Dispersible hydrolytically sensitive nanoparticles for nasal delivery of thyrotropin releasing hormone (TRH)

Nose-to-brain delivery of drugs is affected by nanoparticles (NPs) deposited on the olfactory surface and absorbed directly into the brain. Thyrotropin releasing hormone (TRH), a water soluble drug used for treating suicidal patients, was incorporated into a fast degrading poly(sebacic anhydride) (PSA) NPs. NPs were prepared by a solvent-antisolvent process under strict anhydrous environment to obtain high TRH loading and to avoid premature PSA degradation and TRH release. PSA and TRH were dissolved in a mixture of dichloromethane and ethanol and added dropwise to a dispersion of mannitol particles in heptane as an antisolvent. Mannitol powder was included in the antisolvent, so that formed NPs adhered to the mannitol microparticles for easy isolation and immediate dispersion in water prior to use. The size, surface charge, and morphology of the TRH-PSA NPs were determined using dynamic light scattering (DLS), zeta-potential, and Scanning Electron Microscopy (SEM), respectively. The NPs prepared were uniform and spherical of ~250 nm. Further, the in vitro release profile of TRH from NPs lasted for 12 h with most TRH released within the first hour in water. Concentration dependent cell toxicity studies revealed low toxicity level at low concentrations of the NPs. Surface adsorption of the NPs was also uniform on the cell surface as examined through the odyssey near infrared fluorescence (NIR) images using Indocyanine green (ICG). The NPs are designed to enable direct delivery to the olfactory epithelium using a refillable nasal atomizer that deposits mist onto the olfactory neuro-epithelium.

Improvement of Intestinal Absorption of Thyrotropin-releasing Hormone by Chemical Modification with Lauric Acid

Intestinal absorption of 125I-labelled lauryl thyrotropin-releasing hormone (Lau-TRH), a novel lipophilic derivative of TRH, was examined by rat in-situ closed intestinal loops. At a dose of 1 μmol per rat into the small intestine, a significant increase in percent of dose in plasma radioactivity of Lau-TRH was observed in comparison with that of TRH. A dose-dependent decrease in percent of dose in plasma radioactivity of TRH was noted, suggesting a saturable process of TRH transport. In contrast, the percent of dose in plasma radioactivity of Lau-TRH increased with increasing dose of Lau-TRH. The stability of TRH and Lau-TRH was studied in plasma and rat small intestinal homogenates. Lau-TRH was more stable than TRH in rat plasma. These results suggest that chemical modification of TRH with lauric acid may not only increase the lipophilicity of TRH but also reduce the degradation of TRH, resulting in the increased plasma radioactivity of TRH. On the other hand, Lau-TRH was gradually converted to TRH in the intestinal mucosal homogenate. These findings indicate that chemical modification of TRH with lauric acid might be a useful approach for improving the intestinal absorption of this peptide.

Exploratory neuropharmacological evaluation of a conformationally constrained thyrotropin-releasing hormone analogue

A conformationally constrained peptidomimetic derived from the endocrine and neuroactive tripeptide thyrotropin-releasing hormone (pGlu-His-Pro-NH(2)) was synthesized by convenient solid-phase organic chemistry and evaluated as a potential central nervous system agent. While this ethylene-bridged peptide analogue has been reported to lack the hormonal effect of the native peptide, we have shown in animal models that it possesses central nervous system activity characteristic of thyrotropin-releasing hormone. Compared to control, the peptidomimetic showed significant analeptic and antidepressant-like potencies. Moreover, an enhanced selectivity in antidepressant-like effect was measured when compared to that of the native peptide. Immobilized artificial membrane chromatography and in vitro metabolic stability studies also revealed that this constrained peptidomimetic has higher affinity to the blood-brain barrier than the native peptide and is metabolically stable. Consequently, this structure may be used as a template to design centrally selective and metabolically stable thyrotropin-releasing hormone analogues as potential neuropharmaceutical agents.

Endocrine Function Is Altered in Chronic Migraine Patients with Medication-Overuse

OBJECTIVE

To evaluate the effects of analgesic overuse on endocrine function in patients with chronic migraine and medication-overuse headache (CM-MOH).

BACKGROUND

Chronic migraine is frequently associated with an overuse of symptomatic medications. Drugs currently used in acute migraine attacks are associated with several endocrine effects. At present, the endocrine effects of medication overuse in chronic migraine patients are unknown.

METHODS

Eighteen patients with CM-MOH, diagnosed according to the ICHD-II criteria, and 18 healthy controls received an intravenous administration of GHRH, hCRH, and TRH. Plasma concentrations of GH, TSH, ACTH, and cortisol were measured for a 90-minute period after administration of the specific releasing hormones.

RESULTS

Hormonal basal concentrations were similar in both groups. GH response to GHRH was significantly reduced in patients with CM-MOH in comparison with controls. TRH induced a reduction of TSH concentrations only at the end of the test. After hCRH administration, ACTH and cortisol concentrations were significantly higher in cases than in controls. A significant correlation between duration of the disease and altered hormonal response was found.

CONCLUSIONS

Our study shows that both corticotropic and somatotropic functions are significantly impaired in CM-MOH patients and suggests a role for hormones in the development of chronic migraine.

Centrally Acting and Metabolically Stable Thyrotropin-Releasing Hormone Analogues by Replacement of Histidine with Substituted Pyridinium

Metabolically stable and centrally acting thyrotropin-releasing hormone (TRH) analogues were designed by replacing the central histidine with substituted pyridinium moieties. Their analeptic and acetylcholine-releasing actions were evaluated to assess their potency as central nervous system (CNS) agents. A strong experimental connection between these two CNS-mediated actions of the TRH analogues was obtained in subject animals. The analogue 3-(aminocarbonyl)-1-(3-[2-(aminocarbonyl)pyrrolidin-1-yl]-3-oxo-2-[[(5-oxopyrrolidin-2-yl)carbonyl]amino]propyl)pyridinium (1a) showed the highest (TRH-equivalent) potency and longest, dose-dependent duration of action from a series of homologous compounds in antagonizing pentobarbital-induced narcosis when administered intravenously in its CNS-permeable prodrug form (2a) obtained via reduction of the pyridinium moiety to the nonionic dihydropyridine. The maximum change in hippocampal acetylcholine concentration upon perfusion of the pyridinium-containing tripeptides into the hippocampus of rats was also achieved with 1a. No binding to the endocrine TRH receptor was measured for the TRH analogues reported here; therefore, our design afforded a novel lead for centrally acting TRH analogues. We have also demonstrated the benefits of the prodrug approach on the pharmacokinetics and brain uptake/retention of pyridinium-containing TRH analogues (measured by in vivo microdialysis sampling) upon systemic administration.

Thyrotropin-releasing hormone administration does not affect seizure threshold during electroconvulsive therapy

Despite the fact that a role for thyrotropin-releasing hormone (TRH) in seizure modulation has been consistently hypothesized, the exact nature of this role remains unclear. In this study, we investigated the effects of TRH administration on seizure threshold and seizure duration in 13 depressed inpatients undergoing electroconvulsive therapy (ECT). In a balanced order crossover design, an intravenous bolus of 0.4 mg TRH or placebo was administered immediately before anesthesia, during the first two sessions, in a series of bilateral ECT. In both of these sessions, a threshold titration procedure was applied by using gradual increments of the electrical charge delivered until seizure elicitation, a procedure that has been safely used in the past. Seizure threshold was defined as the lowest energy level required for induction of a grand mal seizure, by use of this titration procedure. Seizure duration was estimated both by simultaneous EEG recording and by the cuff method. Results showed that neither seizure threshold, nor seizure duration (either by cuff or by EEG) differed between the TRH and the placebo conditions, regardless of the order in which TRH or placebo were administered in the two ECT sessions. This was the case regardless of whether the patients had at baseline a blunted TSH response to TRH or not. Our findings do not support a role for TRH on seizure modulation, at least when TRH is administered exogenously. Such an effect, if it exists, could be obscured, however, by several factors, including pharmacokinetics.

Studies on the transport of thyrotropin-releasing hormone (TRH) analogues in Caco-2 cell monolayers

The transport mechanisms of thyrotropin-releasing hormone (TRH) and its pharmacologically active analogues ((3-methyl-His(2))TRH (MeTRH), taltirelin, montirelin, azetirelin) across Caco-2 cell monolayers were characterized. The results of kinetic analysis showed a linear relationship between the concentration (over the range 0.5-10 mM) and apical-to-basolateral transport rate of these agents. The permeability coefficients (P(app)) of these agents were not substantially different from each other, and their P(app) ratios of the basolateral-to-apical over the apical-to-basolateral transport were close to one (0.73-1.23). The cellular transport of [(3)H]MeTRH at low concentrations (3-15 nM) showed a linear relationship between the concentration and transport rate. The transport of [(3)H]MeTRH in Caco-2 cell monolayers was neither affected by TRH nor TRH analogues, and there was little difference in P(app) values between [(3)H]MeTRH and [(14)C]mannitol. The cell-per-medium ratio of [(3)H]MeTRH in the cellular uptake experiment was similar to the value of [14C]mannitol. A large excess of TRH and MeTRH did not significantly influence cell-per-medium ratios of [(3)H]MeTRH in Caco-2 cell monolayers. The k'(IAM) value, which represents lipophilicity, was decreased in the following order: montirelin > taltirelin > TRH > azetirelin, and the values varied from 0.234 to 1.028. These results indicate that a paracellular passive diffusion may be the major route for the transport of TRH and its analogues in Caco-2 cell monolayers.

