Mantle cell lymphoma (MCL) has served as a paradigm of progress among the non-Hodgkin lymphomas over the past 30 years. It was originally defined within the Kiel classification as centrocytic lymphoma, then renamed MCL once the characteristic translocation and resulting cyclin D1 overexpression were identified. These diagnostic markers allowed for the characterization of MCL subtypes as well as the initiation of MCL-focused clinical trials which, in turn, led to regulatory approval of more effective regimens, new therapeutic agents, and an improvement in overall survival (OS) from around 3 years to more than 10 years for many patients.
Despite this progress, virtually all patients relapse, and a cure remains elusive for most. In younger (< 65 to 70 years), medically-fit patients who are transplant-eligible and have symptomatic MCL, a standard of care has been induction chemoimmunotherapy containing high-dose cytarabine followed by ASCT consolidation. For example, a clinical trial of R-CHOP (rituximab, cyclophosphamide, doxorubicin, vincristine, prednisone) alternating with R-DHAP (rituximab, dexamethasone, high-dose cytarabine, cisplatin; 3 cycles each) showed a significant benefit over R-CHOP x 6 cycles; at a median follow-up of 10.6 years, the time-to-treatment failure was 8.4 v 3.9 years . In another trial, all patients received induction R-DHAP (with cisplatin or an alternative platinum agent) x 4 cycles followed by ASCT. Those patients randomized to post-ASCT maintenance rituximab for 3 years had significantly improved, 4-year progression-free survival (PFS) as compared with observation only (83% vs 64%, p < 0.001); maintenance also significantly improved OS.
Although ASCT consolidation followed by maintenance became widely adopted on the basis of these and other clinical trials, important questions remain:
First, MCL is biologically and clinically quite heterogeneous. Several prognostic tools such as the MCL International Prognostic Index (MIPI) scoring system and biomarkers are available to define lower- versus higher-risk subtypes, but none is routinely used for treatment planning . About 15% of MCL patients present with a highly-aggressive blastoid or pleomorphic variant that usually carries a TP53 mutation or deletion. Given the short survival and limited benefit from dose-intensive chemotherapy and ASCT in TP53-mutated MCL, should transplant be avoided in these patients?
Second, if deep remission is achieved following front-line therapy, defined as positron emission tomography (PET) negative and measurable residual disease (MRD) negative, will high-dose chemotherapy and ASCT provide additional benefits or only toxicity? This question is being addressed by the ongoing ECOG 4151 study , a risk-adapted trial in which post-induction MRD-negative patients are randomized to standard ASCT consolidation plus maintenance rituximab vs maintenance only.
Bruton tyrosine kinase inhibitors (BTKi) are now among the most used agents for relapsed MCL. Recent clinical trials testing the integration of a BTKi into first- or second-line therapy have shown increased response rates and variable clinical outcomes and toxicities for the combinations, depending upon the chemotherapy- and non-chemotherapy backbones utilized, as well as the BTKi. Combinations with the BCL2 inhibitor venetoclax plus chemotherapy or BTKi are also showing promise.
The activity of BTKi in MCL led the European MCL Network (EMCL) to design the 3-arm TRIANGLE study to analyze the potential of ibrutinib to improve outcomes when given in conjunction with standard ASCT consolidation, and the ability to replace the need for ASCT. The TRIANGLE results were presented by Dr. Martin Dreyling in the Plenary Session at the December 2022 American Society of Hematology (ASH) Annual Meeting . Transplant-eligible MCL patients < 65 years of age were randomized to the EMCL’s established front-line therapy of alternating R-CHOP/R-DHAP plus ASCT ; the same regimen plus oral ibrutinib given with the R-CHOP induction cycles and then post-ASCT ibrutinib maintenance therapy for 2 years (Arm A+I); or the A+I regimen minus ASCT (Arm I). Maintenance rituximab was allowed in each arm, on the basis of the treating centers’ institutional guidelines. Overall, 54%-58% of patients in each study arm received rituximab maintenance, with no differential benefit in efficacy noted for those so treated.