Pretreatment Plasma Cell Proteasome Levels Predict Responses To Treatment With Carfilzomib, Lenalidomide, and Dexamethasone (CRd) Followed By Lenalidomide Extended Dosing (CRd-R) In Newly Diagnosed Multiple Myeloma Patients

Background Carfilzomib (Cz) is an irreversible proteasome inhibitor with potent anti-myeloma effects resulting in deep clinical responses and durable remissions in the majority of patients. In our two-stage phase II trial of 45 newly diagnosed multiple myeloma (MM) patients using carfilzomib (Cz), lenalidomide (Ln), and dexamethasone (Dx) combination therapy followed by 2 years of Ln maintenance, best responses after a median of 9 completed cycles (range 2-20), included 17-stringent-(s)-complete response-(CR)/1-CR/6-near-(n)-CR (63%), 10-very good partial response (VGPR) (26%), 3-partial response (PR) (8%), and 1-stable disease (SD) (3%) (reported in a separate abstract). In this pre-planned correlative sub-study based on our ongoing clinical CRd trial for newly diagnosed MM patients, we conducted a prospective flow cytometric study designed to characterize pretreatment levels of proteasomes and aggresomes in plasma cells in relation to response to therapy. Methods Bone marrow aspirates were collected at baseline and C1D2 (single agent Cz exposed). Permeabilized CD138 and CD38 positive plasma cells were tested using anti-19S proteasome subunit antibodies (Abcam, Cambridge, UK). In parallel, cells were labeled with ProteoStat Aggresome Detection Reagent (Enzo Life Sciences, Farmingdale, NY). Multicolor flow cytometric acquisition and analysis was performed using BD FACS CANTO and DIVA software. Data was expressed as mean fluorescence intensity (MFI) ratio using isotype-matched controls. Patients’ best responses to therapy after minimum of 4 cycles were correlated to the acquired proteasome and aggresome data. Statistical analysis was performed using DataPrism software. Statistical significance was considered as p-value <0.05. Results Total of 21 patients were assessed: 14 achieved CR (CR group), 4-VGPR, 2-PR and 1-SD (non-CR group). The median plasma cell 19S MFI ratio in the CR group was 20.21 (range 5.21-84.27), and in the non-CR group 6.19 (range 2.88-11.68), p<0.005. The percent of plasma cells in bone marrow aspirates was not statistically significantly different between two groups. 45% of non-CR patients had plasma cell 19S MFI ratio of less than 5, while 65% of CR patients had plasma cell 19S MFI ratio of over 12. After single agent Cz treatment (C1D2) plasma cell 19S proteasome MFI ratio in the CR group decreased in 90% of patients, while aggresome MFI ratio increased compared to baseline. Conversely, in the non-CR group, C1D2 plasma cell 19S MFI ratio increased in 80% of patients, while aggresome MFI ratio remained unchanged or decreased compared to baseline. Conclusions We found the pretreatment florescence intensity (MFI) ratio of 19S proteasome subunit in plasma cells to be significantly higher in MM patients who obtained CR (versus non-CR) when treated with CRd treatment. None of the patients with pretreatment 19S MFI ratio of less than 5 achieved CR; conversely, all patients with pretreatment 19S MFI ratio of over 12 achieved CR. After a single dose of carfilzomib, 90% of CR patients had decreased 19S MFI and increased aggresomes MFI ratio, while 80% of non-CR patients showed opposite changes: increased 19S MFI and unchanged/decreased aggresomes MFI ratio. We conclude that pretreatment plasma cell proteasome levels predict response to the CRd treatment in newly diagnosed MM patients. Disclosures: No relevant conflicts of interest to declare.