NB1011 [E-5-(2-bromovinyl)-2'-
deoxyuridine-5'-(L-methylalaninyl)-phenylphosphoramidate], a
phosphoramidate prodrug of E-5-(2-bromovinyl)-2'-deoxyuridine-5'-monophosphate (BVdUMP), is an investigational new anticancer
drug.
NB1011 targets
thymidylate synthase (TS), which catalyzes the transformation of BVdUMP into cytotoxic reaction products. Due to the elevated levels of TS expression in
tumor cells compared to normal cells, these cytotoxic products are preferentially generated inside
tumor cells, and, as expected,
NB1011 is more toxic to cells with higher levels of TS expression. Therefore,
NB1011 therapy should kill
tumor cells without severely damaging normal cells. Radiolabeled
NB1011 was used to determine the intracellular fate of
NB1011 reaction products and, possibly, the mechanism of action of this
investigational new drug. We found significant incorporation of the radiolabel into cellular macromolecules. In contrast to our expectations that
NB1011 product(s) would be incorporated into
DNA, we discovered that cellular
proteins were the labeled macromolecular fraction. Herein, we report that the intracellular transformation of
NB1011 involves formation of the corresponding monophosphate, TS-dependent transformation into highly reactive intermediates, and subsequent incorporation into cellular
proteins. TS itself appears to escape irreversible inactivation. Our data suggest that
protein modification not
DNA incorporation accounts for the
therapeutic effect of
NB1011. The proposed mechanism is rather unexpected for a
nucleotide analogue and could lead to the discovery of new cellular
protein targets for future
drug design.