Carbocyclic Nucleoside Isosteres as Potential Antitumor

Basic Sciences

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Abstract

My laboratory continues to explore the bicyclo[3.1.0]hexane template as a platform for the construction of conformationally locked nucleosides with the aim of determining the conformational preferences of enzymes involved in nucleoside anabolism and catabolism. This versatile template can be used to synthesize nucleoside analogues constrained into the two possible natural conformations (North and South) adopted by natural nucleosides, which has allowed us to define, for the first time, the antipodal conformational preferences displayed by kinases and polymerases. The application of this knowledge has been successfully applied to the design and synthesis of nucleoside analogues directed to precise therapeutic targets in HIV, herpes and pox viruses. The success of the chemically synthesized North-methanocarbathymidine (N-MCT)-5'-triphosphate as a delay chain terminator of HIV RT and its ability to overcome the excision mechanism of HIV resistance prompted the synthesis of several new analogues capable of circumventing the principal limitation of N-MCT, which is its inability to be activated (phorphorylated) by cellular thymidine kinase (c-tk). Two lines of investigation have been pursued and some of the major findings are highlighted: 1) the successful synthesis of isomeric N-MCT analogues where the fusion site of the cyclopropane ring was relocated to facilitate recognition by c-tk, an enzyme that displays a South penchant. This approach resulted in a compound with 100-fold increased recognition by the related enzyme, herpes thymidine kinase (HSV-tk); however, recognition by the c-tk was still lacking because the relocation of the fusion site of the cyclopropane ring removed a critical OH necessary for enzyme recognition. This finding prompted us to implement the second approach, which has been synthetically more challenging: 2) ring closure metathesis to prepare intermediate precursors of the bicyclo[3.1.0]hexane template with various substitutions (R = OH, CH2OH) at the tip of the fused cyclopropane ring that could simultaneously lead to recognition by c-tk and allow chain elongation by the polymerase from a different position. Functionalization at the tip of the cyclopropane ring has been successfully achieved for R = CH2OH and efforts to prepare the more important, but synthetically more difficult hydroxycyclopronane analogue (R = OH) continue. During this period patent applications for N-MCT as the most potent antiviral agent against human herpes virus 8, the causative agent of Kaposi's sarcoma, and small pox have been submitted. A manuscript describing the former activity is in press and the second one is in preparation. The study on DNA bending continues with the syntheses of segments built with conformationally locked North sugars. This work was completed and published in the Proceedings of the National Academy of Sciences, USA. The study of the unusual properties of identical DNA segments built with Southern units continues.