How the Cancer Environment May Influence Resistance to Immune Therapy

Inflammation is a hallmark of cancer and is associated with the growth and progression of cancer cells. Cancer cells, peripheral stromal and inflammatory cells can together form an inflammatory TME, which in turn promotes proliferation, progression and metastasis of cancer cells [11]. It has been reported that inflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) in the TME are able to enhance PD-L1 expression in cancer cells [23,24], contributing to escape from T cell immune surveillance. Tumor-associated macrophages (TAM) are abundant in the TME, and may take part in regulating inflammation. It has been shown that inflammatory cytokines IL-23 produced by TAMs can induce inflammation and promote tumor growth and progression in a colorectal cancer mouse model [25]. Additionally, macrophage-derived TNF-α can augment the PD-L1 expression in cancer cells [24], thus impeding T cells anti-tumor functions. The presence of IL-6 in the TME enhances the glycolysis activities in cancer associated fibroblasts (CAF), enabling them to generate metabolic intermediates to support cancer cells [26,27]. Similarly, IL-6 also induces glycolytic enzyme expression in cancer cells via signal transducers and activators of transcription 3 (STAT3)/c-Myc signaling [28], and correlated with tumorigenesis, which together suggests that inflammation can induce metabolic reprogramming and promote cancer progression (Figure 1).

Tumor cells consume large quantities of oxygen due to their high rate of proliferation; this leads to the formation of hypoxic regions. Hypoxia is a hallmark of TME and a common characteristic of solid tumors [29], where tumor cells proliferate rapidly to build up solid tumor masses, and accompanied with abnormal formation of blood vessels that may not function properly to supply oxygen into the tumor masses [30]. Tumor cells can adapt to these hypoxic regions by activating HIF-1 transcription factor, enabling tumor cells to shift their metabolic profile from oxidative phosphorylation (OXPHOS) to glycolysis [30]. HIF-1, a crucial factor in regulating angiogenic factors such as vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptor (VEGFR), is overexpressed in several cancers and correlated with metastasis and poor prognosis [31]. The expression of HIF-2α is significantly enhanced in several cancers [32,33,34], and can increase cancer stem-like properties by activating Wnt and Notch pathways and stem cell related markers such as c-Myc, octamer-binding transcription factor 4 (Oct4) and Nanog [35]. Interestingly, HIF-2α is largely expressed and regulates Oct4 in embryonic development stage, which controls division, differentiation and function of stem cells [36]. In line with this, the HIF-2/Oct4 axis has been identified in regulating stemness of embryonic germ stem cells [37,38]. Additionally, HIF-2α can promote hypoxic cancer cells to proliferate via boosting c-Myc transcriptional activity and cell-cycle progression, and together promote proliferation and tumorigenesis [39]. Moreover, HIF-2α has also been reported to elevate the expression Oct4 and Sox2, contributing to stemness and invasiveness characteristics [40].

Under hypoxic conditions, the decreased rate of proteolytic degradation causes HIF-1α to accumulate. Elevated expression of HIF-1α in hypoxic regions may impair the ability of natural killer (NK) cells to upregulate surface activating receptors such as NKp46, NKp30, NKp44, and NKG2D [41]. Depletion of HIF-1α decreases the cytotoxicity in NK cells, but significantly delays tumor growth via stimulating non-productive angiogenesis [42]. The augmented production of adenosine through ectonucleotidases CD39 and CD73 in hypoxic region impedes NK cells cytotoxic activity and cytokine production [43,44], and activation of adenosine A_2A receptor (A_2AR) in NK cells suppress their maturation and proliferation in the TME [45]. Additionally, cancer cells can activate autophagy under hypoxia, which degrades NK-derived granzyme B, thereby impeding NK-mediated tumor lysis [46,47].

