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The addition of fludarabine to cyclophosphamide as a lymphodepleting regimen prior to CD19 chimeric antigen receptor (CAR) T-cell therapy significantly improved outcomes in patients with relapsed/refractory (r/r) B-cell acute lymphoblastic leukemia (B-ALL). Fludarabine exposure, previously shown to be highly variable when dosing is based on body surface area (BSA), is a predictor for survival in allogeneic hematopoietic cell transplantation (allo-HCT). Hence, we hypothesized that an optimal exposure of fludarabine might be of clinical importance in CD19 CAR T-cell treatment. We examined the effect of cumulative fludarabine exposure during lymphodepletion, defined as concentration-time curve (AUC), on clinical outcome and lymphocyte kinetics. A retrospective analysis was conducted with data from 26 patients receiving tisagenlecleucel for r/r B-ALL. Exposure of fludarabine was shown to be a predictor for leukemia-free survival (LFS), B-cell aplasia, and CD19-positive relapse following CAR T-cell infusion. Minimal event probability was observed at a cumulative fludarabine AUCT0−∞ ≥14 mg*h/L, and underexposure was defined as an AUCT0−∞

Background: Anti-thymocyte globulin, which is used in the conditioning of haematopoietic stem-cell transplantation (HSCT) to prevent graft-versus-host disease (GVHD) and graft failure, has highly variable pharmacokinetics. Overexposure to anti-thymocyte globulin leads to poor CD4+ T-cell immune reconstitution, which is associated with inferior overall survival. We hypothesised that individualised anti-thymocyte globulin dosing would promote CD4+ immune reconstitution, while still preventing GVHD and graft failure. Methods: We report the results of a prospective, single-arm, phase 2 clinical trial done at the University Medical Center Utrecht and the Princess Máxima Center for Pediatric Oncology (Utrecht, Netherlands) to investigate individualised dosing of anti-thymocyte globulin for unrelated allogeneic HSCT in paediatric patients. Anti-thymocyte globulin dosing was based on bodyweight, absolute lymphocyte counts before the first dose, and the stem-cell source, with cumulative doses ranging from 2–10 mg/kg. Patients younger than 18 years receiving a first HSCT with a T-cell repleted graft for any indication and a Lansky/Karnofsky performance status of at least 70% were eligible for inclusion. The primary endpoint was CD4+ immune reconstitution (>0·05 × 109 CD4+ T-cells per L twice within 100 days [±3] after transplantation). The primary endpoint needed to be met in 38 of 53 evaluable patients (no death, relapse, or graft failure before day 100). Toxicity was registered according to Common Terminology Criteria for Adverse Events criteria version 4.0. The study is registered with the Dutch Trial Register, NL4836. Findings: Between July 1, 2015, and Aug 22, 2018, 58 patients were included in the study, of whom 51 were evaluable for the primary endpoint. Median follow-up was 25·6 months (IQR 15·0–37·0) and median age was 7·4 years (IQR 2·8–13·2). 29 (50%) of 58 patients were female. CD4+ immune reconstitution was reached in 41 (80%, 95% CI 67–90, in survival analysis) of 51 evaluable patients, hence the study met its primary endpoint. There was no difference in CD4+ immune reconstitution between patients who received different stem-cell sources (87% [95% CI 61–96] in cord blood, 77% [54–89] in bone marrow [p=0·62]). The most common grade 3–5 adverse events were infections (32 [50%] patients had grade 3, two [3%] patients had grade 4, and seven [11%] patients had fatal events) and immunological disorders (seven [11%] patients had grade 3, three [5%] patients had grade 4, and five [8%] patients had fatal events). Two (3%) of 64 patients died of GVHD, which might be indirectly related to the intervention. Interpretation: Individualised dosing of anti-thymocyte globulin led to a significant improvement in early CD4+ immune reconstitution without increasing GVHD and graft failure incidence. Promotion of early CD4+ immune reconstitution by individualising anti-thymocyte globulin dose might improve outcomes of allogeneic HSCT. Funding: Sanofi.

