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

Carisoprodol pharmacokinetics and distribution in the nucleus accumbens correlates with behavioral effects in rats independent from its metabolism to meprobamate

Carisoprodol (Soma®) is a centrally-acting skeletal-muscle relaxant frequently prescribed for treatment of acute musculoskeletal conditions. Carisoprodol's mechanism of action is unclear and is often ascribed to that of its active metabolite, meprobamate. The purpose of this study was to ascertain whether carisoprodol directly produces behavioral effects, or whether metabolism to meprobamate via cytochrome P450 (CYP450) enzymatic reaction is necessary. Rats were trained to discriminate carisoprodol (100 mg/kg) to assess time course and whether a CYP450 inhibitor (cimetidine) administered for 4 days would alter the discriminative effects of carisoprodol. Additionally, pharmacokinetics of carisoprodol and meprobamate with and without co-administration of cimetidine were assessed via in vivo microdialysis combined with liquid-chromatography-tandem mass spectrometry from blood and nucleus accumbens (NAc). The time course of the discriminative-stimulus effects of carisoprodol closely matched the time course of the levels of carisoprodol in blood and NAc, but did not match the time course of meprobamate. Administration of cimetidine increased levels of carisoprodol and decreased levels of meprobamate consistent with its interfering with metabolism of carisoprodol to meprobamate. However, cimetidine failed to alter the discriminative-stimulus effects of carisoprodol. Carisoprodol penetrated into brain tissue and directly produced behavioral effects without being metabolized to meprobamate. These findings indicate that understanding the mechanism of action of carisoprodol independently of meprobamate will be necessary to determine the validity of its clinical uses.

Synthesis and evaluation of in vivo anti-hypothermic effect of all stereoisomers of the thyrotropin-releasing hormone mimetic: Rovatirelin Hydrate

We discovered the orally active thyrotropin-releasing hormone (TRH) mimetic: (4S,5S)-5-methyl-N-{(2S)-1-[(2R)-2-methylpyrrolidin-1-yl]-1-oxo-3-(1,3-thiazol-4-yl)propan-2-yl}-2-oxo-1,3-oxazolidine-4-carboxamide 1 (rovatirelin). The central nervous system (CNS) effect of rovatirelin after intravenous (iv) administration is 100-fold higher than that of TRH. As 1 has four asymmetric carbons in its molecule, there are 16 stereoisomers. We synthesized and evaluated the anti-hypothermic effect of all stereoisomers of 1, which has the (4S),(5S),(2S),(2R) configuration from the N-terminus to the C-terminus, in order to clarify the structure-activity relationship (SAR) of stereoisomers. The (4R),(5R),(2R),(2S)-isomer 16 did not show any anti-hypothermic effect. Only the (4S),(5S),(2S),(2S)-isomer 10, which has the (2S)-2-methylpyrrolidine moiety at the C-terminus showed the anti-hypothermic effect similar to 1. Stereoisomers, which have the (5R) configuration of the oxazolidinone at the N-terminus and the (2R) configuration at the middle-part, showed a much lower anti-hypothermic effect than that of 1. On the other hand, stereoisomers, which have the (4R) configuration of the oxazolidinone at the N-terminus or the (2S) configuration of the C-terminus, have little influence on the anti-hypothermic effect.

Discovery of the Orally Effective Thyrotropin-Releasing Hormone Mimetic: 1-{N-[(4S,5S)-(5-Methyl-2-oxooxazolidine-4-yl)carbonyl]-3-(thiazol-4-yl)-l-alanyl}-(2R)-2-methylpyrrolidine Trihydrate (Rovatirelin Hydrate)

We have explored orally effective thyrotropin-releasing hormone (TRH) mimetics, showing oral bioavailability and brain penetration by structure-activity relationship (SAR) study on the basis of in vivo antagonistic activity on reserpine-induced hypothermia in mice. By primary screening of the synthesized TRH mimetics, we found a novel TRH mimetic: l-pyroglutamyl-[3-(thiazol-4-yl)-l-alanyl]-l-prolinamide with a high central nervous system effect compared with TRH as a lead compound. Further SAR optimization studies of this lead compound led to discovery of a novel orally effective TRH mimetic: 1-{N-[(4S,5S)-(5-methyl-2-oxooxazolidine-4-yl)carbonyl]-3-(thiazol-4-yl)-l-alanyl}-(2R)-2-methylpyrrolidine trihydrate (rovatirelin hydrate), which was selected as a candidate for clinical trials.

