Antibody therapy is a treatment option for several diseases, including multiple myeloma. The logic behind it is relatively simple: A target molecule is selected because of its expression on tumor cells, and the antibody delivers cytotoxic effects. Therapeutic results in multiple myeloma indicate that the anti-CD38 antibodies may have relevant immunotherapeutic properties.

See related article by Moreno et al., p. 3176

In this issue of Clinical Cancer Research, Moreno and colleagues (1) provide an original analysis of the characteristics of a new chimeric anti-CD38 antibody candidate for multiple myeloma therapy.

The initial promise of mAb, heralded as a “magic bullet” for cancer therapy, was not immediately fulfilled. Decades of frustrating results were necessary before the clinical success of anti-CD20 antibodies finally rewarded the efforts of clinicians and pharmaceutical companies, rekindling interest in pursuing the development of antibody-based therapies. This required rethinking the role of antibodies in vivo: it is now accepted that some antibodies block the function of a membrane receptor while others synergize it. Synergistic antibodies occupy a domain that hosts the site of the natural ligand of the molecule (molecular mimicry). When the target lies in close proximity to professional receptors, it can trigger signals (molecular parasitism). CD38, functionally associated with the B-cell receptor (BCR), the T-cell receptor (TCR), and CD16A (the low-affinity IgG FcR; ref. 2), is one example.

These conclusions are relatively straightforward when the function of the target molecule is known. However, such is not the case for CD38, whose precise roles are not yet fully known. The results of antibody therapy following the introduction of daratumumab (3) have provided useful practical and theoretical information about the functions of the target and, at the same time, revealed some unexpected mechanisms of action (MoA) of therapeutic antibodies in vivo. The findings presented by Moreno and colleagues indicate that isatuximab has unique characteristics, only some of which are shared with daratumumab. In fact, given their different MoAs, the two antibodies might be potentially valuable as therapeutic complements or alternatives in patients developing resistance to one of them. The particular benefit provided by isatuximab is its sensitivity to the number of CD38 molecules present on target cells. Isatuximab saturates membrane CD38 and can be internalized. However, antibody-dependent cell cytotoxicity (ADCC), antibody-dependent cell phagocytosis (ADCP), and complement-dependent cytotoxicity (CDC) are triggered only when the number of surface CD38 molecules reaches a threshold. The same thing happens in the induction of direct apoptosis. Isatuximab does not alter the transcription on a high CD38+ myeloma line.

The existence of such a threshold was established by quantifying the number of CD38 molecules expressed both by multiple myeloma and by the majority of normal cell populations. Isatuximab may have a lower depleting power than daratumumab, but this feature could be protective for normal CD38+ cell populations. The action of isatuximab is more complex when studying its effects on natural killer (NK) cells, the major effectors of cytotoxicity. Moreno and colleagues tracked the events taking place in culture after adding isatuximab to multiple myeloma in the presence of peripheral blood mononuclear cells (PBMC) containing NK cells. The results are similar to those reported with daratumumab, with the number of circulating NK cells declining after antibody infusion. Moreno and colleagues show that NK cells rapidly decrease in number after their initial activation by isatuximab. The authors ruled out the possibility that this decrease in NK cells [CD38+ (at a density lower than multiple myeloma) and CD16A+] is due to fratricide cytotoxicity (4). Isatuximab depletes NK cells both in blood and bone marrow along with B progenitors and basophils. No effects were observed in regulatory T-cell (Treg) populations. The experiments also highlighted the existence of cross-talk between NK cells and isatuximab, followed by their activation when exposed to multiple myeloma. This cross-talk includes isatuximab's exploitation of transmembrane signaling, which involves its Fc domain. Indeed, these effects are diminished in the presence of Fc blockers. The same signals were investigated at a molecular level by examining the transcription of NK cells after exposure to a myeloma line in the presence of isatuximab. The 70 genes identified are classified as being involved in chemotaxis, cytolysis, and defense response. One of them, CD137 (Tumor Necrosis Factor Receptor Super Family 9, TNFRSF9), is a gene that controls an inducible costimulatory molecule and also a player in anti-CD20 therapy. The combination of isatuximab with anti-CD137 did not provide the expected prolongation of life of NK cells in this multiple myeloma model. Similar negative results were obtained by combining isatuximab with lenalidomide and proteasome inhibitors. Cocultures with SLAMF7 (a target of an antibody used in multiple myeloma therapy and reported as being involved in ADCP) likewise proved not to alter the depletion of NK cells.

