Prospective Open-Label Phase II Trial of Individualized Anti-Thymocyte Globulin for Improved T-Cell Reconstitution after Pediatric Allogeneic Hematopoietic Cell Transplantation: The Parachute-Study

Population Pharmacokinetics of Total Rabbit Anti-thymocyte Globulin in Non-obese Adult Patients Undergoing Hematopoietic Cell Transplantation for Hematologic Malignancy

BACKGROUND AND OBJECTIVES

Rabbit anti-thymocyte globulin (rATG), a therapeutic polyclonal antibody against human T cells, is commonly used in conditioning therapy prior to allogeneic hematopoietic cell transplantation (HCT). Previous studies successfully developed an individualized rATG dosing regimen based on "active" rATG population PK (popPK) analysis, while "total" rATG can be a more logistically favorable alternative for early HCT outcomes. We conducted a novel popPK analysis of total rATG.

METHODS

Total rATG concentration was measured in adult human-leukocyte antigen (HLA) mismatched HCT patients who received a low-dose rATG regimen (total 2.5-3 mg/kg) within 3 days prior to HCT. PopPK modeling and simulation was performed using nonlinear mixed effect modeling approach.

RESULTS

A total of 504 rATG concentrations were available from 105 non-obese patients with hematologic malignancy (median age 47 years) treated in Japan. The majority had acute leukemia or malignant lymphoma (94%). Total rATG PK was described by a two-compartment linear model. Influential covariate relations include ideal body weight [positively on both clearance (CL) and central volume of distribution], baseline serum albumin (negatively on CL), CD4+ T cell dose (positively on CL), and baseline serum IgG (positively on CL). Simulated covariate effects predicted that early total rATG exposures were affected by ideal body weight.

CONCLUSIONS

This novel popPK model described the PK of total rATG in the adult HCT patients who received a low-dose rATG conditioning regimen. This model can be used for model-informed precision dosing in the settings with minimal baseline rATG targets (T cells), and early clinical outcomes are of interest.

Transplant for non-malignant disorders: an International Society for Cell & Gene Therapy Stem Cell Engineering Committee report on the role of alternative donors, stem cell sources and graft engineering

Hematopoietic stem cell transplantation (HSCT) is curative for many non-malignant disorders. As HSCT and supportive care technologies improve, this life-saving treatment may be offered to more and more patients. With the development of new preparative regimens, expanded alternative donor availability, and graft manipulation techniques, there are many options when choosing the best regimen for patients. Herein the authors review transplant considerations, transplant goals, conditioning regimens, donor choice, and graft manipulation strategies for patients with non-malignant disorders undergoing HSCT.

Improved GRFS after posttransplant cyclophosphamide-based vs ATG-based HLA-mismatched unrelated donor transplant

A common method to prevent graft-versus-host disease after allogeneic hematopoietic cell transplantation (HCT) from an HLA-mismatched unrelated donor (MMUD) is tacrolimus, methotrexate, and antithymocyte globulin (ATG). The use of posttransplant cyclophosphamide (PTCy) showed promise in a prospective trial for MMUD HCT. We compared 1-year graft-versus-host disease–free, relapse-free survival (GRFS) in 128 recipients of prophylaxis based on tacrolimus/methotrexate/ATG (ATG group, n = 46) vs PTCy, mycophenolate mofetil, and tacrolimus or sirolimus (PTCy group, n = 82) after MMUD HCT. Patients receiving HCT from a MMUD mismatched at ≥1 locus among HLA-A, HLA-B, HLA-C, and HLA-DRB1 were included. The 2 groups were well matched for HCT indication, high-risk disease, and HCT comorbidity index, whereas more patients on PTCy received bone marrow (50% vs 26%; P = .01) and >1 locus HLA-mismatched (30.5% vs 2.2%; P = .001) grafts. The 1-year GRFS was 16% (95% confidence interval (CI): 8%-31%) vs 54% (95% CI: 44%-66%; P < .001) in the ATG and PTCy groups, respectively. The multivariable adjusted hazard ratio for GRFS was 0.34 (95% CI: 0.21-0.55; P < .001) with the use of PTCy. The 1-year overall survival in the ATG group was 45% (95% CI: 32%-62%) vs 75% (95% CI: 66%-85%) in the PTCy group (P < .001). Relapse incidence was similar. One-year nonrelapse mortality was greater after ATG-based prophylaxis: 38% (95% CI: 23%-52%) vs 16% (95 CI: 9%-25%), P < .001. In summary, PTCy-based prophylaxis resulted in superior GRFS and overall survival in recipients of MMUD.

