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Abstract P5-16-08: Phase Ib/II trial evaluating safety and efficacy of copanlisib (PI3K inhibitor) and trastuzumab in pre-treated advanced HER2-positive breast cancer: Results from the PantHER study


 Background: De novo and acquired resistance to HER2 directed therapy is frequently encountered. Upregulation of the phosphatidylinositol-3-kinase (PI3K) pathway is an important mediator of treatment resistance. This can occur through an activating mutation of the PIK3CA gene or PTEN loss. PIK3CA mutations are present in approximately 20% of HER2 positive breast cancers and as such, the PI3K pathway has emerged as an attractive target for restoring sensitivity to HER2 directed therapy. Methods: We performed a single arm, multicentre, open label Phase Ib/II trial. Patients (pts) with advanced HER2-positive breast cancer whose disease had progressed on at least 1 line of Trastuzumab/T-DM1 based treatment in the metastatic setting were eligible if they met following criteria: ECOG PS ≤ 2 and adequate organ function. Pts with treated, controlled brain metastases were permitted to enrol. Exclusion criteria included uncontrolled hypertension or diabetes mellitus. Pts on Phase IB were treated according to a 6+6 study design with a dose escalation schedule of Copanlisib IV (level 1 = 45mg, level 2 = 60mg) on Day 1, 8 and 15 of a 28-day cycle along with a fixed dose of Trastuzumab 2mg/kg weekly. Phase II treatment was the MTD (maximum tolerated dose) of Copanlisib in combination with Trastuzumab. Archival tumour tissue, voluntary biopsies and serial plasma samples were collected for genomic sequencing. Primary endpoints were MTD (Phase I) and clinical benefit rate (CBR) which was defined as complete response (CR) or partial response (PR) at any time point; or stable disease (SD) lasting at least 24 weeks (Phase II). Secondary endpoints included safety and tolerability, tumor response rate, duration of response, time to treatment failure (TTF) and progression free (PFS) and overall survival (OS). Results: Twelve pts were enrolled in Phase IB. No dose limiting toxicity was observed. The MTD was established as Copanlisib 60mg and Trastuzumab 2mg/kg. Fourteen pts were enrolled in Phase II (6 pts treated at the MTD in Phase IB were included in the final Phase II analysis resulting in a total of 20 pts). The median number of lines of prior treatment in the metastatic setting was 3 (1-8). The most common grade 3-4 toxicities encountered in the Phase Ib/II cohorts included hypertension (n=7, 27%), hyperglycaemia (n=2, 8%) and vomiting (n=2, 8%). Three pts discontinued treatment due to toxicity. The median follow-up for the Phase II cohort was 7.5 months (95% CI 6.0-14.5). PR was observed in 4 pts (20%) and SD (at any time point) was seen in 8 pts (40%). The CBR was 30% (n=6). The duration of response was 15.0 weeks (95% CI 4.9 - 16.1). The median TTF was 11.9 weeks (95% CI 7.5 - 21.1). The median PFS was 3.0 mo (95% CI 0.2 - 5.8) and OS was 14.0 mo (95% CI 5.2-22.8). At the time of analysis, 9 of 20 patients were alive. PIK3CA mutations were detectable in the archival tissue of 11 of 26 pts (42%). PIK3CA hotspot mutations (H1047R, E542K and E545K) were detectable in the plasma of all 26 pts at various points throughout treatment. Pre and post treatment biopsies of 2 pts in the Phase IB trial revealed somatic mutations in DNAH3 and TRRAP, the latter of which encodes a PI3K-like protein kinase. Targeted next generation sequencing was performed on the circulating tumour DNA of 20 pts in the Phase II cohort taken before, during and after treatment to further validate these findings and to assess for other mechanisms of response or resistance. The final translational results will be presented at the meeting. Conclusions: The combination of Copanlisib and Trastuzumab is a safe and tolerable regimen and is associated with clinical efficacy in a heavily pre-treated metastatic HER2-positive breast cancer population. Translational studies may have identified novel resistance biomarkers in this pt cohort.
 Citation Format: Lisa Prior, Niamh M Keegan, Simon J Furney, Janice M Walshe, Giuseppe Gullo, John Crown, M John Kennedy, Diarmuid Smith, John McCaffrey, Catherine M Kelly, Keith Egan, Jennifer Kerr, Mark Given, Niall Sheehy, Peter O'Donovan, Andres Hernando, Ausra Teiserskiene, Imelda Parker, Elaine Kay, Ray McDermott, Maccon M Keane, Seamus O'Reilly, Liam Grogan, Oscar Breathnach, Patrick G Morris, Sinead Toomey, Bryan T Hennessy. Phase Ib/II trial evaluating safety and efficacy of copanlisib (PI3K inhibitor) and trastuzumab in pre-treated advanced HER2-positive breast cancer: Results from the PantHER study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-16-08.