Mass spectrometric analysis of carisoprodol and meprobamate in rat brain microdialysates

We report the evaluation of several mass spectrometry-based methods for the determination of carisoprodol and meprobamate in samples obtained from the rat brain by in vivo intracranial microdialyis. Among the techniques that aspire to perform analyses without chromatographic separation and thereby increase throughput, chip-based nanoelectrospray ionization and the use of an atmospheric pressure solids analysis probe fell short of requirements because of insufficient detection sensitivity and hard ionization, respectively. Although direct analysis in real time provided the required soft ionization, shortcomings of a tandem mass spectrometry-based assay also included inadequate detection sensitivity and, in addition, poor quantitative reproducibility. Therefore, liquid chromatography coupled with atmospheric pressure chemical ionization tandem mass spectrometry was developed to determine carisoprodol and meprobamate from artificial cerebrospinal fluid as the medium. No desalting and/or extraction of the samples was necessary. The assay, combined with in vivo sampling via intracranial microdialyis, afforded time-resolved concentration profiles for the drug and its major metabolite from the nucleus accumbens region of the brain in rats after systemic administration of carisoprodol. Copyright © 2016 John Wiley & Sons, Ltd.

The prodrug DHED selectively delivers 17β-estradiol to the brain for treating estrogen-responsive disorders

Many neurological and psychiatric maladies originate from the deprivation of the human brain from estrogens. However, current hormone therapies cannot be used safely to treat these conditions commonly associated with menopause because of detrimental side effects in the periphery. The latter also prevents the use of the hormone for neuroprotection. We show that a small-molecule bioprecursor prodrug, 10β,17β-dihydroxyestra-1,4-dien-3-one (DHED), converts to 17β-estradiol in the brain after systemic administration but remains inert in the rest of the body. The localized and rapid formation of estrogen from the prodrug was revealed by a series of in vivo bioanalytical assays and through in vivo imaging in rodents. DHED treatment efficiently alleviated symptoms that originated from brain estrogen deficiency in animal models of surgical menopause and provided neuroprotection in a rat stroke model. Concomitantly, we determined that 17β-estradiol formed in the brain from DHED elicited changes in gene expression and neuronal morphology identical to those obtained after direct 17β-estradiol treatment. Together, complementary functional and mechanistic data show that our approach is highly relevant therapeutically, because administration of the prodrug selectively produces estrogen in the brain independently from the route of administration and treatment regimen. Therefore, peripheral responses associated with the use of systemic estrogens, such as stimulation of the uterus and estrogen-responsive tumor growth, were absent. Collectively, our brain-selective prodrug approach may safely provide estrogen neuroprotection and medicate neurological and psychiatric symptoms developing from estrogen deficiency, particularly those encountered after surgical menopause, without the adverse side effects of current hormone therapies.

Dihydroquinoline Carbamate Derivatives as "Bio-oxidizable" Prodrugs for Brain Delivery of Acetylcholinesterase Inhibitors: [¹¹C] Radiosynthesis and Biological Evaluation

