It was shown previously that three 1,5-diazabicyclo[3.1.0]
hexane-2,4-diones selectively inhibited human Type II
IMP dehydrogenase (IMPDH) from Tmolt4 cell
leukemia [Barnes et al., Biochemistry 2000;39:13641-50]. The agents acted as competitive inhibitors of this
isoform, yet when tested against human Type I at concentrations ranging from 0.5 to 500 microM, Type I was not inhibited. This study focuses on the
antineoplastic activity and cellular effects of one of these agents and two new derivatives containing ethoxycarbonyl substitution at position C6. Agents were studied for antiproliferative activity in human Tmolt4
leukemia (EC(50) 3.3 to 9.2 microM) and alterations in the levels of
enzymes involved with cellular metabolism, including
DNA and
RNA syntheses due to IMPDH inhibition. Results reported here demonstrate that 6-ethoxycarbonyl-3,3-disubstituted-1,5-diazabicyclo[3.1.0]
hexane-2,4-diones are effective inhibitors of
DNA synthesis (30-66% inhibition) due to reductions in
dGTP pool levels. Collectively, the three agents proved to be selective inhibitors of human IMPDH Type II activity (K(i) 11-33 microM), leading to cytotoxicity in a number of suspended and solid
tumor lines, notably MCF-7 (EC(50) 0.7 to 6.0 microM). In addition, negative cytotoxic actions of these agents on WI-38 cell growth, a normal rapidly growing human line, suggest that specific targeting of Type II IMPDH would help to eliminate cell killing in lines where Type I predominates. Furthermore, effects of agents on
DNA synthesis and cell death could be reversed by the addition of exogenous
guanosine to the medium. Results from in vitro studies suggest that the 6-ethoxycarbonyl-3,3-disubstituted-1,5-diazabicyclo[3.1.0]
hexane-2,4-diones may be used as effective
isozyme-selective chemotherapeutic agents.