Background: The standard of care for clinically localized muscle-invasive bladder cancer (MIBC) is neoadjuvant platinum-based combination chemotherapy followed by radical cystectomy (RC). Up to 40% of patients (pts) are ineligible to receive cisplatin and proceed to RC without any neoadjuvant therapy. We and others have demonstrated enrichment of molecular alterations in cell cycle genes in MIBC, including copy number losses of CDKN2A in 41% of pts. Abemaciclib is a unique CDK4/6 inhibitor with single agent activity and a target kinome distinct from other CDK4/6 inhibitors. We have demonstrated that CRISPR knockout of CDKN2A increases susceptibility to abemaciclib in bladder cancer cell lines. Beyond tumor-intrinsic effects, abemaciclib also modulates the tumor microenvironment (TME) via upregulating human endogenous retroviral elements and increasing T cell infiltration. Methods: Cell cycLe inhibitiON to target the EVolution of urOthelial cancer (CLONEVO) is a single arm, window-of-opportunity trial of neoadjuvant abemaciclib which will evaluate tumor cell and TME changes in response to abemaciclib. Enrolled pts must be ineligible for platinum-based neoadjuvant therapy for resectable MIBC. Pts receive abemaciclib (200 mg BID PO) for 4 weeks prior to RC. Tumor tissue collected via transurethral resection of bladder tumor (TURBT) and residual tumor at RC undergo single cell RNA sequencing and whole-exome sequencing. Patient-derived organoids and xenografts are generated for a co-clinical trial of abemaciclib alone or in combination. The primary endpoint is the measurement of changes in cell cycle dynamics. Secondary objectives are assessment of toxicity via NCI CTCAE v 5.0 and pathologic downstaging of MIBC. We will perform targeted sequencing of a panel of cell cycle genes in serial plasma and urine cell free DNA to evaluate changes in the variant allele fractions of somatic alterations. The novel design of this trial allows dynamic in vivo assessment of tumor changes and creates a new paradigm for studying tumor evolution in real time. Clinical trials information: NCT03837821. Clinical trial information: NCT03837821.