A locoregional approach to cancer vaccines, when combined with immunmodulatory antibody therapy, could generate a significant anti-tumor response.
Jacob Ricca, BS
Cancer vaccines comprise a multitude of approaches. The major goal of any approach is to amplify or generate an adaptive immune response against specific tumor antigens. One of the major hurdles of cancer vaccine development is the challenge of identifying an optimal target antigen that could lead to therapeutic immune response in each patient. In situ vaccination approaches using a variety of intratumoral treatment strategies attempt to circumvent this problem by using the patient’s own tumor as a vaccine. These locoregional approaches operate in 2 ways: through localized tumor lysis that releases tumor antigens and danger signals, and through provision of appropriate immunostimulatory signals to activate tumor recognition by the immune system. When combined with systemic immunomodulatory antibody therapy, these strategies present an opportunity to generate and amplify systemic anti-tumor immune response, leading to both local and abscopal therapeutic effects.
Activation of antitumor immune response involves a close interaction between the innate and the adaptive immune systems. Recognition of cancer by the immune system begins with processing and presentation of tumor antigens (TAs) by specialized antigen-presenting cells (APCs), such as dendritic cells (DCs). Antigenic peptides are processed and presented on the surface of APCs within the context of major histocompatibility complex (MHC). The MHC-peptide complex is recognized and bound by a thymus (T) cell expressing a unique T-cell receptor. In the environment of appropriate additional immune costimulatory signals and cytokines, this leads to T-cell activation, proliferation, and differentiation, with eventual migration to the tumor site where the T cells mediate the killing of the tumor cells expressing the specific antigen.
Anton Oseledchyk, MD
Cancer vaccines comprise a multitude of approaches. The major goal of any approach is to amplify or generate an adaptive immune response against specific tumor antigens.1Traditionally, cancer vaccines have involved immunization with a specific tumor antigen, typically a shared tumor antigen that is expressed by tumors of different patients (eg, NY-ESO-1, MAGE A3).1Such approaches proved challenging, as development of anti-vaccine immune response rarely led to clinical benefit, suggesting that perhaps these vaccine targets are not optimal or that other immune pathways need to be targeted in combination. With advances in sequencing and bioinformatic tools, the latest studies have extended these strategies to personalized vaccination approaches using tumor-specific antigens (ie, neoantigens) resulting from DNA mutations, deletions, insertions, and translocations in cancer cells. Studies are currently ongoing to determine whether these approaches would be more successful.2
A major limitation of the traditional vaccination approaches remains the uncertainty of adequate prediction of the optimal antigens that would be appropriately recognized on tumors by T cells. In situ vaccination approaches include locoregional treatment strategies that strive to use the patient’s own tumor as a vaccine through the release of tumor antigens and the provision of appropriate immune signals to activate tumor recognition by the immune system. Such strategies are agnostic to specific TAs and instead rely on the immune system to choose the most immunogenic antigens as targets. These approaches span a multitude of approved and experimental techniques, most of which are summarized in this review.
Dmitriy Zamarin, MD, PhD
Types of In Situ Vaccinations
In addition to direct cytotoxic effects on tumor cells, radiation has been extensively studied for its ability to activate immune response. While direct killing of tumor cells results in tumor antigen release, radiation also leads to upregulation and release of additional inflammatory mediators, collectively known as damage-associated molecular patterns (DAMPs), which include upregulation of cell surface calreticulin and heat shock proteins, and release of intracellular contents such as adenosine triphosphate and high mobility group box 1 protein. In addition, localized cell death results in expression of pro-inflammatory cytokines and chemokines. These components serve as maturation signals for APCs and can thus promote presentation of TAs to the immune system and production of pro-inflammatory cytokines and chemokines.3The overall process has been named immunogenic cell death (ICD) and is thought to be responsible for occasional regression of distant lesions in patients receiving radiotherapy to a single lesion, a process known as the abscopal effect.3Localized radiotherapy alone, however, is rarely enough for effective abscopal clearance.4
Other ablative therapies
Similar to radiation therapy, other ablative therapies, such as radiofrequency ablation (RFA), high intensity-focused ultrasound (HIFU), and cryoablation, can be used as in situ vaccines.5RFA causes tumor necrosis through localized heating, inducing inflammation and circulation of tumor-specific immune cells. HIFU has been shown to increase cytotoxic T-cell response, cause tumor embolization, and induce circulating CD4+ T cells.5Cryoablation causes tumor necrosis through intracellular crystal formation and microvascular injury,6which also leads to ICD and generation of antitumor immune response.
