Shewach Laboratory

The Shewach laboratory studies the metabolism and mechanism of action for chemotherapeutic agents, with the overall goal of developing approaches that will enhance the antitumor activity of these drugs for eventual translation to clinical trials. The two major projects currently being studied are: 1) interaction of chemotherapy drugs with ionizing radiation; and 2) using gene therapy to deliver novel drugs or molecular inhibitors selectively to tumors to enhance antitumor activity while decreasing normal tissue toxicity.

Mechanism of Radiosensitization with Chemotherapy Drugs

The anticancer drug gemcitabine is phosphorylated readily in tumor cells, and both the di- and triphosphate derivatives contribute to the antitumor activity of the drug. The diphosphate is a mechanism-based inhibitor of ribonucleotide reductase, whereas the triphosphate is incorporated into DNA. We hypothesized and then proved that gemcitabine enhances cytotoxicity mediated by ionizing radiation, a combination which has now been shown to have clinical activity. We have established that radiosensitization with gemcitabine is conferred by the inhibition of ribonucleotide reductase and consequent decrease in dATP. We have demonstrated that depletion of dATP causes incorrect nucleotide incorporation into DNA which, if not repaired prior to irradiation, enhances cell death. We are further evaluating the type of damage and repair pathways involved in radiosensitization with gemcitabine. We have now established that other drugs that decrease deoxynucleotides, such as fluorodeoxyuridine (thymidylate synthase inhibitor), also cause radiosensitization by promoting misincorporation of nucleotides in DNA. We are currently evaluating the molecular mechanism by which misincorporated nucleotides cause radiosensitization. In addition, we are attempting to utilize these mechanistic data to improve radiosensitization in vivo while decreasing normal tissue toxicity.

Gene Therapy Studies

The antiviral drug ganciclovir is normally non-toxic to cells, but in the presence of herpes simplex virus thymidine kinase (HSV-TK) it is phosphorylated to its triphosphate which is a potent antitumor agent. Studies in my laboratory have demonstrated that ganciclovir is superior to other related antivirals due to its unique effects on DNA synthesis. Extension of our mechanistic studies led to the demonstration that inhibitors of ribonucleotide reductase synergistically enhanced the antitumor activity of ganciclovir. We have further established the mechanism by which another gene therapy approach, using the antifungal flucytosine and cytosine deaminase, synergizes with HSV-TK and ganciclovir. These mechanism-based approaches produce superior in vivo antitumor activity. Current studies are exploring the molecular mechanism for the synergy with these approaches, and how best to translate our work to clinical trials.