Abstract
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 R1, which contains polythiols and R2, which contains non-heme iron and a free tyrosyl radical. Both the R1 and R2 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 [α-(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.
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