Prodrugs to enhance central nervous system effects of the TRH-like peptide pGlu-Glu-Pro-NH2

Potential prodrugs for the TRH-like tripeptide pGlu-Glu-Pro-NH(2) were synthesized either by esterifying the Glu side-chain of the parent peptide in solution with alcohols in the presence of resin-bound dicyclohexylcarbodiimide or by solid-phase peptide chemistry. Affinities of these ester prodrugs to lipid membranes as predictors of the transport across the blood-brain barrier were compared by immobilized artificial membrane chromatography, and prodrug activation was tested in the brain tissue of experimental animals. Esters of pGlu-Glu-Pro-NH(2) with long-chain primary alcohols emerged as potentially useful prodrugs to improve the central nervous system activity of pGlu-Glu-Pro-NH(2) upon systemic administration, as revealed by the enhancement of analeptic activity in mice.

Synthesis and biological evaluations of brain-targeted chemical delivery systems of [Nva2]-TRH

Various chemical delivery systems for [Nva2]-TRH were synthesized and their CNS activity was investigated and compared with that of a similar chemical delivery system of [Leu2]-TRH, previously studied. Sequential metabolism of the chemical delivery system delivered to the brain, starting with the conversion of the dihydrotrigonellyl (DHT) to the trigonellyl (T+) moiety, will provide the lock-in to the brain of the T+-chemical delivery system, which will undergo hydrolysis of the cholesteryl ester, formation of the Pr-amide and cleavage of the spacer-T+ part, allowing ultimately the sustained release of the active [Nva2]-TRH. The CNS activity was assessed by measuring the extent of antagonizing barbiturate-induced sleeping time in mice. The fully packaged DHT-Pro-Pro-Gln-Nva-Pro-Gly-OCh produced robust antagonism, reducing sleeping time from 89 min to 48 min, similar to the Leu2-analogue (49 min). However, the partially substituted [Nva2]-TRH analogues showed little or no CNS activity. The results indicate that the fully packaged delivery system is necessary to produce the successful brain targeting of the precursor construct and effective release of the Gln-Nva-ProNH2.

Effect of 2-hydroxypropyl-beta-cyclodextrin on the solubility, stability, and pharmacological activity of the chemical delivery system of TRH analogs

To improve the aqueous solubility and stability of the chemical delivery system (CDS) of the thyrotropin-releasing hormone (TRH) analogs, 2-hydroxypropyl-beta-cyclodextrin (HPBCD) has been attempted. TRH analogs were [Leu2]-TRH, [Nva2]-TRH and [Nva2, Pip3]-TRH. Excess amount of CDS was added in various HPBCD in water solutions (0%-50%, pH 6.5). The mixture was saturated by ultra-sonication for 1 h at 15 degrees C and filtered. The concentration of CDS in the filtrate (solubility) was determined with UV detector, and subsequently the stability was investigated. By HPBCD complexation, the aqueous solubility and stability (half-life) of CDS were significantly improved from undetectable levels to about 15 mg/ml and 30 h, respectively. In pH 6.5 and 7.4 HPBCD solution, the degradation of CDS was mainly via acid catalyzed water addition reaction, thus, e.g. [Leu2]-TRH-CDS was more stable in pH 7.4 than in pH 6.5 aqueous solutions. After lyophilizing the saturated CDSs in 50% HPBCD complex solutions, the amount of CDS in the complex was determined as 26.22, 26.79, and 30.34 mg/g for [Leu2]-TRH, [Nva2]-TRH and [Nva2, Pip3]-TRH, respectively. The half-life of [Leu2]-TRH-CDS/HPBCD solid complex at 25 degrees C, 4 degrees C and -15 degrees C was about 100 days, 440 days and no detectable change in three months, respectively. Argon protected condition did not improve the stability of lyophilized [Leu2]-TRH-CDS/HPBCD complex. Dimethyl sulfoxide although increased the solubility of [Leu2]-TRH-CDS in the 50% HPBCD solution by 1.3 times, significantly decreased its stability by 6.6 times. After intravenous administration of CDS (in 30% HPBCD) at a dose of 10 mumole/kg in mice, compared to the vehicle control or the same dose of [Leu2]-TRH (in 30% HPBCD), a significant increase in pharmacological effect (decrease in barbiturate-induced sleeping time) was observed. These results demonstrate the usefulness of cyclodextrin in the formulation of the CDSs of TRH analogs.

Brain receptor binding characteristics and pharmacokinetic-pharmacodynamic analysis of thyrotropin-releasing hormone analogues

Thyrotropin-releasing hormone (TRH) receptor binding in the rat brain after intravenous (i.v.) injections of novel TRH analogues, taltirelin and montirelin, was examined and the data were analyzed in relation to their plasma concentrations which were simultaneously determined. Taltirelin and montirelin inhibited specific [3H]-Me-TRH binding in the rat brain and their Ki values were 311 and 35.2 nM, respectively. The i.v. injection of taltirelin and montirelin (0.1-3 mg/kg) produced a significant reduction in [3H]-Me-TRH binding sites (Bmax values) in the rat brain. The reduction by both agents tended to reach a maximum after 60 min and lasted up to at least 120 min. On the other hand, the i.v. injection of both agents had little significant effect on the apparent dissociation constant (Kd) for [3H]-Me-TRH in the rat brain. Plasma concentrations of taltirelin and montirelin in rats peaked immediately after i.v. injection, and thereafter they decreased with t 1/2 of 23.0 and 14.1 min, respectively. Counter-clockwise hysteresis between the plasma concentration and receptor occupancy of these agents was observed after the i.v. injection of taltirelin and montirelin, and the temporal delay between plasma concentration and brain receptor occupancy was successfully minimized using the "effect compartment" model in combination with the "linear-effect" model. We concluded that taltirelin and montirelin exert a fairly potent effect following sustained occupation of brain TRH receptors under in vivo condition. Thus, both agents could be clinically useful for the treatment of CNS disorders.

Uptake of prolactin and tyroliberin by the heart

The aim of this study was to observe the specificity of prolactin and tyroliberin uptake and thus to determine their affinity for the heart ventricles and atria. Comparison of the uptake of the examined substances revealed that more of these hormones reached the atria than the ventricles. The contents of prolactin and tyroliberin in the atria were statistically significant compared with 125J. The results observed provide evidence for nonuniform prolactin uptake by the heart.

Modification of ocular permeability of peptide drugs by absorption promoters

The purpose of this study was to investigate the effect of absorption promoters on the ocular membrane permeability of thyrotropin-releasing hormone (TRH) and luteinizing hormone-releasing hormone (LHRH) as model peptides. The permeabilities of TRH and LHRH were measured using a two-chamber glass diffusion cell mounted with isolated ocular membranes of albino rabbits. Saponin, EDTA, benzalkonium chloride and paraben were used as absorption promoters. These promoters enhanced the permeability of hydrophilic molecules through the cornea and conjunctiva. The promoting effects of the absorption promoters on the conjunctival drug penetrations were not as strong as those on the corneal penetrations. The different responses of the corneal and conjunctival drug penetrations to these promoters may be useful in controlling the extent and pathway of the ocular and systemic absorptions of instilled drugs. The promotional effects of absorption promoters on the corneal drug penetration apparently increased with an increase in penetrant molecular weights, although those on the conjunctival drug penetrations did not depend on the molecular weights.

Evaluation of the Caco-2 monolayer as a model epithelium for iontophoretic transport

PURPOSE

To assess the Caco-2 monolayer as a model for iontophoresis of drugs across a model epithelium.

METHODS

The apparent permeability co-efficient (Papp) of mannitol, thyrotrophin releasing hormone (TRH), dexamethasone and a range of sizes of fluorescein isothiocyanate (FITC) dextrans across Caco-2 monolayers was measured under passive and electrically stimulated conditions. Trans-epithelial electrical resistance (TEER) was determined throughout. Transmission electron micrographs (TEM) of the monolayers were taken. Confocal laser scanning microscopy (CLSM) was used to visualize the iontophoretic transport route of FITC-Dextran (MW = 20 kDa) across a Caco-2 monolayer.

RESULTS

Application of 14.3 micro-Eq x cm(-2) across the monolayer evoked a transient drop in TEER. The drop in TEER was accompanied by statistically significant increases in fluxes of all the agents in the mucosal to serosal direction except for FD-70. TEM of test samples exhibited tight junction dilatation, in addition to intracellular vacuolisation. The iontophoresis of FD-20 was visualised with confocal laser scanning microscopy and was localised in paracellular spaces of the monolayer.

