Immunotherapy Resistance: Why PD-1/PD-L1 Stops Working (2026)
PD-1 and PD-L1 checkpoint inhibitors can produce remarkably durable responses in some patients. However, resistance remains one of the central challenges in modern oncology. A tumor may never respond to immunotherapy in the first place, or it may respond initially and later progress.
The reason is important: blocking PD-1 or PD-L1 removes one immune-suppressive signal, but it does not guarantee that the immune system can recognize, enter, survive within, and destroy every cancer cell.
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This article explains the major biological mechanisms behind primary and acquired immunotherapy resistance, why PD-L1 alone cannot reliably predict response, how tumor evolution and the tumor microenvironment contribute to treatment failure, and what strategies researchers and oncologists are investigating to overcome resistance.
PD-1/PD-L1 resistance is not a single disease mechanism. It is a systems problem involving tumor genetics, antigen presentation, T-cell function, immune checkpoints, the tumor microenvironment, metabolism, blood supply, cancer evolution and sometimes the gut microbiome.
Table of Contents
- What Is Immunotherapy Resistance?
- How PD-1/PD-L1 Immunotherapy Works
- Primary vs Acquired Resistance
- Major Mechanisms of Resistance
- 1. Loss of Tumor Antigens and Antigen Presentation
- 2. Tumor Heterogeneity and Cancer Evolution
- 3. T-Cell Exhaustion
- 4. Alternative Immune Checkpoints
- 5. The Immunosuppressive Tumor Microenvironment
- 6. Cancer Metabolism and Immune Suppression
- 7. Hypoxia and Abnormal Tumor Blood Vessels
- 8. Interferon and JAK/STAT Signaling
- 9. The Gut Microbiome
- Why PD-L1 Alone Is Not Enough
- TMB, MSI-H, TILs and Other Biomarkers
- What Happens When Immunotherapy Stops Working?
- How Researchers Are Trying to Overcome Resistance
- Combination Immunotherapy
- Targeted Therapy + Immunotherapy
- Radiation + Immunotherapy
- Cancer Vaccines
- TIL, CAR-T and Other Cellular Therapies
- Clinical Trials
- What About Repurposed Drugs?
- Evidence Hierarchy
- The Future of Immunotherapy Resistance
- Frequently Asked Questions
What Is Immunotherapy Resistance?
Immunotherapy resistance occurs when a cancer does not respond adequately to an immune-based treatment or when a cancer that initially responded later begins growing again.
Resistance is commonly divided into two broad categories:
The cancer fails to respond meaningfully from the beginning of treatment.
The cancer initially responds or remains controlled but later progresses despite continued or previous checkpoint blockade.
These categories are clinically useful, but biologically they overlap. A tumor may contain resistant subclones before treatment begins. Immunotherapy can then eliminate sensitive cancer cells while resistant populations survive and expand.
This is one reason cancer should be understood as an evolving ecosystem rather than a static mass of identical cells.
How PD-1/PD-L1 Immunotherapy Works
PD-1 is an inhibitory receptor found on immune cells, particularly activated T cells. PD-L1 is one of its major binding partners and can be expressed by cancer cells as well as other cells in the tumor microenvironment.
When PD-1 interacts with PD-L1, inhibitory signaling can reduce T-cell activity. Cancer cells can exploit this pathway as part of immune escape.
PD-1 and PD-L1 inhibitors interfere with this interaction. The objective is to restore or enhance the ability of T cells to attack cancer.
Examples of PD-1/PD-L1 pathway inhibitors include drugs targeting PD-1 or PD-L1 that are used across multiple cancer types.
However, checkpoint blockade is not equivalent to turning the immune system into an unlimited cancer-killing machine.
PD-1/PD-L1 blockade can release an immune "brake." But the immune system still needs to find the cancer, recognize it, enter the tumor, remain functional, overcome suppressive signals and successfully kill malignant cells.
Primary vs Acquired Immunotherapy Resistance
Primary resistance
Primary resistance can occur when a tumor lacks the biological conditions required for effective immune attack.
