Background: Human epidermal growth factor receptor 2 (HER2) gene amplification or mutations have emerged as oncogenic drivers and therapeutic targets not limited to breast and gastric cancers, but also in a variety of cancers. Despite its considerable therapeutic potential, the evidence has not been established. To address this unmet need, we conducted an organ-agnostic basket trial targeting HER2-amplified solid tumors. Methods: JUPITER is a multicenter, single-arm, phase 2 basket trial for solid tumor patients (pts) with HER2 amplification determined by next-generation sequencing (NGS). Both tissue and liquid NGS results were allowed. HER2 amplification by ISH or HER2 overexpression by IHC were not used for inclusion. Pts had treatment-refractory metastatic tumors, or rare cancers without established standard of care. Breast, gastric, and colorectal cancers were excluded. Pts were treated with intravenous trastuzumab (8 mg/kg loading dose followed by 6 mg/kg) and pertuzumab (840 mg loading dose followed by 420 mg) every 3 weeks until disease progression or any other reason for discontinuation. Tumor response was assessed using RECIST v1.1. Primary endpoint was ORR by blinded independent central review (BICR), and secondary endpoints were ORR assessed by the investigators, progression-free survival (PFS), overall survival (OS), duration of response (DOR), and safety. We set the ORR threshold 5% and expected ORR was 20%. Estimated sample size was 38 patients with one-sided alpha 2.5% and power 80%. Results: Between April 2019 and June 2020, 42 pts were consented, and 40 pts were treated. Median age was 62 (range, 21-86) and 60% were females. The most common diagnosis was biliary tract cancer (20%), followed by salivary ductal carcinoma (12.5%) and endometrial cancer (12.5%). At data cutoff (1 Sep 2021), ORR by BICR was 22.5% (95%CI: 10.8%-38.5%). ORR assessed by the investigator was 25% (95%CI: 12.7%-41.2%). PFS, OS and DOR were not reached at data cutoff; 3 responders remained on treatment. Of 40 pts, 32.5% had grade ≥ 3 adverse events; 10% were treatment-related, including neutropenia, hypertension, peripheral sensory neuropathy and lymphoedema (all grade 3). No treatment-related death was observed. Exploratory biomarker analysis of response and resistance is in progress. Conclusions: Combination therapy with trastuzumab and pertuzumab was well tolerated and showed promising efficacy for the patients with HER2-amplified solid tumors determined by NGS. Clinical trial information: jRCT2031180150.