Absolute Bioavailability and Absorption Profile of Cyanamide in Man

A pharmacokinetic study of cyanamide, an inhibitor of aldehyde dehydrogenase (EC1.2.1.3) used as an adjuvant in the aversive therapy of chronic alcoholism, has been carried out in healthy male volunteers following intravenous and oral administration. Cyanamide plasma levels were determined by a sensitive HPLC assay, specific for cyanamide. After intravenous administration cyanamide displayed a disposition profile according to a two-compartmental open model. Elimination half-life and total plasma clearance values ranged from 42.2 to 61.3 min and from 0.0123 to 0.0190 L · kg ^–1 · min^–1, respectively. After oral administration of 0.3, 1.0, and 1.5 mg/kg ± SEM values of C_max, t_max (median) and AUC were 0.18 ± 0.03, 0.91 ± 0.11, and 1.65 ± 0.27 g · ml ^–1 ; 13.5, 13.5, and 12 min; and 8.59 ± 1.32, 45.39 ± 1.62, and 77.86 ± 17.49 g · ml ^–1 · min, respectively. Absorption was not complete and the oral bioavailability, 45.55 ± 9.22, 70.12 ± 4.73, and 80.78 ± 8.19% for the 0.3, 1.0, and 1.5 mg/kg doses, respectively, increased with the dose administered. The models that consider a first-order absorption process alone (whether with a fixed or variable bioavailability value as a function of dose) or with loss of drug due to presystemic metabolism (with zero-order or Michaelis–Menten kinetics) were simultaneously fitted to plasma level data obtained following 1 mg/kg iv and 0.3, 1.0, and 1.5 mg/kg oral administrations. The model that best fit the data was that with a first-order absorption process plus a loss by presystemic metabolism with Michaelis–Menten kinetics, suggesting the presence of a saturable first-pass effect.