Proteasome inhibitors (PIs) constitute one of the cornerstones of the treatment of multiple myeloma (MM), with bortezomib, carfilzomib and ixazomib being approved for clinical use. Due to the relatively recent introduction of PIs to clinical practice, many aspects of the pleiotropic effects of PIs still remain unexplored, particularly for the second-generation PIs carfilzomib and ixazomib. Since MM still remains incurable and many patients will eventually develop treatment-refractory disease, the search for validated biomarkers to predict treatment response is of great clinical importance. In the first aim of this project, we evaluated the effect of proteasome inhibitors on erythropoiesis. During the follow-up of MM patients treated with PIs in the first part of this project, we observed a consistent and highly significant increase in the reticulocyte count during treatment with carfilzomib-based regimens in patients with relapsed multiple myeloma. This observation was not made in a matched cohort of bortezomib-treated patients. In subsequent ex vivo experiments, we demonstrated that carfilzomib exposure significantly impaired terminal erythroid maturation, independent of erythroid commitment, expansion or differentiation. Our results therefore report the first pharmacologically induced delay in erythroid maturation as a mechanism for carfilzomib-induced reticulocytosis in patients with multiple myeloma. These findings might therefore lead to new therapeutic applications for carfilzomib in disorders of mature erythrocytes, such as sickle cell anaemia. In the second part of this project, we evaluated proteasome activity as a potential biomarker for PI drug sensitivity. For this purpose, we measured proteasome activity in primary myeloma cells, purified from the bone marrow of patients with MM. Baseline proteasome activity was not significantly different in myeloma cells derived from treatment-responsive or –refractory MM patients. The degree of proteasome inhibition by PIs was similar in both groups. As a result, the clinical applicability of proteasome activity as a biomarker for drug sensitivity in MM currently remains limited. Nevertheless, these data also suggest that drug sensitivity to PIs is in part proteasome-independent, indicating that our understanding of PI drug resistance should be further improved. In a follow-up project to the present thesis, we have designed several genome-wide screening experiments using CRISPR-Cas9 technology to gain novel insights in the mechanisms driving drug resistance to PIs in MM.

(BELG J HEMATOL 2020;11(3):133–5)