Effect of oxygen on the cytotoxicity and antitumor activity of etoposide

The selective cytotoxicity of the epipodophyllotoxin etoposide toward normally oxygenated and hypoxic EMT6 mouse mammary tumor cells in culture was examined. Etoposide was much more toxic to normally oxygenated cells. The ratio (hypoxic to oxygenated) of drug concentrations producing 1 log of cell kill was approximately 30:1. Established FSa-11C fibrosarcomas of C3HeB/FeJ mice were treated with 10, 15, or 20 mg etoposide/kg body weight in a 6-day protocol. Fluosol-DA with or without breathing of carbogen (i.e., 95% O2-5% CO2) was added to the treatment program on days 1, 3, and 5. The combination of etoposide-Fluosol-DA-carbogen markedly enhanced tumor growth delay compared to the result with etoposide alone. The dose-modifying effect observed was 1.9 +/- 0.3. With the use of both single-dose and multiple-dose protocols for etoposide and Fluosol-DA with air or carbogen breathing, the survival of bone marrow cells was measured by colony formation in vitro (granulocyte-monocyte colony-forming units). Fluosol-DA and carbogen breathing did not increase the toxicity of etoposide to the bone marrow. Thus the enhancement in antitumor activity produced by the addition of Fluosol-DA and carbogen breathing to etoposide treatment was not accompanied by a concomitant increase in normal tissue toxicity and represents an increase in the therapeutic efficacy of etoposide.