Daratumumab Mechanism of Action: How It Fights Cancer

Daratumumab fights multiple myeloma by binding to a protein called CD38 on the surface of cancer cells and then triggering several distinct immune-mediated attacks against those cells. It was the first therapy designed to target CD38, and what makes it unusual among cancer drugs is that it does not rely on a single killing mechanism. Instead, it recruits natural killer cells, macrophages, and the complement system while simultaneously reshaping the broader immune environment in ways that help the body recognize and attack myeloma on its own. The story of how all these mechanisms work together, and sometimes work against each other, is more nuanced than a simple “antibody finds target, target dies” summary would suggest.

Why CD38 Makes a Good Target

CD38 is a protein found on the surface of many immune cells, but its levels vary dramatically depending on the cell type. Malignant plasma cells in multiple myeloma express CD38 at relatively high levels compared to normal lymphoid and myeloid cells, which carry it at much lower levels.¹ That difference in expression is what makes CD38 a useful handle for a therapeutic antibody: daratumumab can preferentially latch onto myeloma cells while largely sparing healthy tissue. The word “largely” matters here, because CD38 does appear at low levels on normal immune cells, red blood cells, and some non-blood-forming tissues. Those off-target interactions create real clinical consequences, as we will see later.

Recruiting the Immune System to Kill Myeloma Cells

Once daratumumab binds CD38 on a myeloma cell, it does not kill the cell directly in the way chemotherapy poisons a dividing cell. Instead, it acts as a flag, signaling the patient’s own immune system to destroy the tagged cell. The antibody triggers at least four distinct immune killing pathways.

The first is antibody-dependent cellular cytotoxicity, where natural killer cells recognize the tail end of the daratumumab molecule through a receptor called CD16 on their surface. The NK cell then releases toxic granules that punch holes in the myeloma cell membrane. NK cells play a central role during daratumumab therapy through this CD16-mediated process.²

The second pathway involves macrophages, which are large immune cells that physically engulf and digest their targets. Daratumumab promotes macrophage-mediated phagocytosis of myeloma cells through the interaction between the antibody’s tail region and receptors on macrophages, particularly CD64.³ This eating-the-cancer-cell mechanism becomes especially important because, as discussed below, the NK cells that handle the first pathway don’t last long once treatment begins.

The third pathway is complement-dependent cytotoxicity, where daratumumab activates a cascade of blood proteins called complement that forms a pore complex on the myeloma cell surface, effectively blowing it open. And the fourth involves a more subtle process: when daratumumab molecules are crosslinked through nearby immune cells bearing Fc receptors, this crosslinking can trigger programmed cell death (apoptosis) directly in the myeloma cell.⁴

The NK Cell Fratricide Problem

Here is where the elegance of multi-mechanism killing runs into a biological catch-22. NK cells express CD38 on their own surface. So while NK cells are busy killing myeloma cells through daratumumab-mediated ADCC, they are simultaneously being targeted by the same drug. NK cell counts drop rapidly after daratumumab treatment begins.²

Research into how this fratricide works has shown that functional CD16 on the NK cell surface is required not only for NK cells to kill myeloma cells but also for the NK cells themselves to undergo daratumumab-induced death. In experiments with an NK cell line that naturally lacked CD16 but expressed CD38, the cells were resistant to daratumumab-induced apoptosis. When researchers engineered those same cells to express CD16, the resistance disappeared.⁵ The receptor that lets NK cells fight myeloma is the same receptor that lets them be killed by the drug. It is an inherent limitation of how the antibody works.

This rapid depletion of NK cells helps explain why macrophage-mediated killing becomes so important over time. With the NK cell army thinned out, macrophages pick up more of the workload. Research has shown that conditioning macrophages with certain chemotherapy agents can substantially boost their ability to engulf daratumumab-tagged myeloma cells, partly by increasing CD64 receptor expression on the macrophage surface. In these experiments, blocking CD64 attenuated the tumor clearance, confirming that macrophage phagocytosis was driving the effect.⁶

Immune Remodeling Beyond Direct Tumor Killing

Daratumumab does something unexpected for a drug nominally aimed at cancer cells: it reshapes the immune landscape around the tumor. Regulatory T cells, which are immune cells that suppress anti-cancer immune responses, also express CD38. Daratumumab depletes these immunosuppressive cells. In patients receiving the drug, regulatory T cells that were CD38-positive (and more suppressive in lab tests than their CD38-negative counterparts) were reduced, while helper and cytotoxic T-cell counts rose substantially.⁷

