Droplet Digital PCR Quantification of Mantle Cell Lymphoma Follow-up Samples From Four Prospective Trials of the European MCL Network

Minimal residual disease (MRD) has been increasingly investigated in mantle cell lymphoma (MCL), including for individual therapeutic stratification and pre-emptive treatment in clinical trials. Although patient/allele specific real-time quantitative polymerase chain reaction (qPCR) of IGH or BCL1-IGH clonal markers is the gold-standard method, its reliance on a standard curve for relative quantification limits quantification of low-level positivity within the 1E-4 to 1E-5 range; over half of positive MRD samples after treatment fall below the quantitative range (BQR) of the standard curve. Droplet digital PCR (ddPCR), in contrast, allows absolute quantification, including for samples with no baseline determination of tumor infiltration by multicolor flow cytometry (MFC), avoiding the need for a reference standard curve. Using updated, optimized, ddPCR criteria we compared it with qPCR in 416 MRD samples (and with MFC in 63), with over-representation (61%) of BQR results by qPCR, from a total of 166 patients from four prospective MCL clinical trials. ddPCR, qPCR and MFC gave comparable results in MRD samples with at least 0.01% (1E-4) positivity. ddPCR was preferable to qPCR since it provided more robust quantification at positivity between 1E-4 and 1E-5. Amongst 240 BQR samples with duplicate or triplicate analysis, 39% were positive by ddPCR, 49% negative and only 12% remained positive below quantifiable ddPCR limits. The prognostic relevance of ddPCR is currently under assessment in the context of prospective trials within the European MCL Network.