Modulation of agonist stimulated adenylyl cyclase and GTPase activity by L-pro-L-leu-glycinamide and its peptidomimetic analogue in rat striatal membranes

Review on allosteric modulators of dopamine receptors so far

Background Contemporary research is predominantly directed towards allosteric modulators, a class of compounds designed to interact with specific sites distinct from the orthosteric site on G protein-coupled receptors. These allosteric modulators play a pivotal role in influencing diverse pharmacological effects, such as agonism/inverse agonism, efficacy modulation, and affinity modulation. One particularly intriguing aspect is the demonstrated capacity of allosteric modulation to enhance drug selectivity for therapeutic purposes, potentially leading to a reduction in serious side effects associated with traditional approaches. Allosteric ligands, a majority of which fall into the categories of negative allosteric modulators or positive allosteric modulators, exhibit the unique ability to either diminish or enhance the effects of endogenous ligands. Negative allosteric modulators weaken the response, while positive allosteric modulators intensify it. Additionally, silent allosteric modulators represent a distinct class that neither activates nor blocks the effects of endogenous ligands, adding complexity to the spectrum of allosteric modulation. In the broader context of central nervous system disorders, allosteric modulation takes center stage, particularly in the realm of dopamine receptors specifically, D1, D2, and D3 receptors. These receptors hold immense therapeutic potential for a range of conditions spanning neurodegenerative disorders to neurobehavioral and psychiatric disorders. The intricate modulation of dopamine receptors through allosteric mechanisms offers a nuanced and versatile approach to drug development. As research endeavors continue to unfold, the exploration of allosteric modulation stands as a promising frontier, holding the potential to reshape the landscape of drug discovery and therapeutic interventions in the field of neurology and psychiatry.

Stapling Amantadine to Melanostatin Neuropeptide: Discovery of Potent Positive Allosteric Modulators of the D2 Receptors

This work describes the synthesis and pharmacological and toxicological evaluation of melanostatin (MIF-1) bioconjugates with amantadine (Am) via a peptide linkage. The data from the functional assays at human dopamine D2 receptors (hD2R) showed that bioconjugates 1 (EC50 = 26.39 ± 3.37 nM) and 2 (EC50 = 17.82 ± 4.24 nM) promote a 3.3- and 4.9-fold increase of dopamine potency, respectively, at 0.01 nM, with no effect on the efficacy (Emax = 100%). In this assay, MIF-1 was only active at the highest concentration tested (EC50 = 23.64 ± 6.73 nM, at 1 nM). Cytotoxicity assays in differentiated SH-SY5Y cells showed that both MIF-1 (94.09 ± 5.75%, p < 0.05) and carbamate derivative 2 (89.73 ± 4.95%, p < 0.0001) exhibited mild but statistical significant toxicity (assessed through the MTT reduction assay) at 200 μM, while conjugate 1 was found nontoxic at this concentration.

Discovery of New Potent Positive Allosteric Modulators of Dopamine D2 Receptors: Insights into the Bioisosteric Replacement of Proline to 3-Furoic Acid in the Melanostatin Neuropeptide

The control of Parkinson's disease (PD) is challenged by the motor and non-motor fluctuations as well as dyskinesias associated with levodopa long-term therapy. As such, pharmacological alternatives to reduce the reliance on this drug are needed. Melanostatin (MIF-1), a positive allosteric modulator (PAM) of D₂ receptors (D₂R), is being explored as a novel pharmacological approach focused on D₂R potentiation. In this work, 3-furoic acid (3-Fu) was successfully employed as an l-proline (Pro) surrogate, affording two potent MIF-1 analogues, methyl 3-furoyl-l-leucylglycinate (4a) and 3-furoyl-l-leucylglycinamide (6a). In this series, the C-terminal carboxamide moiety was found crucial to enhancing the potency and toxicological profile, yet it is not considered a requisite for the PAM activity. Conformational analysis excludes 4a from adopting the claimed type II β-turn. The discovery and validation of 6a as a lead compound open a new avenue for the development of a novel class of anti-Parkinson therapeutics targeting the D₂R.

