4.1 CD73 and Adenosine Pathway
The adenosine pathway, involving the enzymes CD73 and CD39, plays a
crucial role in shaping the immunosuppressive TME. These enzymes are
highly expressed in various cell types within the TME, including tumor
cells, endothelial cells, and infiltrating immune cells such as Tregs
and stromal cells [359]. Notably, CD73 and CD39 are upregulated in
response to adenosine signaling and the hypoxic conditions commonly
found in tumors. The generation of adenosine through the CD39/CD73
pathway is a key mechanism underlying the immunosuppressive function of
Tregs [359, 360].
The extracellular degradation of ATP by CD39 and CD73 contributes
significantly to immunosuppression. This process reduces ATP-dependent
immune activation and results in the production of adenosine (ADO)
[359, 360]. CD73 activity on the cell surface is the rate-limiting
step in the production of extracellular adenosine, a process that
hinders antitumor immunity and supports tumor progression [361].
Adenosine, a nucleoside molecule, exerts its potent immunosuppressive
effects by binding to specific receptors on immune cells, primarily the
A2A and A2B adenosine receptors. This receptor engagement triggers a
cascade of intracellular signaling events that inhibit immune cell
activation and effector functions, thus enabling tumors to evade immune
detection and destruction [359, 360, 362, 363].
CD73, also known as ecto-5’-nucleotidase , is an is a
glycosyl-phosphatidylinositol-linked cell membrane-bound ecto-enzyme,
encoded by the gene NT5E [364]. Its primary function involves
catalyzing the conversion of extracellular AMP (adenosine monophosphate)
to adenosine. This enzymatic activity leads to the accumulation of
adenosine in the tumor milieu, particularly heightened under conditions
of tissue hypoxia and inflammation typical of solid tumors [359].
Adenosine, upon binding to A2A and A2B receptors (A2AR and A2BR)
expressed on T cells and natural killer T (NKT) cells, monocytes,
macrophages, dendritic cells and NK cells, exerts potent
immunosuppressive effects. These effects include the suppression of
cytotoxic T cell responses, inhibition of dendritic cell maturation and
antigen presentation, promotion of Treg cell differentiation and
function, and reduction in pro-inflammatory cytokine production such as
IFN-γ and TNF-α. [365]. A2AR is up-regulated in macrophages in
response to NF-κB, STAT1 and PPARγ as well as adenosine signaling, and
A2AR activation inhibits the secretion of neutrophil chemokines, thereby
reducing the inflammatory response [359]. Thus, CD73-mediated
adenosine production plays a critical role in fostering an
immunosuppressive environment within tumors, contributing to immune
evasion and supporting tumor progression. CD73 has been shown to play a
role in various cancer processes, such as metastasis [366], tumor
invasion [367], and increased cell proliferation [368].
CD39, also known as ectonucleoside triphosphate
diphosphohydrolase-1 or NTPDase 1, is an ectoenzyme
prominently expressed on immune cells, including Tregs and subsets of
activated T cells [369, 370]. Its primary function involves the
hydrolysis of ATP (adenosine triphosphate) and ADP (adenosine
diphosphate) into AMP (adenosine monophosphate). This enzymatic activity
serves as a critical step in the production of adenosine within the TME.
By generating AMP, CD39 acts upstream of CD73, facilitating the
subsequent conversion of AMP into adenosine [371].
A significant proportion of cancer patients fail to respond to
immunotherapies such as PD-1/PD-L1 and CTLA-4 blockade, indicating that
other immunosuppressive pathways may contribute to immune evasion in
these non-responding tumors [372]. The adenosinergic pathway,
presents a promising new therapeutic approach in cancer immunotherapy,
though still in its early stages [370]. Preclinical studies and
clinical trial data have shown that targeting this pathway is a viable
therapeutic strategy for the future. Small-molecule inhibitors and
monoclonal antibodies targeting CD39, CD73 and A2AR have been developed
for cancer therapy [359, 373]. As small molecules could cross
physiologic barriers in TME, they are better than monoclonal antibodies
(mAb) which are macromolecules. Bastid et al. in their study showed
that, administering a CD39 inhibitor or blocking antibody reduced the
tumor-induced suppression of CD4 and CD8 T-cell proliferation and
enhanced the cytotoxic activity of CTLs and NK cells [374]. In a
lung cancer model study, an anti-CD39 monoclonal antibody, which
inhibits the mouse ectoenzyme CD39, was found to increase CD107a
expression in infiltrating NK cells and stimulate IFN-γ release, leading
to enhanced cancer cell killing and anti-metastatic effects cells
[375]. Simmilarly, in mouse model of melanoma, the administration of
anti-CD39 monoclonal antibody (mAb) stimulated the release of
IFN-γ , resulting in the eradication of cancer cells [375]. A
study by Lu et al. demonstrated that the bifunctional antibody-ligand
trap, ES014 (targeting CD39/TGF-β), effectively inhibited CD39,
preventing the degradation of extracellular ATP, while also neutralizing
autocrine/paracrine TGF-β near target cells leading to restoration of
anti-tumor immunity [376]. Anti-CD73 monoclonal antibody (3F7), in a
mouse model of TNBC, suppressed tumor growth and metastasis [376].
