the Ki of benzodiazepine agonists, including alprazolam, diazepam and clonazepam, indicating an increase in agonist affinity. Only the addition of 3 beta-OH-DHP, an inactive stereoisomer had no effect on the Ki of these agonists. The binding of the benzodiazepine inverse agonist FG 7142 was not significantly affected by these steroids, but the addition of GABA significantly increased the Ki of FG 7142 indicating a decrease in inverse agonist affinity. High concentrations of GABA or bicuculline were able to occlude the 3 alpha-THDOC mediated decrease in alprasolam Ki, indicating a GABA dependent mechanism of binding enhancement. An advantage of using [3H]flumazenil is that neither the Ki nor the Bmax change in the presence of allosteric site modulators, permitting the simple and direct assessment of alterations in benzodiazepine ligand affinity for the GABAA receptor complex by neuroactive steroids.

Triazolam pharmacokinetics after intravenous, oral, and sublingual administration.

This study was designed to evaluate the relative and absolute bioavailability of triazolam, 0.25 mg, after the administration of the marketed oral tablet and a sublingual prototype wafer; an intravenous dose was used as a reference. Twelve men were evaluated in a three-way crossover study; study days were separated by 1 week. A single dose was administered to each subject at approximately 8 a.m.; serial blood samples were obtained for the determination of triazolam concentration. The fraction absorbed relative to intravenous was 20% higher in the sublingual than in the oral treatment (p = 0.0128); the difference between treatments was greatest in the first 2 hours as indicated by the area under the curve from 0 to 2 hours (p < 0.05). The extraction ratio ranged from 0.05 to 0.25, and the predicted availability after oral administration was 86% with a range of 75 to 95%. In contrast, the observed mean absolute availability was 44% (oral) and 53% (sublingual). A potential explanation for this discrepancy between predicted and observed bioavailability is that after oral administration, a fraction of triazolam may be metabolized by cytochrome P450IIIA4 in the gut wall, with a separate fraction subject to first-pass metabolism in the liver. Although this study was not designed to identify sites of triazolam metabolism, the proposed explanation is consistent with the occurrence of P450IIIA4 in the stomach, small intestine, and liver. Doses administered sublingually avoid first-pass metabolism, producing earlier and higher peak concentrations than do doses administered orally.

Acute tolerance to triazolam during continuous and step infusions: estimation of the effect offset rate constant.

Although acute tolerance to selected effects of many benzodiazepines is known to occur, acute tolerance to triazolam has not been documented even in studies that have included pharmacodynamic modeling. The purpose of this investigation was to determine whether acute tolerance to triazolam occurs in humans. Intravenous bolus doses of triazolam were used to individualize two subsequent intravenous infusions: one to achieve and maintain a constant triazolam concentration and one to achieve a series of incremental steady-state concentrations; a placebo treatment was also included. Ten healthy men completed the four single-dose treatments. Serial blood sampling and psychomotor and memory testing were done. In the constant infusion treatment, mean performance impairment was greatest at 1 h and then decreased rapidly despite maintenance of a mean triazolam concentration of 2.48 ng/ml for 9 h. Neither learning nor changes in free concentration account for the observations. Additionally, data from the step-infusion treatment indicate that the triazolam effect-concentration relationship after a single dose can be altered by rate of administration. Because tolerance develops, the administration of drug in small increments results in an increased effect at a lower concentration, with a blunted maximal response. Furthermore, our data suggest intersubject variability in the rate of development of acute tolerance. Patients who develop tolerance more slowly would experience a longer duration of effect. Further study regarding the rate of development of tolerance to specific effects and in different patient populations is warranted.

Diazepam by continuous intravenous infusion for status epilepticus in anticonvulsant hypersensitivity syndrome.

OBJECTIVE: To report a case of status epilepticus in a patient with anticonvulsant hypersensitivity syndrome (AHS) that was controlled successfully using continuous intravenous infusion diazepam. AHS and alternatives for treatment of status epilepticus are also reviewed. DESIGN: Single case report. SETTING: 217-bed children's university hospital. PATIENT: Four-year-old, 20-kg girl, diagnosed with idiopathic tonic-clonic epilepsy, who developed AHS to phenobarbital and phenytoin and status epilepticus unresponsive to lorazepam. RESULTS: Diazepam intravenous infusion at dosages titrated to 8 mg/h was used successfully to control seizures for eight days until signs and symptoms of AHS had resolved and maintenance therapy with valproic acid was started. CONCLUSIONS: Continuous intravenous infusion diazepam is a reasonable therapeutic choice for the management of status epilepticus in a patient with AHS when traditional therapy such as phenytoin and phenobarbital cannot be used.