Pharmacokinetic and pharmacodynamic profile of donepezil HCl following multiple oral doses.

AIM: The aim of this study was to characterize the pharmacokinetics and pharmacodynamics of donepezil HCl, a new, chemically distinct and specific acetylcholinesterase (AChE) inhibitor for the treatment of Alzheimer's disease, following multiple-dose administration. METHODS: This was a double-blind, randomized, placebo-controlled, multiple-dose study in healthy male volunteers (n=27). Three dose levels were investigated in sequential order: 1, 3 and 5 mg. Each dose was administered orally, once a day, for 21 consecutive days. Donepezil concentrations in plasma were quantified by HPLC. Pharmacodynamic activity was determined by the radioenzymatic measurement of erythrocyte membrane acetylcholinesterase (rbc-AChE) inhibition. RESULTS: The pharmacokinetic disposition of donepezil was observed to be dose proportional. The mean terminal disposition half-life was 79.5+/-19.0 h which resulted in a slow approach to steady state (14-21 days). A four- to sixfold increase in donepezil plasma concentration was observed during this time; however, no further increase was evident after achievement of steady state. The mean donepezil plasma concentration at steady state (Css) was 14.2 ng ml(-1). Neither the rate of accumulation nor the rate of clearance was dose dependent. Inhibition of rbc-AChE was directly correlated with donepezil concentration over a wide concentration range, with the higher concentrations showing the expected hyperbolic relationship. Donepezil was well tolerated by all subjects with no clinically significant changes in laboratory or physical parameters observed at any dose. CONCLUSIONS: The pharmacokinetics of donepezil were found to be dose proportional following the administration of multiple doses to healthy volunteers. A predictable relationship was also observed between plasma donepezil concentrations and rbc-AChE inhibition. The half-life of donepezil makes it suitable for once-daily dosing.

Tacrolimus (FK506): validation of a sensitive enzyme-linked immunosorbent assay kit for and application to a clinical pharmacokinetic study.

Tacrolimus (FK506) is a macrolide immunosuppressant approved for the prophylaxis of organ rejection in liver transplant. Immunoassays of low intra- and interday variability and high sensitivity are necessary to adequately characterize terminal elimination phase concentrations in pharmacokinetic studies. A new ELISA kit for the quantitation of tacrolimus in human whole blood has been validated for use in pharmacokinetic studies. Methanol sample extracts were dried and reconstituted in a horseradish peroxidase (HPR)-FK506 conjugate solution. The reconstituted samples and mouse anti-FK506 were added to a microplate, precoated with secondary antibody, and incubated, FK506 and the HPR-FK506 conjugate competed to bind with anti-FK506, which was immobilized by binding to the secondary antibody. Unbound FK506 was washed away, and substrate was added for color development. Once the reaction was stopped with 2 N H2SO4, the plate was read at 450 nm. The linear range was 0.5-60 ng/ml, with a limit of quantitation of 0.5 ng/ml. Interday precision and accuracy were < or = 10.4% C.V. and < or = 3% R.E. for quality control samples. The lack of interference from endogenous compounds was established by parallelism and recoveries of FK506 from six lots of control matrix. Cross-reactivity against the metabolites and analogs were not performed because the kit monoclonal antibody was from the same source as Kobayashi et al (1). The utility and sensitivity of the kit present a good method for the quantitation of tacrolimus in blood from pharmacokinetic studies. The method is robust and has been used to assay tacrolimus in several thousand whole blood samples by multiple analysts.