A first-in-human, Phase 1/2a, open-label study of SQ3370, a first-in-class doxorubicin-based click chemistry therapeutic, in patients with advanced solid tumors

BackgroundWe present the first clinical proof of concept using click chemistry to selectively capture drugs at tumors. SQ3370 combines a clickable pre-targeting agent (intratumorally injected biopolymer, SQL70) and a chemically-attenuated doxorubicin (Dox) protodrug (SQP33) that is activated upon clicking with the biopolymer at the tumor to rapidly release high local concentrations of native doxorubicin. MethodsThis was a Phase 1/2a open-label study in patients with advanced solid tumors (NCT04106492). SQ3370 treatment comprises of an intratumoral SQL70 biopolymer injection followed by 3 or 5 consecutive daily infusions of SQP33 Dox protodrug. The primary endpoints were treatment-emergent adverse events, dose-limiting toxicities (DLTs), and to identify the recommended Phase 2 dose (RP2D). Secondary endpoints included pharmacokinetics, efficacy, and immune profiling. ResultsPhase 1 enrolled 39 patients. SQ3370 administered at 0.38x to 15x the standard Dox dose was well tolerated with no DLTs reported, and mild and manageable myelosuppression observed. Rapid release of Dox was observed at all the dose levels tested, with increasing exposure up to 15x the standard Dox dose; 12x was selected as the RP2D. Phase 2a enrolled 14 soft tissue sarcoma patients at 12x Dox. There was an unconfirmed objective response rate (ORR) of 14.3% (2/14 patients) and disease control rate of 71.4% (10/14; 95% CI: 41.9, 91.6). Immune profiling revealed anti-tumor immune responses, including expansion/activation of cytotoxic CD8+ T-cells. The study was terminated as the prespecified criteria for study continuation of ORR greater than that of standard Dox was not met. ConclusionsSQ3370 is a first-in-class click chemistry-enabled, pre-targeting therapeutic, and the first reported use of in vivo click chemistry in humans. This approach enabled high Dox concentrations at the tumor, with minimal off-target toxicity, unlocking favorable immune responses. Objective clinical activity was observed, but ORR was comparable to standard Dox. Highlights{blacksquare} SQ3370 is a click chemistry-enabled, pre-targeting therapeutic and the first use of in vivo click chemistry in humans {blacksquare}SQ3370 up to15x standard doxorubicin dose in patients with solid tumors was safe with no DLTs reported {blacksquare}In Phase 2a, SQ3370 provided an unconfirmed objective response rate of 14.3% and disease control rate of 71.4% in patients with advanced sarcomas {blacksquare}Tumor size reductions seen in both injected and non-injected lesions, potentially due to systemic anti-tumor responses