Analysis of the in vitro inhibition of murine and human tumor cell growth by pyrazole derivatives and a substituted azabicyclo [3.1.0] hexane-2,4-dione.

3-Ethoxycarbonyl-5-phenyl-1, 3a, 4, 5, 6, 6a-hexahydropyrrolo[3,4-c]pyrazole-4, 6-dione, 2, 2, 6, 6-tetraethyl-1H, 5H-pyrazole[1, 2-a]pyrazole-1, 3, 5, 7-[2H, 6H]-tetraone and 6-ethoxycarbonyl-3-phenyl-3-azabicyclo[3.1.0] hexane-2, 4-dione demonstrated potent cytotoxic activity in the human Tmolt3, Tmolt4 and HL-60 leukemia screens, HuT-78 lymphoma and HeLa suspended uterine carcinoma cell lines. Most notable was the finding that these compounds were significantly more active than the standard cytotoxic agents examined in the MCF-7 breast (ED50 0.2-1.0 microg/ml) and U87MG glioma (ED50 1.3-2. 6 microg/ml) tumor screens. The agents inhibited Tmolt4 leukemia DNA and RVA syntheses after 60 min at 100 microM Multiple enzymes involved with nucleic acid metabolism appeared to be targeted including inhibition of RNA polymerases, ribonucleotide reductase and nucleoside kinase activities, however, inhibition of de novo purine synthesis at the key regulatory enzyme IMP dehydrogenase appeared to be the primary target. The predominant IMPDH isoform (Type II) detected in a number of human cancers, such as leukemias, ovarian and breast, was inhibited by the compounds yielding IC50 values in the microM range. Furthermore, inhibition of IMP dehydrogenase activity led to the selective depletion of dGTP pool levels by two of the compounds. The DNA molecule was not a target of the agents since no alkylation of the bases, cross-linking of the DNA strands or intercalation between base pairs occurred. Yet, the compounds did cause DNA fragmentation after incubating at 100 microuM for 24 h which was consistent with the observed decrease in ct-DNA viscosity.