Effectiveness of murine leukemia chemotherapy according to the immune state: reconsideration of correlations between chemotherapy, tumour cell killing, and survival time.

Cyclophosphamide (CPM) chemotherapy (134 mg/kg) of L1210 leukemia is less efficient in mice previously immunodepressed by antithymocyte serum (ATS) than in non-ATS pretreated mice. On the other hand, administration of a higher dose of CPM (403 mg/kg), which kills a greater number of leukemic cells but induces an immunodepression, according to the skin graft test, results in a shorter survival time than does the administration of a lower dose of CPM (134 mg/kg), capable of killing fewer leukemic cells but not inducing such an immunodepression. Thus, it appears that: (1) the antileukemic effect of the same dose of a chemotherapeutic drug is less efficient in immunodepressed than in nonimmunodepressed hosts, and (2) calculation of the number of neoplastic cells killed by a given chemotherapy by extrapolation from the survival time may lead to erroneous conclusions.