We evaluated the safety, pharmacokinetic profi le, pharmacodynamic effects, and antitumor activity of abemaciclib, an orally bioavailable inhibitor of cyclin-dependent kinases (CDK) 4 and 6, in a multicenter study including phase I dose escalation followed by tumorspecifi c cohorts for breast cancer, non-small cell lung cancer (NSCLC), glioblastoma, melanoma, and colorectal cancer. A total of 225 patients were enrolled: 33 in dose escalation and 192 in tumor-specifi c cohorts. Dose-limiting toxicity was grade 3 fatigue. The maximum tolerated dose was 200 mg every 12 hours. The most common possibly related treatment-emergent adverse events involved fatigue and the gastrointestinal, renal, or hematopoietic systems. Plasma concentrations increased with dose, and pharmacodynamic effects were observed in proliferating keratinocytes and tumors. Radiographic responses were achieved in previously treated patients with breast cancer, NSCLC, and melanoma. For hormone receptor-positive breast cancer, the overall response rate was 31%; moreover, 61% of patients achieved either response or stable disease lasting ≥ 6 months. SIGNIFICANCE:Abemaciclib represents the fi rst selective inhibitor of CDK4 and CDK6 with a safety profi le allowing continuous dosing to achieve sustained target inhibition. This fi rst-in-human experience demonstrates single-agent activity for patients with advanced breast cancer, NSCLC, and other solid tumors. Cancer Discov; 6(7); 740-53.

Clinical resistance mechanisms to CDK4/6 inhibitors in HR+ breast cancer have not been clearly defined. Whole exome sequencing of 59 tumors with CDK4/6i exposure revealed multiple candidate resistance mechanisms including RB1 loss, activating alterations in AKT1, RAS, AURKA, CCNE2, ERBB2, and FGFR2, and loss of ER expression. In vitro experiments confirmed that these alterations conferred CDK4/6i resistance. Cancer cells cultured to resistance with CDK4/6i also acquired RB1, KRAS, AURKA, or CCNE2 alterations, which conferred sensitivity to AURKA, ERK, or CHEK1 inhibition. Besides inactivation of RB1, which accounts for ~5% of resistance, seven of these mechanisms have not been previously identified as clinical mediators of resistance to CDK4/6 inhibitors in patients. Three of these-RAS activation, AKT activation, and AURKA activation-have not to our knowledge been previously demonstrated preclinically. Together, these eight mechanisms were present in 80% of resistant tumors profiled and may define therapeutic opportunities in patients. SignificanceWe identified eight distinct mechanisms of resistance to CDK4/6 inhibitors present in 80% of resistant tumors profiled. Most of these have a therapeutic strategy to overcome or prevent resistance in these tumors. Taken together, these findings have critical implications related to the potential utility of precisionbased approaches to overcome resistance in many patients with HR+ MBC..

AbstractClinical resistance mechanisms to CDK4/6 inhibitors in HR+ breast cancer have not been clearly defined. Whole exome sequencing of 59 tumors with CDK4/6i exposure revealed multiple candidate resistance mechanisms including RB1 loss, activating alterations in AKT1, RAS, AURKA, CCNE2, ERBB2, and FGFR2, and loss of ER expression. In vitro experiments confirmed that these alterations conferred CDK4/6i resistance. Cancer cells cultured to resistance with CDK4/6i also acquired RB1, KRAS, AURKA, or CCNE2 alterations, which conferred sensitivity to AURKA, ERK, or CHEK1 inhibition. Besides inactivation of RB1, which accounts for ∼5% of resistance, seven of these mechanisms have not been previously identified as clinical mediators of resistance to CDK4/6 inhibitors in patients. Three of these—RAS activation, AKT activation, and AURKA activation—have not to our knowledge been previously demonstrated preclinically. Together, these eight mechanisms were present in 80% of resistant tumors profiled and may define therapeutic opportunities in patients.SignificanceWe identified eight distinct mechanisms of resistance to CDK4/6 inhibitors present in 80% of resistant tumors profiled. Most of these have a therapeutic strategy to overcome or prevent resistance in these tumors. Taken together, these findings have critical implications related to the potential utility of precision-based approaches to overcome resistance in many patients with HR+ MBC.

