Degradation of melphalan in vitro: rationale for the use of continuous exposure in chemosensitivity assays

The hydrolysis of melphalan in Cell Culture medium at 37 degrees C has been studied. Degradation of melphalan proceeded via monohydroxy-melphalan (MOH) to dihydroxymelphalan [M(OH)2] with a half-life of 66 min for melphalan and 58 min for MOH. The half-life for melphalan was similar to the terminal half-life of the drug in vivo. The effect of the two metabolites, MOH and M(OH)2, on the chemosensitivity of K562 leukaemia cells during continuous exposure to melphalan was also examined. M(OH)2 had no potentiating effect on melphalan cytotoxicity at concentrations up to 100 micrograms/ml. MOH also had little effect on cell kill at concentrations higher than those commonly achieved during in vitro chemosensitivity assays. The LD50 for 1 h exposure to melphalan was twice that for continuous exposure: this also suggests no interference by MOH and M(OH)2. These data suggest that continuous exposure of melphalan in in vitro chemosensitivity assays is probably preferable to the arbitrary 1 h drug exposure time commonly employed.