The relationship between in vitro cytotoxicity and melphalan induced interstrand-DNA-cross-linking was studied in lymphocytes from healthy persons and from patients with a variety of lymphoproliferative disorders. Interstrand-DNA-cross-link formation, as measured by ethidium bromide fluorescence assay showed a highly significant correlation with in vitro cytotoxicity in normal lymphocytes an in those from patients with lymphoproliferative disorders. However, the melphalan concentration at which such cross-linking occurred, differed significantly when normal lymphocytes and those in patients with lymphoproliferative disorders were compared. The kinetics of interstrand-DNA-cross-link formation and removal following treatment with melphalan also differed, with lymphocytes from patients with lymphoproliferative disorders, particularly those having had prior alkylating agent exposure, showing a more rapid rate of disappearance of cross-links as compared to normal lymphocytes. These findings suggest an increased rate of DNA repair occurring in lymphocytes from patients with lymphoproliferative disorders. In a clinical correlation study in vitro melphalan resistance correlated with in vivo resistance to treatment with alkylating agent based chemotherapy. These results suggest a rapid and simple method for determining alkylator resistance in patients with lymphoproliferative disorders.

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