Background: SY-5609 is an oral, noncovalent, highly selective and potent inhibitor of cyclin-dependent kinase 7 (CDK7), a key regulator of 2 biological processes that play critical roles in driving tumor development: transcription and cell cycle control. Evaluation of SY-5609 as a single agent in PDX models from a range of solid tumors, including breast, ovarian, lung and colorectal tumors, revealed robust antitumor activity including complete regressions, and activity in models known to be resistant to standard of care therapy. Models with genetic alterations in RB pathway genes demonstrated deep ( > 90% TGI) and sustained SY-5609-induced tumor regressions following treatment discontinuation, in contrast to models without genetic alteration in RB pathway genes, suggesting that tumor cells with aberrant cell-cycle control may be particularly sensitive to SY-5609 treatment. The study is designed to evaluate the safety, tolerability, and maximum tolerated dose (MTD) of SY-5609, to characterize the pharmacokinetic (PK), pharmacodynamic (PD), and preliminary antitumor activity of SY-5609, and to explore candidate biomarkers predictive of response to SY-5609. Methods: This is a multi-center, open-label Phase 1 trial expected to enroll approximately 60 adult patients with select advanced solid tumors for which standard treatment is no longer effective. The dose escalation phase of the trial is open to adult patients with ovarian, breast, colorectal, or lung cancer, and patients with any solid tumor histology with molecular evidence of deregulated RB cell cycle control. SY-5609 is being administered orally once daily, for each 4-week cycle. Initially, patients will be enrolled into single-patient accelerated titration cohorts; subsequent cohorts will transition to a 3 + 3 design. Following completion of DLT evaluation at a given dose level, additional patients may be enrolled at that dose to further characterize safety, PK, PD, and early clinical activity. Data from this trial will support dose selection for planned evaluations of antitumor activity of SY-5609 as a single agent and in combination. Clinical trial information: NCT04247126.