Promising reports of preclinical and early clinical data in 2016 are poised to further boost the development of rational combinations of OX40 agonists with checkpoint immunotherapies, surgical resection, radiotherapy, and even the potential for 3-drug cocktails.
The prospect of long-lasting responses and the hope of expanding its impact into unresponsive patient populations continue to make immuno-oncology a dynamic and exciting field. As our understanding of the antitumor immune response grows, so does the immunotherapeutic arsenal.
Beyond the CTLA-4 and PD-1 pathways, the targeting of which has proved most fruitful in the clinic, multiple signaling networks are known to be involved in regulating the activation of T cells, the principal effectors of the antitumor immune response. OX40 is among the most promising emerging checkpoints in development.
OX40 is a costimulatory receptor that binds to its only known ligand, OX40L, initiating cellular signaling events required for full activation of T cells following their recognition of a foreign antigen. Agonists that mimic the effect of OX40L can boost OX40 signaling and potentially overcome suppression of the antitumor immune response in patients with cancer.
The clinical development of OX40 agonists is not new, but their distinct and possibly complementary mechanism of action is helping to forge a new path in combination regimens. Clinical trials are testing OX40 agonists in combination with checkpoint immunotherapies, surgical resection, radiotherapy, and even the potential for 3-drug cocktails. Promising reports of preclinical and early clinical data in 2016 are poised to further boost the development of rational combinations.
Although there are several subsets with distinct roles, the main function of T cells is to coordinate cell-mediated immunity, including the antitumor immune response to malignant cells. Given their potent cytotoxic capabilities, T cells are tightly controlled through a multistep activation process involving a series of receptors expressed on their surface.
First, a T cell must become antigen primed; that is, foreign antigens are presented to the T cell and recognized by its T-cell receptor. Full T-cell activation is then achieved by subsequent costimulatory signals that are generated by the interaction between numerous different receptors on the T-cell surface and their ligands on the antigen presenting cell (APC), facilitated by the formation of close contact between the 2 cells through an immune synapse.
The best-characterized costimulatory receptor is CD28, which is always found on the surface of the T cell and ready to be activated by its ligands, CD80 and CD86, which are located on the APC. Other costimulatory receptors have been identified, including a group that are not automatically expressed but are upregulated following antigen priming. These receptors are thought to provide additional costimulatory signals that are necessary for a long-lasting immune response and for generating immune memory. It is in this group that we find the OX40 protein, also known as CD134.
OX40 is found on the T-cell surface 24 to 48 hours after antigen priming. Like many of the other costimulatory and coinhibitory molecules (the group of receptors that perform the opposing role by deactivating T cells, which includes CTLA-4 and PD-1), OX40 is a member of the tumor necrosis factor (TNF) superfamily of proteins.
Binding of OX40 by OX40L, which is found on the surface of activated APCs, triggers the OX40 signaling pathway. OX40 itself does not have any enzymatic activity; upon activation, it associates with a number of adaptor proteins, including the TNF receptor—associated factors 2, 3, and 5 that activate downstream signaling pathways involving nuclear factor kappa B and c-Jun N-terminal kinase. Ultimately, OX40 signals culminate in enhanced T-cell activation, prolonged T-cell survival, generation of a memory response, prevention of T-cell tolerance, and possibly reduction of the immunosuppressive activity of regulatory T cells.In the absence of any of these activating signals, T cells do not function properly; they fail to proliferate and often become unresponsive, a condition called T-cell anergy, or die. Cancer cells often take advantage of this by increasing the expression of coinhibitory molecules and/or reducing the expression of costimulatory molecules. The cancer cells are thus enabled to co-opt these pathways to subvert the antitumor T cell-mediated immune response.
Several studies have examined the expression of OX40 on T cells that infiltrate the tumor. Although OX40 has been shown to be present on these cells in numerous cancer types, suggesting the T cells have become primed in response to tumor-associated antigens, the expression of OX40L within the tumor is typically low, so the T cells are unlikely to become fully activated. Interestingly, the highest expression of OX40 seems to be on tumor-infiltrating regulatory T cells, which have an immunosuppressive function.
An understanding of these signaling pathways and their dysregulation in cancer has led to the development of monoclonal antibodies that antagonize the coinhibitory PD-1 receptor or its ligand, PD-L1, to “take the brakes” off the immune system.
Turning their attention to costimulatory molecules, researchers have developed a different kind of drug. Agonists can mimic the activity of the ligand, binding to the receptor and triggering downstream signaling. Although the net effect is expected to be the same—the antitumor immune response is boosted— the agonists give the T cells a “go” signal, hitting the gas rather than tampering with the brakes.OX40 agonists have been in development since 2006, predating the approval of the first immune checkpoint inhibitor, the CTLA-4—targeting ipilimumab (Yervoy), by 5 years. The first drug developed was a mouse anti-human monoclonal antibody.
