Abstract B160: A phase 1 open-label, dose-escalation study of central nervous system–penetrant cyclin-dependent kinase (CDK)4/6 inhibitor PRT3645 in patients with select advanced or metastatic solid tumors
Authors: Patnaik Amita , Salkeni Mohamad A. , Patel Manish R. , Basu-Mallick Atrayee , Shepard Dale , Lihou Christine , Sun William , Xavier Jennifer , Sahasranaman Sri , Sharma Neelesh , Hamilton Erika 
Published: December 2023

Abstract
Introduction: CDK4/6 mediate cell cycle progression and are important for cancer initiation, growth, and survival. CDK4/6 inhibitors are approved for patients with advanced HR+/HER2– breast cancer but are ineffective against primary or metastatic brain tumors. PRT3645 is an investigational, highly selective, and potent CDK4/6 inhibitor, designed to achieve high tissue and brain penetration. This phase 1 dose-escalation study (NCT05538572) aims to evaluate safety, preliminary efficacy, and pharmacokinetic/pharmacodynamic profile of PRT3645.
Methods: Patient eligibility criteria include: age ≥18 years; Eastern Cooperative Oncology Group/Karnofsky performance status ≤1/≥80; adequate organ function; histologically confirmed advanced, recurrent, or metastatic solid tumor malignancy: breast cancer (HR+/HER2– or HR+/HER2+), glioblastoma (isocitrate dehydrogenase [IDH] wild type), CDKN2A/B homozygous deleted IDH–mutant astrocytoma, non-small cell lung cancer (KRAS mutant), or select solid tumors with CDK pathway alterations; and progression on, ineligibility for, or refusal of standard therapy. Patients with treated and untreated brain metastases (lesion &lt;1.5 cm) could enroll if they had no neurological symptoms and did not require immediate local treatment. PRT3645 was taken orally once daily for 28 days (1 cycle) with dose escalation employing a Bayesian optimal interval design to evaluate the safety, tolerability, incidence of dose-limiting toxicities, and pharmacokinetic/pharmacodynamic profile.
Results: At data cutoff (May 18, 2023), 8 patients received 20 and 40 mg once daily (first 2 dose levels of dose escalation). No dose-limiting toxicities were observed and the maximum tolerated dose was not reached. Median age was 62 (range, 53-76) years; 50% of patients were male; in these early dose cohorts 75% of patients discontinued therapy (all due to progressive disease; median duration of therapy: 8 [range, 4-16] weeks); 5 patients had glioblastoma, 2 had breast cancer, and 1 had sarcoma. All-cause, any-grade adverse events (AEs) reported in ≥1 patients were anemia and dizziness. One grade 1 AE (fatigue) was considered related to PRT3645. No patients had grade ≥3 AEs, serious AEs, diarrhea, QTcF prolongation, or hepatotoxicity. No patients had dose reductions, interruptions, withdrawals, or deaths due to AEs. Between pre-dose and 2 weeks post first dose, a reduction in phosphorylated Rb (&gt;70% in 3/4 patients) and Ki67 (&gt;60% in 2/4 patients) expression was seen at 40 mg; both markers showed a correlation between histochemical score and drug exposure.
Conclusions: Treatment with the next-generation CDK4/6 inhibitor PRT3645 was associated with a substantial decrease in phosphorylated Rb and Ki67 expression, indicating a high level of target engagement at a low dose. PRT3645 exhibited tolerable dose escalation in the initial 2 cohorts of patients with no significant gastrointestinal or hematologic events reported to date, leveraging its enhanced selectivity profile. Dose escalation is ongoing.
Citation Format: Amita Patnaik, Mohamad A. Salkeni, Manish R. Patel, Atrayee Basu-Mallick, Dale Shepard, Christine Lihou, William Sun, Jennifer Xavier, Sri Sahasranaman, Neelesh Sharma, Erika Hamilton. A phase 1 open-label, dose-escalation study of central nervous system–penetrant cyclin-dependent kinase (CDK)4/6 inhibitor PRT3645 in patients with select advanced or metastatic solid tumors [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B160.