With the aim of improving the efficiency of marketed acetylcholinesterase (AChE) inhibitors in the symptomatic treatment of Alzheimer's disease, plagued by adverse effects arising from peripheral cholinergic activation, this work reports a biological evaluation of new central AChE inhibitors based on an original "bio-oxidizable" prodrug strategy. After peripheral injection of the prodrug 1a [IC50 > 1 mM (hAChE)] in mice, monitoring markers of central and peripheral cholinergic activation provided in vivo proof-of-concept for brain delivery of the drug 2a [IC50 = 20 nM (hAChE)] through central redox activation of 1a. Interestingly, peripheral cholinergic activation has been shown to be limited in time, likely due to the presence of a permanent positive charge in 2a promoting rapid elimination of the AChE inhibitor from the circulation of mice. To support these assumptions, the radiosynthesis with carbon-11 of prodrug 1a was developed for additional ex vivo studies in rats. Whole-body biodistribution of radioactivity revealed high accumulation in excretory organs along with moderate but rapid brain uptake. Radio-HPLC analyses of brain samples confirm rapid CNS penetration of [(11)C]1a, while identification of [(11)C]2a and [(11)C]3a both accounts for central redox activation of 1a and pseudoirreversible inhibition of AChE, respectively. Finally, Caco-2 permeability assays predicted metabolite 3a as a substrate for efflux transporters (P-gp inter alia), suggesting that metabolite 3a might possibly be actively transported out of the brain. Overall, a large body of evidence from in vivo and ex vivo studies on small animals has been collected to validate this "bio-oxidizable" prodrug approach, emerging as a very promising strategy in the rational design of selective central AChE inhibitors.

Design and exploratory neuropharmacological evaluation of novel thyrotropin-releasing hormone analogs and their brain-targeting bioprecursor prodrugs

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

Mass spectrometric detection of neuropeptides using affinity-enhanced microdialysis with antibody-coated magnetic nanoparticles

Microdialysis (MD) is a useful sampling tool for many applications due to its ability to permit sampling from an animal concurrent with normal activity. MD is of particular importance in the field of neuroscience, in which it is used to sample neurotransmitters (NTs) while the animal is behaving in order to correlate dynamic changes in NTs with behavior. One important class of signaling molecules, the neuropeptides (NPs), however, presented significant challenges when studied with MD, due to the low relative recovery (RR) of NPs by this technique. Affinity-enhanced microdialysis (AE-MD) has previously been used to improve recovery of NPs and similar molecules. For AE-MD, an affinity agent (AA), such as an antibody-coated particle or free antibody, is added to the liquid perfusing the MD probe. This AA provides an additional mass transport driving force for analyte to pass through the dialysis membrane and thus increases the RR. In this work, a variety of AAs have been investigated for AE-MD of NPs in vitro and in vivo, including particles with C18 surface functionality and antibody-coated particles. Antibody-coated magnetic nanoparticles (AbMnP) provided the best RR enhancement in vitro, with statistically significant (p < 0.05) enhancements for 4 out of 6 NP standards tested, and RR increases up to 41-fold. These particles were then used for in vivo MD in the Jonah crab, Cancer borealis, during a feeding study, with mass spectrometric (MS) detection. 31 NPs were detected in a 30 min collection sample, compared to 17 when no AA was used. The use of AbMnP also increased the temporal resolution from 4 to 18 h in previous studies to just 30 min in this study. The levels of NPs detected were also sufficient for reliable quantitation with the MS system in use, permitting quantitative analysis of the concentration changes for 7 identified NPs on a 30 min time course during feeding.

Novel thyrotropin-releasing hormone analogs: a patent review

INTRODUCTION

The potential therapeutic applications of thyrotropin-releasing hormone (TRH) have attracted attention, based on its broad-spectrum neuropharmacological action rather than its endocrine properties. These central nervous system (CNS)-mediated effects provide the rationale for use of TRH and its analogs in the treatment of brain and spinal injury, and CNS disorders like schizophrenia, Alzheimer's disease, epilepsy, amyotrophic lateral sclerosis, Parkinson's disease, depression, shock and ischemia.

AREAS COVERED

This review summarizes the patent literature and advances in the discovery and development of novel TRH analogs over the past 20 years. It provides a comprehensive overview of the development of new TRH analogs, giving emphasis to their pharmaceutical profile.

