Amantadine kinetics in healthy elderly men: implications for influenza prevention

Amantadine kinetics were investigated in 10 healthy elderly men 60 to 76 yr old. We calculated a dose that would yield the same trough steady-state plasma amantadine concentration (Cpss; 300 ng/ml) as a 200 mg/day dose taken by young healthy adults; this dose prevents influenza A virus infection and is well tolerated by this population. With a one-compartment open model, kinetic parameters were calculated after a single dose of 25, 50, or 75 mg or the same dose twice a day for 10.5 days. Peak concentration occurred 4.0 to 8.0 hr after dosing, but the calculated AUC was proportional to dose, indicating that relative bioavailability was independent of dose. This was supported by recovery of 88% of the single doses in urine. No change in apparent volume of distribution was found. Log trough Cpss increased with dose. Trough Cpss varied less than 300% for equivalent doses. There was first-order elimination of drug from plasma, with a median t1/2 of 28.9 hr (range 18.5 to 45.0 hr), and elimination was independent of dose and creatinine clearance. The median ratio of renal amantadine clearance to creatinine clearance was 2.07 (range 0.64 to 4.20), suggesting renal tubular secretion. Compared to data from healthy young adults, the t1/2 was doubled and renal drug clearance was diminished in elderly men. To achieve the target trough Cpss of 300 ng/ml, healthy older men must take amantadine at a dose of 1.4 mg/kg/day, and we suggest that this is a rational dose for evaluation of efficacy and safety for influenza A prophylaxis in this population.