Combining immunotherapy and targeted therapies in cancer treatment

Abstract

During the past two decades, the paradigm for cancer treatment has evolved from relatively nonspecific cytotoxic agents to selective, mechanism-based therapeutics. Cancer chemotherapies were initially identified through screens for compounds that killed rapidly dividing cells. These drugs remain the backbone of current treatment, but they are limited by a narrow therapeutic index, significant toxicities and frequently acquired resistance. More recently, an improved understanding of cancer pathogenesis has given rise to new treatment options, including targeted agents and cancer immunotherapy. Targeted approaches aim to inhibit molecular pathways that are crucial for tumour growth and maintenance; whereas, immunotherapy endeavours to stimulate a host immune response that effectuates long-lived tumour destruction. Targeted therapies and cytotoxic agents also modulate immune responses, which raises the possibility that these treatment strategies might be effectively combined with immunotherapy to improve clinical outcomes.

Figure 1. The generation of potent anti-tumor immune responses requires multiple steps

a ǀ Dendritic cell (DC) priming can be accomplished in multiple ways. One example involves the injection of irradiated, cytokine secreting whole tumor cells, after which DCs phagocytose the tumor cells and present tumor antigens in vivo to T cells. In another strategy, DCs can be loaded with tumor cells and matured ex vivo, then re-infused back to the patient. Additionally, tumor cells killed by chemotherapy or targeted therapies may facilitate DC priming and activation in situ. All of these approaches promote in vivo DC priming of tumor specific T cells in a pro-inflammatory context. b ǀ T cell function may be enhanced through the delivery of exogenous costimulatory signals and blocking co-inhibitory signals. Agonistic antibodies to 4-1BB and glucocorticoid induced TNFR related protein (GITR) cross-link these activating receptors and bolster T cell effector responses, while blocking antibodies to the co-inhibitory receptors cytotoxic T lymphocyte antigen 4 (CTLA4) and programmed death 1 (PD1) preclude transduction of negative signals and prevent T cell shutdown and anergy. Both strategies result in boosted T cell effector function, including cytokine release and cytotoxicity. c ǀ Immunosuppressive Tregs and myeloid-derived suppressor cells (MDSCs) secrete numerous tolerogenic cytokines such as interleukin-10 (IL-10) and transforming growth factor-β (TGFβ), inhibiting anti-tumor immune responses. Strategies to inhibit immunosuppressive cytokine secretion or kill Tregs and MDSCs diminish immune suppression and promote T cell mediated tumor destruction.

Figure 2. Targeted agents may boost DC priming and the activities of tumor specific T cells

a ǀ Multiple targeted agents affect dendritic cell (DC)-mediated priming of T cells. Monoclonal antibodies coat tumor cells and promote phagocytosis through Fc receptors, increasing DC presentation of tumor antigens. Fc receptor-mediated opsonization also enhances expression of costimulatory molecules such as CD40, B7-1 (CD80), and B7-2 (CD86) on the DC surface, promoting T cell activation. Janus kinase 2 (JAK2) inhibitors inhibit signal transducer and activator of transcription 3 (STAT3), an immunosuppressive pathway, boosting expression of co-stimulatory molecules. mTOR and glycogen synthase kinase 3β (GSK3β) inhibitors drive T cells towards long-lived memory T cell phenotypes that generate a large pool of enduring anti-tumor T cells, even after completion of immunotherapies. b ǀAgents that increase anti-tumor activity of T cells are shown. Numerous therapies increase the expression of tumor antigens on the tumor cell surface, increasing T cell receptor signaling and T cell activation. Inhibitor of apoptosis protein (IAP) inhibitors reinforce T cell signaling and provide exogenous costimulatory signals, increasing production of inflammatory cytokines. PI3K inhibitors eliminate some pro-survival signals, increasing tumor cell lysis to perforin and granzymes released from cytotoxic T lymphocytes (CTLs). Therapies also increase expression of NKG2D ligands, which serve as additional costimulatory molecules for CTLs as well as activators of natural killer (NK) cells, which also kill tumor targets through the perforin-granzyme pathway. EGFR, epidermal growth factor receptor; HDAC, histone deacetylase; HSP90, heat shock protein 90; MHC I, major histocompatibility class I.

Figure 3. Targeted agents may antagonize immunosuppression in the tumor microenvironment

Multiple factors within tumors promote immune tolerance and curb the anti-tumor immune response. Tumor cells secrete vascular endothelial growth factor A (VEGFA), and VEGF signaling decreases dendritic cell (DC) costimulatory molecule expression and T cell priming, and encourages the formation of myeloid derived suppressor cells (MDSCs). VEGF antagonists, either as a monoclonal antibody (mAb; such as bevacizumab) or small molecule inhibitors (for example, sunitinib) reverse these deleterious effects and promote formation of potent anti-tumor T cells. Tumor cells also produce inflammatory mediators that promote tumorigenesis as well as encourage suppressor cell formation; these can be inhibited with PI3K and BRAF inhibitors respectively. Tregs and MDSCs are two immunosuppressive cell types that dampen immune responses. Tregs secrete immunosuppressive cytokines, whereas MDSCs use indoleamine-pyrrole 2,3-dioxygenase (IDO) to deplete tryptophan and kill effector T cells. Sunitinib and imatinib both decrease the number and effectiveness of these suppressor cell types. Imatinib also directly inhibits IDO, decreasing MDSC suppressive capacity. Sunitinib and janus kinase 2 (JAK2) inhibitors also block the signal transducer and activator of transcription 3 (STAT3) pathway, an immunosuppressive pathway favoring differentiation into regulatory cells and tumor growth. Decreasing STAT3 signaling diminishes formation of regulatory T cells and promotes the formation of effector Th1 T cells secreting interferon-γ (IFNγ).

Figure 4. Critical variables in combining targeted agents and immunotherapy

Immune responses against tumors occur in a step-wise fashion. First, dendritic cells (DCs) must capture tumor antigens and present them to naïve T cells under inflammatory conditions. Naïve T cells then differentiate into effector T cells, which may take up to one week, prior to leaving the lymph node and entering the blood. At this time, some T cells further differentiate into long-lived memory T cells, providing a pool of renewable anti-tumor T cells for an extended period after immunotherapy has ceased. Once in the periphery, tumor cells activate T cells, causing them to secrete inflammatory cytokines and/or cytotoxic granules. Throughout this process, T cells must overcome tumor-derived immunosuppression from myeloid-derived suppressor cells (MDSCs), Tregs, and tumor cell-secreted suppressive molecules. Drugs modulating each of these areas should be delivered just prior to and during the respective critical steps of immune maturation. Therapies boosting DC antigen presentation and initial T cell priming should be delivered prior to vaccines and continued through initial T cell priming. Agents promoting T cell memory formation should be giving during T cell priming and then discontinued after differentiation is complete in order to avoid deleterious effects on effector function. Therapies enhancing T cell function and tumor cell lysis should be given after T cell priming is complete and continued throughout treatment to maximize effector function. Tumor derived immunosuppression constantly antagonizes anti-tumor immune responses; accordingly, therapies designed to mitigate this should be given prior to vaccination and continued throughout treatment. 5-FU, 5-fluorouracil; CTL, cytotoxic T lymphocyte; GSK3β, glycogen synthase kinase 3β; HDAC, histone deacetylase; HSP90, heat shock protein 90; IAP, inhibitor of apoptosis protein; JAK2, janus kinase 2; mABs, monoclonal antibodies.