CONCLUSIONS

The fluxes of mannitol, TRH, dexamethasone, FD-4, FD-10 and FD-20 across the Caco-2 monolayer were significantly enhanced when electric field was applied. The iontophoretic effect appeared to be directly upon tight junctions

Effects of sodium glycocholate and protease inhibitors on permeability of TRH and insulin across rabbit trachea

The permeabilities of thyrotropin-releasing hormone (TRH) and insulin as model peptides were examined to characterize the tracheal epithelial barrier in in vitro experiments using excised rabbit trachea. TRH was not metabolized during 150 min duration of tracheal permeation and the apparent permeability coefficient (Papp) for TRH was about 3 x 10(-7) cm/s. The tracheal permeability of TRH was increased about three times by 10 mM glycocholate as a permeation enhancer. Insulin showed a slight degradation during 150 min duration of tracheal permeation, the Papp for insulin was 7 x 10(-9) cm/s. The tracheal permeability of insulin was significantly increased by 10 mM glycocholate, 1 mM bestatin (aminopeptidase B and leucine aminopeptidase inhibitor), and 10,000 KIU/ml aprotinin (trypsin and chymotrypsin inhibitor). The peptidase activities of rabbit tracheal epithelium were found to be the following; di-peptidyl-aminopeptidase IV (DPP IV) > Leu-aminopeptidase > cathepsin-B > trypsin. These activities were significantly lower than those of jejunal mucosal tissues. These results suggest that the tracheal absorption of peptide drugs through the respiratory tract may contribute to the systemic delivery of these drugs following the pulmonary administration of these drugs by intratracheal insufflation and instillation.

Brain receptor binding characteristics and pharmacokinetics of JTP-2942, a novel thyrotropin-releasing hormone (TRH) analogue

JTP-2942 competed with [3H]-Me-TRH for the binding sites in rat brain in vitro, and its inhibitory effect was approximately 17 times less potent than TRH, as shown by Ki values of 673 and 39.7 nM, respectively. Both JTP-2942 and TRH significantly increased apparent dissociation constant (Kd values) for brain [3H]-Me-TRH binding. Intravenous injection of JTP-2942 (0.3-3 mg/kg) and TRH (3 and 10 mg/kg) produced a significant reduction of [3H]-Me-TRH binding sites (Bmax values) in rat brain. Although the decrease by TRH was maximal 10 min after the injection and declined rapidly with time, the decrease by JTP-2942 (1 and 3 mg/kg) tended to be maximal at 30 min later and it lasted until 120 min. The intravenous injection of JTP-2942 was at least 3 times more potent than that of TRH in decreasing Bmax values for brain [3H]-Me-TRH binding. Plasma concentration of JTP-2942 (0.3-3 mg/kg) after intravenous injection in rats rose with the increase of dose, and it peaked immediately after the injection, thereafter decreasing with t1/2 of 19.3-29.9 min. It is concluded that JTP-2942, compared to TRH, may exert fairly potent and sustained occupation of brain TRH receptors under in vivo condition. Thus, JTP-2942 could be clinically useful for the treatment of CNS disorders.

Changes in metabolism of TRH in euthyroid sick syndrome

OBJECTIVE

The aim of this study was to examine the metabolism of a simple dose, intravenously administered TRH bolus of 200 microg, in patients with euthyroid sick syndrome (ESS).

PATIENTS AND METHODS

A TRH test was performed on ten ESS patients and ten controls upon admission (d1) and after recovery (d2). Blood samples were collected at 0, 10, 20 and 30min after TRH injection. We analyzed the volume of distribution (V(d)), the plasma clearance rate (PCR), the fractional clearance rate (FCR), the half-life (t(1/2)) and the TSH response to the injection of TRH.

RESULTS

All patients had lower tri-iodothyronine (T(3)) levels compared with controls (0.9 +/- 0. 1nmol/l vs 1.9 +/- 0.1 nmol/l; P < 0.0001; mean +/- S.D.; paired t-test). In addition, the V(d) (16.7 +/- 5.9/l vs 30.6 +/- 0.6/l; P < 0.0005) and PCR (2.0 +/- 0.80 l/min vs 3.3 +/- 0.25 l/min; P <0. 0005) were found statistically lowered in patients than in controls, whereas FCR (0.119 +/- 0.01 permin vs 0.110 +/- 0.01 per min; P < 0. 025) was found increased in patients as opposed to controls. The t(1/2) of exogenously administered TRH was increased in ESS compared with controls (7.2 +/- 0.7 min vs 6.3 +/- 0.6 min; P <0.005). TSH response to TRH was found significantly repressed at 10, 20 and 30 min after TRH injection. On d2, these findings had reverted to normal and no changes regarding the kinetics of TRH and the response of TSH could be detected between patients and controls.

CONCLUSIONS

The results demonstrate an impairment of TRH metabolism in ESS. The findings may suggest altered enzymatic activity, responsible for TRH degradation in states of acute ESS. These changes might be involved in the pathogenesis of ESS and represent part of an adaptive mechanism to this syndrome.

Effect of chain length on absorption of biologically active peptides from the gastrointestinal tract

OBJECTIVES

Protein digestion generates many peptides in the gut lumen. Some of these peptides possess biological effects when tested using in vitro systems. It is clear that dipeptides and tripeptides can be absorbed intact from the gastrointestinal tract. However, the fate of larger peptides and small proteins remains unclear. Equally unclear are the biologic potencies of absorbed peptides and the quantity of peptide that must be administered into the gut to produce a biologic effect. Thus, the purpose of this study was to determine the effect of amino acid chain length on the ability of enterally administered peptides to produce biologic effects.

METHODS

Small bowel feeding tubes, jugular catheters, and arterial lines were placed into adult male Sprague-Dawley rats. Rats were administered intravenous (50 microg) and enteral (125 and 500 microg) thyrotropin-releasing hormone (TRH, a tripeptide), intravenous (100 microg) and enteral (100 and 500 microg) luteinizing hormone-releasing hormone (LHRH, a decapeptide), and intravenous (0.5 mg) and enteral (0.5 and 25 mg) insulin (a 51-amino acid polypeptide). The quantity of peptide administered represented less than 0.5% of a rat's normal daily protein intake. The biologic effect of TRH, LHRH, and insulin were assessed using thyroid-stimulating hormone (TSH) response, follicle-stimulating hormone (FSH) response, and glucose. We also measured serum levels of insulin in the rats following enteral insulin administration.

RESULTS

The results indicate that enteral TRH (125 and 500 microg) produced the same TSH response as intravenous TRH. The response to 500 microg enteral LHRH was 50% of the response to intravenous LHRH and the response to 25 mg enteral insulin was 30% of the response to 0.5 mg intravenous insulin. Serum insulin levels increased significantly following both 0.5 and 25 mg enteral insulin.

CONCLUSIONS

These results support the concept that small (di- and tripeptides) and large (10-51 amino acids) peptides generated in the diet can be absorbed intact through the intestines and produce biologic effects at the tissue level. The potency of the enterally administered peptides decreases as the chain length increases. We postulate that absorbed dietary peptides play a role in the modulation of organ function and disease progression.

Effect of penetration enhancers on the permeation of the thyrotropin releasing hormone analogue pGlu-3-methyl-His-Pro amide through human epidermis

The effect of the enhancers, cineole and ethanol, on the transdermal penetration of the tripeptide, pGlu-3-methyl-His2-Pro amide (M-TRH), across human epidermal membrane was studied by flow-through diffusion chambers. The aim of the study was to assess whether the biologically active analogue M-TRH displays similar transdermal penetration properties as those demonstrated earlier for the parental peptide, thyrotropin-releasing hormone (TRH) (Magnusson et al., 1997a Int. J. Pharm. 157, 113-121). Steady-state fluxes with a donor solution of phosphate-buffered saline (PBS) were 0.34 +/- 0.01 microgram/cm2h for M-TRH and 0.27 +/- 0.01 microgram/cm2h for TRH. Measured over 30 h the total amount penetrated was 8.6 +/- 1.0 and 7.8 +/- 1.7 micrograms/cm2, respectively. In the presence of 50% ethanol, the flux of the peptides increased approximately 3-fold. A donor solution of 3% cineole, in combination with 47% ethanol, increased the penetration of M-TRH to 1.60 +/- 0.02 micrograms/cm2h, compared to 0.92 +/- 0.03 microgram/cm2h for TRH, as reported previously. The corresponding total amount penetrated over 30 h was 41.5 +/- 4.9 and 24.9 +/- 1.7 micrograms/cm2, respectively. Our data suggests that enhancers added together with the penetrant can theoretically induce changes in the permeability of the stratum corneum sufficient to promote the transdermal absorption of therapeutically relevant amounts of these peptides. This demonstrates the possibility to deliver classes of compounds that have been viewed as not suitable for transdermal administration.

