A comparison of thyrotropin-releasing hormone with analogs: influence of disposition upon pharmacology

Mapping the energetic and allosteric landscapes of protein binding domains

Allosteric communication between distant sites in proteins is central to biological regulation but still poorly characterized, limiting understanding, engineering and drug development^1-6. An important reason for this is the lack of methods to comprehensively quantify allostery in diverse proteins. Here we address this shortcoming and present a method that uses deep mutational scanning to globally map allostery. The approach uses an efficient experimental design to infer en masse the causal biophysical effects of mutations by quantifying multiple molecular phenotypes-here we examine binding and protein abundance-in multiple genetic backgrounds and fitting thermodynamic models using neural networks. We apply the approach to two of the most common protein interaction domains found in humans, an SH3 domain and a PDZ domain, to produce comprehensive atlases of allosteric communication. Allosteric mutations are abundant, with a large mutational target space of network-altering 'edgetic' variants. Mutations are more likely to be allosteric closer to binding interfaces, at glycine residues and at specific residues connecting to an opposite surface within the PDZ domain. This general approach of quantifying mutational effects for multiple molecular phenotypes and in multiple genetic backgrounds should enable the energetic and allosteric landscapes of many proteins to be rapidly and comprehensively mapped.

Approaches for enhancing oral bioavailability of peptides and proteins

Oral delivery of peptide and protein drugs faces immense challenge partially due to the gastrointestinal (GI) environment. In spite of considerable efforts by industrial and academic laboratories, no major breakthrough in the effective oral delivery of polypeptides and proteins has been accomplished. Upon oral administration, gastrointestinal epithelium acts as a physical and biochemical barrier for absorption of proteins resulting in low bioavailability (typically less than 1-2%). An ideal oral drug delivery system should be capable of (a) maintaining the integrity of protein molecules until it reaches the site of absorption, (b) releasing the drug at the target absorption site, where the delivery system appends to that site by virtue of specific interaction, and (c) retaining inside the gastrointestinal tract irrespective of its transitory constraints. Various technologies have been explored to overcome the problems associated with the oral delivery of macromolecules such as insulin, gonadotropin-releasing hormones, calcitonin, human growth factor, vaccines, enkephalins, and interferons, all of which met with limited success. This review article intends to summarize the physiological barriers to oral delivery of peptides and proteins and novel pharmaceutical approaches to circumvent these barriers and enhance oral bioavailability of these macromolecules.

Noninvasive routes of proteins and peptides drug delivery

Recent advances in the field of pharmaceutical biotechnology have led to the formulation of many protein and peptide-based drugs for therapeutic and clinical application. The route of administration has a significant impact on the therapeutic outcome of a drug. The needle and syringe is a well established choice of protein and peptide delivery which has some drawback related to patient and to formulation such as pain, cost, sterility etc. Thus, the noninvasive routes which were of minor importance as parts of drug delivery in the past have assumed added importance in protein and peptide drug delivery and these include nasal, ophthalmic, buccal, vaginal, transdermal and pulmonary routes. The pharmaceutical scientists have some approaches to develop the formulations for protein and peptide delivery by noninvasive routes. But, due to the physiochemical instability and enzymatic barrier of proteins and peptides there are several hurdle to develop suitable formulation. So there is need of penetration enhancers, enzyme inhibitors and suitable vehicles for noninvasive delivery to increase the bioavailability. In this review, the aim is to focus on the approaches to formulation of protein and peptide based drug administration by noninvasive route.

Is the oral route possible for peptide and protein drug delivery?

Oral delivery of peptides and proteins remains an attractive alternative to parenteral delivery and has challenged various attempts at delivery development. Incorporation of new tools into the delivery systems that can raise membrane permeability of macromolecules is essential to attain high oral bioavailability that is acceptable in clinical applications. In developing oral protein delivery systems with high bioavailability, three practical approaches might be most helpful: (1) modification of the physicochemical properties of macromolecules; (2) addition of novel function to macromolecules; or (3) use of improved delivery carriers. Clearly, it is essential that these approaches maintain the biological activity of the proteins.