BACKGROUND AND OBJECTIVES High-dose melphalan is an integral part of conditioning chemotherapy prior to both autologous and allogeneic hematopoietic cell transplantation. While underexposure may lead to relapse, overexposure may lead to toxicities include mucositis, diarrhea, bone marrow suppression, and rarely sinusoidal obstruction syndrome. In this study, we describe the population pharmacokinetics of high-dose melphalan as a first step towards individualized dosing. METHODS Melphalan samples were collected in patients receiving an allogeneic or autologous hematopoietic cell transplantation between August 2016 and August 2020 at the Memorial Sloan Kettering Cancer Center. A population-pharmacokinetic model was developed using NONMEM. RESULTS Based on a total of 3418 samples from 452 patients receiving a median cumulative dose of 140 mg/m2, a two-compartment population-pharmacokinetic model was developed. Fat-free mass was a covariate for clearance, central volume of distribution, and inter-compartmental clearance, while glomerular filtration rate predicted clearance. Simulation studies showed that based on fixed body surface area-based dosing, renal impairment has a higher impact in increasing melphalan exposure compared with obesity. CONCLUSIONS The proposed model adequately describes the population pharmacokinetics of melphalan in adult patients receiving a hematopoietic cell transplantation. This model can be used to define the therapeutic window of melphalan, and subsequently to develop individualized dosing regimens aiming for that therapeutic window in all patients.

High-dose melphalan is one of the main cytotoxic DNA alkylating agents and is used in many transplantation conditioning regimens. Studies have shown a wide range of drug exposure when a traditional weight-based dose of melphalan is used. The optimal melphalan dose in BEAM (carmustine, etoposide, cytarabine, and melphalan), which results in maximum efficacy with acceptable toxicity, is unknown. In this pharmacokinetic (PK) analysis of 105 patients with lymphoma undergoing treatment with BEAM and autologous hematopoietic cell transplantation, we initially estimated melphalan exposure as area under the curve (AUC) by a noncompartmental analysis and subsequently compared it with a newly developed 2-compartment population-PK model. The 2 models correlated closely with each other. We found that the traditional fixed weight-based dosing of propylene glycol-free (captisol-enabled) melphalan in BEAM results in a wide variation in exposure as estimated by both models. Higher melphalan exposure was significantly associated with increased metabolic toxicities but did not seem to impact progression-free survival. Although our study suggests a melphalan AUC of 8 mg·h/L as a potential target in BEAM, larger prospective studies using personalized PK-directed melphalan dosing are needed to determine the optimal melphalan exposure in lymphomas.

We conducted a multicenter prospective single-arm phase 1/2 study that assesses the outcome of αβ T-cell depleted allogeneic hematopoietic stem cell transplantation (allo-HSCT) of peripheral blood derived stem cells from matched related, or unrelated donors (10/10 and 9/10) in adults, with the incidence of acute graft-versus-host disease (aGVHD) as the primary end point at day 100. Thirty-five adults (median age, 59; range, 19-69 years) were enrolled. Conditioning consisted of antithymocyte globulin, busulfan, and fludarabine, followed by 28 days of mycophenolic acid after allo-HSCT. The minimal follow-up time was 24 months. The median number of infused CD34+ cells and αβ T cells were 6.1 × 106 and 16.3 × 103 cells per kg, respectively. The cumulative incidence (CI) of aGVHD grades 2-4 and 3-4 at day 100 was 26% and 14%. One secondary graft failure was observed. A prophylactic donor lymphocyte infusion (DLI) (1 × 105 CD3+ T cells per kg) was administered to 54% of the subjects, resulting in a CI of aGVHD grades 2-4 and 3-4 to 37% and 17% at 2 years. Immune monitoring revealed an early reconstitution of natural killer (NK) and γδ T cells. Cytomegalovirus reactivation associated with expansion of memory-like NK cells. The CI of relapse was 29%, and the nonrelapse mortality 32% at 2 years. The 2-year CI of chronic GVHD (cGVHD) was 23%, of which 17% was moderate. We conclude that only 26% of patients developed aGVHD 2-4 after αβ T-cell–depleted allo-HSCT within 100 days and was associated with a low incidence of cGVHD after 2 years. This trial was registered at www.trialregister.nl as #NL4767.