Thyroid volumetric measurement and quantitative thyroid scintigraphy in dogs

Thyroid volumetric measurement combined with quantitative 99mTc-per-technetate thyroid scintigraphy was performed in 62 clinical canine patients having suspected thyroid abnormalities. Euthyroid dogs (n = 22) had a total thyroid size of 3.60 +/- 1.36 cm3, the thyroid/salivary gland region of interest (ROI) ratio was 2.01 +/- 0.55, the thyroid/background ROI ratio was 3.86 +/- 0.90, and 20-min thyroid radioactivity uptake was 1.17 +/- 0.71% of the injected dose (I. D.). By Student's unpaired test, thyroid size of the hypothyroid group (n = 36) was not statistically different from that of the euthyroid dogs, but all other quantitative data (e.g., thyroid/salivary gland ROI ratio = 1.08 +/- 0.56, thyroid/background ROI ratio = 2.32 +/- 0.70, and 20-min thyroid radioactivity uptake = 0.34 +/- 0.22% of the I. D.) were significantly (p < 0.001) lower in hypothyroid than in euthyroid dogs. Evaluating the above-listed quantitative data of separated thyroid lobes by Student's paired test, there was no significant difference between the left and the right lobe either in the euthyroid or in the hypothyroid group.

Nasal delivery of peptides: an in vitro cell culture model for the investigation of transport and metabolism in human nasal epithelium

We investigated the transport- and metabolism properties of three peptides in monolayers of human nasal epithelial cells. The effective permeability coefficients of thyrotropin-releasing hormone, met-enkephalin and human recombinant insulin were found to be 4.5, 4.4 and 0.4 x 10(-7) cm/s, respectively. The permeability was inversely proportional to the molecular weight and one order of magnitude lower than in excised nasal mucosa of rabbits. The metabolic cleavage of thyrotropin-releasing hormone (TRH) to the free acid by cytosolic prolyl-endopeptidase was also detected in human nasal cell monolayers, suggesting that ca. 10% of the total amount of TRH is transported via a transcellular pathway. Met-enkephalin is a substrate for aminopeptidases, located on the apical membrane of nasal epithelial cells. Metabolites and enzyme activity are comparable with literature data. Our studies demonstrate that not only morphological, but also functional properties of human nasal epithelial cells are preserved under in vitro conditions. Such a cell culture model based on human nasal cells could be beneficial for the characterization of peptide transport on a cellular level and for investigation of the absorption enhancer mechanism. Further studies are necessary, however, to establish correlations between in vitro permeabilities in cell cultures and nasal drug absorption in animals and humans.

Radioimmunoassay for TA-0910, a new stable thyrotropin releasing hormone analogue and its metabolite, TA-0910 acid-type, in human plasma and urine

Radioimmunoassay (RIA) was investigated for the determination of TA-0910 and its main metabolite, TA-0910 acid-type, in human plasma and urine. TA-0910 is a new metabolically stable analogue of thyrotropin releasing hormone (TRH). Antiserum was raised in the rabbit against the 1-fluoro-2,4-dinitrophenyl derivative of TA-0910 or TA-0910 acid-type conjugated to keyhole limpet hemocyanin (KLH). The radioligand was prepared by iodination with 125I of the histidine imidazole ring of TA-0910 or TA-0910 acid-type. Cross-reactivities of anti-TA-0910 or TA-0910 acid-type antiserum for TA-0910, its metabolite and related compounds were low. The calibration range was 0.02-5 ng ml-1 using 100 microliters human plasma or urine. Inter-day variations of TA-0910 and TA-0910 acid-type assay in plasma were 3.5-15.5 and 1.8-9.4%, respectively. The variations of the assay in urine were the same as those in plasma. The recovery of TA-0910 and TA-0910 acid-type spiked in plasma or urine samples was approximately 100%. Furthermore, this method was applied to the determination of TA-0910 and TA-0910 acid-type in human plasma and urine samples, for the evaluation of the pharmacokinetics of TA-0910 in humans. From the results it was demonstrated that he developed RIA was useful for the determination of TA-0910 and TA-0910 acid-type in human plasma and urine, and was applicable to pharmacokinetic studies in humans.

[Design and development of controlled release of drugs from injectable microcapsules]

Monolithic microcapsules were designed and developed for controlled release of leuprorelin for one month following a single injection. Copoly (DL-lactic/glycolic) acid (PLGA) of copolymer ratio of 75/25 and average molecular weight of 14,000 was suitable for achieving steady serum leuprorelin levels in rats and dogs for 4 weeks. The clinical efficacy of these injectable microcapsules of leuprorelin has been widely proved for prostate cancer, endometriosis, and other sex hormone dependent diseases in about sixty countries. The interaction between the basic functional group of the drug and the carboxylic end group of PLGA was found to be the most important factor in preparing the microcapsules with a small initial burst, as shown with thyrotropin releasing hormone (TRH) and a water-soluble GPIIb/IIIa antagonist (TAK-029).

[Improvement of intestinal absorption of peptide and protein drugs by chemical modification with fatty acids]

It is well known that the oral bioavailability of peptide and protein drugs is generally poor because they are extensively degraded by proteases in the gastrointestinal tract and impermeable through the intestinal mucosa. Therefore, various approaches have been examined to overcome the delivery problems of these peptides and to improve their absorption via the gastrointestinal tract. Of these approaches, a potentially useful approach to solve these delivery problems may be chemical modification of peptides and proteins to produce prodrugs and analogues. Thus, it is plausible that this approach may protect peptides against degradation by peptidases and other enzymes present at the mucosal barrier and renders the peptides and proteins more lipophilic, resulting in increased bioavailability. From these standpoints, we synthesized lipophilic derivatives of peptides and proteins such as thyrotropin-releasing hormone (TRH), tetragastrin (TG), calcitonin and insulin by chemical modification with fatty acids. The pharmacological activities of these derivatives were relatively high as compared with the native peptides. A significant increase in the intestinal absorption of these derivatives of peptides was observed in comparison with native peptides. Overall, the effects of acylation on the intestinal absorption of these peptides were more predominant in the large intestine than those in the small intestine. In addition, these derivatives were more stable than the parent peptides in homogenates of the various intestinal mucosae. We also examined the intestinal transport characteristics of TG and its acyl derivatives using Caco-2 cell monolayers in order to assess the contribution of enzymatic and transport barriers on their intestinal absorption. The degradation clearance of TG on the apical membrane was decreased by chemical modification with fatty acids. In addition, the permeability clearance of TG was improved by the acylation. On the other hand, the intestinal absorption of thyrotropin releasing hormone (TRH), which is transported by a carrier-mediated process, was also enhanced by chemical modification with lauric acid. In summary, this chemical modification approach may be useful to improve the intestinal absorption of peptide and protein drugs.

Maternal-to-fetal transfer of thyrotropin-releasing hormone in vivo

OBJECTIVE

Our purpose was to determine the transplacental transfer of thyrotropin-releasing hormone at the time of fetal blood sampling.

STUDY DESIGN

Four hundred micrograms of thyrotropin-releasing hormone was given intravenously to 13 pregnant women between 24 and 35 weeks' gestation and maternal-to-fetal transfer of thyrotropin-releasing hormone was determined at fetal blood sampling 1 to 93 minutes later. The fetal thyrotropic response to thyrotropin-releasing hormone was determined by measuring thyroid-stimulating hormone, thyroxine, and prolactin. For comparison, endogenous fetal and maternal levels of thyrotropin-releasing hormone, thyroid-stimulating hormone, thyroxine, and prolactin levels were determined in a further 20 patients undergoing fetal blood sampling between 19 and 35 weeks' gestation. The concentration of thyrotrophin-releasing hormone was measured by radioimmunoassay and thyroid-stimulating hormone, thyroxine, and prolactin by chemiluminescence assay.

RESULTS

Thyrotropin-releasing hormone was undetectable in the maternal circulation, whereas endogenous levels were detectable in the fetus from 19 weeks' gestation (median 150; range 50 to 276 pmol/L) and did not correlate with gestational age. After thyrotropin-releasing hormone injection as an intravenous bolus, peak levels in the mother were attained at 3 minutes (50,000 pmol/L). Maximal transplacental transfer of thyrotropin-releasing hormone occurred within 5 minutes of maternal administration but accounted in fetal blood for only 0.01% of initial dose administered (median 250; 30 to 550 pmol/L). Thyrotropin-releasing hormone-stimulated fetal peak thyroid-stimulating hormone levels occurred within 13 minutes and were higher than maternal values (p < 0.001). There was no change in fetal prolactin level with thyrotropin-releasing hormone therapy.

CONCLUSION

Although maternally administered thyrotropin-releasing hormone crosses the placenta sparingly, it still elicits a thyroid-stimulating hormone but not a prolactin response in the human fetus.

