External quality assessment demonstrates that PD-L1 22C3 and SP263 assays are systematically different

Abstract

PD-L1 inhibitors are part of first line treatment options for patients with advanced non-small cell lung cancer. PD-L1 immunohistochemistry (IHC) assays act as either a companion or a complementary diagnostic. The purpose of this study is to describe the experience of external quality assurance (EQA) provider UK NEQAS ICC and ISH with the comparison of different PD-L1 assays used in daily practice. Three EQA rounds (pilot, run A and run B) were carried out using formalin fixed paraffin embedded samples with sample sets covering a range of epitope concentrations, including 'critical samples' near to clinical threshold cut-offs. An expert panel (n = 4) evaluated all returned slides simultaneously and independently on a multi-header microscope together with the participants own in-house control material. The tonsil sample was evaluated as 'acceptable' or 'unacceptable', and for the other samples the percentage of PD-L1 stained tumour cells were estimated in predetermined categories (<1%, 1 to <5%, 5 to <10%, 10 to <25%, 25 to <50%, 50 to <80%, 80 to 100%). In the pilot and the two subsequent runs the number of participating laboratories was 43, 69 and 76, respectively. The pass rate for the pilot run was 67%; this increased to 81% at run A and 82% at run B. For two 'critical samples', in runs A and B, 22C3 IHC had significantly higher PD-L1 expression than SP263 IHC (p < 0.001), whilst the PD-L1 scores for the other six samples were similar for all assays. In run A the laboratory developed tests (LDTs) using 22C3 scored lower than the commercial 22C3 tests (p = 0.01). After the initial testing, improvement in performance of PD-L1 IHC is shown for approved and LDT PD-L1 assays. Equivalency of approved PD-L1 22C3 and SP263 assays cannot be assumed as the scores cross the clinically relevant thresholds of 1% and 50% PD-L1 expression.

Keywords: PD-L1; companion diagnostic assays; external quality assessment; immunohistochemistry; non-small cell lung cancer; predictive testing.

Figure 1

The relationship between epitope concentration and intensity is shown with the black line 35. Critical samples are positioned around a threshold of the IHC test 8. The approximate PD‐L1 intensities for 7 of the 8 distributed EQA samples (B–H) are shown along the black line (A, tonsil [not shown]). Note that samples outside the critical range are less likely to result in a different outcome, i.e., remain negative or positive.

Figure 2

A graphical display of the lines representing the likely difference between SP263 and 22C3 approved tests. The arrows denote the threshold of the test distinction between positive (+) and negative (−). Note that the SP263 is positive at a slightly higher epitope concentration than 22C3.