The addition of rabbit anti-human thymocyte globulin (ATG) to the conditioning regimen prior to allogeneic hematopoietic cell transplantation has significantly reduced the risk of graft-versus-host disease (GvHD) and graft failure. However, ATG has a small therapeutic window. Overexposure of ATG post-HCT hampers T cell immune reconstitution and has been associated with increased relapse rates and viral reactivations, whereas underexposure has been associated with an increased incidence of GvHD, both of which lead to increased mortality. Therapeutic drug monitoring of T cell binding ATG plasma levels provides a means to optimize dosing for patients at high risk for graft failure to ensure timely T cell immune reconstitution and subsequently increase survival chances. This manuscript describes the first liquid chromatography tandem-mass spectrometry (LC-MS/MS) method to quantify the pharmacologically active fraction of polyclonal ATG in plasma. This was achieved through immunoaffinity purification of active ATG from plasma with Jurkat T cells. After the binding and washing, samples were eluted, denatured, and trypsin-digested. Signature peptides originating from the IgG constant chain were measured with LC-MS/MS. Critical method parameters were optimized, and the method was successfully validated following European Medicines Agency (EMA) guidelines. The method covered the therapeutic range of ATG and was validated at a lower limit of quantification (LLOQ) of 1 AU/mL with an overall CV and bias of 11.8% and − 2.5%, respectively. In conclusion, we developed a LC-MS/MS-based method to quantify active polyclonal rabbit ATG in human plasma. We suggest that this novel assay can be used to monitor and optimize dosing of ATG in clinical practice. Electronic supplementary material The online version of this article (10.1208/s12248-020-0419-6) contains supplementary material, which is available to authorized users.

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Residual antithymocyte globulin (ATG; Thymoglobulin) exposure after allogeneic hematopoietic (stem) cell transplantation (HCT) delays CD4+T-cell immune reconstitution (CD4+IR), subsequently increasing morbidity and mortality. This effect seems particularly present after cord blood transplantation (CBT) compared to bone marrow transplantation (BMT). The reason for this is currently unknown. We investigated the effect of active-ATG exposure on CD4+IR after BMT and CBT in 275 patients (CBT n = 155, BMT n = 120; median age, 7.8 years; range, 0.16-19.2 years) receiving their first allogeneic HCT between January 2008 and September 2016. Multivariate log-rank tests (with correction for covariates) revealed that CD4+IR was faster after CBT than after BMT with 10 active-ATG × day/mL exposure severely impaired CD4+IR after CBT (P< .001), but not after BMT (P= .74). To decipher these differences, we performed ATG-binding and ATG-cytotoxicity experiments using cord blood- and bone marrow graft-derived T-cell subsets, B cells, natural killer cells, and monocytes. No differences were observed. Nevertheless, a major covariate in our cohort was Filgrastim treatment (only given after CBT). We found that Filgrastim (granulocyte colony-stimulating factor [G-CSF]) exposure highly increased neutrophil-mediated ATG cytotoxicity (by 40-fold [0.5 vs 20%;P= .002]), which explained the enhanced T-cell clearance after CBT. These findings imply revision of the use (and/or timing) of G-CSF in patients with residual ATG exposure.

Human herpesvirus 6 (HHV6) viremia is a common cause of morbidity following allogeneic hematopoietic cell transplantation (HCT). We previously associated T-cell reconstitution with HHV6 viremia. Here, we investigated whether HHV6 viremia affects T-cell reconstitution after HCT in a time-dependent retrospective analysis. We included 273 pediatric patients (0.1-22.7 years; median follow-up, 58 months) receiving a first HCT between 2004 and 2014. HHV6 was screened weekly in plasma via polymerase chain reaction and occurred in 79 patients (29%) at a median time of 19 days after transplant. Main outcome of interest was immune reconstitution (IR) (CD3/CD4/CD8 T cells), measured biweekly until 12 weeks and monthly thereafter. Cox proportional-hazard models were used with IR and HHV6 as time-dependent variables in multivariate analysis with serotherapy in conditioning, graft source, graft-versus-host disease, age, and other viruses (Epstein-Barr virus, cytomegalovirus, and adenovirus) as covariates. Only patients with very high HHV6 viremia (>105 copies/mL) showed hampered CD4+ (hazard ratio [HR], 0.913; 95% confidence interval [CI], 0.892-0.934; P < .001) and CD8+ (HR, 0.912; 95% CI, 0.891-0.933; P < .001) reconstitution in comparison with patients without HHV6, from ∼6 months after HCT. Especially naive CD4+ IR was affected (P = .028) but not effector memory CD4+ IR (P = .33). Interestingly, T-cell reconstitution was improved in patients treated with antivirals (HR, 1.572; 95% CI, 1.463-1.690; P < .001). These findings suggest that HHV6 viremia affects late but not early T-cell reconstitution.

