Harmonizing PD-L1 testing in metastatic triple negative breast cancer

Immunotherapy (IO) is rapidly reshaping the treatment landscape for solid tumors, with survival improvements demonstrated in multiple disease types. In the context of metastatic triple-negative breast cancer (mTNBC), the use of IO is based on the results from two phase III clinical trials: Impassion130 and KEYNOTE-355. In both trials, the addition of IO to first-line chemotherapy has shown an improvement in clinical outcomes restricted to patients with PD-L1 expressing tumors. Impassion130 demonstrated a statistically significant improvement in progression-free survival (PFS) with the addition of atezolizumab to nab-paclitaxel in the intention-to-treat (ITT) population, with a greater improvement in the PD-L1 + population, namely PD-L1 stained tumor-infiltrating immune cells (IC) covering ≥1% of the tumor area, as determined by the VENTANA PD-L1 (SP142) as a companion diagnostic test (CDx) [1]. Although the Food and Drug Administration (FDA) granted accelerated approval for the combination of atezolizumab and nab-paclitaxel as frontline treatment of PD-L1 + mTNBC, this study failed to demonstrate a statistically significant improvement in the co-primary endpoint of overall survival (OS) in ITT population due to its hierarchical statistical design, despite a numeric difference in OS [2]. This ultimately led to the withdrawal of the indication in the United States by the agent’s developer. Thus, atezolizumab will no longer be available for the treatment of PD-L1+ mTNBC in the United States, while it is still available in Europe at the time of writing. In contrast, in KEYNOTE-355, the addition of pembrolizumab to chemotherapy improved both progression-free survival (PFS) and overall survival (OS) compared to chemotherapy alone in PD-L1+ (combined positive score, CPS ≥ 10) mTNBC patients. PD-L1 status was defined using the IHC 22C3 pharmDx (Dako North America, Inc.) and a specific scoring system (CPS) [3,4]. Based on these results, the FDA granted full approval to pembrolizumab for locally recurrent unresectable or mTNBC expressing PD-L1 as assessed with 22C3 Dako CDx with a CPS ≥ 10, and the same agent may soon gain approval in Europe as well. Two clinical trials, two drugs, and two CDx to answer the same question: how to identify mTNBC patients who might benefit the most from IO? It is worth noting that such discordance pervades the selection of patients for IO across various tumor histologies [5]. Indeed, there are four anti-PD-(L) 1 agents approved for PD-L1-restricted indications, together with four distinct CDx to assess the expression of PD-L1: nivolumab (28–8, Dako), pembrolizumab (22C3, Dako), atezolizumab (SP142, Ventana), and durvalumab (SP263, Ventana) [5]. Moreover, the complexity of PD-L1 assessment itself presents many key sources of variability (Figure 1) [5]. First of all, PD-L1 expression can be evaluated only on tumor cells (TC), only on IC, or on both (as for CPS). In addition, each CDx uses a different method for scoring calculation (see Table 1) and there is no consensus for the cut-off value, even for the same CDx across different tumor histologies [5]. Another important source of variability is the potential intratumoral heterogeneity in PD-L1 expression within each single lesion, or between primary and metastatic samples (as well as among different metastatic anatomic locations) [6]. Last but not least, different assays use different immunohistochemistry (IHC) staining protocols, primary antibody clones (SP142, SP263, and 22C3), IHC platforms, and detection kits [5]. Hence, the next question easily arises: do different assays ultimately identify different patient populations? Across tumor histologies, SP142 stains fewer TC and IC compared to other clones [7]. In mTNBC, a hint is given by a post hoc analysis from Impassion130, where IHC for PD-L1 status using SP263 and 22C3 was evaluated for analytical concordance with SP142 and patient-associated outcomes in a subset of tumor samples [8]. Prevalence rates using PD-L1 IC ≥ 1% as a threshold for SP142, SP263, and 22C3 were, respectively, 46.4% (95% confidence interval [CI] = 42.5% to 50.4%), 74.9% (95% CI = 71.5% to 78.3%), and 73.1% (95% CI = 69.6% to 76.6%). The prevalence of PD-L1 22C3 CPS ≥ 1 was 80.9%. The analytical concordance, namely overall percentage agreement (OPA) for IC ≥ 1% (+), between SP142 and SP263 or 22C3 was 69.2% and 68.7%, respectively. Even if almost all SP142+ cases were classified as PD-L1+ by other assays (double positive), 29.6% of SP263+ and 29.0% of 22C3+ cases were SP142– (single positive) [8]. Importantly, the IO clinical activity in SP263+ and 22C3+ patients could be mainly ascribed to double-positive cases rather than single-positive cases. Thus,