Ibrutinib-Lenalidomide-Rituximab in Patients with Relapsed/Refractory Mantle Cell Lymphoma: Final Results from the Nordic Lymphoma Group MCL6 (PHILEMON) Phase II Trial

Introduction: In spite of improvements in treatment of mantle cell lymphoma (MCL), this is still considered an incurable lymphoma entity, and the majority of patients eventually relapse. Ibrutinib is a very active agent in MCL, but in vitro has been shown to partially antagonize the activity of rituximab, by suppression of NK cell activity and subsequent ADCC. Lenalidomide, on the other hand, improves rituximab-induced ADCC. In this multi-centre open-label phase II trial, we evaluated safety and efficacy of this triplet combination in patients with relapsed or refractory MCL.

Methods: Patients with MCL, relapsing after or refractory to at least one rituximab-containing chemotherapy regimen, WHO PS 0-3, and measurable disease were eligible. The primary endpoint was maximal overall response rate (ORR) measured with CT and PET/CT. Minimal residual disease (MRD) monitoring by PCR was performed during follow-up, according to EuroMRD criteria. Ion Torrent sequencing of the most frequently mutated genes in MCL was performed on frozen tumor cells from bone marrow at time of relapse. Health-related quality of life was assessed by the EORTC-QLQ C30 questionnaire before and during treatment.

Treatment schedule: Induction phase: Up to twelve 28-day cycles with: Lenalidomide 15 mg p o daily, days 1-21, Ibrutinib 560 mg p o days 1-28, Rituximab 375 mg/m² i v day 1 in cycle 1, then 1400 mg s c (or 375 mg/m²i v) days 8, 15 and 22 in cycle 1, then day 1 in cycles 3, 5, 7, 9 and 11. Maintenance phase: For patients in CR, PR or SD, not in need of other treatment, given until progression, cycle duration 56 days. Ibrutinib: 560 mg p o days 1-56, 2. Rituximab 1400 mg s c (or 375 mg/m²i v) day 1 of each cycle.

Results: Accrual of 50 pts was completed in June 2016, at 10 centres in Sweden, Norway, Denmark and Finland. The median age was 69.5 years, with a median MIPI score of 6.2. Patients had received a median of two previous regimens, four had progressed after single agent ibrutinib, and three had received prior allo-SCT. A TP53 mutation was detected in 11 of 49 evaluable cases (22%), 8 cases were of blastoid/pleomorphic histology, and 22 of 40 evaluable cases had a Ki67 >30%. Treatment emergent-AEs of any grade in ≥20% of patients were rash (24%) and fatigue (20%). Five pts (10%) experienced rash grade 3, mainly during cycle 1. Hematological toxicity was generally of low grade, apart from grade 3-4 neutropenia in 5 patients. One patient died due to possible treatment-related toxicity (septic shock). In total, 27 patients achieved CR (54%) and 10 PR (20%). Among evaluable patients with a TP53 mutation, blastoid/pleomorphic histology or Ki67 >30%, the CR rates were 7/11 (64%), 15/8 (62%) and 11/22 (50%), respectively. After a median follow-up of 40 months, the median PFS is 18 months (95% CI 6.5-28), and median OS 47 months (95% CI 30-64). Patients with a detectable TP53 mutation at relapse (n=11) had a median PFS of 13 months (95% CI 4.2-21), whereas pts without a TP53 mutation had a median PFS of 34 months (95% CI 8.3-60). Of the 28 patients evaluable for MRD at 6 months, 15/27 (56%) patients achieved molecular remission in blood and 12/28 (43%) in bone marrow. After 12 months, MRD-negativity in BM was 68% (13/19). Out of 4 patients with TP53-mutated MCL, 2 were MRD-negative in BM after 12 months, as well as 2 out of 4 patients with blastoid/pleomorphic histology. By self-reported HRQOL, a lower level of emotional functioning (EF), as well as a higher level of pain (PA) at baseline, was associated with inferior PFS. In addition, low EF was associated with inferior OS. By a Cox regression multivariable analysis, including MIPI, TP53, histology, Ki67, EF and PA, only MIPI was prognostic for PFS or OS with this regimen.

Conclusions: The combination of ibrutinib, lenalidomide and rituximab has been shown to be an active and well tolerated regimen in this cohort of high risk R/R MCL, associated with a high rate of molecular remission. The activity in TP53 mutated MCL is lower than in unmutated disease, but this regimen may still serve as an option for a bridge to an allogeneic transplantation or CAR-T therapy in this category of patients.