Pharmacokinetics of zimelidine

Summary

The systemic availability of a new antidepressant, zimelidine, and of its pharmacologically active metabolite, norzimelidine, was studied in six healthy male volunteers. Three single doses of zimelidine (25 mg and 100 mg orally and 25 mg i.v.) and two single doses of norzimelidine (25 mg orally and i. v.) were given to each volunteer allowing at least seven days between administrations. Plasma concentrations of zimelidine and norzimelidine were determined in serial blood samples by HPLC. Following oral zimelidine peak plasma concentrations of the metabolite were attained about 3 h after dosing. Oral administration of norzimelidine itself resulted in a plasma concentration profile for this compound that was similar to that observed after oral zimelidine. Utilising the plasma concentration data following intravenous infusion of each compound, the elimination half-lives for zimelidine and norzimelidine were calculated to be 5.1 h (range 4.3–6.0) and 15.5 h (range 10.6–22.9) respectively. The total body clearances of the 2 compounds were similar at 0.52 l · min⁻¹ (range 0.26–0.70) for zimelidine and 0.56 l · min⁻¹ (range 0.28–0.83) for norzimelidine. The substantially longer elimination half-life of norzimelidine was apparently the result of a larger volume of distribution (9.4 l · kg⁻¹; range 7.8–11.4) for this metabolite, as compared to zimelidine (3.21 · kg⁻¹; range 1.6–4.9). The calculated bioavailability of zimelidine was 26% (range 9.1–39) after the 25 mg oral dose, and 29% (range 14–46) after the 100 mg dose. The bioavailability of norzimelidine was 66% (range 36–91). However, oral administration of zimelidine resulted in as much or more norzimelidine reaching the systemic circulation, as the oral administration of norzimelidine itself. This is important as a large part of the activity of the drug may be due to the metabolite.