A Phase 1 Study of SHR6390, A Cyclin-dependent Kinase 4/6 Inhibitor in Patients with Advanced Breast Cancer

Background: SHR6390 is a novel inhibitor of cyclin-dependent kinase 4/6 which demonstrated promising anti-tumor potency in preclinical models. This rst-in-human study was conducted to evaluate the tolerability, pharmacokinetics, safety, and preliminary antitumor activity of SHR6390 in patients with advanced breast cancer (ABC). Methods: In this open-label, phase 1 study, patients who had failed standard therapy were enrolled to receive oral SHR6390 in 3 + 3 dose-escalation pattern at doses of 25−175 mg. Eligible patients were given a single-dose of SHR6390 in week 1, followed by once daily continuous doses for three weeks, and one week off in 28-day cycles. Based on the tolerability, pharmacokinetics, and activity data revealed from the dose-escalation phase, three dose cohorts were selected to expand to 8−10 patients. The primary endpoints were maximum tolerated dose (MTD) and pharmacokinetics. Results: Between Apr 15, 2016 and Dec 21, 2018, 40 patients were enrolled; all were diagnosed of hormone receptor-positive and HER2-negative ABC. SHR6390 100 mg, 125 mg, and 150 mg cohorts were expanded to 10 patients. No dose limiting toxicity was observed and the MTD was not reached. Adverse events (AEs) of grade 3 or 4 were observed in 22 (55.0%) of 40 patients, being neutropenia (52.5%), leukopenia (35.0%), thrombocytopenia (5.0%), and hypertension (2.5%). No serious AEs were reported. At the doses of 50−175 mg, steady state areas under the concentration-time curve and peak concentration increased almost proportionally with dose. The disease control rate (DCR) was 62.5% (25/40, 95% CI: 45.8−77.3). Two patients (5%; 125 mg and 150 mg cohorts) achieved partial response, with responses lasting 169 and 356+ days, respectively. Among the three expansion cohorts, the 150 mg cohort had the numerically highest DCR of 80.0% (95% CI: 44.4 ‒ 97.5) and longest median progression-free survival of 8.4 months (95% CI: 2.1 ‒ not reached). Conclusions: SHR6390 showed acceptable safety prole and dose-dependent plasma exposure in patients with ABC. The recommended phase 2 dose was 150 mg. Preliminary evidence of clinical activity was observed, which warrants further validation. The of dose-escalation and dose-expansion In the dose-escalation phase, to establish the (MTD) for In the dose-expansion phase, three selected dose cohorts to further characterize the PK parameters and preliminary anti-tumor activity of


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addition, selective CDK 4/6 inhibition allows preferential inhibition of oncogenic events while sparing toxicity in normal tissues and therefore represents an appealing therapeutic strategy for cancer 3,4 .
Endocrine therapy is the cornerstone in the treatment of hormone receptor (HR)-positive advanced breast cancer (ABC). Nevertheless, acquired resistance to endocrine therapy inevitably develops during the course of treatment 5 . Estrogen-mediated hyperactivity of the CCND-CDK4/6 axis is a central feature of HR-positive breast cancer, and the tumors usually retain a functional Rb 6 , which can be targeted by CDK 4/6 inhibitors. To date, three CDK 4/6 inhibitors including palbociclib, ribociclib, and abemaciclib have been approved by the US Food and Drug Administration in combination with endocrine therapy as the rst-and second-line treatment for HR-positive and human epidermal growth factor receptor 2 (HER2)negative ABC. In pivotal trials, the addition of these CDK 4/6 inhibitors to standard endocrine therapy substantially improved progression-free survival (PFS) and overall survival [7][8][9][10][11][12][13][14][15] . Abemaciclib has also been approved as a monotherapy for HR-positive and HER2-negative breast cancer progressing on prior endocrine therapy or chemotherapy in the metastatic setting 11 .
SHR6390 is a novel, highly selective, small molecule CDK4/6 inhibitor that demonstrated anti-tumor activity in a variety of in-vitro and xenograft models primarily via Rb-dependent cytostasis 16,17 . In vivo xenografts, SHR6390 generally showed similar or slightly better anti-tumor potency compared with palbociclib, without inducing noticeable toxicity 17 . Moreover, in HR-positive breast cancer cell lines and xenografts, SHR6390 could overcome acquired drug resistance to endocrine therapy 17 . Based on these preclinical evidences, we conducted a rst-in-human, phase 1 trial to assess the safety, tolerability, pharmacokinetics (PK) and preliminary e cacy of oral SHR6390 in patients with HR-positive and HER2negative ABC.

Study design
This open-label, phase 1 trial in patients with ABC was conducted in China (ClinicalTrials.gov identi er: NCT02684266). The study consisted of dose-escalation and dose-expansion phases. In the doseescalation phase, the primary objective was to establish the maximum tolerated dose (MTD) for oral SHR6390. In the dose-expansion phase, three selected dose cohorts were expanded to further characterize the safety pro le, tolerability, PK parameters and preliminary anti-tumor activity of SHR6390.