EXPERT OPINION

The use of TRH in the treatment of various CNS disorders has been proven clinically. However, TRH itself is a poor drug candidate due to its short plasma half-life (5 min), poor biopharmaceutical properties (low intestinal and CNS permeability) and endocrine side effect. Nevertheless, researchers have come up with metabolically stable, more potent and selective TRH analogs and prodrugs. Taltirelin, one of the TRH analogs, has been approved under the trade name of Ceredist(®) in Japan for the treatment of spinocerebellar degeneration. Several other TRH analogs are in various stages of preclinical or clinical development.

Prodrug design for brain delivery of small- and medium-sized neuropeptides

The blood-brain barrier (BBB) represents multiple barriers for drug delivery from the circulation. Peptides potentially useful to treat maladies of the brain are especially limited in their ability to cross the BBB due to several shortcomings. Specific delivery strategies have been conceived to outwit the BBB to target neuropeptides into the brain. It should be noted, however, that no unified method is possible for true brain-targeting of these fascinating biomolecules due to their structural features, properties, and intricate interplays among factors governing their entrance into and retention within the brain. In most brain-targeting prodrug approaches, a lipophilic and bioreversible moiety(ies) is covalently attached to the peptide that results in the complete loss of the innate biological activity of the parent peptide (prodrugs are inactive per definition) but significantly improves brain uptake and metabolic stability in the plasma and the interstitial fluid. Once the peptide prodrug has crossed the BBB, specific enzymes liberate the parent agent from its prodrug in the brain. To illustrate the applicability of the prodrug strategy for brain delivery of small neuropeptides, pGlu-Glu-Pro-NH(2), [Glu(2)TRH], a thyrotropin-releasing hormone (TRH) analogue with a vast array of central activities, was chosen as an example. An ester prodrug provided significantly improved brain delivery compared to the unmodified parent peptide. The synthesis, in vitro and in vivo evaluations of this prodrug as specific examples are given for typical exploratory prodrug validation.

"All in the mind"? Brain-targeting chemical delivery system of 17β-estradiol (Estredox) produces significant uterotrophic side effect

Here we revisit the peculiarly named redox chemical delivery system concept. This unique prodrug approach has long been claimed to be capable of targeting 17β-estradiol (E2), which has numerous beneficial central effects, into the brain without detrimental peripheral hormonal exposure. Using a well-established protocol to monitor E2's antidepressant-like effect, we show that the administration of this chemical delivery system incorporated into hydroxypropyl-β-cyclodextrin (i.e., Estredox), indeed, triggers a transient antidepressant-like behavior in ovariectomized mice. At the same time, even an acute dose of the carefully purified chemical delivery system produces significant circulating E2 levels and uterotrophic side effects for several days after drug administration. For the first time, we also unequivocally show by liquid chromatography coupled with tandem mass spectrometry that the uterus of the Estredox-treated animals contains a large quantity of E2 compared to that of the control group. These thus far unexposed yet consequential peripheral side effects brought about by Estredox call for a thorough and unbiased reassessment of the extent of brain-targeting of the hormone via the chemical delivery system approach.

Protective effects of TRH and its analogues against various cytotoxic agents in retinoic acid (RA)-differentiated human neuroblastoma SH-SY5Y cells