This is a genuinely distinct benefit from the direct tumor-killing mechanisms. By removing the brakes that regulatory T cells put on anti-tumor immunity, daratumumab allows the patient’s own T cells to expand and diversify. The T-cell repertoire shifts in ways that could promote better long-term immune surveillance against remaining myeloma cells. Some researchers believe this immunomodulatory effect may be a key reason daratumumab-containing regimens produce such deep and durable responses, because the drug is not just killing the cancer cells it finds today but helping the immune system hunt down the ones that survive.

Does Blocking CD38’s Enzyme Activity Matter?

CD38 is not just a surface marker; it is an enzyme that participates in cellular signaling by converting certain molecules. This raised the question of whether daratumumab might have a second mode of action by shutting down CD38’s enzymatic functions and depriving myeloma cells of important survival signals. The short answer, based on current evidence, is probably not. Research showed that daratumumab inhibits one of CD38’s enzymatic activities (its ADPR cyclase function) but does not block the other (its NAD+ hydrolase function) in myeloma cells expressing CD38. The investigators concluded that neither daratumumab nor newer nanobody-derived antibodies provide a meaningful second mode of action through enzyme inhibition.⁸ This stands in contrast to isatuximab, the other approved anti-CD38 antibody, which binds closer to CD38’s active site and produces stronger enzymatic inhibition.

How Myeloma Cells Develop Resistance

Myeloma cells are not passive targets. Over the course of daratumumab treatment, they adapt. The most consistent change observed is a reduction in CD38 expression on the myeloma cell surface. In patients from the GEN501 clinical study, CD38 levels on myeloma cells dropped significantly during treatment compared to baseline.⁹ Less CD38 means fewer docking sites for the antibody, which translates to weaker immune activation against the tumor. When patients stop daratumumab, CD38 expression tends to recover, suggesting this is a dynamic, reversible adaptation rather than a permanent genetic change.¹⁰

Resistance also involves the complement system. Myeloma cells at the time of disease progression show increased expression of CD55 and CD59, proteins that protect cells from complement attack.¹⁰ By armoring themselves against complement, the surviving myeloma cells neutralize one of daratumumab’s four killing pathways. The combination of lower CD38 and higher complement inhibitors effectively starves daratumumab of the signals it needs to do its job.

Why Combination Therapy Works Better

Daratumumab is rarely used alone. It is almost always paired with other myeloma drugs, and the mechanistic reasons for this are well-documented. Lenalidomide, an immunomodulatory drug, acts synergistically with daratumumab. In lab experiments using bone marrow samples from patients who were refractory to lenalidomide and bortezomib, the combination of daratumumab and lenalidomide killed more myeloma cells than would be expected if each drug worked independently. The synergy comes from lenalidomide’s ability to activate immune effector cells involved in antibody-dependent killing.¹¹ Bortezomib, a proteasome inhibitor, also improved myeloma killing when combined with daratumumab, though its contribution was additive rather than synergistic.¹²

Cyclophosphamide, lenalidomide, and bortezomib all alter what myeloma cells secrete, and exposing macrophages to this altered secretome significantly boosted daratumumab-specific clearance. The killing was partially blocked by inhibiting actin polymerization, confirming that macrophage phagocytosis was a driving mechanism behind the combination benefit.¹³ In practical terms, these combinations partially compensate for the NK cell depletion problem by redirecting more of the tumor killing through macrophages.

How Daratumumab Differs From Isatuximab

Isatuximab, the second approved anti-CD38 antibody, targets the same protein but binds to a completely different spot. Structural studies show that daratumumab binds to a discontinuous region on CD38 located opposite the enzyme’s active site, with all six of its antigen-recognition loops involved in the contact. The binding sites of daratumumab and isatuximab do not overlap at all, and the two antibodies induce different structural changes in the CD38 protein.¹⁴

These structural differences translate into functional ones. Because isatuximab binds near CD38’s catalytic site, it more potently blocks the enzyme’s activity and can trigger direct cytotoxicity through reactive oxygen species. Daratumumab, binding away from the active site, exerts stronger Fc-dependent immune effects, including a process called trogocytosis, where immune cells strip CD38 and other adhesion molecules off the myeloma cell surface. This trogocytosis-mediated stripping of VLA-4 (an adhesion protein) may help overcome the drug resistance that myeloma cells acquire by clinging to the bone marrow microenvironment.¹⁵ The two drugs are not interchangeable, and growing evidence suggests they have different strengths in different clinical situations.