Examples include:
- Few or ineffective tumor-specific T cells
- Poor antigen presentation
- Low tumor immunogenicity
- Failure of T cells to enter the tumor
- Strong immunosuppressive signaling
- Abundant regulatory immune cells
- Immunosuppressive myeloid cells
- Physical barriers within the tumor microenvironment
- Metabolic conditions that impair immune-cell function
- Intrinsic tumor alterations that interfere with immune recognition
Acquired resistance
Acquired resistance is different because the immune system initially demonstrates some degree of effectiveness.
The tumor may then evolve under immune pressure.
Sensitive cancer-cell populations may disappear while resistant populations survive. Cancer cells may also acquire changes that reduce antigen presentation, interfere with interferon signaling, alter immune interactions or increase alternative immunosuppressive pathways.
This process is closely related to immunoediting—the concept that immune pressure can shape tumor evolution by eliminating susceptible cancer-cell populations while selecting cells capable of immune escape.
Major Mechanisms of PD-1/PD-L1 Resistance
There is no single molecular explanation for immunotherapy failure. Major mechanisms operate at several levels.
Antigen loss, genetic evolution, defective antigen presentation and immune-evasive mutations.
Exhaustion, dysfunction, inadequate activation or inability to maintain effective cytotoxic activity.
Suppressive macrophages, regulatory T cells, myeloid-derived suppressor cells, fibroblasts and abnormal vasculature.
Metabolic state, inflammation, microbiome composition and other host factors.
1. Loss of Tumor Antigens and Antigen Presentation
For T cells to kill a cancer cell, the immune system generally needs to recognize tumor-derived antigens presented through appropriate antigen-presentation machinery.
If cancer cells lose important antigens or interfere with antigen presentation, releasing the PD-1 brake may not be sufficient.
Some tumors can alter components of the antigen-presentation pathway, including molecules associated with MHC class I presentation and beta-2-microglobulin.
This creates an important distinction:
Checkpoint blockade cannot fully restore an immune response if the tumor has become difficult for T cells to recognize in the first place.
Research into antigen presentation is therefore an important part of understanding acquired resistance.
2. Tumor Heterogeneity and Cancer Evolution
Cancers are rarely composed of genetically identical cells.
A tumor may contain multiple subclones with different mutations, antigens, growth rates and immune vulnerabilities.
Immunotherapy may eliminate the most visible or immunogenic populations while leaving behind cells with traits that permit immune escape.
Those surviving populations can subsequently expand.
This is one reason tumor heterogeneity is increasingly recognized as a major component of treatment resistance.
The concept also connects immunotherapy resistance with the broader biology of cancer treatment resistance.
3. T-Cell Exhaustion
T cells exposed to persistent antigen stimulation can enter states commonly described as T-cell exhaustion.
Exhausted T cells are not necessarily completely inactive. Rather, they can exhibit altered function, reduced proliferative capacity and expression of multiple inhibitory receptors.
PD-1 is one of these inhibitory pathways, which explains why PD-1 blockade can restore function in some contexts.
But exhaustion is more complicated than a single receptor.
Persistent antigen exposure can involve multiple inhibitory pathways and changes in T-cell state, differentiation and metabolism.
Consequently, simply adding more PD-1 blockade does not necessarily overcome every form of T-cell dysfunction.
4. Alternative Immune Checkpoints
When PD-1 signaling is blocked, tumors and immune cells may remain regulated by other inhibitory pathways.
Important research targets include:
An inhibitory receptor expressed on several immune-cell populations and an important target of next-generation immunotherapy.
An immune checkpoint involved in regulation of T-cell and natural-killer-cell responses.
Another inhibitory pathway associated with dysfunctional or exhausted immune states.
A distinct checkpoint pathway that regulates T-cell activation and can be targeted in combination with PD-1 blockade in selected cancers.
This is one rationale behind combination immunotherapy: different checkpoints may control different stages or states of the immune response.
However, more immune activation is not automatically better. Combination therapies can increase immune-related adverse events and must be evaluated through clinical evidence.
5. The Immunosuppressive Tumor Microenvironment
The tumor is not simply a collection of cancer cells.