[Repeated dose toxicity studies of taltirelin tetrahydrate (TA-0910) with oral administration to dogs]

Taltirelin tetrahydrate (TA-0910), novel thyrotropin-releasing hormone (TRH) analogue, was orally administered to dogs as dose levels 0.5, 5, and 50 mg/kg for 13 weeks and 0.15, 1.5 and 15 mg/kg for 52 weeks. Blood concentrations of test substance measured in 52-week study revealed that absorption of TA-0910 was with dose-dependent manner and not changed through the treatment period. These toxicokinetics suggested that there were no alterations on metabolism of TA-0910 with repeated treatment. The animals receiving 5 or 50 mg/kg showed decrease in body weight or suppression of body weight gain, and decrease in food intake (13-week study). As an abnormality in general conditions, vomiting and salivation (5 mg/kg or more, both in 13- and 52-week studies), increase in behavior as water intake (5 mg/kg or more, 13-week study), and hyperlocomotion (50 mg/kg) were observed. Elevating GPT values were noted temporally in the animals treated with 5 mg/kg or more (both in 13- and 52-week studies) without abnormal findings in histopathology. The thyroid weights were increased in treated animals receiving 5 or 50 mg/kg in 13-week study, but no histopathological changes were noted. Electron microscopy revealed dilatation of granular endoplasmic reticulums in follicular cells of thyroid from 50 mg/kg group in 13-week study. It was concluded that no-effect levels of 13- and 52-week studies were 0.5 mg/kg and 1.5 mg/kg, respectively.

Transport and metabolism of thyrotrophin-releasing hormone across the fetal membrane

To determine the transfer and metabolism of TRH by human fetal membranes, the bidirectional transport and uptake of TRH was investigated by adding 125I-labeled TRH (100,000 cpm) or commercial TRH either to the maternal or the fetal compartment of an in vitro model of cultured human fetal membranes obtained from term and preterm placenta. Transmembrane transfer was also studied in the presence of 200 microM p-hydroxy-mercuriphenyl-sulphonic acid (p-HMSA), a dipeptidase enzyme inhibitor. Creatinine and heparin were used as an internal markers. Metabolites of TRH were separated from intact molecules by gel filtration on Sephadex G-10. The structural integrity of the membrane was confirmed by electron microscopy. The transmembrane transfer of radiolabeled and commercial TRH were comparable across both preterm and term placenta. When transport was studied from the maternal to fetal side, the maternal concentration of TRH declined rapidly from 100% at time 0 to 19.31 +/- 2.26% at 8 h with a concomitant increase in the fetal concentration from undetectable to a maximum of 2.56 +/- 0.38% with a fetomaternal ratio of 0.16 +/- 0.01. Transfer of TRH from the fetal to maternal compartment was similar to that of maternal to fetal. Chromatography of maternal and fetal media showed that TRH was metabolized by the membrane into small molecular weight fragments. Treatment of the membrane with p-HMSA increased TRH transport from the maternal to fetal compartment to 18.12 +/- 0.91 (P < 0.001) with an fetomaternal ratio of 0.35 +/- 0.02 (P < 0.001). Although transmembrane transfer of TRH from the fetal to maternal side was also increased by p-HMSA, levels achieved were less than that from maternal to fetal (12.26 +/- 1.50%; P < 0.05). These results suggest that the human fetal membrane acts as an enzymatic barrier to the bidirectional transfer of TRH from 24 weeks gestation.

[Synthesis and pharmacological action of TRH analog peptide (Taltirelin)]

Taltirelin hydrate[1-methyl-(S)-4,5-dihydroorotyl-L-histidyl-L-prolineamide tetrahydrate] is a new orally active thyrotropin releasing hormone (TRH) peptide analog synthetized from aspartic acid. From preclinical studies with mice and rats, Taltirelin hydrate was found to be highly stable in the blood and brain as compared with TRH. Furthermore, the CNS stimulating actions of Taltirelin hydrate such as antagonistic actions against pentobarbital-induced anesthesia and reserpine- induced hypothermia were found to be about 100 times stronger and about 8 times longer-lasting as compared with those of TRH. Meanwhile, the affinity of Taltirelin hydrate for TRH-receptors was about 10 times lower, and the endocrine action was about 5 times less potent than those of TRH. Therefore, high CNS-selectivity and long-lasting action of Taltirelin hydrate would be attributed to its high stability in the body and low affinity for TRH-receptors. Oral administrations of Taltirelin hydrate ameliorated consiousness impairment, memory impairment and motor dysfunction in several models. The clinical studies for patients with spinocerebellar degeneration are in progress.

Pharmacokinetics and pharmacodynamics of TRH during pregnancy

OBJECTIVE

To determine the pharmacokinetics and pharmacodynamics of thyrotropin-releasing hormone (TRH) in pregnant women.

METHODS

Twenty-four pregnant and eight nonpregnant women were given 400 micrograms TRH as either intravenous infusion or bolus. Serial venous samples were collected for TRH, TSH, thyroxine, and prolactin assay.

RESULTS

When given as bolus, mean (+/- standard error of the mean) peak plasma concentration (50 +/- 5.2 and 73 +/- 5.1 ng/mL, P < .01), elimination half life (4.3 +/- 0.3 and 6.3 +/- 0.4 minutes, P < .001), and area under the curve (156.4 +/- 14.8 and 340.1 +/- 32.8 ng/mL/minute, P < .001) in pregnant subjects were reduced compared with controls, whereas plasma clearance (45.4 +/- 6.5 and 23.6 +/- 2.1 mL/kg/minute, P < .01) and volume of distribution (27.8 +/- 1.8 and 19.0 +/- 1.3% body weight, P < .01) were increased. When given by infusion, steady-state concentration (6.6 +/- 0.5 and 9.8 +/- 0.9 ng/mL, P < .01) and elimination half-life (4.6 +/- 0.5 and 6.3 +/- 0.3 minutes, P < .05) were lower in pregnant subjects than in controls. Thyrotropin-releasing hormone kinetics were independent of mode of administration. Although basal TSH and thyroid hormone concentrations were similar in patients and controls, the TSH response to TRH was blunted in pregnant subjects compared with controls (9.3 +/- 0.6 and 16.4 +/- 1.4 microIU/mL, P < .001). The basal (3187 +/- 488 and 147 +/- 16 mIU/L) and maximal prolactin response (6193 +/- 426 and 1316 +/- 106 mIU/L) were increased in pregnant subjects compared with controls (P < .001).

CONCLUSION

The peak plasma concentration and elimination half-life of TRH are reduced during pregnancy because of the increased volume of distribution and rapid clearance. Mode of administration does not affect TRH pharmacokinetics, but the maternal pharmacodynamic response differs in patients receiving bolus compared with infusion.

Urinary excretion of the TRH-like peptide pyroglutamyl-glutamyl-prolineamide in rats

TRH-like immunoreactivity (TRH-LI) was estimated in methanolic extracts of rat tissues and blood by RIA using antiserum 4319, which binds most peptides with the structure pGlu-X-ProNH2, or antiserum 8880, which is specific for TRH (pGlu-His-ProNH2). TRH-LI (determined with antiserum 4319) and TRH (determined with antiserum 8880) contents were 8 and 8 ng/g in brain, 216 and 222 ng/g in hypothalamus, 6.5 and 6 ng/g in pancreas, 163 and 116 ng/g in male pituitary, 105 and 77 ng/g in female pituitary, 1 and 0.1 ng/g in salivary gland, 61 and 42 ng/g in thyroid, 12 and 3 ng/g in adrenal, 3 and 0.3 ng/g in prostate, and 11 and 0.8 ng/g in ovary respectively. Blood TRH-LI (antiserum 4319) and TRH (antiserum 8880) levels were 31 and 18 pg/ml in male rats, and 23 and 10 pg/ml in female rats respectively. Unextracted serum obtained from blood kept for at least 1 h at room temperature no longer contained authentic TRH but still contained TRH-LI (males 20.3 +/- 3.1, females 15.9 +/- 3.0 pg/ml; means +/- S.E.M.). Isocratic reverse-phase HPLC showed that TRH-LI in serum is largely pGlu-Glu-ProNH2 (< EEP-NH2), a peptide previously found in prostate and anterior pituitary. In urine, TRH-LI (antiserum 4319) and TRH (antiserum 8880) levels were 3.21 +/- 0.35 and 0.32 +/- 0.04 ng/ml in male rats and 3.75 +/- 0.22 and 0.37 +/- 0.04 ng/ml in female rats respectively (means +/- S.E.M.). Anion-exchange chromatography on QAE-Sephadex showed that urine of normally fed rats contains both basic/neutral TRH-LI (b/n TRH-LI) and acidic TRH-LI (aTRH-LI) in a ratio of approximately 40:60, and further analysis by HPLC indicated that aTRH-LI represents < EEP-NH2. Analysis of food extracts and urine from fasted rats demonstrated that b/n TRH-LI is derived from food particles spilled by the rats during urine collection, while aTRH-LI is endogenously produced. While urinary aTRH-LI levels were higher in female than in male rats (2.99 +/- 0.41 vs 2.04 +/- 0.20 ng/ml), the daily urinary excretion was similar in both sexes (females 15.6 +/- 1.4, males 19.5 +/- 2.0 ng/day). Intravenously injected < EEP-NH2 disappeared from serum with a half-life of approximately 1 h, and was recovered unchanged and quantitatively in urine. In contrast, when < EEP-NH2 was administered with food, only approximately 0.5% was recovered in urine. The urinary clearance rate of serum TRH-LI amounted to 0.52 +/- 0.10 ml/min in males and 0.34 +/- 0.05 ml/min in females. In view of the presence of < EEP-NH2 in the anterior pituitary gland, and the regulation of its content in parallel with gonadotrophins, we examined the possibility that serum < EEP-NH2 is of pituitary origin and correlates with gonadotrophin secretion. However, treatments that alter pituitary < EEP-NH2 content and gonadotrophin release had no effect on serum TRH-LI or urinary aTRH-LI. In conclusion, the TRH-like peptide < EEP-NH2 is present in rat serum and is excreted into the urine. Moreover, < EEP-NH2 in serum and urine is not derived from rat food and is probably not of pituitary origin.

