Covalent sequestration of phosphoramide mustard by metallothionein - An invitro study

Acquired drug resistance is one of the most important problems in cancer ch emotherapy. One of the proposed mechanisms for these phenomena is the seque stration of alkylating agents by metallothionein in vivo. This research sho ws that metallothionein can covalently sequester phosphoramide mustard, the active form of cyclophosphamide in vitro. On-line electrospray mass spectr ometry reveals that it is phosphoramide, not nornitrogen mustard that alkyl ates metallothionein, although the metallothionein/nornitrogen mustard addu ct was isolated as the major adduct. Tandem mass spectrometric experiments were performed on an isolated drug-modified tryptic peptide. The alkylation occurred predominantly at Cys48 of metallothionein. These results provide further evidence that overexpression of metallothionein can detoxify the ac tive form of the drugs.