In hypoxic regions, TAMs are shaped into tumor promoting phenotype, and upregulate platelet-derived growth factor (PDGF) and VEGF to support the growth of cancer cells [48]. It has been reported that TAMs reside in hypoxic tumor regions and significantly upregulate the expression of regulated in development and DNA damage responses 1 (REDD1), which inhibits mammalian target of rapamycin (mTOR) activity [13]. The inhibited mTOR may further impede glycolysis in TAMs, leading to the formation of abnormal blood vessels and facilitating metastasis [13]. TAMs may also facilitate tumor hypoxia by competing for available oxygen in the TME, impeding T cell infiltration [49].

Limited oxygen in hypoxic regions may lower expression levels of major histocompatibility complex II (MHC-II), CD80 and CD86, as well as levels of proinflammatory cytokines such as IL-1β, IL-6, and TNF-α in dendritic cells (DC), thus impairing the maturation and functions of DCs [50]. Additionally, differentiation of DCs under hypoxic conditions may impede their antigen uptake and alter their chemokine expression [51], which may further affect DCs ability for triggering immune responses.

A recent study has reported that hypoxia in the TME induces T cell mitochondria dysfunction, which decreases the production of ATP and mitochondrial OXPHOS activities, leading to T cell exhaustion [14]. T cell exhaustion may be associated with the downregulation of mitochondrial fusion protein mitofusin 1 and upregulation of miR-24 [14]. Additionally, the ability to proliferate and to produce interferon-gamma (IFN-γ) is attenuated in CD8⁺ T cells under hypoxic conditions [52]. Reintroduction of oxygen can restore the cytokine producing capacity of T cells [52]. It has been reported that under the hypoxic region, HIF-2α downregulates Fas-ligand expression and induce the expression of A_2AR in natural killer T cells, leading to immunosuppression [53]. In addition, studies in a colitis-associated colon cancer mouse model have demonstrated that hypoxic conditions reduce differentiation of CD4⁺ effector T cells, while elevating the number and activity of regulatory T cells (T_regs) [54]. These findings suggest that a hypoxic TME may have detrimental effects on effector T cells, and may attenuate anti-tumor responses (Figure 1).

Increased production of lactic acid by cancer cells can cause acidification of the TME since lactate and H⁺ are transported outward by monocarboxylate transporter-4 (MCT-4) [55]. The decreased pH in acidic TMEs promotes tumor growth and metastasis [56]. Recently, lactate has been suggested to be a key player in cancer; it has been associated with the development of malignancies, immune escape, and regulation of cytokine release [57] (Figure 1).

It has been shown that when co-cultured with MCF7 breast cancer cells, CAFs are induced to express MCT-4 [58]. The increased glycolysis in CAFs may produce excessive lactate that is transported via MCT-4 leading to acidification of the TME.

Interestingly, a report by Colegio et al., sheds some light on the effect of lactic acid on TAMs. They show that cancer-derived lactic acid induces the expression of VEGF in TAMs, and skews TAMs toward a M2-like phenotype [59]. They also demonstrate that lactate upregulates the expression of arginase 1 (ARG1) in TAMs [59]. ARG1 plays a critical role in tumor promotion, as the ARG1-dependent pathway is responsible for generating cell proliferating substrates. It was also shown that extracellular acidosis can promote macrophage polarization toward a tumor-promoting phenotype in a prostate cancer model [60]. Neutralizing the tumor-secreted acids could reduce the pro-tumor phenotype of TAMs and impede tumor progression [60]. Interestingly, low extracellular pH may also be involved in regulating inflammatory cytokines and phagocytic activity in monocytes and macrophages [61].

Lactate, an oncometabolite in the TME, is a robust regulator of T cells [55]. It has been reported that CAF-derived lactate decreases the population of anti-tumoral CD4⁺ T cells, while increasing the population of T_regs in a prostate cancer model [62]. This reduction in anti-tumoral CD4⁺ T cells might result from lactate dependent SIRT1-mediated T-bet deacetylation [62]. On the other hand, lactate might enhance the activity of NF-kB and Foxp3 expression, which induces naïve T cells to polarize into T_regs and creates an immunosuppressive TME to sustain cancer progression [62]. Additionally, a recent study demonstrates that accumulation of lactate can upregulate the expression of lactate transporter SLC5A12 in CD4⁺ T cells, increasing IL-17 production and fatty acid synthesis (FAS) and reducing glycolysis [63]. Another study revealed that, an acidic environment can significantly impede secretion of IFN-γ and TNF-α by T cells, preventing them from generating proinflammatory cytokines [64]. The inhibition of glycolysis is also observed in acidic conditions, abrogating the activation of T cells [64]. Administration of bicarbonate can neutralize tumor acidity, increasing T cell infiltration and enhancing the efficacy of immunotherapy [64].