Antithymocyte globulin exposure in CD34+ T-cell depleted allogeneic hematopoietic cell transplantation

In CD34⁺ TCD allogeneic-HCT, optimum post-HCT rATG exposure decreases NRM driven by faster CD4⁺ IR and improves survival.

Personalized rATG exposure using a PK-directed strategy may improve survival after allogeneic CD34⁺ TCD HCT.

Traditional weight-based dosing results in variable rabbit antithymocyte globulin (rATG) clearance that can delay CD4⁺ T-cell immune reconstitution (CD4⁺ IR) leading to higher mortality. In a retrospective pharmacokinetic/pharmacodynamic (PK/PD) analysis of patients undergoing their first CD34⁺ T-cell–depleted (TCD) allogeneic hematopoietic cell transplantation (HCT) after myeloablative conditioning with rATG, we estimated post-HCT rATG exposure as area under the curve (arbitrary unit per day/milliliter [AU × day/mL]) using a validated population PK model. We related rATG exposure to nonrelapse mortality (NRM), CD4⁺ IR (CD4⁺ ≥50 cells per µL at 2 consecutive measures within 100 days after HCT), overall survival, relapse, and acute graft-versus-host disease (aGVHD) to define an optimal rATG exposure. We used Cox proportional hazard models and multistate competing risk models for analysis. In all, 554 patients were included (age range, 0.1-73 years). Median post-HCT rATG exposure was 47 AU × day/mL (range, 0-101 AU × day/mL). Low post-HCT area under the curve (<30 AU × day/mL) was associated with lower risk of NRM (P < .01) and higher probability of achieving CD4⁺ IR (P < .001). Patients who attained CD4⁺ IR had a sevenfold lower 5-year NRM (P < .0001). The probability of achieving CD4⁺ IR was 2.5-fold higher in the <30 AU × day/mL group compared with 30-55 AU × day/mL and threefold higher in the <30 AU × day/mL group compared with the ≥55 AU × day/mL group. In multivariable analyses, post-HCT rATG exposure ≥55 AU × day/mL was associated with an increased risk of NRM (hazard ratio, 3.42; 95% confidence interval, 1.26-9.30). In the malignancy subgroup (n = 515), a tenfold increased NRM was observed in the ≥55 AU × day/mL group, and a sevenfold increased NRM was observed in the 30-55 AU × day/mL group compared with the <30 AU × day/mL group. Post-HCT rATG exposure ≥55 AU × day/mL was associated with higher risk of a GVHD (hazard ratio, 2.28; 95% confidence interval, 1.01-5.16). High post-HCT rATG exposure is associated with higher NRM secondary to poor CD4⁺ IR after TCD HCT. Using personalized PK-directed rATG dosing to achieve optimal exposure may improve survival after HCT.

Clofarabine-fludarabine-busulfan in HCT for pediatric leukemia: an effective, low toxicity, TBI-free conditioning regimen

CloFluBu-conditioning results in encouraging EFS for ALL and AML, with low TRM, limited incidence of aGvHD and GF, and no cases of VOD.

Minimal residual disease status prior to transplantation impacted outcome due to increased relapse risk in both AML and ALL patients.

