MicroDose

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 I¼g), (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 t1/2 of 45.1 hours, clearance CL of 1.38 L/h, and volume of distribution V of 90.1 L for 100 I¼g and t1/2 of 35.7 hours, CL of 1.3 L/h, and V of 123 L for 10 mg), midazolam (t1/2 of 4.87 hours, CL of 21.2 L/h, V of 145 L, and oral bioavailability F of 0.23 for 100 I¼g and t1/2 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 I¼g and 50 mg). For warfarin, clearance was reasonably well predicted (0.17 L/h for 100 I¼g and 0.26 L/h for 5 mg), but the discrepancy observed in distribution (67 L for 100 I¼g 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. © 2006 American Society for Clinical Pharmacology and Therapeutics.

Abstract: A microdose study of metformin was conducted to investigate the predictability of drug-drug interactions at the therapeutic dose (ThD). Healthy subjects received a microdose (100 µg) or ThD (250 mg) of metformin orally, with or without a potent and competitive multidrug and toxin extrusion (MATE) inhibitor, pyrimethamine (50 mg, p.o.), in a crossover fashion. Pyrimethamine significantly reduced the renal clearance of metformin by 23 and 35% at the microdose and ThD, respectively. At ThD, but not at microdose, it caused significant increases in the maximum concentration (C(max)) and area under the plasma concentration-time curve (AUC) of metformin (142 and 139% of control values, respectively). Human canalicular membrane vesicles showed pyrimethamine-inhibitable metformin uptake. Pyrimethamine did not affect plasma lactate/pyruvate after ThD of metformin but significantly reduced the renal clearance of creatinine, thereby causing elevation of plasma creatinine level. This microdose study quantitatively predicted a drug-drug interaction involving the renal clearance of metformin at ThD by pyrimethamine. Pyrimethamine is a useful in vivo inhibitor of MATE proteins.

Abstract: A lower incidence of dementia in bipolar patients treated with lithium has been described This metal inhibits the phosphorylation of glycogen-synthase-kinase 3-α and β, which are related to amyloid precursor protein processing and tau hyperphosphorylation in pathological conditions, respectively Following the same rationale, a group just found that lithium has disease-modifying properties in amnestic mild cognitive impairment with potential clinical implications for the prevention of Alzheimer's Disease (AD) when a dose ranging from 150 to 600 mg is used As lithium is highly toxic in regular doses, our group evaluated the effect of a microdose of 300 μg, administered once daily on AD patients for 15 months In the evaluation phase, the treated group showed no decreased performance in the mini-mental state examination test, in opposition to the lower scores observed for the control group during the treatment, with significant differences starting three months after the beginning of the treatment, and increasing progressively This data suggests the efficacy of a microdose lithium treatment in preventing cognitive loss, reinforcing its therapeutic potential to treat AD using very low doses