Acidity and lactate in the TME have also been demonstrated to impede the cytotoxic functions of NK cells. A recent report indicates that the activities of T cells and NK cells are impaired via inhibition of nuclear factor of activated T cells (NFAT) mediated by lactate dehydrogenase A (LDHA)-associated lactic acid production and intracellular acidification. The inhibited NFAT in T cells and NK cells restricts their IFN-γ production [65], abrogating their anti-tumor responses. Likewise, tumor-derived lactic acid causes intracellular acidification in liver-resident NK cells, leading to dysfunction of mitochondria, and inducing apoptosis of NK cells in biopsies from colorectal liver metastasis patients [66].

CAFs are especially abundant in solid tumors, and have various functions in the TME, such as promoting cancer growth and metastasis, as well as regulating the extracellular matrix (ECM) [67]. CAFs can directly promote tumor growth by secreting stromal cell-derived factor 1 (SDF-1), which mediates the recruitment of endothelial progenitor cells, thereby promoting angiogenesis [68]. Additionally, CAFs play a role in the assembly of fibronectin, which is involved in metastasis, and express other major ECM components that promote tumor progression [69].

ECM is a non-cellular component produced by the secretion of intracellular resident cells that provides both biochemical and structural support. Alterations towards both degradation and stiffness of ECM can promote tumor growth and progression [70]. In solid tumors, tissue containing high amounts of ECM proteins may stiffen the stroma, thus promoting further malignancy [71]. It was recently reported that stiffened ECM may enhance glycolysis and glutamine metabolism in both cancer cells and CAFs, demonstrating the metabolic interplay between these cell types [72]. Aspartate secreted by CAFs is utilized by cancer cells for nucleotide biosynthesis to maintain proliferation. While glutamate secreted by cancer cells is utilized by CAFs in balancing redox state through the glutathione pathway to remodel ECM [72].

Rapidly proliferating cancer cells have high biosynthetic demands and generate nutrient-deficient TME [73]. The scarcity of nutrients, including glucose, amino acids and fatty acids, may induce immune cells to undergo metabolic reprogramming that can affect their fate and functions [73,74] (Figure 1).

Immune checkpoint proteins are mediators of the immune system, and are mainly two types of signals: co-stimulatory signals and co-inhibitory signals. These immune checkpoints are crucial for balancing self-tolerance and autoimmunity and work by sending signals to regulate immune cells [141]. Cancer cells, however, can utilize this regulatory mechanism to escape from immune surveillance. During the activation of T cells, co-inhibitory CTLA-4 is significantly upregulated and impedes T-cell receptor (TCR) signaling by competing with the co-stimulatory receptor CD28 for B7 ligands B7-1 (CD80) and B7-2 (CD86) that expressed by antigen presenting cells (APCs) [5]. It has been reported that T_regs express high levels of CTLA-4, and that these high-expressing CTLA-4 T_regs can be activated by binding to B7 ligands on APCs to exert immunosuppression [142], and also limit the availability of B7 ligands that are necessary for T cell activation. PD-1 is another important co-inhibitory checkpoint for balancing immune responses to chronic pathogens and cancer cells [143]. Upon activation, PD-1 expression in T cells is significantly upregulated and delivers inhibitory signals via binding to PD-1 ligands (PD-L1 and PD-L2) expressed by APCs or cancer cells [143], leading to dampened immune responses. It has become clearer that several neoplasms evade the immune surveillance by upregulating PD-L1 expressions that can bind to PD-1 expressed by T cells, contributing to T cell exhaustion [144]. The activation of PD-1 signaling in T cells may regulate their cytokines production such as IFN-γ, TNF-α, and IL-2, and also proliferation and cellular differentiation [145,146,147]. By utilizing this negative regulating pathway, cancer cells are able to survive and proliferate to sustain the neoplastic formation.

