The purpose of this study was to evaluate the effect of a concomitant infusion of ornithine on the difluoromethylornithine (DFMO)-induced thrombocytopenia and antitumor activity. Male Fischer 344 rats with either a transplantable fibrosarcoma or Ward colon tumor were given a 12-day continuous infusion of DFMO (2000 mg/kg/day) alone or with ornithine. The dose of ornithine was defined as the molar ratio to DFMO. A continuous infusion of DFMO significantly reduced circulating platelet counts to

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... 16% of the control. Concomitant ornithine treatment at a molar ratio of 0.2-0.5 resulted in protection of the rat from thrombocytopenia while the antiproliferative activity of DFMO against the fibrosarcoma or Ward colon tumor was unaffected. At a higher ornithine: DFMO molar ratio (0.7), the DFMO-induced inhibition of tumor growth was blocked. Tissue polyamine levels suggest a different sensitivity of tumor and normal tissue to DFMO. Concomitant ornithine resulted in a greater increase in the polyamine levels of normal tissues, compared with the tumor. These results suggest that ornithine can selectively inhibit DFMO-induced thrombocytopenia while not affecting the antitumor activity.