Failure of orally administered RA233 to influence B16 melanoma growth or metastasis

Possible prophylactic antitumor and/or antimetastatic effects of long-term oral administration of a potent inhibitor of platelet aggregation, the pyrimido-pyrimidine derivative RA233, were assessed using four phenotypically distinct clones of the mouse B16 melanoma. The clones tested included: a poorly tumorigenic, very slowly growing and poorly metastatic population (G3.15); a moderately tumorigenic and slowly growing population that frequently metastasizes to the lungs (G3.5); a highly tumorigenic, moderately growing and highly metastatic population (G3.12); and a highly tumorigenic and rapidly growing population that is generally nonmetastatic but can be slightly metastatic when tumors are initiated by very small numbers of cells (G3.26). Addition of 0.5 mg/ml RA233 to the drinking water continuously from the time of subcutaneous injection of cultured tumor cells until death from tumor growth, which resulted in a daily uptake of 80-100 mg/kg of drug per mouse, failed to significantly influence the tumorigenicities, tumor growth rates, metastatic incidences, or metastatic burdens of any of these clones. RA233 at doses equivalent to those delivered daily to experimental animals strongly inhibited ADP-induced aggregation of homologous C57BL/6 mouse platelets and exhibited selective anti-proliferative effects on cultured cells. Although RA233 prolonged bleeding times, pharmacokinetic analysis indicated that clearance of RA233 from mice was so rapid that achievement of sustained circulating levels sufficient to influence tumor cells or platelet-tumor cell interactions by oral administration was unlikely.