TRH (thyroliberin) and its analogues were reported to possess neuroprotective effects in cellular and animal experimental models of acute and chronic neurodegenerative diseases. In the present study we evaluated effects of TRH and its three stable analogues, montirelin (CG-3703), RGH-2202 and Z-TRH (N-(carbobenzyloxy)-pGlutamyl-Histydyl-Proline) on the neuronally differentiated human neuroblastoma SH-SY5Y cell line, which is widely accepted for studying potential neuroprotectants. We found that TRH and all the tested analogues at concentrations 0.1-50 μM attenuated cell damage induced by MPP(+) (2 mM), 3-nitropropionate (10 mM), hydrogen peroxide (0.5 mM), homocysteine (250 μM) and beta-amyloid (20μM) in retinoic acid differentiated SH-SY5Y cells. Furthermore, we demonstrated that TRH and its analogues decreased the staurosporine (0.5 μM)-induced LDH release, caspase-3 activity and DNA fragmentation, which indicate the anti-apoptotic proprieties of these peptides. The neuroprotective effects of TRH (10 μM) and RGH-2202 (10 μM) on St-induced cell death was attenuated by inhibitors of PI3-K pathway (wortmannin and LY294002), but not MAPK/ERK1/2 (PD98059 and U0126). Moreover, TRH and its analogues at neuroprotective concentrations (1 and 10 μM) increased expression of Bcl-2 protein, as confirmed by Western blot analysis. All in all, these results extend data on neuroprotective properties of TRH and its analogues and provide evidence that mechanism of anti-apoptotic effects of these peptides in SH-SY5Y cell line involves induction of PI3K/Akt pathway and Bcl-2. Furthermore, the data obtained on human cell line with a dopaminergic phenotype suggest potential utility of TRH and its analogues in the treatment of some neurodegenerative diseases including Parkinson's disease.

[Glu2]TRH dose-dependently attenuates TRH-evoked analeptic effect in mice

Thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH(2)) and the structurally related [Glu(2)]TRH (pGlu-Glu-Pro-NH(2)) are endogenous peptides with a plethora of actions in the central nervous system. Many centrally-mediated effects of TRH are shared with those of [Glu(2)]TRH, although the involvement of different receptors is presumed. The analeptic action is the best-known TRH-related central nervous system effect. While [Glu(2)]TRH itself is analeptic, its co-administration with TRH into mice produced a dose-dependent attenuation of TRH-evoked reversal of barbiturate-induced sleeping time. This finding is in agreement with our previous observations that [Glu(2)]TRH significantly attenuates TRH-induced hippocampal extracellular acetylcholine release. Taken together, [Glu(2)]TRH may be considered as a negative modulator for the cholinergic effect of TRH in the mouse brain.

Effects of TRH and its analogues on primary cortical neuronal cell damage induced by various excitotoxic, necrotic and apoptotic agents

The tripeptide thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH2) has been shown to possess neuroprotective activity in in vitro and in vivo models. Since its potential utility is limited by relatively rapid metabolism, metabolically stabilized analogues have been constructed. In the present study we investigated the influence of TRH and its three stable analogues: Montirelin (MON, CG-3703), RGH-2202 (L-6-keto-piperidine-2carbonyl-l-leucyl-l-prolinamide) and Z-TRH (N-carbobenzyloxy-pGlutamyl-Histydyl-Proline) in various models of mouse cortical neuronal cell injury. Twenty four hour pre-treatment with TRH and its analogues in low micromolar concentrations attenuated the neuronal cell death evoked by excitatory amino acids (EAAs: glutamate, NMDA, kainate, quisqualate) and hydrogen peroxide. All the peptides showed neuroprotective action on staurosporine (St)-evoked apoptotic neuronal cell death, but this effect was caspase-3 independent. Interestingly, in mixed neuronal-glial cell preparations only MON decreased St- and glutamate-evoked neurotoxicity. None of the peptides inhibited the doxorubicin- and lactacystin-induced neuronal cortical cell death, agents acting via activation of death receptor (FAS) or inhibition of proteasome function, respectively. Furthermore, we found that neither inhibitors of PI3-K (wortmannin, LY 294002) nor MAPK/ERK1/2 (PD 098059, U 0126) were able to inhibit neuroprotective properties of TRH and MON in St model of apoptosis. The protection mediated by TRH and MON it that model was also not connected with influence of peptides on the pro-apoptotic GSK-3beta and JNK protein kinase expression and activity. Further studies showed that calpains, calcium-activated proteases were induced by Glu, but not by St in cortical neurons. Moreover, the Glu-evoked increase in spectrin alpha II cleavage product induced by calpains was blocked by TRH. The obtained data showed that the potency of TRH and its analogues in inhibiting EAAs- and H(2)O(2)-induced neuronal cell death from the highest to lowest activity was: MON>TRH>Z-TRH>RHG. Interestingly, all peptides were active against St-induced apoptosis, however, on concentration basis MON was far more potent than the other peptides. None of the peptides inhibited Dox- and LC-evoked apoptotic cell death. Additionally, the data exclude potential role of pro-survival (PI3-K/Akt and MAPK/ERK1/2) and pro-apoptotic (GSK-3beta and JNK) pathways in neuroprotective effects of TRH and its analogues on St-induced neuronal apoptosis. Moreover, the results point to involvement of the inhibition of calpains in the TRH neuroprotective effect in Glu model of neuronal cell death.

