Mechanism of Antitumor Action of Pyrimidinones in the Treatment of B16 Melanoma and P388 Leukemia

Abstract This study was undertaken in an attempt to understand the mechanism of antitumor action of pyrimidinones alone and in combination with cyclophosphamide (CY). Pyrimidinones such as 2-amino-5-bromo-6-(3-fluorophenyl)-4(3 H )pyrimidinone (ABMFPP) were relatively nontoxic toward murine L1210 leukemia cell growth in vitro with the concentration of drug required for a 50% inhibition of cell growth being >50 µg/ml. In contrast, ABMFPP showed anti-B16 melanoma activity in vivo which was sensitive to X-irradiation of the hosts. These results collectively suggest that pyrimidinones may act differently from conventional cytotoxic antitumor agents. Multiple i.p. injections of ABMFPP (125 mg/kg/injection) significantly augmented the cytotoxicity of both natural killer cells and macrophages in peritoneal exudates. The augmentation of both effector cell populations was delayed, but was more pronounced when animals received a dose of CY (100 mg/kg) prior to ABMFPP injections. The combination of CY and ABMFPP also showed a synergistic anti-P388 leukemia effect which appeared to be related to the initial reduction of the tumor burden by CY and the marked augmentation of the cytotoxicity of both natural killer cells and macrophages by ABMFPP. The antitumor activity of ABMFPP against B16 melanoma was almost completely eliminated when animals received a dose of 400 rads X-irradiation 5 days prior to tumor inoculation or a dose of 200 rads X-irradiation followed by several injections of anti-asialo monosialoganglioside antibody. The administration of anti-asialo monosialoganglioside alone also markedly reduced the anti-B16 melanoma activity of ABMFPP. The magnitude of reduction of the antitumor effect of ABMFPP by radiation and/or anti-asialo monosialoganglioside antibody directly correlated with the inhibition of the ABMFPP-mediated augmentation of immune responses. These results strongly suggest that the antitumor effect of ABMFPP alone or in combination with CY is at least in part mediated through its augmentation of natural killer cell and/or macrophage activities.