Intratumoral Immunotherapy and In-Situ Vaccination: Local Shots, Systemic Responses?

The Central Idea: Turning a Tumor Into Its Own Vaccine

A tumor is both the target of treatment and a repository of patient-specific antigens. Intratumoral immunotherapy attempts to exploit this. An injected agent may lyse tumor cells, activate innate immune sensing, recruit or mature antigen-presenting cells, release inflammatory mediators, and expand tumor-reactive lymphocytes. Those lymphocytes may then leave the injected site and attack uninjected deposits.[3,6]

This sequence underpins the phrase in situ vaccination. The term is useful as a mechanistic model, but it can also overstate what has been proved. A local inflammatory reaction is not, by itself, a vaccine effect. Shrinkage of the injected lesion is not evidence of systemic immunity. Even regression of an uninjected lesion does not establish a survival advantage.

For clinical interpretation, the question is not simply whether a local injection can provoke an immune response. It is whether that response is sufficiently broad, durable, and clinically meaningful to control disease that was never injected.

What Counts as a Systemic Response?

Intratumoral trials often report several different outcomes, and they should not be treated as equivalent.[5,7,8]

Evidence level	What it measures	What it can support	What it cannot prove by itself
Injected-lesion response	Shrinkage or disappearance of treated tumors	Direct local antitumor activity	Systemic immunity
Uninjected superficial or nodal response	Regression outside the injected field	A biologic effect beyond direct drug placement	Control of visceral disease or survival benefit
Uninjected visceral response	Regression in organs such as lung or liver	Stronger evidence of a systemic antitumor effect	The contribution of each component in a combination regimen
Whole-patient objective response rate and duration	RECIST-based overall disease response	Clinically interpretable activity across measurable disease	Comparative benefit without a control group
Progression-free or overall survival	Time-to-event patient outcomes	The most consequential evidence of clinical benefit	Mechanism of action
Immune-cell, cytokine, or gene-expression changes	Pharmacodynamic activity	Target engagement and biologic plausibility	Patient benefit without clinical outcome correlation

The most persuasive “local shot, systemic response” evidence combines whole-patient responses with documented regression of uninjected lesions, including visceral lesions, and durable follow-up. Randomized comparisons remain essential when the aim is to determine whether adding the intratumoral component improves outcomes over an active systemic regimen.

The U.S. Landscape in 2026

Two oncolytic immunotherapies now define the U.S. clinical landscape in melanoma, but their indications, evidence bases, routes, and limitations are different.[1-4]

Feature	Talimogene laherparepvec	Vusolimogene oderparepvec-wtpg plus nivolumab
Common shorthand	T-VEC; Imlygic	RP1; Tudriqev plus nivolumab
U.S. indication	Local treatment of unresectable cutaneous, subcutaneous, and nodal melanoma lesions recurrent after initial surgery	Adults with unresectable advanced cutaneous melanoma that progressed on a PD-1-blocking antibody-containing regimen
Regulatory basis	Regular approval	Accelerated approval based on response rate and durability
Injection targets	Cutaneous, subcutaneous, or nodal lesions that are visible, palpable, or detectable by ultrasound	Superficial lesions and, when appropriate, image-guided deep or visceral lesions
Schedule summary	Initial 10^6 PFU/mL dose, then 10^8 PFU/mL 3 weeks later and every 2 weeks; maximum 4 mL per treatment	Every 2 weeks for 8 consecutive doses: 10^6 PFU/mL at week 1, then 10^7 PFU/mL; maximum 10 mL per dose; nivolumab begins at week 3
Main efficacy context	Durable response in OPTiM, with responses observed in some uninjected lesions	Single-arm IGNYTE data in anti-PD-1-experienced melanoma
Central limitation	Label states no demonstrated overall-survival benefit or effect on visceral metastases	Component contribution and comparative survival benefit are not established; confirmatory trial is required
Key safety domains	Herpetic infection and transmission, injection-site complications, immune-mediated events; contraindicated in immunocompromised and pregnant patients	Accidental exposure and HSV-1 infection, injection complications including deep-organ procedures, immune-mediated events from nivolumab; pregnancy and exposure precautions are product-specific

PFU indicates plaque-forming units.

