PROJECT SUMMARY (ABSTRACT) Pruritus, the urge to scratch due to an unpleasant sensation (acute and chronic), is the most frequent symptom in skin diseases like Atopic Dermatitis (AD). Although the origin of itch is not fully elucidated, treating chronic itch is an important part of caring for AD patients’ well-being. Pruritus affects sleep, mood, personal relationships and can significantly reduce quality of life. In fact, the negative impact of chronic itch on patient’s well-being is reported to be like that of chronic pain. AD is typically caused by multiple pruritogenic mediators. Antihistamines may have utility in treating acute pruritus, but have not been effective in chronic pruritus or AD. The management of chronic pruritus in AD represents an unmet medical need, currently without effective treatment options, and underscores the need for new therapies against novel targets within non-histaminergic pathways. An important component of the pathophysiologic mechanism of non-histaminergic chronic pruritus involves the increase of 5-HT levels in the innervated dermis/epidermis. Elevated levels of 5-HT in patients with AD (serum) and chronic eczema (skin) have been reported. Recent studies demonstrate the serotonergic 5-HT7 receptor plays an important role in TRPA1 Ca2+ flux mediated pruritus. 5-HT7 and TRPA1 receptors are co-expressed on a subset of primary afferent sensory neurons in the skin. Both receptors are functionally coupled where 5-HT7 stimulation results in opening of TRPA1 channels promoting neuronal depolarization and action potential firing, and ultimately triggering itch-evoked scratching. In vivo studies strongly support that 5-HT signaling contributes to the pathogenesis of pruritus through 5-HT7-TRPA1 signaling. Praeventix has confirmed the role of 5- HT7/TRPA1 blockade in the MC903 induced AD model. These data strongly demonstrate selective inhibition of the 5HT7/TRPA1 signaling pathway will translate to a meaningful suppression of the itch/scratch cycle. Praeventix has identified clinical lead PRA-523, a potent selective 5-HT7 antagonist that inhibits Ca2+ flux in 5- HT7/TRPA1-HEK293 cells. PRA-523 exhibits peripherally restricted pharmacokinetics and good dermal penetration/permeability that is ideal for a topically administered product. Studies in the MC903 induced AD model demonstrated topically applied PRA-523 significantly and dose dependently reduced scratching behavior in both male and female mice. These data strongly demonstrate that selective inhibition of the 5-HT7/TRPA1 signaling pathway will translate to meaningful suppression of pruritus. This proposal aims to complete the initial IND enabling studies to permit the advancement of PRA-523 into GLP toxicology studies, develop the clinical formulation and manufacture the drug product needed for the planned nonclinical studies. This work will be performed at high quality contract research organizations in the USA, overseen by the experienced team at Praeventix.