Since the discovery of immune checkpoint proteins, immune checkpoint inhibitors have revolutionized the approach to cancer treatment. Antibodies that block the PD-1 and PD-L1 axis or CTLA-4 have been developed and are able to produce durable clinical responses and prolong overall survival in cancer patients [148,149]. There are currently 7 immune checkpoint inhibitors approved by the FDA, including the CTLA-4 inhibitor ipilimumab; PD-1 inhibitors nivolumab, pembrolizumab, and cemiplimab; and PD-L1 inhibitors avelumab, durvalumab, and atezolizumab [1]. Patients with certain specific types of tumors may have durable clinical responses [2,3,4], however, the majority of clinical responses to ICIs remain unsatisfactory since some proportion of patients who received ICIs might have developed resistance to checkpoint therapy. One possible explanation is that cancer cells have been shown to disable antigen presentation naturally or induced by therapeutic strategies with the robust T cell immune surveillance, enabling cancer cells to evade from immuno-recognition [150,151,152]. Gene mutation in antigen-presenting protein beta-2-microglobulin (B2M) leads to the loss of MHC I presentation in cancer cells [150,152], contributing to escape from CD8⁺ T cell immune surveillance. Another possible explanation is that cancer cells exert genetic mutation in IFN-γ related signaling pathways Janus kinase 1 (JAK1) or Janus kinase 2 (JAK2), which make cancer cells less susceptible to T cell-mediated IFN-γ tumor suppression [150,153]. Another current possible explanation is the deregulation of immune-metabolism. Cancer cells and immunosuppressive cells in the TME can secrete a variety of cytokines or metabolites that may directly or indirectly impede anti-cancer immunity via altering their metabolic profiles [22,154,155]. It is promising that recent studies have identified several additional immune checkpoint targets including inhibitory pathway targets LAG-3 (lymphocyte activating gene-3), TIM-3 (T-cell immunoglobulin and mucin domain-3), TIGIT (T-cell immunoglobulin and ITIM domain), and VISTA (V-domain Ig-containing suppressor of T cell activation); and stimulatory pathway targets OX40 (CD134), ICOS (inducible T-cell co-stimulator), and GITR (glucocorticoid-induced tumor necrosis factor receptor-related protein) [156,157]. These novel targets may possibly lead to improved clinical outcomes with the use of immune checkpoint therapy, however, these novel targets will not be further discussed since this review mainly focus on the current FDA-approved ICIs.

The application of ICIs that block the PD-1 and PD-L1 axis or CTLA-4 has yielded remarkable clinical responses for a subset of patients. However, some patients do not respond to this immunotherapy, which may be the result of either primary (de-novo) resistance or acquired resistance [154]. Recently, a growing number of studies have shed light on acquired resistance that may be a result of deregulation of immuno-metabolism [22,154,155]. The efficacy of ICI monotherapy may be limited by the immunosuppressive TME. Therefore, several ongoing clinical trials have emphasized targeting metabolic circuits in combination with ICIs to enhance anti-tumor responses.

An ongoing phase III clinical trial in head and neck squamous cell carcinoma (NCT03358472↗) has demonstrated that treatment with combination IDO-1 inhibitor epacadostat and anti-PD-1 antibody pembrolizumab resulted in a lower mortality rate than treatment with pembrolizumab alone (17.14 vs. 21.05%). Combination epacadostat and pembrolizumab also resulted in a lower rate of serious adverse events compared with pembrolizumab alone (35.29 vs. 42.11%). Similarly, in a phase II complete trial (NCT03322540↗) the same combination resulted in a lower mortality rate compared with pembrolizumab alone (17.33 vs. 22.08%). However, a completed phase III trial in unresectable or metastatic melanoma found that combination pembrolizumab and epacadostat failed to yield better clinical outcomes compared to pembrolizumab alone, as they observed no significant differences in progression-free survival or overall survival [158].

