Treating MRD Positivity in Multiple Myeloma: An Allogeneic GM-CSF-Based Vaccine in Combination with Lenalidomide Induces Long-Term Remissions in Patients with Low Disease Burden
Authors: Biavati Luca , Huff Carol Ann , Ferguson Anna , Rudraraju Lakshmi , Zucchinetti Cristina , Gittelman Rachel , Johnson Sarah , Sanders Catherine , Ali Abbas Abbas , Noonan Kimberly , Borrello Ivan M. 
Published: February 2021

Introduction:
Novel therapies have increased the complete response (CR) rates in multiple myeloma (MM) with corresponding improvement in progression free survival (PFS). However, relapse occurs in a significant number of patients. Emerging evidence strongly correlates minimal residual disease (MRD) negativity with improved clinical outcome. Nonetheless, definitive clinical guidance on the management of MRD-positive MM is currently lacking. Here we present the results of a phase II trial evaluating an allogeneic GM-CSF vaccine in combination with lenalidomide (Len) in patients with MM in near CR (NCT01349569).
Methods:
To be enrolled, patients required a sustained near CR (defined as no measurable M-spike and positive immunofixation, IFE) for at least four months while on a Len-containing regimen. The vaccine consisted of two allogeneic, commercially available MM cell lines (U266 and H929) coupled to a GM-CSF producing leukemia cell line (K562, GVAX®). Patients received four vaccinations over 6 months in combination with Len maintenance for at least 1 year, although in most patients Len was continued until disease progression. The primary endpoints of this trial were eradication of detectable disease and conversion to CR. The secondary endpoints were safety, time to response and immune monitoring of vaccine- and MM-specific T cell responses. Bone marrow (BM) and peripheral blood (PB) samples were collected at pre-established timepoints and cryopreserved for subsequent analyses. The ImmunoSEQ® assay (AdaptiveBiotechnologies, Seattle, WA) was used to sequence T cell receptors (TCR) and the resulting data was used to analyze TCR repertoire metrics. The ImmunoSEQ® assay was also used to sequence the IGH and IGK/L B-cell receptors for MRD quatification. Deep phenotyping of T cells and immune monitoring were performed by flow cytometry.
Results:
Fifteen MM patients were enrolled. The primary enpoint of the trial was met as 53% (n = 8/15) of patients converted to a true CR (negative IFE) at a median time of 11.6 months (p = 0.011 when compared to the 25% threshold for futility). With a median follow-up of 5.13 years from enrollment, the median PFS could not be estimated as only 6 patients (40%) experienced disease relapse. At the time of the analysis, the median OS was 7.8 years from enrollment (95% CI: 4.2-7.8 years, n = 6/15, 40%).
MRD testing was performed on 7 patients. The disease burden threshold of 105/106 cells was arbitrarily used to evaluate the clinical significance of high-level and low-level MRD positivity. Interestingly, all 3 patients with high-level MRD experienced relapse within 1 year of vaccination (median = 4.8 months, range: 2.8 - 9.5 months), while median PFS for low-level MRD patients was significantly prolonged (median = 84.15 months, range: 51.9 - 97.3 months, p = 0.01). Consistently, high-level MRD was associated with increased likelihood of clinical relapse (hazard ratio, HR = 25.79, 95% CI: 2.17 - 306.4). Immune phenotyping of BM CD8+ T cells identified a CD27- DNAM1-/low subpopulation whose abundance was associated with prolonged MM remission. This subset included senescent, effector and exhausted CD8+ T cells and was nearly absent in patients with high-level MRD. Analysis of T-cell repertoires identified vaccine-specific T-cell clonotypes that expanded in both BM and PB of all patients and persisted for up to seven years. Accordingly, polyfunctional T cell responses against vaccine-specific and MM-related antigens were increased and persisted 7 years after enrollment.
Despite the correlation between MRD negativity and improved clinical outcome, 47% of patients subsequently developed a detectable M-spike that did not meet criteria for disease relapse nor required any change in treatment. We identified a bone marrow-resident T cell population likely responsible for the establishment of an immune equilibrium mediating long-term disease control.
Conclusions:
Collectively, these data demonstrate that an allogeneic MM vaccine in combination with Len effectively stimulates antitumor immunity and supports long-term remissions. Although MRD negativity is a critical factor in maintaining remission, the establishment of a MM-MGUS immune equilibrium is likely crucial in the setting of MRD+ MM to prevent disease progression. To our knowledge, this is the first study attempting to treat MRD+ MM in an effort to further improve disease response as well as prevent relapse.

Disclosures
Sanders: Adaptive Biotechnologies: Current equity holder in private company. Ali:Celgene: Membership on an entity's Board of Directors or advisory committees. Noonan:Aduro: Patents & Royalties. Borrello:WindMIL Therapeutics: Other: Founder , Research Funding; Aduro: Patents & Royalties; Celgene: Research Funding.