Chronic anti-ataxic actions of the novel thyrotropin-releasing hormone (TRH) analog, TA-0910, during and after repeated administration in Rolling mouse Nagoya: behavioral and pharmacokinetic studies

Anti-ataxic effects of TA-0910, a novel thyrotropin-releasing hormone analog, in mice of the ataxic mutant mouse strain Rolling mouse Nagoya (RMN) are sustained beyond its 2-week oral administration period (Kinoshita et al., Eur. J. Pharmacol., 274, 65-72, 1995). We examined the concentration of TA-0910 in the central nervous system (CNS) of RMN after repeated administration in an attempt to clarify the mechanism of the sustained effect of the drug. Repeated administration of TA-0910 (3 mg/kg/d, i.p.) for 2 weeks produced a long-lasting ameliorating effect on ataxia in RMN, and this effect was maintained until 3 weeks after drug withdrawal. The concentrations of TA-0910 in the cerebrum and brain stem 24 h after the final administration were twice the concentration observed 24h after single administration. The cerebellum cencentration of TA-0910 was more than 4 times that observed 24 h after final administration. After repeated administrations, the drug concentrations in the brain tissues gradually decreased, but the drug was still detectable in the cerebrum and brain stem 3 weeks after withdrawal. However, these concentrations of TA-0910 3 weeks after withdrawal were as low those observed 24 h after single administration when there were no anti-ataxic effects. These observations suggest that the long-lasting ameliorating effect on the ataxia during and after repeated administration of TA-0910 is not ascribable to the drug remaining in the CNS of RMN.

Pharmacokinetic profile and endocrine effects of posatirelin treatment in healthy elderly subjects

The pharmacokinetics and endocrine effects of a therapeutic dose (10 mg/day) of posatirelin (L-pyro-2-aminoadipyl-L-leucyl-L-prolinamide) were investigated in healthy elderly subjects. Posatirelin was given once daily by intramuscular injection for 7 days. Pharmacokinetic parameters were estimated using a model-independent approach. The plasma concentrations of free triiodotyronine (FT3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) and the circadian rhythms of prolactin and cortisol were considered as indicator variables of endocrine response to posatirelin administration. Posatirelin was well tolerated and no significant adverse effects were observed during the study. Peak plasma concentration (Cmax), time of peak plasma concentration (tmax), area under the plasma concentration-time curve from time zero to infinity (AUC0-infinity), elimination half-life (t1/2), and total clearance (CI/F) were measured after single-dose intramuscular injection (day 1) and after multiple-dose administration (day 7). There were no significant changes in these parameters after multiple-dose administration (day 7). Posatirelin induced a progressive reduction in basal TSH levels and maximum response. There were no significant changes during treatment in the time at which basal levels of FT3 and FT4 occurred, and these levels remained within the normal range throughout the study. The circadian rhythms of cortisol and prolactin were not influenced by posatirelin treatment. The pharmacokinetics of posatirelin were not time dependent, and the drug did not accumulate after multiple-dose administration. Short-term treatment with posatirelin did not induce clinically relevant endocrine consequences in healthy elderly subjects.

Response surface method: a novel strategy to optimize iontophoretic transdermal delivery of thyrotropin-releasing hormone

PURPOSE

To maximize the iontophoretic transdermal delivery rate of thyrotropin-releasing hormone (TRH) facilitated by periodically monophase-pulsed current across excised skin.

METHODS

The pH of the buffer, the ionic strength in the solution, the frequency of the periodically monophase-pulsed current and the current on/off ratio were chosen as the key variables. A response surface method was applied to optimize the transdermal delivery rate of TRH under different operational conditions.

RESULTS

The optimum operating conditions were achieved via experimentation based on the response surface method by systematically adjusting the pH of the buffer, the ionic strength in the solution, the current amplitude, frequency and the active temporal ratio of the pulsed current. The rate of permeation of TRH crossing the skin during iontophoresis varied from two to ten-fold, depending on operating conditions.

CONCLUSIONS

Only a few steps, two in this work, were needed to reach the optimal. The response surface near the region of the maximal point was thoroughly described with a quadratic function. A maximal transdermal rate of permeation of TRH, 103.2 micrograms h-1 cm-2, was obtained when the donor solution was at pH = 7.0, ionic strength = 0.037, and with a periodically monophase-pulsed current iontophoresis with duty cycle = 75%. The effect of pulse frequency was not statistically significant.

Pharmacokinetics of the new thyrotropin releasing hormone analogue montirelin hydrate. 2nd communication: distribution and transfer into the fetus and milk after a single intravenous administration and pharmacokinetics and enzyme induction after repeated intravenous administration to rats

Pharmacokinetics of 14C-labeled montirelin hydrate (CAS 90243-66-6, NS-3) in rats was studied after single or repeated intravenous administration. 1. Radioactivity concentrations in tissues after single administration to male and female rats were highest in the kidney followed (in this order) by plasma, liver, blood, pancreas, uterus (female rats), lung and skin and low in various brain sites 5 min after administration. The concentrations in most tissues were practically parallel to those in plasma over the 24-h period after administration. After decreasing rapidly the concentrations rose slightly for 10 h and then decreased gradually. 2. Five min after single administration to male rats, the concentration of the main metabolite CNK-6004 (deamidated product) was lower in the plasma, but higher in the liver and kidney than the NS-3 concentration. From 0.5 to 2 h after administration, the concentration of CNK-6004 was higher than that of NS-3 in the plasma, liver and kidney, accounting for 33-64% of the radioactivity concentration. 3. After administration to rats on the 18th day of pregnancy, the radioactivity concentrations in the fetal whole body and fetal tissues peaked later than those in the maternal plasma, tissues and placenta. The maximum concentration in the fetal tissues was 2% or less of that in the maternal plasma. 4. After administration to lactating rats, the radioactivity concentration in milk reached the maximum 10 h after administration, and decreased gradually in parallel with the concentration in the plasma 24 to 168 h after administration. 5. During repeated once daily administration for 10 days, the radioactivity concentration in the plasma 24 h after each administration reached practically steady state level after the 7th administration and decreased with a half-life of 38.1 h after the last administration. 6. The radioactivity concentrations in most tissues after the last administration were not significantly different from those after a single administration. Decreased elimination of the radioactivity was observed in the white fat and skin, in which radioactivity levels were higher than those in the other tissues a long time after the last dose. 7. Excretion of the radioactivity in the urine and feces during repeated administration was constant after the 2nd administration. The excretion by 168 h after the last administration was 66.9 and 14.3% of the cumulative dose in the urine and feces, respectively (total: 81.2%). 8. The composition of metabolites in the plasma, liver, kidney and urine after the last administration did not differ markedly from that after a single administration. Once daily repeated administration for 7 days had no effect on the liver drug metabolizing enzyme activities.

Pharmacokinetics of the new thyrotropin releasing hormone analogue montirelin hydrate. 3rd communication: identification of metabolites in rat urine

The metabolism of montirelin hydrate (CAS 90243-66-6, NS-3) was studied in rats after intravenous administration of 14C-labeled or unlabeled NS-3. 1. Four radioactive metabolites (M-1 to M-4) were found in the urine after administration of 14C-NS-3. M-3 (major metabolite) and M-2 showed the same Rf values as (-)-N-[[(3R,6R)-6-methyl-5-oxo-3-thiomorpholinyl]carbonyl]-L-histi dyl-L- proline (CNK-6004) and (+)-N-[[(3R,6R)-6-methyl-5-oxo-3- thiomorpholinyl]carbonyl]-L-histidine (CNK-6001), respectively. 2. M-3 and M-2 were purified from the urine after administration of unlabeled NS-3, and their chemical structures were identified by mass spectrometry, 1H-nuclear magnetic resonance spectroscopy, thin-layer chromatography and high performance liquid chromatography. Consequently, M-3 was identified as CNK-6004 formed by deamidation at a prolinamide moiety of NS-3, and M-2 as CNK-6001 formed by deprolination of CNK-6004.

