The recognition that programmed death ligand 1 expression on tumor cells and tumor associated immune cells represents a shared immune evasion mechanism exploited across diverse cancer histologies has established PD L1 as a cross cutting therapeutic target whose relevance extends far beyond any single tumor type. The development of PD 1 and PD L1 blocking antibodies has consequently generated the broadest expansion of approved oncological indications in pharmaceutical history, with pembrolizumab alone receiving regulatory approvals spanning more than two dozen distinct cancer types and indications. This breadth of clinical activity reflects the fundamental biology of PD 1/PD L1 checkpoint engagement as a universal mechanism of adaptive immune resistance co opted by tumors across histological boundaries, creating a therapeutic vulnerability that transcends traditional tumor type categorization.
The biomarker landscape for PD L1 guided immunotherapy has evolved substantially since the first approvals of checkpoint inhibitors, progressing from early observations of PD L1 positivity as a general indicator of potential immunotherapy responsiveness to the development of standardized assays, validated scoring systems, and clinically meaningful expression thresholds that guide specific treatment decisions. Different assays calibrated for different checkpoint inhibitors, different PD L1 scoring methodologies including tumor proportion score and combined positive score, and different expression thresholds defining clinically meaningful positivity create a complex biomarker ecosystem that requires careful attention to assay specific details when interpreting PD L1 data in clinical contexts. The continued evolution of biomarker science toward composite molecular profiles and complementary biomarkers beyond PD L1 will further refine the identification of patients most likely to benefit from checkpoint inhibition.
Tumor Agnostic Pembrolizumab Approvals
The approval of pembrolizumab for MSI high or mismatch repair deficient solid tumors in 2017 established the first tumor agnostic indication in oncology, recognizing that a molecular characteristic rather than histological origin could define a therapeutically relevant patient population across cancers. This regulatory paradigm was subsequently extended with the approval of pembrolizumab for tumor mutational burden high solid tumors with ten or more mutations per megabase, providing an additional molecular characteristic that predicts immunotherapy responsiveness across cancer types. These tumor agnostic approvals represent a fundamental shift in oncological regulatory thinking that has implications for clinical trial design, diagnostic testing practices, and the treatment of patients with rare cancers for whom histology specific data may never be generated in sufficient quantities to support conventional regulatory approval.
The practical implementation of tumor agnostic pembrolizumab therapy requires that comprehensive molecular profiling including MSI and mismatch repair status, tumor mutational burden assessment, and PD L1 expression be performed at diagnosis for patients with advanced cancer who are being considered for immunotherapy. Next generation sequencing based comprehensive genomic profiling panels that simultaneously assess all these parameters, along with driver alterations, copy number changes, and other clinically relevant molecular features, provide the most complete and efficient molecular characterization, enabling integrated treatment planning that accounts for all actionable molecular findings in a single testing episode.
Esophageal Cancer and PD L1 Guided Treatment
Esophageal squamous cell carcinoma and adenocarcinoma both demonstrate PD L1 expression in substantial proportions of cases and have been the subjects of multiple pembrolizumab trials demonstrating clinical benefit. The KEYNOTE 590 trial established pembrolizumab combined with fluorouracil and cisplatin as a standard first line regimen for locally advanced or metastatic esophageal cancer including esophageal squamous cell carcinoma and esophagogastric junction adenocarcinoma, demonstrating significant overall survival improvement particularly in patients with high PD L1 combined positive scores. Esophageal squamous cell carcinoma demonstrated the largest relative survival benefit from pembrolizumab addition, consistent with the high PD L1 expression rates and immunological features characteristic of squamous tumors at this anatomical site.
The KEYNOTE 181 trial provided second line pembrolizumab data for esophageal cancer, demonstrating superior overall survival compared to investigator’s choice chemotherapy specifically in patients with esophageal squamous cell carcinoma with PD L1 combined positive scores of ten or greater. These results reinforced the importance of PD L1 scoring as a treatment selection guide in esophageal cancer and established pembrolizumab as the preferred second line treatment for PD L1 high esophageal squamous cell carcinoma, an indication that has been incorporated into clinical practice guidelines internationally. The consistent pattern of greater pembrolizumab benefit with higher PD L1 expression observed across multiple tumor types reinforces the clinical value of PD L1 testing as a biomarker guiding immunotherapy decisions.
