Abstract 391: Implications of Akt inhibition for neoadjuvant radiotherapy: improving the rectal cancer treatment

Resistance to therapy is a major obstacle to a favorable outcome in cancer treatment. For rectal cancer, neoadjuvant chemoradiotherapy (nCRT) can lead to complete tumor regression in a significant proportion of patients (up to 40%) but approximately 60% of patients have only partial or no tumor remission after nCRT. A number of works searching for a differential gene expression signature has failed. As those gene signatures relies on the sample set, there is an increasing agreement that biological understanding is necessary to produce a signature in terms of pathways and biological processes. In this regard we applied pathway enrichment analysis to RNA-Seq data in order to identify biological pathways involved in tumor resistance and discriminate rectal cancer patients that is refractory to neoadjuvant treatment. Mitochondrial oxidative phosphorylation and phosphatidylinositol signaling were identified as the main pathways involved in resistance to nCRT in patients with rectal cancer. Both pathways have been reported as involved in resistance to radiotherapy and chemotherapy in solid tumors but according to the status of gene regulation observed and the literature, there is evidence that Akt pathway could be central in promoting resistance.Therefore, we evaluated the status of Akt activation by immunohistochemistry of pre-treatment biopsies of rectal tumors as well as by immunoblot of colorectal cancer cell lines utilizing antibodies against phosphorylated Akt (pS473). As a result, we observed an increased proportion of Akt phosphorylation in the resistant cell line, SW480, and in patients with TRG 0-2 in comparison to those with TRG 3-4 and in the sensitive cell line, SW48. To evaluate the implication of Akt in promoting nCRT resistance, we performed colony formation assays using Akt inhibitors as radiosensitizers. We observed an additive effect using MK2206 at sub-doses in combination to radiotherapy (2 Gy) in the resistant cell line. In fact, Akt inhibition improved in approximately 40% the tumoricidal effects of radiotherapy. Moreover, at IC50 dose this effect is even higher, reaching 60% the improvement of radiotherapy. Furthermore, the treatment of MK2206 with radiotherapy alone or in combination with 5-FU have similar effectiveness. In this setting, as Akt is involved in a range of cellular functions that promote tumor progression and metastasis, we propose that combination of Akt inhibitor and radiotherapy as neoadjuvant treatment would maximize tu