Minimal residual disease in mantle cell lymphoma: are we ready for a personalized treatment approach?

Mantle cell lymphoma (MCL) is nowadays recognized as a spectrum of diseases, characterized by significantly different treatment responses and outcomes. Some predictors of clinical and biological outcome have been established and validated over recent years, and these are either assessable at baseline (mainly MCL international prognostic indexes, Ki-67 proliferative index and genomic aberrations) or during treatment (functional imaging and minimal residual disease, MRD). MRD is defined as the minimal traceable persistence of lymphoma cells after a successful treatment. Many methods to monitor MRD have been published; however, the most sensitive and the most commonly used and best standardized approach in MCL is represented by the allele-specific oligonucleotide (ASO) quantitative polymerase chain reaction (qPCR) method.1 The most relevant prospective trials investigating the impact of MRD on MCL patient outcome are listed in Table 1A.

Table 1A.Prospective clinical trials investigating the impact of minimal residual disease (MRD) on patients’ outcome in mantle cell lymphoma (MCL) patients.

The clinical role of MRD analysis in MCL is reflected according to four major aspects (Figure 1).

Figure 1.The clinical role of minimal residual disease analysis in mantle cell lymphoma.

MRD provides early feedback on the efficacy of the clearance of different induction regimens

The dynamics and stability of tumor shrinkage after treatment can currently be precisely tracked by MRD kinetics; these data might be useful as an early in vivo predictor of the anti-lymphoma effect of a new compound.1096 Moreover, MRD can be used as a surrogate end point for progression-free survival (PFS) comparing the efficacy of different treatments in randomized trials, thus accelerating the development, and eventually the approval, of new drugs. For example, the superior outcome of the cytarabine-containing experimental arm of the “MCL Younger” phase III trial of the European MCL Network was heralded by a higher rate of MRD clearance many years before publication of the final results.155

MRD can provide an early prediction of disease recurrence

Even in the context of an incurable disease like MCL, the deepness of treatment response measured by MRD widely reflects patient outcome in large, prospective trials.53 The predictive role of MRD analysis in MCL was confirmed in different patient subsets (both younger and elderly), treatment strategies (autologous transplantation and conventional immuno-chemotherapy), tissues (bone marrow and peripheral blood) and time points (end of induction and during maintenance treatment).4

MRD allows for risk stratification of patients after treatment

MRD describes the efficacy of therapy and presence of even minimal, resistant tumor clones; thus, this approach identifies patients at higher risk of recurrence after an apparently successful treatment. Actually, persistence of MRD positivity or recurrence after a transient MRD negativity precedes clinical relapse, with a median time lag of 18 months.16

MRD might drive pre-emptive treatment

As MRD positivity predicts upcoming clinical relapse, this approach can guide treatment tailoring, with the aim of preventing or delaying overt disease progression. In a number of prospective reports, a pre-emptive rituximab treatment of MRD positive patients was able to reconvert them to MRD negativity, with the possibility of also prolonging their PFS.178

Nevertheless, some limitations still hamper the widespread use of MRD analysis in clinical routine. The two major obstacles as far as methodology is concerned are the need for patient-specific primers and standardization issues. At present, the ASO-qPCR strategy relies upon either the clonal rearrangement of the IGH gene or the BCL-1/IGH rearrangement, derived from the t(11;14); both of these DNA sequences are unique for each B-cell clone, so individual primers are required for each patient to guarantee a reliable sensitivity. Thus a “one-fits-all” easy-to-use in vitro diagnostic medical device (IVD-kit) is not conceivable in MRD diagnostics so far, and access to an experienced and dedicated laboratory is mandatory. In addition, a rigorous standardization of the methods is essential in order to provide comparable results among different centers. Only selected laboratories across Europe have so far been certified by the Euro MRD consortium, a standardization group regularly performing quality control rounds for MRD analysis in leukemia and lymphoma (http://www.euromrd.org).18 Only by standardized analyses in this laboratory network can reliable MRD results be obtained in large, international clinical trials, such as the recently launched TRIANGLE trial (EudraCT n. 2014-001363-12), sponsored by the European MCL Network. Moreover, another limitation of the current technique is that 10–15% of patients still lack a reliable molecular marker for MRD. For the moment, in MCL, an IGH-based marker is available in approximately 70% of cases and a BCL-1/IGH marker in approximately 35–40%, with some overlapping cases.4 In particular, patients with low or absent bone marrow invasion often do not carry a marker and cannot, therefore, be analyzed for MRD. In addition, hypermutated IGH genes may hamper an optimal primer design. Finally, although the predictive role of MRD has been established in MCL,53 evidence for the usefulness of subsequent treatment tailoring based on the MRD results is unfortunately still scarce due to the lack of MRD-driven phase III trials in MCL.178 Moreover, no MRD data are available yet in the context of the new targeted treatments, such as the Bruton’s tyrosine kinase inhibitor ibrutinib.