Synthesis, Pharmacological, and Biological Evaluation of 2-Furoyl-Based MIF-1 Peptidomimetics and the Development of a General-Purpose Model for Allosteric Modulators (ALLOPTML)

This work describes the synthesis and pharmacological evaluation of 2-furoyl-based Melanostatin (MIF-1) peptidomimetics as dopamine D₂ modulating agents. Eight novel peptidomimetics were tested for their ability to enhance the maximal effect of tritiated N-propylapomorphine ([³H]-NPA) at D₂ receptors (D₂R). In this series, 2-furoyl-l-leucylglycinamide (6a) produced a statistically significant increase in the maximal [³H]-NPA response at 10 pM (11 ± 1%), comparable to the effect of MIF-1 (18 ± 9%) at the same concentration. This result supports previous evidence that the replacement of proline residue by heteroaromatic scaffolds are tolerated at the allosteric binding site of MIF-1. Biological assays performed for peptidomimetic 6a using cortex neurons from 19-day-old Wistar-Kyoto rat embryos suggest that 6a displays no neurotoxicity up to 100 μM. Overall, the pharmacological and toxicological profile and the structural simplicity of 6a makes this peptidomimetic a potential lead compound for further development and optimization, paving the way for the development of novel modulating agents of D₂R suitable for the treatment of CNS-related diseases. Additionally, the pharmacological and biological data herein reported, along with >20 000 outcomes of preclinical assays, was used to seek a general model to predict the allosteric modulatory potential of molecular candidates for a myriad of target receptors, organisms, cell lines, and biological activity parameters based on perturbation theory (PT) ideas and machine learning (ML) techniques, abbreviated as ALLOPTML. By doing so, ALLOPTML shows high specificity Sp = 89.2/89.4%, sensitivity Sn = 71.3/72.2%, and accuracy Ac = 86.1%/86.4% in training/validation series, respectively. To the best of our knowledge, ALLOPTML is the first general-purpose chemoinformatic tool using a PTML-based model for the multioutput and multicondition prediction of allosteric compounds, which is expected to save both time and resources during the early drug discovery of allosteric modulators.

The Dopamine Allosteric Agent, PAOPA, Demonstrates Therapeutic Potential in the Phencyclidine NMDA Pre-clinical Rat Model of Schizophrenia

PAOPA, a potent analog of prolyl-leucyl-glycinamide, has shown therapeutic potential at the preclinical stage for dopaminergic related illnesses, including animal models of schizophrenia, Parkinson’s disease and haloperidol-induced extrapyramidal movement disorders. PAOPA’s unique allosteric mechanism and dopamine D2 receptor specificity provide a unique composition of properties for the development of potential therapeutics for neuropsychiatric illnesses. We sought to investigate PAOPA’s therapeutic prospects across the spectrum of schizophrenia-like symptoms represented in the established phencyclidine-induced rat model of schizophrenia, (5 mg/kg PCP twice daily for 7 days, followed by 7 days of drug withdrawal). PAOPA was assessed for its effect on brain metabolic activity and across a battery of behavioral tests including, hyperlocomotion, social withdrawal, sensorimotor gating, and novel object recognition. PAOPA showed therapeutic efficacy in behavioral paradigms representing the negative (social withdrawal) and cognitive-like (novel object recognition) symptoms of schizophrenia. Interestingly, some behavioral indices associated with the positive symptoms of schizophrenia that were ameliorated in PAOPA’s prior examination in the amphetamine-sensitized model of schizophrenia were not ameliorated in the PCP model; suggesting that the deficits induced by amphetamine and PCP—while phenotypically similar—are mechanistically different and that PAOPA’s effects are restricted to certain mechanisms and systems. These studies provide insight on the potential use of PAOPA for the safe and effective treatment of schizophrenia.

