Safety, Pharmacokinetics, and Preliminary Activity of CDK4/6 Inhibitor FCN-437c in Chinese Female Patients with HR+HER2- Advanced Breast Cancer (ABC) From Phase Ia Study

Jian Zhang Fudan University Shanghai Cancer Center Xiaojia Wang Zhejiang Cancer Hospital Xian Wang Sir Run Run Shaw Hospital Aimin Hui Fosun Pharma USA Inc. Zhuli Wu Beijing Fosun pharmaceutical Research and Development Co., Ltd Ling Tian Avanc Pharmaceutical Co., Ltd. Changjiang Xu Beijing Fosun Pharmaceutical Research and Development Co., Ltd. Yuchen Yang Beijing Fosun Pharmaceutical Research and Development Co,. Ltd https://orcid.org/0000-0002-47220786 Wenjing Zhang Beijing Fosun Pharmaceutical Research and Development Co., Ltd Xichun Hu (  xchu2009@hotmail.com ) Fudan University Shanghai Cancer Center


Introduction
Breast cancer (BC) has become the most common life-threatening malignancies in women worldwide. Although many approaches have been developed for the diagnosis and treatment of BC, the 5-year survival rate of metastatic BC remains at 27% [1]. CDK4/6 has been found to play an important role in cell proliferation and are often dysregulated in BCs, particularly in HR-positive BCs [2,3]. Cyclin D1 is a transcriptional target of ER and forms complexes with CDK4/6 [4]. Activation of the CDK4/6-cyclinD1 complex contributes to the hyperphosphorylation of retinoblastoma (Rb) protein, which causes inactivation of the cell growth-inhibitory by releasing E2F transcription factors and the cell-cycle progression from G1 to S phase [5,6]. Because of the essential role of this pathway in cell cycle regulation, inhibition of CDK4/6 has been regarded as a promising target for anti-tumor therapies. Small molecule CDK4/6 inhibitors may block tumor cell growth by binding to ATP-binding domain of CDK4/6 kinase and dephosphorylate Rb protein, resulting in cell cycle arrest in G1 phase [7].
The emergence of second generation selective CDK4/6 inhibitors has targeted tumors with the expression of CDK4/6 with meaningful prolongation of progression-free survival over endocrine therapy alone [8]. To date, three orally bioavailable CDK4/6 inhibitors, palbociclib (Ibrance, PD0332991), ribociclib (Kisqali, LEE011) and abemaciclib (Verzenio, LY2834219), have been FDA-approved as standard of care against HR + HER2-metastatic breast cancer in combination with aromatase inhibitors (AI) as initial therapy and with fulvestrant after disease progression following rst line endocrine therapy or as monotherapy for heavily pre-treated patients. However, the treatment with CDK4/6 inhibitors is limited for patients with brain metastases. Only abemaciclib demonstrated a con rmed objective intracranial response of 6% in a phase II study in 58 patients with brain metastases secondary to HR + HER2-metastatic breast cancer and 38% of the patients showed a decrease in the sum of intracranial target lesions. Intracranial clinical bene t rate (CR + PR + SD persisting for ≥ 6 months) was 25% and median PFS was 4.4 months (95% CI, 2.6-5.5) [9].
FCN-437c is an oral, second generation, potent CDK4/6 dual inhibitor which selectively inhibits the kinase activities of CDK4 and CDK6 kinases and had no inhibitory activity against CDK1, CDK2 or CDK5 kinases. In in vitro studies, FCN-437c showed inhibitory effects on cell proliferation in human breast cancer cell lines MCF7 and MCF/ARO, which was comparable to or greater than ribociclib and palbociclib. FCN-437c also showed a synergistically in vitro and in vivo anti-tumor effect in combination with fulvestrant on MCF7 cell line and xenograft models comparable to ribociclib and palbociclib, while that on MCF7/ARO xenograft models FCN-437c was more potent than ribociclib and palbociclib. In addition, FCN-437c represents favorable physical and pharmacokinetic (PK) properties with good penetration through blood brain barrier, and an acceptable toxicity pro le in non-clinical studies.

