Pharmacokinetics of 5,6-dihydro-5-azacytidine (NSC-264880) in the foxhound.

An HPLC analytical method was applied to the determination of plasma concentrations of 5,6-dihydro-5-azacytidine (NSC 264880, DHAC) in two foxhounds after a rapid intravenous infusion of 300 mg/kg DHAC. The dose employed is the mouse equivalent LD10 dose which results in mild reversible toxicity in the dog. The decline in DHAC plasma concentrations was greater than three log decades after dosing. The plasma concentration-time data was computer-fitted by NONLIN to a three compartment open model with first-order elimination using the equations for a short intravenous infusion. The half-lives corresponding to the three exponential phases were: t1/2 alpha = 5.78 min, t1/2 beta = 1.57 h and t1/2 gamma = 22.0 h in dog 1 and t1/2 alpha = 7.41 min, t1/2 beta = 2.25 h and t1/2 gamma = 21.6 h in dog 2. The terminal phase of the plasma concentration time profiles represented a minor contribution (2.2-3.2%) to the total area under the curve. The plasma concentration time data for the first 12 h after dosing was computer fitted to a two compartment open model. The initial and terminal half-lives determined from the two compartment fits were similar to the t1/2 alpha and t1/2 beta values of the fits to the three compartment open model. Similar total body clearance values were calculated from the areas under the plasma concentration time curves from time zero to infinity for the computer fits to the three and two compartment models, respectively. Thus, for practical purposes, it appears feasible to define adequately the total body clearance of DHAC by analysis of the plasma concentration time data during the time interval in which the plasma concentration is described as a bi-exponential equation. Renal excretion of the parent drug is the principal route of excretion of DHAC from the dog.