Simple SummaryPatients suffering from multiple myeloma receive many lines of treatment including chimeric antigen receptor T cell (CART) therapy. Myeloma patients may develop devastating secondary malignancies such as myelodysplastic syndrome (MDS). The causative relationship between various anti-myeloma treatments and MDS is not fully understood. We report a study of five CART-treated patients myeloma patients who had MDS (four only after CART and one already prior to CART). We found that all these patients had all their MDS-related molecular changes (mutations) already prior to CART even if they did not have overt MDS in bone marrow evaluation before CART. No new mutations developed after CART, but the frequency of pre-existing mutations did increase. Our study presents evidence that anti-myeloma CART therapy may promote the expansion of pre-existing MDS clones rather than causing the development of new ones.AbstractMultiple myeloma (MM) is characterized by recurrent relapses. Consequently, patients receive multiple therapy lines, including alkylating agents and immune modulators, which have been associated with secondary malignancies such as myelodysplastic syndrome (MDS). Anti-B-cell maturation antigen (BCMA) chimeric antigen receptor T cell (CART) therapy is efficacious in patients with relapsed/refractory (R/R) MM. However, the long-term complications, particularly MDS, are not well understood. Whether CART therapy causes or promotes MDS has not been thoroughly investigated. In this study, we explored the causal relationship between MDS and CART therapy. We retrospectively examined the prevalence of MDS-related morphological and mutational changes before and after administration of CART therapy in five patients. Among them, four developed MDS after CART therapy, while one had pre-existing MDS prior to CART. None of the four patients who developed post-CART MDS showed morphological MDS changes prior to CART therapy. However, all four patients exhibited molecular alterations associated with MDS in their pre-CART as well as post-CART therapy bone marrow. No new mutations were observed. Our findings provide initial evidence suggesting that anti-BCMA CART therapy in MM may promote expansion of pre-existing MDS clones rather than causing development of new clones.