Advancing the utility of thyrotropin-releasing hormone (TRH) as a CNS agent

Thyrotropin-releasing hormone (TRH) is a small tripeptide having the sequence pGlu-His-Pro-NH₂. It was initially discovered and studied for its role within multiple neuroendocrine pathways, yet TRH also influences broad neurological effects throughout the CNS by acting as a neurotransmitter and neuromodulator, mediating effects on feeding behavior, thermogenesis, locomotor activation, and autonomic regulation. TRH’s ability to alter brain chemistry, behavior, and physiology implies great potential in treating neurological and psychological disorders, however, pharmacological applications have been largely unrealized due to peripheral exposure and brain delivery shortcomings following systemic administration of the peptide. Therefore, our laboratory has focused on developing a medicinal chemistry-based prodrug approach to overcome these limitations. Our TRH prodrugs are designed to exhibit favorable physiochemical properties to increase brain permeability and subsequent bioactivation by the preferentially brain expressed enzymes, prolyl oligopeptidase and glutaminyl cyclase. TRH’s ability to increase acetylcholine release is well documented and its neurochemical assessment has been utilized in determining the CNS delivery of prodrug-derived TRH. Our laboratory’s recent discovery of the first functional TRH antagonist (pGlu-βGlu-Pro-NH₂) and its allosteric binding site was also substantiated with neuropharmacological tests that assessed the attenuation of TRH’s central cholinergic actions, which should prove useful in the development and validation of future prodrug designs. Altogether, these advancements establish not only the first TRH prodrug capable of delivering the metabolically liable TRH to the brain, but also a mechanistic elucidation of various TRH receptor-binding modalities.