The addition of fludarabine to cyclophosphamide as lymphodepleting regimen prior to adoptive transfer of CD19 chimeric antigen receptor (CAR) T cells significantly improves CAR T cell expansion and correlates with longer B cell aplasia and a decreased probability of developing a CD19+ relapse (Gardner, 2017). Dosing of fludarabine is currently based on body surface area. We previously showed that this leads to a highly variable plasma exposure that correlates with clinical outcome after allogeneic hematopoietic cell transplantation (Langenhorst, 2019). We therefore hypothesized that optimal exposure of fludarabine might be of clinical importance in the CD19 CAR T setting. An observational cohort analysis was conducted with data from 26 consecutive patients receiving tisagenlecleucel as treatment for refractory/relapsed B cell acute lymphoblastic leukemia (B-ALL; table 1). Prior to CAR T cell infusion, patients received fludarabine on 4 consecutive days at a daily dosage of 30 mg/m 2 and cyclophosphamide on 2 consecutive days at a daily dosage of 500 mg/m 2. Fludarabine concentrations were measured longitudinally after fludarabine infusion using a liquid chromatography mass spectrometry method. The total exposure (Area Under the Curve (AUC 0−∞)) was subsequently determined using a fludarabine population pharmacokinetic model (Langenhorst, 2019). The study was performed in accordance with the Declaration of Helsinki. The primary outcome parameter was leukemia free survival, defined as the time between CAR19 T cell infusion and the moment of measurable leukemic blasts (>5% or >0.01% by two subsequent measurements). The effect of fludarabine on leukemia free survival and the secondary outcome measures CD19+ relapse and B cell aplasia were explored using martingale residuals and further identified by fitting univariable Cox Proportional Hazards models. In addition, Kaplan Meier and cumulative incidence curves were plotted and compared with log-rank tests. To compare CAR T cell numbers over time in peripheral blood, the AUCs were computed and compared between exposure groups with the Mann-Whitney test. Analyses were performed using R4.03 with packages pknca, survival and survminer. The fludarabine AUC 0−∞ was highly variable, resulting in a large range of 8.7-21.8 mg*h/L. Exposure of fludarabine was shown to be a predictor for leukemia free survival, B cell aplasia, and CD19+ relapse following CAR T cell infusion. Minimal event probability was observed at a cumulative fludarabine exposure ≥14 mg*h/L and underexposure was therefore defined as an AUC 0−∞ To our knowledge, this is the first study describing the effect of fludarabine exposure on outcome in a cohort of paediatric and young adults receiving CD19 CAR T cell therapy as treatment for B-ALL. These preliminary results suggest that optimizing fludarabine exposure may have a relevant impact on leukemia free survival following CAR T cell therapy. However, it should be noted that multivariate regression models are needed to show consistency of the relationship between fludarabine exposure and outcome. The limited number of patients did not allow for inclusion of potential covariates that may influence clinical outcome into the analysis. Therefore, our results need to be confirmed in a larger cohort. In conclusion, clinical outcome in patients receiving CAR19 T cell therapy might be improved by the optimization of fludarabine exposure in the lymphodepleting regimen. LD and FC contributed equally to this study. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Introduction Rabbit anti-thymocyte globulin (ATG) is used in allogeneic hematopoietic cell transplantation (HCT) to prevent graft-versus-host-disease (GvHD) and graft failure (GF). Its main and unpredictable toxicity includes poor immune reconstitution associated with increased relapse, viral reactivations and subsequently higher mortality. Early ( Methods We performed an open label, phase II historically controlled non-randomized prospective clinical trial investigating individualized versus fixed dosing of ATG (Thymoglobulin). Individualized dosing was based on the results from a previous validated population PKPD model1,2 with cumulative doses varying between 2 to 10 mg/kg starting based on weight, recipient lymphocyte counts before first dose of ATG and stem cell source, starting day -9. Primary endpoint was successful CD4+ T-cell reconstitution (CD4+ >50/mm3 at 2 consecutive timepoints within 100 days after HCT1) in patients alive without relapse or graft failure Results We included 58 patients between 2015-2018, and compared to 112 historical controls (table 1). CD4+ reconstitution was significantly better in the individualized dosing group: 83% versus 54% in the fixed dosing group; HR 2.4 (95% CI 1.6-3.6), p Conclusions Individualized ATG dosing significantly increases the chance of rapid immune reconstitution without affecting the incidence of aGvHD and graft failure. Together, this is an important step towards predictable CD4+ T cell reconstitution and improved survival changes.