Interestingly, a phase I clinical trial in renal cell carcinoma patients (NCT02655822↗) demonstrated that administration of A_2AR antagonist combined with atezolizumab provided positive clinical outcomes. More than 72% of patients in this trial were resistant or refractory to anti-PD-1 and PD-L1 therapy, and the majority of patients had PD-L1-negative tumors, which makes monotherapy with anti-PD-1 and PD-L1 unlikely to provide significant benefit [159]. This study showed that the combination of anti-PD-L1 antibody atezolizumab with the A_2AR antagonist ciforadenant increased recruitment of cytotoxic T cells to tumor regions, and increased the diversity of T-cell receptors, which together prolonged overall survival of patients [159].

An ongoing phase I/II trial (NCT02903914↗) is targeting the arginine pathway with arginase inhibitor CB-1158 alone or in combination with anti-PD-1 antibody pembrolizumab. Since arginine is required for the activation and proliferation of T cells, treatment with arginase inhibitor is a potential strategy to provide bioavailable arginine for T cells. The preliminary results show that >90% of arginase is inhibited and arginine levels increase up to 4-fold [160]. The increased arginine levels are able to trigger immune responses and might synergize with ICIs. Several other trials are also testing the efficacy of various metabolic interventions in combination with ICIs (Table 1).

The metabolism of arginine plays a crucial role in the activation of T cells and regulates immune responses [97,106]. Arginase inhibitors may prevent arginine from degradation via inhibiting ARG1 that expressed mainly by DCs, TAMs, and MDSCs, leaving adequate amounts of arginine for T cell activation. It has recently been shown that CB-1158 (arginase inhibitor) can rescue the suppressed proliferation of T cells mediated by myeloid cells in vitro [162], and can increase the number of tumor-infiltrating CD8⁺ T cells and NK cells, reducing tumor growth, in vivo [162]. CB-1158 is currently under investigation in combination with pembrolizumab for the treatment of solid tumors (NCT02903914↗), and a possible underlying mechanism is that bioavailable arginine is scarce in the TME, limiting the efficacy of ICIs. With the use of CB-1158, it may provide bioavailable arginine for T cell activation, and the combination use of pembrolizumab can block PD-1 and PD-L1 ligation, thus abrogating inhibitory signals, which together may enhance T cell-mediated anti-cancer activities. Additionally, the inhibition of NOS may block the immunosuppressive activities in MDSCs and enhances anti-tumoral effects [163], indicating a NOS inhibitor may impede the production of NO in MDSCs, and might enhance the efficacy of ICIs since T cell is less suppressed and can exert effector functions. Currently, a NOS inhibitor, NG-Monomethyl-L-Arginine (L-NMMA), in combination with pembrolizumab, is being explored for the treatment of triple-negative breast cancer (NCT04095689↗).

Tryptophan can be metabolized into kynurenine by the enzyme IDO, and correlates with immunosuppressive properties [91,92,93]. A study has demonstrated that silencing IDO in DCs by small interfering RNA (siRNA) increases T cell proliferation and CD8⁺ T cell activity, while decreasing the number of T_regs [164]. The immunosuppressive effects on T cells exerted by TAMs are blocked via pre-treating IDO inhibitors to TAMs [165]. Importantly, a recent study demonstrated that with the administration of IDO inhibitors in combination with ICIs, CD8⁺ T cells were able to produce IL-2, TNF-α, and IFN-γ significantly comparing to either ICIs or IDO inhibitor alone, indicating that the combination treatment of IDO inhibitor with ICIs may enhance polyfunctional T cells in the TME [166]. The combination use of IDO inhibitor with ICIs significantly attenuates tumor growth, mainly via reactivation of T cells, while also increasing IL-2 production and proliferation of CD8⁺ T cells [166]. Therefore, targeting tryptophan metabolism with IDO inhibitors may reduce suppressive signals from immunosuppressive cells, and restore T cell-mediated anti-tumor responses. Several ongoing trials are targeting tryptophan metabolism with ICIs for the treatment of certain cancers (Table 1).

