Once-daily intravenous busulfan for 47 pediatric patients undergoing autologous hematopoietic stem cell transplantation: a single center study

Total Body Irradiation Forever? Optimising Chemotherapeutic Options for Irradiation-Free Conditioning for Paediatric Acute Lymphoblastic Leukaemia

Total-body irradiation (TBI) based conditioning prior to allogeneic hematopoietic stem cell transplantation (HSCT) is generally regarded as the gold-standard for children >4 years of age with acute lymphoblastic leukaemia (ALL). Retrospective studies in the 1990's suggested better survival with irradiation, confirmed in a small randomised, prospective study in the early 2000's. Most recently, this was reconfirmed by the early results of the large, randomised, international, phase III FORUM study published in 2020. But we know survivors will suffer a multitude of long-term sequelae after TBI, including second malignancies, neurocognitive, endocrine and cardiometabolic effects. The drive to avoid TBI directs us to continue optimising irradiation-free, myeloablative conditioning. In chemotherapy-based conditioning, the dominant myeloablative effect is provided by the alkylating agents, most commonly busulfan or treosulfan. Busulfan with cyclophosphamide is a long-established alternative to TBI-based conditioning in ALL patients. Substituting fludarabine for cyclophosphamide reduces toxicity, but may not be as effective, prompting the addition of a third agent, such as thiotepa, melphalan, and now clofarabine. For busulfan, it's wide pharmacokinetic (PK) variability and narrow therapeutic window is well-known, with widespread use of therapeutic drug monitoring (TDM) to individualise dosing and control the cumulative busulfan exposure. The development of first-dose selection algorithms has helped achieve early, accurate busulfan levels within the targeted therapeutic window. In the future, predictive genetic variants, associated with differing busulfan exposures and toxicities, could be employed to further tailor individualised busulfan-based conditioning for ALL patients. Treosulfan-based conditioning leads to comparable outcomes to busulfan-based conditioning in paediatric ALL, without the need for TDM to date. Future PK evaluation and modelling may optimise therapy and improve outcome. More recently, the addition of clofarabine to busulfan/fludarabine has shown encouraging results when compared to TBI-based regimens. The combination shows activity in ALL as well as AML and deserves further evaluation. Like busulfan, optimization of chemotherapy conditioning may be enhanced by understanding not just the PK of clofarabine, fludarabine, treosulfan and other agents, but also the pharmacodynamics and pharmacogenetics, ideally in the context of a single disease such as ALL.

Review of the Pharmacokinetics and Pharmacodynamics of Intravenous Busulfan in Paediatric Patients

We aimed to review the pharmacokinetics (PK) of intravenous busulfan in paediatric patients, identify covariate factors influencing exposure, investigate evidence of changes in PK behaviour over time, and correlate exposure with efficacy and toxicity outcomes. A literature review was undertaken of original research published between 2007 and 2019, investigating the PK and pharmacodynamics (PD) of intravenous busulfan in patients ≤ 18 years of age. The review identified 41 publications characterising the PK, and 45 publications describing the PD, of busulfan. Median typical clearance (CL) was 0.22 L/h/kg and median typical volume of distribution was 0.69 L/kg. Patient weight, age, glutathione-S-transferase A1 (GSTA1) genotype and busulfan dosing day/time were the most commonly identified factors affecting CL. Of nine studies investigating changes in CL, seven reported reduced CL over the 4-day course of treatment. Exposure monitoring methods and therapeutic targets were heterogeneous across studies. Relationships between busulfan exposure and patient outcomes were observed in five studies. One study observed a cumulative area under the concentration-time curve over all days of treatment of between 78 and 101 mg/L·h, and two studies observed an average concentration at first dose of < 600 ng/mL improved overall survival, transplant-related mortality, or relapse. One study observed increased sinusoidal obstructive syndrome with maximum busulfan concentration > 1.88 ng/mL. Patient weight, age and GSTA1 genotype are important covariates to consider when individualising busulfan therapy. Reduced busulfan CL over time may need to be accounted for, particularly in patients not receiving phenytoin co-therapy. Standardised monitoring of busulfan exposure over the entire course of treatment and further investigation of the role of busulfan metabolites and pharmacogenomics is warranted.

Haploidentical stem cell transplant with post-transplantation cyclophosphamide and mini-dose methotrexate in children

BACKGROUND

Haploidentical stem cell transplantation (haplo-SCT) is an option for patients without human leukocyte antigen-matched related or unrelated donor. Post-transplantation cyclophosphamide (PTCy) is an effective method of graft versus host disease (GVHD) prophylaxis and permits the use of T-cell replete grafts in settings were ex vivo manipulation is not feasible.