Allosteric and biased g protein-coupled receptor signaling regulation: potentials for new therapeutics

G protein-coupled receptors (GPCRs) are seven-transmembrane proteins that participate in many aspects of the endocrine function and are important targets for drug development. They transduce signals mainly, but not exclusively, via hetero-trimeric G proteins, leading to a diversity of intracellular signaling cascades. Ligands binding at the hormone orthosteric sites of receptors have been classified as agonists, antagonists, and/or inverse agonists based on their ability to mainly modulate G protein signaling. Accumulating evidence also indicates that such ligands, alone or in combination with other ones such as those acting outside the orthosteric hormone binding sites (e.g., allosteric modulators), have the ability to selectively engage subsets of signaling responses as compared to the natural endogenous ligands. Such modes of functioning have been variously referred to as "functional selectivity" or "ligand-biased signaling." In this review, we provide an overview of the current knowledge regarding GPCR-biased signaling and their functional regulation with a focus on the evolving concept that receptor domains can also be targeted to allosterically bias signaling, and discuss the usefulness of such modes of regulation for the design of more efficient therapeutics.

PAOPA, a potent dopamine D2 receptor allosteric modulator, prevents and reverses behavioral and biochemical abnormalities in an amphetamine-sensitized preclinical animal model of schizophrenia

Allosteric modulators are emerging as new therapeutics for the treatment of psychiatric illnesses, such as schizophrenia. Conventional antipsychotic drugs are typically dopamine D2 receptor antagonists that compete with endogenous dopamine at the orthosteric site, and block excessive dopamine neurotransmission in the brain. However, they are unable to treat all symptoms of schizophrenia and often cause adverse motor and metabolic side effects. The binding profile of allosteric modulators differs, as they interact with their receptor at a novel binding site and their activity is determined by physiological signaling. In collaboration, our laboratories have synthesized and evaluated over 185 compounds for their allosteric modulatory activity at the dopamine D2 receptor. Of these compounds, PAOPA is among the most potent allosteric modulators, and has been shown to be effective in treating the MK-801 induced preclinical animal model of schizophrenia. The objective of this study was to evaluate PAOPA's ability to prevent and reverse behavioral abnormalities in an amphetamine-sensitized preclinical animal model of schizophrenia. Amphetamine sensitized rats were given PAOPA during sensitization and following sensitization to determine whether PAOPA is able to prevent and reverse behavioral abnormalities. Furthermore, changes in post-mortem dopamine levels were measured by high performance liquid chromatography in various brain regions. The results presented demonstrate that PAOPA is able to prevent and reverse behavioral and biochemical abnormalities in an amphetamine-sensitized animal model of schizophrenia.

Specific binding of photoaffinity-labeling peptidomimetics of Pro-Leu-Gly-NH2 to the dopamine D2L receptor: evidence for the allosteric modulation of the dopamine receptor

The present study was undertaken to investigate the mechanistic role of l-prolyl-l-leucyl-glycinamide (PLG) in modulating agonist binding to the dopamine D(2L) receptor. Competition and displacement assays indicate that the photoaffinity-labeling peptidomimetics of PLG, 3(R)-[(4(S)-(4-azido-2-hydroxy-benzoyl) amino-2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide hydrochloride (1a) and 3(R)-[(4(S)-(4-azido-2-hydroxy-5-iodo-benzoyl)amino-2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide hydrochloride (1b) bind at the same site as PLG. Autoradiography was used to establish the covalent binding of [(125)I]-1b to an approximately 51kDa protein in bovine striatal membranes. Western blot analysis with a dopamine D(2L)-specific antibody, in combination with autoradiography, following a two-dimensional gel separation, suggested this approximately 51kDa protein to be the dopamine D(2L) receptor. Further evidence for binding of 1b to dopamine D(2L) was provided by samples immunoprecipitated with the D(2L) antibody. These samples were analyzed by western blotting in parallel with autoradiography of [(125)I]-1b labeled protein. Both methods revealed bands at approximately 51kDa. Furthermore, PLG is shown to compete with 1b for binding to the dopamine D(2L) receptor as determined by autoradiography, as well as competition experiments with PLG and 1a. Collectively, these findings suggest the successful development of a photoaffinity-labeling agent, compound 1b, that has been used to elucidate the interaction of PLG specifically with the dopamine D(2L) receptor.

