Background: Transforming growth factor-beta 1 (TGFβ1) is a key mediator of primary resistance to programmed cell death protein 1 (PD-1) pathway blockade. SRK-181 is a fully human, highly potent and selective monoclonal antibody that inhibits latent TGFβ1 activation. SRK-181 has minimal or no binding to latent TGFβ2 and TGFβ3 isoforms or to active TGFβ growth factors. In mouse tumor models (bladder, melanoma, and breast cancer), SRK-181 in combination with anti-PD1 therapy overcame primary anti-PD-1 resistance and showed survival benefit. No cardiotoxicities (valvulopathy) were observed with SRK-181 in 4-week GLP nonclinical toxicology studies. Thus, the potency and selectivity of SRK-181 may overcome PD-1 inhibitor resistance and toxicity of non-selective TGFβ pathway approaches. Methods: The DRAGON trial NCT04291079 is an ongoing multicenter, open-label, phase 1 study of SRK-181 administered by IV infusion every 3 weeks (Q3W) alone or in combination with anti-PD-(L)1 in patients (pts) with locally advanced or metastatic solid tumors. The study comprises 3 parts: Part A of the study follows a standard 3+3 dose escalation trial design to evaluate safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD) and antitumor activity of SRK-181 alone (Part A1) or in combination with the anti-PD-(L)1 agent that is approved for the respective tumor indication (Part A2). Part A1 and Part A2 will determine maximum tolerated dose (MTD) or maximum administered dose and Recommended Phase 2 Dose (RP2D) for Part B. Part B (expansion phase) will evaluate combination treatment of SRK-181 with anti-PD-(L)1 in pts with non-small cell lung cancer, urothelial carcinoma (UC), melanoma or other advanced solid tumors, to confirm the tolerability of the RP2D and to evaluate the antitumor activity of combination treatment. Pts in Part A2 and Part B must have previously received an anti-PD-(L)1 therapy approved in their tumor indication and considered non-responders (best response of stable disease or disease progression) to anti-PD-(L)1 monotherapy. Pts in Part B must have received the most recent dose of the prior anti-PD-(L)1 within 6 months of study enrollment (9 months for UC cohort). Safety, PK, PD and efficacy data will be collected and monitored throughout the study. Detailed translational PD and predictive biomarker studies for SRK-181 will include a novel digital pathology analysis of CD8 to assess the alteration of immune profile in tumor microenvironment and TGFb pathway biomarkers, such as quantitative analysis of tumor phospho-Smad2 and circulating levels of TGFb1 ligand. As of Feb 01 2021, dose escalation has proceeded to the highest planned dose of 2400 mg Q3W in Part A1 (monotherapy) and to 800 mg Q3W in Part A2 (anti-PD-(L)1 combination). Additional planned doses in Part A2 are 1600 mg and 2400 mg Q3W. Clinical trial information: NCT04291079