Topoisomerase I mediated DNA damage leads to activation of the S and G2 cell cycle checkpoints along with the p53 trails, assessed in. However, interpretation of these pathways is complex due to the various mechanisms associated with cell cycle inhibition, these in turn vary in accordance with levels of topoisomerase I poisons. Depending on the dose of topoisomerase I poison and the cell form, different checkpoints have been found AP26113 to be triggered. Treatment with low dose concentrations of topoisomerase I poisons, which are therapeutically possible, effects in S phase arrest followed by a G2 arrest, although higher doses result in an elevated S phase arrest followed by arrest at G2. These dose dependent ramifications of topoisomerase I toxins have now been proposed to be a result of changes in gene expression patterns and cell cycle response. Inhibitors targeting both topoisomerase I and Hsp90 have been assayed with a number of organizations. Nevertheless. Nevertheless the results have already been unclear. Treatment combining gemcitabine and the Chk1 chemical UCN 01 in HeLa, OVCAR3 and ML 1 cells was found to be additive, combining TPT and UCN 01 also had an additive effect on breast cancer produced cells with mutant or lazy p53, mixed CPT and UCN 01 therapy was found to cause an increase in DNA damage in p53 HCT116 cells compared to their wild type counterparts. Furthermore, synergy following combined Hsp90 and Metastasis topoisomerase I inhibition with 17AAG and the active metabolite of IRT, SN 38, was demonstrated in p53 _HCT116 cells, while in p53 HCT116 cells the mixture was found to be antagonistic. On the other hand, synergy was seen in p53 HCT116 cells in addition to HeLa and T98G when combining 17AAG with SN 38, and extended the potential mechanism to more than simply removal of Chk1. This illustrates ab muscles important point that Hsp90 inhibition results in the simultaneous destruction of several proteins. Several studies used Doxorubicin Adriamycin the widely established pair of isogenic mobile lines HCT116 wild type and knock out for p53. These cells were therefore used by us as our type cell line, with the aim of dissecting the process underlying mixtures of clinically effective topoisomerase I poisons with Hsp90 inhibitors. We describe a typical underlying p53 separate system behind the observed combination synergistic drug effect. We show that concurrent treatment with a Hsp90 chemical and topoisomerase I poison TPT can reverse TPT induced upregulation of the anti apoptotic protein Bcl2. The isogenic human a cancerous colon cell lines, HCT116 p53 wild type and p53 knock out were a gift from Prof. T. Vogelstein. Cells were maintained in McCoys 5A medium supplemented with 10 % foetal calf serum at 37 8C in a five hundred CO2 enriched humidified environment.