Peptide Vaccines Plus Checkpoint Inhibitors
Personalized Cancer Vaccines
80% ORR
Objective response rate in metastatic melanoma patients treated with an IDO/PD-L1 peptide vaccine plus nivolumab, with 43% achieving complete responses.
Kjeldsen et al., Nature Medicine, 2021
Kjeldsen et al., Nature Medicine, 2021
View as imageCheckpoint inhibitors changed cancer treatment. Anti-PD-1 antibodies like pembrolizumab and nivolumab can produce durable remissions in some patients with melanoma, lung cancer, and other solid tumors. The problem: fewer than 20% of patients with most solid tumor types respond to checkpoint blockade alone.[1] The immune system needs tumor-specific T cells to attack cancer, and checkpoint inhibitors can only "release the brakes" on T cells that already exist. Personalized cancer vaccines offer a solution: they prime new T cells against specific tumor targets, creating the army that checkpoint inhibitors then unleash.
This combination approach, vaccines to generate tumor-killing T cells plus checkpoint inhibitors to sustain them, has moved from theory to clinical data in melanoma, hepatocellular carcinoma, lung cancer, and other cancers. The results so far suggest that neither strategy alone captures the full potential of the immune system against cancer.
Key Takeaways
- A peptide vaccine targeting IDO and PD-L1 combined with nivolumab produced an 80% objective response rate in metastatic melanoma, with 43% complete responses and 26-month median progression-free survival (Kjeldsen et al., 2021)
- Vaccine formulation determines whether checkpoint inhibitor combinations work: water-in-oil emulsions trapped T cells at injection sites and blocked synergy with anti-CTLA-4 and anti-PD-L1, while short-lived formulations did not (Hailemichael et al., 2018)
- In mouse hepatocellular carcinoma models, adding a neoantigen vaccine to anti-PD-1 therapy increased tumor-infiltrating CD8+ T cells and slowed tumor growth beyond what either treatment achieved alone (Yang et al., 2023)
- A helper peptide vaccine plus PD-1 blockade in treatment-naive melanoma patients showed T cell responses associated with prolonged overall survival, but PD-1-experienced patients had lower response rates (Vavolizza et al., 2022)
- TLR9 plus STING agonist adjuvants boosted neopeptide T cell responses and improved checkpoint blockade efficacy in tumor models by increasing CD8+ T cell infiltration (Castro Eiro et al., 2024)
Why checkpoint inhibitors alone fall short
Checkpoint inhibitors work by blocking proteins that suppress T cell activity. PD-1 on T cells binds PD-L1 on tumor cells, sending a "stand down" signal. CTLA-4 competes with the co-stimulatory receptor CD28 to dampen T cell activation. Blocking these pathways with antibodies like nivolumab (anti-PD-1), pembrolizumab (anti-PD-1), or ipilimumab (anti-CTLA-4) allows T cells to attack tumors they would otherwise tolerate.
But this only works when tumor-reactive T cells already exist in or near the tumor. In cancers with low mutational burden, the immune system may never have generated strong T cell responses against the tumor. Even in highly mutated cancers, the tumor microenvironment can suppress T cell infiltration and function through multiple mechanisms beyond PD-1/PD-L1 signaling.[2]
Leclerc et al. (2019) reviewed this gap in lung cancer specifically: anti-PD-1 monotherapy produces objective responses in fewer than 20% of non-small cell lung cancer patients. Many tumors classified as "cold," meaning they lack significant T cell infiltration, fail to respond regardless of PD-L1 expression levels.[1] The challenge is not just releasing immune brakes; it is generating the immune response in the first place.
The rationale: vaccines prime, checkpoints sustain
The logic behind combining peptide vaccines with checkpoint inhibitors follows a two-step model that researchers have described as "fueling the engine and releasing the brake."[3]
Step 1: Peptide vaccines generate tumor-specific T cells. A peptide vaccine presents tumor-associated antigens or neoantigens to the immune system, triggering dendritic cells to activate CD4+ helper and CD8+ cytotoxic T cells against those targets. This increases the pool of T cells capable of recognizing and killing tumor cells.
Step 2: Checkpoint inhibitors prevent T cell exhaustion. Once vaccine-primed T cells encounter tumor cells, the tumor microenvironment attempts to shut them down through PD-L1 expression and other inhibitory signals. Checkpoint blockade prevents this suppression, allowing vaccine-induced T cells to sustain their attack.
Maeng and Berzofsky (2019) outlined this strategy in their review of cancer vaccine optimization: vaccines convert immunologically "cold" tumors into "hot" ones by driving T cell infiltration, and checkpoint inhibitors maintain the infiltrating T cells in an active state.[3] Neither approach alone captures both sides of the equation.
Clinical evidence in melanoma
Melanoma has been the testing ground for peptide vaccine and checkpoint inhibitor combinations, largely because it was the first cancer where checkpoint blockade demonstrated clear efficacy.
