Outcomes of a novel rituximab-based non-myeloablative conditioning regimen for hematopoietic cell transplantation in severe aplastic anemia

Allogeneic hematopoietic stem cell transplantation (HCT) is recommended for older patients (>40 years) with severe aplastic anemia (SAA) who fail first-line immunosuppressive therapy and patients <40 years who have an available matched related donor or fail first-line immunosuppressive if an human leukocyte antigen (HLA)-identical sibling donor is not available [1]. At many institutions, including Vanderbilt University Medical Center (VUMC), the standard reduced intensity conditioning regimen for these patients ineligible for an ablative regimen includes cyclophosphamide (Cy, 30 mg/kg × 4 doses), fludarabine, anti-thymocyte globulin (ATG), and low-dose total body irradiation (TBI) as it is associated with lower toxicity and high immunosuppressive activity [2,3,4,5]. However, a strong age effect is consistently reported in transplantation of older adults with SAA, even among those transplanted from an HLA-identical sibling, and is a major predictor of survival [6]. This makes transplantation of older adults with SAA without use of a matched related donor a challenging procedure with a high risk of transplant-related mortality that increases with age [4, 6].

It is well documented that SAA patients are particularly prone to infections [4]. SAA patients who undergo HCT with a thymoglobulin-containing regimen or receive a cord blood graft are at especially high risk of Epstein–Barr virus (EBV) reactivation and post-transplant lymphoproliferative disorder [7]. In patients receiving unrelated donor transplants for hematologic malignancies, a single flat dose of rituximab was added on day +5 to thymoglobulin-containing conditioning regimens to prevent EBV reactivation [7]. This resulted in significantly lower reactivation rates, lower incidence of acute graft-versus-host disease (GVHD), and a trend towards improved overall survival [7, 8]. This strategy has been employed in other thymoglobulin-containing SAA conditioning regimens with encouraging results [7]. Furthermore, rituximab demonstrates benefit as primary or secondary treatment for SAA and immune platelet refractoriness due to its immunomodulatory effects [9,10,11,12]. Therefore, it is likely that inclusion of rituximab in SAA conditioning regimens may enhance disease control, particularly in non-myeloablative settings.

In 2014, based on initial positive outcomes in allogeneic HCT for lymphomas, VUMC instituted a novel non-myeloablative rituximab-based outpatient conditioning regimen for allogeneic transplantation as standard of care for SAA patients with significant co-morbidities precluding use of a myeloablative regimen. The regimen consists of fludarabine, cyclophosphamide, and rituximab (FCR) with low-dose TBI and ATG in those transplanted from non-haploidentical donors (Fig. 1a, b). Following successful administration of these regimens in the first three patients (≥40 years of age), the regimen was instituted as the standard for all SAA patients, regardless of age. Therefore, we conducted a retrospective analysis to assess the efficacy and safety of these FCR conditioning regimens among patients transplanted for SAA.

Fig. 1
figure1

a FCR conditioning regimen schema—matched related, matched unrelated, or 1-allele mismatch unrelated donor. b FCR conditioning regimen schema—haploidentical donor

Utilizing institutional registries, we identified SAA patients who underwent first allogeneic HCT between August 2014 and May 2017. All included patients underwent transplantation utilizing an FCR regimen as the institution-approved standard of care. Post-transplant immune suppression included tacrolimus and methotrexate or tacrolimus and mycophenolate mofetil. The Institutional Review Board approved this analysis and waived informed consent. Assessment of GVHD was performed following standard grading practices.

Patient demographics and transplantation data were analyzed descriptively. Endpoints included time to engraftment, incidence of acute and chronic GVHD, transplant-related mortality, and infectious complications, including a specific assessment of viral reactivation incidence. Based on its recently demonstrated utility, we analyzed 1-year GVHD-free/relapse-free survival (GRFS) in the study cohort compared to a historical cohort of all SAA patients (n = 27) transplanted at our institution in the past 10 years who did not receive an FCR regimen [5]. The difference between curves was determined using Mantel–Cox log-rank tests, and SPSS version 23 (IBM, Armonk, NY) was used for all analyses.

Eleven SAA patients underwent HCT from 1 August 2014 to 5 May 2017 (Table 1). Most were males and the median age was 41 years (range: 19–64). All patients received transplants from either matched or haploidentical donors using a peripheral blood stem cell source, except one patient who received a bone marrow graft. At median follow-up of 302 days post-transplant (range: 49–911), all patients were alive with 100% donor peripheral blood chimerism and bone marrow biopsies documenting normal trilineage hematopoiesis. Average time to neutrophil and platelet engraftment was 15.4 and 16 days, respectively; median number of inpatient hospital days was 11 days (range: 3–29). By day 100 post-transplant, 54.5% of patients developed some degree of acute GVHD. However, by date of last follow-up, no evaluable patients (10/11) developed chronic GVHD, and only two patients experienced viral reactivation (1 = CMV, 1 = CMV + EBV). Unadjusted Kaplan–Meier estimates of 1-year GRFS were 81.8% (95% CI: 59.1–104.7%) in the study group versus 61.5% (95% CI: 42.9–80.1%) in the historical group (P = 0.256) (Fig. 2).