However, many technical innovations have recently been introduced in the MRD field, and these have the potential to overcome the issues of applicability and sensitivity described above. The droplet digital PCR (ddPCR), a 3-generation, end point, quantitative PCR has been shown to provide comparable results to ASO-qPCR for MRD monitoring in MCL with the advantage that it is less labor intensive. Moreover, since it does not require a standard curve for tumor quantification, ddPCR might provide reliable and sensitive MRD results also in cases in which the classical approach has failed.19 Currently, a totally innovative approach is represented by the application of next-generation sequencing (NGS) techniques to the MRD field. The LymphoSIGHT™ approach was first published for MRD detection in acute lymphoblastic leukemia,20 and was subsequently shown to be feasible also in MCL.21 Its main advantages rely on the fact that it does not require patient-specific reagents (being thus suitable for an IVD-kit), it can provide additional MRD targets for patients lacking a “classical” molecular marker, it should easily reach high sensitivity levels, and might overcome some false-negative results (e.g. deciphering the clonal evolution issues). However, until now, this promising NGS technology has been available as a commercial tool only in US. Nevertheless, many laboratories are currently implementing alternative NGS-based approaches for MRD and an international development and standardization effort is ongoing within the EuroClonality-NGS laboratory consortium (http://www.euro-clonality.org/wp-content/uploads/2015/03/EuroClonality-NGS.pdf).22 Moreover, further promising NGS-based approaches for the identification of new molecular markers are being studied and might effectively provide an MRD target for each patient in the near future (Targeted Locus Amplification and Rapid Capture techniques).2423 Finally, MRD targeting on plasmatic, circulating tumor DNA is extremely promising as a means to track lymphoma clones residing outside the peripheral blood or bone marrow compartments.25 Despite all of these encouraging data, a large-scale validation of each of these new technologies is required before their introduction into clinical practice.

Nowadays, MRD evaluation is mostly based on ASO-qPCR in the most important ongoing European clinical trials and, as explorative investigation, as part of clinical routine in a few selected centers. However, for the moment, the published diagnostic and treatment guidelines discourage any clinical decision-making based on MRD results,26 mainly due to limitations of technical standardization and to the lack of prospective, randomized trials evaluating modified therapy according to MRD. Reliable and reproducible MRD data can currently be guaranteed only by the standardized methodological guidelines of the Euro MRD laboratory network. Therapeutic decisions based on MRD results obtained by different, as yet not validated techniques may even compromise the patient’s long-term outcome. Moreover, convincing data on the real usefulness of such an MRD-based treatment modulation are not yet available in lymphoma, with only retrospective evidence or evidence derived from a single, phase II trial available so far.178 Some MRD-driven phase II and III trials are ongoing in MCL and also in follicular lymphoma to assess the clinical impact of personalized treatment, with first results eagerly awaited in the near future (reviewed by Dogliotti and Ferrero27 and summarized for MCL in Table 1B).

Table 1B.Current MRD-driven trials in MCL.

In conclusion, given that there is compelling evidence as to the predictive role of MRD in MCL, and ongoing standardization and methodological efforts are highly advanced, MRD analysis in MCL is already included in the majority of prospective trials. The next steps will consist of large trials investigating an MRD-driven tailored therapeutic approach (Table 1B). Thanks to the expected results and the development of rapidly evolving MRD techniques, we foresee that, in the near future, the huge research efforts over the last years will finally translate into personalized treatment strategies as part of clinical routine, leading to a further benefit for lymphoma patients in general.