MV is an enveloped negative-sense ssRNA virus in the family Paramyxoviridae. The most studied strain is the attenuated Edmonston strain (MV-Edm), which has mutations in two accessory proteins (C and V) leading to tumour selectivity, genetic stability and is non-transmissible
[53][30]. In vitro, a GFP-tagged MV-Edm was able to effectively lyse myeloma cells, whilst having no effect on phytohemagglutinin (PHA)-stimulated peripheral blood lymphocytes
[54][31]. In vivo, intratumoral MV-Edm treatment of subcutaneous ARH-77 tumours resulted in all tumours regressing. Intravenous administration had a similar effect
[54][31]. MV-Edm was further modified to encode human thyroidal sodium/iodine symporter (MV-NIS) and infection of myeloma cells with MV-NIS showed oncolysis in vitro
[55][32]. In vivo, three subcutaneous xenograft models were tested (two MV-sensitive, one MV-resistant). The two MV-sensitive xenografts regressed completely after one intravenous dose of MV-NIS. The MV-resistant xenograft was unresponsive to MV-NIS infection alone, but when combined with iodine-123 (123I), resulted in enhanced tumour regression compared to MV-NIS
[55][32]. These studies have limitations, as xenograft models typically have a defective immune response, they were conducted in a subcutaneous setting, and it is possible that 123I could suppress normal haematopoiesis in patients. Therefore, to better target MV to myeloma, the H protein of an MV variant was mutated so it lacked the ability to bind to its receptors CD46 and SLAM. Hummel et al.
[56][33] then attached a single chain variable fragment (scFV) based on a mouse mAb known as Wue-1 which binds to CD138, which is expressed on healthy plasma cells and primary MPCs. MV-Wue infected and killed primary MPCs but also infected healthy plasma cells which could be a concern. However, in lymphoma patients, the widespread use of the well tolerated anti-CD20 mAb rituximab causes long lasting B cell depletion after administration. Therefore, eradicating plasma cells after MV-Wue administration should not generate toxicity problems.