Pharmacokinetics of melphalan in isolated perfusion of the limbs

The pharmacokinetics of melphalan (L-phenylalanine mustard) in isolated perfusion treatment of patients with melanoma in the limbs have been studied at standardized pseudo-physiological perfusion conditions. Perfused tissue volumes ranged from 2.1 to 16 liters as measured by water displacement. A fixed dose of 10 mg of the drug per liter of perfused tissue was applied. The resulting variation in total melphalan dosage gave rise to varying drug concentrations in the perfusate as the extracorporeal system was operated with a fixed volume of priming fluid. Perfusion with melphalan was applied for 60 min. Concentrations of intact drug were assayed by high-performance liquid chromatography. The melphalan concentrations vs. time for each patient were fitted to a biexponential equation using a nonlinear least-squares computer program. Mean half-lives of 4.7 +/- 0.3 and 53.0 +/- 1.6 min were obtained for the alpha and beta phase, respectively. The hybrid constant alpha remained virtually constant with increasing dose (i.e., increasing drug concentration) and beta also appeared independent of dose levels. With increasing tissue volume plasma clearance was found to diminish per unit of tissue volume. This phenomenon, and a levelling off of the steady-state distribution volume with increasing volume of perfused tissue, are discussed in terms of a possibly restricted transfer of drug from intravascular to the extracellular space and of the possibility of saturation of cellular uptake systems in the bulk of the limb tissues.