Project Summary/AbstractWhile immune checkpoint inhibitors (ICIs) have transformed the landscape of cancer treatment paradigm theresponse rate is limited to a small subset of cancer patients (~20%). For colorectal cancer (CRC) the secondleading cause of cancer-related deaths in US only patients (~4%) with mismatch-repair-deficient or microsatelliteinstability-high tumors can respond to ICIs leaving the vast majority of CRC patients with limited to no clinicalbenefit. Chemotherapy has been increasingly manifested to contribute significantly to the overall antitumorefficacy when combined with ICIs via switching the tumors from immune-cold to immune-hot. However owingto the poor solubility and pharmacokinetics limited tumor accumulation and non-specific toxicities to healthytissues the utility of chemotherapeutics in enhancing the efficacy of ICIs has been considerably hindered. Torender a safer and more efficacious chemotherapy-enabled immune response to cooperate with ICIs our long-term goal is to develop an innovative and multifunctional liposomal nanotherapeutic platform via conjugatinganticancer agents to the backbone phospholipid of liposome. We have developed a phospholipid-derivedcamptothecin (CPT) liposome (Camptothesome) nanoplatform which significantly prolonged blood circulationtime enhanced tumor uptake and therapeutic efficacy and minimized systemic toxicities compared to free CPT.Moreover Camptothesome potentiated the anti-CRC efficacy of PD-L1/PD-1 inhibitors resulting in partialeradication of tumors in immunocompetent mice. To improve the efficacy of this combined therapy we usedCamptothesome to co-deliver an inhibitor targeting another independent immune checkpoint Indoleamine 23-dioxygenase (IDO1) which markedly enhanced anti-CRC efficacy and immunity. To further strengthen thedelivery efficiency and explore the potential of this nanoplatform in enhancing PD-L1/PD-1 blockade therapy inthis proposal we will:Aim 1: Improve the Camptothesome system for enhanced therapeutic delivery.Aim 2: Determine the tumor delivery efficiency and pharmacokinetics of the improved co-delivery systemin murine CRC models.Aim 3: Define antitumor effects of the improved co-delivery system with or without PD-L1/PD-1 blockadein murine CRC models. The mechanistic action for the in vivo antitumor efficacy and immune responses of thecombined therapy will also be elucidated. Successful completion of this proposal will result in an innovative andmultifunctional nanotherapeutic platform for improved and safe CRC immunochemotherapy. Moreover giventhat IDO1 is expressed in diverse cancer cells and the broad applicability of this nanoplatform to other anticancerdrugs our combination nanotherapeutic system has the potential to revolutionize cancer treatment paradigms.