These products should not be generalized into a class-wide rule. Viral backbone, transgenes, dose, injection technique, systemic partner, treatment setting, disease biology, and trial design can all alter the balance between local control and systemic benefit.

What T-VEC Proved, and What It Did Not

Talimogene laherparepvec is a modified herpes simplex virus type 1 engineered to replicate preferentially in tumors and express granulocyte-macrophage colony-stimulating factor. Its pivotal OPTiM trial compared intralesional T-VEC with subcutaneous GM-CSF in unresectable stage IIIB-IV melanoma. T-VEC improved the durable response rate, which became the principal basis for approval.[1,9]

The lesion-level data were important. Complete resolution occurred in 47% of injected lesions, 22% of uninjected nonvisceral lesions, and 9% of uninjected visceral lesions.[7] Those findings showed that treatment can be followed by regression beyond the injected tumor. They also showed a steep gradient: direct local effects were strongest, distant superficial effects were less frequent, and visceral effects were least frequent.

The label preserves this distinction. T-VEC is indicated for local treatment of injectable melanoma lesions, and it has not demonstrated an overall-survival benefit or an effect on visceral metastases.[1] Clinicians should therefore avoid translating lesion-level evidence into an unqualified claim of systemic disease control.

Combination studies reinforced both the platform’s promise and its limits. In a randomized phase 2 trial, T-VEC plus ipilimumab improved objective response compared with ipilimumab alone.[10] However, the much larger phase 3 MASTERKEY-265 trial did not show a statistically significant improvement in progression-free survival or overall survival when T-VEC was added to pembrolizumab, despite a numerically higher objective response rate.[5]

This is one of the field’s clearest lessons: increased response activity does not necessarily become a time-to-event benefit.

What Changed With Tudriqev Plus Nivolumab

On August 6, 2026, the FDA granted accelerated approval to vusolimogene oderparepvec-wtpg, an intratumoral modified HSV-1 oncolytic immunotherapy, in combination with nivolumab for adults with unresectable advanced cutaneous melanoma that progressed on a PD-1-blocking antibody-containing regimen.[2-4]

The construct is designed to replicate in tumors, lyse malignant cells, express a fusogenic glycoprotein, and produce GM-CSF. The label describes tumor-cell lysis, antigen release, inflammatory cell death, and increased T-cell infiltration as elements of its pharmacologic activity.[3]

This approval differs from the original T-VEC paradigm in several clinically important ways:

1. It addresses PD-1-experienced disease. The indication is specifically for progression after a PD-1-containing regimen, an area with substantial unmet need.[2-4]
2. The indication includes the combination. Vusolimogene oderparepvec-wtpg is approved with nivolumab, not as monotherapy.[2-4]
3. Deep and visceral injection may be used. The prescribing information permits image-guided injection of selected deep or visceral lesions, with additional procedural precautions and post-procedure monitoring.[3]
4. The approval is accelerated. Continued approval may depend on verification of clinical benefit in a confirmatory trial.[2-4]

Reading the IGNYTE Numbers Correctly

The FDA label’s efficacy analysis included 91 patients with at least one noninjected lesion. The objective response rate was 24.2% (95% CI, 15.8%-34.3%), and the median duration of response was 14.1 months (95% CI, 10.7 months to not reached). Among responders, 86.1% had a response lasting at least 6 months, and 54.6% had a response lasting at least 12 months.[3]

A peer-reviewed IGNYTE analysis reported results for a broader 140-patient cohort, including an objective response rate of 32.9%, a complete response rate of 15.0%, and a median duration of response of 33.7 months. Responses were reported in injected and noninjected lesions, including visceral sites.[8]

Those figures should not be blended as though they came from one analysis. The FDA label and the journal publication use different analysis populations and data conventions. For clinical communication, the label-defined numbers are the appropriate reference for the approved indication, while the publication provides additional context on the broader trial experience.