Prexasertib, a checkpoint kinase 1 inhibitor, demonstrated single-agent activity in patients with advanced squamous cell carcinoma (SCC) in the dose-escalation portion of a phase I study (NCT01115790). Monotherapy prexasertib was further evaluated in patients with advanced SCC. Patients were given prexasertib 105 mg/m as a 1-hour infusion on day 1 of a 14-day cycle. Expansion cohorts were defined by tumor and treatment line. Safety, tolerability, efficacy, and exploratory biomarkers were analyzed. Prexasertib was given to 101 patients, including 26 with SCC of the anus, 57 with SCC of the head and neck (SCCHN), and 16 with squamous cell non-small cell lung cancer (sqNSCLC). Patients were heavily pretreated (49% ≥3 prior regimens). The most common treatment-related adverse event was grade 4 neutropenia (71%); 12% of patients had febrile neutropenia. Median progression-free survival was 2.8 months [90% confidence interval (CI), 1.9-4.2] for SCC of the anus, 1.6 months (90% CI, 1.4-2.8) for SCCHN, and 3.0 months (90% CI, 1.4-3.9) for sqNSCLC. The clinical benefit rate at 3 months (complete response + partial response + stable disease) across tumors was 29% (23% SCC of the anus, 28% SCCHN, 44% sqNSCLC). Four patients with SCC of the anus had partial or complete response [overall response rate (ORR) = 15%], and three patients with SCCHN had partial response (ORR = 5%). Biomarker analyses focused on genes that altered DNA damage response or increased replication stress. Prexasertib demonstrated an acceptable safety profile and single-agent activity in patients with advanced SCC. The prexasertib maximum-tolerated dose of 105 mg/m was confirmed as the recommended phase II dose. .

Purpose: The PI3K/mTOR pathway is frequently aberrated in cancer. LY3023414 is a potent and selective ATP-competitive inhibitor of class I PI3K isoforms, mTOR, and DNA-PK. Here we report the dose-escalation results of the first-in-human phase I study of LY3023414.Experimental Design: A 3þ3 dose escalation for once-daily and twice-daily oral dosing of LY3023414 was followed by an expansion cohort for CYP3A4 drug-drug interaction (DDI) assessment. The primary objective was to determine the recommended phase 2 dose (RP2D). Additional objectives included safety, pharmacokinetics/pharmacodynamics, and antitumor activity.Results: Forty-seven patients with solid tumors received LY3023414 at once-daily (20-450 mg) or twice-daily dosing (150-250 mg). Dose-limiting toxicities were observed at 450 mg once-daily (thrombocytopenia, hypotension, hyperkalemia) in three of three patients, 250-mg twice-daily dosing (hypophosphatemia, fatigue, mucositis) in three of four patients, and in one of 15 patients at 200 mg twice-daily (nausea). Common related AEs included nausea (38%), fatigue (34%), and vomiting (32%) and were mostly mild or moderate. LY3023414 pharmacokinetics demonstrated dose-dependent increase in exposure with ! 90% target inhibition at doses !150 mg. DDI analysis demonstrated LY3023414 to be a weak inhibitor of CYP3A4. Durable partial response was observed in a patient with endometrial cancer harboring PIK3R1 and PTEN truncating mutations, and 13 additional patients (28%) had a decrease in their target lesions by up to 30%.Conclusions: LY3023414 has a tolerable safety profile and single-agent activity in patients with advanced cancers. The RP2D of LY3023414 monotherapy is 200 mg twice daily based on safety, tolerability, and pharmacokinetic/pharmacodynamic data. Clin Cancer Res; 1-10. Ó2018 AACR.