Treosulfan Exposure Predicts Thalassemia-Free Survival in Patients with Beta Thalassemia Major Undergoing Allogeneic Hematopoietic Cell Transplantation

Abstract

A toxicity-reduced conditioning regimen with treosulfan, fludarabine, and thiotepa in patients with high-risk β-thalassemia major has significantly improved hematopoietic stem cell transplantation (HCT) outcomes. However, complications resulting from regimen-related toxicities (RRTs), mixed chimerism, and graft rejection remain a challenge. We evaluated the dose-exposure-response relationship of treosulfan and its active metabolite S, S-EBDM, in a uniform cohort of patients with β-thalassemia major to identify whether therapeutic drug monitoring (TDM) and dose adjustment of treosulfan is feasible. Plasma treosulfan/S, S-EBDM levels were measured in 77 patients using a validated liquid chromatography with tandem mass spectrometry method, and the pharmacokinetic parameters were estimated using nlmixr2. The influence of treosulfan and S, S-EBDM exposure, and GSTA1/NQO1 polymorphisms on graft rejection, RRTs, chimerism status, and 1-year overall survival (OS), and thalassemia-free survival (TFS) were assessed. We observed that treosulfan exposure was lower in patients with graft rejection than those without (1,655 vs. 2,037 mg•h/L, P = 0.07). Pharmacodynamic modeling analysis to identify therapeutic cutoff revealed that treosulfan exposure ≥1,660 mg•hour/L was significantly associated with better 1-year TFS (97% vs. 81%, P = 0.02) and a trend to better 1-year OS (90% vs. 69%, P = 0.07). Further, multivariate analysis adjusting for known pre-HCT risk factors also revealed treosulfan exposure <1,660 mg•h/L (hazard ratio (HR) = 3.23; 95% confidence interval (CI) = 1.12-9.34; P = 0.03) and GSTA1*B variant genotype (HR = 3.75; 95% CI = 1.04-13.47; P = 0.04) to be independent predictors for inferior 1-year TFS. We conclude that lower treosulfan exposure increases the risk of graft rejection and early transplant-related mortality affecting TFS. As no RRTs were observed with increasing treosulfan exposure, TDM-based dose adjustment could be feasible and beneficial.

Figure 1

Plots showing treosulfan and S, S-EBDM concentrations vs. timepoints in the study cohort. Spaghetti plots representing (top left) treosulfan and (top right) S, S-EBDM concentrations vs. timepoints acquired for patients included in the study. Plasma treosulfan and S, S-EBDM levels (mg/L) were plotted against five study timepoints (0-hour pre-treosulfan infusion, EOI (end of treosulfan infusion), and 2, 4, and 24 hours post-treosulfan infusion). Log plasma concentration-time profile (bottom) of both treosulfan and S, S-EBDM for patients included in the study. Mean log-transformed treosulfan and S, S-EBDM concentrations were plotted against actual study timepoints.

Figure 2

Correlation analysis: (left) treosulfan with S, S-EBDM exposure, and (right) treosulfan clearance with age. Left: correlation analysis between the (x-axis) treosulfan and (y-axis) S, S-EBDM exposure in each patient showed a statistically moderate positive linear relationship (R² = 0.17, P = 0.0002). Right: Correlation analysis between (x-axis) age and (y-axis) treosulfan clearance in each patient showing absence of correlation.

Figure 3

NQO1 / GSTA1*B genotypes influence treosulfan and S, S-EBDM Exposure. Association between NQO1 and GSTA1*B genotypes with treosulfan and S, S-EBDM exposure. *Wild-type: homozygous reference genotype; Het/Homo: heterozygous and homozygous genotype; the P value was calculated by Mann–Whitney U-test. *Asterisks indicate the level of the significance (P value); *Means P < 0.05. Log-transformed (y-axis) treosulfan and S, S-EBDM AUC were plotted against (x-axis) NQO1 / GSTA1*B genotypes. AUC, area under the concentration-time curve.

Figure 4

Role of treosulfan AUC in predicting the probability of 1-year TFS and OS. Kaplan–Meier curve for (left) 1-year TFS and (right) OS in patients above and below the upper success probability treosulfan AUC cutoff of 1,660 mg•hour/L. AUC, area under the concentration-time curve; OS, overall survival; TFS, thalassemia-free survival.