The trial was single-arm and evaluated a combination. It therefore cannot determine how much benefit came from the oncolytic virus, nivolumab rechallenge, their interaction, patient selection, or other factors. The FDA required a randomized confirmatory study, IGNYTE-3, comparing the combination with physician’s-choice therapy and using overall survival as the primary endpoint. The approval letter lists final study completion for September 2030 and final report submission for March 2031.[4]

Beyond Oncolytic Viruses

“In situ vaccination” is a strategy, not a single technology. The investigational field includes innate immune agonists, cytokine or gene-delivery systems, cellular products, and combinations with checkpoint blockade, radiation, or local ablation.

Platform	Intended local effect	Illustrative clinical finding	Current interpretation
TLR9 agonists	Activate plasmacytoid dendritic cells and type I interferon pathways	Vidutolimod plus pembrolizumab produced durable responses in a phase 1b melanoma study after PD-1 resistance.[11]	Biologically active, but early-phase activity does not establish comparative benefit
TLR9 agonist plus CTLA-4 blockade	Broaden innate activation and T-cell priming	In phase 3 ILLUMINATE-301, tilsotolimod plus ipilimumab did not improve response or overall survival over ipilimumab alone.[12]	A negative randomized test of the in situ vaccination hypothesis
STING agonists	Trigger innate immune sensing and interferon signaling	Intratumoral MIW815 plus spartalizumab showed pharmacodynamic activity but limited objective responses in a phase 1b study.[13]	Target engagement was not sufficient for broad clinical efficacy
IL-12 gene delivery	Produce local proinflammatory cytokine expression with reduced systemic exposure	Intratumoral electroporation of an IL-12 plasmid plus pembrolizumab showed responses and immune activation in a phase 2 melanoma study.[14]	Promising signal that still requires larger comparative validation

The pattern is consistent across platforms: local immune activation can be measured, and selected patients can experience distant responses, but many programs fail to improve randomized clinical outcomes.

Why a Biologically Active Local Therapy May Fail Systemically

Several bottlenecks can interrupt the proposed sequence from injected tumor to whole-patient benefit.

1. The injected lesion may not represent the full disease

Metastases within one patient can differ in antigen expression, clonality, interferon signaling, antigen presentation, stromal architecture, and immune-cell composition. An immune response generated against one deposit may not recognize or penetrate all others.

2. Local priming may not overcome distant immune suppression

Tumor-reactive T cells must expand, enter the circulation, home to distant lesions, cross abnormal vasculature, persist, and remain functional. Myeloid suppression, regulatory T cells, inhibitory ligands, metabolic stress, and poor antigen presentation can block any of those steps.

3. Injection coverage may be inadequate

A limited number of accessible lesions may be treated while a large burden of biologically aggressive disease remains untreated. The procedure can debulk or inflame selected tumors without changing the trajectory of rapidly progressive visceral disease.

4. Disease tempo may outpace immune priming

Immune responses take time. Patients with impending organ compromise, rapidly enlarging tumors, or high-risk central nervous system disease may not have a sufficient therapeutic window for an injection-based immune strategy to work.

5. The systemic partner may already be near its efficacy ceiling

Adding a local agent to an effective checkpoint inhibitor may increase lesion-level responses without producing enough incremental benefit to improve progression-free or overall survival. MASTERKEY-265 is a practical example.[5]

6. Surrogate endpoints may not capture net benefit

Objective response and duration are clinically meaningful, especially in refractory disease, but they do not automatically predict longer survival. Procedure burden, treatment delays, toxicity, subsequent therapy, and nonresponding disease sites all affect the outcome.

Patient Selection Is Both Biologic and Procedural

A clinician considering an approved intratumoral regimen should evaluate more than whether a lesion can technically be reached.