Arachidonic acid is oxidized into PGE₂ by the cyclooxygenase (COX) enzymes, and can regulate both innate and adaptive immunity [137]. COX-2 has been found to be overexpressed in many types of cancers, and to correlate with the promotion of carcinogenesis [167]. It has been reported that high levels of COX-2 in TAMs is crucial in maintaining M2-like phenotype, and can induce COX-2 expression in cancer cells, which promotes proliferation and survival of cancer cells [168]. In addition, cancer-derived PGE₂ can suppress the cytotoxicity and differentiation of NK cells [169], while also dampening NK-mediated recruitment of DCs [170]. Moreover, the presence of COX-2 in TME can increase the accumulation of T_regs [171], while T_regs can also express COX-2 and produce PGE₂, which is required to induce Foxp3 expression in T_regs and further inhibit T cell-mediated responses [172]. Together, the COX-2 and PGE₂ pathway can promote immunosuppressive signals in TME and boost tumor promotion. Thus, inhibition of the COX-2 and PGE₂ axis is a potential therapeutic target (Figure 2).

COX enzyme inhibitors such as aspirin and celecoxib may serve as anti-inflammatory agents that interfere with COX-2 mediated inflammatory responses in cancer [173]. It has been reported that administration of selective COX-2 inhibitor, celecoxib, can suppress macrophage infiltration and tumorigenesis [174]. They also found out that administration of celecoxib significantly decrease CXC chemokine ligand 2 (CXCL2) and N-cadherin expression in gastric tumor mouse model [174], indicating the use of COX enzyme inhibitor may suppress tumor engraftment and metastasis. Additionally, it has been shown that COX enzyme inhibitors in combination with anti-PD-1 treatment exhibit a synergistic effect in reducing tumor growth compared with either COX enzyme inhibitors or anti-PD-1 antibodies alone [175]. Taken together, these findings suggest that the combination of COX enzyme inhibitors with ICIs could be a therapeutic strategy for cancer treatment. Currently, these combination therapies are under examination in several clinical trials (Table 1).

Glutamine can be converted to glutamate by GLS, and can be harnessed by cancer cells to promote proliferation [117,118]. A recent study in a tumor-bearing mouse model showed that restriction of glutamine could efficiently eradicate tumors and prolong survival via increasing CD8⁺ T cell activity [176]. GLS inhibitors including BPTES (bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl)ethyl sulfide) and CB-839 have also been demonstrated to suppress tumor growth in liver and breast cancer mouse models [177,178]. Additionally, GLS inhibitor BPTES can impeded M2-related gene expression in IL-4-treated macrophages [124]. Another study showed that combination treatment of GLS inhibitors with ICIs significantly decreased tumor volume in an ICI-resistant-tumor mouse model, and the possible underlying mechanism may via enhancing CD8⁺ T cell activities and by decreasing ARG1-expressing myeloid cells [179]. This study suggests that GLS inhibitors can enhance the efficacy of ICIs, by reinforcing the cytotoxic activities of CD8⁺ T cells and reshaping the immunosuppressive TME. There are several ongoing clinical trials that are investigating GLS inhibitors combined with ICIs for the treatment of solid tumors or advanced tumors (Table 1).

Ectonucleotidases CD39 and CD73 both engage in the adenosinergic pathway [180], and have been used as prognostic biomarkers in several types of cancers [181]. T_regs, macrophages and DCs are found to co-express CD39 and CD73 [182,183], enabling them to generate adenosine. It has been reported that extracellular levels of adenosine in the tumor environment are able to suppress anti-tumor immune responses [184]. In addition, the generation of adenosine via CD39 and CD73 in cancer cells may contribute to the recruitment of TAMs, and since TAMs also express elevated CD39 and CD73 this results in amplifying the immunosuppressive adenosine level [185]. The activation of A_2AR or A_2BR may also result in immunosuppression [186]. Together, the adenosinergic pathway appears to regulate immune responses and create an immunosuppressive TME [181,184,185,186]. Therefore, inhibiting the adenosine pathway might result in a less immunosuppressive TME and may boost immune responses (Figure 2).

