In vitro and in vivo studies with the drug combination thioTEPA and cyclophosphamide (CPA) were carried out using the MCF-7 human breast carcinoma cell line and the EMT6 mouse mammary carcinoma cell line. In vitro, survival curves were essentially linear. The EMT6 cell line was less sensitive to thioTEPA than the MCF-7 cell line, with concentrations which reduce cell survival to 10% of 440 and 140 μm, respectively. The response of both cell lines to 4-hydroperoxycyclophosphamide was similar. Simultaneous and immediate sequential treatments with these drugs produced supraadditive cell killing of both cell lines, although the magnitude of the supraadditivity was greater in the MCF-7 cell line than in the EMT6 cell line. Both of these drugs appeared to be as effective as thiol-depleting agents as is diethyl maleate. By DNA alkaline elution, there was a pattern of increasing DNA cross-linking similar to the increasing levels of cytotoxicity of this drug combination with increasing thioTEPA concentrations. In the EMT6 tumor in vivo, the maximally tolerated combination therapy (5 mg/kg × 6 thioTEPA and 100 mg/kg × 3 CPA) produced about 25 days of tumor growth delay which was not significantly different than expected for additivity of the individual drugs. The survival of EMT6 tumor cells after treatment of the animals with various single doses of thioTEPA and CPA was assayed. Tumor cell killing by thioTEPA produced a very steep, linear survival curve through 5 logs. The tumor cell survival curve for CPA out to 500 mg/kg gave linear tumor cell kill through almost 4 logs. In all cases, the combination treatment tumor cell survivals fell well within the envelope of additivity. Both of these drugs are somewhat less toxic toward bone marrow cells by the granulocyte-macrophage colony-forming unit in vitro assay method than to tumor cells. The combination treatments were subadditive or additive in bone marrow granulocyte-macrophage colony-forming unit killing. When bone marrow is the dose-limiting tissue, there is a therapeutic advantage to the use of this drug combination.

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Supported by National Cancer Institute Grant 1P01-CA38493 and grants from the Mathers Foundation and Lederle Laboratories, Pearl River, NY.

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