Background: Sotorasib, a specific, irreversible KRAS^G12C inhibitor, has been approved in multiple countries for adults with KRAS p.G12C-mutated locally advanced or metastatic NSCLC who received prior systemic therapy based on the global phase 1/2 CodeBreaK100 trial. Here we describe putative mechanisms of acquired resistance to sotorasib from the largest single dataset evaluated to-date. Methods: Patients with advanced KRAS p.G12C-mutated NSCLC or CRC from the CodeBreaK100 Ph1/2 trial who received sotorasib monotherapy at 960 mg once daily were analyzed for efficacy. Primary endpoint was objective response rate (ORR) assessed by central review. To investigate biomarkers of resistance to sotorasib, an exploratory endpoint was defined to examine acquired genomic alterations at disease progression. Plasma samples collected at baseline and progression were analyzed for genomic alterations with the 23-gene Resolution Bioscience ctDx Lung test for NSCLC and the 74-gene Guardant 360 ctDNA test for CRC. Acquired genomic alterations were defined by their absence at baseline and presence at progression. Results: In 174 pts with NSCLC and 91 pts with CRC-treated with sotorasib, the ORR were 41% and 12% respectively. Median progression-free survival and median overall survival were 6.3 months (mos) and 12.5 mos for NSCLC pts and 4.2 mos and 13.4 mos for CRC pts (median follow-up: 22.5 mos NSCLC; 12.5 mos CRC). A total of 67 NSCLC pts and 45 CRC pts had a plasma sample sequenced both at baseline and at progression. At least one new acquired genomic alteration at progression was detected in 19 (28%) NSCLC pts and in 33 (73%) CRC pts (Table). The acquired genomic alterations were heterogeneous in both NSCLC and CRC, with variants detected across multiple genes and pathways. The most prevalent putative pathway of resistance in both NSCLC and CRC was the receptor tyrosine kinase (RTK) pathway. Secondary RAS alterations occurred more frequently in CRC versus NSCLC pts (16% vs. 3%). Conclusions: Based on the largest descriptive dataset to-date, diverse mechanisms of acquired resistance occur in KRAS p.G12C-mutated NSCLC and CRC pts treated with sotorasib. New RTK pathway alterations frequently emerged at progression, highlighting the potential role for combining sotorasib with upstream inhibitors of RTK, such as SHP2 or EGFR inhibitors. Serial plasma DNA analysis revealed acquired resistance patterns that support the development of KRAS^G12C inhibitor combination therapies. Clinical trial information: NCT03600883.