CD46–ADC Reduces the Engraftment of Multiple Myeloma Patient-Derived Xenografts

SIMPLE SUMMARY: Multiple myeloma (MM) is incurable, implying that the disease cells are inherently resistant to current agents and inevitably lead to relapse. One strategy to improve the treatment paradigm is to pursue novel drug targets that are associated with relapsed disease. Towards this end, we developed a novel antibody–drug conjugate (ADC) that targets the cell surface complement inhibitor CD46. Here, we study the potential of this new agent to affect disease initiation. The gene encoding for CD46 is amplified on chromosome 1q in the majority of relapsed myeloma patients, as well as cells containing stem-like aldehyde dehydrogenase activity. We demonstrate the curative potential of CD46–ADC via its ability to abrogate disease engraftment of primary MM cell xenografts. In combination with a recent clinical trial, this study supports the continued study of CD46 as a therapeutic target in MM. ABSTRACT: An antibody–drug conjugate (ADC) targeting CD46 conjugated to monomethyl auristatin has a potent anti-myeloma effect in cell lines in vitro and in vivo, and patient samples treated ex vivo. Here, we tested if CD46–ADC may have the potential to target MM-initiating cells (MM-ICs). CD46 expression was measured on primary MM cells with a stem-like phenotype. A patient-derived xenograft (PDX) model was implemented utilizing implanted fetal bone fragments to provide a humanized microenvironment. Engraftment was monitored via serum human light chain ELISA, and at sacrifice via bone marrow and bone fragment flow cytometry. We then tested MM regeneration in PDX by treating mice with CD46–ADC or the nonbinding control–ADC. MM progenitor cells from patients that exhibit high aldehyde dehydrogenase activity also have a high expression of CD46. In PDX, newly diagnosed MM patient samples engrafted significantly more compared to relapsed/refractory samples. In mice transplanted with newly diagnosed samples, CD46–ADC treatment showed significantly decreased engraftment compared to control–ADC treatment. Our data further support the targeting of CD46 in MM. To our knowledge, this is the first study to show preclinical drug efficacy in a PDX model of MM. This is an important area for future study, as patient samples but not cell lines accurately represent intratumoral heterogeneity.