Pharmacokinetics of the new thyrotropin releasing hormone analogue montirelin hydrate. 1st communication: plasma concentrations, metabolism and excretion after a single intravenous administration to rats, dogs and monkeys

The plasma concentrations, metabolism and excretion were studied in rats, dogs and monkeys after a single intravenous administration of 14C-montirelin hydrate (CAS 90243-66-6, 14C-NS-3). 1. The plasma concentration of radioactivity and the area under curve (AUC0-infinity) increased with the dose after intravenous administration of 0.25, 1 and 4 mg/kg to male rats. 2. Until 4 h after intravenous administration of 0.25 mg/kg, the plasma concentration of radioactivity decreased with half-lives of 0.512, 0.771 and 0.786 h in male rats, dogs and monkeys, respectively. The plasma concentration of radioactivity in rats increased 6-10 h after administration, suggesting enterohepatic circulation. 3. The plasma concentration of NS-3 in various animal species decreased biphasically after intravenous administration. The elimination half-lives, t1/2 beta, in rats, dogs and monkeys were 0.324, 0.679 and 0.682 h, respectively, and the steady state volumes of distribution, Vdss, were 0.248, 0.319 and 0.306 l/kg, respectively. 4. The binding of NS-3 to serum protein was less than 3% in rats, dogs, monkeys and humans. 5. Within 48 h after intravenous administration of 1 mg/kg to male rats, the excretion of radioactivity in urine, feces and expired air was 67.3, 15.0 and 14.8% of the dose, respectively (total 97.1%). The biliary excretion of radioactivity was 26.6% within 24 h after dosing. 6. No sex-related difference was found in plasma concentration of radioactivity or the excretion of radioactivity in urine, feces and expired air after intravenous administration of 1 mg/kg to male and female rats. 7. In male dogs, the excretion of radioactivity in urine and feces was 92.3 and 5.6% of the dose, respectively, during 72 h after intravenous administration of 0.25 mg/kg (total 97.9%). 8. In male monkeys, the excretion of radioactivity in urine and feces was 91.7 and 2.9% of the dose, respectively, during 72 h after intravenous administration of 0.25 mg/kg (total 94.6%). 9. The urinary excretion of NS-3 and its main metabolite CNK-6004 (deamidation product) during 24 h after intravenous administration was 31.4 and 25.3% of the dose in male rats, 45.4 and 30.1% in male dogs, and 20.2 and 48.0% in male monkeys, respectively. The biliary excretion of NS-3 and CNK-6004 in male rats was 1.1 and 17.4%, respectively, during 24 h after administration. 10. The radioactivity in the bile collected from donor rats that had received 14C-NS-3 was easily absorbed from the intestinal tract of recipient rats after intraduodenal administration of the donor's bile.

Permeability of NS-3, a thyrotropin-releasing hormone analogue, into the brain after its systemic administration in rats: a microdialysis study

The concentration of NS-3 (montirelin hydrate, CG 3703), a thyrotropin-releasing hormone (TRH) analogue, in the cerebral cortex of urethane-anaesthetized rats was measured after its systemic administration (1 mg kg-1, i.v.), using in-vivo microdialysis coupled with a radioimmunoassay. The concentration in microdialysates was highest (24 nM) during the first 20 min after injection, and it fell below the detection limit (3.5 nM) 100 min after treatment. The maximal interstitial concentration was estimated to be 0.51 microM. From these results, it is suggested that NS-3 can readily penetrate into the brain.

Assessment of dose and time responses to TRH and thyrotropin in healthy dogs

Changes in total thyroxine (T4 [TT4]), free T4 (FT4) and total tri-iodothyronine (T3 [TT3]) in serum after the intravenous administration of different doses of thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) were measured in six healthy beagles. Significant (P < 0.05) elevations in serum TT4, FT4 and TT3 were observed at each sampling time (two, four, five, six, seven, eight and 10 hours) after administration of 1, 3 or 5 iu (total dose) TSH and peak mean responses were observed six to eight hours after injection. At six hours after injection the mean TT4, FT4 and TT3 levels were approximately 2.6, 3.9, and 1.5 times basal levels, respectively, and there were no significant differences between the three doses of TSH, Significant (P < 0.05) elevations in serum TT4 and FT4 but not TT3 were observed at each sampling time (two, four, five, six, seven and eight hours) after the administration of TRH. Peak mean responses were observed at four hours after injection at which time TT4 and FT4 levels were approximately 1.7 and 1.9 times basal levels, respectively. No significant differences were observed between the four doses of TRH used (100, 200, 300, and 600 micrograms total dose). Concentrations of TT4, FT4 and TT3 were significantly (P < 0.05) higher following the administration of TSH compared with TRH, and the response to TRH showed greater individual variation.

Influence of the dosing interval on prolactin release after remoxipride

1. The prolactin response following administration of the D2-dopamine receptor antagonist remoxipride was studied in eight healthy male volunteers. The purpose of the study was to investigate the duration of a refractory period of prolactin release following two doses of remoxipride. A further aim was to compare the prolactin response following remoxipride and thyrotropin release hormone (TRH) during the refractory period. The subjects received two 30 min intravenous (i.v.) infusions of remoxipride 50 mg with different time intervals between the two doses, in a randomized six period crossover design. The time intervals between the two remoxipride doses were 2, 8, 12, 24 and 48 h. On one occasion the remoxipride dose was followed by an i.v. injection of TRH after 2 h. 2. The plasma peak prolactin concentrations obtained after the first remoxipride dose correspond to a maximal release of prolactin according to earlier studies. A small second peak of prolactin was observed after 2 h. The release was gradually increased with longer time intervals between the consecutive doses. The refractory period for a second prolactin release similar to the first one after remoxipride was found to be 24 h for most of the subjects. 3. TRH resulted in a faster and higher increase in prolactin response of a shorter duration than after remoxipride administered 2 h after the first dose.

The electrophoretic mobility of tripeptides as a function of pH and ionic strength: comparison with iontophoretic flux data

Capillary electrophoresis (CE) is an extremely efficient separations tool which can also be used to determine fundamental molecular parameters, e.g., the electrophoretic mobility of a molecule. We have studied the changes in the CE estimated electrophoretic mobility of thyrotropin releasing hormone (TRH) as a function of pH and ionic strength. Further, we have used CE to estimate the mobilities of two synthetic analogs of TRH to examine the behavior of positive (basic) and negative (acidic) peptides under the conditions of this work. These data were then compared with literature values of iontophoretic flux of these molecules under similar formulation conditions. Our results suggest that CE could potentially assist formulation optimization for the iontophoretic delivery of peptides.

Transepithelial transport and metabolism of thyrotropin-releasing hormone (TRH) in monolayers of a human intestinal cell line (Caco-2): evidence for an active transport component?

Cell culture models for gastrointestinal transport and metabolism are important mechanistic tools. Our studies of Caco-2 monolayers demonstrate heterogeneity in transport characteristics depending on passage number and origin of the cells. In accordance with data obtained in animals and humans, TRH shows a carrier-mediated, saturable transport component, which operates parallel to a passive pathway in Caco-2 cells at passage number 89-99. At low TRH concentrations (< 3 mM) active transport becomes prominent, as demonstrated by the temperature dependence of TRH transport and inhibition experiments. The Michaelis-Menten parameters of the active, saturable transport component are: Km = 1.59 mM and Vmax = 1.84 microM/min. The pH optimum was determined to be at pH 6.0. On the other hand an exclusively paracellular passive route was found with Caco-2 cells at passage number 30-34. These results are also in agreement with observations made by others in cell culture experiments. The aspect of rigorously characterizing the specific Caco-2 clone under investigation is emphasized, especially when active transport mechanisms are suspected.

Intestinal absorption of azetirelin, a new thyrotropin-releasing hormone (TRH) analogue. I. Possible factors for the low oral bioavailability in rats

Absorption of azetirelin, a new thyrotropin-releasing hormone (TRH) analogue, from the gastrointestinal (GI) tract was evaluated. The bioavailability of this compound after oral administration was considerably poor in rats. Studies were undertaken to elucidate the mechanisms for this low oral bioavailability of azetirelin. The plasma azetirelin levels following intravenous and hepatoportal vein injection were virtually identical over the dose range of 0.02-0.1 mg/kg, indicating a minor contribution of the hepatic first-pass metabolism of this drug. Azetirelin was stable against peptide hydrolases both in luminal fluid and intestinal mucosal homogenates, whereas its degradation occurred when incubated with cecal contents under an anaerobic condition. In addition, complete degradation of azetirelin during the GI transit was disclosed by analyzing the fecal sample collected after oral administration of [14C]azetirelin. These results suggested that gut bacteria may be responsible for the hydrolysis of azetirelin in the GI tract. The low intestinal permeability of azetirelin was revealed by a modified everted gut experiment in various segments of the rat intestine. The poor membrane transport characteristics of azetirelin may be due to its high hydrophilicity. From these results, it was suggested that the insufficient oral bioavailability of azetirelin may be mainly attributed to its low intestinal permeability due to a lack of lipophilicity, and also to the degradation of the peptide by intestinal microflora.

