| Transient kinetic studies with nucleoside reverse transcriptase (RT) inhibitors of human immunodeficiency virus and key polymerases that cause these agents effects on the host cell have proven useful in gaining a pharmacological understanding of these compounds giving novel insights for future drug design. Structure activity relationships have been created that relate structural features of nucleotide analogs to their incorporation by RT, their target for antiviral activity, and mitochondrial polymerase gamma, located in the mitochondria where these compounds often cause unwanted toxicity. This work shows that analogs with a (+)-beta-D-2',3' -didehydro-2',3'-dideoxy ribose ring are the best tolerated alternative substrates among those examined for RT. Results strengthen the hypothesis that these compounds' ability to mimic natural deoxynucleotides in the RT active site leads to a reduction in the number of drug resistance mutations in the viral pol gene (which encodes RT) and a decrease in the magnitude of these mutations' effect on drug activity. These findings led to the suggestion that (+)-beta-D-2',3'-didehydro-2 ',3'-dideoxyguanosine (D4G) would be a superior nucleoside inhibitor relative to clinically relevant guanosine analogs. However, D4G's poor solubility, instability under acidic conditions and high cytotoxicity make it a poor drug candidate despite its potent anti-HIV activity. In a novel use of a guanosine prodrug approach, D4G was modified at the 6 position of the guanine ring with a cyclopropyl substituted secondary amine (to make cyclo-D4G) in order to impart increased acid stability to the nucleoside. This compound also has the unique attribute among guanosine prodrugs of being less cytotoxic than its parent compound, and displays several chemical properties that may improve its activity in vivo. Initial results against methionine 184 to valine drug resistant virus suggest that cyclo-D4G will have good activity against mutant viruses resistant to other nucleoside analogs similar to that of its thymidine counterpart (+)-beta-D-2' ,3'-didehydro-2',3 '-dideoxythymidine (d4T or Stavudine). Further studies are underway with other drug resistance mutations to test this hypothesis. Cyclo-D4G's favorable attributes, D4G triphosphate's relatively poor incorporation by mitochondrial polymerase gamma and advantageous interactions with approved anti-HIV agents make cyclo-D4G a promising drug candidate and further research should allow for the determination if clinical development is warranted. |