It contains immune cells, fibroblasts, endothelial cells, extracellular matrix, blood vessels, signaling molecules and metabolites.
This ecosystem is called the tumor microenvironment (TME).
The TME can strongly influence whether checkpoint blockade succeeds or fails.
Important suppressive components include:
- Regulatory T cells
- Myeloid-derived suppressor cells
- Immunosuppressive macrophage populations
- Cancer-associated fibroblasts
- Suppressive cytokines
- Extracellular matrix barriers
- Abnormal tumor vasculature
- Hypoxic regions
- Immunosuppressive metabolites
Modern immunotherapy research therefore increasingly looks beyond the cancer cell and T cell to the entire tumor ecosystem.
6. Cancer Metabolism and Immune Suppression
Immune cells and cancer cells compete for nutrients and resources inside tumors.
Cancer cells can consume substantial quantities of glucose and other nutrients while producing metabolites that alter the surrounding environment.
Hypoxia, lactate accumulation, altered amino-acid availability and other metabolic changes can influence immune-cell function.
This creates an important intersection between cancer metabolism and immunotherapy resistance.
Metabolic pathways under investigation include:
- Glucose metabolism
- Glycolysis
- Glutamine metabolism
- Lactate signaling
- Fatty-acid metabolism
- Adenosine signaling
- AMPK and mTOR pathways
- Mitochondrial metabolism
- Hypoxia-related signaling
However, mechanistic plausibility does not automatically establish that modifying these pathways improves outcomes in patients. Many metabolic approaches remain investigational.
See also: Cancer as a Metabolic and Immune Disease.
7. Hypoxia and Abnormal Tumor Blood Vessels
Solid tumors can develop regions with inadequate oxygen delivery.
Hypoxia can influence cancer-cell behavior, angiogenesis, metabolism and immune-cell function.
Abnormal tumor blood vessels can also make it difficult for immune cells to efficiently enter the tumor.
This helps explain why a tumor can contain immune cells yet still behave as an "immune-cold" or poorly responsive tumor.
Research is investigating whether modifying the tumor vasculature or hypoxic microenvironment can improve immunotherapy responses.
8. Interferon and JAK/STAT Signaling
Interferon signaling plays an important role in communication between tumor cells and the immune system.
Genetic alterations affecting interferon-related pathways can interfere with the ability of cancer cells to respond appropriately to immune attack.
Changes involving the JAK/STAT signaling system have been associated with resistance to immune checkpoint blockade in some cancers.
These findings demonstrate that resistance can arise from mutations inside cancer cells that alter how they respond to immune pressure.
9. The Gut Microbiome
The gut microbiome is another area of active research in cancer immunotherapy.
Gut bacteria can influence immune development, inflammation, metabolism and systemic immune responses.
Studies have associated aspects of microbiome composition with response to immune checkpoint blockade, although translating these observations into standardized clinical interventions remains challenging.
One bacterium that has received particular attention is Akkermansia muciniphila.
Research has suggested associations between certain microbial profiles and improved responses to checkpoint blockade, but microbiome-based treatment remains an evolving field.
Probiotics, dietary changes and fecal microbiota transplantation should not currently be assumed to improve immunotherapy outcomes for every patient. Clinical evidence is disease-specific and intervention-specific.
Why PD-L1 Alone Is Not Enough
PD-L1 testing is clinically important in several cancers, but PD-L1 is not a universal predictor of immunotherapy benefit.
There are several reasons.
- PD-L1 expression can vary between tumor regions.
- PD-L1 can change over time.
- Different laboratory assays and scoring systems are used.
- PD-L1 expression can occur on immune cells as well as tumor cells.
- Some PD-L1-negative tumors respond to immunotherapy.
- Some PD-L1-positive tumors fail to respond.
- PD-L1 does not measure T-cell function.
- PD-L1 does not fully characterize the tumor microenvironment.
- PD-L1 does not measure tumor heterogeneity.
PD-L1 is one piece of the immunotherapy decision-making process—not a complete description of the tumor's immune biology.
TMB, MSI-H, TILs and Other Biomarkers
Researchers are increasingly moving toward multi-dimensional biomarker models rather than relying on a single measurement.
