Ofatumumab Plus Chemotherapy Followed By G-CSF For Mobilization Of Peripheral Blood Stem Cells (PBSC) In Patients With B-Cell Non-Hodgkin’s Lymphomas (NHL)

The in-vivo use of anti-CD20 mAbs during PBSC mobilization has the potential to improve the cure rates following autologous stem cell transplantation in patients with NHL (Khouri et al. J Clin Oncol 2005). Ofatumumab, a type I human antibody, binds to a different epitope of CD20 than rituximab. We studied the feasibility of PBSC mobilization following ofatumumab with ifosfamide/etoposide (a standard chemo-regimen at our center). In addition, as the very late antigens (VLA)-4 and VLA-6 are expressed on hematopoietic stem cells (HSC) and both play a role in homing and engraftment of HSC, we studied these markers on HSC progenitor (CD34+) cells.

Eligibility criteria included patients with chemosensitive NHL with<5% marrow involvement at study entry, a platelet count of > 100,000/mm³ and an ANC > 1500/mm^3. Ofatumumab was given at 1000 mg on day 1 and 2000 mg of day 8. Chemotherapy (days 2-4) and G-CSF were used as per standard protocols at our center. Each patient provided a PB sample prior to mobilization and an aliquot of leukapheresis product (LP) after mobilization for detection of leukocyte subsets including T-, B-, natural killer (NK)- and dendritic cell (DC)-subsets, and VLA-4 and VLA-6 molecules on CD34⁺ PBSC by multicolor FACS analyses. Differences between median values of leukocyte subsets between LP and PB were determined using the Wilcoxon Signed Rank test.

Seventeen patients were studied; Median age was 52 years (range, 25-68). Histologies included: Diffuse large cell (n=9), Follicular (n=4), Mantle cell (n=3) and double-hit lymphoma (n=1). Median number of prior chemotherapies received was 2 (range, 1-4). Fourteen (82.3%) patients mobilized PBSC after a median of 3 (range, 1-6) apheresis. WBC count at day one of collection was 19.6 (range, 4.8-31) k/μL. Three (17.6%) patients failed collection of at least 2x10⁶ CD34+cells/kg but then mobilized successfully after receiving Plerixafor. All patients were able to receive an autologous transplantation with BEAM. Median number of CD34 infused was 5.56 x10^6/kg(range, 3.79-8.85). Median times to recovery of ANC > 500/mm³ and a platelet count of >20,000/mm³ were 11 days (range, 10-12) and 10 days (range, 9-15), respectively, following transplantation. With a median follow-up time of 9 months (range, 2-11 months) post-transplant, 16 (94%) patients remain alive in remission and 1 patient died of progression. Three patients experienced grade 2 toxicity with ofatumumab (hypertension, rash, and bone pain). None had > grade 2. Correlative studies were undertaken in 14 patients. LP had a significantly higher median WBC than paired PB samples (31.1 vs. 4.3 k/uL; p=0.001) but a significantly lower median percentage of lymphocytes (3.0% vs. 18.5%; p=0.001). Among the lymphocytes, LP had significantly higher percentages of CD3⁺ CD4⁺ T cells (56.9% vs. 46.7%; p=0.048) and CD3^-CD16^-CD56⁺ NK cells (13.7% vs. 7.24%; p=0.022), respectively than paired PB samples. There were no significant differences in total T (CD3⁺), T suppressor/cytotoxic (CD3⁺ CD8⁺) B (CD19⁺), and CD16⁺ NK cells.

In addition, total dendritic cells (DC: Lin^-HLA-DR⁺), and DC subsets comprised of myeloid-derived DC (mDC: CD11c⁺CD123^-) and plasmacytoid-derived DC (pDC: CD11c^-CD123⁺) were assessed in LP and PB samples. Compared with PB, LP had a significantly higher percentage of total DC (0.40 vs. 0.32; p=0.022) but significantly lower percentages of mDC (17.25 vs. 61.9; p=0.001) and pDC (13.35 vs. 21.85; p=0.035), respectively.

The median percentage of HSC progenitors (CD34⁺ CD38^dim) in PB was significantly lower in PB than in LP (12.3% vs. 46.4%, p=0.001). Moreover, LP specimens were assesssed for VLA-4/CD49d and VLA-6/CD49f expression. Median percentage of VLA-4 expression was significantly higher in LP than in PB samples (99.05% vs. 55.65%, p= 0.001). In contrast, the median percentage of VLA-6 expression was significantly lower in LP than in PB (22.15% vs. 60.3%, p=0.003).

PBSC mobilization with ofatumumab is feasible with minimal toxicity. This resulted in an increase in the recruitment of T-helper, NK, and DC subsets in the leukopheresis product, compared with levels in PB. Although others have reported a positive association between the expression of VLA-4 on CD34⁺ CD38^dim cells and time to engraftment, we were unable to confirm this in our study. Since this is most likely due to the small sample size, additional studies are warranted.

Khouri:GlaxoSmithKline: Research Funding. Off Label Use: Ofatumumab in stem cell mobilization.