Background: SY-5609 is an oral, highly selective and potent noncovalent inhibitor of CDK7, a key regulator of cell cycle progression and transcription. In this Phase 1 FIH study (NCT04247126), the single agent (SA) continuous daily dosing (CDD) regimen maximum tolerated dose (MTD) was 3 mg/day. Subsequently, enhanced tolerability was demonstrated using an intermittent 7 day on/7 day off (7/7) schedule at higher dose levels (Sharma 2021). SY-5609 preclinical data and early clinical activity provided the rationale for evaluating SY-5609 + gemcitabine (gem) +/- nab-paclitaxel (np) in PDAC. Herein, we present data from the ongoing SA dose escalation and SY-5609 + gem +/- nab-paclitaxel (np) safety lead-ins (SLIs). Methods: The 7/7 schedule was used to further evaluate SA escalation in pts with select solid tumors. In the PDAC combination SLIs, SY-5609, starting with a dose of 4 mg, is escalated using the 7/7 schedule in combination with biweekly gem (1000 mg/m²) +/- np (125 mg/m²). Evaluations included safety, clinical activity (RECIST v1.1), PK, and induction of peripheral blood PD marker POLR2A (Papadopoulos 2020). Results: As of 13 Jan 2023, 87 pts have been enrolled (70 SA, 14 SY-5609 + gem, 3 SY-5609 + gem/np), including 30 pts across the 4, 5, 6, 7, and 10 mg SA 7/7 cohorts included in the safety summary. MTD for the 7/7 schedule has not been reached. The 15 mg SA cohort is now enrolling. Most SA 7/7 AEs were low-grade; AEs included (>20%) nausea, diarrhea, fatigue, anorexia, and thrombocytopenia. No new safety signals were observed in the SY-5609 4 mg and 5 mg + gem and SY-5609 4 mg + gem/np cohorts; 1 pt (SY-5609 5 mg + gem) had a DLT of Gr3 diarrhea. The 10 mg + gem cohort is now enrolling. Most SY-5609 + gem +/- np AEs were low grade. AEs in the SLIs were consistent with the safety profile of either SY-5609 or gem +/- np and included (>20%) fatigue, anorexia, nausea, abdominal pain, and constipation. In the SA cohorts (response-evaluable, n=29), 100% (3/3) of pts in the 10 mg 7/7 cohort had stable disease (SD) as the best response, compared to 19% (5/26) SD rate in 4 to 7 mg SA 7/7 cohorts. Disease control rate in the combination SLIs (response-evaluable, n=11) was 45% (5/11), including 1 confirmed partial response (4 mg + gem) in a patient with liver metastases who had a reduction in CA 19-9 from 60,357 U/mL to 968 U/mL, and SD in 4 pts (3 in 5 mg + gem, 1 in 4 mg + gem/np). SA and SLI PK data support an emerging exposure-activity relationship. POLR2A PD marker data from SA and SLI cohorts correlate with exposure. Conclusions: To date, intermittent dosing of SY-5609 on the 7/7 schedule has acceptable tolerability alone and in combination with gem +/- np. Encouraging preliminary clinical activity and the emerging exposure-activity relationship supports the ongoing escalation of SY-5609 in SA and combination SLI cohorts; updated results will be presented. Clinical trial information: NCT04247126.