TPS1103 Background: The mTOR inhibitor Ev, combined with Ex, is a mainstay in the treatment of post-menopausal women with advanced HR+/HER2- BC. However, the activity of Ev is limited by counter-regulatory feedback mechanisms in cancer cells, involving reactivation of insulin-like growth factor (IGF)/mTOR survival signaling. Combining Ev with the humanized IGF-1 and IGF-2 ligand-blocking antibody Xe abrogates this feedback, thus intensifying inhibition of tumor growth; this leads to a pronounced effect in patients with non-visceral (e.g., bone and lymph node) metastases. The phase II XENERA-1 trial evaluates the combination of Xe with Ev and Ex in post-menopausal women with HR+/HER2- BC. Methods: XENERA-1 (NCT03659136) is a double-blind, placebo-controlled, randomized study to assess the efficacy and safety of Xe in combination with Ev and Ex, in post-menopausal women with HR+/HER2- locally advanced/mBC and non-visceral disease. The population comprises post-menopausal mBC patients who have progressed on ≤1 previous line of a non-steroidal aromatase inhibitor, with or without a CDK 4/6 inhibitor, who may have received fulvestrant. Other inclusion criteria are: an Eastern Cooperative Oncology Group Performance Status of 0 or 1; adequate organ function; and non-visceral disease (absence of brain, liver, lung, peritoneal or pleural metastases). Patients are randomized (1:1) to receive Xe (1000 mg/week, iv) or placebo (weekly, iv), in combination with Ev (10 mg/day) and Ex (25 mg/day). Treatment will continue until disease progression, unacceptable toxicity or other reasons. The primary endpoint is progression-free survival according to RECIST 1.1 criteria, by independent review. Secondary endpoints are: overall survival; disease control; duration of disease control; objective response; and time to progression of pain/intensification of palliation. Safety and pharmacokinetic endpoints, and exploratory biomarkers will also be evaluated. The first patient was enrolled in January 2019; target enrollment is 80 patients in 12 countries. Clinical trial information: NCT03659136.