Safety, tolerability, and pharmacokinetics of BAT8001 in patients with HER2-positive breast cancer: An open-label, dose-escalation, phase I study

2.1 Patients

This open-label, single-center, dose-escalation, phase I study prospectively enrolled patients with advanced or metastatic breast cancer histopathologically or cytologically diagnosed and tested HER2-positive (IHC 3+ and/or ISH +) who sought treatment between March 2017 to May 2018 at the Sun Yat-sen University Cancer Center (Guangzhou, Guangdong, China). The inclusion criteria were as follows: (1) aged 18–70 years old; (2) had at least one measurable tumor according to the response evaluation criteria in solid tumors version 1.1 (RECIST 1.1) [23]; (3) had a performance status (PS) of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) scale; (4) life expectancy of at least three months; (5) adequate hematological, heart, lung, hepatic and renal function; (6) left ventricular ejection fraction (LVEF) ≥ 50%; (7) cumulative anthracycline dose ≤360 mg/m² doxorubicin or equivalent. Key exclusion criteria included a medical history of serious lung diseases, cardiac insufficiency (grade ≥3 symptomatic congestive heart failure according to Common Terminology Criteria for Adverse Events [CTCAE] v4.03), myocardial infarction, unstable angina, severe arrhythmia, symptomatic central nervous system, or brain metastases.

This study was registered at ClinicalTrials.gov (NCT04189211) and was performed in accordance with the principles of the Declaration of Helsinki and the Chinese Good Clinical Practice and local legislation. It was approved by the Ethics Committee of Sun Yat-sen University Cancer Center (Approval No.: A2016-052). All patients provided written informed consent to the study.

2.2 Study design and procedures

A “3+3” protocol was used for dose escalation to identify the maximum tolerated dose. BAT8001 was administered by intravenous infusion on day 1, once every 21 days, and treatment cycles were repeated until disease progression or unacceptable toxicity. The initial cohort at dose level of 1.2 mg/kg enrolled 3 patients, with 100% dose escalations to the second cohort, 50% dose escalations to the third cohort, and 33% dose escalations to subsequent dose levels. Doses were escalated up to 6.0 mg/kg. At least 3 patients were evaluated at each dose level. For any cohort, if one patient experienced dose-limiting toxicity (DLT) during the DLT assessment window (21-day period following first dose of BAT8001), that cohort patient number was expanded to up to 6 patients. If 2 or more patients experienced DLT, further enrollment at that dose level and dose escalation were ceased and the previous dose level was considered as the maximum-tolerated dose (MTD).

2.3 Study assessments

Safety evaluations were conducted at each study visit. Safety assessments included documentation of adverse events (AEs), physical examination, vital signs, laboratory test assessments, radiographs, electrocardiogram (ECG) and left ventricular function by echocardiogram, and ECOG PS. Toxicities were graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE) v4.03.

DLT was defined as any of the following treatment-related AE occurring during cycle 1 of each dose level: grade-3 or worse non-hematologic and non-hepatic major organ toxicity (excluding grade 3 diarrhea, nausea, or vomiting in the absence of premedication that responded to therapy); grade ≥4 thrombocytopenia or anemia, or grade ≥4 neutropenia lasting 4 days or associated with fever; LVEF ≤45% and decreased ≥10% from baseline; grade ≥3 cardiac toxicity, new segmental wall-motion abnormalities or cardiac troponin I elevation to 0.2 ng/mL; grade ≥3 elevations in total bilirubin, hepatic transaminases levels; serum aminotransferase >3ULN and total bilirubin (TBIL) >2ULN; patients with baseline grade 2 hepatic transaminase level ≥10×ULN. All safety analyses were descriptive only.

Serum was collected for pharmacokinetic (PK) analysis on day 1–5, 7, 9, 11, 14 and 18 after the first administration of the study drug (cycle 1); day 1 for cycles 2 and 3; and day 1–5, 7, 9, 11, 14, 18 and 21 for cycle 4. Pharmacokinetics of BAT8001, total anti-HER2 antibody, and maytansine derivative (payload) were evaluated using standard non­compartmental methods using WinNonlin version 6.2 (Certara USA Inc; Princeton, NJ, USA).

