Blockade of Adenosine Generation
As previously described, CD73 is an nucleotidase that converts AMP, generated from CD39- or CD38/CD203-mediated catabolism of ATP or NAD respectively, to adenosine. Its central role in adenosine generation is underscored by the fact that CD73-deficient mice display drastically decreased interstitial levels of adenosine, not only at steady state, but also upon induction of trauma or hypoxia (409, 410). CD73 knock-out mice exhibit hindered tumor growth and metastatic spreading (378–380, 387) and mice inoculated with tumor cells lacking CD73 survive longer than mice inoculated with tumor cells expressing this ecto-enzyme (378, 388). Indeed, administration of anti-CD73 monoclonal antibodies (mAb) (368, 378–386) or of a CD73-specific pharmacologic inhibitor (378, 379, 381, 383, 384, 387–389) impairs tumor growth (368, 378–382, 385, 387, 389) and metastasis (368, 379, 380, 384, 386) while increasing survival (378, 382, 384, 388). Of note, CD73 can also act as an adhesion/signaling molecule to promote metastasis in a catalytic-activity independent manner (386, 411, 412). Mechanistically, the aforementioned treatments have been shown to promote intra-tumoral accumulation of CD8+ T cells (381, 382, 385, 389), B cells (381) as well as of Th1- and Th17-associated cytokines (381) while decreasing the levels of intra-tumoral VEGF (383) and the presence of Tregs (389). Of note, even though metastasis can be modestly inhibited by anti-CD73 therapy in an immune-system independent fashion (368, 386), most of the antitumor effect of CD73 blockade is due to alleviation of A2AR-mediated immunosuppression (368).
No doubt encouraged by these pre-clinical studies, four anti-CD73 mAbs are currently being evaluated as monotherapies in small scale trials targeting a variety of solid tumors. In July 2015, MedImmune launched a first in-human trial (NCT02503774) evaluating the human anti-CD73 mAb Oleclumab, which allosterically prevents CD73 from assuming its catalytically active conformation (413). In June 2016, Bristol-Myers Squibb (BMS) launched a Phase I/IIa trial (NCT02754141) to assess the efficacy of BMS-986179, a human IgG2-IgG1 hybrid mAb that not only inhibits CD73-exerted AMP hydrolysis but also induces CD73 internalization (414). In April 2018, Corvus Pharmaceuticals initiated clinical evaluation (NCT03454451) of their humanized anti-CD73 mAb, CPI-006, which directly competes with AMP for the CD73 active site (415). Finally, in July 2018, Novartis listed a Phase I/Ib trial (NCT03549000) evaluating the efficacy of SRF373/NZV930, a human mAb that impedes CD73 activity via a currently undisclosed mechanism, and was pre-clinically developed by Surface Oncology before being exclusively licensed to Novartis for further clinical development.
CD39 also critically contributes to the generation of extracellular adenosine from ATP as evidenced by the fact that deficiency of this enzyme results in significantly decreased adenosine content in tissues, not only at steady state, but also upon ischemia induction (80). Similar to studies with CD73-deficient mice, tumor growth and metastasis are reduced in CD39-null mice (391, 416). In addition, intraperitoneal delivery of a CD39 inhibitor in immunocompetent mice reduces tumor growth rates (391). Administration of an anti-CD39 mAb increased the survival of immuno-deficient mice inoculated with patient-derived tumors (390), indicating that CD39 can also promote tumor growth or metastasis in an immune system independent manner. In terms of mechanisms, several studies have demonstrated that in vitro inhibition of CD39 activity by pharmacologic inhibitors (45, 47, 62) or blocking mAbs (45, 417, 418) results in enhanced functionality of T cells (45, 47, 62, 418) and NK cells (45, 47, 418), as well as decreased Treg-mediated suppression of T cell proliferation (47, 417). Even though restriction of CD39 activity in vitro conclusively alleviates adenosine-induced immunosuppression, a surprisingly small number of studies demonstrate effectiveness of this approach within tumor-bearing mice. Finally, while humanized mAbs targeting CD39, such as IPH52 (Innate Pharma) have been developed, clinical studies exploring CD39 blockade/inhibition have not been launched.
As previously mentioned, the concerted activity of CD38 and CD203a, can functionally replace CD39 toward the generation of extracellular adenosine. Further substantiating the soundness of CD38-blockade as a cancer treatment, immunocompetent CD38-null mice display reduced tumor growth (419) whereas tumors devoid of this ectonucleotidase grow slower both in immuno-competent (96) as well as in immuno-deficient mice (97). Indeed, administration of CD38 mAbs retards tumor growth (96, 420). Interestingly, tumors derived from anti-CD38 mAb-treated mice encompass more CD8+ T cells and less Tregs and MDSCs (96). Moreover, increased fraction of CD8+ T cells infiltrating these tumors display an effector memory phenotype while less of these cells are double positive for the exhaustion markers PD-1 and TIM3 (96). Three anti-CD38 mAbs, Daratumumab (Janssen Biotech), Isatuximab (Sanofi), and MOR202 (Morphosys) are being clinically evaluated. Daratumumab was FDA-approved in 2015 for treating multiple myeloma patients, while to date the most advanced testing of Isatuximab and MOR202 as monotherapies are respectively the Phase II trials NCT01084252, NCT02960555, and NCT02812706, as well as the Phase I/IIa trial NCT01421186. Of note, in addition to modulating the enzymatic activity of CD38, these mAbs also have the capacity to induce cytotoxicity through diverse mechanisms, such as induction of complement activation, Ab-dependent cellular cytotoxicity (ADCC) or phagocytosis, and programmed cell death (420). Albeit extensive clinical experience of utilizing the aforementioned mAbs against CD38-overexpressing hematologic malignancies, the recently launched trial NCT03473730 constitutes the first application of a CD38-specific mAb in patients with solid tumor malignancies.
Another approach for limiting the intratumoral interstitial adenosine is the oxygenation of the TME (293). As mentioned, hypoxia promotes build-up of extracellular adenosine at least by inducing upregulation of CD39 and CD73 as well as downregulation of adenosine transporters. Indeed, in pre-clinical models, respiratory hyperoxia (60% oxygen) lowers intra-tumoral adenosine levels (9), tumor growth rates (9), metastasis formation (293) and increases survival of tumor-bearing mice (9, 293). Mechanistically, this treatment boosts MHC-I levels on the tumor-cell surface (9), the presence of CD8+, CD69+, or CD44+ cells within the TME (293) and reduces the presence of Tregs (293) as well as the latter's capacity to express CD39, CD73, CTLA-4, or FoxP3 (293). Moreover, increased oxygenation of tumors not only averts angiogenesis through reduction of VEGF concentration (9), but also dampens expression of molecules associated with immune dysfunction, such as TGF-β, CD39, CD73, A2AR, A2BR and COX-2 (9, 293), the rate-limiting enzyme of PGE2 biosynthesis, while increasing the mRNA levels of pro-inflammatory agents, such as IL-2, and IL-12a (293).