¹Xceleron, York; Preclinical Development Pharmacokinetics, Schering, Berlin; Servier Research Group, Courbevoie; F. Hoffmann-La Roche, Basel; Eli Lilly, Indianapolis; Pharma Bio-Research Group, Zuidlaren; and Centre for Applied Pharmacokinetic Research, School of Pharmacy and Pharmaceutical Sciences, University of Manchester, Manchester.

Correspondence: R. Colin Garner, DSc, Xceleron, York Biocentre, Innovation Way, Heslington, York YO10 5NY, United Kingdom. E-mail: colin.garner@xceleron.com

^*Supported by the Consortium for Resourcing and Evaluating AMS Microdosing (CREAM) trial consisting of Xceleron, Schering, Institut de Recherches Internationales Servier, F. Hoffmann-La Roche, Eli Lilly, and Pharma Bio-Research Group.

Received 22 December 2005; Accepted 1 May 2006.

Abstract

Objectives: A volunteer trial was performed to compare the pharmacokinetics of 5 drugs—warfarin, ZK253 (Schering), diazepam, midazolam, and erythromycin—when administered at a microdose or pharmacologic dose. Each compound was chosen to represent a situation in which prediction of pharmacokinetics from either animal or in vitro studies (or both) was or is likely to be problematic.

Methods: In a crossover design volunteers received (1) 1 of the 5 compounds as a microdose labeled with radioactive carbon (carbon 14) (100 g), (2) the corresponding ¹⁴C-labeled therapeutic dose on a separate occasion, and (3) simultaneous administration of an intravenous ¹⁴C-labeled microdose and an oral therapeutic dose for ZK253, midazolam, and erythromycin. Analysis of ¹⁴C-labeled drugs in plasma was done by use of HPLC followed by accelerator mass spectrometry. Liquid chromatography–tandem mass spectrometry was used to measure plasma concentrations of ZK253, midazolam, and erythromycin at therapeutic concentrations, whereas HPLC–accelerator mass spectrometry was used to measure warfarin and diazepam concentrations.

Results: Good concordance between microdose and therapeutic dose pharmacokinetics was observed for diazepam (half-life [t_½] of 45.1 hours, clearance [CL] of 1.38 L/h, and volume of distribution [V] of 90.1 L for 100 g and t_½ of 35.7 hours, CL of 1.3 L/h, and V of 123 L for 10 mg), midazolam (t_½ of 4.87 hours, CL of 21.2 L/h, V of 145 L, and oral bioavailability [F] of 0.23 for 100 g and t_½ of 3.31 hours, CL of 20.4 L/h, V of 75 L, and F of 0.22 for 7.5 mg), and development compound ZK253 (F = <1% for both 100 g and 50 mg). For warfarin, clearance was reasonably well predicted (0.17 L/h for 100 g and 0.26 L/h for 5 mg), but the discrepancy observed in distribution (67 L for 100 g and 17.9 L for 5 mg) was probably a result of high-affinity, low-capacity tissue binding. The oral microdose of erythromycin failed to provide detectable plasma levels as a result of possible acid lability in the stomach. Absolute bioavailability for the 3 compounds examined yielded excellent concordance with data from the literature or data generated in house.

Conclusion: Overall, when used appropriately, microdosing offers the potential to aid in early drug candidate selection.