Study design and treatment:
This is a phase Ia, multi-center, open-label, single arm dose-escalation clinical trial of FCN-437c to treat the female patients with advanced HR + HER2-breast cancer. The primary objective of the study was to evaluate the safety, tolerability and to determine the MTD of FCN-437c as a single agent. In addition, this study evaluated the pharmacokinetics characteristics of FNC-437c as a single dose and continuous dose, and preliminary anti-tumor activity.
Dose-escalation was conducted following a 3 + 3 study design with starting dose of 50mg. 50 mg, 100 mg, 200 mg, 300 mg, 450 mg and 600 mg per day were set as main dose level for escalation. Patients were administered a single oral dose of FCN-437c under fasting conditions for PK run in of 7 days, and following a continuous treatment once daily for 21 days and 7 days break in a 28-day cycle. DLT was evaluated during the DLT observation period, including the PK run in period (7 days) and the rst cycle (28 days). MTD was considered as the highest dose level with no more than 33% DLT of assessable patients during the DLT evaluation period. All screened patients provided a signed informed consent form (ICF) and agreed to comply with the study protocol. This study was conducted in accordance with the Declaration of Helsinki and guidelines for Good Clinical Practice as de ned by the International Conference on Harmonization.

Patients:
Adult female patients (≥ 18 years old) must have a histological or cytological con rmation of HR + HER2advance breast cancer, who had disease progression after standard therapy or for whom no standard therapy is available, was eligible for this study. Patients should have at least one measurable lesion based on RECIST Version 1.1 or only have bone metastatic lesions. Life expectancy should exceed at least 12 weeks and the Eastern Cooperative Oncology Group (ECOG) performance status score was 0 or 1. Patients who recently underwent major surgery, chemotherapy, radiotherapy, antibodies or other investigational drugs within 28 days, or patients previously treated with any CDK4/6 inhibitor were excluded from this study. Patients were also excluded if they had uncontrolled central nervous system metastases or presented with cardiac dysfunction such as average QTc > 470 ms in 3 sequential 12-lead ECGs, arrhythmia with clinical signi cance according to NYHA (New York Heart Association) Grade 3 or 4 congestive heart failure or have potential for prolongation of the QTc interval.
Safety assessments: DLT was assessed during DLT observation period (7 days PK run in and 28 days since C1D1). DLT was de ned as (1)hematological toxicities: Grade 4 neutropenia lasting ≥ 3 days; Grade 3 thrombocytopenia with hemorrhage; Grade 3 or 4 febrile neutropenia (> 38℃ for 1 hour or > 38.3℃) (2) non-hematological toxicities (alopecia excluded): Grade 2 increased AST/ALT with Grade 2 increased total bilirubin; QTc interval ≥ 501 ms (mean value of at least two ECGs) or QTc interval prolongation ≥ 60 ms from baseline; Grade 3 nausea, vomiting and/or diarrhea, electrolyte disturbances lasting for more than 3 days, which cannot be controlled or recovered to Grade 1 with supportive care; and other non-speci ed ≥ Grade 3 non hematological toxicities. Radiographic tumor assessment was performed at screening and once every 8 weeks (± 7 days) until disease progression, intolerable toxicity, death. E cacy endpoints including ORR (con rmed partial or complete response), CBR (objective response or stable disease for ≥ 24 weeks) and investigator-assessed PFS was assessed based on RECIST Version 1.1 Statistical Methods: Sample size was estimated based on 3 + 3 study design. All statistical analyses were performed with SAS® 9.2 or higher version (SAS Institute, Inc., Cary, N.C., USA) except for the calculation of pharmacokinetic parameters with Phoenix® WinNonlin 6.4 or higher version (Pharsight Corp., Certara, Princeton, NJ, USA). DLT was summarized by counts and percentages. Safety data were summarized using descriptive statistics in patients who received ≥ 1 dose of study treatment (as-treated population). For e cacy, con rmed objective response rate (ORR), clinical bene t rate (CBR) and 95% Clopper Pearson con dence interval will be calculated. Progression-free survival (PFS), duration of response (DOR) and overall survival (OS) for survival analysis will be carried out by using Kaplan-Meier curve according to the investigator's evaluation.

Patients and treatment
Between Feb 13th. 2019 to Apr.15th 2020, 17 patients were enrolled in 3 study centers in Mainland China and received FCN-437c at doses ranging from 50 to 450mg in a 3-week-on and 1-week-off schedule: 50mg (n = 3), 100mg (n = 3), 200mg (n = 3), 300 mg (n = 6) and 450 mg (n = 2) mg. Most of the patients were ≤ 65 years, with a median age of 45.0 years. Most of patients are ECOG performance status grade 1 (84.2%). Patients were heavily pretreated, 100% failed to prior both hormone therapy and chemotherapy. A summary of patient characteristics is provided in Table 1.
All of 12 discontinued patients were due to disease progression, the other 5 patients are still on the treatment. Five patients had completed the study, 4 deaths and 1 completed the one-year follow-up per protocol.
As of the cut-off date on Aug. 10th 2020, the median follow-up duration was 8.71 months (95%CI: 4.53-10.84). Median planned treatment duration was 112 days (56, 216), and the median actual treatment duration was 81days (43, 157). The mean relevant dose intensity was 85.4%, 70.6% of patients received over 80% of the planned dose.