Tumor Mutational Burden (TMB)
TMB estimates the number of somatic mutations within a tumor.
Higher mutational burden can increase the probability that a tumor produces recognizable neoantigens, but TMB is an imperfect predictor. A high TMB does not guarantee that those mutations generate effective immune recognition or that T cells can successfully attack the tumor.
For a detailed explanation, see Tumor Mutation Burden (TMB) Explained.
MSI-H and dMMR
Microsatellite instability-high (MSI-H) tumors and tumors with mismatch-repair deficiency (dMMR) can contain high numbers of mutations and have demonstrated particular sensitivity to immune checkpoint blockade in several cancer settings.
However, even biomarker-defined groups are not biologically identical, and resistance can still develop.
See: MSI-H and Immunotherapy.
Tumor-Infiltrating Lymphocytes
TILs are immune cells found within or around tumors.
The presence, density, location and functional state of TILs can provide information about the immune context of a tumor.
However, TIL assessment is complex and is not yet a universal stand-alone biomarker across all cancers.
Other emerging biomarkers
- Immune gene-expression signatures
- Neoantigen load
- HLA status
- Antigen-presentation machinery
- Interferon signaling
- ctDNA dynamics
- T-cell receptor clonality
- Tumor immune microenvironment profiles
- Spatial transcriptomics
- Multi-omics signatures
What Happens When Immunotherapy Stops Working?
Radiographic progression does not always mean that the immune therapy has failed immediately.
Oncologists may need to consider the clinical context, imaging pattern, symptoms and timing of progression.
In selected situations, apparent early progression can reflect immune-related inflammatory changes rather than straightforward tumor growth. This phenomenon is one reason response assessment during immunotherapy can be more complicated than with conventional cytotoxic therapy.
When genuine progression is established, the next step depends heavily on:
- Cancer type
- Stage
- Previous treatments
- Duration and depth of immunotherapy response
- Biomarker profile
- Presence of actionable genomic alterations
- Site and pattern of progression
- Performance status
- Prior treatment toxicity
- Available clinical trials
How Researchers Are Trying to Overcome Immunotherapy Resistance
There is no universal treatment for checkpoint inhibitor resistance.
Instead, researchers are pursuing several strategies based on the biological cause of resistance.
Improve antigen presentation or introduce new tumor antigens through vaccines and cellular therapies.
Target exhaustion pathways and other inhibitory mechanisms.
Target suppressive myeloid cells, fibroblasts, abnormal vasculature or immunosuppressive signals.
Use rational combinations involving immunotherapy, targeted therapy, chemotherapy, radiation or cellular therapy when supported by clinical evidence.
Combination Immunotherapy
Combining immune checkpoint inhibitors can produce stronger immune activation in selected cancers.
A major example is combining PD-1/PD-L1 blockade with CTLA-4 blockade.
The biological rationale is that the two pathways regulate different aspects of T-cell activation.
Other checkpoint combinations involving targets such as LAG-3 and TIGIT are being investigated.
More checkpoint inhibition does not automatically mean better treatment. Combination immunotherapy can increase immune-related toxicities, including inflammation affecting organs such as the colon, liver, lungs, endocrine glands and other tissues.
Combination treatment should therefore be selected according to cancer type, evidence, patient characteristics and clinical guidelines.
Targeted Therapy + Immunotherapy
Combining targeted therapies with immunotherapy is an active area of research and clinical development.
The rationale varies by cancer type.
Targeted therapies may alter tumor-cell signaling, antigen expression, tumor vasculature or the immune microenvironment.
However, the interaction between targeted therapy and immunotherapy can be complex.
For example, in some oncogene-driven cancers, immune checkpoint inhibitors may not be appropriate in certain treatment settings despite PD-L1 expression.
This is why treatment should be based on the complete molecular profile rather than PD-L1 alone.
For example, current ASCO guidance for advanced NSCLC with actionable driver alterations distinguishes treatment strategies from those used in tumors without driver alterations.
Radiation + Immunotherapy
Radiation can kill tumor cells and potentially alter the immune environment.
