R-13

R-13 (BrAD-R13, Braegen-01) is a synthetic bis-methylcarbamate prodrug of 7,8-dihydroxyflavone (7,8-DHF; tropoflavin), designed to overcome the poor oral bioavailability and rapid hepatic conjugation of the parent flavone while preserving its selective agonist activity at the tropomyosin receptor kinase B (TrkB) receptor, the principal high-affinity signaling receptor for brain-derived neurotrophic factor (BDNF). The compound was identified through systematic medicinal chemistry optimization of ester and carbamate modifications on the 7,8-DHF catechol ring by Chen, Ye, and colleagues at Emory University School of Medicine and Zhejiang University, with R-13 selected from among twenty candidate derivatives as the sole compound satisfying all screening criteria for gastric acid stability, intestinal absorption, hydrolyzability in liver microsomes and plasma, and adequate membrane permeability. Following oral administration, R-13 is hydrolyzed through a monophenol intermediate (designated T1) to release 7,8-DHF, which binds the extracellular domain of TrkB and triggers receptor dimerization, autophosphorylation, and activation of the downstream Akt and ERK/MAPK signaling cascades. In mice, R-13 at 36 mg/kg oral dose produces a maximum plasma concentration of 129 ng/mL at 30 minutes, with an elimination half-life of approximately 220 minutes and oral bioavailability of 10.5 percent (compared to 4.6 percent for the parent 7,8-DHF), and sustains brain 7,8-DHF concentrations above 8 ng/g for at least 4 hours. The pharmacological consequences of sustained TrkB activation are broad: R-13 represses asparagine endopeptidase (AEP, also termed delta-secretase), a protease implicated in the pathological cleavage of both amyloid precursor protein and tau in Alzheimer's disease, thereby reducing amyloid-beta deposition and neurofibrillary tangle formation. Chronic oral administration of R-13 in the 5XFAD transgenic mouse model of Alzheimer's disease dose-dependently restored dendritic spine density, enhanced long-term potentiation, reduced senile plaque burden, attenuated neuroinflammatory cytokine levels (interleukin-1-beta, interleukin-6, tumor necrosis factor alpha), and reversed spatial and working memory deficits in the Morris water maze without demonstrable toxicity at 12 weeks of continuous dosing at up to 43.6 mg/kg/day. Additional preclinical evidence supports efficacy in the SOD1-G93A transgenic mouse model of amyotrophic lateral sclerosis, where R-13 preserved motor neuron counts, reduced gastrocnemius muscle atrophy, attenuated glial activation, and enhanced mitochondrial biogenesis through AMPK/PGC-1-alpha/Nrf1/Tfam pathway activation. In a peripheral nerve transection model, oral R-13 produced axon regeneration and functional electromyographic recovery superior to the parent 7,8-DHF. R-13 has also been characterized as preventing ovariectomy-induced bone loss through TrkB/Akt-mediated inhibition of AEP and upregulation of osteoprotegerin. Braegen Pharmaceutical (Shenzhen, China) has completed a Phase 1 clinical trial of BrAD-R13 in Alzheimer's disease, the first clinical evaluation of a TrkB agonist prodrug in this indication, with plans for Phase 2 efficacy trials. The compound is not approved in any jurisdiction for any indication. This monograph reviews the chemistry, synthesis, and prodrug design of R-13; the TrkB receptor pharmacology and downstream signaling; the comprehensive preclinical pharmacokinetic profile; the evidence base across Alzheimer's disease, amyotrophic lateral sclerosis, peripheral nerve injury, bone metabolism, and neuropsychiatric indications; sourcing and handling for research applications; stack-interaction considerations; adverse-event signal; and a comparative assessment of five alternative TrkB agonist and BDNF-mimetic candidates against R-13 on five competency standards.