Upstream Bio, Inc.

We are a clinical-stage biotechnology company developing treatments for inflammatory diseases, with an initial focus on severe respiratory disorders. We are developing verekitug, the only known antagonist currently in clinical development that targets the receptor for Thymic Stromal Lymphopoietin ( TSLP ), a cytokine which is a clinically validated driver of inflammatory response positioned upstream of multiple signaling cascades that affect a variety of immune mediated diseases. Preclinical and clinical data to date demonstrate verekitug’s highly potent inhibition of the TSLP receptor, which we believe will translate to a differentiated product profile, including improved clinical outcomes, substantially extended dosing intervals and the potential to treat a broad spectrum of patients. We have advanced this highly potent monoclonal antibody into separate Phase 2 trials for the treatment of severe asthma, including a long-term safety and efficacy extension study ( Phase 2 LTE ), chronic rhinosinusitis with nasal polyps ( CRSwNP ), and chronic obstructive pulmonary disease ( COPD ). We reported positive top-line results in our CRSwNP Phase 2 trial in September 2025 and positive top-line results in our severe asthma Phase 2 trial in February 2026. We initiated our Phase 2 COPD trial in July 2025. We plan to initiate dosing in Phase 3 trials in both severe asthma and CRSwNP in the first quarter of 2027, prioritizing a Phase 3 development strategy that focuses on maximizing efficacy in both indications, without biomarker restriction, with quarterly at-home administration.