Tumor response assessments were repeated every two cycles and evaluated as per the Response Evaluation Criteria in Solid Tumors (RECIST v1.1) [23]. Partial response (PR) or complete response (CR) was confirmed on the response 4 weeks after the criteria for response were first met. All patients who received at least one complete cycle (3 weeks) of BAT8001 and had a follow-up assessment were evaluable for tumor response.

2.4 Endpoints

The primary outcomes of this study were to evaluate the safety and the tolerability of intravenous (IV) BAT8001 and to determine the MTD or recommended dose for subsequent clinical trials.

Secondary outcomes included pharmacokinetics and therapeutic efficacy evaluations. The PK parameters included the maximum serum concentration (C_max) or C_max at steady-state (C_max, ss), the trough plasma concentration at steady state (C_min, ss), the time to C_max (T_max), the terminal elimination half-life (t_1/2), the area under the concentration-time curve to the last sampling time (AUC_0-t), the area under the concentration-time curve to infinity (AUC_0-∞), clearance (CL), terminal phase distributed volume (V_z), steady-state volume of distribution (V_ss). Anti-tumor efficacy assessment included the proportion of patients achieving an objective response (including CR and PR), duration of response (DOR, measured from the date of first objective response [either complete or partial response] to the first date that progressive disease was documented as per the RECIST v1.1; Any participants who did not progress by the cutoff date were censored at their final tumor assessment date) and progression-free survival (PFS, defined as the time from the date of first dose of study drug to the date of the first objective documentation of disease progression by radiography or death by any cause, whichever occurred sooner).

2.5 Statistical analysis

Total sample size for the dose escalation analysis was not predetermined, and was dependent on practical considerations. For the safety analysis, the evaluable population included patients who received at least one dose of study drug. The pharmacokinetic evaluable population included all patients who received at least one dose of study drug and had at least one post­treatment serum concentration of BAT8001. The efficacy evaluable population included all patients who received at least one dose of study drug and for whom both baseline and post­treatment data for tumor measurement per RECIST were available.

Demographic, safety, and pharmacokinetic data were summarized with descriptive statistics. Point estimates and 95% exact binomial confidence interval (CI) were determined. Kaplan Meier method was used to estimate the median for time­to­event endpoints including duration of response and PFS, with corresponding 95% CIs calculated. For objective responses, the number, proportion, and corresponding two-sided Clopper-Pearson exact 95% CIs were summarized. SAS version 9.3 and higher (SAS Institute, Cary, NC, USA) was used for all statistical analyses.

3.1 Patients and treatment

A total of 29 female patients with metastatic breast cancer (median age, 49.0 years; range, 27–65 years) were enrolled. The patients’ demographics and baseline characteristics are summarized in Table 1. All patients had an ECOG PS of 0 (10/29, 34.5%) or 1 (19/29, 65.5%). Most of the patients were heavily pretreated: 19 (65.5%) received 3 or more previous chemotherapy regimens in the metastatic setting. Twenty-two (75.9%) received prior anti-HER2 treatment. Fifteen (51.7%) patients with estrogen-receptor-positive (ER+) breast cancer received prior endocrine therapy before receiving BAT8001 treatment.

3.2 Safety

In the dose-escalation study, 29 patients were evaluable for MTD determination of BAT8001. BAT8001 dose was escalated from 1.2 mg/kg to 6.0 mg/kg (1.2 mg/kg [n  = 3], 2.4 mg/kg [n  = 3]; 3.6 mg/kg [n  =  10]; 4.8 mg/kg, [n  = 7]; 6.0 mg/kg [n  = 6]). The DLTs observed in cycle 1 were grade-4 thrombocytopenia and grade-3 elevated transaminase. Grade-4 thrombocytopenia occurred in 1 patient who was prescribed with 6.0 mg/kg, 2 patients who was prescribed with 4.8 mg/kg, and 1 patient who was prescribed with 3.6 mg/kg of BAT8001. Grade-3 elevated alanine aminotransferase and aspartate aminotransferase occurred in 1 patient who was prescribed with 6.0 mg/kg. As 2 of 7 patients in the 4.8 mg/kg dose group experienced a DLT, 3.6 mg/kg was determined to be the MTD of BAT8001.All patients had at least one drug-related AE reported in this study. The incidence of AEs is summarized in Table 2. The most frequent drug-related AEs during treatment were increased aspartate aminotransferase (AST; 25/29, 86.2%), thrombocytopenia (24/29, 82.8%), increased alanine aminotransferase (16/29, 55.2%), increased γ-glutamyl transferase (14/29, 48.3%), anemia (13/29, 44.8%), increased alkaline phosphatase (11/29, 37.9%), elevated low-density lipoprotein (11/29, 37.9%), hyperuricemia (10/29, 34.5%), and fever (10/29, 34.5%). Grade 3 or greater AE occurred in 14 (48.3%) patients, including thrombocytopenia in 12 (41.4%) patients, increased AST in 4 (13.8%) patients, increased γ-glutamyl transferase in 2 (6.9%) patients, increased alanine aminotransferase in 2 (6.9%) patients, diarrhea in 2 (6.9%) patients. Transient thrombocytopenia was the only grade 4 BAT8001-related AE (6/29, 20.7%). One patient in the 6.0 mg/kg dose group experienced a severe AE (spinal compression fracture and spinal cord compression) which required treatment interruptions, but were considered not related to the BAT8001 treatment. No patient discontinued the study treatment because of drug-related AEs. No death was reported in this study. Overall, an increased occurrence of grade 3 and grade 4 AEs was observed in higher dose cohorts than in lower doses.

