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Mechanism of action of a new anti-CD38 antibody: enhancing myeloma immunotherapy
Fabio Malavasi and Angelo Corso Faini
Department of Medical Science
University of Torino, Italy
Running title:
Anti-CD38 antibody in vivo therapy: not only target ligation
Corresponding author:
Fabio Malavasi, M.D.
Department of Medical Sciences
University of Torino Medical School
Via Santena, 19
10126 TORINO
Italy
Phone: (+39) 011-860-7552
E-mail: [email protected]
Funding:
F.M. receives supports from the Fondazione Ricerca Molinette (Torino, Italy).
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Conflicts of interests:
F.M. has received honoraria for lectures and participation on the advisory boards of Janssen, Tusk
Therapeutics, Takeda and Sanofi, along with research agreements from Janssen and Tusk
Therapeutics.
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Summary
Antibody therapy is a treatment option for several diseases, including multiple myeloma (MM).
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 MM indicate that
the anti-CD38 antibodies may have relevant immunotherapeutic properties.
Commentary on Moreno et al, this issue
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 MM therapy.
The initial promise of monoclonal antibodies (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 BCR, TCR and CD16A (the low-affinity
IgG FcR) (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 (MoAs) of therapeutic antibodies in vivo. The
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findings presented by Moreno et al. 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 a
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 cytoxicity (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 MM 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 NK cells,
the major effectors of cytotoxicity. Moreno et al. tracked the events taking place in culture after
adding Isatuximab to MM in the presence of 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 et al. 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 MM ) and CD16A+] is due to fratricide cytotoxicity (4). Isatuximab depletes
NK cells both in blood and bone marrow (BM) along with B progenitors and basophils. No effects
were observed in Treg populations. The experiments also highlighted the existence of cross-talk
between NK cells and Isatuximab, followed by their activation when exposed to MM. 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
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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 co-stimulatory
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 MM model. Similar negative
results were obtained by combining Isatuximab with lenalidomide and proteasome inhibitors. Co-
cultures with SLAMF7 (a target of an antibody used in MM 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 et al. 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 MM 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 raise 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 ATP, with the production of
cADPR and ADPR, cytoplasmic messengers regulating Ca2+. In selected environments such as the
myeloma niche in BM, CD38 may also use NAD+, especially in acidic conditions. pH modification is
one of the evasion strategies implemented by MM and induces the expression of CD203a, an
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ectonucleotide pyrophosphatase/phosphodiesterase 1 (NPP1), also known as Plasma Cell-1 (PC-1).
This ectoenzyme cooperates with CD38 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 (figure
1). The formation of a trimolecular complex (the “scorpion effect”) may lead to activatory or
inhibitory signals, according to the target cells (8).
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 physiological regulator of homeostasis of
IgG and albumin (9), may offer some related insights.
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The contribution by Moreno et al. is noteworthy in that it links the threshold of in vivo levels of
CD38 to the predicted efficacy of antibody treatment in MM. 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 co-expressed 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 MM are paralleled by its
wide acceptance by patients, another powerful argument for seeking further advances in the field.
Acknowledgements
F.M. is greatly indebted to Professor M. Cragg (University of Southampton, UK) for suggestions
and to Laura McLean and Marzia Roccia for editorial assistance in the preparation of the
manuscript.
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REFERENCES
1) Moreno et al, Clinical Cancer Research, this issue
2) Malavasi F, Deaglio S, Funaro A, Ferrero E, Horenstein AL, Ortolan E, et al Evolution and function of the ADP ribosyl cyclase/CD38 gene family in physiology and pathology. Physiol Rev. 2008 Jul;88(3):841-86 3) van de Donk NWCJ, Richardson PG, Malavasi F. B CD38 antibodies in multiple myeloma: back to the future. Blood. 2018 Jan 4;131(1):13-29 4) Wang Y, Zhang Y, Hughes T, Zhang J, Caligiuri MA, Benson DM et al. Fratricide of NK Cells in Daratumumab Therapy for Multiple Myeloma Overcome by Ex Vivo-Expanded Autologous NK Cells. Clin Cancer Res. 2018 Aug 15;24(16):4006-4017 5) Deckert J, Wetzel MC, Bartle LM, Skaletskaya A, Goldmacher VS, Vallée F et al. SAR650984, a novel humanized CD38-targeting antibody, demonstrates potent antitumor activity in models of multiple myeloma and other CD38+ hematologic malignancies. Clin Cancer Res. 2014 Sep 1;20(17):4574-83 6) Roghanian A, Teige I, Mårtensson L, Cox KL, Kovacek M, Ljungars A, et al. Antagonistic human FcγRIIB (CD32B) antibodies have anti-tumor activity and overcome resistance to antibody therapy in vivo. Cancer Cell. 2015 Apr 13;27(4):473-88 7) Malavasi F, Castella B, Schiavoni I, Incarnato D, Oliva S, Horenstein AL CD38 and antibody therapy: what can basic science add? [Abstract]. Blood 2016; 128:SCI-36 8) Hogarth PM, Pietersz GA. Fc receptor-targeted therapies for the treatment of inflammation, cancer and beyond. Nat Rev Drug Discov. 2012 Mar 30;11(4):311-31 9) Pyzik M, Rath T, Lencer WI, Baker K, Blumberg RS. FcRn: The Architect Behind the Immune and Nonimmune Functions of IgG and Albumin. J Immunol. 2015 May 15;194(10):4595-603
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Figure 1. The left side of the figure illustrates the key effects of the anti-CD38 Isatuximab MoA on
the tumor target and on the main functional effectors (NK cells). On the right, a diagram of 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 Fab and via FcR, through the so-called scorpion effect. Most
steps in the diagram have been experimentally validated and others await verification.
Abbreviations: ITAM, immunoreceptor tyrosine-based activation motif; ITIM, immunoreceptor
tyrosine-based inhibitory motif
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Figure 1:
© 2019 American Association for Cancer Research
Isatuximab MoA Hypothesis for an extension of MoA
FcR+
e�ectorFcR+
e�ector
ITAM ITAM
ITIM ITIM
FcR FcRCD38 CD38
Scorpion e�ect
CD38
E�ects onFcR- target
Myeloma
CD38
Signals
ADCCCDCet al.
or or
- Quantitative levels of antibody
Isatuximab
IsatuximabIsatuximab
Isatuximab
Isatuximab
Isatuximab
Deprivation of B-cell precursorsand basophils
ADCC (on high and low CD38 levels)CDC (on high CD38 levels)ADCP (on high CD38 levels)Direct apoptosis (on high CD38 levels)Sensitization of high CD38 cells tobortezomib+dexamethasone in thepresence of stromaTranscription unaltered
Events followed by exhaustion andphagocytosis
Ex vivo depletion of CD38+ cells, witha Fc-dependent activation (andmodification of transcription)
Myeloma
NK cells
Bone marrow
- Activation/di�erentiation and number of target molecules
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Published OnlineFirst March 7, 2019.Clin Cancer Res Fabio Malavasi and Angelo Corso Faini myeloma immunotherapyMechanism of action of a new anti-CD38 antibody: enhancing
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