Syntheses and antitumor activities of potent inhibitors of ribonucleotide reductase: 3-amino-4-methylpyridine-2-carboxaldehydehyde-thiosemicarbazone (3-Amp), 3-amino-pyridine-2-carboxaldehyde-thiosemicarbazone (3-Ap) and itswater-soluble prodrugs

The reductive conversion of ribonucleotides to deoxyribonucleotides by ribo nucleotide reductase (RR) is a crucial and rate-controlling step in the pat hway leading to the biosynthesis of DNA, since deoxyribonucleotides are pre sent in extremely low levels in mammalian cells. Mammalian ribonucleotide r eductase (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 activ e site of the enzyme. Currently, there are two broad classes of RR inhibito rs. The first class includes nucleoside analogs which bind to the R-1 subun it of the enzyme, several of which are in development. Among those, Gemcita bine and MDL 101,731 have demonstrated impressive efficacy against various solid tumors. Gemcitabine has now been approved for the treatment of pancre atic cancer and non-small cell lung cancer. The most promising second class of inhibitors of RR includes HCTs [alpha-(N)-heterocyclic carboxaldehyde t hiosemicarbazones, e.g., 3-AP and 3-AMP], which exert enzyme inhibitory eff ect through high affinity binding with non-heme iron. Based on the clinical success achieved by Gemcitabine, it seems reasonable that a strong inhibit or of RR, which is essential for cellular replication, would be a useful ad dition to the existing therapeutic agents against cancer. In this chapter, we wish to report several highly efficient syntheses for b oth 3-AP and 3-AMP based upon palladium mediated Stille/Suzuki/Heck couplin g reactions. Based upon the in vivo efficacy profile observed with these tw o agents, 3-AP was chosen over 3-AMP as the candidate for further optimizat ion with the intention to improve its biological and pharmaceutical propert ies. In this vein, we have completed the synthesis of two water soluble pho sphate containing prodrugs and one disulfide-linked prodrug of 3-AP. As exp ected, 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 mu rine melanoma xenograft models, the newly prepared phosphate prodrugs displ ayed 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 pro drugs prepared indeed demonstrated improved pharmaceutical, biological and toxicity profiles over the parent 3-AP. Efforts directed towards further op timization of 3-AP prodrugs as novel anticancer agents is clearly warranted .