Synthesis and evaluation of novel pyridine based PLG tripeptidomimetics

Analogues of the pyridine based PLG (Pro-Leu-Gly-NH(2)) peptidomimetic were synthesized and evaluated as dopamine modulating agents. Modifications in the position corresponding to the leucine side chain in PLG afforded derivatives , and , substituted with H, Me and Bn instead of the isobutyl group, respectively. Changes in the proline residue produced derivative , substituted with a symmetrical piperidine ring instead of the pyrrolidine ring and , in which the pyrrolidine ring is connected to the pyridine ring via a hydroxymethyl group instead of a keto function. The peptidomimetics were tested for their ability to enhance the maximal effect of N-propylapomorphine (NPA) at dopamine D2 receptors in the functional cell-based R-SAT assay. Compounds , , and , produced a statistically significant increase in the maximal NPA response at 10 nM (117 +/- 6%, 118 +/- 6%, and 116 +/- 3%, respectively), which is similar to the effect of PLG in this assay, whereas was able to potentiate the response to a similar extent at 1 nM concentration (115 +/- 5%). All derivatives produced a bell-shaped dose-response curve and none of the compounds were active at the D2 receptor alone, which indicates that the mechanism behind the activity of both the pyridine based mimetics and PLG is the same. Interestingly, l-Pro-d-Leu-Gly-NH(2) was found to be more potent than PLG and produced a 119 +/- 1% increase in the NPA response at 1 nM.

Allosteric Modulation of G Protein–Coupled Receptors

The past decade has witnessed a significant growth in the identification of allosteric modulators of G protein-coupled receptors (GPCRs), i.e., ligands that interact with binding sites that are topographically distinct from the orthosteric site recognized by the receptor's endogenous agonist. Because of their ability to modulate receptor conformations in the presence of orthosteric ligand, allosteric modulators can "fine-tune" classical pharmacological responses. This is advantageous in terms of a potential for engendering greater GPCR subtype-selectivity, but represents a significant challenge for detecting and validating allosteric behaviors. Although allosteric sites need not have evolved to accommodate endogenous ligands, there are a number of examples of where such modulators have been shown to contribute to physiological or pathophysiological processes. Studies are also beginning to unravel the structural basis of allosteric modulation of GPCRs. It remains to be determined whether such modulation represents interactions within monomers versus across dimers.

Design and synthesis of photoaffinity-labeling ligands of the L-prolyl-L-leucylglycinamide binding site involved in the allosteric modulation of the dopamine receptor

Pro-Leu-Gly-NH(2) (PLG), in addition to its endocrine effects, possesses the ability to modulate dopamine D(2) receptors within the central nervous system. However, the precise binding site of PLG is unknown. Potential photoaffinity-labeling ligands of the PLG binding site were designed as tools to be used in the identification of the macromolecule that possesses this binding site. Six different photoaffinity-labeling ligands were designed and synthesized on the basis of the gamma-lactam PLG peptidomimetic 1. The 4-azidobenzoyl and 4-azido-2-hydroxybenzoyl photoaffinity-labeling moieties were placed at opposite ends of PLG peptidomimetic 1 to generate a series of ligands that potentially could be used to map the PLG binding site. All of the compounds that were synthesized possessed activity comparable to or better than PLG in enhancing [(3)H]-N-propylnorapomorphine agonist binding to dopamine receptors. Photoaffinity ligands that were cross-linked to the receptor preparation produced a modulatory effect that was either comparable to or greater than the increase in agonist binding produced by the respective ligands that were not cross-linked to the dopamine receptor. The results indicate that these photoaffinity-labeling agents are binding at the same allosteric site as PLG and PLG peptidomimetic 1.