This has generated interest in combining radiation with checkpoint blockade.
Researchers are studying whether radiation can increase antigen release and improve immune recognition.
The concept is sometimes discussed in connection with the abscopal effect, in which treatment of one tumor site is associated with immune-mediated responses at distant sites.
Although biologically intriguing, clinically meaningful abscopal responses remain uncommon and should not be assumed to occur routinely.
Cancer Vaccines
Therapeutic cancer vaccines aim to direct the immune system toward tumor-specific antigens or neoantigens.
This approach could potentially address one of the fundamental limitations of checkpoint blockade:
Checkpoint inhibitors improve the activity of existing immune responses, whereas vaccines may help create or strengthen tumor-specific immune recognition.
Personalized neoantigen vaccines are an important area of clinical research.
They remain investigational for many cancer types and settings.
TIL, CAR-T and Other Cellular Therapies
Cellular immunotherapies attempt to provide patients with immune cells capable of recognizing and attacking cancer.
Tumor-Infiltrating Lymphocyte Therapy
TIL therapy uses immune cells that have already entered a patient's tumor.
These cells can be isolated, expanded outside the body and then administered back to the patient following a preparative treatment regimen.
The FDA granted accelerated approval to lifileucel (Amtagvi) in 2024 for adults with unresectable or metastatic melanoma previously treated with a PD-1 blocking antibody, with additional requirements for BRAF V600-positive disease.
This represents an important example of how treatment after PD-1 resistance can move beyond simply switching from one checkpoint inhibitor to another.
CAR-T and TCR-based therapies
CAR-T therapy has produced major successes in several blood cancers, while researchers continue working to make engineered cellular therapies more effective against solid tumors.
TCR-based therapies and other engineered immune-cell approaches are also under development.
These approaches remain highly specialized and cancer-specific.
Clinical Trials: Often the Most Important Option After Resistance
When standard therapies have been exhausted or resistance has developed, a well-designed clinical trial may provide access to treatments specifically intended to overcome resistance.
Trials may investigate:
- New checkpoint inhibitors
- Dual- and multi-checkpoint blockade
- LAG-3 or TIGIT inhibitors
- Personalized cancer vaccines
- Bispecific antibodies
- TIL therapy
- CAR-T and TCR therapies
- Oncolytic viruses
- STING-pathway agonists
- Myeloid-cell targeting
- Metabolic interventions
- Combination targeted therapy and immunotherapy
- Radiation-immunotherapy combinations
Clinical-trial eligibility should always be evaluated by the patient's oncology team.
What About Repurposed Drugs?
Drug repurposing is an important research strategy because existing drugs may have biological effects that are relevant to cancer immunity.
Examples investigated in preclinical or early clinical research include drugs affecting:
- Metabolism
- Inflammation
- Autophagy
- Angiogenesis
- Myeloid-cell signaling
- Cellular stress
- DNA repair
- Immune signaling
However, an important evidence distinction must be maintained.
A drug that changes PD-1/PD-L1 signaling, tumor metabolism or immune-cell behavior in cultured cells or animal models does not automatically improve survival in patients.
The same principle applies to ivermectin, mebendazole, fenbendazole, metformin, statins, doxycycline, niclosamide and other repurposed agents that have been investigated in cancer research.
Human clinical trials are required to determine whether a proposed combination actually improves patient outcomes and whether its risks are acceptable.
OneDayMD's repurposed cancer drug evidence series should therefore distinguish clearly between mechanistic hypotheses, preclinical evidence, case reports, observational evidence and randomized clinical trials.
Evidence Hierarchy for Immunotherapy Resistance
Not all evidence deserves equal weight.
Best evidence for whether a treatment improves clinically meaningful outcomes.
Useful evidence, although generally less definitive than randomized comparisons.
Can identify associations but are vulnerable to confounding and selection bias.
Useful for hypothesis generation but cannot establish treatment efficacy.
Important for mechanism and early therapeutic development but not proof of human benefit.
Useful for molecular hypotheses but far removed from clinical effectiveness.
The lower levels of the evidence hierarchy are valuable for discovering possibilities—but they should not be presented as equivalent to clinical evidence.
