Acyclic Guanosine Analogs Inhibit DNA Polymerases .alpha., .delta., and .epsilon. with Very Different Potencies and Have Unique Mechanisms of Action

Abstract

Acyclovir triphosphate, ganciclovir triphosphate and penciclovir triphosphate inhibited DNA polymerases alpha, delta, and epsilon. Each triphosphate preferentially inhibited pol delta, although ganciclovir triphosphate was the most impressive of the three; the Ki for inhibition of pol delta was 2 microM (competitive with dGTP), while the Kis for inhibition of pol alpha and epsilon were 80 and 140 microM, respectively. Each of the compounds was polymerized by pol alpha, delta, and epsilon. Incorporation of acyclovir triphosphate resulted in immediate chain termination, whereas incorporation of ganciclovir triphosphate often allowed polymerization of additional dNTPs. Interestingly, chain termination most often occurred after polymerization of just one additional dNTP onto the ganciclovir monophosphate. All three compounds were very weak inhibitors of DNA primase. Acyclovir triphosphate, however, was a unique inhibitor of the pol alpha-catalyzed elongation of primase-synthesized primers. Immediately after DNA primase synthesized a primer, pol alpha frequently incorporated acyclovir triphosphate with consequent chain termination. If, however, pol alpha did not immediately polymerize acyclovir triphosphate onto the primase-synthesized primer, further dNTPs were readily added and acyclovir triphosphate was incorporated much less frequently.

GCV phosphates are transferred between HeLa cells despite lack of bystander cytotoxicity

“…[12][13][14][15][16] Upon further phosphorylation by endogenous cellular kinases to its cytotoxic triphosphate form, GCVTP then competes with endogenous dGTP for incorporation into DNA. 14,[17][18][19][20][21] We have demonstrated previously that GCV incorporation into DNA is associated with a delayed S-phase hypothesized to lead to cell death. 9 Success of this approach in animal tumor models has led to clinical trials in a variety of tumor types.…”

Temozolomide enhances herpes simplex virus thymidine kinase/ganciclovir therapy of malignant glioma

“…Furthermore, it was shown that GCVtriphosphate inhibits DNA polymerase , an enzyme involved in repair of DNA cross -links. 33 Measurement of synergy between HSV -tk / GCV and TMZ An important issue in interpreting the results of this study concerns the methods that were used for assessment of synergy. The CI -isobologram method of ChouTalalay 21,22,25 is the only method employing actually derived equations from the mass action principle offering quantitative analyses of drug interactions by first defining the additive effect.…”

Hydroxyurea significantly enhances tumor growth delay in vivo with herpes simplex virus thymidine kinase/ganciclovir gene therapy

“…1 The antitumor activity is the result of HSVtkexpressing tumor cells activating GCV to its cytotoxic triphosphate derivative which competes with endogenous dGTP for incorporation into DNA interfering with cellular DNA synthesis. 2,3 HSVtk/GCV cancer gene therapy has been successful in vitro with a number of tumor cell lines and has produced complete tumor regressions in animal models, [4][5][6][7][8][9][10] which has been the impetus for the development of clinical trials for the treatment of brain and ovarian tumors. 11 This form of therapy not only benefits from, but is also dependent on a phenomenon know as the 'bystander effect', whereby a small percentage of HSVtk-expressing cells causes GCV-mediated cell death to tumor cells which are not expressing the transgene.…”