An experimental study by Jin et al. found that the combination of tumor
CD73 knockdown with tumor-specific T-cell transfer successfully cured
all tumor-bearing mice. Notably, adoptive T-cell immunotherapy alone
provided no therapeutic benefit in mice with tumors that did not undergo
CD73 knockdown [377]. Another study in tumor-bearing mouse models,
showed that anti-CD73 antibodies can amplify the anticancer effects of
both anti-CTLA-4 and anti-PD-1 immunotherapies in MC38-OVA (colon) and
RM-1 (prostate) subcutaneous tumors and established metastatic 4T1.2
breast cancer. The activity of anti-PD-1 mAb was also enhanced by
anti-CD73 mAb, against 3-methylcholanthrene (MCA)-induced fibrosarcomas
[378]. In a study conducted by Perrot et al., it was observed that
the antibodies IPH5201 and IPH5301, which target the human
membrane-associated and soluble forms of CD39 and CD73 respectively,
effectively inhibited the hydrolysis of immunogenic ATP into
immunosuppressive adenosine. The mechanism of action involved
stimulating dendritic cells and macrophages, as well as restoring the
activation of T cells isolated from cancer patients [379]. Drugs
that block the A2AR-mediated adenosinergic pathway could boost antitumor
immunity by counteracting the effects of extracellular adenosine
generated by both tissue cells and Tregs. Pharmacological treatment of
mice with A2AR antagonists enhanced antitumor T-cell activity, leading
to greater inhibition of tumor growth, destruction of metastases, and
reduced neovascularization of cancerous tissues [380].
Current clinical trials are evaluating adenosinergic pathway targets
either as monotherapy or in combination therapy:
- CD39 inhibitors like, anti-CD39 monoclonal antibody [NCT05508373, JS019] [NCT05234853, PUR001,], CD39
antagonist [NCT04261075, IPH5201] [NCT04336098, SRF617]
[NCT05075564, ES002023], anti-CD39/TGF-β bispecific
antibody [NCT05381935, ES014], CD39 antagonist with
chemotherapy [NCT04336098, Combination SRF617 with pembrolizumab,
gemcitabine, albuminbound paclitaxel], Anti-CD39 antibody
with immunotherapy [NCT04306900, Combination TTX-030 with
immunotherapy and/or chemotherapy]
- CD373 inhibitors like, Anti-CD73 monoclonal antibody [NCT05431270, PT199] [NCT05174585, JAB-BX102]; CD73
antagonist [NCT04797468, HLX23] [NCT04104672, AB680]
[NCT03736473, MEDI9447 (oleclumab)] [NCT04148937, LY3475070]
[NCT03549000, NZV930] [NCT02754141, BMS-986179]
[NCT05227144, ORIC-533]; Anti-CD73 antibody with
immunotherapy [NCT05174585, JAB-BX102 with pembrolizumab]
[NCT05431270, PT199 with an anti-PD-1 monoclonal antibody]
[NCT03549000, NZV930 with PDR001] [NCT04672434, Sym024 with
Sym021]; Anti-CD73 antibody with chemotherapy [NCT05143970, IPH5301 with chemotherapy and trastuzumab]
[NCT04572152, AK119 with AK104] [NCT05119998, IBI325 with
sintilimab] [NCT04940286, oleclumab with gemcitabine,
nab-paclitaxel, durvalumab]; Anti-CD73-TGFβ-Trap antibody
with chemotherapy [ NCT03954704, dalutrafusp (GS-1423) with mFOLFOX6
regimen]; CD73 antagonist with immunotherapy [NCT04148937, LY3475070 with pembrolizumab] [NCT02754141,
BMS-986179 with nivolumab (BMS-936558)] [NCT04989387, INCA00186
with INCB106385 and/or retifanlimab]
- A2AR antagonist [NCT05501054, Ciforadenant (CPI-444)]
[NCT04969315, TT-10] [NCT03207867, Taminadenant (NIR178)]
[NCT02403193, PBF-509] [NCT05117177, Inupadenant (EOS100850)]
[NCT04580485, INCB106385] [NCT04478513, AZD4635]; A2AR
and A2BR antagonist [NCT04262856, Etrumadenant (AB928)];Anti-CD39 antibody with A2AR and A2BR [NCT05177770, SRF617
with AB928 (Etrumadenent) and AB122 (zimberelimab)];Anti-CD73 antibody with A2AR antagonist [NCT04089553,
AZD4635 with durvalumab or oleclumab (MEDI9447)] [NCT03454451,
CPI-006 with ciforadenant or pembrolizumab] [NCT03381274,
oleclumab (MEDI9447) with AZD4635]; A2AR antagonist with
chemotherapy [NCT05403385, inupadenant (EOS100850) with
Chemotherapy]; A2AR antagonist with immunotherapy [NCT04580485, INCB106385 with immunotherapy] [NCT05501054,
lpilimumab, nivolumab with ciforadenant (CPI-444)] [NCT03549000,
NZV930 with PDR001 and /or NIR178] [NCT02403193, taminadenant with
PDR001] NCT03207867, NIR178 with PDR001] [NCT04895748, DFF332,
spartalizumab with taminadenant].