Background: The RAS/MAPK pathway is dysregulated in a broad range of cancers including CRC and PDAC, resulting in downstream activation of ERK1/2. ERAS-007 is a novel, orally bioavailable inhibitor of ERK. Palbo is an oral CDK4/6 inhibitor that inhibits cellular proliferation, an essential feature of tumor growth. Both in vitro & in vivo data exploring the combination of ERAS-007 and palbo in a panel of CRC and pancreatic CDX and/or PDX models have shown promising activity to support the potential combinatorial clinical benefit in RASm CRC and PDAC pts. Methods: HERKULES-3 is a Phase 1b/2 study to assess safety, tolerability, PK, and preliminary clinical activity of ERAS-007 combinations targeting the MAPK pathway in pts with advanced GI cancers. Within this study, we are currently evaluating the safety, tolerability, and PK of escalating doses of ERAS-007 twice daily-once a week (BID-QW) (75, 100 mg) in combination with palbo once daily (QD) (75, 100, 125 mg) in pts with KRASm/NRASm CRC or KRASm PDAC. Results: As of 30 November 2022, 30 pts were dosed with the combination of palbo and ERAS-007. Treatment emergent AEs (TEAEs) occurring in ≥20% pts were diarrhea (40%), nausea (40%), anemia (33%), vision blurred (27%), fatigue (23%), and neutrophil count decreased (20%). ERAS-007 treatment related AEs (TRAEs) were reported in 19 pts (63%), with the most frequently reported as diarrhea (40%), nausea (33%), and vision blurred (27%). Grade (Gr) ≥3 TEAEs were reported in 12 pts (40%), including 3 related to ERAS-007 (neutrophil count decreased, diarrhea and dermatitis acneiform). Neutrophil count decreased and anemia were the only Gr 3 events reported in ≥2 pts. No Gr 4 events were reported. Two Gr 5 events unrelated to any drugs (hemorrhage intracranial and malignant pleural effusion) and one Gr 5 event (anemia) related to palbo were reported. Two pts discontinued ERAS-007 due to TEAEs (Gr 5 malignant pleural effusion and Gr 2 neutrophil count decreased). Three pts reported 4 DLTs: 1 pt at 75mg ERAS-007 & 125mg palbo (Gr 3 maculopapular rash & Gr 4 sepsis), 1 pt at 100mg ERAS-007 & 100mg palbo (Gr 3 dermatitis acneiform), and 1 pt at 100mg ERAS-007 & 125mg palbo (Gr 3 thrombocytopenia). Based on preliminary PK, no clinically relevant PK interactions were identified between ERAS-007 and palbo. The evaluation of clinical activity is ongoing. Conclusions: ERAS-007 in combination with palbo in pts with KRASm/NRASm CRC or KRASm PDAC shows expected preliminary safety with reversible and manageable AEs. The highest dose evaluated and cleared by the safety review committee to date is ERAS-007 100 mg BID-QW in combination with the approved monotherapy dose of palbo 125 mg QD. Clinical trial information: NCT05039177.