MEDI2228, a Novel Bcma Antibody-PBD Conjugate, Sensitizes Human Multiple Myeloma Cells to NK Cell-Mediated Cytotoxicity and Upregulates CD38 Expression in MM Cells

MEDI2228, an antibody-drug conjugate (ADC) comprised of an anti-BCMA antibody site-specifically conjugated to a DNA cross-linking pyrrolobenzodiazepine dimer, is currently under clinical development for the treatment of human multiple myeloma (MM) (NCT03489525). MEDI2228 induces DNA damage responses (DDR) prior to apoptosis, as demonstrated by phosphorylation of ATM/ATR, CHK1/2, and gH2AX in MM cells regardless of p53 status and responsiveness to current MM therapies including bortezomib and IMiDs. Since activation of DDR alters expression of ligands for NKG2D receptors critical for NK-mediated immune surveillance, we here examined whether the ATM/ATR-CHK1/2 signaling cascades activated by MEDI2228 treatment would increase NKG2D ligands in MM cells. Using real-time quantitative RT-PCR and flow cytometry analysis, we found that treatment with MEDI2228 increased the expression of major histocompatibility complex (MHC) class I chain-related proteins A and B (MICA/B) in MM cell lines (n>5) and CD138+ MM cells from patients with relapsed and refractory disease (n=4). In addition, expression of the MHC class I molecules/NKG2D ligands ULBP-1, -3, -2/5/6 increased following MEDI2228 treatment. Next, we evaluated NK cell-mediated lysis of MM target cells (n>3) with or without pretreatment with MEDI2228 and found increased NK cell-mediated lysis of MEDI2228-pretreated vs control MM cells in an effector-target ratio-dependent manner. In parallel, we examined whether MEDI2228 stimulates STAT1- and IFN-related signaling pathways since they are activated by DDR and play a crucial role in innate and adaptive immunity. We found that MEDI2228 treatment significantly increases phosphorylation of STAT1 in H929 and its derived IMiD-resistant cells, and further augments expression of IFN-induced genes (IFITs), IFIT1, 2, 3, and 5, which have been shown to inhibit proliferation and promote apoptosis in cancer cells. Significantly, CD38 is upregulated by MEDI2228 treatment, with increased mRNA expression as well as membrane expression detected by flow cytometry in MM cell lines and MM cells from newly diagnosed and refractory patients (n=5). Consequently, MEDI2228-pretreated MM cells (n>3) are more susceptible to NK cell-mediated antibody-dependent cellular cytotoxicity (ADCC) by daratumumab, which targets CD38. Taken together, our data show that MEDI2228-induced DDR primes MM cells to NK cell-mediated cytotoxicity by increasing expression of MICA/B in MM cells to enhance binding and activating NK cytolytic activity. Simultaneously, MEDI2228 induces IFN-stimulated genes, including CD38, resulting in enhanced MM cell lysis by daratumumab. These results indicate additional mechanisms of anti-MM activity for MEDI2228 and suggest that a combination of MEDI2228 and anti-CD38 mAbs may further improve outcome for MM patients.