Endometrial Cancer and Immune Checkpoint Inhibition
Endometrial cancer represents a tumor type with particularly rich opportunities for immunotherapy based on the high prevalence of mismatch repair deficiency, with approximately 20 to 30 percent of endometrial cancers demonstrating MSI high status. The KEYNOTE 158 endometrial cancer cohort demonstrated substantial pembrolizumab activity in MSI high endometrial cancer, with objective response rates of approximately 57 percent and complete response rates of approximately 21 percent, exceptional results in a platinum refractory patient population. The MSI high subset of endometrial cancer represents one of the most immunotherapy responsive solid tumor populations identified, and pembrolizumab has become a critical treatment option for these patients in later lines of therapy.
Beyond the MSI high subgroup, the combination of pembrolizumab with lenvatinib, a multi kinase inhibitor targeting VEGFR, FGFR, and other kinases, has demonstrated remarkable activity in mismatch repair proficient endometrial cancer, a population that has historically shown limited responsiveness to single agent checkpoint inhibition. The KEYNOTE 775 trial demonstrated that pembrolizumab plus lenvatinib significantly improved overall survival and progression free survival compared to chemotherapy in platinum refractory endometrial cancer regardless of MSI status, establishing this combination as a standard second line treatment for endometrial cancer. The synergistic interaction between VEGFR inhibition and PD 1 blockade, potentially mediated through anti angiogenic effects that normalize the tumor vasculature and improve T cell infiltration, has opened a new therapeutic direction for immunotherapy refractory solid tumors.
Tumor Microenvironment and PD L1 Expression Dynamics
PD L1 expression in tumors is not a static intrinsic property but a dynamic feature regulated by complex interactions between tumor cells, immune cells, and the cytokine milieu of the tumor microenvironment. Interferon gamma, released by activated T cells infiltrating the tumor, potently induces PD L1 expression on tumor cells through JAK STAT signaling, creating an adaptive feedback mechanism in which anti tumor immune activity generates the very immunosuppressive signal that ultimately limits its own effectiveness. This adaptive regulation means that PD L1 expression can change over the course of disease and in response to treatments that alter immune activation within the tumor, with implications for the predictive value of baseline PD L1 measurements obtained at a single timepoint.
Spatial and temporal heterogeneity of PD L1 expression within individual tumors further complicates biomarker interpretation, as different regions of the same tumor may display markedly different PD L1 expression levels depending on local immune infiltration patterns and the proximity to blood vessels and necrotic zones. Sampling variability from small biopsies may therefore result in PD L1 scores that do not reflect the overall expression status of the entire tumor, introducing uncertainty into biomarker based treatment decisions. The development of non invasive imaging approaches using PD L1 targeted radiotracers for positron emission tomography holds promise as a method for assessing whole tumor and whole body PD L1 expression status without the sampling limitations of tissue biopsy, potentially improving the accuracy and clinical utility of PD L1 as a predictive biomarker.
Combination Strategies Across Tumor Types
The clinical development of pembrolizumab in PD L1 expressing cancers has increasingly moved from monotherapy toward rational combination strategies that address the biological mechanisms of primary and acquired resistance to checkpoint inhibition. Combinations of Keytruda with chemotherapy, targeted agents, anti angiogenic therapy, antibody drug conjugates, and other checkpoint inhibitors targeting complementary pathways such as CTLA 4, LAG 3, and TIM 3 are being evaluated across multiple tumor types. The combinations that have demonstrated the most consistent clinical benefit across diverse tumor types share the common feature of addressing additional components of the tumor microenvironment or tumor biology that limit the efficacy of PD 1 blockade alone.
The integration of pembrolizumab with antibody drug conjugates, which deliver potent cytotoxic payloads to tumor cells with high precision through antibody targeting, has produced particularly striking results in multiple tumor types. Antibody drug conjugates induce immunogenic cell death that releases tumor antigens and damage associated molecular patterns, potentially priming or boosting T cell responses that are then amplified by concurrent PD 1 blockade. This mechanistic synergy between antibody drug conjugates and checkpoint inhibition has generated remarkable response rates in bladder cancer, cervical cancer, endometrial cancer, and other tumor types that are reshaping treatment paradigms and establishing new combination standards of care that leverage the complementary biology of targeted cytotoxicity and immune checkpoint inhibition.
Conclusion
PD L1 expression as a biomarker guiding pembrolizumab use across advanced cancer types has established immunotherapy as a broadly applicable treatment strategy whose benefits extend across the spectrum of solid and hematological malignancies. Keytruda and its active ingredient pembrolizumab have accumulated the most extensive clinical evidence base of any oncological agent in history, with approvals spanning more than two dozen indications that reflect the universal relevance of PD 1/PD L1 checkpoint biology across cancer histologies. The continued refinement of biomarker selection, the development of rational combination strategies, and the extension of immunotherapy into earlier disease settings will further expand the patient populations that benefit from the transformative clinical impact that pembrolizumab has demonstrated across the landscape of advanced cancer treatment.
