Abstract PD10-05: Activity of atezolizumab (atezo) plus paclitaxel (pac) in metastatic triple-negative breast cancer (mTNBC) according to Burstein molecular subtype: Analysis of the IMpassion131 trial

Abstract Background: In contrast to the IMpassion130 trial evaluating atezo + nab-pac [Schmid, NEJM 2018], the randomized phase 3 IMpassion131 trial (NCT03125902) did not demonstrate significantly improved progression-free survival (PFS; primary endpoint) and showed no improvement in overall survival (OS; secondary endpoint) with the addition of atezo to pac as first-line therapy for mTNBC in either the PD-L1+ or the intention-to-treat (ITT) population [Miles, Ann Oncol 2021]. In IMpassion130, enhanced PFS and OS improvement with atezo + nab-pac were seen in the basal-like immune-activated (BLIA) subtype, whereas potential resistance to atezo + nab-pac was observed in the luminal androgen receptor (LAR) subtype [Emens, ASCO 2021]. LAR may be more prevalent in Asian populations [Ding, Oncotarget 2019], which represented one-third of patients enrolled in IMpassion131 and could have influenced the overall result. PFS outcomes were numerically worse in Asian vs non-Asian subgroups in IMpassion131 [Miles, Ann Oncol 2021]. To investigate potential reasons for observing different effects in IMpassion130 and IMpassion131, we explored the prevalence and impact on clinical outcomes of TNBC molecular subtypes and race in IMpassion131. Patients and Methods: Patients with mTNBC (no prior systemic therapy or ≥12 months since [neo]adjuvant chemotherapy) were randomized 2:1 to atezolizumab 840 mg or placebo (days 1, 15), both with paclitaxel 90 mg/m2 (days 1, 8, 15), every 28 days until disease progression or unacceptable toxicity, stratified by tumor PD-L1 status, prior taxane, liver metastases, and geographic region. Molecular subtypes according to Burstein classification were determined by RNA sequencing (Illumina TruSeq RNA Access) of baseline tumor samples. Prevalence of Burstein molecular subtypes [Burstein, Clin Cancer Res 2015] was assessed in Asian (of whom 69% were enrolled in China) and non-Asian populations; clinical outcomes were assessed according to molecular subtype and race using Cox proportional hazards analysis. All analyses were performed using the final data cut-off (Sep 4, 2020; median follow-up duration: 14.4 months). Results: The biomarker-evaluable population (BEP; n=471) was representative of the ITT population (n=651) with respect to baseline characteristics and PFS hazard ratio (HR; 0.75 [95% CI 0.61-0.92] vs 0.81 [95% CI 0.68-0.96], respectively). Distribution of molecular subtypes in the BEP was: 30% BLIA, 41% basal-like immune suppressed (BLIS), 24% LAR, 5% mesenchymal (MES), similar to IMpassion130. Among the BLIA samples, 82% were PD-L1+ and 18% PD-L1-; corresponding percentages were 41% vs 59% for BLIS, 31% vs 69% for LAR, and 32% vs 68% for MES. Compared with non-Asian patients, the Asian subgroup (n=117) included more LAR (31% vs 22%) and fewer MES (1% vs 6%) samples; this was particularly pronounced in the cohort enrolled in China (n=79; 37% LAR, 0% MES). PFS was improved with atezo + pac in the BLIA subtype (HR 0.66, 95% CI 0.45-0.97). None of the Burstein subgroups derived OS benefit from atezo + pac. Findings were similar irrespective of PD-L1 status. Direction of effect for PFS and OS favored the placebo + pac arm in the LAR Asian subgroup (n=30). Conclusion: In these exploratory analyses, the distribution of molecular subtypes and enhanced effect of atezo + pac in the BLIA subtype are consistent with findings from IMpassion130. The lack of improved efficacy with the combination of atezo + pac in the IMpassion131 trial cannot be explained by overrepresentation of a Burstein subtype less sensitive to atezo in the trial population. Citation Format: Fabrice André, Regula Deurloo, Aditi Qamra, David Cameron, Joseph Gligorov, Andreas Schneeweiss, Carlos Barrios, Binghe Xu, Ching-Wei Chang, Luciana Molinero, Shilpen Patel, Andrea Liptrot, Leilani Morales, David Miles, Joyce O’Shaughnessy. Activity of atezolizumab (atezo) plus paclitaxel (pac) in metastatic triple-negative breast cancer (mTNBC) according to Burstein molecular subtype: Analysis of the IMpassion131 trial [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 PD10-05.