NS-3, a TRH-analog, reverses memory disruption by stimulating cholinergic and noradrenergic systems

The effects of a TRH-analog, N[[(3R,6R)-6-methyl-5-oxo-3-thiomorpholinyl]carbonyl]-L-histidyl-L - prolinamide tetrahydrate (NS-3, CG3703, montirelin hydrate) were compared with those of physostigmine on learning and memory disruption in the passive avoidance response (PAR) induced by either electrolytic lesion of the nucleus basalis magnocellularis (NBM) or by treatment with the noradrenergic neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) in rats. a) In NBM-lesioned rats, both NS-3 and physostigmine significantly reversed disruption of memory consolidation examined 15 min after the training session when these drugs were injected IP immediately after the training session. In addition, reversal by NS-3 (0.1 mg/kg) of the disruption of memory was observed even in the retention test conducted 24 h after the training session. b) NS-3 (0.5 mg/kg) significantly reversed the disruption of memory retrieval, when the drug was administered 15 min before the test session. c) DSP4 (50 mg/kg IP) caused memory disruption when the retention tests were conducted between 1 and 48 h after the acquisition session. NS-3 (0.1 mg/kg), but not physostigmine, significantly reversed the disruption of memory induced by DSP4 treatment. These findings suggest that the consistent antiamnestic action of NS-3 is due to the enhancement of both central cholinergic and noradrenergic systems, possibly via facilitation of the release of these transmitters.

NS-3(CG3703), a TRH analog, ameliorates scopolamine-induced memory disruption in rats

The effects of a metabolically stable TRH analog, N-[[(3R, 6R)-6-methyl-5-oxo-3-thiomorpholinyl]carbonyl]-L-histidyl-L- prolinamide tetrahydrate (NS-3, CG3703) on the scopolamine-induced memory disruption in maze performance tests were investigated in rats. a) In the delayed nonmatching-to-sample (DNMS) task using a T-maze, NS-3 (0.3 mg/kg) produced a significant reversal of the marginal disruption of choice accuracy induced by scopolamine (0.3 mg/kg) at the short (5 s) and long (120, 480 s) interval delays. Physostigmine (0.5 mg/kg) produced a significant reversal only at a 5-s interval delay. b) In the eight-arm radial maze task, NS-3 (0.3 mg/kg) significantly reversed the deficit of choice accuracy induced by scopolamine (0.3 mg/kg), whereas neither TRH (3-30 mg/kg) nor physostigmine (0.1-1 mg/kg) had any effect. The consistent reversal of these maze-learning performances by NS-3, but not by TRH or physostigmine, may be due to its potent enhancement of cholinergic and noradrenergic neuronal activities.

NS-3 (CG3703), an analog of thyrotropin-releasing hormone, ameliorates cognitive impairment in rats

The effects of thyrotropin-releasing hormone (TRH) and its analog, N-[[(3R,6R)-6-methyl-5-oxo-thiomorpholinyl] carbonyl]-L-histidyl-L- prolinamide tetrahydrate (NS-3, CG3703) on disturbance of memory of a passive avoidance response (PAR) and an escape response in rats were investigated. NS-3 improved amnesia caused by scopolamine, electroconvulsive shock (ECS), and cycloheximide (CXM), but TRH improved only the ECS-induced amnesia. NS-3 reversed learning deficits caused by hypercapnia, but TRH had no effect. These differences in the effect between NS-3 and TRH may be due to their biological half-life in rat plasma. These results suggest that NS-3 possesses more potent antiamnestic effects than TRH in rats.

Transport of thyrotropin-releasing hormone across rat alveolar epithelial cell monolayers