CD38 is a phylogenetically ancient molecule whose functions as an ectoenzyme, an adhesion molecule, and a receptor are well known (2). The work by Moreno and colleagues significantly expands investigation of the actual role of CD38. It is still difficult to reconcile the use of CD38 as a tumor target given its almost ubiquitous presence on the surface of normal cells, including B, T, and myeloid regulatory populations. Other cells (e.g., erythrocytes and platelets) also express CD38, although at very low densities. It is also unclear how the same antibody can deliver toxic hits to the tumor, while also driving different effects on positive and negative effectors. Differences in the surface levels of the molecule, very high on multiple myeloma and low on effectors, may account for its distinct functional effects. The interactions taking place between the target epitope of the therapeutic antibodies and the different IgG Fc receptors (FcR) give rise to alternative hypotheses. The epitope recognized by isatuximab encompasses the catalytic domain of the molecule and is different from that of daratumumab (5). The main CD38 substrate is NAD+, with the production of cADPR and ADPR, cytoplasmic messengers, regulating Ca2+ levels. In selected environments such as the myeloma niche in bone marrow, CD38 may also use NADP, especially in acidic conditions. pH modification is one of the evasion strategies implemented by multiple myeloma and induces the expression of CD203a, an ectonucleotide pyrophosphatase/phosphodiesterase 1 (NPP1), also known as plasma cell-1 (PC-1). This ectoenzyme cooperates with CD38 to produce AMP from ADPR, which is catalyzed by CD73 to produce adenosine, a potent immune suppressant. Isatuximab is reportedly one of the most efficient inhibitors of the enzymatic features exerted by CD38, only now being evaluated for its therapeutic potential.

While the role of the IgG FcRs has been analyzed in different tumor models and with different antibodies (6), there has been no systematic analysis of its role in anti-CD38 therapy to date. The structural differences between isatuximab (chimeric) and daratumumab (human) may account for distinct functional interactions with the IgG FcRs. Experience with individual FcRs shows that the membrane dynamics of myeloma cells change when daratumumab is presented by antibody-armed FcR+ effectors, mimicking the events taking place in vivo (7). The outcome of the interactions of the two different antibodies may explain the differences observed in terms of membrane dynamics, with isatuximab leading to internalization and daratumumab to generation of membrane-derived vesicles.

It seems intuitively difficult to rationalize a scenario in which the antibody kills the tumor, depletes cells populations, activates T effectors, and blocks suppressors. A hypothesis that may help in explaining it is that the distinct signals may result from the ligation of the target molecule by the antigen-binding site and from the simultaneous engagement of the IgG Fc domain by FcRs (Fig. 1). The formation of a trimolecular complex (the “scorpion effect”) may lead to activatory or inhibitory signals, according to the target cells (8).

Figure 1.

The left side of the figure illustrates the key effects of the anti-CD38 isatuximab on the tumor target and on the main functional effectors (NK cells). On the right, a diagram contains a hypothetical extension of the MoA by the major therapeutic antibodies specific for target molecules of lymphoid neoplasia. The hypothesis rests mainly on the functional network implemented between the antibodies and their IgG Fc receptors (FcR) expressed at various levels by lymphoid and myeloid effectors. Included is the possibility that the antibodies may react simultaneously on the same cell via their binding site and also via FcR by implementing the so-called scorpion effect. Most steps in the diagram have been experimentally validated and others await verification. ITAM, immunoreceptor tyrosine-based activation motif; ITIM, immunoreceptor tyrosine-based inhibitory motif.

Figure 1.

The left side of the figure illustrates the key effects of the anti-CD38 isatuximab on the tumor target and on the main functional effectors (NK cells). On the right, a diagram contains a hypothetical extension of the MoA by the major therapeutic antibodies specific for target molecules of lymphoid neoplasia. The hypothesis rests mainly on the functional network implemented between the antibodies and their IgG Fc receptors (FcR) expressed at various levels by lymphoid and myeloid effectors. Included is the possibility that the antibodies may react simultaneously on the same cell via their binding site and also via FcR by implementing the so-called scorpion effect. Most steps in the diagram have been experimentally validated and others await verification. ITAM, immunoreceptor tyrosine-based activation motif; ITIM, immunoreceptor tyrosine-based inhibitory motif.

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The fate of the therapeutic antibody may also be an issue. The life of daratumumab in vivo is longer than that of normal IgG. An attractive hypothesis is that the low-CD38 density erythrocytes and platelets act as a carrier of the therapeutic antibodies in the biological fluids, protecting them from elimination. Recent studies on neonatal FcR, the physiologic regulator of homeostasis of IgG and albumin (9), may offer some related insights.

The contribution by Moreno and colleagues is noteworthy in that it links the threshold of in vivo levels of CD38 to the predicted efficacy of antibody treatment in multiple myeloma. The peculiar ability of isatuximab to react only with cells expressing high levels of CD38 may lead to the definition of signals in NK cells capable of sustaining their activation and duration in vivo. The relationship between a given target for a therapeutic antibody and the FcR coexpressed on the target cell raises the possibility that the “scorpion effect” might be exploited when engineering the Fc domain to enhance the therapeutic effects.

No less importantly, the clinical benefits of antibody-mediated therapy in multiple myeloma are paralleled by its wide acceptance by patients, another powerful argument for seeking further advances in the field.

F. Malavasi reports receiving commercial research grants from Janssen and Tusk Biotherapeutics, speakers bureau honoraria from Takeda, Sanofi, Tusk Biotherapeutics, and Janssen, and is a consultant/advisory board member for Janssen, Tusk Biotherapeutics, Takeda, and Sanofi. No potential conflicts of interest were disclosed by the other author.

F. Malavasi is greatly indebted to Professor M. Cragg (University of Southampton, Southampton, United Kingdom) for suggestions and to Laura McLean and Marzia Roccia for editorial assistance in the preparation of the manuscript. F. Malavasi received support from the Fondazione Ricerca Molinette (Torino, Italy).

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