Study Design

All study participants were enrolled from a single institution, the National University Cancer Institute, Singapore. Patients were enrolled between April 2015 and December 2017. Data were collected up till December 2018. This trial was approved by the institutional review board prior to commencement. This single-arm, open-label phase I trial was conducted in three sequential cohorts: varlitinib administered in combination with carboplatin are under the curve (AUC) 1.5 and paclitaxel 80 mg/m² dosed weekly in cohort A, varlitinib in combination with single-agent paclitaxel 80 mg/m² dosed weekly in cohort B, and varlitinib in combination with paclitaxel 80 mg/m² dosed weekly and subcutaneous trastuzumab 600 mg administered every 3 weeks (cohort C; Table 1 of the Electronic Supplementary Material [ESM]). The initial starting dose of varlitinib was 500 mg twice daily administered with a continuous daily dosing schedule. This dose was selected as the starting dose based on previous studies of varlitinib as monotherapy and as combination therapy with docetaxel. A dose de-escalation study design was planned to be employed in the event the 500-mg twice-daily dose was found to be intolerable in combination with weekly carboplatin and paclitaxel. As the eventual intention is to develop the combination as neoadjuvant therapy in HER2+ breast cancer, if the triplet combination was deemed to be intolerable, it was planned to open cohorts studying the combination of varlitinib with weekly paclitaxel, dropping carboplatin from the combination as paclitaxel is the standard chemotherapy backbone drug in HER2+ breast cancer. Depending on the number of patients developing dose-limiting toxicities (DLTs), the dose of varlitinib could be escalated in these cohorts to find the MTD. In some of the subsequent dose levels, varlitinib was administered intermittently, 4 days per week, starting 24–36 h after and ending 24–36 h before each weekly paclitaxel dose administration.

Using a modified 3+3 design, patients were enrolled in cohorts of three to six patients. If two or more of six patients developed DLTs, three subjects were enrolled into the next lower dose cohort. If DLTs were observed in one of three subjects in the cohort, or if two or more subjects experienced drug-related adverse events (AEs) of more than or equal to grade 2, an additional three subjects were enrolled at the same dose level. Cohorts were enrolled at de-escalating dose levels until the MTD was identified, defined as the dose level at which DLTs developed in fewer than one of three, or two of six patients.

Dose-limiting toxicity was defined as any of the following: grade 4 neutropenia lasting ≥ 7 days, grade 3 and above febrile neutropenia or neutropenic infection, grade 4 thrombocytopenia, grade 3 thrombocytopenia lasting for ≥ 7 days, grade 3 and above nausea, vomiting or diarrhea despite optimal use of anti-emetic and anti-diarrheal medications, grade 2 abnormal alanine transaminase or aspartate transaminase in the presence of grade 2 raised bilirubin attributed to study treatment with or without signs and symptoms of drug-induced liver injury, or any other grade 3 or 4 treatment-related non-hematological toxicities except alopecia. If patients received less than 75% of the intended varlitinib dose because of any toxicities during cycle 1, they were also deemed to have a DLT. Patients who had received at least four cycles of varlitinib in combination with chemotherapy and are benefitting from the treatment regimen (sustained stable disease or objective response) and who experience unacceptable toxicity from paclitaxel/carboplatin may discontinue paclitaxel/carboplatin and be maintained on anti-HER2 therapy alone with single-agent varlitinib (with or without trastuzumab for cohort C) until documented disease progression or unacceptable toxicity.