Background and Objective Alemtuzumab (Campath®) is used to prevent graft-versus-host disease and graft failure following pediatric allogeneic hematopoietic cell transplantation. The main toxicity includes delayed immune reconstitution, subsequent viral reactivations, and leukemia relapse. Exposure to alemtuzumab is highly variable upon empirical milligram/kilogram dosing. Methods A population pharmacokinetic (PK) model for alemtuzumab was developed based on a total of 1146 concentration samples from 206 patients, aged 0.2–19 years, receiving a cumulative intravenous dose of 0.2–1.5 mg/kg, and treated between 2003 and 2015 in two centers. Results Alemtuzumab PK were best described using a two-compartment model with a parallel saturable and linear elimination pathway. The linear clearance pathway, central volume of distribution, and intercompartmental distribution increased with body weight. Blood lymphocyte counts, a potential substrate for alemtuzumab, did not impact clearance. Conclusion The current practice with uniform milligram/kilogram doses leads to highly variable exposures in children due to the non-linear relationship between body weight and alemtuzumab PK. This model may be used for individualized dosing of alemtuzumab. Electronic supplementary material The online version of this article (10.1007/s40262-019-00782-0) contains supplementary material, which is available to authorized users.

Introduction: We report the first analysis of a multicenter prospective single-arm phase I/II study that assesses the safety and feasibility of transplantation of TCRalpha/beta depleted stem cells from matched related or unrelated donors using the CliniMACS plus System (Miltenyi Biotec, Germany) in combination with a non-myeloablative conditioning in adult patients (trial nr NL48606.000.14). The conditioning regimen consisted of ATG (Thymoglobulin®) 1.5mg/kg i.v. days -12 to -9; fludarabine i.v. 40 mg/m2 days -5 to -2 and busulfan i.v. (Busivex®) days -5 to -2 (cumulative AUC of 80-90mg*h/L), followed by 28 days of mycophenolic acid. All 35 subjects were eligible to address the primary endpoint which is the incidence of acute GVHD at day 100. Results: 2 centers enrolled 10 female and 25 male patients (median age 59 years, range 19 - 69 years), including 9 AML, 4 ALL, 2 CML, 5 MM, 1 NHL, 10 MDS, 4 MPN. Diseases were in CR (n=17), VGPR (n=1), PR (n=5) or non-remission (n=12). 4 subjects had one or more previous allogeneic SCTs. Donors included 4 MRD, 24 10/10 matched MUD and 7 9/10 matched MUD. 8 male patients received stem cells of female donors. The median number of CD34+ cells and αβ TCR cells/kg was 6.1x 10^6 (range, 1.9-10) and 14.5x10^3 (range, 0-136), respectively. One primary graft failure was observed. Primary engraftment of ANCs > 500 cells/μL was reached at a median of 14 days (range 9 - 48 days) and of platelets > 20 at a median of 17 days (range 10 - 99) days. The median time of follow-up of this first analysis was 190 days (range 110-400 days). 3/35 patients developed acute GVHD grade III-IV during the first 100 days, 9/35 patients developed acute GVHD grade II-IV and 16/35 patients grade I-IV at day 100. At day 100 the cumulative incidence (CI) of CMV was 31%, of EBV 36% and of BK cystitis 20%. The combined CI of CMV/EBV/BK infections with a CTC-AE grade III-V was 53% at day 100. Immune reconstitution was rapid with a median of 227 (range 34 - 1626) CD3+ cells/µl on day 100 but varied substantially between patients. The median numbers of CD3/CD4+ and CD3/CD8+ at day 100 post transplant were 60 (range 30 - 120) and 124 (range 5 - 1450) cells/µl. The median numbers of NK were 289 (range 32-1140) at day 30 and 128 (range 24 - 1228) cells/µl at day 100 post transplant. The median numbers of γδ T cells were 38 (range 4-143) at day 30 and 50 (range 8-556) cells/µl at day 100 post transplant. 26 out of 35 patients were alive, 2 patients died of a relapse, 2 of GVHD and 5 of infectious complications. Kaplan Meier estimates of the OS are 88% (+/- 6%) at day 100 and 68% (+/- 9%) at 1Y. Kaplan Meier estimates of the EFS are 77% (+/- 7%) at day 100 and 52% (+/- 10%) at 1Y. 2 patients developed a relapse before day 100 and 5 patients before 1Y. It is noted that the outcome of 5 patients with MM was very poor in this cohort (3 patients developed a relapse and 2 patients died of complications). Conclusion: Allo-SCT of αβ T cell depleted PBMCs form matched related or unrelated donors, in combination with early ATG and a non-myeloablative conditioning regimen, resulted in favorable rates of primary engraftment (34/35), a CI of aGVHD II-IV of 26% with only minimal immuno-suppression and an encouraging early disease control (Fig 1). OS survival was mainly impacted by NRM (Fig 1), by which the majority of complications were of infectious nature. While all patients with MM suffered from NRM or relapse, all other disease categories had a rather favorable outcome. Reducing inter-individual variations in immune reconstitution early after transplantation will be key to further improve outcomes. Disclosures Minnema: Amgen: Consultancy; Takeda: Consultancy; Celgene: Consultancy, Research Funding; Janssen: Consultancy; Servier: Consultancy. Kuball:Novartis: Research Funding; Miltenyi Biotec: Research Funding; Gadeta (www.gadeta.nl): Consultancy, Equity Ownership, Patents & Royalties: on gd T cells and receptors and isolation strategies, Research Funding.