It was recently shown that inhibition of CD39 enhances the activity of NK cells and inhibits cancer cell metastasis [187]. Likewise, co-inhibition of CD73 and A_2AR in leukocytes was shown to limit tumor initiation, growth, and metastasis in a tumor-bearing mouse model [188]. Another study has demonstrated potential therapeutic effects of combining CD39 inhibitors with ICIs, showing a significant decrease in tumor size and prolonged survival in a melanoma mouse model [189]. Similarly, ICI treatment showed a positive therapeutic response in a CD39-deficient tumor-bearing mouse model [190]. Recently, the A_2AR antagonist, ciforadenant (CPI-444), was administrated in combination with ICIs to different types of tumor mouse models [191]. The combination treatment of ciforadenant with anti-PD-1 improved tumor regression compared to treatment with anti-PD-1 alone [191], and one possible mechanism may due to a decrease in PD-1 expression in A_2AR antagonist treated CD8⁺ T cells since decreasing PD-1 expression in CD8⁺ T cells may lower the threshold and increase the sensitivity of anti-PD-1 therapy [192]. A clinical trial combining ciforadenant with atezolizumab is now ongoing (NCT02655822↗). Many other adenosine-associated inhibitors combined with ICIs are also being tested (Table 1).

Glucose is largely consumed by cancer cells and may impede T cell-mediated anti-cancer activities [20]. Recently, researchers have discovered some types of cancers may upregulate OXPHOS to promote metastasis and progression [89,90]. Metformin, a widely prescribed drug for type II diabetes, is also considered as an OXPHOS inhibitor [193] that can stimulate the AMP activated protein kinase (AMPK) signaling pathway while inhibiting mTOR, and consequently exhibiting anti-tumorigenic effects [194,195]. Metformin inhibited tumor growth in a colorectal cancer patient-derived xenograft mouse model [196], via an increase in apoptotic Bax levels and a decrease in anti-apoptotic Bcl-2 levels [197]. Interestingly, metformin also inhibits oxygen consumption by cancer cells, thereby increasing the availability of oxygen in cancer regions and reducing intratumoral hypoxia [198]. The capacity of CD8⁺ T cells to secrete effector cytokines is enhanced by administration of metformin with anti-PD-1 compared with either metformin or anti-PD-1 alone [198]. Therefore, metformin combined with ICIs may have synergistic effects in stimulating T cell functions and eradicating cancer cells. Several clinical trials are now testing metformin in combination with ICIs to see if this can yield better clinical outcomes than monotherapy (Table 1).

FAS and FAO are both important factors that engage in the lipid metabolism and may exert regulation of immune cells [125,126]. Inhibition of FASN has been observed to upregulate expression of CD36, which can compensate for the anti-tumor effects of FASN inhibition [199]. Therefore, inhibiting CD36 should improve the efficacy of FASN-targeted therapy. A recent study shows that genetic deletion of CD36 in T_regs can shift their metabolic profile from OXPHOS to glycolysis, and can induce apoptosis of intratumoral T_regs [133]. They also demonstrate that in a melanoma-bearing mouse model combination treatment of anti-CD36 antibody combined with anti-PD-1 shows a stronger capability to restrict cancer growth compared with either anti-CD36 or anti-PD-1 alone [133]. Administration of anti-CD36 with ICIs has not yet been studied in clinical trials. Importantly, the ligand-activated nuclear transcription factor PPARα has been shown to regulate lipid metabolism and FAO [200], thus, a selective PPARα antagonist may be effective in shifting intracellular metabolism from FAO to glycolysis [201], skewing FAO-dependent M2 macrophages toward an M1 phenotype [127]. The PPARα antagonist TPST-1120 in combination with nivolumab is being studied for the treatment of advanced cancers (NCT03829436↗).

Data is contained within the article.