The Future of Immunotherapy Resistance
The next generation of cancer immunotherapy is likely to be less about finding one universal "immune booster" and more about identifying the specific reason a patient's tumor is resistant.
This represents a transition from one-size-fits-all immunotherapy toward precision immuno-oncology.
Future treatment selection may integrate:
- PD-L1 expression
- TMB
- MSI/MMR status
- Tumor genomics
- Antigen presentation
- T-cell receptor sequencing
- TIL characteristics
- Spatial immune profiling
- ctDNA dynamics
- RNA expression profiles
- Microbiome data
- Metabolic features
- Artificial intelligence and machine learning
Dynamic biomarkers may become especially important because tumors change during treatment.
A biopsy taken before therapy may not accurately describe the tumor months later.
Longitudinal monitoring using approaches such as ctDNA could eventually help clinicians detect molecular evolution before radiographic progression becomes obvious in selected settings.
Immunotherapy Resistance Is a Systems Problem
The most useful conceptual model is to think of resistance as an interaction between several biological systems.
Mutations and evolving cancer subclones determine what the immune system can recognize.
Tumor antigens must be processed and presented appropriately.
T cells and other immune populations must remain capable of effective anti-tumor activity.
Suppressive cells, fibroblasts, blood vessels and extracellular matrix can restrict immune activity.
Competition for nutrients and accumulation of suppressive metabolites can influence immune function.
Gut microbial ecology may influence systemic immunity and response to checkpoint blockade.
What Should Patients Ask When Immunotherapy Stops Working?
Patients facing progression during or after immunotherapy can discuss questions such as:
- Is this definitely progression, or could additional evaluation be useful?
- Could a new biopsy provide important information?
- Has the tumor's molecular profile changed?
- Is repeat molecular testing appropriate?
- Would ctDNA testing be informative in this cancer?
- Are there actionable mutations?
- Has the tumor's PD-L1 status changed?
- Is MSI/MMR or TMB testing available and relevant?
- Are there clinical trials specifically targeting resistance?
- Would a different treatment class be more appropriate?
- Is combination immunotherapy appropriate for this cancer?
- Would targeted therapy, chemotherapy, radiation or cellular therapy be appropriate?
The correct answer depends on the cancer type, molecular profile, prior treatment and overall clinical condition.
Frequently Asked Questions
Why does immunotherapy stop working?
Immunotherapy can stop working because cancer cells evolve, lose recognizable antigens, alter antigen presentation, activate alternative immune-suppressive pathways, change the tumor microenvironment or develop other mechanisms of immune escape. Multiple mechanisms can occur simultaneously.
What is primary immunotherapy resistance?
Primary resistance means a cancer fails to respond meaningfully to immunotherapy from the beginning of treatment.
What is acquired immunotherapy resistance?
Acquired resistance occurs when a tumor initially responds or remains controlled but later progresses. Cancer evolution and immune selection can contribute to this process.
Does high PD-L1 guarantee that immunotherapy will work?
No. PD-L1 can help predict benefit in certain cancers and treatment settings, but it is not a perfect biomarker. Some PD-L1-positive tumors do not respond, while some PD-L1-negative tumors can respond.
Is TMB a better biomarker than PD-L1?
Neither is universally superior. TMB and PD-L1 measure different aspects of tumor biology, and their usefulness varies by cancer type, assay, treatment and clinical context.
Can MSI-H tumors become resistant to immunotherapy?
Yes. MSI-H/dMMR status is associated with strong responses to checkpoint blockade in several settings, but it does not guarantee permanent disease control. Resistance can still develop.
What is a cold tumor?
A "cold" tumor generally describes a tumor with limited effective immune infiltration or an immune environment that is poorly conducive to anti-tumor activity. Cold tumors may be less responsive to checkpoint blockade alone.
Can cancer stem cells cause immunotherapy resistance?
Cancer stem-cell populations are being investigated as contributors to treatment resistance, tumor recurrence and immune escape. However, the clinical importance and best therapeutic targets remain active areas of research.
Can changing from one PD-1 inhibitor to another overcome resistance?
