Quercetin and 6-Br-quercetin: antioxidant properties and off target screening results advance glycosylated 6BrQ as developmental candidates for Alzheimer’s disease

Abstract Quercetin is an abundant dietary flavonol with interesting in vitro properties that include substrate-selective positive allosteric modulation (PAM) of the activity of the receptor type protein tyrosine phosphatase D (PTPRD) and substantial antioxidant actions. Its in vivo activities include reducing incidence of Alzheimer’s disease (AD) and reducing AD neurofibrillary pathology in mouse models. Structure-activity studies have identified quercetin analogs with improved in vitro and in vivo properties, including the improved PTPRD PAM 6-bromoquercetin (6BrQ). However, there is no comparison of the antioxidant properties of 6BrQ to those of quercetin. There is no systematic screening for activities of quercetin or of 6BrQ using a panel of targets for most currently-used drugs. We now report that both quercetin and 6BrQ provide equivalent results in cyclic voltammetric and biochemical antioxidant assays. We also report that neither 10 -7 M quercetin nor 6BrQ provides any significant (>50%) effects on any of the 104 assays in a Eurofins off-target screening panel. At 10 -5 M, both quercetin and 6BrQ exert significant effects in assays for glycogen synthase kinase 3 (GSK3β) as well as those for serotonin 5HT2B receptor, adenosine transport, adenosine A2A receptors, cyclooxygenases COX1 and COX2, phosphodiesterases PDE3A and 4D2 and PPAR gamma. These data extend prior characterization of quercetin’s biochemical effects, provide novel results for 6BrQ and support the likelihood that both quercetin and 6BrQ can a) directly inhibit GSK3, b) reduce GSK3 activities via enhancement of its dephosphorylation by PTPRD and c) display modest numbers of off target activities at high concentrations, several of which could conceivably contribute to anti-AD activities. These results advance bioavailable glycosylated prodrugs that can be metabolized to 6BrQ as developmental candidates for AD.