BackgroundCancer immunotherapies have been very successful in recent times; however, they benefit only a subset of patients and have varying response rates across tumor types. Conversely, conventional chemotherapies are effective in a large group of patients, but have limited dosing capabilities, lack specificity, and often result in systemic adverse events. Here, we present SQ3370, a novel approach that activates doxorubicin (Dox) at the tumor site while avoiding systemic toxicities commonly associated with the therapy, and also potentially activates an immune response against tumors. SQ3370 is based on a local intratumoral injection of a prodrug-capturing biomaterial (SQL70) followed by 5 daily systemic infusions of an attenuated form of Dox (SQP33). Mutually-reactive click chemistry groups in the 2 components allow the capture and release of active Dox at the tumor site. While conventional Dox is known to induce immune activation¹ and enhance tumor responsiveness to checkpoint inhibitors,² its benefit is limited by cumulative dose cardiotoxicity. We safely administered SQ3370 in dogs at 8.95-times the veterinary clinical dose of Dox with minimal side effects including cardiotoxicity and immunosuppression. In syngeneic mouse models, SQ3370 improved overall survival and induced a robust anti-tumor response against the biomaterial-injected lesion compared to conventional Dox. Surprisingly, SQ3370 also induced regression of the non-injected tumor and enhanced T-cell infiltration in both injected and noninjected tumors. We hypothesize that activating Dox at a local site with SQ3370 promotes activation of the native immune system against the tumor. Thus, SQ3370 represents a new therapeutic modality to treat solid tumors by using a drug with known efficacy, Dox, and expanding its therapeutic window. SQ3370 could potentially also benefit patients with widely disseminated or micro-metastatic lesions.MethodsSQ3370-001 (NCT04106492), the first-in-human Phase 1 study, is currently open in the United States and Australia to treat patients with advanced solid tumors. SQ3370-001 is enrolling patients ≥ 18 years of age with an injectable local or metastatic lesion, for which published data indicates responsiveness to anthracyclines. Patients must be relapsed or refractory following standard of care therapy and must not have received more than 225 mg/m² of Dox (or equivalent anthracycline). Each cycle will be for 21 days with no limit on total cycles. Primary objectives include determining the safety, tolerability, and recommended Phase 2 dose. Additional objectives include assessment of the pharmacokinetic profile, preliminary efficacy per RECIST 1.1, and immune response.ResultsN/AConclusionsN/AAcknowledgementsThe authors would like to thank the National Institutes of Health (NIH), the National Science Foundation (NSF), and Y Combinator.Ethics ApprovalThis study was approved by:1. The Institutional Review Board (IRB) of Stanford University; eProtocol Number: 54928.2. The IRB of The University of Texas MD Anderson Cancer Center; IRB ID Number: 2020-0185_MOD001.3. Western IRB, on behalf of The Angeles Clinic and Research Institute and Henry Ford Health System IRB Office; IRB Tracking Number: 20200758.4. Bellberry Limited Human Research Ethics Committee, on behalf of Royal North Shore Hospital and Chris O’Brien Lifehouse; Application Number: 2019-10-848.ReferencesMattarollo SR, Loi S, Duret H, Ma Y, Zitvogel L, Smyth MJ. Pivotal role of innate and adaptive immunity in anthracycline chemotherapy of established tumors. Cancer Res 2011;71:4809–4820.Zitvogel L, Galluzzi L, Smyth MJ, Kroemer G. Mechanism of action of conventional and targeted anticancer therapies: reinstating immunosurveillance. Immunity 2013;39:74–88.