CYP26 Inhibition Increases Retinoic Acid Concentrations in Target Tissues and Alters Retinoid Signaling
Authors: Stevison Faith , Tripathy Sasmita , Kent Travis , Hogarth Cathryn A. , Griswold Michael D. , Isoherranen Nina 
Published: November 2023

The active metabolite of vitamin A, all‐trans‐retinoic acid (atRA), regulates many physiological processes including lipid homeostasis, adiposity and spermatogenesis. Clinically atRA is used in the treatment of acute promyelocytic leukemia but use is associated with high relapse rates likely due to autoinduction of CYP26s, the major enzymes responsible for atRA clearance. To overcome autoinduction and increase efficacy, retinoic acid metabolism blocking agents (RAMBAs) were developed. Subsequently, inhibition of CYP26s has become a drug development strategy with the aim to increase endogenous levels of atRA in other diseases.The RAMBA talarozole (TLZ) is a selective and potent CYP26 inhibitor. The objective of this work was to determine whether TLZ treatment increases endogenous atRA concentrations in retinoid target tissues as predicted from in vitro CYP26 inhibition data and TLZ disposition in mice, and to establish whether TLZ treatment alters retinoid signaling related to fatty acid oxidation in the liver and spermatogonial differentiation in the testesTalarozole was administered to mice as a single dose (2.5 mg/kg) or as multiple doses for three days. Serum TLZ concentrations and serum, liver and testes atRA concentrations were measured by LC‐MS/MS. Inhibition of CYP26 and changes in atRA concentrations in each tissue were predicted based on CYP26 activity in vitro and TLZ disposition. Markers of fatty acid oxidation in the liver and spermatogonial differentiation in the testes were measured following TLZ treatment.A maximum 84% inhibition of CYP26 activity at 0.5 hours post‐dose was predicted based on TLZ Cmax of 80 nM and a Ki of 1 nM following a single dose of TLZ. Due to the short TLZ half‐life (2.2 hrs) CYP26 activity was predicted to return to 100% by 12 hours. In agreement with the predictions, atRA concentrations were increased by 82, 63 and 60% at 4 hours post‐dose in the serum, liver and testes, respectively, and concentrations returned to baseline by 24 hours. Following multiple doses of TLZ, liver CYP26 mRNA and activity were increased suggesting autoinduction of CYP26 due to increased atRA concentrations. In agreement, atRA concentrations were elevated in serum and liver at all timepoints measured. This increase in atRA concentrations was associated with increased mRNA of the mitochondrial biogenesis markers PGC‐1β and NRF‐1 in comparison to control mice. atRA signaling in the testes was increased following multiple doses of TLZ based on Stra8 expression, a marker of atRA signaling, and spermatogonial differentiation, but the increase in testes atRA concentrations was not significant.This study shows that administration of a RAMBA to inhibit the enzymes primarily responsible for atRA elimination (CYP26s) increases atRA concentrations in the serum, liver and testes of mice and increases the expression of genes that are markers of retinoid signaling in the target tissues. These studies suggest that blocking atRA metabolism in vivo via CYP26 inhibition may be a viable drug target in select tissues.Support or Funding InformationThis work is support by NIH grants R01GM111772 and T32GM007750.