Liarozole markedly increases all trans-retinoic acid toxicity in mouse limb bud cell cultures: a model to explain the potency of the aromatic retinoid (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthylenyl)-1-propenyl] benzoic acid

The remarkable toxicity of (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthylenyl)-1-propenyl] benzoic acid (TTNPB) compared to all trans-retinoic acid (tRA) is due to multiple factors, including reduced affinities for cytosolic binding proteins (CRABPs), resistance to metabolism, and prolonged nuclear receptor activation. To further investigate the role of half-life in retinoid toxicity, experiments were performed to determine whether, and to what extent, inhibition of tRA metabolism by liarozole increased its toxicity comparable to that of TTNPB in the mouse limb bud system. Liarozole is a known inhibitor of tRA 4-hydroxylation (CYP26). In the absence of liarozole, the IC50 for inhibition of chondrogenesis by tRA was 140 nM compared to 0.3 nM for TTNPB, a 467-fold difference. Following the addition of liarozole (10(-6) M) to limb bud cultures, the potency of tRA to inhibit chondrogenesis was increased approximately 14-fold (IC50 of 9.8 nM). Although liarozole markedly increased toxicity of tRA in mouse limb bud micromass cultures, tRA metabolism was inhibited only about 10%. These results indicate that a relatively minor decrease in the metabolism of tRA in the mouse limb bud system is associated with a marked enhancement of toxicity that is likely related to the prolongation of tRA half-life during a critical period of development. Thus, the prolonged half-life of TTNPB is the most significant factor contributing to the remarkable teratogenicity of this synthetic aromatic retinoid.