Alberto L. Horenstein

Introduction: Multiple myeloma (MM) develops within a profoundly immunosuppressive bone marrow (BM) microenvironment. CD38, a multifunctional ectoenzyme highly expressed on malignant plasma cells, contributes to this niche by degrading nicotinamide adenine dinucleotide (NAD ⁺ ) into ADPR, which fuels adenosine (ADO) production through CD38/CD203a/CD73 enzymatic pathway. CD38_targeting monoclonal antibodies (mAbs), including daratumumab (DARA) and isatuximab (ISA), exert antitumor activity through direct cytotoxicity and immune modulation; however, resistance to these agents remains a major clinical challenge. Understanding how CD38 enzymatic activity and adenosinergic metabolism evolve during therapy is essential for improving treatment efficacy. Methods: This study investigates (i) the enzymatic functions of CD38, (ii) the in vitro effects of DARA and ISA on CD38_mediated NAD ⁺ degradation in primary MM cells and a representative MM cell line, and (iii) the in vivo dynamics of ADO and its metabolite inosine (INO) in BM and peripheral blood (PB) plasma from MM patients receiving DARA monotherapy. Results: In vitro , both DARA and ISA promoted NAD⁺ degradation with accumulation of ADPR. In vivo , ADO concentrations in BM plasma remained consistently in the micromolar range and declined only modestly during treatment, whereas INO progressively increased, leading to a gradual attenuation of the BM -PB gradient. These findings indicate that adenosinergic metabolism remains active during DARA therapy, likely sustained by reduced CD38 expression due to antibody_driven internalization or microvesicle (MV) release, together with adenosine deaminase (ADA) -mediated ADO degradation. Discussion: Despite CD38_targeted therapy, ADO concentrations in the BM remain well above the activation thresholds of P1 purinergic receptors, suggesting persistent adenosinergic immunosuppression. This sustained ADO production may contribute to a tolerogenic BM niche that promotes immune evasion and therapeutic resistance. These results support the rationale for combining CD38_directed antibodies with agents targeting adenosinergic signaling to enhance antitumor immunity and improve clinical outcomes in MM.

Adenosinergic signaling is an important regulator of tissue homeostasis and extracellular accumulation of adenosine (Ado) and is associated with different pathologies, such as cancer. In non-small-cell lung cancer (NSCLC), a subset of CD133/CXCR4+ cancer stem cell (CSCs) has been demonstrated to initiate bone metastases. Here we investigated how NSCLC CSCs interact with osteoclasts (OCs) and osteoblasts (OBs) by modulating Ado production and OC activity. We proved that CSC-spheres, generated in vitro from NSCLC cell lines, express CD38, PC-1, and CD73, enzymes of the non-canonical adenosinergic pathway, produce high level of Ado, and down-regulate A1R and A3R inhibitory receptors, while expressing A2AR and A2BR. To address the Ado role and modulation of the in-bone pre-metastatic niche, we performed co-cultures of CSC-spheres with OCs and OBs cells. Firstly, we verified that active OCs do not activate non-canonical the adenosinergic pathway, conversely to OBs. OCs co-cultured with CSC-spheres increase Ado production that is related to the OC resorption activity and contributes to T-cell suppression. Finally, we proved the efficacy of anti-CD73 agents in blocking NSCLC cell migration. Overall, we assessed the importance of adenosinergic signaling in the interaction between CSCs and OCs at the pre-metastatic niche, with therapeutic implications related to Ado production.

This medical review addresses the hypothesis that CD38/NADase is at the center of a functional axis (i.e., intracellular Ca²⁺mobilization/IFNγ response/reactive oxygen species burst) driven by severe acute respiratory syndrome coronavirus 2 infection, as already verified in respiratory syncytial virus pathology and CD38 activity in other cellular settings. Key features of the hypothesis are that 1) the substrates of CD38 (e.g., NAD⁺and NADP⁺) are depleted by viral-induced metabolic changes; 2) the products of the enzymatic activity of CD38 [e.g., cyclic adenosine diphosphate-ribose (ADPR)/ADPR/nicotinic acid adenine dinucleotide phosphate] and related enzymes [e.g., poly(ADP-ribose)polymerase, Sirtuins, and ADP-ribosyl hydrolase] are involved in the anti‐viral and proinflammatory response that favors the onset of lung immunopathology (e.g., cytokine storm and organ fibrosis); and 3) the pathological changes induced by this kinetic mechanism may be reduced by distinct modulators of the CD38/NAD⁺axis (e.g., CD38 blockers, NAD⁺suppliers, among others). This view is supported by arrays of associative basic and applied research data that are herein discussed and integrated with conclusions reported by others in the field of inflammatory, immune, tumor, and viral diseases.

Monoclonal antibodies (mAbs) were initially considered as a possible “magic bullet” for in vivo elimination of tumor cells. mAbs represented the first step: however, as they were murine in nature (the earliest experience on the field), they were considered unfit for human applications. This prompted the development of techniques for cloning the variable regions of conventional murine antibodies, genetically mounted on human IgG. The last step in this years-long process was the design for the preparation of fully human reagents. The choice of the target molecule was also problematic, since cancer-specific targets are quite limited in number. To overcome this obstacle in the planning phases of antibody-mediated therapy, attention was focused on a set of normal molecules, whose quantitative distribution may balance a tissue-dependent generalized expression. The results and clinical success obtained with anti-CD20 mAbs revived interest in this type of strategy. Using multiple myeloma (MM) as a tumor model was challenging first of all because the plasma cells and their neoplastic counterpart eluded the efforts of the Workshop on Differentiation Antigens to find a target molecule exclusively expressed by these cells. For this reason, attention was turned to surface molecules which fulfill the requisites of being reasonably good targets, even if not specifically restricted to tumor cells. In 2009, we proposed CD38 as a MM target in virtue of its expression: it is absent on early hematological progenitors, has variable but generalized limited expression by normal cells, but is extremely high in plasma cells and in myeloma. Further, regulation of its expression appeared to be dependent on a variety of factors, including exposure to all-trans retinoic acid (ATRA), a potent and highly specific inducer of CD38 expression in human promyelocytic leukemia cells that are now approved for in vivo use. This review discusses the history of human CD38, from its initial characterization to its targeting in antibody-mediated therapy of human myeloma.