No abstract

#403 Background: Triple negative breast cancer (TNBC: negative for ER, PR and HER2) presents a significant clinical challenge. The frequent occurrence of mutations in TP53, the gene encoding the p53, in TNBC provides an opportunity for therapeutic intervention. p53 is required for cells to respond to DNA damage in the G1 phase of the cell cycle. Tumors with a defective p53 pathway fail to arrest (or undergo apoptosis) in G1 and rely on p53-independent pathways at S and G2 for their survival following DNA damage. Therefore, a potential therapeutic strategy for TP53 mutant tumors is to inhibit the S- and G2-checkpoints. Chk1 is a key regulator of both the S- and G2- checkpoints as such, p53 mutant cells are absolutely dependent on Chk1 activity to respond to DNA damage. This property makes Chk1 a potential therapeutic target in p53 defective tumors. Therefore, we tested the combination of irinotecan (DNA damaging agent) and UCN-01 (non-specific Chk1 inhibitor) in a preclinical model of TNBC. This combination was chosen based on preliminary results obtained in patients with TNBC on a Phase I trial conducted at our institution. UCN-01 is also a potent inhibitor of PDK1 and therefore inhibits the PI3K pathway. Given that PTEN is frequently deficient in TNBC, we also monitored components of the PI3K pathway in our preclinical model of TNBC.
 Material and Method: Tumor biopsies from patients with TNBC were engrafted into the humanized mammary fat pad of immunodeficient NOD/SCID mice. TP53 was sequenced in each engrafted tumor explant and the integrity of the p53 pathway was determined by monitoring p53 stabilization and p21 induction following DNA damage. Three independent TNBC tumor explants, one wild-type and two mutant for TP53 were analyzed for their response to irinotecan and UCN-01 either as single agents or in combination. UCN-01 was administered 24h post irinotecan in mice treated with the combination. Mice were sacrificed 48 h later and tumors were harvested and analyzed for cell cycle arrest (geminin, pCdk1), DNA damage (pChk1, gamma H2AX), apoptosis (cleaved caspase 3), checkpoint bypass (pHistone H3) and the PI3K pathway (pS6) by Western blotting and immunohistochemistry.
 Results: Expression profiling revealed that the characteristics of the tumor are preserved in the tumor explants suggesting this is a valid model system to study experimental therapy for TNBC. Irinotecan induced similar levels of DNA damage in TP53 wild-type and TP53 mutant TNBCs. UCN-01 potently inhibited levels of pS6 independent of p53 status. Strikingly, the combination of irinotecan and UCN-01 selectively induced checkpoint bypass and apoptosis in p53 mutant TNBCs.
 Conclusion: p53 status is a significant predictor of response to combination therapies involving DNA damage followed by Chk1 inhibition. Tumors, like TNBC, that frequently lack a functional p53 pathway may be effectively treated using this strategy. Studies are underway to test different chemotherapy agents and more selective Chk1 and PI3K inhibitors in this preclinical model of TNBC. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 403.