It has been recently shown that epithelial cell monolayers of rat type II pneumocytes cultivated on tissue culture-treated polycarbonate Transwell filters are tight (> 2,000 ohm-cm2) and exhibit morphological and phenotypic characteristics of in vivo type I pneumocytes. We studied, utilizing these tight monolayers, the transepithelial transport of thyrotropin-releasing hormone (TRH). Either [125I]TRH or [3H]TRH was used to measure the transalveolar epithelial radiolabel fluxes across the monolayer. Radiochromatography was performed, utilizing reverse-phase HPLC techniques, to determine the presence of TRH and its subspecies in dosing, donor and receiver fluids. The apparent permeability coefficient (Papp) estimated from 125I-radiolabel fluxes was approximately 1.7 x 10(-7) cm/sec in both the apical-to-basolateral (AB) and basolateral-to-apical (BA) directions. In contrast, the Papp for 3H-radiolabels was approximately 4.2 x 10(-7) cm/sec in both directions. Radiochromatography results indicated that neither apical nor basolateral receiver fluid collected at the end of 4 h flux studies contained metabolites of [125I]TRH or [3H]TRH. In the presence of 1,000-fold excess of unlabeled TRH in the donor fluid, the Papp of neither [125I]- or [3H]-TRH in either direction was altered. These data taken together provide evidence for restricted diffusional transport of intact TRH across the rat alveolar epithelial cell monolayer, most likely via paracellular pathways. Thus, it appears that TRH delivery via pulmonary alveolar epithelium in the distal airspaces of the mammalian lung may be feasible without significant interference from peptidase activities.

Prodrugs of peptides. 6. Bioreversible derivatives of thyrotropin-releasing hormone (TRH) with increased lipophilicity and resistance to cleavage by the TRH-specific serum enzyme

Bioreversible derivatization of TRH (pGlu-His-Pro-NH2) to protect the tripeptide against rapid enzymatic inactivation in the systemic circulation and to improve the lipophilicity of this highly hydrophilic peptide was performed by N-acylation of the imidazole group of the histidine residue with various chloroformates. Whereas TRH was rapidly hydrolyzed at its pGlu-His bond in human plasma by a TRH-specific pyroglutamyl aminopeptidase serum enzyme, the N-alkoxycarbonyl derivatives were resistant to cleavage by the enzyme. On the other hand, these derivatives are readily bioreversible as the parent TRH is formed quantitatively from the derivatives by spontaneous hydrolysis or by plasma esterase-catalyzed hydrolysis. In addition to protecting the parent TRH against rapid inactivation in the circulation and hence potentially prolonging the duration of action of TRH in vivo, the N-alkoxycarbonyl prodrug derivatives were much more lipophilic than TRH as assessed by octanol-buffer partitioning. This property may enhance prodrug penetration of the blood-brain barrier and various other biomembranes compared to the parent peptide.

Kinetics and pattern of degradation of thyrotropin-releasing hormone (TRH) in human plasma

The kinetics and mechanism of degradation of the tripeptide TRH (pGlu-His-Pro-NH2) and its various primary and secondary degradation products (TRH-OH, His-Pro-NH2, and His-Pro) have been determined in human plasma at 37 degrees C. The rates of degradation of both TRH and TRH-OH (pGlu-His-Pro) showed mixed zero-order and first-order kinetics. At low substrate concentrations first-order kinetics occurred, with TRH and TRH-OH degradation half-lives of 9.4 and 27 min, respectively. The initial step in the plasma-catalyzed degradation of TRH is due to hydrolysis of the pGlu-His bond by the TRH-specific pyroglutamyl aminopeptidase serum enzyme, resulting in the exclusive formation of histidyl-proline amide (His-Pro-NH2). Using specific HPLC methods the major degradation route (67%) of this dipeptide in human plasma was hydrolysis of the peptide bond to yield His and Pro-NH2, whereas deamidation to yield His-Pro accounted for 29% of the total degradation. A minor pathway (less than or equal to 4%) was spontaneous cyclization to yield cyclo(His-Pro). Both His-Pro-NH2 and His-Pro degraded by first-order kinetics and faster than TRH, with half-lives of 5.3 and 2.2 min, respectively, in 80% plasma.

