The Role of Pathology in Predicting Immunotherapy Response in Urogenital Tumors

Introduction

The immune system has an important role in tumor pathogenesis, tumor growth and tumor metastasis.[] Immune checkpoint inhibitors (ICI) are recently introduced in the management of various cancers, such as non-small cell lung cancer, renal cell carcinoma (RCC), urothelial carcinoma (UC), and head and neck squamous cell carcinoma. Several immunotherapeutic drugs have been approved by the Food and Drug Administration (FDA) and European Medicines Agency (EMA) in genitourinary cancers.[]

There are three main immunotargets as follows: Cytotoxic T-lymphocyte antigen-4 (CTLA4), programmed death 1 (PD-1) and programmed death ligand-1 (PD-L1). These three targets are under investigation for therapeutic, predictive and prognostic purposes in genitourinary cancers. PD-1 is a negative immune checkpoint protein that is expressed on activated T-cells.[] PD-L1, one of several ligands of PD-1 receptor, is expressed in the tissue. Binding of PD-1 and PD-L1 results in the inhibition of cytotoxic T cell response. Overexpression of PD-L1 on the surface of tumor cells and tumor-infiltrating immune cells, and its engagement with PD-1 on T cells suppress T-cell-mediated tumor cytotoxicity. These inhibitory signals help the tumor to evade the immune system and to grow. [,] The therapeutic blockade of the PD-1/PD-L1 pathway has been shown as an efficient and safe option for the management of cancer patients with impressive clinical outcomes.[-]

There is a need for predictive biomarkers of immunotherapy to optimize treatment, to limit side-effects and to reduce the costs of therapy. Thus, tissue-based PD-L1 assays have been developed as a companion biomarker for ICIs.[-] Although PD-L1 immunohistochemistry is widely used in the prediction of immunotherapy response, this methodology is not perfect. There are several other predictive biomarkers under investigation, including tumor mutation burden, mismatch repair status, gene expression profiles, tumor infiltrating lymphocytes (TILs) and PD-L2.

This review focuses on the role of pathologic applications in the selection of patients with genitourinary cancer (mainly RCC and UC) who are potentially responsive to immunotherapy.

PD-L1
The predictive value of expression of PD-L1 on tumor cells by immunohistochemistry is controversial. For each approved PD-1/PD-L1 drug, there is a paired immunohistochemistry- based PD-L1 assay. The presence of variance in PD-L1 assays and quantitative immunostaining thresholds poses a clinical challenge in routine practice. Critical issues for PD-L1 immunohistochemistry may be summarized in four categories []:

- Cellular component to be scored: tumor cells and/ or tumor-infiltrating immune cells
- Positivity threshold that is clinically meaningful
- Intratumoral and intertumoral heterogeneity of PD-L1 expression
- Preanalytic technical challenges

The first three challenges listed above vary among different tumor types. Most data on genitourinary tumors accumulate in RCCs and UCs. The results of the PD-L1 assay may be discordant for an individual patient depending on the assay used. In one study, durvalumab was investigated in the second-line treatment of UC patients. The authors showed that PD-L1 results in only one component; immune cells or tumor cells did not cleanly separate responders and non-responders. However, when they took into account the PD-L1 status either in immune cells or tumor cells (?25%), a clear distinction could be made between responder and non-responder groups of the patients.[] PD-L1 positivity in ?5% of tumor cells has been shown to be associated with a response to avelumab in the second line.[] In another study on atezolizumab among second-line treatment of advanced UC patients, PD-L1 expression on only immune cells was associated with response and longer survival.[,] PD-L1 expression did not act as a predictive marker in the first-line treatment setting with atezolizumab.[] The data on the predictive value of PD-L1 on response rate to pembrolizumab show conflicting results in the first and second lines for advanced urothelial carcinoma.[,]

Similar conflicting results have been shown for RCC patients. Response to nivolumab was not related to PD-L1 expression in advanced RCC patients in one study.[] Treatment benefit of nivolumab was observed irrespective of PD-L1 status. Researchers also showed that although PD-L1 was not predictive, it may be prognostic.[-] Recent data also showed discordant results between primary and metastatic tumors, and, low and high-grade components of a tumor.[] Another challenging issue is that prior treatment of VEGF and mTOR inhibitors modifies PD-L1 expression.[,]

PD-L1 expression testing additionally has technical challenges since PD-L1 testing usually is performed on archival specimens.[,] Technical challenges include the time of fixation in formalin, variation in the affinity of the antibody used and standardization of the amplification systems. It is important to remember that the philosophy of "tissue is the issue" in pathology practice and standardization of the routine procedures in a pathology laboratory is essential.

A summary of FDA-approved drugs and paired companion PD-L1 testing for UCs and RCCs is given in the Table 1. None of the immune checkpoint inhibitors has been approved for advanced prostate cancer yet.[] Although metastatic castration-resistant prostate carcinomas show high PD-L1 expression in one third of biopsies, mostly in neuroendocrine and small cell carcinomas of the prostate,[] response to PD-L1 inhibitors is not related to PD-L1 expression.[,]

Table 1 Summary of the literature data on immunohistochemical assays of PD-L1 with matched immune checkpoint drugs

Tumor Mutation Burden (TMB) and Mismatch Repair Status
Higher mutation burden is believed to be associated with an increase in the tumor antigens and these antigen- rich tumor cells may be recognized by the immune system more easily. TMB is defined as the total number of coding and somatic mutations, but also include insertions/deletions.[] Preanalytic factors related to routine tissue preparing in a pathology laboratory may influence the results of TMB analysis. These factors are fixatives, fixation time, and tumor purity.

A better response to immunotherapy was observed in tumors with higher TMB, such as melanoma [] and non-small cell lung cancer.[] Because of the presence of similar limiting factors for both PD-L1 assays and TMB, it is suggested to use both predictive markers as complementary to each other.[]

RCC has been shown to have an intermediate level of TMB and a high prevalence of insertions/deletions. This specific type of mutations may result in a high expression of neoantigens and better recognition by the immune system.[,] Further studies showed better prediction of immunotherapy in metastatic UCs with high TMB.[,]

Cancers deficient in DNA mismatch-repair (dMMR) or microsatellite instability (MSI-high) demonstrate better benefit from immunotherapy. As expected, tumors with higher TMB also show dMMR or MSI-high status.[,] Based on clinical trials, the FDA has approved pembrolizumab therapy for solid tumors with dMMR or MSI-high, irrespective of tumor origin.[]

MSI-high status is a major feature of hereditary non-polyposis colorectal cancer (HNPCC) syndrome. Among HNPCC syndrome patients, upper tract UCs are placed in the third rank.[] Mismatch repair status and its relation to response to immunotherapy among genitourinary tumors are still under investigation.

Conclusion

References