7,8-Dihydroxyflavone

7,8-Dihydroxyflavone (7,8-DHF), also designated tropoflavin, is a naturally occurring flavone first isolated from the leaves of Godmania aesculifolia and subsequently identified in Tridax procumbens, Primula vulgaris, and other plant species. The compound was characterized in 2010 by Jang et al. at Emory University as the first orally bioactive, blood-brain-barrier-penetrant small-molecule agonist of the tropomyosin receptor kinase B (TrkB), the principal high-affinity signaling receptor for brain-derived neurotrophic factor (BDNF) [1]. 7,8-DHF binds the extracellular domain of TrkB with a dissociation constant of approximately 320 nM by filter binding assay and approximately 15.4 nM by surface plasmon resonance, triggering receptor dimerization, autophosphorylation at tyrosine residues 706/707, and activation of the downstream PI3K/Akt and MAPK/ERK signaling cascades that mediate neuronal survival, synaptic plasticity, and long-term potentiation [1, 2]. The compound displays selectivity for TrkB over the related neurotrophin receptors TrkA and TrkC and does not activate TrkB kinase-dead mutants, confirming that the phosphorylation signal arises from the receptor itself rather than from off-target tyrosine kinases [2].

Pharmacokinetic characterization in rodents reveals oral bioavailability of approximately 4.6 percent, a plasma half-life of approximately 134 minutes, rapid brain penetration with peak brain concentrations at 10 minutes after oral dosing, and primary hepatic metabolism through glucuronidation, sulfation, and catechol-O-methyltransferase-mediated methylation [3, 4]. The O-methylated metabolites (7-methoxy-8-hydroxyflavone and 7-hydroxy-8-methoxyflavone) retain TrkB agonist activity in vitro and in vivo, extending the effective pharmacodynamic window beyond the parent compound [5]. The modest oral bioavailability prompted the development of the prodrug R13 (a carbamate ester derivative) by the Ye laboratory, which increases oral bioavailability to approximately 10.5 percent and extends the plasma half-life to approximately 220 minutes [6]. R13 has entered Phase 1 clinical evaluation for Alzheimer's disease, representing the most advanced clinical translation of TrkB agonist pharmacology from this scaffold.

Preclinical pharmacology is extensive. In Alzheimer's disease models, 7,8-DHF reduces BACE1 elevation, decreases amyloid-beta deposition, restores hippocampal synaptic density, and rescues spatial and working memory deficits in 5XFAD, APP/PS1, and Tg2576 transgenic mice at oral doses of 5 mg/kg/day [7, 8, 9]. In Parkinson's disease models, the compound protects dopaminergic neurons from MPTP- and rotenone-induced degeneration and improves motor function [10]. In depression models, 7,8-DHF reverses learned helplessness, chronic mild stress, and social defeat stress phenotypes through restoration of hippocampal and prefrontal cortical TrkB-BDNF signaling [11, 12]. Additional preclinical efficacy has been demonstrated in models of Huntington's disease, amyotrophic lateral sclerosis, traumatic brain injury, post-traumatic stress disorder, Rett syndrome, fragile X syndrome, retinal ganglion cell degeneration, and diet-induced obesity [13, 14, 15, 16, 17, 18, 19].

Safety characterization in chronic rodent studies at 5 mg/kg/day for periods up to 6 months has revealed no pathological changes in major organs, no hematological abnormalities, and no observable toxicity at doses producing robust TrkB activation [2]. A 7-month oral dosing study in a non-human primate model of Parkinson's disease similarly reported no toxic reactions [20]. No human clinical trial data for the parent compound 7,8-DHF have been published; clinical development has proceeded through the prodrug R13. This monograph reviews the chemistry, natural sources, and structure-activity relationships of 7,8-DHF; the TrkB receptor pharmacology in molecular and cellular detail; the pharmacokinetic profile including metabolism and prodrug development; the preclinical evidence base across neurodegenerative, neuropsychiatric, and metabolic indications; sourcing and quality verification; reconstitution and handling; stack-interaction considerations; adverse-event and safety characterization; and a comparative assessment of five TrkB-targeted candidates against 7,8-DHF on five competency standards.