Patients
Eligible patients were aged 18-65 years, with histologically con rmed ABC who failed standard therapy. Other inclusion criteria included Eastern Cooperative Oncology Group performance status 0-1, life expectancy ≥ 3 months, adequate bone marrow function (hemoglobin > 110 g/L, neutrophils > 2.0×10 9 per L and platelets > 100×10 9 per L), adequate liver or renal function (total bilirubin < 1.5 times the upper limit of normal [ULN], alanine transaminase [ALT] and aspartate aminotransferase [AST] ≤ 1.5 times ULN [≤ 5 times ULN in the presence of liver metastases] and creatinine ≤ 1 ULN), and adequate cardiac function (left ventricular ejection fraction ≥ 50% and Fridericia corrected-QT interval < 450 ms in males or < 470 ms in females). The key exclusion criteria were prior or current treatment with CDK4/6 targeted therapy, cytotoxic chemotherapy within three weeks (six weeks for mitomycin C or nitrosamine) or any other anti-tumor therapy within three weeks (except for endocrine therapy, which had to be discontinued before the time of informed consent), untreated or uncontrolled/unstable brain metastases (as judged by the investigator) or requirement of long-term use of steroids.

Procedures
For dose escalation, a traditional 3 + 3 design was used with three to six patients enrolled per dose level and the escalation continued until two or more patients had dose-limiting toxicities (DLTs) in one dose cohort during the DLT assessment period (from the administration of the rst study dose to the end of the rst cycle). The starting dose of SHR6390 was 25 mg and was escalated by 25 mg increments up to 175 mg (25 mg, 50 mg, 75 mg, 100mg, 125 mg, 150 mg and 175 mg) in a modi ed Fibonacci schema. At 25 mg dose level, one patient was enrolled as a sentinel; if no grade ≥ 2 toxicity was observed during the sentinel patient's DLT assessment period, escalation could proceed to 50 mg dose level immediately. DLT was de ned as an adverse event (AE) that met any of the following criteria: grade 4 hematologic toxicity, grade 3 neutropenia with infection or fever ≥ 38.5℃, grade 3 thrombocytopenia with apparent clinical bleeding tendency, grade ≥ 3 non-hematologic toxicity (excluding untreated nausea, vomiting, and diarrhea or AEs considered tolerable by the patients, such as alopecia) or grade ≥ 2 cardiac or renal toxicity. Based on the tolerability, PK, and anti-tumor activity revealed from dose-escalation phase, three dose cohorts were selected to expand to 8-10 patients.
All patients were rst given a single dose of oral SHR6390. After a ≥ 7-day washout period, the patient was then administered once-daily continuous doses of SHR6390 for three weeks in 28-day cycles. Dose interruption or reduction was permitted after the initiation of the second cycle of treatment. Patients who achieved complete response [CR], partial response [PR] or stable disease [SD]) at rst e cacy assessment after receiving two cycles of SHR6390 could continue treatment until disease progression, intolerable toxicity, withdrawal of patient consent or investigator decision.

Endpoint
The primary endpoint was MTD of oral SHR6390 and pharmacokinetic parameters in patients with ABC. MTD was the highest dose level at which <33% of the patients reported a DLT within the rst cycle of multiple dosing. The secondary endpoints included safety and the investigator-assessed objective response rate (ORR, de ned as the proportion of patients with CR and PR as the best overall response) and disease control rate (DCR, de ned as the proportion of patients with CR, PR or SD ≥ 6 weeks) per RECIST v1.1.

Assessment
Safety was regularly monitored from the time of informed consent until 30 days after the last dose of study drug. AEs were graded according to Common Terminology Criteria for Adverse Events v4.0. Tumor response was assessed using computerized tomography or magnetic resonance imaging at baseline and every two cycles after the start of multiple once-daily dosing. The response was classi ed as CR, PR, SD or progressive disease (PD) according to RECIST v1.1 by the investigators 18 . A CR or PR was required to be con rmed with a subsequent scan four weeks after the initial assessment.
For PK analysis, blood samples were collected in all enrolled patients from predose and at intervals up to 24 h (0.

Statistics
The DLT analysis set included all patients who received at least one dose of the study drug and completed the rst cycle of treatment or discontinued study drug due to AE during the rst cycle of treatment. PK was analyzed in patients who received at least one dose of the study drug and had evaluable post-treatment PK data. Safety and e cacy were analyzed in all patients who received at least one dose of the study drug. Descriptive statistics were reported for the safety and e cacy outcomes and PK parameters. The ORR and DCR were calculated, with the corresponding 95% Clopper-Pearson con dence intervals (CIs) provided. The Kaplan-Meier method was used to analyze PFS, and the median PFS (month) was estimated. The two-sided 95% CI of the median PFS was calculated by the Brookmeyer Crowley method. PK parameters were analyzed using the non-compartmental model with WinNonlin 7.0 (Pharsight, Mountain View, CA). Statistical analyses were conducted with SAS 9.4 (SAS Institute Inc., Cary, NC).

