Abstract OT3-06-01: Phase Ib clinical trial of coPANlisib in combination with Trastuzumab emtansine (T-DM1) in pre-treated unresectable locally advanced or metastatic HER2-positive breAst cancer (BC) “PANTHERA”-CTRIAL-IE 17-13

Background: The phosphoinositide 3 kinase (PI3K) pathway is important in the oncogenic function of HER2. Aberrant activation of PI3K is implicated in resistance to trastuzumab and other HER2-targeted therapies and is frequent, with up to 22% of HER2 positive breast cancer having a PIK3CA mutation. Copanlisib is a pan-class 1 PI3K inhibitor administered i.v. with low nanomolar activity against both PI3Kα and PI3Kβ. Copanlisib has been shown to re-sensitise trastuzumab resistant cell lines to trastuzumab with synergism seen in some cell lines between copanlisib and HER2 targeted therapy. Trial design: This is a phase Ib open label, single arm adaptive, multi-centre trial of copanlisib in combination with T-DM1. Eligible patients will receive T-DM1 at 3.6mg/kg i.v. on day 1 of a 21-day cycle plus copanlisib. Copanlisib will be administered i.v. according to the dose escalation scheme (dose level 1 is 45mg on days 1 and 8, dose level 2 is 60mg on days 1 and 8, dose level 3 is 60mg on days 1, 8, and 15). Dose level -1 will be 45 mg on day 1 in case dose de-escalation is needed. We will enrol 3 to 6 patients per dose level. All patients in each level must have completed at least the first cycle of therapy before enrolment in the next dose level. Patients not completing the first cycle for a reason other than toxicity will be replaced. Dose escalation and determination of the Maximum Tolerated Dose (MTD) will be based on the occurrence of Dose Limiting Toxicities (DLT). Eligibility criteria: Eligible patients are those with unresectable locally advanced or metastatic HER2-positive BC who previously received trastuzumab and a taxane, separately or in combination. Participants must have adequate organ function and ECOG PS ≤ 2 Objectives: The primary objective is to determine the MTD for copanlisib in combination with T-DM1 in patients with pre-treated unresectable locally advanced or metastatic HER2-positive BC. Secondary objectives include evaluating the safety, efficacy and cardiotoxicity in patients treated with this regimen. Exploratory objectives include examining for predictive biomarkers in tumour tissue and blood or plasma and to examine molecular tumour adaptation to clinical trial therapy. Statistical methods : Patients will be accrued in cohorts of 3 patients according to a standard 3+3 algorithm, with dose escalation and determination of MTD based on the occurrence of DLT, using the usual threshold probability of 33%. The final dose level will be expanded to include a total of 6 additional patients (expansion cohort). Present accrual and target accrual: The trial will start accrual in October 2018. Maximum of 24 patients will be enrolled. Citation Format: Hassan A, Gullo G, O9Reilly S, Ruiz-Borrego M, Toomey S, Grogan L, Breathnach O, Morris PG, Walshe JM, Crown J, O9Mahony D, Falcon A, Egan K, Hernando A, Teiserskiene A, Kelly CM, Coate L, Hennessy BT. Phase Ib clinical trial of coPANlisib in combination with Trastuzumab emtansine (T-DM1) in pre-treated unresectable locally advanced or metastatic HER2-positive breAst cancer (BC) “PANTHERA”-CTRIAL-IE 17-13 [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr OT3-06-01.