Identification of Novel Immune Cell Populations in Lenalidomide Refractory Relapsed Multiple Myeloma Patients Treated with Pomalidomide and Low Dose Dexamethasone

The ALLG MM14 trial evaluated the impact of low dose dexamethasone (LoDEX) withdrawal in lenalidomide (LEN) refractory and relapsed (RR) multiple myeloma (MM) patients achieving initial disease control with pomalidomide (POM) and LoDEX re-induction. As previously reported, patients continuing with POM LoDEX had superior progression free survival (PFS) compared to maintenance with POM alone, however, this early PFS benefit was lost and by 18m was reversed to favour POM only. In patients who received post-progression therapy, more durable responses (second PFS: 12.7m vs 4.6m, p=0.034) and superior survival (OS: 19.4m vs 12.5m, p=0.092) were seen in those previously treated with POM alone. Here we present findings from the preliminary correlative immune studies of this trial.

Aims To undertake mass cytometry (CyTOF) based immune profiling in patients with advanced MM receiving treatment with POM LoDEX.

Methods MM14 was a multicentre, open-label, randomised phase 2 study of LEN refractory RRMM patients who had received ≥ 2 prior lines of therapy. Patients were treated with POM 4mg d1-21 (28d cycle) and LoDEX (40mg weekly). After 4 cycles (induction), patients with stable disease or better (≥SD) were randomised to receive maintenance with ongoing POM-LoDEX or POM alone. Therapy continued until toxicity/progression. PBMCs were collected at baseline and sequentially while on treatment. Cells were barcoded using the Cell-ID 20-Plex Pd barcoding kit (Fluidigm) followed by staining with sub-set/function defining antibodies (targeting myeloid, B, T and NK cells: CD16, CD24, CD11c, CD45RO, CD314, CD38, CD336, HLA-DR, CD14, CD56, CD158a, CD27, CD28, CD159a, CD8, CD19, CD45RA, CD11b, CD4, IgD, CD335, FOXP3, CD25, CD66b, CD3, CD337, CD20, CD158b, CD127 CD57, CD197, CD194, CD304 and CD279). Samples were acquired on the Helios instrument. Data were clustered in the VORTEX package. Significant differences in cluster frequency were assessed by Mann-Whitney test for statistical significance. Cluster phenotypes were determined and validated via multiple visualisation approaches. CD3-CD19-CD56+ NK cells were pre-gated from patient datasets. We then performed Boolean gating using seven NK cell activation/inhibitory markers - CD158a, CD158b, CD159a, CD314, CD335, CD336 and CD337. Boolean populations that comprised 3% or greater of the total NK cell population (median) were then compared. A Mann-Whitney test was used to determine statistical significance.

Results 154 patients from 11 Australian sites were enrolled. The median number of prior treatment lines was 4.5, 82.5% were double refractory. 78 patients who achieved ≥SD were randomised to maintenance: POM n = 40, Pom LoDEX n = 38. CD336+CD20+ cells ("NK-B-cells") were identified in the pre-induction samples of all patients and were significantly more frequent in responders (median 2% of total cells) than in non-responders (0.8% of total cells, p<0.0001). These cells also variably expressed CD19, IgD, HLA-DR, CD158b, CD38 and CD45RA. Preliminary validation of this observation has also been successfully undertaken in an independent cohort of MM patients utilising multi-parameter flow cytometry. In the patients who achieved ≥SD, 5 out of the 8 large clusters (each at least 3% [median] of total nucleated cells evaluated) that were significantly enriched (p<0.0001) following POM LoDEX induction were neutrophil populations. These populations all expressed CD66b but with variable expression of CD24, CD16, CD11c, CD11b and CD45RO. Inhibited NK cells (CD3-CD19-CD56+) based on CD159a, CD314 and CD158a expression were enriched pre-induction and significantly decreased following POM LoDEX (p<0.0001), while activated NK cells expressing CD337 and CD336 and no inhibitory receptors were significantly increased following POM LoDEX (p<0.0001).

Conclusion Utilising CyToF, we have identified a novel "NK B cell" population in RRMM patients, with a higher baseline frequency of these cells being associated with a greater likelihood of response to POM LoDEX. Importantly, we have also confirmed the presence of these cells in an independent MM cohort. Moreover, subsequent to POM LoDEX exposure we have demonstrated the enrichment of heterogeneous neutrophil populations as well as an increase in activated NK cells and commensurate decrease in inhibited NK cells. These novel observations may provide new insights into the mechanisms of action of pomalidomide in MM.