Design and Exploratory Neuropharmacological Evaluation of Novel Thyrotropin-Releasing Hormone Analogs and Their Brain-Targeting Bioprecursor Prodrugs

Abstract

Efforts to take advantage of the beneficial activities of thyrotropin-releasing hormone (TRH) in the brain are hampered by its poor metabolic stability and lack of adequate central nervous system bioavailability. We report here novel and metabolically stable analogs that we derived from TRH by replacing its amino-terminal pyroglutamyl (pGlu) residue with pyridinium-containing moieties. Exploratory studies have shown that the resultant compounds were successfully delivered into the mouse brain after systemic administration via their bioprecursor prodrugs, where they manifested neuropharmacological responses characteristic of the endogenous parent peptide. On the other hand, the loss of potency compared to TRH in a model testing antidepressant-like effect with a simultaneous preservation of analeptic activity has been observed, when pGlu was replaced with trigonelloyl residue. This finding may indicate an opportunity for designing TRH analogs with potential selectivity towards cholinergic effects.

Brain Delivery of Thyrotropin-Releasing Hormone via a Novel Prodrug Approach

“…While the prodrug did not perform significantly better than the direct administration of TRH in this paradigm under the experimental conditions employed [16,34] we feel that these exploratory results are encouraging especially in the context of potential preclinical therapeutic advantage considering the excellent in vitro stability of the prodrug in plasma (Table 2). Therefore, we anticipate that prodrug administration will be devoid of significant TRH exposure to the periphery in follow up studies.…”

“…In the PST, the immobility represents a “depressive mood” and, when mice receive a test agent with antidepressant-like effect, the immobility time is reduced compared to that of the control group, as mice spend more time swimming in order to escape. TRH has been shown by us [6,13,16,34] and others [40] to exhibit such a central effect. As shown in Figure 2, a statistically significant decrease in the immobility time was established compared to mice receiving vehicle only (immobility time of 205 ± 4 s), when the prodrug was injected to the animals (immobility time of 176 ± 3 s).…”

“…Another exploratory evaluation of the prodrug design employed in vivo microdialysis sampling from the extracellular space of the rat cortex for monitoring ACh release by TRH [34,35], when the prodrug was perfused with or without the presence of the brain-permeable POP inhibitor [30]. TRH is well-known for its ability to increase ACh production, and this cholinergic mechanism has been implicated in many CNS effects of the peptide [1,6].…”

“…For the assessment of antidepressant-like activity after subcutaneous (s.c.) administration of ( 1 ), our validated and previously published PST protocol was used [16,34]. TRH (3 µmol/kg body weight) was the positive control.…”

[β-Glu2]TRH Is a Functional Antagonist of Thyrotropin-Releasing Hormone (TRH) in the Rodent Brain

“…Several endogenous and synthetic TRH-like peptides with the pGlu-Xaa-Pro-NH 2 ([Xaa 2 ]TRH) sequence have been investigated, where Xaa denotes any natural and unnatural amino acid residues [20][21][22]. For example, [Glu 2 ]TRH (1b; Figure 1) exhibits various CNS actions characteristic to TRH without endocrine effects, which is preferable in the context of CNS drug development [23]. We have previously shown that, unlike TRH, this peptide did not increase acetylcholine (ACh) turnover in the rat hippocampus [24].…”