In a phase I clinical trial, intravenous administration of the antibody at 3 different doses (0.1 mg/kg, 0.4 mg/kg, or 2 mg/kg) in 30 patients with metastatic cancer was well tolerated. Although it had a potent immune-stimulating effect that was accompanied by the regression of at least 1 metastatic lesion in 30% of patients, the agent failed to demonstrate an objective response by Response Evaluation Criteria in Solid Tumors (RECIST). Its efficacy was thought to be hindered in part by the generation of anti-mouse antibodies in patients, which limited repeat dosing.
Seven OX40 agonists are now in development (Table), 6 of which take the form of fully human monoclonal antibodies to address the mouse antibody issue. One OX40L-Fc fusion protein, MEDI6383, is also undergoing clinical evaluation; this links 2 OX40L molecules to part of the fragment crystallizable (Fc) region of immunoglobulin. In preclinical testing, the fusion protein appears to have stronger effects than OX40 antibodies, possibly because it may also activate dendritic cells and vascular endothelial cells in addition to T cells.Many studies have examined how OX40 agonists work. They likely have numerous mechanisms of action that have yet to be fully elucidated. What is known from both preclinical and clinical studies is that they improve the effector functions of several different subsets of T cells, predominantly CD4-positive and CD8-positive populations. OX40 agonists do this by increasing both the proliferation and survival of these cells and by increasing cytokine production.
Preclinical studies have also shown that OX40 agonists might orchestrate their anticancer activity by depleting the number of T-regulatory cells, which, as mentioned previously, express high levels of OX40.
The distinct and potentially complementary ways in which OX40 agonists appear to kill tumor cells is proving to have important implications for their optimal use. Although their antitumor activity as single agents in clinical trials has been limited, they may be ideally suited to combination therapy with a variety of other anticancer strategies.There is a significant rationale for combining OX40 agonists with checkpoint inhibitors and other immunotherapies, as well as substantial preclinical evidence of synergistic activity for these combinations. In a mouse model of ovarian cancer, for example, the combination of an OX40 agonist and a PD-L1 inhibitor resulted in a statistically significant improvement in survival compared with either drug administered alone.
Several clinical trials involving these combinations are ongoing. The results of a phase I, open-label, multicenter trial (NCT02410512) of the PD-L1 inhibitor atezolizumab (Tecentriq) in combination with the OX40 antibody MOXR0916 were reported at the 2016 American Society of Clinical Oncology Annual Meeting.
Patients with locally advanced or metastatic solid tumors were treated with escalating doses of MOXR0916 in combination with a fixed 1200-mg dose of atezolizumab every 3 weeks. A total of 51 patients were enrolled, most having non—small cell lung cancer (NSCLC), renal cell carcinoma (RCC), ovarian cancer, gastroesophageal cancer, and soft tissue sarcoma. The combination regimen was well tolerated; the most common adverse events (AEs) were nausea, pyrexia, fatigue, rash, and chills, with only a single occurrence of a serious AE that was resolved upon treatment.
At data cut-off, there were 2 partial responses: 1 in a patient with urothelial carcinoma and 1 with RCC. Efficacy analyses in expansion cohorts of patients with melanoma, RCC, NSCLC, urothelial carcinoma, and triple-negative breast cancer are ongoing, using a MOXR0916 dose of 300 mg every 3 weeks.
One of the limitations of PD-1/PD-L1—targeted therapies is that they are not particularly effective against tumors that do not provoke much of an immune response. Combining them with OX40 agonists that promote the effector functions of tumor-infiltrating T cells could help to overcome this, especially since the subsets of T cells that it impacts are those that also typically express the PD-1 receptor. The 2 types of drugs also may complement each other because OX40 signaling enhances interferon gamma production by T cells and cancer cells have been shown to increase the expression of PD-L1 in response to this cytokine.
Preclinical data also suggest that OX40 agonists could be combined with other types of therapy. When used with surgical resection, OX40 agonists could help to boost the antitumor immune response, which could clear any residual cancer cells. Radiation and chemotherapy have the potential to provoke an antitumor immune response by releasing tumor-associated antigens and other immune-related molecules, which could enhance the effects of OX40 agonists.
There is also the potential for 3-drug combinations, since OX40 agonists appear to have a favorable safety profile. Given the efficacy already demonstrated by combinations of checkpoint inhibitors with distinct mechanisms of action, the addition of OX40-targeting drugs with yet another unique mechanism of action is of particular interest.
Pfizer has announced plans to collaborate with the National Cancer Institute and Merck KGaA on a clinical trial of triplet immunotherapy. The study would test the combination of Pfizer’s monoclonal antibodies targeting OX40 (PF-04518600) and 4-1BB (PF-05082566) with Merck KGaA’s avelumab, a PD-L1 inhibitor.
Meanwhile, the results of a preclinical trial examining the combination of a CTLA-4 inhibitor, a 4-1BB agonist, and an OX40 agonist were recently reported at the 2016 American Society of Hematology Annual Meeting. When administered as a cocktail via an intratumoral injection into a mouse lymphoma and colon cancer model, the combination demonstrated significant antitumor effects; tumors were cleared in between 70% and 100% of mice and survival was significantly extended.