Minimal Residual Disease (MRD) at 10^-6 Measured By Next Generation Flow (NGF) during Daratumumab Consolidation Therapy: Analysis at 18 Months Follow up of DART4MM Study (Daratumumab Treatment For Multiple Myeloma Eradication)

Background. New drugs with or without autologous stem cell transplantation (ASCT) can induce deep CR responses. MRD can be now considered in the response evaluation by the IMWG and many studies propose it as surrogate for survivals. Multiparametric flow cytometric assays have now been replaced by advanced assays that permit to assess simultaneously more than 8 markers in a single tube. In particular, Euro-flow consortium has developed NGF, a novel high sensitive and standardized approach for MM MRD evaluation that is based on the use of 2 single 8-color tubes, containing all the markers needed to distinguish normal vs MM PCs. However, it is necessary to work on fresh samples and to acquire 10⁷ cell/sample, so to have the possibility to evaluate the Limit Of Quantification (LOQ) and the Limit Of Detection (LOD). The LOQ is calculated as 50 clonal plasmacells among 10⁷ nucleated cells; the LOD as 20 clonal plasmacells among 10⁷ nucleated cells.

Aim. DART4MM is a single arm, multicenter, prospective study that evaluate Daratumumab effect on MM patients who already achieved VGPR/CR but MRD positive by NGF after a first line therapy (ASCT, VMP) (Gozzetti et al. IMW 2019). The purpose was to analyze 10.000.000 cells for MRD evaluation and reach at least 10^-6 level.

Patients and Methods. Next generation flow (NGF) is centralized and measured at Siena University Hospital with two 8 colors tubes panel developed by the EuroFlow Consortium (BD OneFLOW Tm PCSTe BD OneFLOW Tm PCD. BD BioSciences) with detection of MRD with a sensitivity (≥ 1 in 105 /10-6). Daratumumab 16 mg/kg administered at weekly intervals for 8 weeks, then every 2 weeks for an additional 8 weeks, will be given to 50 MM patients who achieved a VGPR or more defined as per IMWG criteria and MRD-positivity (by NGF). Daratumumab starts at least 12 weeks from ASCT and at least 4 weeks after VMP. Free light chain (FLC) and CT/PET are evaluated at time 0 and every 6 months. NGF is done on marrow aspirate at time 0, at 2 months and every 6 months for 2 years. Primary endpoint is achievement of MRD negativity at 6 months: if patients are MRD negative after 6 months of therapy, treatment is stopped. Otherwise treatment will continue every 4 weeks up to 2 years. Rapid infusion was allowed from the third dose (cycle 1, day 15) if no serious IRR was seen in the previous infusion (second). The infusion rate was calculated to deliver 20% of the dose over 30 min (200 mL/hr), and then the rate was increased to deliver the remaining 80% over 60 min (450 mL/hr). This resulted in a 90 min estimated infusion time (total volume 550 mL).

Results. Recruitment started at the end of December 2018. 70 patients were screened until July 2020 at 5 centers in Italy. At least 10 million cells were analyzed for sensitivity at flow for each sample. 31/70 (44%) resulted MRD positive and eligible. M/F =15/16, median age was 61 (range 48-68).Three patients were excluded from the protocol because of consent withdraw. Previous therapy were single ASCT (21 patients), double ASCT 3 patient, VMP (3 patients), KRD (1 patient). ISS stage was I in 8 patients, II in 9 patients, and III in the other 6 patients. Cytogenetics/FISH analysis at diagnosis was done in 25/28 patients : it was negative for 17p deletion, t(14q) and 1q amplification in 16 patients, 2 had t(4;14) , 5 had t(11;14), 2 had del 17p, 1 del 13q, +11 in 2. Grade 2 reaction (moderate infusion-related reactions) during first daratumumab infusion was seen in 10/28 (35%) patients and promptly resolved with corticosteroids administration and temporary infusion interrumption. More than 200 rapid infusions were given to 16 patients. No serious adverse event was registered. 22/28 (79%) patients completed 8 weeks of treatment (2 months) and evaluated MRD. 17/28 (60%) completed 6 months of therapy. MRD negativity was reached at 6 months in 9/17patients (53%). Interestingly 9/13 (62%) patients treated previously with ASCT were MRD negative (10^-6) after 6 months of Dara and stopped treatment. 12 patients reached 12 months of follow up: 2/12 patients are still MRD negative at 10^-7 (6 lost MRD negativity).

Conclusions. Follow up will continue with marrow evaluation for MRD every 6 months until 2 years. Having at disposition high quality BM samples for MRD evaluation can ameliorate our assays, even to 10^-6 or 10^-7 and it is crucial to have a good coordination between clinicians and laboratories so to improve the accuracy, sensitivity, and specificity of MM MRD detection in MM patients.