No abstract

Inverse agonists are ligands that are capable of repressing basal receptor activity in the absence of an agonist. We have designed a series of C-1-substituted acetylenic retinoids that exhibit potent antagonism of retinoic acid receptor (RAR)-mediated transactivation. Comparison of these related retinoid antagonists for their ability to repress basal RAR transcriptional activity demonstrates that the identity of the C-1 substituent differentiates these ligands into two groups: RAR inverse agonists and neutral antagonists. We show that treatment of cultured human keratinocytes with a RAR inverse agonist, but not a RAR neutral antagonist, leads to the repression of the serum-induced differentiation marker MRP-8. While RAR-selective agonists also repress expression of MRP-8, cotreatment with a RAR inverse agonist and a RAR agonist results in a mutual repression of their individual inhibitory activities, indicating the distinct modes of action of these two disparate retinoids in modulating MRP-8 expression. Our data indicate that RARs, like beta2-adrenoreceptors, are sensitive to inverse agonists and that this new class of retinoids will provide insight into the molecular mechanisms of RAR function in skin and other responsive tissues.

Synthetic retinoids, ligands for the RAR and RXR members of the steroid/thyroid superfamily of nuclear hormone receptors, are used for the treatment of psoriasis, acne, photoaging and cancer. Retinoid mechanisms of action for these conditions largely involve effects on epithelial differentiation and modulation of inflammation with some impact on the immune system. Retinoid medicinal chemistry in recent years has identified ligands highly specific for one of the three RAR subtypes (RAR-alpha) and for the RXR family of receptors, as well as antagonists for the RARs, RARalpha and the RXRs. Structure-activity relationships among the novel retinoid classes are reviewed along with potential therapeutic activities and side effects. RAR-alpha specific retinoids inhibit cancer cell growth but lack other retinoid toxicities, including skin irritation now ascribed to RAR-gama. RXR-specific retinoids lower blood glucose in animal models of type 2 diabetes albeit with a potential for mild hypothyroidism. Function-selective retinoids, especially a class of RAR antagonists called inverse agonists, have unexpected gene regulatory activity. Given the diverse properties and tissue distributions of the retinoid receptors, synthesis of additional classes of receptor-specific and function-selective ligands has the potential to produce novel therapeutic applications.

The synthesis and characterization of chiral RXR selective ligands are described. The enantiomeric acids 2 and 3 were synthesized employing an enantioselective cylopropanation procedure as the key step. Compound 2, with an S,S configuration at C-9 and C-10, is a potent RXR agonist devoid of any RAR activity. The R,R enantiomer 3 is a weak RXR agonist and has demonstrable RAR activity in the receptor transactivation assays. The potent RXR activity of 2 was further confirmed in a hyperglycemic animal model (db/db mice). Compound 2 lowered glucose by 50% by day 7 at 2 mg/kg, whereas 3 had no effect at the same dosage. This further supports the contention that RXR mediated gene transcription is involved in the antidiabetic effects of RXR ligands.

A novel series of RORγ inhibitors was identified starting with the HTS hit 1. After SAR investigation based on a prospective consideration of two drug-likeness metrics, ligand efficiency (LE) and fraction of sp 3 carbon atoms (Fsp 3 ), significant improvement of metabolic stability as well as reduction of CYP inhibition was observed, which finally led to discovery of a selective and orally efficacious RORγ inhibitor 3z.KEYWORDS: Th17, immunological diseases, nuclear receptor, RORγ, ligand efficiency (LE), fraction of sp 3 carbon atoms (Fsp 3 )T wo decades after the discovery of Th1 and Th2 cells, a third subset of T helper cells called Th17 cells was identified and has drawn considerable attention since it was suggested to play a central role in the pathogenesis of various autoimmune diseases such as psoriasis and rheumatoid arthritis. 1,2 Among several regulatory pathways in which Th17 development and function are involved, the one regulated by the nuclear receptor RORγ appears to be crucial for controlling the differentiation and function. 3 Given its validity as an emerging drug target for treatment of immunological diseases, many research groups have made significant efforts in the discovery of RORγ modulators in recent years. 4−19 Since starting our RORγ inhibitor program in 2003, we discovered several structurally diverse hits after a HTS campaign. 20 From these hits we selected compound 1 as the first hit-to-lead series for optimization. In addition to being reasonably potent against RORγ (hLUC EC 50 = 1.7 μM, FRET EC 50 = 0.85 μM), compound 1 also demonstrated >20-fold selectivity over five nuclear receptors (hRORα, hFXR, hRXRα, hPR, and hPPARγ) and was structurally unique in comparison to other nuclear receptor modulators. 16−18 However, this compound has several drawbacks. For example, the microsomal stability in liver microsomes is poor with only 18% remaining at 10 min in human liver microsomes. It also has a modest time-dependent human CYP3A4 inhibition (IC 50 = 4 μM) probably due to some reactive metabolites formed by the oxidation of 1. The ligand efficiency is only 0.25, far below the literature consensus value (0.30) for a drug-like molecule. 21 The concept of ligand efficiency (LE) was first introduced by Kuntz 22 and is widely accepted as a reliable index of drug-like qualities. 23 Improvement of LE inevitably results in lower molecular weight and higher potency. We reasoned that a strategy of increasing LE and lowering the lipophilicity should therefore significantly improve the drug-like properties of compound 1. In addition, compound 1 is a rather flat molecule with a fraction of saturated carbons (Fsp 3 ) of 0.24. Fsp 3 is a newer index representing drug-likeness. 24 Lovering et al. pointed out that a decrease of Fsp 3 value would result in an increased incidence of CYP inhibition. 25 The desired Fsp 3 value is over 0.47 according to the literature. 24 Thus, we considered that improvement of the poor Fsp 3 value of compound 1 would be a rational way to overcome the CYP inhibi...