APOPTOTIC CELL-DEATH DURING TREATMENT OF LEUKEMIAS

The apoptosis-associated DNA strand breaks were detected in situ, in i ndividual leukemic cells in peripheral blood and bone marrow of over 1 10 patients with different types of leukemia (ALL, AML, CML in blastic crisis, APL), prior to and during routine chemotherapy. The DNA stran d breaks were labeled with digoxigenin- or biotin-conjugated dUTP in t he reaction catalyzed by exogenous terminal deoxynucleotidyl transfera se, and the cells, counterstained for DNA, were analyzed by bivariate flow cytometry. The proportion of cells with DNA strand breaks prior t o therapy, most likely reflecting spontaneous apoptosis, varied from 0 .1 to 16%, but in the large majority of cases was below 3%. Administra tion of drugs of different classes, which included DNA topoisomerase I (Topotecan) and II (mitoxantrone, VP-16) inhibitors, antimetabolite ( ara-C) or microtubule poison (Taxol), all triggered the appearance of cells with extensive DNA breakage, typical of apoptosis, to up to 80%. The peak of the response, measured as maximal percent of cells with D NA strand breaks, which varied between individual patients by as much as factor 10, was generally seen between 8 to 24 h after the initial a dministration of DNA topoisomerase inhibitors, and somewhat later (48- 72 h) during the response to Taxol or ara-C. Thus, the data show that the response to treatment with a variety of drugs, in terms of inducti on of apoptosis, can be conveniently measured by the present method. T he prognostic value of the apoptotic index, before, as well as during treatment, is being estimated for each type of leukemia, in the ongoin g prospective studies.