Question	Why it matters
Is there at least one safely injectable lesion?	Intratumoral treatment requires reliable access, imaging support when needed, and a route that avoids major vessels, nerves, airways, bone erosion, or other high-risk anatomy
Is there measurable uninjected disease?	It permits assessment of a systemic effect and may be required by the evidentiary context of a specific regimen
What is the tempo of progression?	Rapid visceral progression or imminent organ compromise may favor treatment with a faster or more established systemic effect
Is the indication exactly matched?	T-VEC and vusolimogene oderparepvec-wtpg plus nivolumab have different populations and regulatory bases
Can the patient safely receive the viral product?	Viral exposure, herpetic infection risk, immunocompromise, pregnancy, wound care, and caregiver exposure require product-specific review
Can the patient safely receive the systemic partner?	Nivolumab adds immune-mediated risks involving multiple organs and may require corticosteroids, treatment interruption, or discontinuation
Is an interventional team available?	Deep or visceral injection may require radiology, anesthesia or sedation planning, biosafety procedures, and post-procedure monitoring
Can response be assessed across injected and uninjected sites?	Whole-patient imaging and lesion tracking are essential to distinguish local control from systemic benefit
Is a clinical trial available?	Most nonapproved intratumoral platforms remain investigational and are best used within protocol-defined care

The trials reviewed here have not established a single cross-platform predictive biomarker for routine clinical use.[5,8,11-14] Lesion accessibility, disease distribution, prior checkpoint exposure, tumor biology, and host immune competence remain practical parts of selection, but their predictive value is product- and trial-specific.

Safety Is Not Merely Local

Intratumoral delivery may reduce systemic exposure to some agents, but it does not make treatment risk-free.

Viral and exposure risks

Both approved products are replication-competent HSV-1-based therapies. Accidental exposure can occur through contact with treated lesions, dressings, body fluids, or needlestick injury. Safe administration includes product-specific handling, personal protective equipment, lesion covering, waste disposal, and instructions for patients and close contacts.[1,3]

T-VEC is contraindicated in immunocompromised and pregnant patients.[1] The newer product’s label lists no formal contraindications, but it contains warnings regarding accidental exposure, herpetic infection, and embryo-fetal risk. Clinicians should follow the exact label rather than extrapolate contraindications or precautions from one viral product to another.[3]

Injection and procedure risks

Pain, inflammation, infection, impaired healing, bleeding, necrosis, and damage to nearby structures can occur. Image-guided lung, liver, kidney, or other deep-organ injections add risks such as pneumothorax, hemorrhage, organ injury, and post-procedure complications. The vusolimogene oderparepvec-wtpg label requires procedural planning and 2-4 hours of monitoring after injection into the lung, liver, or kidney.[3]

Systemic immune toxicity

When an intratumoral agent is combined with a checkpoint inhibitor, the systemic partner’s immune-mediated adverse reactions remain fully relevant. Pneumonitis, colitis, hepatitis, endocrinopathies, nephritis, dermatologic reactions, neurologic toxicity, and other immune complications may require prompt evaluation, corticosteroids, treatment interruption, or permanent discontinuation, depending on the systemic agent’s prescribing information.[3,15]

Antiviral treatment can be clinically relevant

Acyclovir and related antivirals may reduce the effectiveness of HSV-1-based oncolytic therapies. Their use should be considered carefully when herpetic infection is suspected or confirmed, balancing infection management against possible attenuation of the therapeutic virus.[1,3]

A Practical Clinical Approach

1. Confirm the disease and treatment setting. Identify histology, stage, resectability, prior therapies, prior checkpoint exposure, current disease tempo, and whether the proposed regimen matches an approved indication.
2. Map every lesion. Distinguish injectable from noninjectable disease, superficial from deep or visceral targets, symptomatic from asymptomatic sites, and lesions that would create excessive procedural risk.
3. Define the therapeutic objective. Clarify whether the goal is control of locoregional disease, generation of a systemic response, checkpoint resensitization, symptom relief, or trial participation.
4. Choose evidence-appropriate endpoints. Track injected lesions, uninjected lesions, overall RECIST response, durability, progression-free survival, symptoms, corticosteroid exposure, hospitalizations, and treatment burden.
5. Assess viral, immune, and procedural safety. Review immune status, pregnancy potential, herpetic history, close-contact exposure, organ-specific checkpoint risks, anticoagulation, sedation needs, wound care, and access to urgent evaluation.
6. Coordinate the treatment team. Medical oncology, surgical oncology, dermatology, interventional radiology, pharmacy, infection prevention, nursing, pathology, and radiology may all be involved.
7. Reassess early when disease is discordant. Growth of uninjected visceral disease should not be dismissed because an injected lesion is shrinking. Whole-patient clinical status remains the decisive frame.

