Spirogermanium: a new investigational drug of novel structure and lack of bone marrow toxicity.
Spirogermanium (NSC 192965) is a new metallic investigational anticancer drug of novel heterocyclic structure. Although its mode of action has not been fully elucidated, it appears that spirogermanium is not a phase or cell cycle specific drug and inhibits DNA, RNA and protein synthesis, the protein synthesis being the most susceptible to this agent. Spirogermanium has shown cytotoxic activity in vitro against several human tumor cell lines at concentrations (1 micrograms/ml) that were also found toxic to the cultured rat neurons. Although spirogermanium has no effect on normal bone marrow colony forming cells in mice, dogs, or man, it has revealed cytotoxic activity in vitro against human myeloid leukemia cell line K 562 at clinically achievable concentrations. These in vitro findings, indicating selective cytotoxic activity against leukemic cells suggest this drug as a candidate for clinical studies in acute and chronic leukemias. Spirogermanium has revealed activity in vivo against intraperitoneally implanted Walker 256 sarcoma, 13762 mammary adenocarcinoma, and 11095 prostatic carcinoma in rats, but no antitumor activity in vivo was found in the murine tumors used in the past by the NCI screen (L 1210 and P 388 leukemia, B 16 melanoma, Lewis lung carcinoma). Spirogermanium is remarkable for its lack of bone marrow toxicity confirmed in preclinical toxicology and clinical studies; moderate, predictable, and reversible CNS toxicity is dose-limiting. Activity in malignant lymphoma, ovarian cancer, breast cancer, large bowel cancer, and prostatic cancer was reported in the clinical studies. The drug is currently under clinical investigation against the wide spectrum of solid tumors and malignant lymphomas. The dose of 80-120 mg/m2, given by 60' infusion three times a week, is currently used and tolerated in Phase II clinical studies. The recently introduced five days continuous infusion schedule has been also under clinical investigation and the doses of 250-300 mg/m2/day are recommended for Phase II studies. Of interest are results reported in this paper of spirogermanium in vitro preferential activity against the resistant strains of Plasmodium falciparum at clinically achievable concentrations suggesting this drug as a possible new antimalarial agent of novel structure.