Treosulfan Metabolite (S, S-EBDM) Pharmacokinetics Influences Regimen Related Toxicity in Patients with Beta Thalassaemia Major Undergoing HSCT

A toxicity reduced conditioning regimen containing Treosulfan (Treo), Fludarabine (Flu), Thiotepa in patients with class III high risk thalassemia major (TM) has significantly improved transplant outcomes compared to the historical Busulfan/cyclophosphamide based myeloablative regimen (Mathews et al, 2013). However, complications related to mixed chimerism, rejection and regimen related toxicity remain. Analysis of Treo pharmacokinetics (PK) in TM showed significant inter individual variation but no association with HSCT outcome (Mohanan et al, 2018). This may be attributed to the fact that only the pro-drug Treo was measured and not its active metabolites. Treo, administered as a prodrug, is converted non-enzymatically to its active metabolite monoepoxide 2S, 3S-1,2-epoxybutane-3,4-diol-4-methanesulfonate (S, S-EBDM) and (2S,3S)- 1,2:3,4-diepoxybutane (S, S-DEB) that causes cytotoxicity. We have now assessed the PK of both Treo and S, S-EBDM and the factors which may influence the inter-individual variation in PK, and their impact on HSCT outcome in patients with TM.

Fifty-six patients with TM undergoing HSCT between January 2016 and June 2019 with Flu/Treo based conditioning regimen (Flu 40mg/m²/day x 4 days as a 1hr infusion and Treo as 14g/m²/day x 3 days infused at 5g/hr) were included in this analysis. Selected functional polymorphisms in NAD(P)H Quinone Dehydrogenase-1 (NQO1) and Glutathione S-transferase (GST) genes that showed association with Treo PK based on our previous report (Pai et al, Abstract. Proceedings of BMT Tandem Meetings 2018) were screened. Plasma Treo and S, S-EBDM were measured using an LC-MS/MS method. Treo and S, S-EBDM PK were estimated using nonlinear mixed effects modeling via Monolix 5.0.1. The covariates tested were age, sex, body weight, BSA, serum ferritin, NQO1 3'UTR (rs10517) and GSTA1*B polymorphisms. The influence of Treo & S, S-EBDM PK and polymorphisms on graft rejection, regimen related toxicities (RRT), chimerism status, overall and event-free survival were estimated using logistic regression analysis.

A two-compartment model best described the PK of Treo and S, S-EBDM. Two and 11-fold inter-individual variation was observed in Treo and S, S-EBDM PK respectively. Among the covariates tested, Treo Cl decreased with age (p=0.03) while, S, S-EBDM apparent Cl increased in patients with NQO1 3'UTR variant (p=0.02) and decreased in patients with GSTA1*B variant genotype (p=0.02). Neither Treo AUC nor clearance showed association with HSCT outcome. However, high S, S-EBDM AUC (213. Vs 130 mg*hr/mL, p=0.016) and low S, S-EBDM apparent Cl (68 Vs 122 L/hr/m², p=0.013) were significantly associated with increased incidence of sinusoidal obstruction syndrome (SOS). Also, on quartile analysis, patients with S, S-EBDM AUC >210 mg*hr/mL and Treo/S, S-EBDM AUC ratio >18 mg*hr/mL showed increased incidence of SOS (p=0.006 & p=0.026 respectively). Multivariate analysis adjusting for known clinical risk factors also showed in addition to age, only high S, S-EBDM AUC to be significantly associated with increased incidence of SOS (p=0.013). No association between rejection or mixed chimerism and Treo/S, S-EBDM PK was observed, probably due to very low incidence of graft rejection in this cohort (4/56; 7%).

Our study suggests that active metabolite S, S-EBDM influences SOS following HSCT in TM patients. It is well acknowledged that Treo to S, S-EBDM conversion is a non-enzymatic nucleophilic substitution reaction and there is a difference in the disposition of Treo and S, S-EBDM in different organs including the liver (Romanski et al, 2017). Treo metabolites exert their cytotoxic activity by both DNA-DNA and DNA-protein crosslinks in these organs. The role of Treo metabolites in exhausting essential cellular proteins like GSH and NQO1 as well as the effect of GST/NQO1 variants in influencing Treo metabolism in liver which may then impact the incidence and severity of SOS need further evaluation.