Bendamustine and PBSC Mobilization

Why the Concern Exists

• Direct DNA cross-linking and strand breaks in hematopoietic stem and progenitor cells (HSPCs)
• Damage to the bone marrow microenvironment/niche, reducing the pool of mobilizable CD34+ cells
• Prolonged bone marrow suppression compared to pure alkylators (e.g., cyclophosphamide), because the nucleoside-like component impairs DNA repair in HSPCs

1. Does prior exposure to bendamustine impair subsequent PBSC mobilization?
2. Can bendamustine itself be used as a mobilizing chemotherapy regimen?

1. Bendamustine as a Prior Therapy - Impact on Mobilization

• ASH 2018 abstract (Blood 2018;132:4556) - A retrospective analysis specifically concluded that "bendamustine adversely affects stem cell mobilization" among patients proceeding to autologous stem cell transplant, with mobilization failure (yield <1 x10⁶ CD34+/kg on apheresis day 1) significantly higher in bendamustine-exposed patients.
• SWOG S1106 RCT (Chen et al., Br J Haematol 2017, PMID 27992063) - This randomized phase II trial in mantle cell lymphoma compared rituximab-bendamustine (RB) vs. rituximab-hyperCVAD induction before auto-HCT. Notably, the RH arm was closed early due to a 29% mobilization failure rate - the RB arm (with bendamustine) did NOT cause the same problem. This indirectly underscores that while bendamustine is usable in certain contexts, other heavily myelotoxic regimens carry even higher risk.

• Number of bendamustine cycles - risk increases with cumulative dose
• Bendamustine dose per cycle (120 mg/m² vs. 60-90 mg/m²)
• Time interval between last bendamustine dose and mobilization attempt - shorter intervals increase failure risk
• Concurrent fludarabine or other nucleoside analogs - compounds the stem cell toxicity
• Prior radiotherapy to marrow-bearing sites

• A waiting period of at least 6-12 months after the last bendamustine dose is generally recommended before mobilization attempts; some guidelines suggest up to 12 months for patients who received multiple cycles
• Patients with >4 cycles of BR should be considered high-risk for mobilization failure
• Use of plerixafor (CXCR4 antagonist) is strongly recommended, either preemptively or reactively (triggered by low circulating CD34+ on day of planned apheresis)

2. Bendamustine as a Mobilizing Chemotherapy Agent (BED Regimen)

• BED regimen (Bendamustine 120 mg/m² d1-2 + Etoposide 200 mg/m² d1-3 + Dexamethasone 40 mg d1-4) followed by G-CSF:
  • Multiple myeloma (Green et al., Bone Marrow Transplant 2016, PMID 27214069): Phase II, n=34. 100% of patients successfully mobilized, median CD34+ yield 21.6 x10⁶/kg (range 9.2-55.5 x10⁶/kg); 88% required only a single apheresis session. Six serious non-hematologic adverse events were observed (neutropenic fever, bone pain, renal insufficiency).
  • Non-Hodgkin lymphoma (Greenbaum et al., Blood Res 2018, PMID 30310789): Phase II subset, n=6 (DLBCL, FL, PMBL, NK/T-cell). All patients mobilized successfully, median CD34+ yield 7.9 x10⁶/kg (range 4.4-17.3 x10⁶/kg), median 1.5 days of apheresis. Engraftment kinetics comparable to other regimens.
• BPV (Bendamustine 60 mg/m² + Bortezomib + Prednisone) as induction before mobilization:
  • Poenisch et al. (J Cancer Res Clin Oncol 2015, PMID 25976868): n=35 newly diagnosed MM. After BPV induction (median 2 cycles), mobilization with cyclophosphamide 4g/m² + G-CSF was successful in all patients. Median CD34+ yield 13.5 x10⁶/kg. 54% required only a single apheresis. Engraftment successful in 34/35 patients. PFS at 18 months 87%.

3. Mantle Cell Lymphoma - A Disease-Specific Nuance

• BR induction (6 cycles) before transplant raised concerns initially, but the SWOG S1106 data showed the RB arm had no excessive mobilization failures - the problem was on the R-hyperCVAD arm (29% failure, study closed early)
• A pilot study of sequential BR + cytarabine/rituximab ± acalabrutinib in MCL (Patel et al., Clin Lymphoma Myeloma Leuk 2023, PMID 37183115) confirmed all patients successfully underwent stem cell harvest, supporting its use as a platform for future transplant trials

Summary Table

Clinical Bottom Line

• Cumulative dose and timing are the most important determinants - more cycles and shorter intervals to mobilization increase failure risk
• Patients with heavy prior bendamustine exposure should have plerixafor incorporated preemptively rather than reactively
• G-CSF alone is often insufficient in bendamustine-pretreated patients; chemomobilization with cyclophosphamide or the BED regimen may be preferred
• When bendamustine is the planned mobilizing agent (BED), results are surprisingly favorable, likely because patients are not heavily pretreated with it at that point
• Remobilization after primary failure is possible in >70% of cases with plerixafor-based rescue, but a ≥4-week break is required

Acalabrutinib and PBSC Mobilization

The Short Answer

Mechanism: How BTK Relates to Stem Cell Mobilization

• Disrupt CXCR4/SDF-1 signaling in HSPCs, potentially altering their retention in the marrow
• Increase circulating HSPCs - this is actually observed with ibrutinib (lymphocytosis and redistribution are hallmarks; analogous effects occur on HSPCs)
• Reduce VCAM-1/VLA-4 adhesion - BTKi are known to interfere with integrin-mediated bone marrow retention

Clinical Evidence

• Sequential BR (bendamustine/rituximab) alternating with cytarabine/rituximab, with acalabrutinib added in FitMCL 2.0
• Primary endpoint: stem cell mobilization success rate
• Result: all patients in both cohorts (with and without acalabrutinib) successfully underwent stem cell harvest
• These pilot data supported the launch of the NCTN trial EA4181 comparing induction regimens ± acalabrutinib in transplant-eligible MCL

• Acalabrutinib 100 mg BID continuously combined with alternating BR and cytarabine/rituximab x 6 cycles in untreated MCL
• Primary endpoint was specifically stem cell mobilization success rate (target >2 × 10⁶ CD34+/kg)
• G-CSF support provided; leukapheresis after cycle 6
• Confirmed that mobilization success after acalabrutinib-containing chemoimmunotherapy was feasible as the study primary outcome (results published as part of the FitMCL pooled data above)

• Phase Ib in treatment-naive and relapsed/refractory MCL
• ORR 94.4% (TN) and 85.0% (R/R); CR 77.8% (TN)
• Hematologic toxicity reported (expected with BR), but no specific mobilization failure data reported since this was not a transplant-directed trial

Contrast with Ibrutinib

Key Practical Points

1. Acalabrutinib does not directly damage hematopoietic stem cells - there is no alkylating or nucleoside-analog mechanism to impair the CD34+ pool
2. Prior or concurrent acalabrutinib does not appear to cause mobilization failure in the MCL pilot data - 100% collection success when combined with BR/cytarabine-based induction
3. The bigger mobilization risk in these patients is still the bendamustine component (see previous discussion), not the BTK inhibitor
4. Holding acalabrutinib before mobilization is not universally required, but some centers hold it 3-5 days before apheresis to minimize any potential CXCR4-mediated effects on niche retention (analogous to the practice with ibrutinib in CLL)
5. Data remain limited - acalabrutinib-specific mobilization data come from small pilot studies in MCL; there is no large prospective trial dedicated to this question

Bottom Line