Table 1 Patient characteristics and outcomes
Fig. 2
figure2

Kaplan–Meier estimate of 1-year graft-versus-host disease-free, relapse-free survival (GRFS)

SAA patients of any age have multiple factors to consider in constructing optimal transplantation strategies, making a decision to institute aggressive intervention, such as HCT, difficult [1, 3]. Timing of transplantation, donor selection, graft source, and conditioning regimen all significantly impact outcomes [1, 3].

In SAA, HCT first became standard care primarily for young patients with an available matched related donor using a conditioning regimen of cyclophosphamide (50 mg/kg/day × 4) paired with ATG, yielding survival rates of 65–95% [13]. The necessity and tolerability of Cy 200 mg/kg is controversial in this population as larger doses result in significant early toxicities, such as mucositis and hemorrhagic cystitis and have considerable long-term effects, particularly cardiotoxicity. Therefore, when broadening HCT patient pools to include older or sensitized recipients and use of alternative donors, several modifications were required to create less toxic, more immunosuppressive conditioning regimens [4]. To accomplish this, many centers adopted a non-myeloablative conditioning regimen of Cy (40–120 mg/kg cumulative dose), fludarabine, ATG, and TBI [2,3,4]. While this approach showed 79% 5-year survival, it was associated with high rates of infectious complications, particularly EBV reactivation [14]. Our study utilized a conditioning approach adjusted to further reduce toxicity with a cumulative cyclophosphamide dose of 2250 mg/m2 (equivalent to 75 mg/kg). Transplant-related toxicity was low, as demonstrated by lack of transplant-related mortality and ability to administer the conditioning regimen in the outpatient setting with minimal inpatient days required within 100 days of transplantation.

In our study, approximately half of the patients, including the single subject who received a bone marrow graft, developed acute GVHD; however, the majority developed only low grade (I–II) and no patients developed chronic GVHD by date of last follow-up. Two patients (patients 2 and 3) tapered off all immunosuppression by date of last follow-up and eight others (patients 1, 4, and 6–9) were completing tapers of tacrolimus and/or prednisone from prior GVHD treatment. Consequently, only one patient (patient 5) remained on therapeutic doses of tacrolimus and high-dose steroids (prednisone 20 mg/day) by last follow-up. Of note, patients 10 and 11 had not reached day 180 post-transplant by date of last follow-up and were therefore ineligible for immunosuppression discontinuation. Finally, in comparing GRFS events, death without relapse or GVHD accounted for the greatest proportion of events in the historical cohort while grade III acute GVHD accounted for the two GRFS events in the study population. This analysis was limited by its small sample size; however, the observed trend is encouraging and supports the efficacy and tolerability of this novel non-myeloablative regimen.

EBV reactivation is a well-known post-HCT complication in SAA and addition of a single dose of rituximab has been added to many modern regimens to prevent reactivation [7]. Despite this, average reactivation rates remain at 50% [4, 7]. Incorporating weekly scheduled doses in our regimen has the potential to further decrease likelihood of EBV reactivation, enhance GVHD prophylaxis, and prevent disease relapse through immunomodulation [9,10,11,12]. One patient receiving haploidentical transplantation reactivated EBV but the viral load was low and did not require therapeutic intervention.

Previous data demonstrated that an interval of less than 2 years between diagnosis and transplant is the most significant factor impacting SAA survival [15]. In our study, two-thirds of patients proceeded to HCT within 1 year after SAA diagnosis. Additionally, the majority of patients in our cohort failed front-line immunosuppressive therapy, while those previously untreated were younger with severe disease and transplanted within 3 months of SAA diagnosis per treating physician discretion. Moreover, time from diagnosis to transplantation plays a significant role in available donor options [15]. One patient in our cohort was transplanted quickly—within 70 days following diagnosis—via a haploidentical donor. The ability to successfully perform haploidentical transplantation in SAA is an exciting advancement as it expands the available donor pool and allows for quicker transplantation, which is often necessary in SAA due to high risk of infection and mortality [15].

Lastly, we believe the positive results from use of novel HCT treatment strategies at our institution make these regimens appealing for use in SAA patients of any age as they are feasible for outpatient administration and can be used with a wide variety of donor and stem cell sources while limiting transplant-related toxicities, infectious complications, and GVHD rates. Our initial outcomes are equivalent or superior to those previously published in the literature and compared to historical data [1,2,3,4, 6]. While we recognize the sample size is a limitation, we feel these encouraging results justify further prospective investigation of this approach in a larger SAA population.

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Acknowledgements

The authors would like to acknowledge the contribution of Justin D. Gatwood, PhD, MPH to this manuscript.

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Correspondence to Katie S. Gatwood.

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KSG and BNS serve on a Speakers Bureau for Jazz Pharmaceuticals. The remaining authors declare that they have no conflict of interest.

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Gatwood, K.S., Culos, K.A., Binari, L.A. et al. Outcomes of a novel rituximab-based non-myeloablative conditioning regimen for hematopoietic cell transplantation in severe aplastic anemia. Bone Marrow Transplant 53, 795–799 (2018). https://doi.org/10.1038/s41409-018-0124-6

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