6MHP helper peptide vaccine plus PD-1 blockade
Vavolizza et al. (2022) reported results from MEL64, a phase I/II trial testing a vaccine containing six melanoma-associated helper peptides (6MHP) combined with PD-1 blockade. The trial enrolled two groups: patients who had never received anti-PD-1 therapy and patients who had already progressed on it.[4]
In PD-1-naive patients, the combination was safe, increased intratumoral lymphocytes, and induced T cell responses associated with prolonged overall survival. The PD-1-experienced group showed lower T cell response rates. This finding corroborates earlier observations that delaying cancer vaccines until after checkpoint blockade fails may reduce vaccine efficacy, possibly because prior PD-1 therapy depletes or exhausts the very T cell populations vaccines need to expand.
Thymosin alpha-1 and checkpoint synergy
Danielli et al. (2018) followed metastatic melanoma patients long-term who had received thymosin alpha-1 (Ta1) combined with dacarbazine. The investigators noted that Ta1's immunomodulatory properties, including enhanced dendritic cell maturation and T cell activation, could synergize with checkpoint inhibition. Patients who later received ipilimumab showed overall survival outcomes that warranted further investigation of the sequence.[5] The data came from a small retrospective analysis, and the contribution of Ta1 versus sequential treatment effects remains unclear.
EVX-01 personalized neoantigen vaccine
Mork et al. (2022) reported on EVX-01, a personalized peptide-based neoantigen vaccine with a novel adjuvant (CAF09b), in patients with metastatic melanoma. The vaccine induced neoantigen-specific T cell responses in treated patients. While the trial was designed primarily to assess safety and immunogenicity rather than efficacy, the personalized approach demonstrated that identifying patient-specific mutations and synthesizing corresponding peptide vaccines was feasible in a clinical setting.[6]
Evidence in hepatocellular carcinoma
Hepatocellular carcinoma (HCC) represents a particularly challenging target for checkpoint monotherapy. Checkpoint inhibitors produce modest response rates in HCC, making it a compelling cancer type for combination approaches.
Yang et al. (2023) demonstrated in a mouse HCC model that neoantigen vaccination augmented the antitumor effects of anti-PD-1 therapy. Mice receiving both the neoantigen vaccine and anti-PD-1 antibody showed significantly greater tumor growth inhibition and increased CD8+ T cell infiltration compared to either treatment alone.[7] The combination worked by expanding the pool of neoantigen-specific T cells (via vaccination) while preventing their exhaustion in the tumor microenvironment (via anti-PD-1).
Charneau et al. (2021) reviewed peptide-based vaccine strategies for HCC more broadly, noting that the combination with checkpoint inhibitors addresses a fundamental limitation of each therapy: vaccines alone may generate T cells that become exhausted upon encountering the immunosuppressive liver tumor environment, while checkpoint inhibitors alone cannot overcome the absence of pre-existing tumor-reactive T cells in many HCC patients.[8]
Baretti et al. (2025) reported results from a phase 1 trial of a therapeutic peptide vaccine targeting the chimeric DNAJ-PKAc protein in fibrolamellar HCC, a rare liver cancer affecting young patients. The vaccine demonstrated safety, immunogenicity, and preliminary evidence of clinical activity, establishing that peptide vaccines can target fusion protein neoantigens in liver cancers.[9]
Evidence in lung cancer and other solid tumors
HPV-associated cancers
Lee et al. (2022) tested flagellin-adjuvanted HPV E7 long peptide vaccines in combination with anti-PD-1 therapy in a mouse tumor model. The combination significantly enhanced tumor suppression compared to either treatment alone. Longer peptides (containing both CD4+ and CD8+ T cell epitopes) performed better than short peptides when combined with anti-PD-1, suggesting that peptide length and epitope design directly influence combination efficacy.[10]
Neoantigen vaccines in advanced NSCLC
Oosting et al. (2022) described the development of FRAME-001, a personalized tumor neoantigen-based vaccine formulation designed for a phase II trial in advanced non-small cell lung cancer. The approach involved identifying patient-specific neoantigens through whole-exome sequencing and RNA sequencing, then manufacturing GMP-grade peptide vaccines for combination with standard immunotherapy.[11] The trial design reflects the growing consensus that personalized vaccines may convert checkpoint-resistant lung tumors into checkpoint-responsive ones.