How Future Trials Should Be Judged

The next generation of intratumoral studies should answer questions that early trials often could not:

• Does the intratumoral component improve an active systemic comparator?
• Are noninjected visceral responses prospectively measured and independently reviewed?
• Is benefit consistent across tumor burden, metastatic site, and prior checkpoint exposure?
• Can biomarkers identify patients with antigen release but impaired T-cell trafficking or effector function?
• Does treating multiple lesions improve antigen breadth, or does it merely increase procedural burden?
• Are overall survival, patient-reported outcomes, hospital use, and quality of life improved?
• Can the local agent’s contribution be separated from the systemic partner?
• Is the treatment feasible outside highly specialized centers?

A positive early-phase response signal should be considered a hypothesis for randomized testing, not the endpoint of development.

Limitations of the Evidence

The field is heavily weighted toward melanoma, a tumor in which visible or image-guided lesions are often available, and checkpoint inhibitors already have established activity.[5,8,9,11-14] Results may not transfer to less immunogenic tumors, poorly accessible disease, or patients with rapidly progressive visceral burden.

Many studies are single-arm, use heterogeneous injection strategies, combine a local agent with an active systemic drug, or analyze injected and uninjected lesions differently. Published trial populations and FDA label populations may also differ. Cross-trial comparisons are therefore particularly unreliable.

Finally, “in situ vaccination” encompasses products with distinct mechanisms. A viral oncolytic therapy, a TLR agonist, a STING agonist, and a cytokine gene-delivery system should not be treated as interchangeable because they share an injection route.

Clinical Bottom Line

Local shots can produce systemic responses. Regression of uninjected and even visceral lesions has been documented with intratumoral immunotherapy.[7,8,11]

That finding is not the same as proven survival benefit. T-VEC’s label retains explicit limitations, and randomized studies have shown that biologic activity or improved response does not necessarily improve progression-free or overall survival.[1,5,12]

The 2026 approval of vusolimogene oderparepvec-wtpg plus nivolumab is clinically important but conditional. It provides a new option for a defined anti-PD-1-experienced melanoma population, while confirmatory randomized evidence is still pending.[2-4]

The most defensible conclusion is that in situ vaccination has neither failed nor been broadly validated. It has produced genuine systemic antitumor effects in selected settings. Its clinical value remains product-specific, population-specific, and endpoint-specific.

