FDA evaluations using in vitro metabolism to predict and interpret in vivo metabolic drug-drug interactions: impact on labeling

Recent advances in in vitro metabolism methods have led to an improved ability to predict clinically relevant metabolic drug-drug interactions. To address the relationships of in vitro metabolism data and in vivo metabolism outcomes, the Office of Clinical Pharmacology and Biopharmaceutics in the Center for Drug Evaluation and Research, Food and Drug Administration, evaluated a number of recently approved new drug applications. The goal of these evaluations was to determine the contribution of in vitro metabolism data in (1) predicting in vivo drug-drug interactions, (2) determining the need to conduct an in vivo drug-drug interaction study, and (3) incorporating findings into drug product labeling. Ten cases are presented in this article. They fall into two major groups: (1) in vitro data were predictive of in vivo results, and (2) in vitro data were not predictive of in vivo results. Discussion of these cases highlights factors limiting predictability of in vivo metabolic interactions from in vitro metabolism data. The integration of these findings into drug product labeling is also discussed.

Clinical pharmacokinetics and pharmacodynamics of buspirone, an anxiolytic drug

Buspirone is an anxiolytic drug given at a dosage of 15 mg/day. The mechanism of action of the drug is not well characterised, but it may exert its effect by acting on the dopaminergic system in the central nervous system or by binding to serotonin (5-hydroxytryptamine) receptors. Following a oral dose of buspirone 20 mg, the drug is rapidly absorbed. The mean peak plasma concentration (Cmax) is approximately 2.5 micrograms/L, and the time to reach the peak is under 1 hour. The absolute bioavailability of buspirone is approximately 4%. Buspirone is extensively metabolised. One of the major metabolites of buspirone is 1-pyrimidinylpiperazine (1-PP), which may contribute to the pharmacological activity of buspirone. Buspirone has a volume of distribution of 5.3 L/kg, a systemic clearance of about 1.7 L/h/kg, an elimination half-life of about 2.5 hours and the pharmacokinetics are linear over the dose range 10 to 40 mg. After multiple-dose administration of buspirone 10 mg/day for 9 days, there was no accumulation of either parent compound or metabolite (1-PP). Administration with food increased the Cmax and area under the plasma concentration-time curve (AUC) of buspirone 2-fold. After a single 20 mg dose, the Cmax and AUC increased 2-fold in patients with renal impairment as compared with healthy volunteers. The Cmax and AUC were 15-fold higher for the same dose in patients with hepatic impairment compared with healthy individuals. The half-life of buspirone in patients with hepatic impairment was twice that in healthy individuals. The pharmacokinetics of buspirone were not affected by age or gender. Coadministration of buspirone with verapamil, diltiazem, erythromycin and itraconazole substantially increased the plasma concentration of buspirone, whereas cimetidine and alprazolam had negligible effects. Rifampicin (rifampin) decreased the plasma concentrations of buspirone almost 10-fold.

Disposition of drugs in cystic fibrosis. I. Sulfamethoxazole and trimethoprim

The disposition of sulfamethoxazole and trimethoprim, after constant rate intravenous administration (10 mg/kg/hr sulfamethoxazole and 2 mg/kg/hr trimethoprim for 1 hour), was investigated in adult patients with cystic fibrosis (n = 7) and in age-matched healthy subjects (control subjects, n = 8). The total plasma clearance of sulfamethoxazole was found to be increased in cystic fibrosis (0.0262 +/- 0.0064 L/hr/kg) when compared with that found in control subjects (0.0188 +/- 0.0043 L/hr/kg). This increase in clearance was found to be primarily attributable to an increase in the metabolic clearance of sulfamethoxazole to N4-acetylsulfamethoxazole (0.00903 +/- 0.00247 versus 0.00355 +/- 0.00049 L/hr/kg) with the renal clearance of sulfamethoxazole remaining unchanged. These conclusions were not altered when the pharmacokinetic parameters were computed for the unbound drug or when they were normalized with respect to body surface area. These data indicate that, in cystic fibrosis, the enzymes mediating the metabolism of sulfamethoxazole to N4-acetylsulfamethoxazole, N-acetyltransferase(s), may be induced, activated, or both, or that the uptake of sulfamethoxazole by cells that metabolize sulfamethoxazole to N4-acetylsulfamethoxazole is enhanced. The total plasma clearance of trimethoprim was also found to be increased in cystic fibrosis (0.1808 +/- 0.0440 L/hr/kg) when compared with that found in control subjects (0.1139 +/- 0.0193 L/hr/kg). In contrast to sulfamethoxazole, this increase in clearance was found to be primarily attributable to an increase in the renal clearance of trimethoprim (0.1240 +/- 0.0299 versus 0.0720 +/- 0.0166 L/hr/kg). These data indicate that the tubular secretion of trimethoprim may be enhanced in cystic fibrosis.