A pharmacokinetic study of doxapram in patients and volunteers

1. Following intravenous bolus injections or brief infusions in healthy volunteers, plasma concentrations of doxapram declined in a multi-exponential fashion. The mean half-life from 4-12 h was 3.4 h (range 2.4-4.1h), the mean apparent volume of distribution was 1.5 1 kg-1 and the whole body clearance was 370 ml min-1. 2. Enteric-coated capsules of doxapram base were absorbed rapidly after an initial delay, and the systemic availability was about 60%. 3. Doxapram is extensively metabolized and less than 5% of an i.v. dose was excreted unchanged in the urine in 24 h. A metabolite (AHR 5955) was present in plasma in amounts comparable to the parent compound and had a similar half-life. 4. The disposition of doxapram appears to be similar in healthy volunteers and patients with respiratory failure. 5. The previously held belief that plasma concentrations fall rapidly when an infusion is stopped is only true following short duration infusions. The pharmacokinetic properties of doxapram are such that steady-state plasma concentrations will not be achieved for many hours with the recommended constant rate infusion régime.