We prospectively studied clofarabine-fludarabine-busulfan (CloFluBu)-conditioning in allogeneic hematopoietic cell therapy (HCT) for lymphoid and myeloid malignancies and hypothesized that CloFluBu provides a less toxic alternative to conventional conditioning regimens, with adequate antileukemic activity. All patients receiving their first HCT, from 2011-2019, were included and received CloFluBu. The primary endpoint was event-free survival (EFS). Secondary endpoints were overall survival (OS), graft-versus-host disease (GvHD)-relapse-free survival (GRFS), treatment-related mortality (TRM), cumulative incidence of relapse (CIR), acute and chronic GvHD (aGvHD and cGvHD), and veno-occlusive disease (VOD). Cox proportional hazard and Fine and Gray competing-risk models were used for data analysis. One hundred fifty-five children were included: 60 acute lymphoid leukemia (ALL), 69 acute myeloid leukemia (AML), and 26 other malignancies (mostly MDS-EB). The median age was 9.7 (0.5 to 18.6) years. Estimated 2-year EFS was 72.0% ± 6.0 in ALL patients, and 62.4% ± 6.0 in AML patients. TRM in the whole cohort was 11.0% ± 2.6, incidence of aGvHD 3 to 4 at 6 months was 12.3% ± 2.7, extensive cGvHD at 2 years was 6.4% ± 2.1. Minimal residual disease-positivity prior to HCT was associated with higher CIR, both in ALL and AML. CloFluBu showed limited toxicity and encouraging EFS. CloFluBu is a potentially less toxic alternative to conventional conditioning regimens. Randomized prospective studies are needed.

Serotherapy as Graft-Versus-Host Disease Prophylaxis in Haematopoietic Stem Cell Transplantation for Acute Lymphoblastic Leukaemia

Serotherapy comprising agents such as anti-thymocyte globulin, anti-T-lymphocyte globulin, and the anti-CD52 monoclonal antibody alemtuzumab is used widely to reduce the incidence of graft-versus-host disease (GvHD) after paediatric haematopoietic stem cell transplantation (HSCT). The outcome of transplants using matched unrelated donors now approaches that of matched sibling donors. This is likely due to better disease control in recipients, the use of donors more closely human-leukocyte antigen (HLA)-matched to recipients, and more effective graft-versus-host disease (GvHD) prophylaxis. The price paid for reduced GvHD is slower immune reconstitution of T cells and thus more infections. This has led to studies looking to optimise the amount of serotherapy used. The balance between prevention of GvHD on one side and prevention of infections and relapse on the other side is quite delicate. Serotherapy is given with chemotherapy-/radiotherapy-based conditioning prior to HSCT. Due to their long half-lives, agents used for serotherapy may be detectable in patients well after graft infusion. This exposes the graft-infused T cells to a lympholytic effect, impacting T-cell recovery. As such, excessive serotherapy dosing may lead to no GvHD but a higher incidence of infections and relapse of leukaemia, while under-dosing may result in a higher chance of serious GvHD as immunity recovers more quickly. Individualised dosing is being developed through studies including retrospective analyses of serotherapy exposure, population pharmacokinetic modelling, therapeutic drug monitoring in certain centres, and the development of dosing models reliant on factors including the patient's peripheral blood lymphocyte count. Early results of "optimal" dosing strategies for serotherapy and conditioning chemotherapy show promise of improved overall survival.

Immune Monitoring After Allogeneic Hematopoietic Cell Transplantation: Toward Practical Guidelines and Standardization

Hematopoietic cell transplantation (HCT) is often a last resort, but potentially curative treatment option for children suffering from hematological malignancies and a variety of non-malignant disorders, such as bone marrow failure, inborn metabolic disease or immune deficiencies. Although efficacy and safety of the HCT procedure has increased significantly over the last decades, the majority of the patients still suffer from severe acute toxicity, viral reactivation, acute or chronic graft-versus-host disease (GvHD) and/or, in case of malignant disease, relapses. Factors influencing HCT outcomes are numerous and versatile. For example, there is variation in the selected graft sources, type of infused cell subsets, cell doses, and the protocols used for conditioning, as well as immune suppression and treatment of adverse events. Moreover, recent pharmacokinetic studies show that medications used in the conditioning regimen (e.g., busulphan, fludarabine, anti-thymocyte globulin) should be dosed patient-specific to achieve optimal exposure in every individual patient. Due to this multitude of variables and site-specific policies/preferences, harmonization between HCT centers is still difficult to achieve. Literature shows that adequate immune recovery post-HCT limits both relapse and non-relapse mortality (death due to viral reactivations and GvHD). Monitoring immune parameters post-HCT may facilitate a timely prediction of outcome. The use of standardized assays to measure immune parameters would facilitate a fast comparison between different strategies tested in different centers or between different clinical trials. We here discuss immune cell markers that may contribute to clinical decision making and may be worth to standardize in multicenter collaborations for future trials.