A radioimmunoassay for TA-0910, a new metabolically stable thyrotrophin-releasing hormone analogue

TA-0910 [1-methyl-(S)-4,5-dihydroorotyl-L-histidyl-L-prolineamide] is a metabolically stable analogue of thyrotrophin-releasing hormone (TRH) and is under clinical investigation as a central nervous system function modulator. A method for determination of its plasma concentrations by radioimmunoassay (RIA) was established. TA-0910 was conjugated to bovine serum albumin and keyhole limpet haemocyanin (KLH) with bis-diazotized benzidine and 1,5-difluoro-2,4-dinitrobenzene as bridging agents. Anti-TA-0910 antisera were prepared by immunizing rabbits with the TA-0910 conjugates and Freund's complete adjuvant. The radiolabelled TA-0910 for RIA was prepared by introducing 125I into the histidine imidazole ring of TA-0910 by the Na125I/chloramine-T method, and purified by reversed phase high-performance liquid chromatography to give a specific radioactivity of 81.4 TBq mmole-1. As the result of testing the cross-reactivity of the antisera with assumed TA-0910 metabolites and TRH, a TA-0910-selective antiserum was obtained from a rabbit immunized with TA-0910-dinitrophenyl-KLH. RIA using this antiserum and the radiolabelled TA-0910 afforded a determination range of 10 pg approximately 5 ng ml-1 plasma. By using this RIA, the time courses of plasma concentrations of unchanged TA-0910 after oral and intravenous administration of TA-0910 were obtained in rats.

Controlled release of thyrotropin releasing hormone from microspheres: evaluation of release profiles and pharmacokinetics after subcutaneous administration

The drug-release kinetics of thyrotropin releasing hormone (TRH) containing copoly(dl-lactic/glycolic acid) (PLGA) microspheres were evaluated both in vitro and in vivo. The drug was encapsulated in PLGA using an in-water drying method through a water in oil in water emulsion. The drug release from the PLGA microspheres in vitro correlated well with that in vivo, and pseudo-zero-order release kinetics were observed. The pharmacokinetics of TRH following administration of this controlled-release parenteral dosage form have been also examined in rats. Following a transient increase in the plasma level due to an initial burst, steady-state plasma levels were observed. The duration of drug release estimated from the plasma level was comparable with the results in the in vitro and in vivo release studies. The steady-state plasma levels correlated well with the levels predicted from the pharmacokinetic parameters following a single subcutaneous or intravenous injection of TRH solution. The results of this study confirm the previously reported in vivo sustained release of TRH achieved with this drug-delivery system.

In vitro and in vivo evaluation of thyrotrophin releasing hormone release from copoly(dl-lactic/glycolic acid) microspheres

In vitro and in vivo release of thyrotrophin releasing hormone (TRH) from copoly(dl-lactic/glycolic acid) (PLGA) microspheres were evaluated. Factors affecting the TRH release from the microspheres were examined to clarify the release mechanisms by changing the medium composition in the in vitro release test. The hydrolysis rate of PLGA, the matrix-forming substance in the microspheres, was faster in acidic medium than in neutral medium. The release rate of TRH from the PLGA microspheres increased with the increase in the degradation rate of PLGA. A decrease in an osmolarity of the medium also caused an increase in the TRH release rate even though no significant change in PLGA degradation was observed. The effect of osmolarity appears to be characteristic of water-soluble drug-containing microspheres composed of hydrophobic polymer. The release rate of TRH from PLGA microspheres was largely affected by the medium composition in the in vitro release test. A proper choice of medium was found to be important for the estimation of in vivo release. The in vivo release rate of TRH from the PLGA microspheres following administration to rats correlated with the in vitro release in pH 7, 1/30 M buffer.

Thyrotropin-releasing hormone (TRH) uptake in intestinal brush-border membrane vesicles: comparison with proton-coupled dipeptide and Na(+)-coupled glucose transport

The mechanism of thyrotropin-releasing hormone (pGlu-His-Pro-NH2; TRH) uptake across the luminal membrane of intestinal enterocytes was investigated using brush-border membrane vesicles (BBMV) from rabbit duodenum and jejunum and rat upper small intestine. [14C]Glucose accumulated within the intestinal vesicles (at 10 sec), in the presence of an inwardly directed Na+ gradient, 7- to 14-fold higher than equilibrium values (65 min). The vesicles also accumulated the dipeptide [14C]Gly-Sar. Dipeptide uptake was greatest in the presence of both an inwardly directed proton gradient and an inside negative membrane potential. The H(+)-dependent Gly-Sar transport was not affected by the presence of an excess (46-fold) of cold TRH. In contrast to the observations with glucose and Gly-Sar, the uptake of [3H]TRH after 10 or 60 sec (in each of the vesicle preparations) was not enhanced by either Na+ or H+ gradient conditions. The absence of vesicular accumulation of TRH was not due to peptide hydrolysis. For example, after a 60-sec incubation with rabbit jejunal BBMV no degradation of the tripeptide was evident. After 65 min, 6% of [3H]TRH had undergone degradation, by deamidation, to form TRH-OH. These studies provide no evidence for the oral absorption of TRH by a Na(+)- or H(+)-dependent carrier system in the brush-border membrane. Previous observations of TRH absorption in vivo may be accounted for by passive absorption of the peptide combined with its relative resistance to luminal hydrolysis.

Passive transepithelial absorption of thyrotropin-releasing hormone (TRH) via a paracellular route in cultured intestinal and renal epithelial cell lines

Transport studies using intestinal brush-border membrane vesicles isolated from rats and rabbits have failed to demonstrate proton- or Na(+)-dependent carrier-mediated transport of thyrotropin-releasing hormone (TRH), despite a pharmacologically relevant oral bioavailability. To examine the hypothesis that reported levels of oral bioavailability reflect predominantly a paracellular rather than transcellular route for transepithelial transport of TRH, we have studied TRH transport in cultured epithelial cell types of intestinal (Caco-2 and T84) and renal (MDCK I, MDCK II, and LLC-PK1) origin, whose paracellular pathways span the range of permeability values observed in natural epithelia. Transport of TRH across monolayers of intestinal Caco-2 cells was similar to the flux of mannitol (approximately 1-4% per 4 hr), and unlike other putative substrates for the di-/tripeptide carrier, apical-to-basolateral transport was not increased by the presence of an acidic pH in the apical chamber. TRH transport did not show saturation, being uneffected in the presence of 20 mM cold TRH. In each cell type studied TRH and mannitol transport were similar and positively correlated with the conductance of the cell layers, consistent with a passive mechanism of absorption. This evidence suggests that, providing that a peptide is resistant to luminal hydrolysis, small but pharmacologically significant amounts of peptide absorption may be achieved by passive absorption across a paracellular route.

Thyrotropin-releasing hormone: pharmacokinetic and pharmacodynamic properties in chronic renal failure

The pharmacokinetics of thyrotropin-releasing hormone (TRH) were determined following a single i.v. administration in ten patients with chronic renal failure (CRF) maintained on chronic hemodialysis and in six normal subjects. A TRH-test (200 micrograms) was performed in all subjects on nondialysis days and was followed by sequential venous blood sampling at 0, 2, 5, 10, 20, 30 and 60 min. Plasma TRH and serum concentrations of TSH, T4, FT4 and T3 were measured by specific and sensitive RIA's. Serum thyroid hormone concentrations were lower in the hemodialysis patients than in the normals (p < 0.001). Basal TRH and TSH levels were similar in patients and in controls, however, a blunted response of TSH to TRH in CRF (3.8 +/- 2.4 vs. 11.2 +/- 2.6 mU/l, p < 0.001) was observed. Mean peak TRH concentrations (Cmax) were 34.445 (11.085, SD) fmoles/ml in CRF and only (13,400 (1.020) in the normals 2 min after TRH administration (tmax). The mean elimination half-life (t1/2) of TRH was 16 min in CRF and 6.5 min in normals (p < 0.001). The metabolic clearance rate (MCR) was markedly lowered in CRF, 58.3 (19.1) compared to normals (82.2 [15.3] l/m2/day, p < 0.001). The area under the plasma concentration-time curve (AUC) was 57.529 (28.562) fmoles.ml-1.min in CRF and 37.339 (5.026) (p < 0.005) in normals. These findings indicate that the pharmacokinetic properties of TRH are impaired in CRF. The kidney might be an important catabolic organ for exogenous TRH. Dosing schedules of TRH require possible adaptation to renal function.

Transport of thyrotropin releasing hormone in rabbit buccal mucosa in vitro

The transport of thyrotropin releasing hormone (TRH) in rabbit buccal mucosa in vitro has been investigated with respect to (a) rate and type of metabolism of TRH on mucosal and serosal sides of buccal mucosa, (b) mechanism of TRH transport including charge effect on its permeability, and (c) pathway and rate-limiting regions of TRH movement. In addition, the integrity of excised buccal mucosa has been evaluated for purposes of in vitro solute diffusion experiments using tissue ATP level data, transmission electron microscopy, and TRH transport kinetic data. The results indicate that excised rabbit buccal mucosa can be used for TRH diffusion studies for approximately 6 hr. In addition, TRH apparently traverses buccal mucosa by simple diffusion with a steady-state permeability of about 10(-7) cm/sec, and this permeability is independent of pH. Moreover, the primary pathway appears to be via the intercellular space in the rate-limiting barrier, i.e., the upper 50 microns of the epithelium. Finally, TRH is degraded predominantly by deamidase activity, which is followed by, to a lesser degree, carboxypeptidase metabolism.