Characterizing the Fate of Anti-CS1 Nanobody Displaying Extracellular Vesicles in Multiple Myeloma

Extracellular vesicles (EVs) are promising delivery vehicles capable of transporting therapeutic agents across biological barriers. However, native EVs primarily accumulate in liver, spleen and lungs, limiting targeted delivery to disease sites. To enhance their targeting efficiency for the plasma cell cancer multiple myeloma (MM), localized in the bone marrow (BM), we engineered HEK293-derived EVs to display a nanobody (Nb) against the MM cell surface marker CS1. We confirmed enrichment of the Nb construct on engineered EVs and binding of α-CS1 EVs to CS1. In vitro, we found enhanced α-CS1 EV uptake by MM cell cultures. In vivo, we first compared the biodistribution of HEK293-derived native EVs in healthy and MM-bearing mice. Although native EVs reached the BM in both groups, MM-bearing mice showed increased liver and lung accumulation together with reduced BM delivery. α-mCS1 EV delivery to the BM of MM-bearing mice was only slightly increased compared to native EVs, while off-target accumulation also increased. At the cellular level, no changes in EV delivery to MM cells were detected. In conclusion, while CS1 targeting enhances in vitro EV uptake by MM cells, in vivo biodistribution remains suboptimal. Further optimization is needed to improve EV-based drug delivery for MM.