Prodrugs of peptides. IV: Bioreversible derivatization of the pyroglutamyl group by N-acylation and N-aminomethylation to effect protection against pyroglutamyl aminopeptidase

Various N-acyl derivatives and N-Mannich bases of the model compound L-pyroglutamyl benzylamide were synthesized to assess their suitability as prodrug forms for the N-terminal pyroglutamyl residue occurring in several peptides, with the aim of improving peptide delivery characteristics. Whereas pyroglutamyl benzylamide was rapidly hydrolyzed by pyroglutamyl aminopeptidase, the N-acyl derivatives and N-Mannich bases (N-aminomethyl derivatives) were totally resistant to cleavage by the enzyme. On the other hand, these derivatives are readily bioreversible, the conversion to the parent pyroglutamyl amide taking place either by spontaneous hydrolysis at physiological pH, as demonstrated for the N-Mannich bases, or by plasma enzymes, as shown for the N-acyl derivatives. The results suggest that by appropriate N-acylation or N-aminomethylation it may be feasible to protect pyroglutamyl-containing peptides against cleavage by pyroglutamyl aminopeptidase and to obtain a release of the parent peptide in the organism, hence improving the delivery characteristics of such peptides.

The regulation of TRH and serotonin receptors: chronic TRH and analog administration in the rat

Thyrotropin releasing hormone (TRH) and serotonin (5-HT) are co-transmitters in spinal and medullary neurons. To study the functional significance of this relationship and the regulation of TRH receptors, we chronically administered TRH and analogs MK-771 and CG-3509 to rats at a dose which evoked behavioral abnormalities. TRH reduced specific binding at spinal (24%) and hypothalamic (31%) TRH receptors and decreased TRH stimulated motor behaviors, such as rearing and cage crossing (locomotion). 5-HT1 and 5-HT2 receptors were unaffected except for an 11% increase in specific binding at spinal 5-HT1 sites. 5-HT and NE concentrations measured by HPLC were not altered in brainstem or hippocampus. In contrast, spinal TRH receptor specific binding increased 11% in rats treated intracisternally with 5,7-dihydroxytryptamine, but the effect was not significant. In competition studies in vitro, MK-771, TRH, and CG-3509 had no activity at 5-HT2 receptors in neocortex, brainstem, or spinal cord, and little activity at 5-HT1 sites (IC 50s greater than 100 uM). A mixed competitive and non-competitive binding profile at 5-HT1 sites resulted in the presence of 100 uM TRH. Conversely, 5-HT agonists had minimal effect at TRH receptors in spinal cord. These data suggest reciprocal or independent regulation of 5-HT1 and TRH receptors in co-transmitter and non-cotransmitter regions, respectively, in response to chronic TRH administration.

The comparative pharmacology of the behavioral syndromes induced by TRH and by 5-HT in the rat

1. The relationship of the behavioral syndromes induced by the co-transmitters thyrotropin releasing hormone (TRH) and serotonin (5-HT) has not been previously studied with drugs selective for 5-HT receptor subtypes. 2. Both the TRH analog MK-771 (in naive rats) and 5-hydroxytryptophan (in rats with 5,7-dihydroxytryptamine [DHT] lesions) evoked reciprocal forepaw tapping, Straub tail, hunching, hindlimb abduction, and shaking behavior. Sniffing and rearing were features of the MK-771 but not the 5-HT syndrome. 3. 5-HTP potentiated MK-771-induced hyperthermia. 4. MK-771 evoked two types of shaking behavior, head shakes (HS) and wet-dog shakes (WDS). Neither independently was dose-related, unlike total shaking behaviors. 5. MK-771-induced shaking behavior was pharmacologically dissociated from other MK-771-evoked behaviors. A 5-HT1A agonist (8-OH-DPAT) blocked WDS, but like putative 5-HT1B (RU 24969) and 5-HT2 (DOI) agonists and the 5-HT antagonists methysergide (non-selective), ritanserin (5-HT2 selective), and l-propranolol (5-HT1 selective), it did not block other antagonists behavioural effects of MK-771. 6. Ipsapirone, a 5-HT1A-active drug purported both as an agonist and as an antagonist, inhibited MK-771-evoked WDS, like 8-OH-DPAT, but did not induce the serotonin syndrome, unlike 8-OH-DPAT. 7. DHT-treated rats were behaviorally supersensitive to 10 mg/kg MK-771 as indicated by a significantly shortened latency of onset of WDS and greater frequency of abnormal forepaw movements. The same rats were also supersensitive to 50 mg/kg 5-HTP to a significantly greater degree. 8. These data suggest behavioral relatedness of the TRH and 5-HT syndromes, but distinctive pharmacologic features and presumed mechanisms of action.