Phase Ib Dose-Finding Study of Varlitinib Combined with Weekly Paclitaxel With or Without Carboplatin ± Trastuzumab in Advanced Solid Tumors

BACKGROUND: Varlitinib is a highly potent, small-molecule, pan-HER inhibitor targeting HER1, HER2, and HER4. It has demonstrated activity in gastric, biliary tract, and breast cancers. OBJECTIVE: We conducted a phase Ib dose confirmation study to determine safety and early efficacy signals of varlitinib in combination with chemotherapy (paclitaxel ± carboplatin) ± subcutaneous trastuzumab. METHODS: Eligible patients had advanced or metastatic solid tumors. A 3+3 dose de-escalation study design was used and pharmacokinetic analyses of varlitinib and paclitaxel were performed. RESULTS: Thirty-seven patients were enrolled into eight cohorts with median 4 (0–14) prior lines of palliative systemic therapies. Carboplatin area under the curve 1.5 and paclitaxel 80 mg/m(2) weekly with varlitinib 500 mg twice daily continuously was de-escalated over four dose levels to 300 mg twice daily intermittently (4 days on, 3 days off) due to dose-limiting toxicities, most commonly neutropenia, febrile neutropenia, and electrolyte disturbances, with the triplet combination deemed intolerable and unable to be developed further. Varlitinib was then combined with paclitaxel alone; the recommended phase II dose of varlitinib was 300 mg twice daily intermittently. The addition of subcutaneous trastuzumab 600 mg was safe with no dose-limiting toxicities. Thirty-one patients were evaluable for response: 35.5% partial response, 41.9% stable disease. Twenty patients had HER2+ metastatic breast cancer with a median of 4 (0–14) treatment lines, 8/20 continued on single-agent varlitinib after completing chemotherapy for a median of 5.1 (range 2.0–13.3) months. A pharmacokinetic analysis showed that plasma exposure of varlitinib was dose dependent. Varlitinib administration did not significantly affect the maximum concentration or area under the curve of paclitaxel. CONCLUSIONS: The recommended phase II dose of varlitinib with paclitaxel is 300 mg twice daily intermittently dosed. This is active in HER2+ metastatic breast cancer. Subcutaneous trastuzumab can be added safely to varlitinib and paclitaxel. This combination is currently being evaluated as neoadjuvant therapy in HER2+ breast cancer (NCT02396108). Clinical Trial Registration: NCT02396108, date of registration: 25 March, 2015. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s11523-022-00867-0.