Comparison of pharmacokinetic and safety profiles of amantadine 50- and 100-mg daily doses in elderly nursing home residents.

Pharmacokinetic and adverse event profiles of 50- and 100-mg amantadine doses administered daily for up to 21 days for influenza prophylaxis were compared in 82 elderly nursing home residents (mean age 85 yrs, 68% female). We sought to determine if a standard daily dose of 50 mg would achieve mean steady-state trough serum concentrations (CPSSt) of 300 ng/ml and be associated with a lower frequency of adverse events than 100-mg doses. Statistically significant relationships were found between CPSSt and dosage (in mg/kg/day) and serum creatinine. Adverse events were more common with the 100-mg dose (24% vs 14%); 94% occurred in women. Amantadine CPSSt and apparent clearance were not significantly different between sexes. Thirty-nine residents (89%) receiving 50 mg daily achieved CPSSt below 300 ng/ml compared to 42% receiving 100 mg. Standard daily amantadine doses of 50 mg may not achieve adequate CPSSt in elderly nursing home residents, but 100 mg may lead to excessive CPSSt and adverse events, especially in the presence of renal impairment.