Prodrugs of thyrotropin-releasing hormone and related peptides as central nervous system agents

Prodrug design for brain delivery of small- and medium-sized neuropeptides was reviewed, focusing on thyrotropin-releasing hormone and structurally related peptides as examples. We have summarized our most important advances in methodology, as well as assessed the benefits and limitations of bioreversible chemical manipulation techniques to achieve targeting of the parent molecules into the central nervous system. The value of prodrug-amenable analogues as potential drug-like central nervous systems agents was highlighted.

Factors that contribute to the misidentification of tyrosine nitration by shotgun proteomics

The high selectivity and throughput of tandem mass spectrometry allow for rapid identification and localization of various posttranslational protein modifications from complex mixtures by shotgun approaches. Although sequence database search algorithms provide necessary support to process the potentially enormous quantity of MS/MS spectra generated from large scale tandem mass spectrometry experiments, false positive identifications of peptide modifications may exist even after implementation of stringent identification criteria. In this report, we describe factors that lead to misinterpretation of MS/MS spectra as well as common chemical and experimental artifacts that generate false positives using the proteomics-based identification of tyrosine nitration as an example. In addition to the proposed manual validation criteria, the importance of peptide synthesis and subsequent MS/MS characterization for validation of peptide nitration demonstrated by several examples from earlier publications is also presented.

Progress in drug delivery to the central nervous system by the prodrug approach

This review describes specific strategies for targeting to the central nervous system (CNS). Systemically administered drugs can reach the brain by crossing one of two physiological barriers resistant to free diffusion of most molecules from blood to CNS: the endothelial blood-brain barrier or the epithelial blood-cerebrospinal fluid barrier. These tissues constitute both transport and enzymatic barriers. The most common strategy for designing effective prodrugs relies on the increase of parent drug lipophilicity. However, increasing lipophilicity without a concomitant increase in rate and selectivity of prodrug bioconversion in the brain will result in failure. In these regards, consideration of the enzymes present in brain tissue and in the barriers is essential for a successful approach. Nasal administration of lipophilic prodrugs can be a promising alternative non-invasive route to improve brain targeting of the parent drugs due to fast absorption and rapid onset of drug action. The carrier-mediated absorption of drugs and prodrugs across epithelial and endothelial barriers is emerging as another novel trend in biotherapeutics. Several specific transporters have been identified in boundary tissues between blood and CNS compartments. Some of them are involved in the active supply of nutrients and have been used to explore prodrug approaches with improved brain delivery. The feasibility of CNS uptake of appropriately designed prodrugs via these transporters is described in detail.

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.

Novel neuroprotective tripeptides and dipeptides

It has long been recognized that thyrotropin-releasing hormone (TRH) and certain TRH analogues are neuroprotective in a variety of animal models of CNS trauma. In addition to these neuroprotective actions, TRH and most TRH analogues have other physiological actions that may not be desirable for treatment of acute injury, such as analeptic, autonomic, and endocrine effects. We have developed a series of dual-substituted TRH analogues that have strong neuroprotective actions, but are largely devoid of these other physiological actions. In addition, we have developed a family of cyclized dipeptides (diketopiperazines), structurally somewhat related to a metabolic product of TRH, that appear even more effective as neuroprotective agents in vitro and in vivo, and may have nootropic properties. Here, we review these novel tripeptide and dipeptide compounds.