Depth of Clinical Response

The multi-pronged mechanism of daratumumab translates into something oncologists can measure: deep, sustained remissions. Minimal residual disease testing looks for leftover myeloma cells at sensitivities far beyond standard lab work. In the MAIA and ALCYONE trials studying newly diagnosed patients, daratumumab-based combinations produced higher rates of sustained minimal residual disease negativity compared to standard therapy. In MAIA, about 11% of patients on the daratumumab combination achieved MRD negativity lasting at least a year, compared to roughly 2% on standard treatment. Similar patterns held in ALCYONE.¹⁶ In relapsed disease, analyses of the POLLUX and CASTOR trials confirmed that daratumumab-based combinations produced higher rates of sustained MRD negativity, which translated into longer remissions and better outcomes.¹⁷

Infection Risk From Collateral Immune Damage

The same broad immune effects that make daratumumab effective also create a significant safety trade-off. Because the drug depletes CD38-expressing immune cells beyond just myeloma cells, it can substantially lower immunoglobulin levels. In one large study, average IgG levels dropped from about 530 mg/dL before treatment to about 350 mg/dL afterward, and the proportion of patients with severe hypogammaglobulinemia nearly tripled.¹⁸ Low immunoglobulin levels leave patients more vulnerable to infections. Patients who already had low IgG before starting daratumumab faced higher infection rates, with severe hypogammaglobulinemia carrying the greatest risk.¹⁹ This is a direct consequence of CD38 being present on normal plasma cells that produce antibodies, not just on myeloma cells. Some patients receive immunoglobulin replacement therapy to mitigate the risk.

Interference With Blood Typing

One of the more unusual practical consequences of daratumumab’s mechanism has nothing to do with how well it kills cancer. Because red blood cells express CD38 at low levels, daratumumab circulating in a patient’s bloodstream can bind to reagent red blood cells used in pre-transfusion testing. This causes a false-positive reaction that looks like the patient has antibodies against donor blood, creating what blood bank staff see as a panagglutination result across all test panels.²⁰

The interference is not harmless to ignore. Many myeloma patients have received multiple blood transfusions and may have developed real antibodies against certain red blood cell types. The false-positive from daratumumab masks those real antibodies, making it impossible to tell whether the patient genuinely needs antigen-matched blood or whether the test result is just an artifact of the drug. Standard ABO and Rh(D) blood typing remain unaffected, but the additional compatibility testing that ensures safe transfusion can be delayed substantially.²¹ Blood banks now use workarounds, such as treating reagent cells with a chemical that destroys CD38 on their surface, to eliminate the interference. Patients starting daratumumab are typically advised to have their blood typed before the first dose.

Subcutaneous Delivery and Practical Shifts

Daratumumab was originally given as an intravenous infusion, which meant long chair times. The first dose could take around seven hours because of the need to monitor for infusion-related reactions, which occurred in roughly half of patients during initial IV dosing. A subcutaneous formulation, combined with an enzyme called hyaluronidase that helps the drug disperse under the skin, cut administration time to a matter of minutes. Infusion-related reactions dropped to under 10% with the subcutaneous version, and the volume of fluid went from several hundred milliliters down to about 15 mL, while efficacy and safety remained comparable.²² For a drug that patients may receive for years, that difference in treatment burden is substantial.

Use in AL Amyloidosis

Because AL amyloidosis is caused by the same type of abnormal plasma cell that drives multiple myeloma, the logic of targeting CD38 extends to this disease as well. The malignant plasma cells in AL amyloidosis produce misfolded antibody fragments that deposit as amyloid in organs like the heart and kidneys, causing progressive damage. Early clinical data from phase I and phase II studies showed that daratumumab is well tolerated in this population and produces rapid, deep responses.²³ It has since become a standard part of frontline therapy for AL amyloidosis, representing one of the clearest examples of a mechanistic insight in one blood cancer translating directly to another.