Background: TP-1287 is a an orally bioavailable phosphate prodrug of alvocidib, a cyclin dependent kinase 9 (CDK9) inhibitor. TP-1287 exhibits potent inhibition of intracellular kinases including CDK9. Inhibition of CDK9 leads to downregulation of the BCL-2 family member, MCL-1, which in turn inhibits tumor growth in preclinical animal models of prostate, breast, and lung carcinomas. Methods: This is a multicenter, Phase 1, dose escalation study using a standard 3+3 design with a modified Fibonacci scheme to examine the safety and clinical activity of TP-1287 in patients with advanced solid tumors. Patients will be added at the maximum tolerated dose (i.e. expansion cohort) to test TP-1287 as a single agent in patients with castrate resistant prostate cancer. Results: Twenty-two patients who were enrolled between December 2018 and January 2020 received a range of doses from 1 mg QD to 11 mg BID over 7 cohorts. Data are available for 20 patients as of the data cutoff date. TP-1287 plasma PK C_max and AUC increased in near linear fashion over cohorts 1 thru 6, reaching 80 ng/mL and 499.3 ng*h/mL in cohort 6 for C_max and AUC, respectively. TP-1287 treatment resulted in dose-dependent reductions of phospho-RNA Pol II, consistent with CDK9 inhibition, as measured by a flow cytometric assay assessing pharmacodynamic changes in phosphorylation state in PBMCs. The most frequently observed Grade 3 AE was unrelated anemia in 2 patients. All other events of Grade 3 (9 events/7 patients) and Grade 4 (1 event/seizure with new CNS mets) were unlikely related or unrelated. Clinical benefit was seen in one sarcoma patient with PR (15+cycles), one RCC patient with SD (7+cycles) and 2 bladder cancer patients with SD (6 and 8 cycles). Conclusions: These findings suggest that TP-1287 is tolerated as a monotherapy in patients with heavily pretreated, relapsed, refractory solid tumors and further clinical development in selected indications is warranted. Clinical trial information: NCT03298984.