Background Intravenous-busulfan (IV-busulfan) combined with therapeutic drug monitoring to guide dosing improves outcomes after allogeneic hematopoietic cell transplantation (allo-HCT). The best method to estimate busulfan exposure and the optimal exposure in children/young adults remains unclear. We therefore evaluated three approaches to estimate IV-Bu exposure (expressed as cumulative-area-under-the-curve; AUC) and associated busulfan-AUC with clinical outcomes in children/young adults undergoing allo-HCT. Methods In this retrospective analysis, patients (0.1–30.4 years) receiving busulfan-based conditioning regimen from 15 centers were included. Cumulative AUC was calculated by numerical integration using non-linear mixed effect modeling (AUCNONMEM), non-compartmental analysis (AUC0-infinity and AUC to the end of the dose interval AUC0-tau) and by individual centers using a variety of approaches (AUCcenter). Main outcome of interest was event-free survival (EFS). Other outcomes of interest were overall survival, graft-failure, relapse, transplantation related mortality (TRM), acute toxicity (veno-occlusive disease (VOD) and/or acute graft versus-host disease (aGvHD), chronic GvHD (cGvHD) and cGVHD-free event-free survival (GEFS). Propensity score adjusted cox proportional hazard models, Weibull models, and Fine-Gray competing risk regressions were used. Results 674 patients were included (41% malignant, 59% non-malignant) Estimated 2-year EFS was 69.7%. The median busulfan AUCNONMEM was 74.4 mg*h/L (CI95% 31.1–104.6 mg*h/L). The median AUCNONMEM correlated poorly with AUCcenter (R2 = 0.254). Patients with optimal IV-busulfan AUC of 78–101 mg*h/L showed 81% EFS at 2 years compared to 66.1% and 49.5% in the low (<78 mg*h/L) and high (>101 mg*h/L) busulfan AUC group respectively (P=0.011). Graft-failure/relapse occurred more frequently in the low AUC group (HR=1.75 P<0.001). Acute toxicity, cGvHD and TRM was significantly higher in the high AUC group (HR 1.69, 2.99 and 1.30), independent of indication. Interpretation These results demonstrate that improved clinical outcomes may be achieved by targeting the busulfan-AUC to 78–101 mg*h/L using a new validated pharmacokinetic-model for all indications.

A prospective multicenter study was conducted to characterize the pharmacokinetics (PK) and pharmacodynamics (PD) of fludarabine plasma (f-ara-a) and intracellular triphosphate (f-ara-ATP) in children undergoing hematopoietic cell transplantation (HCT) and receiving fludarabine with conditioning. Plasma and peripheral blood mononuclear cells (PBMCs) were collected over the course of therapy for quantitation of f-ara-a and f-ara-ATP. Nonlinear mixed-effects modeling was used to develop the PK model, including identification of covariates impacting drug disposition. Data from a total of 133 children (median age, 5 years; range, .2 to 17.9) undergoing HCT for a variety of malignant and nonmalignant disorders were available for PK-PD modeling. The implementation of allometric scaling of PK parameters alone was insufficient to describe drug clearance, particularly in very young children. Renal impairment was predicted to increase drug exposure across all ages. The rate of f-ara-a entry into PBMCs (expressed in pmoles per million cells) decreased over the course of therapy, resulting in 78% lower f-ara-ATP after the fourth dose (1.7 pmoles/million cells [range, .2 to 7.2]) compared with first dose (7.9 pmoles/million cells [range, .7 to 18.2]). The overall incidence of treatment-related mortality (TRM) was low at 3% and 8% at days 60 and 360, respectively, and no association with f-ara-a exposure and TRM was found. In the setting of malignancy, disease-free survival was highest at 1 year after HCT in subjects achieving a systemic f-ara-a cumulative area under the curve (cAUC) greater than 15 mg*hour/L compared to patients with a cAUC less than 15 mg*hour/L (82.6% versus 52.8% P = .04). These results suggest that individualized model-based dosing of fludarabine in infants and young children may reduce morbidity and mortality through improved rates of disease-free survival and limiting drug-related toxicity. ClinicalTrials.gov Identifier: NCT01316549

Mycophenolic acid (MPA)is the active component of mycophenolate mofetil (MMF). Low MPA exposure is associated with a higher incidence of acute GVHD and possibly worse engraftment. Therapeutic plasma targets have been proposed in hematopoietic cell transplantation (HCT), however, are difficult to achieve in adult patients with MMF doses of 2 g/day. Mycophenolate pharmaco-kinetics was prospectively studied in adults undergoing nonmyeloablative HCT who received MMF 3 g/day with CYA. The first 15 individuals received 1.5 g every 12 h and the second 15 received 1 g every 8 h. Sampling was performed in each patient with i.v. and oral administration. There were no differences in total or unbound MPA 24-h cumulative area under the curves (AUCs), concentrations at steady state (Css)or troughs between the two dosing regimens (all P>0.01). The previously proposed total MPA Css target of 3 μg/ml and trough ≥1 μ/ml were achieved in only 13–27% and 20–53% of patients, respectively, on 3 g/day. However, the 3 g/day regimens readily achieved satisfactory unbound 24-h cumulative AUC targets of 0.600 μg*h/ml in 87–100% of subjects. There appears to be no significant difference in daily MPA exposure when MMF of 3 g/day is divided into two or three equal doses.

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