Multiple assays to assess tumour PD-L1 expression have been, or are currently being, developed. These include: VENTANA PD-L1 (SP263) Assay, Dako PD-L1 IHC 22C3 pharmDx, Dako PD-L1 IHC 28-8 pharmDx, VENTANA PD-L1 (SP142) Assay and laboratory developed tests (LDTs).1-7 These assays utilise a number of antibody clones, including: SP263, 22C3, 28-8, SP142, E1L3N and 73-10.1-7
All of the commercially available assays utilise automated platform-specific systems, which facilitate greater standardisation of PD-L1 IHC staining compared with LDTs.6
PD-L1 immunohistochemistry staining procedure
These steps are specific to the assay used.
PD-L1 assay design
The use of controls is important to monitor the assay and confirm whether tissues have been correctly prepared and processed.1
Negative reagent control
A negative reagent control needs to be performed on each patient sample tested in order to confirm acceptable background staining.1
Positive control tissue
A tissue control should be included with each staining procedure. Positive control tissue can be stained as a separate slide in each run (run control) or as an on slide control next to the patient sample being tested (slide control) depending on manufacturer requirements. Control tissue should be fixed in a timely manner and processed alongside patient tumour samples.1–4
Control tissue should also be used to verify antibody specificity prior to antibody use in any diagnostic procedure. This process should be repeated for each new antibody batch, or whenever there is a change in assay parameters.6 Negative reagent controls should be evaluated in each to confirm acceptable background staining.2
Laboratory developed tests require additional validation steps and extensive testing to determine their accuracy before any implementation in clinical routine.9,10
The tiers of technical validation and their respective tissue tools are summarized in the figure below.9
Reproduced with permission from Cheung et al. Evolution of Quality Assurance for Clinical Immunohistochemistry in the Era of Precision Medicine: Part 4: Tissue Tools for Quality Assurance in Immunohistochemistry. Appl Immunohistochem Mol Morphol 2017;25(4):228–230. http://journals.lww.com/appliedimmunohist/pages/default.aspx
Tissue tools in relation to tiers of technical validation of the immunohistochemistry protocol. iCAPCs indicates Immunohistochemistry Critical Assay Performance Controls; LOD, limit of detection; TMA, tissue microarray.
Immunohistochemistry Critical Assay Performance Controls (iCAPCs) are informative for both validation and daily QC monitoring of IHC protocols.9 Appropriately selected iCAPCs can be used to characterise IHC protocols, including technical sensitivity and specificity and have been described as ‘Gold standard controls’, based on expert agreement.10
For further reading, please see: Principles of Analytic Validation of Immunohistochemistry Assays by the College of American Pathologists, available here.
The challenges associated with multiple assays
The approval of numerous PD-L1 IHC assays in oncology indications is associated with a significant challenge to patients and care providers with respect to clinical application of PD-L1 testing and treatment decision-making.11
Running a different test for each drug is impractical due to limited tumour tissue and the requirement for a rapid turnaround time.6,10
Equally, using one test for every drug is impractical:1–7
Such challenges have led to the fruition of a number of comparability initiatives.7,11–15
A number of cross-industry comparability initiatives are underway to help the clinical and testing community understand the comparative analytical performance of each PD-L1 assay.7,11–15
The currently available comparative data are promising, providing evidence that physicians may, in the future, be able to use the tests interchangeably.7,11–15 Further research is needed to establish this in practice.
Offering a choice of validated PD-L1 tests may facilitate reliable, quality testing and informed treatment decisions.