Usually, simply switching between drugs targeting the same pathway is not expected to overcome a biological resistance mechanism. Treatment after progression should be based on cancer type, previous therapy, biomarkers, resistance mechanisms and available evidence.
Can ivermectin, fenbendazole or mebendazole overcome immunotherapy resistance?
These drugs have generated preclinical hypotheses involving cancer-cell signaling, metabolism or immune-related pathways, but there is currently insufficient clinical evidence to establish them as treatments for PD-1/PD-L1 resistance. They should not replace proven cancer therapy.
What is the most promising approach to overcoming immunotherapy resistance?
There is no single universal solution. Current research includes rational combination therapies, new immune checkpoints, personalized vaccines, cellular therapies, tumor-microenvironment targeting, biomarker-guided treatment and strategies designed around the specific resistance mechanism.
Related OneDayMD Topics
Why some tumors contain active immune responses while others resist immune infiltration.
How TMB is used as an immunotherapy biomarker—and its limitations.
Why mismatch-repair deficiency and microsatellite instability can predict checkpoint inhibitor sensitivity.
The possible role of CSCs in metastasis, recurrence and treatment resistance.
How cancer metabolism and immune biology interact.
Explore the wider immunotherapy evidence library.
Conclusion
PD-1/PD-L1 immunotherapy does not fail for one single reason.
Resistance can arise because cancer cells evolve, immune recognition is impaired, T cells become dysfunctional, alternative checkpoints become dominant, suppressive cells accumulate, the tumor microenvironment becomes hostile, metabolism changes or the tumor develops entirely new immune-evasion strategies.
This explains why the future of immunotherapy is unlikely to be simply "more PD-1."
The emerging model is more sophisticated:
Identify the resistance mechanism → measure the relevant biology → select a rational treatment strategy → monitor how the tumor evolves.
That approach is moving oncology toward increasingly personalized treatment, in which PD-L1 is only one part of a much larger picture involving tumor genomics, TMB, MSI/MMR, TILs, ctDNA, the tumor microenvironment, metabolism and immune-cell function.
For patients whose cancer progresses after immunotherapy, the most important question is therefore not simply "What other immunotherapy can I try?"
It is:
"Why did this tumor become resistant, and is there an evidence-based treatment that specifically addresses that biology?"
Selected References
- National Cancer Institute. Immune Checkpoint Inhibitors. NCI Cancer Treatment Information.
- Roerden M, Spranger S. Cancer immune evasion, immunoediting and intratumour heterogeneity. Nature Reviews Immunology. 2025.
- Aliazis K, et al. The tumor microenvironment’s role in the response to immune checkpoint blockade. Nature Cancer. 2025;6:924–937.
- Holder AM, et al. Defining clinically useful biomarkers of immune checkpoint inhibitors in solid tumours. Nature Reviews Cancer. 2024;24:498–512.
- Budczies J, et al. Tumour mutational burden: clinical utility, challenges and emerging improvements. Nature Reviews Clinical Oncology. 2024;21:725–742.
- Lopez de Rodas M, et al. Biological and clinical significance of tumour-infiltrating lymphocytes in the era of immunotherapy. Nature Reviews Clinical Oncology. 2025;22:163–181.
- Butterfield LH, Najjar YG. Immunotherapy combination approaches: mechanisms, biomarkers and clinical observations. Nature Reviews Immunology. 2024;24:399–416.
- Kumagai S, Itahashi K, Nishikawa H. Regulatory T cell-mediated immunosuppression orchestrated by cancer: towards an immuno-genomic paradigm for precision medicine. Nature Reviews Clinical Oncology. 2024;21:337–353.
- National Cancer Institute. The Story of Immune Checkpoint Inhibitors and Immunotherapy. 2026.
- U.S. Food and Drug Administration. FDA approval of lifileucel (Amtagvi) for unresectable or metastatic melanoma previously treated with PD-1 blockade. 2024.
- ASCO. Therapy for Stage IV Non–Small Cell Lung Cancer Without Driver Alterations: ASCO Living Guideline, Version 2026.3.1. Journal of Clinical Oncology. 2026.
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