P1.07-002 G1T28, a Cyclin Dependent Kinase 4/6 Inhibitor, in Combination with Topotecan for Previously Treated Small Cell Lung Cancer: Preliminary Results: Topic: Drug Treatment Alone and in Combination with Radiotherapy

Chemotherapy-induced bone marrow and immune system toxicity causes significant acute and long-term consequences. G1T28 is a potent and selective CDK4/6 inhibitor (CDK4/6i) in development to reduce chemotherapy-induced myelosuppression and preserve immune system function in small cell lung cancer (SCLC) patients. Hematopoietic stem and progenitor cells (HSPC) are dependent upon CDK4/6 for proliferation, and preclinical models demonstrated that transient G1T28-induced G1 cell cycle arrest renders them resistant to chemotherapy cytotoxicity, allowing faster hematopoietic recovery, preservation of long-term stem cell and immune system function, and enhancement of chemotherapy anti-tumor activity. Objectives of this ongoing multicenter Phase 1b/2a study are to assess the dose limiting toxicities (DLTs), safety, hematological profile, PK, and anti-tumor activity of G1T28 in combination with topotecan (NCT02514447). The study consists of a limited open-label, dose-finding portion (Part 1; up to 40 patients), and an open-label, single-arm expansion portion (Part 2; 28 patients). Eligible patients had histologically/cytologically confirmed SCLC, adequate organ function, ECOG performance status 0-2, 1-2 prior lines of chemotherapy, and no symptomatic brain metastases. G1T28, at a starting dose of 200 mg/m2 (derived from the Phase 1a healthy volunteer study and expected to maintain HSPC G1 arrest beyond topotecan exposure), was administered IV prior to IV topotecan on days 1-5 every 21-days. 21 patients (median age 68, 5 females, 20 white and 1 African-American) have been enrolled across 5 cohorts. DLTs due to Grade 3/4 myelotoxicity occurred in the first two cohorts and were associated with supra-therapeutic topotecan exposures due to decreased topotecan clearance by G1T28. Reducing the topotecan dose achieved exposures in the therapeutic range and was well tolerated. No episodes of febrile neutropenia or bleeding have occurred to date. For the 17 evaluable patients, there were 5 PR, 8 SD, and 4 PD. In the 6 platinum refractory patients there were 1 PR, 3 SD, and 2 PD. G1T28, a novel CDK4/6i, combined with topotecan for previously treated SCLC patients has been well tolerated, without any episodes of febrile neutropenia or bleeding. There are encouraging early signs of anti-tumor activity, with a response rate of 29% overall (36%, 4/11 in sensitive and 17%, 1/6 in refractory) and a clinical benefit rate (CR+PR+SD) of 76% overall (82%, 9/11 in sensitive and 67%, 4/6 in refractory). This novel approach, allowing the administration of chemotherapy with preservation of hematopoietic function and cellular immunity, could potentially improve treatment outcomes of patients with CDK4/6-independent tumors. Updated data will be presented.