Synthesis and allosteric modulation of the dopamine receptor by peptide analogs of l-prolyl-l-leucyl-glycinamide (PLG) modified in the l-proline or l-proline and l-leucine scaffolds

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.

Copper catalyzed Shono-type oxidation of proline residues in peptide

Since the initial report in 1975, the Shono oxidation has become a powerful tool to functionalize the α position of amines, including proline derivatives, by electrochemical oxidation. However, the application of electrochemical Shono oxidations is restricted to the preparation of simple building blocks and homogeneous Shono-type oxidation of proline derivatives remains challenging. The late-stage functionalization at proline residues embedded within peptides is highly important as substitutions about the proline ring are known to affect biological and pharmacological activities. Here, we show that homogenous copper-catalyzed oxidation conditions complement the Shono oxidation and this general protocol can be applied to a series of formal C-C coupling reactions with a variety of nucleophiles using a one-pot procedure. This protocol shows good tolerance toward 19 proteinogenic amino acids and was used to functionalize several representative bioactive peptides, including captopril, enalapril, Smac, and endomorphin-2. Last, peptide cyclization can also be achieved by using an appropriately positioned side-chain hydroxyl moiety.

Concise Overview of Glypromate Neuropeptide Research: From Chemistry to Pharmacological Applications in Neurosciences

Neurodegenerative diseases of the central nervous system (CNS) pose a serious health concern worldwide, with a particular incidence in developed countries as a result of life expectancy increase and the absence of restorative treatments. Presently, treatments for these neurological conditions are focused on managing the symptoms and/or slowing down their progression. As so, the research on novel neuroprotective drugs is of high interest. Glypromate (glycyl-l-prolyl-l-glutamic acid, also known as GPE), an endogenous small peptide widespread in the brain, holds great promise to tackle neurodegenerative diseases such as Parkinson's, Alzheimer's, and Huntington's, s well as other CNS-related disorders like Rett and Down's syndromes. However, the limited pharmacokinetic properties of Glypromate hinder its clinical application. As such, intense research has been devoted to leveraging the pharmacokinetic profile of this neuropeptide. This review aims to offer an updated perspective on Glypromate research by exploring the vast array of chemical derivatizations of more than 100 analogs described in the literature over the past two decades. The collection and discussion of the most relevant structure-activity relationships will hopefully guide the discovery of new Glypromate-based neuroprotective drugs.

Allosteric Modulators of Dopamine D2 Receptors for Fine-Tuning of Dopaminergic Neurotransmission in CNS Diseases: Overview, Pharmacology, Structural Aspects and Synthesis

Allosteric modulation of G protein-coupled receptors (GPCRs) is nowadays a hot topic in medicinal chemistry. Allosteric modulators, i.e., compounds which bind in a receptor site topologically distinct from orthosteric sites, exhibit a number of advantages. They are more selective, safer and display a ceiling effect which prevents overdosing. Allosteric modulators of dopamine D₂ receptor are potential drugs against a number of psychiatric and neurological diseases, such as schizophrenia and Parkinson's disease. In this review, an insightful summary of current research on D₂ receptor modulators is presented, ranging from their pharmacology and structural aspects of ligand-receptor interactions to their synthesis.

A Convenient On-Site Oxidation Strategy for the N-Hydroxylation of Melanostatin Neuropeptide Using Cope Elimination

A convenient synthetic protocol for the unprecedented N-hydroxylation of proline residue in Melanostatin (MIF-1) neuropeptide is reported. This methodology is grounded on the incorporation of N-(cyanoethyl)prolyl residue followed by on-site oxidation by Cope elimination with m-chloroperbenzoic acid, exploring the unrecognized dual role of the cyanoethyl group as an effective N-protecting group under peptide synthesis conditions and as a suitable leaving group during the chemoselective on-site N-oxidation. Following this protocol N-hydroxy-MIF-1 is obtained in 78% global yield from N-(cyanoethyl)-l-proline. This synthetic approach opens a new avenue for access to N-hydroxylated Melanostatin analogues with direct application in neurochemistry and Parkinson’s research.

