T Cell Immunoprofiling of Patients with Relapsed and/or Refractory Myeloma Who Receive Daratumumab Monotherapy: Longitudinal Analysis during 7 Cycle Follow-up of the Rebuild Phase 2 Study

Preliminary evidence for T cell receptor (TR) repertoire renewal and increased TR clonality has been reported by our group (Vlachonikola et al., ASH 2019) in multiple myeloma (MM) patients (pts) receiving daratumumab monotherapy within the context of the REBUILD study, an ongoing prospective, multicenter, non-comparative, open-label, phase II study in pts with relapsed and/or refractory MM (RRMM) who have had ≥2 prior lines of therapy, including lenalidomide and a proteasome inhibitor. Herein, we report the results from the longitudinal analysis of the TR repertoire employing next generation sequencing (NGS) and multi-color flow cytometry in 24 pts who completed 3 cycles (n=24) and 6 cycles (n=11/24) of daratumumab monotherapy, in order to assess the immunomodulatory effects of daratumumab. We assessed 59 peripheral blood samples collected at screening (SCR, n=24), on Day 1 of Cycle 4 (C4, n=24) and Day 1 of Cycle 7 (C7, n=11). Patients were grouped based on best responses at C4 into responders (i.e. pts with partial response [PR, n=7] and very good PR [VGPR, n=8]), and non-responders (i.e. pts with minimal response [MR, n=2], stable disease [SD, n=5], or progressive disease [PD, n=2]). TRBV-TRBD-TRBJ gene rearrangements were subjected to paired-end NGS and raw reads (n=13,886,646 | median 239,969/sample) were processed through a purpose-built bioinformatics pipeline. Productive TRBV-TRBD-TRBJ rearrangements were taken into consideration (n=6,324,986 | median 100,738/sample) for the computation of clonotypes (i.e. TRB rearrangements with identical TRBV gene usage and amino acid complementarity-determining region 3 sequence). Overall, 325,789 distinct clonotypes (median 4,535 clonotypes/sample) were analyzed. The TR repertoire displayed clonal T cell expansions in both groups (responders/non-responders) in all pre/post-treatment timepoints. Clonality increased after treatment for both responders and non-responders in all assessed timepoints, with statistical significance at C4 in both groups (median cumulative frequency of the 10 most expanded T cell clonotypes/sample in responders: 31.6% pre-treatment vs 43% C4 post-treatment, p=0.009; and, in non-responders: 19.8% pre-treatment vs 39.6% C4 post-treatment, p=0.009). In both groups, the clonotype repertoire appeared to be renewed. Interestingly, in the responders' group a significant shift was noticed in the major clonotype repertoire at screening vs C4. In particular, the 10 most expanded clonotypes/sample