Background Allogeneic hematopoietic stem cell transplantation (aHSCT) is a curative treatment for numerous benign and malignant hematologic and immunological processes. Rabbit anti-thymocyte globulin (rATG) was introduced into conditioning regimens to reduce the risk of GVHD and graft failure. High exposure (AUC) of rATG before stem cell infusion leads to reduced graft failure and GVHD while low post-transplant exposure (AUC) is associated with improved T-cell reconstitution. Initial lymphocyte counts also affect the exposure to rATG. Hence the dosing and timing of rATG is crucial to balance the risks and benefits of rATG. At our center in the United States, we implemented individualized dosing and timing of rATG like that previously reported by authors in the Netherlands. ATG dosing is based on body weight, stem cell source, and absolute lymphocyte. Objectives To study rates of overall survival (OS), graft failure and acute and chronic GvHD in patients who received individualized dosing of rATG Methods We performed a retrospective chart review of consecutive pediatric patients who underwent aHSCT between January 2015 and June 2018 and received rATG (Thymoglobulin). All patients received individualized ATG dosing. Results A total of 18 patients were included (median age-9 years, range 15 months to 21 years; 14 malignant, 4 non-malignant); matched sibling (n=1), unrelated (n= 10 complete match, n=5 mismatched 9/10, n=1 mismatched 7/8) and haplo-identical transplant (n=1). Graft source included bone marrow (n=15), peripheral blood stem cells (n=2) and cord blood (n=1). Conditioning regimens were variable depending on donor source and indication for transplant, with fludarabine, busulfan and rATG being the most commonly utilized regimen (50%). GVHD prophylaxis for most patients was tacrolimus with mini-methotrexate. Seventeen of the 18 transplants resulted in neutrophil engraftment at a median of 17 days (1 graft failure: 6%; who died in neutropenia of fungal infection). One patient relapsed after +100d. Severe acute GVHD (≥ grade III) occurred in 1/18 patients (6%) while grade II-IV acute GVHD was noted in 4 patients (23%). Five patients developed chronic GVHD, but only two developed extensive cGVHD (12%), one of which was known to be due to non-compliance of medications. Overall survival for our cohort is 88%, with a median follow up of 1.3 years. Conclusion Use of an individualized dosing of rATG is feasible in a pediatric aHSCT patient population and resulted in minimal acute and chronic graft versus host disease with very little graft failure in patients undergoing aHSCT for a wide variety of indications. Our results compare favorably with those of authors first reporting this approach to rATG dosing.

Unrelated umbilical cord blood transplantation (UCBT) exhibits a low risk of graft-versus-host-disease (GvHD) and has unique potent anti-virus and anti-leukemia effects. Anti-thymocyte globulin (ATG) in the conditioning regimen for UCBT is successful in reducing graft rejection and GvHD. Nevertheless, this beneficial effect of ATG coincides with its detrimental effect on immune reconstitution. The latter directly relates to a high incidence of viral infections and leukemia relapses. ATG has been used in transplant patients for over 30 years. In recent years, the knowledge on the mechanisms of action of ATG and its implementation in the UCBT setting has increased dramatically. Important data became available showing the highly variable pharmacokinetics (PK) of ATG and its consequence on outcome measures. Here, we review the effects of ATG on immune reconstitution and subsequent outcomes after UCBT, and describe the mechanisms causing these effects. We highlight the importance of optimizing ATG exposure before and after UCBT and discuss strategies to maintain the 'good' and overcome the 'bad and ugly' effects of ATG on UCBT outcome.

