Risk-Adapted Induction and Maintenance with Ofatumumab in Previously Untreated Patients with Chronic Lymphocytic Leukemia (CLL) / Small Lymphocytic Lymphoma (SLL)

Introduction: Pre-treatment cytogenetics and post-treatment minimal residual disease (MRD) status are predictive biomarkers in CLL. We hypothesized the two markers can guide a risk-adapted treatment strategy to achieve deep and durable responses in previously untreated CLL/SLL.

Methods and Patients: 31 subjects were enrolled. Risk assessment was performed before treatment and after induction. Based on pre-treatment FISH, patients were categorized into either high-risk (with deletion 17p or 11q) or standard-risk (no deletion 17p or 11q) and treated with up to 6 cycles of OFC (Ofatumumab 300mg i.v. on cycle 1 day 1, 1000mg i.v. on cycle 1 day 8 and on day 1 of subsequent cycles; Fludarabine 25mg/m² i.v. on day 1-3; Cyclophosphamide 250mg/m² i.v. on day 1-3;) or OF (Ofatumumab given as described for the high-risk group; Fludarabine 25mg/m² /d i.v. on day 1-5), respectively. Three months after completion of chemoimmunotherapy, a second risk stratification was performed based on presence or absence of MRD. MRD negative (MRD^neg) state was defined as <1 monoclonal B cell among 10⁴ leukocytes based on 8-color peripheral blood flow cytometry (PBFC). MRD^neg patients were observed and MRD positive (MRD^pos) patients received ofatumumab maintenance (1000mg i.v. every two months for 4 cycles). Serial PBFCs were performed pre- and post-induction, pre- and post-maintenance, and at 24 months. The primary endpoint was progression free survival at 24 months.

Results: 25 patients who completed ≥3 cycles of chemoimmunotherapy were considered evaluable; median age 60 years (range 26-79), 56% male, 48% in high Rai stage, 68.6% having unmutated IGHV. Nine (36%) patients received OFC, 16 (64%) received OF. 72% of all patients required dose reductions and 64% completed 6 cycles. After induction, 15 (60%) patients were MRD^pos and received maintenance, while 10 MRD^neg patients were observed. More patients on the OFC arm were MRD^neg than on the OF arm (66.7 vs. 25%, P=0.087). At a median follow-up of 32 months, 7 patients had progressed. The projected PFS was 95% at 24 months (85-100%), 82.5% at 32 months (66-100%), and 75% at 36 months (56-100%), with no significant difference between subgroups divided by treatment regimen or MRD status after induction. The estimated median PFS was 36 months for MRD^pos vs. 49 months for MRD^neg patients (P=0.13). At 24 months, 3 out of 7 MRD^neg patients maintained MRD negativity without further treatment. For MRD^pos patients, the number of residual CLL cells appeared to decrease during maintenance ofatumumab and two patients became transiently MRD^neg. However, the change in MRD burden was not statistically significant (median CLL cell count per 10⁴ leukocytes: 50 vs. 36 at pre- vs. post-maintenance, P=0.13) and none of the patients receiving maintenance was MRD^neg at 24 months. The median CLL count per 10⁴ leukocytes increased from 36 at post-maintenance to 260 at 24 months (P=0.047). Taken together, MRD after chemoimmunotherapy was not eradicated with additional doses of ofatumumab. We hypothesized that loss of CD20 expression through trogocytosis (Fc-receptor mediated "stripping" of CD20 from CLL cells) or internalization may limit the efficacy of ofatumumab. In fact, among MRD^pos patients, CD20 expression was negative by PBFC in 90% of patients at post-induction and in all patients at post-maintenance. On bone marrow biopsies, immunohistochemical staining for CD20 using an antibody against an intracellular epitope confirmed CD20 loss or demonstrated substantially decreased CD20 expression compared to matching baseline samples.

Conclusion: Pre-treatment risk stratification with FISH cytogenetics can guide the choice of induction therapy. Maintenance therapy with four additional doses of ofatumumab in MRD^pos patients did not improve the depth of response but appeared to reduce the risk of disease progression. Loss of CD20 expression on residual tumor cells may limit the efficacy of prolonged therapy with an anti-CD20 monoclonal antibody.