3H-Imidazo[4,5-b]pyridine-6-carboxylic acid derivatives as rexinoids with reduced teratogenicity.

Several retinoid X receptor (RXR) ligands (rexinoids), such as bexarotene (1), exhibit teratogenicity, which is a serious impediment to their clinical application. We considered that rexinoids with a lower level of maximal RXR transcription activation (i.e., partial agonists) and lower lipid solubility might show weaker adverse side effects. Based on this idea, we modified our previously reported pentamethyltetralin-type RXR partial agonists 5 and 6 to reduce their lipophilicity. Here, we report a new RXR partial agonist, 3-(3,5,5,8,8-pentamethyl-5,6,7,8-tetrahydronaphthalen-2-yl)-2-(trifluoromethyl)-3H-imidazo[4,5-b]pyridine-6-carboxylic acid (8, CATF-PMN), which showed greatly reduced teratogenicity in zebrafish embryos.

Double-blind, placebo-controlled, proof-of-concept trial of bexarotene Xin moderate Alzheimer's disease.

BACKGROUND: We assessed the impact of retinoid X receptor (RXR) agonist bexarotene on brain amyloid measured by amyloid imaging in patients with Alzheimer's disease (AD) in a proof-of-concept trial. METHODS: Twenty patients with AD [Mini Mental State Examination (MMSE) score 10-20 inclusive] with positive florbetapir scans were randomized to receive 300 mg of bexarotene or placebo for 4 weeks. The amyloid imaging result was the primary outcome. Whole-population analyses and prespecified analyses by genotype [apolipoprotein E ε4 (ApoE4) carriers and ApoE4 noncarriers] were conducted. Secondary outcomes included scores on the Alzheimer's Disease Assessment Scale-Cognitive subscale, Alzheimer's Disease Cooperative Study-Activities of Daily Living scale, MMSE, Clinical Dementia Rating scale, and Neuropsychiatric Inventory. Serum amyloid-ß (Aß) peptide sequences Aß1-40 and Aß1-42 measurements were collected as biomarker outcomes. RESULTS: There was no change in the composite or regional amyloid burden when all patients were included in the analysis. ApoE4 noncarriers showed a significant reduction in brain amyloid on the composite measure in five of six regional measurements. No change in amyloid burden was observed in ApoE4 carriers. There was a significant association between increased serum Aß1-42 and reductions in brain amyloid in ApoE4 noncarriers (not in carriers). There were significant elevations in serum triglycerides in bexarotene-treated patients. There was no consistent change in any clinical measure. CONCLUSIONS: The primary outcome of this trial was negative. The data suggest that bexarotene reduced brain amyloid and increased serum Aß1-42 in ApoE4 noncarriers. Elevated triglycerides could represent a cardiovascular risk, and bexarotene should not be administered outside a research setting. RXR agonists warrant further investigations as AD therapies. TRIAL REGISTRATION: ClinicalTrials.gov identifier NCT01782742 . Registered 29 January 2013.