Experiments to determine the hydrolysis and protein binding of melphalan (L-phenylalanine mustard, L-PAM) were carried out in vitro for therapeutic concentration of the drug: the decrease in L-PAM concentration in plasma and whole blood during 24 h incubation at 37 degrees C was only 5% due to hydrolysis. Serum protein binding was about 90%, whereby 60% and 20% of this binding was due to interactions with albumin and acid alpha 1-glycoprotein, respectively. Immunoglobulins did not participate in the binding of L-PAM. The covalently bound part of L-PAM in serum was 30% in the concentration range of 1-30 micrograms/ml. The binding of dihydroxymelphalan (DOH) in serum did not exceed 20%. Glucocorticoids used in combination with L-PAM for treating multiple myeloma did not influence its protein binding. Our study with 35 sera from 15 patients with multiple myeloma shows that high levels of paraproteins do not increase but may decrease the binding of L-PAM, resulting in an elevated concentration of free drug.

A sensitive high-performance liquid chromatographic assay has been developed for the measurement of the alkylating cytostatic drug melphalan (4-[bis(2-chloroethyl)amino]-L-phenyl-alanine, or L-phenylalanine-mustard, L-PAM) and its two hydrolysis products, monohydroxy melphalan (MOH) and dihydroxy melphalan (DOH). A reversed-phase phenyl column and a mobile phase consisting of acetonitrile/citrate buffer made possible an isocratic separation and quantification. N,N-[bis(2-hydroxy-ethyl)]toluidine has been synthesized as an internal standard structurally related to DOH. A new, accurate "kinetic" calibration procedure enabled us to determine even the concentration of the unstable MOH. The lower limit of quantification was 30 ng/ml for L-PAM and 20 ng/ml for both DOH and MOH with fluorescence detection. The use of this method is illustrated by some pharmacokinetic data in systemic and locoregional melphalan therapy.