Multiple myeloma (MM) is a hematologic malignancy characterized by the proliferation of abnormal plasma cells within the bone marrow [1] which has seen promising advancements with monoclonal antibodies targeting the CD38 protein (daratumumab/isatuximab) [2, 3]. While these therapies have demonstrated substantial efficacy in enhancing MM patient survival, the lack of predictive factors for response to anti-CD38 therapy poses a considerable challenge. Furthermore, it is noteworthy that these antibodies may migrate along with serum proteins once treatment commences, potentially complicating the interpretation of laboratory tests [4], being detectable within serum protein electrophoresis, and/or in serum immunofixation/immunosubtraction (IF) [4]. This phenomenon can pose challenges in accurately assessing the quality of the therapeutic response achieved. On the other hand, while often underestimated, the detection or absence of these antibodies in laboratory tests could serve as a marker for the concentration/presence of the antibody in the patient. To explore the latter point, we conducted a retrospective study (within the MMVision and VISIUMM studies, approved by our internal ethical committee with the number 02/2022 and 1300 12/2023) to evaluate the association between the appearance of positive IgGk (i.e. the therapeutic antibody) at IF (measured at day 1 of each cycle) and clinical parameters/outcome measures in 87 non-IgGk MM patients treated with daratumumab or isatuximab in three different hematology centers. The patient cohort included 34 IgA (22 kappa and 12 lambda), 38 IgG lambda, 1 IgD, 1 IgM, 10 light chains, and 3 low/non-secreting MM. Main patients’ characteristics and treatment schedules are reported in Table 1 and in Supplementary Table 1. Interestingly, a positive IgGk IF was observed in 42/87 patients, after a median of three treatment courses. Our results demonstrated a significant association between positive IgGk IF and a higher rate of CR/VGPR responses to anti-CD38 therapy (chi-square p = 0.03) (Fig. 1A). Furthermore, we investigated whether these results could impact patient outcomes. Interestingly, we found that patients who developed IgGk IF + had improved progression-free survival (PFS) compared to those who did not (median PFS not reached, versus 21.83 months respectively, HR: 0.10, p < 0.01) (Fig. 1B and Supplementary Fig. 1A, the latter showing results excluding patients who received fixed-duration Daratumumab). Additionally, this variable remained significant in a multivariate Cox regression model (Supplementary Fig. 1B). Of note, survival comparison between IgGk and non-IgGk MM patients showed no differences in terms of PFS (IgGk MM n = 54, supplementary Fig. 2A). Next, we explored clinical and laboratory parameters for their association with positive IF appearance (all variables and analysis, including administration routes comparisons, reported in Supplementary Table 2 and supplementary Fig. 3 and 4), with a high BMI (p = 0.03), higher hemoglobin levels (p = 0.02), lower CRP levels (p = 0.04), and lower monoclonal component levels (p = 0.03) emerging as the most significantly associated factors (Fig. 2). In multivariate analysis (generalized linear model, supplementary Fig. 2B), only BMI evaluation maintained its statistical significance (interestingly, hemoglobin and monoclonal protein resulted highly correlated, supplementary Fig. 5), thus supporting the idea that these results could depend, at least in part, on drug bio-availability (of note, only 10 patients received the majority of treatment courses in an iv route). Along the same line, iv Daratumumab already demonstrated an increased half-life in patients with a BMI > 30 [5]. Systemic absorption of daratumumab, as well as other monoclonal antibodies (mAbs) after subcutaneous injection, occurs primarily through the lymphatic system, influenced by factors such as temperature, pH, interstitial fluid composition, lymphatic capillary density, and molecular characteristics including size, charge density, and immunogenicity [6, 7]. FcRn and non-specific binding affect transport, while presystemic catabolism may limit mAb availability in the central compartment [8]. Recent studies suggest that obese patients have lower blood volume per kilogram and lower IgG clearance, which could extend the half-life of mAbs [9, 10]. These mechanisms, alongside the increased feasibility of subcutaneous administration in obese patients, may improve drug bioavailability and enhance daratumumab detection by immunofixation [5, 8,9,10]. Overall, the appearance of therapeutic antibodies in serum, as detected by immunofixation, likely reflects a combination of factors, including drug pharmacokinetics, immune response dynamics, and tumor biology [1]. Our findings suggest that checking these parameters may be crucial in predicting and monitoring responses to anti-CD38 therapy as well as optimizing treatment strategies for MM patients. Further research is needed to elucidate the specific mechanisms underlying this phenomenon and its potential role as a predictive factor for treatment response in multiple myeloma patients.

Response rate (A) and progression-free survival (PFS) (B) of the patients analyzed based on the appearance of IgGk positive immunofixation

Dot plot of laboratory parameters significantly associated with the appearance of IgGk positive immunofixation (BMI body mass index, Hb hemoglobin, CRP C-reactive protein, CM monoclonal component)