Reactions of 4-bis(2-chloroethyl)aminophenylacetic acid (phenylacetic acidmustard) in physiological solutions

4-Bis(2-chloroethyl)aminophenylacetic acid (phenylacetic acid mustard, 2) i s the major metabolite of the cancer chemotherapeutic agent chlorambucil (1 ). Although its high antitumor activity and high toxicity to normal tissues have been known for a long time, no detailed chemical data on its reaction s in aqueous media have been available. According to the present results 2 is decomposed in aqueous solutions by the same mechanism as other aromatic and aliphatic nitrogen mustards: an intramolecular, rate determining attack of the unprotonated nitrogen to form an aziridinium ion intermediate is fo llowed by attack of an external nucleophile. Species 2 is considerably more stable in whole plasma than in plasma ultrafiltrate (t(1/2) at 37 degrees C 10 h and 37 min, respectively). Also the product distribution is complete ly different: while in protein-depleted plasma the only reaction is hydroly sis, in plasma the predominant reactions are non-covalent and covalent bind ing of 2 to albumin. The present information is important when clinical eva luation of chlorambucil, or in vitro evaluation of phenylacetic acid mustar d, is performed in order to determine efficacy and bioavailability of these compounds.