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]pyrazol e-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] hex ane-2, 4-dione demonstrated potent cytotoxic activity in the human Tmolt(3) , Tmolt(4) and HL-60 leukemia screens, HuT-78 lymphoma and HeLa suspended u terine carcinoma cell lines. Most notable was the finding that these compou nds were significantly more active than the standard cytotoxic agents exami ned in theMCF-7 breast (ED50 0.2-1.0 mug/ml) and U87MG glioma (ED50 1.3-2. 6 mug/ml) tumor screens. The agents inhibited Tmolt4 leukemia DNA and RNA s yntheses after 60 min at 100 muM Multiple enzymes involved with nucleic aci d metabolism appeared to be targeted including inhibition of RNA polymerase s, ribonucleotide reductase and nucleoside kinase activities, however, inhi bition of de novo purine synthesis at the key regulatory enzyme IMP dehydro genase appeared to be the primary target. The predominant IMPDH isoform (Ty pe II) detected in a number of human cancers, such as leukemias, ovarian an d breast, was inhibited by the compounds yielding IC50 values in the muM ra nge. Furthermore, inhibition of IMP dehydrogenase activity led to the selec tive depletion of dGTP pool levels by two of the compounds. The DNA molecul e was not a target of the agents since no alkylation of the bases, cross-li nking of the DNA strands or intercalation between base pairs occurred. Yet, the compounds did cause DNA fragmentation after incubating at 100 muM for 24 h which was consistent with the observed decrease in ct-DNA viscosity.