Development of a New Family of Conformationally Restricted Peptides as Potent Nucleators of β-Turns. Design, Synthesis, Structure, and Biological Evaluation of a β-Lactam Peptide Analogue of Melanostatin

Novel enantiopure (i)-(beta-lactam)-(Gly)-(i+3) peptide models, defined by the presence of a central alpha-alkyl-alpha-amino-beta-lactam ring placed as the (i+1) residue, have been synthesized in a totally stereocontrolled way by alpha-alkylation of suitable N-[bis(trimethylsilyl)methyl]-beta-lactams. The structural properties of these beta-lactam pseudopeptides have been studied by X-ray crystallography, Molecular Dynamics simulation, and NOESY-restrained NMR simulated annealing techniques, showing a strong tendency to form stable type II or type II' beta-turns either in the solid state or in highly coordinating DMSO solutions. Tetrapeptide models containing syn- or anti-alpha,beta-dialkyl-alpha-amino-beta-lactam rings have also been synthesized and their conformations analyzed, revealing that alpha-alkyl substitution is essential for beta-turn stabilization. A beta-lactam analogue of melanostatin (PLG amide) has also been prepared, characterized as a type-II beta-turn in DMSO-d6 solution, and tested by competitive binding assay as a dopaminergic D2 modulator in rat neuron cultured cells, displaying moderate agonist activity in the micromolar concentration range. On the basis of these results, a novel peptidomimetic design concept, based on the separation of constraint and recognition elements, is proposed.

Iso-lactam and reduced amide analogues of the peptidomimetic dopamine receptor modulator 3(R)-[(2(S)-pyrrolidinylcarbonyl)amino]-2-oxo-1-pyrrolidineacetamide

An analogue of the highly potent gamma-lactam Pro-Leu-Gly-NH(2) peptidomimetic, 3(R)-[(2(S)-pyrrolidinylcarbonyl) amino]-2-oxo-1-pyrrolidineacetamide (2), 4(R)-[[2(S)-pyrrolidinylcarbonyl]amino]-2-oxo-1-pyrrolidineacetamide (3), in which the lactam carbonyl moiety has been placed in a different position with respect to the 3-amino group was synthesized. Also, a series of analogues of 2, compounds 4-6, were synthesized in which each of the amide bonds of 2 were systematically replaced with a reduced amide bond surrogate. The analogues were tested for their ability to enhance the binding of [3H]N-propylnorapomorphine to dopamine receptors in a functional in vitro assay utilizing bovine striatal membranes. Peptidomimetic 3 was shown to be more potent than 2, while 4 and 5 were significantly less effective than 2. Peptidomimetic 6 had a pharmacological profile similar to that of 2.

Pro-Leu-glycinamide and its peptidomimetic, PAOPA, attenuate haloperidol induced vacuous chewing movements in rat: A model of human tardive dyskinesia

In the present experimental paradigm, we examine the effect of L-prolyl-L-leucyl-glycinamide (PLG) co-administration with haloperidol on vacuous chewing movements (VCM) in rats-a model of tardive dyskinesia (TD) in humans. We examined the dose dependent induction of VCM through both injected and orally administered PLG (MIF-1). Our results show significant levels of VCM attenuation (P<0.05) in rats treated with 10mg/kg of PLG. Doses of 1 and 100mg/kg were ineffective. Reductions were present in both orally treated and injected rats. We also examined the therapeutic effect of a peptidomimetic of PLG-PAOPA. PAOPA was able to produce similar behavioral effects to PLG at a dose, which was 100-fold lower than the effective dose of PLG. These results suggest that PLG may play a role in D2 receptor expression and function, as well as providing a therapy for neuroleptic induced TD.