Background: The KRAS^G12C mutation is found in approximately 13% of lung adenocarcinomas and 1–3% of other solid tumors, but there is no approved therapy that targets this mutation. AMG 510 is a novel small molecule that specifically and irreversibly inhibits KRAS^G12C by locking it in an inactive GDP-bound state. Methods: This phase 1, first-in-human, open-label, multicenter study (NCT03600883) is evaluating the safety, tolerability, PK, and efficacy of AMG 510 in adult patients (pts) with locally-advanced or metastatic KRAS^G12C mutant solid tumors. The primary endpoint is safety; key secondary endpoints include PK, ORR (assessed every 6 weeks [wks]), DOR, and PFS. Key inclusion criteria: KRAS^G12C mutation identified through DNA sequencing, measurable or evaluable disease, ECOG PS ≤2, life expectancy >3 months (mo). Key exclusion criteria: active brain metastases, myocardial infarction within 6 mo. A dose exploration will determine the maximum tolerated dose (MTD) or recommended phase 2 dose (RP2D). A dose expansion will enroll pts with NSCLC, CRC, and other advanced solid tumors carrying the KRAS^G12C mutation. AMG 510 will be given PO until disease progression, intolerance, or withdrawal of consent. Results: 22 pts (8 men, 14 women; median age 55.5 y) were enrolled in the first 3 dose cohorts. Tumor types: 6 NSCLC, 15 CRC, 1 other. Most pts (n=17) had ≥3 prior lines of treatment (tx). Median tx duration was 28 d (range: 8–134). 5 pts reported 10 treatment-related AEs (grade 1, n=9; grade 2, n=1); there were no DLTs. Tumor response was evaluated in 9 pts (4 with ≥2 assessments); 13 pts have not reached their first assessment.1 pt had a PR (NSCLC at wks 6 and 12, tx ongoing), 6 pts had SD (4 CRC and 2 NSCLC; median tx duration 9.7 wks [range: 6.3–19.1], tx ongoing), 2 pts had PD. 20 pts are continuing to receive AMG 510. A second PR (NSCLC at wk 6, tx ongoing) was reported after data cutoff. Conclusions: AMG 510 has been well tolerated at the dose levels tested and has shown antitumor activity when administered as monotherapy to patients with advanced KRAS^G12C mutant solid tumors. MTD has not been determined, and enrollment into the dose exploration is ongoing. Clinical trial information: NCT03600883