Background: Interleukin-12 (IL-12) is a proinflammatory cytokine that has shown promise as an immunotherapy for cancer. Although it has demonstrated potent antitumor activity in mouse models of cancer, development of IL-12 in the clinical setting has been limited by severe toxicities with systemic administration. XTX301 is a half-life extended, masked and tumor-activated IL-12. XTX301 is designed to be pharmacologically inactive in non-tumor tissue when circulating systemically, and unmasked by matrix metalloproteases (MMPs) preferentially active in the tumor microenvironment (TME). The masking domain is engineered to be removed via cleavage at a protease cleavage site in the XTX301 linker by MMPs. XTX301 is designed to produce a localized anti-tumor immune response while limiting exposure of the active form of XTX301 in non-tumor tissue, and thereby improving the therapeutic index of IL-12. Using a murine analog of XTX301 (mXTX301), tumor growth inhibition was demonstrated in vivo in murine models with a single dose of mXTX301. In mouse tumors, mXTX301 stimulated the infiltration of cytotoxic CD8+ T cells and induced pro-inflammatory gene expression signatures. Preclinical data have demonstrated tumor-selective activity of mXTX301 in mice with limited and reversible peripheral toxicities of XTX301 in non-human primates [Malkova et al., AACR 2023]; these data are supportive of further clinical development. Methods: XTX301-01 trial (NCT05684965) is a first-in human, multicenter, Phase 1, open label study to establish the safety and tolerability of monotherapy XTX301, and to determine a recommended phase 2 dose (RP2D). Additional objectives include assessment of pharmacokinetics (PK), pharmacodynamics (PD), immunogenicity and anti-tumor activity of XTX301. The study will enroll patients with solid tumors, including lymphoma, that have locally advanced or metastatic disease and have failed standard therapy, standard therapy does not confer survival benefit, or standard therapy is not available. Patients in part 1A will receive XTX301 monotherapy every 3 weeks via intravenous infusion in an outpatient setting, using a 3+3 dose escalation design. The starting dose for XTX301 is 5 ug/kg. Part 1B will further examine multiple doses of XTX301 as monotherapy in patients with select advanced solid tumors to further characterize PK, PD and dose response relationships. Part 1B will require mandatory fresh tumor biopsies before and after initiation of study drug administration to fully characterize the PD profile and PK of XTX301 in the tumor. Enrollment to the study is planned to begin in the first quarter of 2023. Clinical trial information: NCT05684965.