METHODS

A retrospective study among patients younger than 18 years, with a history of hematologic malignancies who underwent haplo-SCT between 2012 and 2016. All patients received a preparative regimen of fludarabine, busulfan, and 400 cGy total body irradiation or melphalan. Post-transplant GvHD prophylaxis consisted either of PTCy (50 mg/kg on Days + 3 and + 4) and cyclosporine (CSA) plus mycophenolate (MMF) (15 mg/kg/dose, thrice daily, per os), or mini-dose methotrexate (MTX; 5 mg/m² dose) on Days + 5, +7, +10, and + 15.

RESULTS

A total of 52 children were included, whose median age was 9 years (interquartile range, 4.9-14; range, 1.2-17 years), and 63% were males. The most common complications were cytomegalovirus reactivation (57%) and hemorrhagic cystitis (36%). The acute GVHD prophylaxis was PTCy, CSA, and mini-dose MTX in 42 (81%) patients, and 10 (19%) patients received PTCy, CSA, and MMF. The cumulative incidence of acute GvHD II-IV, acute GvHD III-IV, and chronic GvHD were 42%, 8.5%, and 19%, respectively. Grades I-IV acute GvHD occurred in 100% of the patients who received prophylaxis with CSA and MMF, and 62% who received CSA and mini-dose MTX (p = .055). The transplant-related mortality at 100 days was 18%. The 5-year overall and event-free survival were 59% and 57%, respectively.

CONCLUSIONS

Haplo-SCT with PT/Cy can be an available, safe, and feasible option for children with hematologic malignancies; meanwhile, the use of mini-dose of MTX was associated with lower rates of acute GVHD. However, our results require further support from prospective randomized studies to improve the efficacy of this prophylactic strategy.

Maximal concentration of intravenous busulfan as a determinant of veno-occlusive disease: a pharmacokinetic-pharmacodynamic analysis in 293 hematopoietic stem cell transplanted children

Veno-occlusive disease (VOD) is a severe adverse reaction to busulfan-containing regimens used in the preparation of children for hematopoietic stem cell transplantation (HSCT). We conducted a retrospective analysis of data to examine determinants of VOD in children who received IV busulfan for HSCT conditioning. Busulfan PK parameters as well as various indices (maximal concentration-Cmax, area under the concentration-time curve-AUC) were estimated using a validated Bayesian approach. The influence of available PK, demographic, and clinical variables on the incidence of VOD was evaluated by using logistic regression and classification and regression tree (CART) analyses. Among the 293 patients included, the mean age was 6.5 years and the mean actual body weight was 26.3 kg. The incidence of VOD was 25.6%. Busulfan Cmax as well as weight <9 kg or age <3 years were identified as independent predictors of VOD in logistic regression analysis. CART analysis identified busulfan Cmax over the entire regimen as the strongest predictor of VOD. This study suggests that busulfan-associated VOD is in part a concentration-dependent reaction. In addition, the youngest children showed the highest risk of VOD. These findings may have important implications for busulfan dosing and therapeutic drug monitoring practice in HSCT children.

HLA haploidentical hematopoietic cell transplantation using clofarabine and busulfan for refractory pediatric hematological malignancy

Haploidentical hematopoietic cell transplantation (HCT) conditioning with clofarabine and target area under the blood concentration-time curve (AUC)-based busulfan adjustment was performed in three patients with refractory pediatric leukemia. The target AUC for two patients who had already received multiple transplantations was 3600 and 4000 μmol min/L, and that for the patient with Down's syndrome was 3000 μmol min/L. Regimen-related toxicity was well tolerated in all cases. All three maintained cytological remission throughout the follow-up period (between 31 and 167 weeks). Thus, haploidentical HCT conditioning with clofarabine and target AUC-based busulfan adjustment may be a preferable option for children with recurrent or refractory pediatric leukemia.

Intravenous busulfan: a guide to its use as conditioning treatment before transplantation of haematopoietic progenitor cells

With intravenous administration of busulfan (Busilvex®), the therapeutic range of the drug is achieved in more paediatric patients than after oral busulfan administration. In clinical trials in paediatric and adult patients, intravenous busulfan-based preconditioning treatment regimen prior to haematopoietic progenitor cell transplantation (HPCT) provided high rates of engraftment, low transplant-related mortality rates and good overall survival and progression-free survival outcomes. Intravenous busulfan was considered to have an acceptable tolerability profile.

Intravenous busulfan: a guide to its use as conditioning treatment before transplantation of haematopoietic progenitor cells

With intravenous administration of busulfan (Busilvex®), the therapeutic range of the drug is achieved in more paediatric patients than after oral busulfan administration. In clinical trials in paediatric and adult patients, intravenous busulfan-based preconditioning treatment regimen prior to haematopoietic progenitor cell transplantation (HPCT) provided high rates of engraftment, low transplant-related mortality rates and good overall survival and progression-free survival outcomes. Intravenous busulfan was considered to have an acceptable tolerability profile.