Esophageal cancer
Daiko et al. (2020) conducted an exploratory study of S-588410, a cancer peptide vaccine comprising five HLA-A*24:02-restricted peptides, in esophageal cancer patients. The vaccine induced tumor-infiltrating lymphocytes and altered the tumor microenvironment, providing mechanistic evidence that peptide vaccines can increase immune cell infiltration even in tumors not traditionally considered immunogenic.[12]
Why vaccine formulation matters for combination success
One of the most important findings in this field comes from Hailemichael et al. (2018), who demonstrated that vaccine formulation determines whether checkpoint inhibitor combinations succeed or fail. In the landmark ipilimumab registration trial, patients who received a gp100 peptide vaccine in a water-in-oil emulsion (Incomplete Freund's Adjuvant) showed no benefit from the combination, and the vaccine arm actually performed worse than ipilimumab alone.[2]
The explanation: the water-in-oil emulsion created persistent antigen depots at injection sites that trapped vaccine-induced T cells, sequestering them away from the tumor. These T cells underwent chronic antigen stimulation and exhaustion at the injection site rather than migrating to and attacking the tumor. Short-lived vaccine formulations that released antigen rapidly did not cause this trapping effect and synergized effectively with anti-CTLA-4 and anti-PD-L1 therapy.
This finding reshaped how researchers design vaccine-checkpoint combinations. The vaccine must generate T cells that can traffic to the tumor, not remain stuck at the injection site.
Adjuvant strategies that enhance the combination
The choice of adjuvant in a peptide vaccine directly impacts how well it synergizes with checkpoint blockade.
Castro Eiro et al. (2024) showed that combining TLR9 and STING agonist adjuvants with neopeptide vaccines produced potent T cell immunity and improved checkpoint blockade efficacy in a tumor model. The dual-adjuvant approach increased the frequency and cytotoxic capacity of CD8+ T cells infiltrating tumors, addressing the T cell deficit that limits checkpoint inhibitor monotherapy.[13]
Song et al. (2025) developed virus-inspired peptide vaccine formulations using structurally distinct STING agonist "drugamers" and demonstrated that different STING agonist structures induced discrete antitumor immune responses. Some formulations preferentially activated CD8+ T cells, while others biased toward CD4+ responses, suggesting that adjuvant design can be tuned to complement specific checkpoint inhibitor mechanisms.[14]
Shared versus personalized neoantigens in combination therapy
A practical question for clinical application: do combination vaccines need to be personalized for each patient, or can shared tumor antigens achieve similar results?
Peterson et al. (2020) compared personal and shared frameshift neoantigen vaccines in a mouse mammary cancer model and found that personal neoantigen vaccines generated stronger tumor-specific immune responses. However, shared neoantigen vaccines still produced measurable antitumor immunity, and the investigators noted that creating personalized vaccines at clinical scale remains challenging in terms of time, cost, and manufacturing logistics.[15]
Ashi et al. (2022) reviewed both mutant and non-mutant neoantigen approaches and concluded that combining multiple antigen types in a single vaccine could maximize the chance of generating T cell responses that synergize with checkpoint blockade. Non-mutant antigens, such as cancer-testis antigens and overexpressed self-antigens like WT1, face the additional challenge of central tolerance but can target a broader patient population.[16]
This question intersects with the broader design of KRAS peptide vaccines and HER2 peptide vaccines, where specific shared mutations or overexpressed proteins serve as targets across patient populations.
Limitations and open questions
The clinical data for peptide vaccine-checkpoint inhibitor combinations is encouraging but incomplete. Most trials to date have been phase I or I/II, designed to assess safety and immunogenicity rather than definitive efficacy. Several specific gaps remain:
Sequencing and timing. The MEL64 trial data suggest that vaccines may work better when given before or alongside checkpoint inhibitors rather than after checkpoint failure. But the optimal sequence, whether to vaccinate first and add checkpoint blockade later, start both simultaneously, or cycle between them, has not been established in randomized trials.
Patient selection. Not all patients respond to combination approaches. HLA type restricts which peptide epitopes a given patient can present to T cells. Tumor mutational burden influences the number of available neoantigen targets. Biomarkers to predict which patients will benefit from vaccination plus checkpoint blockade are still in development.
Manufacturing and access. Personalized neoantigen vaccines require tumor sequencing, neoantigen prediction, peptide synthesis, and quality testing for each individual patient. This process currently takes weeks to months, during which time the patient's cancer may progress. Shared antigen approaches are faster to manufacture but may sacrifice some efficacy.
Long-term outcomes. Whether vaccine-checkpoint combinations produce more durable responses than checkpoint inhibitors alone remains unclear. Longer follow-up from ongoing trials will be critical. The mechanism (expanding a broader repertoire of tumor-specific T cells) suggests durability, but this remains theoretical until confirmed by data.
Understanding how tumors evade peptide vaccines through antigen loss, HLA downregulation, and other immune escape mechanisms will be essential for improving combination strategies.
The Bottom Line
Combining peptide vaccines with checkpoint inhibitors addresses a fundamental gap in each approach: vaccines generate tumor-specific T cells that checkpoint inhibitors alone cannot create, while checkpoint blockade prevents the exhaustion of vaccine-induced T cells in the tumor microenvironment. Clinical data from melanoma and preclinical data from hepatocellular carcinoma and other cancers support the biological rationale. The field is still in early-phase trials, and critical questions about optimal sequencing, patient selection, vaccine formulation, and long-term durability remain unanswered.