INTRODUCTION: The conditioning regimen used in cord blood transplantation (CBT) may significantly impact the outcomes. Variable pharmacokinetics (PK) of drugs used may further influence outcome. Individualized dosing takes inter-patient differences in PK into account, tailoring drug dose for each individual patient in order to reach optimal exposure. Dose individualization may result in a better predictable regimen in terms of safety and efficacy, including timely T cell reconstitution, which may result in improved survival chances. AREAS COVERED: Conditioning regimens used in CBT varies significantly between and within centres. For busulfan, individualized dosing with therapeutic drug monitoring has resulted in better outcomes. Anti-thymocyte globulin (ATG), used to prevent rejection and GvHD, significantly hampers early T-cell reconstitution (IR). Timely IR is crucial in preventing viral reactivations and relapse. By individudalizing ATG, IR is better predicted and may prevent morbidity and mortality. EXPERT OPINION: Individualization of agents used in the conditioning regimen in CBT has proven its added value. Further fine-tuning, including new drugs and/or comprehensive models for all drugs, may result in better predictable conditioning regimens. A predictable conditioning regimen is also of interest/importance when studying adjuvant therapies, including immunotherapies (e.g. cellular vaccines or engineered T-cell) in a harmonized clinical trial design setting.

Leukemia-free survival in myeloid leukemia, but not in lymphoid leukemia, is predicted by early CD4+ reconstitution following unrelated cord blood transplantation in children: a multicenter retrospective cohort analysis

Background and Objectives: To prevent graft-versus-host disease and rejection in hematopoietic cell transplantation (HCT), children receive Thymoglobulin®, a polyclonal antibody acting mainly by depleting T cells. The therapeutic window is critical as over-exposure may result in delayed immune reconstitution of donor T cells. In this study, we describe the population pharmacokinetics of Thymoglobulin® as a first step towards an evidence-based dosing regimen of Thymoglobulin® in pediatric HCT. Methods: Serum active Thymoglobulin® concentrations were measured in all pediatric HCTs performed between 2004 and 2012 in two pediatric HCT centers in The Netherlands. Population pharmacokinetic analysis was performed using NONMEM® version 7.2. Results: A total of 3,113 concentration samples from 280 pediatric HCTs were analyzed, with age ranging from 3 months to 23 years old. The cumulative Thymoglobulin® dose was 10 mg/kg in 94 % of the patients given in 4 consecutive days. A model incorporating parallel linear and concentration-dependent clearance of Thymoglobulin® was identified. Body weight [for linear clearance (CL) and central volume of distribution] as well as lymphocyte count pre-Thymoglobulin® infusion (for CL) were important covariates. As such, the current dosing regimen results in higher exposure in children with a higher bodyweight and/or a lower lymphocyte count pre-Thymoglobulin® infusion. Conclusion: This model can be used to develop an individual dosing regimen for Thymoglobulin®, based on both body weight and lymphocyte counts, once the therapeutic window has been determined. This individualized regimen may contribute to a better immune reconstitution and thus outcome of allogeneic HCT.

The outcome of allogeneic hematopoietic stem cell transplantation (HSCT) is strongly affected by the kinetics of reconstitution of the immune system. This study compared the effects of antithymocyte globulin (ATG) and alemtuzumab on various outcome parameters after HSCT. The study cohort consisted of 148 children, with a median age of 9.6 years (range, .4 to 19.0), who underwent HSCT for malignant and benign hematological disorders in a single HSCT unit. Conditioning included ATG (n = 110) or alemtuzumab (n = 38). Cox proportional hazard regression analysis showed that alemtuzumab significantly delayed the recovery of CD3+ T cells and CD4+as well as CD8+ T cell subsets (P ≤ .001) and natural killer (NK) cells (P = .008) compared with ATG. In both ATG- and alemtuzumab-treated patients, shorter drug exposure lead to significantly faster recovery of T cells. Alemtuzumab was associated with lower donor chimerism 3 and 6 months after transplantation and a higher risk of disease relapse (P = .001). The overall survival and event-free survival risks were significantly lower for alemtuzumab-treated patients (P = .020 and P < .001, respectively). Patients who received alemtuzumab showed a trend to lower risk of acute graft-versus-host disease, more human adenovirus, and less Epstein-Barr virus reactivations compared with patients who received ATG. These data indicate that children treated with alemtuzumab as part of the conditioning regimen have a slower T cell and NK cell reconstitution compared with those treated with ATG, which compromises the overall and event-free survival. Prolonged length of lympholytic drug exposure delayed the T cell recovery in both ATG- and alemtuzumab-treated patients. Therefore, we recommend detailed pharmacokinetic/pharmacodynamic (PK/PD) analyses in a larger cohort of patients to develop an algorithm aiming at optimization of the serotherapy containing conditioning regimen.

