Integrating Immunotherapy and Targeted Therapy in Cancer Treatment: Mechanistic Insights and Clinical Implications

Abstract

Small-molecule targeted therapies have demonstrated outstanding potential in the clinic. These drugs are designed to minimize adverse effects by selectively attacking cancer cells while exerting minimal damage to normal cells. Although initial response to targeted therapies may be high, yielding positive response rates and often improving survival for an important percentage of patients, resistance often limits long-term effectiveness. On the other hand, immunotherapy has demonstrated durable results, yet for a limited number of patients. Growing evidence indicates that some targeted agents can modulate different components of the antitumor immune response. These include immune sensitization by inhibiting tumor cell-intrinsic immune evasion programs or enhancing antigenicity, as well as direct effects on immune effector and immunosuppressive cells. The combination of these two approaches, therefore, has the potential to result in synergistic and durable outcomes for patients. In this review, we focus on the latest advances on integrating immunotherapy with small-molecule targeted inhibitors. In particular, we discuss how specific oncogenic events differentially affect immune response, and the implications of these findings on the rational design of effective combinations of immunotherapy and targeted therapies.

Figure 1

Generation of an immune suppressive tumor microenvironment. (1) Regulatory T-cells (T_Regs) suppress immune response via direct cell contact and humoral mechanisms. T_Regs constitutively express CTLA-4, which binds to CD80 and CD86 on antigen-presenting cells, such as dendritic cells (DCs), leading to impaired DC maturation and blocking binding of CD80/CD86 to CD28 on conventional T-cells, thereby preventing co-stimulation and T-cell activation. Moreover, T_Regs can directly target effector T-cells (T_Eff) and natural killer (NK) cells for destruction by secreting cytotoxic granzymes and perforin. Secretion of inhibitory cytokines such as TGFβ, IL-10 and IL-35 further inhibit anti-tumor immune response. (2) Tumor-associated macrophages (TAMs) are a major component of the immune infiltrate in solid tumors. Chronic inflammation within the TME and production of IL-4 and IL-13 by T_H2 cells and IL-10 by T_Regs induce pro-tumorigenic macrophage polarization. In turn, TAMs exacerbate immune suppression by releasing cytokines such as IL-10 and TGFβ that suppress T_Eff and NK cells but stimulate T_Regs. Pro-tumorigenic TAMs also up-regulate metabolic enzymes such as IDO-1 and Arg-1, which can severely affect the composition of the immune infiltrate by competing for catabolism of nutrients. In addition, TAMs can directly inhibit T-cells by expressing immune checkpoint ligands PD-L1 and PD-L2. (3) Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells that accumulate in response to chronic inflammation and fail to differentiate into mature cells. MDSCs secrete significant levels of IL-10 and TGFβ, thereby inducing T_Reg accumulation and pro-tumorigenic macrophage polarization, while simultaneously inhibiting T_Eff and NK cells function and activation. Furthermore, MDSCs promote metabolic stress by dramatically depleting nutrients needed for T-cell function. (4) Oncogenic events can directly and indirectly inhibit immune response by multiple mechanisms: a) Numerous cytokines secreted by tumor cells, including TGFβ, IL-10 and the pro-angiogenic molecule VEGF, promote recruitment of immune suppressive cells. b) Down-regulation of pro-inflammatory chemokines, including CCL3, CCL4 and CCL5, and CXCR3 ligands such as CXCL9 and CXCL10 result in decreased DC and T-cell recruitment and impaired T-cell priming/activation. c) Expression of PD-L1 and direct inhibition of T-cell effector function by tumor cells has been observed in numerous cancer types. PD-L1 can be induced by multiple non-exclusive mechanisms, including by cytokines such as type I and II IFNs, TNFα and IL-10, and specific oncogenic events, including chromosomal amplification and up-regulation by oncogenic signaling. d) Decreased immunogenicity may result from defects in antigen presentation and/or defective response to IFNγ, which can occur due to genomic inactivation or downregulation of class I MHC and MHC-related molecules (e.g. B2M) or of genes related to the IFNγ pathway. e) Tumor cells also exert strong metabolic stress on the immune infiltrate by competing for nutrients and secreting byproducts that negatively affect immune effector function. (Reviewed on (1,4,6).)

