Increased Expression of the Sialyltransferase Gene ST3GAL1 Predicts Lack of Sustained MRD Negativity and Increased Risk of Progression in Newly Diagnosed, Transplant Eligible Multiple Myeloma Patients in the Maintenance/Observation Phase of Cassiopeia Study

Background: Hypersialylation induces immune evasion through expression of ligands for inhibitory Siglec receptors. Hypersialylation inhibits NK cytotoxicity in MM and in other cancers increased expression of the sialyltransferase ST3GAL1 was associated with macrophage polarisation as well as inhibition of complement dependent cytotoxicity (CDC). We recently reported that increased expression of ST3GAL1 was associated with inferior PFS in patients with autologous stem-cell transplant (ASCT)-eligible newly diagnosed MM patients treated with bortezomib, thalidomide, and dexamethasone (VTd) +/- Dara as induction and consolidation within the CASSIOPEIA study. In patients receiving induction with Dara-VTd the estimated 36-month PFS was 83.9% (95% CI: 78.1-90.2) if ST3GAL1 expression was < median compared to 72.9% (95% CI: 65.9-80.6) if ST3GAL1 expression was > median: HR 1.67 (95% CI, 1.10-2.53) p value = 0.0169. This report is focused on part 2 of the CASSIOPEIA study, in which patients were randomized to maintenance Dara or observation, with the intention of assessing the impact of ST3GAL1 expression on PFS and achievement of long-term sustained MRD negativity.

Methods: In part 2 patients still on study who had PR or better were randomly assigned (1:1) to Dara 16 mg/kg intravenously every 8 weeks or observation only for up to 2 years. The part 2 primary endpoint was PFS from second randomisation. RNA-seq data were generated from sorted CD138+ bone marrow cells. MRD was assessed using bone marrow aspirates via multiparametric flow cytometry (8-colour EuroFlow) at a sensitivity threshold of 10^-5 (D100 post consolidation) and next-generation sequencing at a sensitivity threshold of 10^-5 (maintenance phase; clonoSEQ assay [version 2.0]; Adaptive Biotechnologies, Seattle, WA, USA). In preliminary experiments MM cell lines (MM.1S, H929) +/- desialylation were co-cultured with healthy donor macrophages and immunophenotyped. CD38 positive Raji cells were exposed to Dara +/- treatment with bisialidase (PALLEON Pharma, MA, USA) in the presence of complement or macrophages to evaluate CDC and antibody dependent cellular phagocytosis (ADCP), respectively.

Results: MRD data at D100 post consolidation were available for 489 patients with RNAseq data: 288 Patients were MRD neg (59%) and 201 MRD pos (41%). ST3GAL1 expression level at baseline did not differ significantly according to MRD response at D100. Analyzing the whole population, with known ST3GAL1 and known arms, and stratifying the analysis on the 4 subgroups, increased expression of ST3GAL1 was found to have a significant association with PFS (0.0017). In a multivariable model including MRD (10^-5), ST3GAL1 (> median) and del17p, ST3GAL1 was significantly associated with PFS (p=0.0188) but lost significance with the addition of t(4;14), (p=0.1031). Finally, expression of ST3GAL1 significantly affected the outcome of MRD negative patients. Considering only MRD negative patients at day 100 (n=288), ST3GAL1 < median was associated with a higher likelihood of achieving sustained MRD (10^-5) at week 105 (p=0.0090). A greater number of patients than expected with low values of ST3GAL1 among patients achieved sustained MRD negativity (n=156/192). In contrast in patients who lost MRD negativity more patients than expected had high levels of ST3GAL1: n=24/36. In patients who were MRD negative but had ST3GAL1 expression > median, PFS was significantly shorter than in patients who were MRD negative but had ST3GAL1 < median: HR=1.65 [1.04;2.61]; p=0.03. Following co-culture with MM cell lines, macrophages acquired a pro-tumorigenic phenotype (e.g. PD-L1 induction +/- loss of HLA-DR) along with increased expression of Siglec receptors. These changes were abrogated by desialylation of MM cells. Desialylation of Raji cells enhanced ADCP with Dara in co-cultures with macrophages. Additionally, we observed improved CDC following desialylation of Raji cells.

Conclusions: Within part 2 of CASSIOPEIA, increased expression of ST3GAL1 is significantly associated with inferior PFS and predicts lack of sustained MRD and disease progression in patients MRD negative (10^-5) prior to the onset of maintenance therapy or observation. Our preliminary data suggest that desialylation strategies, currently in clinical development, could help achieve deep and durable remissions in MM patients receiving current Dara based quadruplet treatment approaches.