Pharmacokinetics of physostigmine in man following a single application of a transdermal system

1. The pharmacokinetics of physostigmine were investigated in a three-way cross-over design in six healthy, male volunteers comparing a physostigmine transdermal system (PTS), an oral solution and an i.v. infusion. 2. A single application of the patch over 24 h produced detectable plasma drug concentrations after a mean lag-time of 4 h. Thereafter, the drug was absorbed continuously from the PTS and putative therapeutic plasma concentrations were measured over approximately 18 h. 3. A mean absolute bioavailability of 36% was determined for the transdermal system and 3% for the oral solution. In comparison with the oral solution, interindividual variability of pharmacokinetics was less with the PTS. 4. The mean amount of physostigmine released from the transdermal system after 24 h was 5.7 mg. Because of extensive metabolism, only 2.2 mg of physostigmine were detected systemically. 5. After removing the PTS, the mean apparent half-life of elimination was 4.9 h, compared with 0.5 h for the i.v. infusion. This indicates continued drug absorption from a skin depot. 6. Physostigmine was well tolerated by the volunteers. With the PTS, a mild erythema was observed at the area of application, disappearing within a few hours.