Abstract P3-10-04: Efficacy of RIDRPI103, a reactive oxygen species (ROS) activated prodrug in treatment of breast cancer

The Phosphatidylinositol-3 kinases (PI3Ks) is a family of lipid kinases encoded by PIK3C isoform genes. PI3K phosphorylates phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-triphosphate (PIP3), leading to AKT phosphorylation along with other proteins containing a PH domain. Phosphorylation of AKT stimulates protein synthesis and cancer cell growth. PI3K pathway is hyper-activated in >60% of clinical breast cancer patients due to aberrations in the genes encoding HER2, PTEN, PIK3CA, or AKT1-3 leading to de novo and acquired treatment resistance. There has been intense interest in developing drugs that target PI3K. However, drugs targeting PI3K activity are toxic, due to the physiological roles of PI3Ks in basic cellular processes. We have developed a novel technology, called RIDR (ROS-Induced Drug Release) which is a self-cyclizing reagent linked to a PI3K inhibitor (PI-103) to eject PI-103 in a controlled manner under oxidative stress in highly aggressive breast cancers including triple negative breast cancer cell lines (TNBCs) and other ER+, HER2+ breast cancer cell lines with activating PIK3CA mutations. We evaluated the efficacy of RIDR-PI-103 (5-100 µM) or PI-103 (0-5 µM) in normal fibroblasts, T47D, MCF-7, MDA-MB-361, BT474, MDA-MB-453 and MDA-MB-231 breast cancer cell lines. We noted that IC50 of PI-103 is 3.34 µM whereas IC50 of RIDR-PI-103 is >100 µM in normal fibroblasts, indicating the toxicity of the PI-103 alone but not the RIDR-PI-103 in normal fibroblasts. Our data indicated that ~30-40 µM RIDR-PI-103 significantly inhibited T47D, MDA-MB-231, MDA-MB-361 and MDA-MB-453 cell proliferation whereas higher concentrations of the drug were effective in BT474 and MCF-7 breast cancer cell lines. Our quantitative PCR data showed that the antioxidant catalase mRNA levels are statistically lower compared to normal fibroblasts in these cancer cell lines indicating that the efficacy of RIDR-PI-103 could be correlated to catalase expression. Doxorubicin is a clinically relevant chemotherapy known to induce reactive oxygen species (ROS) in breast cancer cell lines. We are currently assessing the levels of biomarkers for ROS in formalin-fixed paraffin embedded breast tumors treated without or with chemotherapy including 8-hydroxy-29-deoxyguanosine (8-oxo-dG) and 4-hydroxy-2-noneal (4HNE). Our data indicated that doxorubicin significantly sensitized MDA-MB-453, MDA-MB-361 and MDA-MB-231 cells to RIDR-PI-103 indicated by significant lower IC50 values of the combined drug treatment versus the single agent. Doxorubicin and RIDR-PI-103 showed a synergistic effect in MDA-MB-361 and MDA-MB-231 cells to inhibit cancer cell proliferation. Doxorubicin was more effective than docetaxel, another chemotherapeutic drug, in sensitizing these cells to growth inhibition. Thus, this novel combination of the ROS-activatable PI3K inhibitor prodrug and chemotherapy provides strong justification for its continued development and future clinical trials for patients stricken with PI3K driven tumors. In other experiments, we show that a recent FDA-approved CDK4/6 inhibitor, palbociclib, sensitized ER+ breast cancer cell lines (T47D and MDA-MB-361) to RIDR-PI103 using cell viability assays. Experiments are ongoing to determine the mechanism of action of these novel drug combinations that could be potentially translated to clinic for treatment of breast cancer patients. Citation Format: Rosalin Mishra, Hima Patel, Haizhou Zhu, Aaron White, Long Yuan, Samar Alanazi, Kurt Hodges, Edward J Merino, Joan T Garrett. Efficacy of RIDRPI103, a reactive oxygen species (ROS) activated prodrug in treatment of breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-10-04.