A Phase II Study of Daratumumab in Patients with High-Risk MGUS and Low-Risk Smoldering Multiple Myeloma

Introduction : Daratumumab (Dara) is an anti-CD38 monoclonal antibody that is approved for use in patients with newly diagnosed and relapsed multiple myeloma (MM). We hypothesized that early therapeutic intervention with Dara in patients with high-risk MGUS (HR-MGUS) or low-risk SMM (LR-SMM) would lead to eradication of the tumor clone by achieving deep responses, resulting in prevention of progression to MM. We present results of our phase II, single arm study of Dara in HR-MGUS and LR-SMM.

Methods : Patients enrolled on this study met eligibility for either HR-MGUS or LR-SMM. HR-MGUS is defined as <10% bone marrow plasma cells and <3g/dL M protein and at least 2 of the following 3 high-risk criteria: Abnormal serum free light chain ratio (SFLC) of <0.26 or >1.65, M protein ≥ 1.5g/dL or non-IgG M protein. LR-SMM is defined by one of the following 3 criteria: M protein ≥3g/dL, ≥10% bone marrow plasma cells, SFLC ratio <0.125 or >8. Dara (16mg/kg) was administered intravenously on a weekly schedule for cycles 1-2, every other week cycles 3-6, and monthly during cycles 7-20. The primary objective of this study was to determine the proportion of patients who achieve very good partial response (VGPR) or greater after 20 cycles of Dara. Secondary objectives included duration of response, safety, and rates of minimal residual disease (MRD)-negativity in VGPR or greater patients. Correlative studies included assessing changes in immune microenvironment, evaluating clonal heterogeneity using deep sequencing, and determining association of genomic aberrations correlating with either response to therapy or progression of disease.

Results : At the time of data cutoff, a total of 42 patients were enrolled on this study from 2018 to 2020 with participation of 5 sites. The median age for all patients at enrolment was 60 years (range 38 to 76), with 22 males (52.4%) and 20 females (47.6%). Majority of patients enrolled were classified as LR-SMM (n = 37, 88.1%) and the remaining 5 patients had HR-MGUS (11.9%).

41 patients have started treatment and are included in toxicity assessment, and 40 patients have at least completed 16 cycles (range 6-20). Grade 3 toxicities were rare and only experienced in 5/41 patients including diarrhea (n =1/41; 2%), flu like symptoms (n = 1/41; 2%), headache (n=1/41; 2%), and hypertension (n=2/41; 5%). Most common toxicities of any grade included fatigue (n = 24/41, 51%), cough (n = 19/41, 46%), nasal congestion (n = 18/41, 44%), headache (n = 14/41, 34%), hypertension (n = 11/41, 27%), nausea (n = 13/41, 32%), and leukopenia (n = 13/41, 32%). No patients have discontinued therapy due to toxicity.

Minimal response or better was observed in 82.9% of patients (34/41) and PR or better was observed in 51.2% of patients (21/41). This included overall CR (n = 4, 9.8%), VGPR (n = 1, 2.4%), PR (n = 16, 39.0%), MR (n = 13, 31.7%), and SD (n = 7, 17.1%). In the 40 patients who completed at least 16 cycles, response rates were as follows: MR or better 85% (34/40), PR or better 52.5% (21/40) and VGPR or better 12.5% (5/40). Median time to VGPR was 7 months. Median overall survival and progression-free survival have not been reached and no patients have progressed to overt multiple myeloma while on study.

Conclusion : Dara is very well tolerated among patients with HR-MGUS and LR-SMM with minimal toxicities. Responses are seen in majority of patients. Early therapeutic intervention in this precursor patient population appears promising but longer follow up is required to define the role of single agent Dara in preventing progression to MM, therefore avoiding more toxic interventions in this low-risk patient population.