Topotecan selectively enhances the radioresponse of human small-cell lung carcinoma and glioblastoma multiforme xenografts in nude mice.

OBJECTIVE

To evaluate the therapeutic efficacy of different combinations of the DNA topoisomerase I-targeting drug, topotecan (TPT), with radiation for treatment of two human tumor xenografts.

METHODS

The small cell lung carcinoma 54A and glioblastoma multiforme U87 were transplanted into nude mice. Equal i.p. injections of TPT and/or equal fractions of tumor irradiation were administered daily, for 5 consecutive days. When combined, TPT was injected at different constant time intervals prior to or after each radiation fraction. The tumor growth delay and changes in skin radiation reaction by TPT were evaluated. Tumor oxygenation was measured using the Eppendorf pO(2) histography.

RESULTS

The tumor growth delay induced by such chemoradiotherapy was independent of interval and sequencing of the agents for either tumor model. The efficacy of TPT alone or in combination with radiation was always dose-dependent, although of different magnitude in the two xenografts. In 54A xenografts, TPT alone induced longer growth delay, but its combined effect with radiation was not more than additive. In contrast, U87 responded less to TPT alone, however the drug and radiation interacted synergisticly in this tumor model. Using both a radiobiological approach (tumor irradiation under normoxia vs. clamp hypoxia conditions) and the polarographic electrode measurements, it was shown that TPT did not modify tumor oxygenation and, thus, unlikely modulated oxygen-related tumor radiosensitivity. In contrast to tumors, TPT virtually unchanged skin radiation reaction.

CONCLUSIONS

Our data suggest that TPT, when combined with radiation treatment of tumors, provides a therapeutic gain without substantial local and systemic adverse effects.