Role of fludarabine exposure on post-transplant outcomes in high-risk β-thalassemia patients undergoing allogeneic HCT with thiotepa/ fludarabine/ treosulfan conditioning Free

Fludarabine (Flu) in combination with Treosulfan (Treo) and Thiotepa (Thio) (TFT) has been used as a toxicity-reduced conditioning regimen in patients with high-risk β-Thalassemia undergoing HCT, resulting in better outcomes in patients compared to the Bu/Cy-based regimen (Mathews et al., 2013). Our group has previously reported the PK and PD of Flu/Treo-containing regimen (Mohanan et al., 2019; Pai et al., 2023). More recently, there has been an increase in the use of TFT regimen, especially in high-risk Thal at our center, hence the need to explore the impact of Flu PK on HCT outcomes in this group, which has not been reported. Fludarabine exposure and dose optimization in HCT has been attempted by few groups (Ivaturi et al., 2017, Dekker et al., 2024, Scordo et al., 2023). Here, we compared the association between Flu exposure with transplant outcomes in patients with β- Thal.

A total of 245 Class III low-risk or high-risk patients who received TFT as a conditioning regimen were enrolled between 2012 and 2024. All patients received Thio 8mg/kg/day x 1 day (day -1) Flu 40mg/m²/day x 4 days (day – 5 to -2) as a 1-hr infusion and Treo as 14g/m²/day x 3 days at the rate of 5g/hr (day -5 to -3) infused post Flu). Plasma samples were collected and stored at various points (pre infusion, end of infusion, and 2hr, 6hr and 24 post infusion) for Flu PK analysis using LC-MS. Non-linear mixed effects modeling analysis was performed with Monolix (version 2023R1) using the SEAM method. A two-compartment PK model was used to describe the data. The individual post hoc parameter values were used to estimate the area under the concentration curve (AUC). The inter-individual variability of the parameters was assumed to be log-normally distributed. A proportional residual error model was used with an assumed normal distribution of the residuals.

Patient characteristics and post-transplant outcomes up to one year were documented from the clinical databases. Engraftment was achieved at a median of 16 days (11- 43 days) in 233 (95%) patients. Any deaths occurring within the first 100 days post-HCT were regarded as transplant-related mortality (TRM). Early TRM (TRM≤D30) are deaths occurring within 30 days post-HCT. One-year thalassemia-free survival (TFS) was defined as the time from transplant to the occurrence of an event (rejection or death within a year). Patients were characterized into three groups based on Flu exposure: low AUC (<18 mg*h/L), optimal AUC (18-20 mg*h/L), and high AUC (>20 mg*h/L) (Scordo et al., 2023). Flu exposure cut-off for Allo HCT in Thalassemia has not been previously described and therefore we chose target AUC based on available literature. Descriptive statistics and Pearson Chi-Square analysis were performed to assess the association of Flu exposure with transplant outcomes.

In our cohort, 142 patients (58%) were in the low AUC, 23 (9%) in the optimal AUC and 80 (33%) in the high AUC groups. Rejection occurred in a total of 13 patients (5.3%), 8 of whom were in the low AUC group, and 5 in the high AUC, and this association was not statistically significant (p=0.474). Incidence of GvHD was recorded in a total of 67 (27.3%) patients, of which 41 patients were in the low AUC group with no significant difference in Flu AUC among the three groups (p=0.259). TRM day ≤30 occurred in a total of 19 patients (7.8%), of which 10 deaths were in the low AUC group with no statistically significant differences between the three groups as tested by Chi-square analysis (p=0.595). We also observed that Flu exposure had no direct effect on influencing 1yr OS (p=0.817) and TFS (p=0.748). We then compared the outcomes between patients with Flu AUC below or above median (median AUC 15.2mg*hr/L). Similar analysis was done comparing the association of Flu AUC with outcomes, in those above (n=121) and below (n=124) median. However, this too revealed no differences in outcomes based on exposure.

Our study suggests that Flu exposure does not directly influence transplant outcomes in high-risk patients with Thalassemia receiving the Thio/Flu/Treo regimen. However, low exposure to Flu may be a surrogate indicator of poor outcomes. More robust patient-specific parameters (pre-transplant risk factors), the role of concomitantly administered drugs such a Treo and other pharmacodynamic measures should be considered along with the PK of Flu to better explain and predict transplant outcomes.