Figure 2

Integration of clinical and animal studies in translational immuno-oncology. (1) Experimental design informed by clinical observations to maximize translational potential. (2) Development of animal models that recapitulate genetic abnormalities found in the clinic. In this regard, immunocompetent, syngeneic mouse models provide the current gold standard. Emerging technologies, such as mouse models engrafted with humanized immune systems can improve the clinical relevance of pre-clinical studies and maximize translational feasibility. (3) Mechanistic studies are a crucial component of modern tumor immunology research. Complementing classical gain- and loss-of-function experiments, powerful technologies such as single-cell RNA sequencing (scRNA-Seq), cytometry by time-of-flight (CyTOF) and highly multiplex tissue cyclic immunofluorescence (t-CyCIF) have greatly enhanced our ability to interrogate the nature and degree of interplay between tumor and immune cells. Future work will undoubtedly entail more robust integration of single-cell expression analyses with single-cell spatial relationships within tissues. (4-6) Iterative rounds of target prediction (4), pre-clinical evaluation (5) and refining hypothesis (6) are needed to identify promising targets of clinical relevance. (7) Results from pre-clinical studies are used to inform and support the design of clinical trials for promising combinations.

Figure 3

Tumor-intrinsic molecular mechanisms of immune suppression driven by specific oncogenic events. Selected examples are depicted based on mechanistic studies on animal models. Of note, co-occurring oncogenic events affect immune suppressive mechanisms, thus increasing immune heterogeneity between cancer cases. Additional mechanisms linked to each example have also been described but could not be included in this diagram due to space constraints. A, MYC has been shown to promote T-cell and natural killer (NK) cell exclusion, and infiltration of tumor-associated macrophages (TAMs), while also directly inhibiting T-cells and phagocytic macrophages via upregulation of PD-L1 and CD47. B, Mutant KRas has been shown to promote recruitment of myeloid-derived suppressor cells (MDSCs) to the TME through upregulation of CXCL3. C, Mutant EGFR has been shown to up-regulate PD-L1 in tumor cells and to induce recruitment of TAMs and MDSCs. D, Loss of PTEN is associated with increased production of immune suppressive cytokines, which promote the establishment of an immune suppressive tumor microenvironment (TME) and inhibit T-cell infiltration. E, β-Catenin has been shown to inhibit secretion of CCL4 by tumor cells, hence preventing activation of CD103+ dendritic cells (DCs) and subsequent cytotoxic T lymphocyte (CTL) activation. F, Loss of LKB1 down-regulates the STING pathway in tumor cells, thereby preventing release of type I IFNs in response to cytoplasmic double-stranded DNA (dsDNA), which would otherwise stimulate immune response. G, STAT3 signaling in tumor cells induces upregulation of multiple cytokines that contribute to the establishment of an immune suppressive TME by stimulating suppressive immune cells and inhibiting effector cells. H, Dysregulated NOTCH promotes an immune suppressive TME via multiple anti-inflammatory cytokines. I, FAK has been shown to stimulate regulatory T-cells (T_Regs) by upregulating numerous cytokines.

Figure 4

Immune modulation by small molecule targeted therapies. A, Targeted therapies have been shown to affect multiple aspects of cancer immunity, including inhibition of anti-inflammatory mechanisms and promotion of pro-inflammatory mechanisms, up-regulation of antigen presentation, and direct modulatory effects on immune cells. Some targeted agents have been designed to specifically target immune sub-populations. For example, PI3Kγ and CSFR1 inhibitors are used to deplete tumor-associated macrophages (TAMs), and CXCR1/2 inhibitors are used to inhibit myeloid-derived suppressor cells (MDSCs). Other drugs, such as BRAF, PI3K, FAK and KRAS^G12C inhibitors, were found to affect immune-related mechanisms in addition to their intended cytotoxic effect on tumor cells largely because oncogenic signaling from tumor cells modulates immune response. In the case of PARP inhibitors, enhanced immunogenicity seems to be a corollary of its primary effect on inducing irreparable DNA damage; however, engagement of a robust immune response is required for effective response. And in the case of CDK4/6 inhibitors, unexpected effects in tumor antigenicity as well as direct effects on immune suppressive and immune effector cells have been reported by independent research groups. B-D, Summary of immune modulatory effect of selected examples of targeted therapies currently under clinical investigation in combination with immune checkpoint blockade immunotherapy.