The effects of chemotherapy on the immune system are very heterogeneous, as systemic chemotherapy has both negative and positive influence on systemic immune cells. Although chemotherapy is, in general, associated with immunosuppression, multiple studies have demonstrated positive effects of chemotherapy on the immune system. Systemically, such effects include preferential depletion of specific immunosuppressive cell subsets, such as regulatory T cells and myeloid-derived suppressor cells.7On the tumor level, chemotherapy acts as an in situ vaccine through induction of ICD.7It is important to note that different chemotherapy agents can lead to different immune effects and that not all agents have been associated with ICD. For example, oxaliplatin, but not cisplatin, has been demonstrated to stimulate antitumor immune response in mouse tumor models.8Like radiotherapy and many others on this list, immunotherapeutic efficacy of chemotherapy is limited when used alone.
Advances in understanding of tumor immune recognition and escape have led to identification of multiple mechanisms governing T-cell activation and inhibition. Such mechanisms include a range of immune stimulatory (eg, a co-stimulatory transmembrane glycoprotein receptor on activated T cells [OX40], glucocorticoid-induced tumor necrosis factor receptor related gene [GITR], CD40, CD137), and inducible co-stimulator (ICOS) and inhibitory receptors (eg, cytotoxic T lymphocyte antigen 4 [CTLA-4], programmed cell death protein 1 [PD-1], lymphocyte-activation gene 3 [LAG-3], T-cell immunoglobulin and mucin protein-3 [TIM- 3]), which are currently being explored in clinical trials using immunomodulatory antibodies targeting these receptors alone or in combination. Agents targeting PD-1 and its ligand programmed death-ligand 1 (PD-L1) are the most advanced in clinical development and have already been approved for a variety of indications.9Some of these agents, in particular CTLA-4 blocking antibodies and CD40 agonists, can be associated with significant toxicities with systemic administration. Several studies have demonstrated that intratumoral administration of these agents, such as anti-CTLA-4, anti-OX40, and anti-CD40, can be successful in regressing distant tumors in animal models through induction of systemic antitumor immunity.10-12Such therapies given intratumorally can thus be considered in situ vaccines and have a potential to offer the benefit of immunomodulatory antibody therapy while mitigating systemic toxicity of these drugs. Early data in patients with melanoma combining intratumoral interleukin-2 (IL-2) and the anti-CTLA-4 antibody ipilimumab demonstrated good safety and signal of efficacy.13
Intratumoral injections of cytokines involved in immune modulation can induce in situ vaccination effects. Several cytokines have been explored in this setting, with IL-2, IL-12, and granulocyte- macrophage colony-stimulating factor (GM-CSF) as examples that have undergone extensive clinical testing. IL-2 is normally produced by T cells and controls the expansion and differentiation of T cells. It is approved for systemic treatment of metastatic melanoma and renal cell cancer, although its administration is limited by significant systemic toxicity. Intratumoral administration of IL-2 has shown significant activity in preclinical models, and several studies with intratumoral IL-2 using different delivery methods are currently ongoing.12IL-12 is produced by myeloid APCs and promotes T cell and natural killer (NK) cell activation, which has been demonstrated to result in significant antitumor activity in pre-clinical models.
While systemic treatment with the cytokine had significant toxicities, intratumoral administration of IL-12 using different vector strategies has demonstrated promising activity in preclinical and clinical studies.14GM-CSF is normally produced by a variety of immune cells, including macrophages, T cells, and NK cells, as well as non-immune cells such as endothelial cells and broblasts. GM-CSF controls DC maturation and function and has been explored for anti-cancer therapy. Intratumoral injection of GM-CSF has been investigated in several trials in melanoma with promising activity in early studies.15,16
TLR agonists and STING agonists
Toll-like receptors (TLRs) are pattern recognition receptors (PRRs) found on leukocytes and APCs that bind to danger signals, such as pathogen-asso- ciated molecular patterns and DAMPs, resulting in APC activation, enhanced antigen presentation, and initiation of immune responses.4Intratumoral TLR agonists targeting different TLRs have been explored in many preclinical and clinical studies.12The TLR7 agonist, imiquimod, is approved for treatment of genital warts and basal cell carcinoma.17Intratumoral TLR9 agonist, in combination with radiation, demonstrated promising activity against injected and distant lesions in lymphoma.18The stimulator of interferon genes (STING) pathway is responsible for expression of interferon-β by tumor-resident DCs, which is required for spontaneous tumor-initiated T-cell priming. Intratumoral injection of STING agonists, such as 5,6-dimethylxanthenone-4-acetic acid (DMXAA), has yielded promising results in pre-clinical studies, providing tumor regression and long-lived immunologic memory.19
