Since chemotherapy resistance is probably multifactorial, we studied the toxicity and efficacy of adding to etoposide in sequence 5 resistance modulators in the treatment of resistant solid tumors. In cohort 1, metronidazole and ketoconazole were given with i.v. etoposide 100 mg/m(2)/day x 5 days. Because of excessive toxicity, cohort 2 received just metronidazole with etoposide, and metronidazole doses were reduced. Subsequent patient cohorts had the following drugs added to etoposide plus metronidazole: ketoconazole (cohort 3), dipyridamole (cohort 4), tamoxifen [cohort 5 (with etoposide 75 mg/m(2)/day) and 6 (with etoposide 60 mg/m(2)/day)], and cyclosporin (cohort 7). Hence, cohort 7 received daily x 5 i.v. etoposide 60 mg/m(2)/day plus 5 resistance modulators. Forty patients were treated, of whom 38 were evaluable for toxicity. Metronidazole resulted in augmentation of both central neurotoxicity and peripheral neuropathy. Sequential addition of each of dipyridamole, tamoxifen, and cyclosporin appeared to increase hematological toxicity. Some patients also experienced reversible hepatic and renal toxicity. Partial responses were seen in adrenocortical and small cell lung cancers, and minor responses with symptomatic improvement were seen in adrenocortical, small cell lung, non-small cell lung and colorectal carcinomas. Further evaluation of this approach may be warranted in patients with minimal prior chemotherapy exposure.

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