Heavy/light chain ratio for the assessment of minimal residual disease in myeloma patients achieving complete response

Polyclonal antibodies against the conformational epitopes between the heavy and light chains (HLC) of immunoglobulin (Ig) were introduced as a diagnostic tool in multiple myeloma (MM) and other monoclonal gammopathies in 2009 (Bradwell et al, 2009). They separately identify the two different light chain types of each immunoglobulin class (IgGj, IgGk, IgAj, IgAk, IgMj and IgMk), allowing the quantification of the monoclonal component. Similarly to free light chain (FLC), the ratio of monoclonal and polyclonal immunoglobulins of the same isotype, for example IgGj/ IgGk, can also be measured (HLC ratio), thus enabling the monoclonal component to be monitored during therapy and disease evolution. HLC and HLC ratios may be particularly useful for monitoring the presence of monoclonal components that are difficult to identify using serum protein electrophoresis (SPE) (Paolini et al, 2015), especially in oligosecretory MM or when it migrates in the b region, as frequently observed in IgA MM (Boyle et al, 2014). The International Myeloma Working Group (IMWG) has recently published new consensus criteria for assessing response and minimal residual disease (MRD) in MM, outlining, along with the pivotal relevance of next-generation flow, next-generation sequencing and imaging plus negative MRD, the potential role of HLC assay and the need of its further investigation, particularly in patients achieving complete response (CR) after treatment (Kumar et al, 2016). In order to contribute to this specific issue, we conducted a single centre, prospective study of HLC ratio, and its comparison with FLC ratio, for the evaluation of MRD and early detection of relapse in MM patients achieving immunofixation-negative CR, according to the 2006 IMWG criteria (Durie et al, 2006), after initial treatments including novel agents. Twenty-five consecutive patients were evaluated. Mean age was 63 years (range 43–82) and 14 patients were male. Immunoglobulin isotype was IgG or IgA in 14 and 11 patients, respectively, with 20 patients showing kappa and 5 lambda light chains. According to the International Staging System, seven patients had stage 1, ten stage 2 and eight stage 3. Fourteen patients not eligible for autologous stem cell transplantation (ASCT) received a bortezomib-based treatment, which mainly constituted the combination of bortezomib, melphalan and prednisone (VMP), while 11 patients underwent ASCT after induction therapy with bortezomib, thalidomide and dexamethasone (VTD). With a median follow-up of 52 months (range 21–92), overall survival (OS) of the entire cohort was 61 months (95% confidence interval [CI] 52–80) and progression-free survival (PFS) was 26 months (95% CI 12–38). Immunoglobulin HLC pairs (IgGj/IgGk and IgAj/IgAk) and FLC were analysed on serum samples at diagnosis and at the time of CR (after 6–9 induction cycles in patients not eligible for ASCT, and after ASCT in those undergoing such a procedure), using Hevylite and Freelite commercial kits on a SPAplus analyser (Binding Site, Birmingham, UK); IgGj/ IgGk, IgAj/IgAk and j/k ratios were then calculated. In this selected population of patients analysed after achievement of high quality response (CR) to frontline treatment, seven (28%) samples showed abnormal HLC ratio and 14 samples (56%) had an abnormal FLC ratio. The different combinations of normal/abnormal ratios identified four different groups of patients, which are reported in Table I. FLC normalization in CR was significantly associated with better progression-free survival (PFS) (43 months, 95% CI 14–45, vs. 12 months, 95% CI 9–35, P = 0 049; Fig 1A). These data are in agreement with 2016 IMWG consensus, that has maintained, among standard response criteria, the value of “stringent CR” in patients with immune-fixation negative response (Kumar et al, 2016), though this has been recently questioned (Mart ınez-L opez et al, 2015). Interestingly, Tacchetti et al (2017) recently confirmed a relationship between relapse defined by increasing levels of FLC and shorter time to second-line therapy, risk of second progression and risk of death as compared to a relapse with increase in M-protein only. In contrast with FLC, no impact on PFS was observed in our series using the HLC assay (26 months, 95% CI 10–38, vs. 20 months, 95% CI 10–34, P = 0 51; Fig 1B), even when selecting IgA MM (data not shown). Notably, the negative effect of abnormal FLC ratio at CR on PFS was not mitigated by concomitant normalization of the HLC ratio (43 months,