Pharmacokinetics characteristics
The plasma concentration-time data and pharmacokinetics pro le of FCN-437c were determined in all 17 patients following single dose on D1 and multiple dose on C1D21, which are shown in Fig. 1 and Table 3, respectively. The correlation of exposure and dose after multi-dose administration are shown in Table 4.
On Single dose administration  DR: dose ratio; ER: exposure ratio. The dose of 50mg was set to 1 and the other doses were compared to 50 mg. The exposure (C max AUC 0−∞ AUC 0 − 24 C av−ss 24h Trough concentration) of 50mg was set to 1, and the exposure of the other doses were compared to the exposure of 50mg respectively.

E cacy
Fifteen patients had post baseline assessment. Nine stable disease was observed (60%), no response was reported (ORR 0%) and 6 patients' disease progressed. One SD maintained over 6 months at data cut-off date and CBR was 6.7%. Median PFS was 3.91 months (95%CI: 2.07-7.62), while median OS was not reached due to short follow-up period. The tumor shrinkage and maintain time are referred to Fig. 2 and the response data summary in Table 5. Patients in different dose level groups also experienced mild to moderate non-hematological AEs, all of which were Grade 1 or 2. Prolonged electrocardiogram QT interval, hypoalbuminemia and rash were the most common observed non-hematological toxicities. It is nice to see that only G1 or G2 QTcF prolongation happening to 5 patients (29.4%) in this phase 1a study. Ribociclib has been reported with high prevalence of prolongation of QTc interval. Based on the results from MONALEESA-7, an increase of more than 60 ms from baseline in the QTcF interval occurred in 32 (10%) of 335 patients in the ribociclib group and led to dose interruptions or reductions in 13 (4%) of the patients [12].
By contrast, abemaciclib is reported with less frequent hematological toxicities such as Grade 3/4 neutropenia found only in 21.1% patients, but higher incidence of diarrhea, nausea, fatigue and creatinine increased [13,14], which are not very common with FCN-437c treatment, only 3,2 and 4 patients reported diarrhea, nausea and fatigue respectively, all of which are G1 or G2, no creatinine increase was reported as study drug related in this study. As of the three marketed CDK4/6 inhibitors, combination therapies with endocrine agents (aromatase inhibitors or fulvestrant) signi cantly improved PFS and OS over the placebo plus endocrine agents in patients with HR + HER2-metastatic breast cancer in rst-and second-line settings, regardless of endocrine therapy strategies, treatment lines, the number of metastatic sites or menopausal status [15,16]. However, minor antitumor activities were demonstrated with palbociclib and ribociclib as single agent.
Barely patients achieved partial response and approximately 30% of the patients had stable disease in phase I trials [17][18][19]. In phase II trial MONARCH-1, abemaciclib alone showed a con rmed objective response rate at 19.7% and clinical bene t rate at 42.4% among 132 heavily pre-treated patients, with a median PFS of 6.0 months and a median OS of 17.7 months [20]. Therefore, abemaciclib has become the only CDK4/6 inhibitor approved as monotherapy for the treatment of patients with HR + HER2-advanced breast cancer with disease progression following prior endocrine therapy and chemotherapy.
Similar to palbociclib and ribociclib, FCN-437c as monotherapy showed modest tumor activity, however, there seems to be high potential anti-tumor capability of the combination with hormone therapy, given 60% of the patients exhibited stable disease with long disease control period by monotherapy, which is likely to be better than that from palbociclib and ribociclib phase 1 studies, warranting further exploration of the combination therapy in phase 2 study.
In conclusion, this study indicated a favorable safety pro le of oral administration of single agent FCN-437c on a 3/1 schedule and the toxicities were generally tolerable and manageable. Major AEs were hematological toxicities and 200mg was determined to be recommended dose for further development based on the comprehensive assessment of safety and pharmacokinetics results. 60% of the patients exhibited stable disease with long disease control period by monotherapy warranted to explore the clinical antitumor activity of the combinations with other endocrine therapies. A phase II trial of FCN-437c is now ongoing in combination with letrozole or fulvestrant in female advanced/metastatic HR + HER2-breast cancer patients.

Declarations
Funding: All funding was provided by Avanc Pharmaceutical Co., Ltd.

Con icts of interest/Competing interests:
The authors declare no potential con icts of interest.
Availability of data and material: