The reductive conversion of
ribonucleotides to
deoxyribonucleotides by
ribonucleotide reductase (RR) is a crucial and rate-controlling step in the pathway leading to the biosynthesis of
DNA, since
deoxyribonucleotides are present in extremely low levels in mammalian cells. Mammalian
ribonucleotide reductase (RR) is composed of two dissimilar
proteins, often referred to as R(1), which contains polythiols and R(2), which contains non-
heme iron and a free tyrosyl radical. Both the R(1) and R(2) subunits contribute to the active site of the
enzyme. Currently, there are two broad classes of RR inhibitors. The first class includes
nucleoside analogs which bind to the R1 subunit of the
enzyme, several of which are in development. Among those,
Gemcitabine and
MDL 101,731 have demonstrated impressive efficacy against various solid
tumors.
Gemcitabine has now been approved for the treatment of
pancreatic cancer and
non-small cell lung cancer. The most promising second class of inhibitors of RR includes HCTs [alpha--(N)-heterocyclic carboxaldehyde
thiosemicarbazones, e.g., 3-AP and 3-
AMP], which exert
enzyme inhibitory effect through high affinity binding with non-
heme iron. Based on the clinical success achieved by
Gemcitabine, it seems reasonable that a strong inhibitor of RR, which is essential for cellular replication, would be a useful addition to the existing therapeutic agents against
cancer. In this chapter, we wish to report several highly efficient syntheses for both 3-AP and 3-AMP based upon
palladium mediated Stille/Suzuki/Heck coupling reactions. Based upon the in vivo efficacy profile observed with these two agents, 3-AP was chosen over 3-AMP as the candidate for further optimization with the intention to improve its biological and pharmaceutical properties. In this vein, we have completed the synthesis of two water soluble
phosphate containing
prodrugs and one
disulfide-linked
prodrug of 3-AP. As expected, bioconversion study using either
alkaline phosphatase or
glutathione showed that these
prodrugs were indeed converted to the parent 3-AP. When evaluated against the murine M-109 lung
carcinoma as well as the B16-F10 murine
melanoma xenograft models, the newly prepared
phosphate prodrugs displayed improved efficacy and safety profiles than that found with the parent. More significantly, the ortho-
phosphate prodrug 21 demonstrated impressive antitumor effect using once-a-day dosing regimen. In summary, the results disclosed herein demonstrated that some of 3-AP
prodrugs prepared indeed demonstrated improved pharmaceutical, biological and toxicity profiles over the parent 3-AP. Efforts directed towards further optimization of 3-AP
prodrugs as novel
anticancer agents is clearly warranted.