Background Viral reactivations (VRs) after hematopoietic cell transplantation (HCT) contribute to significant morbidity and mortality. Timely immune reconstitution (IR) is suggested to prevent VR. Objectives We studied the relation between IR (as a continuous predictor over time) and VR (as a time-varying predictor) and the relation between VR and other clinical outcomes. Methods In this retrospective analysis all patients receiving a first HCT between January 2004 and September 2014 were included. IR (CD3/CD4/CD8 T, natural killer, and B cells) was measured biweekly until 12 weeks and monthly thereafter. Main outcomes of interest were VR of adenovirus, EBV, human herpesvirus 6 (HHV6), cytomegalovirus (CMV), and BK virus screened weekly. Clinical outcomes included overall survival (OS), event-free-survival, nonrelapse mortality (NRM), and graft-versus-host disease. Cox proportional hazard and Fine and Gray competing risk models were used. Results Two hundred seventy-three patients (age, 0.1-22.7 years; median follow-up, 58 months) were included. Delayed CD4 reconstitution predicted reactivation of adenovirus (hazard ratio [HR], 0.995; P = .022), EBV (HR, 0.994; P = .029), and HHV6 (HR, 0.991; P = .012) but not CMV ( P = .31) and BK virus ( P = .27). Duration of adenovirus reactivation was shorter with timely CD4 reconstitution, which was defined as 50 × 10 6 cells/L or greater within 100 days. Adenovirus reactivation predicted lower OS (HR, 2.17; P = .0039) and higher NRM (HR, 2.96; P = .0008). Concomitant CD4 reconstitution abolished this negative effect of adenovirus reactivation (OS, P = .67; NRM, P = .64). EBV and HHV6 reactivations were predictors for the occurrence of graft-versus-host disease, whereas CMV and BK virus reactivation did not predict clinical outcomes. Conclusion These results stress the importance of timely CD4 reconstitution. Strategies to improve CD4 reconstitution can improve HCT outcomes, including survival, and reduce the need for toxic antiviral therapies.

In haploidentical (haplo)-cord blood (CB) transplantations, early haplo donor engraftment serves as a myeloid bridge to sustainable CB engraftment and is associated with early neutrophil recovery. The conditioning regimens as published for haplo-cord protocols usually contain serotherapy, such as rabbit antithymocyte globulin (ATG) (Thymoglobulin, Genzyme, Cambridge, MA). However, reducing or omitting serotherapy is an important strategy to improve early immune reconstitution after transplantation. The need for serotherapy in successful haplo-cord transplantation, defined as having a haplo-derived myeloid bridge to CB engraftment, has not been investigated before. Two consecutive cohorts of patients underwent transplantation with haplo-CB. The first group underwent transplantation with haplo-CB for active infection and/or an underlying condition with expected difficult engraftment without a conventional donor available. They received a single unit (s) CB and haplo donor cells (CD34(+) selected, 5 x 10(6) CD34(+)/kg). The second cohort included patients with poor-risk malignancies, not eligible for other treatment protocols. They received a sCB and haplo donor cells (CD19/alpha beta TCR-depleted; 5 x 10(6) CD34(+)/kg). Retrospectively in both cohorts, active ATG (Thymoglobulin) levels were measured and post hematopoietic cell transplantation area under the curve (AUC) was calculated. The influence of ATG exposure for having a successful haplo-myeloid bridge (early haplo donor engraftment before CB engraftment and no secondary neutropenia) and transplantation-related mortality (TRM) were analyzed as primary endpoints. Twenty patients were included (16 in the first cohort and 4 in the second cohort). In 58% of evaluable patients, there was no successful haplo-derived myeloid bridge to CB engraftment, for which a low post-transplantation ATG exposure appeared to be a predictor (P