Induced bacterial infection has been used since 1891, when Dr William Coley injected tumors with live Streptococcus pyogenes cultures after linking spontaneous tumor regression with concurrent bacterial infection.20Activation of PRRs by bacterial components induces activation and recruitment of innate and adaptive immune responses that can result in tumor recognition and systemic antitumor immunity. To this end, Bacillus Calmette-Guérin is approved for treatment of nonmuscle-invasive bladder cancer21and has been explored for intralesional therapy of other cancers. Several bacterial agents have been explored for intratumoral therapy in preclinical models.22Intralesional administration of Clostridia novyi spores has demonstrated anti-tumor response and regression in patients, partly mediated by cytotoxic T cells and DC activation.23
Oncolytic viruses selectively infect and lyse tumor cells, releasing TAs and danger signals, eliciting a strong innate immune response and maturation of APCs. Additionally, engineering of oncolytic viruses to express genes for cytokines or TAs can further increase their efficacy. In a phase III trial, talimogene laherparepvec (T-VEC), an engineered herpes virus expressing GM-CSF, was shown to be superior to GM-CSF alone in advanced melanoma patients, with objective response rates (ORRs) of 26% versus 6% and durable response rates of more than 6 months in 16% of patients.24CAVATAK is a nonen- gineered cocksackievirus A21, the activity of which relies on potent activation of innate immune responses in the infected tumors. In a phase II trial in 57 patients with stage IIIC-IV melanoma who were treated intralesionally with weekly CAVATAK, a 28% ORR and a 1-year OS of 75% was observed,25with abscopal effects among the responders.26Promising results with DNX-2401, an engineered adenovirus, have also been achieved in patients with glioblastoma. A single intratumoral injection of DNX-2401 resulted in median survival of 11 months and durable complete responses (CRs) of more than 1.5 years in 3 of 37 patients.27
Synergy of Immunomodulatory Antibodies and in situ Vaccines
Recent studies have demonstrated that the efficacy of immunomodulatory antibodies, such as the ones targeting PD-1 and CTLA-4, can be potentiated when combined with in situ vaccines. In preclinical models, combination with oncolytic viruses elicited responses in immunologically silent tumors.28,29In patients with advanced melanoma, the combination of ipilimumab (antiCTLA-4) and T-VEC showed a response rate of 50%, while being well tolerated.30Concomitant intratumoral application of ipilimumab and IL-2 resulted in abscopal effects in 8 of 9 patients evaulated.13When ipilimumab was combined with cryoablation, it increased cytokine levels and the amount of intratumoral CD4+ effector cells in stage I-II breast cancer, which has inherently low immune infiltration.31Combinations of immune checkpoint blockade and radiotherapy are currently subject to approximately 70 open trials and were recently reviewed.32Combination of pembrolizumab (antiPD-1) with T-VEC in patients with unresectable stage IIIB-IV melanoma led to 48% ORR and 14% complete response (CR) as well as an acceptable toxicity profile.33Pembrolizumab in combination with CAVATAK is currently being tested in a phase I/III expansion trial in patients with advanced melanoma (NCT02565992) and in a phase I trial including other advanced tumor types (NCT02043665).
Advantages and Limitations
In situ vaccines present several obvious advantages over traditional vaccines. First, they present an “off the shelf” approach to personalized vaccination, obviating the need for identification and synthesis of patient-specific antigenic targets. Second, such vaccination has a potential to generate an immune response against a broader antigen repertoire, thus decreasing the possibility of emergence of resistant clones that lose antigen expression. Third, the intratumoral delivery route allows for the use of agents that would otherwise be toxic if given systemically. An obvious limitation of in situ vaccination is a potential challenge with tumor accessibility for intratumoral administration, especially with repetitive dosing protocols. With advances in modern interventional radiology, endoscopy, and surgical techniques, however, such studies are becoming more feasible. Fong and colleagues have demon- strated that oncolytic herpes virus can be delivered to liver metastases via hepatic arterial infusion.34Infusion through intraperitoneal or intrapleural catheters can be used for patients with peritoneal and pleural disease, respectively. Direct intratumoral injection through catheter implantation has been shown to be possible in glioma.27
With improved understanding of tumor cells and the immune microenviroment came the recognition that therapies targeting tumor and immune cells directly within the patient can, in essence, generate cancer vaccines in vivo. Such approaches encompass a range of experimental and local therapeutic techniques, some of which have traditionally not been thought of as immune therapies. With provision of appropriate local immunostimulatory signals, such in situ vaccination approaches can enhance tumor recognition by the innate and the adaptive immune systems and present an opportunity to generate an immune response against a broad repertoire of TAs. When combined with systemic immunomodulatory antibody therapy, in situ vaccination presents a powerful strategy to generate a strong systemic anti-tumor immune response and to extend the benefit of immunotherapy to a broader range of patients and cancer types.