References

1. DailyMed. IMLYGIC (talimogene laherparepvec) injection, suspension: U.S. prescribing information. Revised November 2024; DailyMed updated April 4, 2025. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=64ffb680-ea8c-42fc-9649-9e8c0eb77ddb. Accessed August 12, 2026.
2. U.S. Food and Drug Administration. FDA grants accelerated approval to vusolimogene oderparepvec-wtpg with nivolumab for advanced melanoma. August 6, 2026. https://www.fda.gov/drugs/resources-information-approved-drugs/fda-grants-accelerated-approval-vusolimogene-oderparepvec-wtpg-combination-nivolumab-melanoma. Accessed August 12, 2026.
3. U.S. Food and Drug Administration. TUDRIQEV (vusolimogene oderparepvec-wtpg) suspension for intratumoral injection: prescribing information. Initial U.S. approval 2026. https://www.fda.gov/media/194127/download. Accessed August 12, 2026.
4. U.S. Food and Drug Administration. TUDRIQEV accelerated approval letter. August 6, 2026. https://www.fda.gov/media/194128/download?attachment=. Accessed August 12, 2026.
5. Chesney JA, Ribas A, Long GV, et al. Randomized, double-blind, placebo-controlled, global phase III trial of talimogene laherparepvec combined with pembrolizumab for advanced melanoma. J Clin Oncol. 2023;41(3):528-540. https://doi.org/10.1200/JCO.22.00343. PMID: 35998300. https://pubmed.ncbi.nlm.nih.gov/35998300/.
6. Ribas A, Dummer R, Puzanov I, et al. Oncolytic virotherapy promotes intratumoral T cell infiltration and improves anti-PD-1 immunotherapy. Cell. 2017;170(6):1109-1119.e10. https://doi.org/10.1016/j.cell.2017.08.027. PMID: 28886381. https://pubmed.ncbi.nlm.nih.gov/28886381/.
7. Andtbacka RHI, Ross M, Puzanov I, et al. Patterns of clinical response with talimogene laherparepvec in patients with melanoma. Ann Surg Oncol. 2016;23(13):4169-4177. https://doi.org/10.1245/s10434-016-5286-0. PMID: 27342831. https://pubmed.ncbi.nlm.nih.gov/27342831/.
8. Wong MK, Milhem MM, Sacco JJ, et al. RP1 combined with nivolumab in advanced anti-PD-1-failed melanoma (IGNYTE). J Clin Oncol. 2025;43(33):3589-3599. https://doi.org/10.1200/JCO-25-01346. PMID: 40627813. https://pubmed.ncbi.nlm.nih.gov/40627813/.
9. Andtbacka RHI, Kaufman HL, Collichio F, et al. Talimogene laherparepvec improves durable response rate in patients with advanced melanoma. J Clin Oncol. 2015;33(25):2780-2788. https://doi.org/10.1200/JCO.2014.58.3377. PMID: 26014293. https://pubmed.ncbi.nlm.nih.gov/26014293/.
10. Chesney J, Puzanov I, Collichio F, et al. Randomized, open-label phase II study evaluating the efficacy and safety of talimogene laherparepvec in combination with ipilimumab versus ipilimumab alone in patients with advanced, unresectable melanoma. J Clin Oncol. 2018;36(17):1658-1667. https://doi.org/10.1200/JCO.2017.73.7379. PMID: 28981385. https://pubmed.ncbi.nlm.nih.gov/28981385/.
11. Ribas A, Medina T, Kirkwood JM, et al. Overcoming PD-1 blockade resistance with CpG-A Toll-like receptor 9 agonist vidutolimod in patients with metastatic melanoma. Cancer Discov. 2021;11(12):2998-3007. https://doi.org/10.1158/2159-8290.CD-21-0425. PMID: 34326162. https://pubmed.ncbi.nlm.nih.gov/34326162/.
12. Diab A, Ascierto PA, Maio M, et al. Randomized, open-label, phase III study of tilsotolimod in combination with ipilimumab versus ipilimumab alone in patients with advanced refractory melanoma (ILLUMINATE-301). J Clin Oncol. 2025;43(15):1800-1809. https://doi.org/10.1200/JCO.24.00727. PMID: 40048691. https://pubmed.ncbi.nlm.nih.gov/40048691/.
13. Meric-Bernstam F, Sweis RF, Kasper S, et al. Combination of the STING agonist MIW815 (ADU-S100) and PD-1 inhibitor spartalizumab in advanced/metastatic solid tumors or lymphomas: an open-label, multicenter, phase Ib study. Clin Cancer Res. 2023;29(1):110-121. https://doi.org/10.1158/1078-0432.CCR-22-2235. PMID: 36282874. https://pubmed.ncbi.nlm.nih.gov/36282874/.
14. Algazi AP, Twitty CG, Tsai KK, et al. Phase II trial of IL-12 plasmid transfection and PD-1 blockade in immunologically quiescent melanoma. Clin Cancer Res. 2020;26(12):2827-2837. https://doi.org/10.1158/1078-0432.CCR-19-2217. PMID: 32376655. https://pubmed.ncbi.nlm.nih.gov/32376655/.
15. DailyMed. OPDIVO (nivolumab) injection: U.S. prescribing information. https://dailymed.nlm.nih.gov/dailymed/fda/
    fdaDrugXsl.cfm?setid=f570b9c4-6846-4de2-abfa-4d0a4ae4e394&type=display. Accessed August 12, 2026.