Patient characteristics and deposition
Between Apr 15, 2016 and Dec 21, 2018, 58 patients were screened. Among them, 40 patients were enrolled and received SHR6390. All patients were diagnosed of HR-positive and HER2-negative stage IV ABC. 45.0% of patients had at least three lines of prior chemotherapies and 55.0% had at least two prior endocrine therapies ( Table 1). The median follow-up was 7.0 months (range, 2.7 − 35.0). The main reason for treatment discontinuation was disease progression (28 patients, 70.0%). At data cutoff (Jun 28, 2019), treatment was still on-going in 10 patients (25.0%). No deaths were reported.  The median duration of treatment exposure was 123 days (range, 56 − 1063). No patients discontinued treatment due to AEs. All patients in the study had at least one treatment-emergent AE (TEAE) and the majority were transient laboratory abnormalities and low grade non-hematologic AEs ( Table 2). TEAEs of grade 3 or 4 were observed in 22 (55.0%) of 40 patients, being neutropenia (52.5%), leukopenia (35.0%), thrombocytopenia (5.0%), and hypertension (2.5%). No serious AEs or grade 5 AEs were reported. Table 2 Treatment-emergent adverse events occurring in ≥ 20% of patients. PK PK parameters following single and multiple dosing of SHR6390 are presented in Table 3 (Table 3).       The ndings from this rst-in-human, phase 1 trial showed that oral SHR6390 with a 3-week on and 1week off dosing regimen was well-tolerated in patients with ABC, with no MTD observed up to the highest dose tested. TEAEs were generally manageable with no treatment discontinuation due to AEs or serious AEs documented. At doses of 50-175 mg, plasma exposure increased almost proportionally with dose. Preliminary evidence of clinical activity of SHR6390 in HR-positive and HER2-negative ABC was also observed, with a DCR of 62.5%.
During dose-escalation and expansion, no DLT was observed in all patients within the rst cycle of treatment. With extended exposure beyond cycle 1 of treatment, a total of two cases of grade 4 hematologic AEs (neutropenia; one in 100 mg and one in 150 mg cohort) were observed, suggesting a general low rate of late onset severe toxicity. Consistent with the safety pro le of other CDK 4/6 inhibitors (palbociclib and ribociclib) [19][20][21] , the most frequent grade 3 or 4 AEs of SHR6390 were neutropenia and leukopenia, observed in 52.5% and 35.0% of patients respectively. All the hematologic AEs were managed with dose interruption and/or reduction and standard supportive care (granulocyte colony-stimulating factor etc.) and the abnormalities resolved immediately with no related complications such as fever or infection. The myelosuppressive effects of SHR6390 were considered an on-target effect due to the involvement of CDK6 in the proliferation of hematological precursors 22  Despite the initial success of endocrine therapy in the treatment of HR-positive and HER2-negative ABC, nearly all patients will develop acquired drug resistance with prolonged treatment 5 20,23 . Within the limitations of cross-trial comparison, the anti-tumor activity of single agent SHR6390 reported in this trial was comparable to abemaciclib and palbociclib in treatment of ABC. In general, our results were encouraging considering that the e cacy data of SHR6390 were derived from a heavily pre-treated population with refractory disease and from single agent treatment. Given the established synergic effects of SHR6390 with letrozole, anastrozole and fulvestrant in a phase Ib trial (NCT03481998, Hengrui data on le) and the purpose to further improve anti-tumor activities, SHR6390 is currently under evaluation in combination with endocrine therapy for HR-positive, and HER2-negative ABC in two phase 3 trials: one in combination with letrozole or anastrozole in the frontline setting (NCT03966898) and one in combination with fulvestrant in the later-line setting (NCT03927456).
In summary, SHR6390 showed acceptable safety pro le and dose-dependent plasma exposure in patients with ABC. The recommended phase 2 dose was determined to be 150 mg QD. Preliminary evidence of clinical activity of SHR6390 was observed.

Declarations
Ethics approval and consent to participate This trial was conducted in accordance with the principles of the Declaration of Helsinki and Good Clinical Practice guidelines. The study protocol was approved by the ethics committee of the Cancer Hospital of the Chinese Academy of Medical Sciences (Beijing, China) and regulatory authorities. All patients provided written informed consent before enrollment.

Consent for publication
Not applicable Availability of data and materials All data generated or analyzed during this study are included in this published article and its supplementary information les.

Funding
The study was sponsored by Jiangsu Hengrui Medicine Co., Ltd.