Deferiprone Glucuronidation by Human Tissues and Recombinant UDP Glucuronosyltransferase 1A6: An in Vitro Investigation of Genetic and Splice Variants

Tissue iron overload constitutes a major health problem for people who require regular blood transfusions, such as those with beta-thalassemia major. Deferiprone is a hydroxypyridinone iron chelator used therapeutically to remove this excess iron and prevent tissue damage. Deferiprone is metabolized by UDP-glucuronosyl-transferases (UGTs) into deferiprone 3-O-glucuronide (DG), but a systematic evaluation of the contribution of individual human UGTs and the impact of genetic variations of UGTs have not been conducted. Sixteen human UGT1A and UGT2B were studied for deferiprone glucuronidation, and their clearances were compared in human tissue samples. DG was measured by liquid chromatography coupled with mass spectrometry. DG was primarily produced in vitro by UGT1A6, and a second glucuronide metabolite was discovered. UGT1A6, as well as liver and kidney human microsomes, had similar kinetic profiles and clearance (Cl-int = 1.4-3.0 mu l/min/mg), but clearance by intestinal microsomes was much lower (0.04 mu l/min/mg). Binding of deferiprone to microsomal preparations was not significant. Genetic variants of UGT1A6 had K-m values similar to the reference protein (UGT1A6*1), but their V-max values were reduced by 25 to 70%. The UGT1A6 splice variant isoform 2, detected in the liver and kidney, had no transferase activity for deferiprone. When UGT1A6_i2 was coexpressed with the classic UGT1A6_i1 isoform, velocity was reduced for deferiprone but remained similar for 4-nitrophenol or serotonin glucuronidation. In conclusion, deferiprone glucuronidation seems to depend almost totally on UGT1A6, especially in the liver. Genetic variations and differences in the expression of splice variants represent a potential source of variation in deferiprone metabolism.