3.3 Pharmacokinetics

Plasma and serum samples for pharmacokinetic analyses were available for all 29 patients who received BAT8001 doses ranging from 1.2 to 6.0 mg/kg. The serum concentrations of BAT8001 over cycle 1 for the 1.2-, 2.4-, 3.6-, 4.8-, and 6.0-mg/kg treatment groups are shown in Figure 1A. A multi-exponential decline in serum concentrations of BAT8001 was observed, and systemic exposure increased with respect to dose across the 1.2- to 6.0-mg/kg dose range after the cycle 1. The calculated pharmacokinetic parameters for each cohort over cycle 1 are summarized in Table 3. There was no significant accumulation of BAT8001 after IVinjection once every 21 days. Very low concentrations (<1 ng/mL) of free maytansine derivative were detected at all dose levels, and systemic clearance of total anti-HER2 antibody was slower than clearance of BAT8001 (Figure 1B and Supplementary Table S1 and S2). Of these 29 patients, only one had an anti-drug antibody response, with no observed impact on pharmacokinetics of BAT8001.

3.4 Efficacy

All 29 enrolled patients in the 1.2- to 6.0-mg/kg cohorts were considered evaluable for efficacy. Patients were evaluated for tumor response every two cycles according to the RECIST 1.1 criteria. A summary of best overall response is shown in Figure 2. Of the 29 evaluable patients, 12 (41.4%; 95% CI = 23.5%–61.1%) achieved an objective response, 10 of which were confirmed. A total of 24 (82.8%; 95% CI = 64.2%–94.2%) patients achieved disease control. In 7 patients previously treated with trastuzumab and lapatinib, 2 (28.6%; 95% CI = 3.7%–71.0%) patients achieved an objective response and 4 (57.1%; 95% CI = 18.1%–90.1%) patients achieved disease control. For 13 patients previously treated with trastuzumab alone, the objective response was observed in 6 (46.2%; 95% CI = 19.2%–74.9%) patients, including 1 unconfirmed overall response, and disease control was observed in 11 (84.6%; 95% CI = 54.6%–98.1%) patients. For the 3.6 mg/kg dose (MTD) cohort (n = 10), objective response was observed in 4 (40.0%; 95% CI = 12.2%-73.8%) patients, including 1 unconfirmed overall response, and disease control was observed in 8 (80%; 95% CI = 44.4%–97.5%) patients. Notably, 1 patient achieved CR at the 3.6 mg/kg dose (MTD) cohort, and for whom the duration of response was 9 months.

At the study cutoff date, 10 (34.5%) patients had received more than 6 months of treatment and 2 (6.9%) patients were still on treatment (Figure 3). Patients with a partial response were mostly on 3.6mg/kg or higher dose cohorts, except for 1 patient who received 1.2mg/kg of BAT8001 and remained on treatment after 18.6 months since initial dosing (Figure 2 and 3). Of the 12 patients who achieved an objective response, their median duration of response was 4.30 months (95% CI = 0–14.24 months). Of the 29 efficacy-evaluable patients, 26 (89.7%) had a PFS event and their median PFS was 4.30 months (95% CI = 2.51–6.09 months; Figure 4).