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.

Synthesis, Pharmacological, and Biological Evaluation of MIF-1 Picolinoyl Peptidomimetics as Positive Allosteric Modulators of D2R

This work describes the synthesis and pharmacological evaluation of picolinoyl-based peptidomimetics of melanocyte stimulating hormone release inhibiting factor 1 (MIF-1) as dopamine modulating agents. Eight novel peptidomimetics were tested for their ability to enhance the maximal effect of tritiated N-propylapomorphine ([³H]-NPA) at dopamine D₂ receptors (D₂R). Methyl picolinoyl-l-valyl-l-alaninate (compound 6b) produced a statistically significant increase in the maximal [³H]-NPA response at 0.01 nM (11.9 ± 3.7%), which is close to the effect of MIF-1 in this assay at same concentration (18.3 ± 9.1%). Functional assays measuring cAMP mobilization in the presence of dopamine corroborate the activity of peptidomimetic 6b as a positive allosteric modulator (PAM) of D₂R. In this assay, 6b produced a typical bell-shaped dose-response curve similar to that of the parent neuropeptide (18.3 ± 7.1% for 6b vs 15.4 ± 5.5% for MIF-1, both at 0.1 nM). Dose-response curves for dopamine in the presence of 6b show EC₅₀ (0.33 ± 0.21 μM for 6b vs 0.17 ± 0.07 μM for MIF-1) and E_max (86.0 ± 5.4% for 6b vs 93.6 ± 4.4% for MIF-1) comparable to those of MIF-1, both at 0.01 nM. Furthermore, peptidomimetic 6b was tested for agonist activity at the human D₂R and the results show that it displays no intrinsic agonism effect, endorsing its activity as a PAM of D₂R. Cytotoxic and neurotoxic assays were performed for peptidomimetic 6b using HEK 293T cells and cortex neurons from 19 day old Wistar-Kyoto rat embryos, respectively, suggesting this analogue displays no toxicity effect in these assays up to 100 μM. Conformational energy minimization for 6b shows that this peptidomimetic cannot adopt the postulated type-II β-turn bioactive conformation, endorsing the possibility of an extended bioactive conformation as claimed by other researchers as a second bioactive conformation of MIF-1. Overall, the pharmacological and toxicological profile of peptidomimetic 6b together with its favorable druglike properties and structural simplicity makes it a potential lead compound for further development and optimization.

Allosteric Modulation of Class A GPCRs: Targets, Agents, and Emerging Concepts

G-protein-coupled receptors (GPCRs) have been tractable drug targets for decades with over one-third of currently marketed drugs targeting GPCRs. Of these, the class A GPCR superfamily is highly represented, and continued drug discovery for this family of receptors may provide novel therapeutics for a vast range of diseases. GPCR allosteric modulation is an innovative targeting approach that broadens the available small molecule toolbox and is proving to be a viable drug discovery strategy, as evidenced by recent FDA approvals and clinical trials. Numerous class A GPCR allosteric modulators have been discovered recently, and emerging trends such as the availability of GPCR crystal structures, diverse functional assays, and structure-based computational approaches are improving optimization and development. This Perspective provides an update on allosterically targeted class A GPCRs and their disease indications and the medicinal chemistry approaches toward novel allosteric modulators and highlights emerging trends and opportunities in the field.

Mechanistic insights for the transprotection of tertiary amines with Boc2O via charged carbamates: access to both enantiomers of 2-azanorbornane-3-exo-carboxylic acids

Development of a synthetic methodology for the selective transprotection in hindered tertiary amines using Boc₂O under N–C hydrogenolysis catalyzed by Pd/C: access to both enantiomers of 2-azanorbornane-3-exo-carboxylates.

Perturbation Theory/Machine Learning Model of ChEMBL Data for Dopamine Targets: Docking, Synthesis, and Assay of New l-Prolyl-l-leucyl-glycinamide Peptidomimetics

Predicting drug-protein interactions (DPIs) for target proteins involved in dopamine pathways is a very important goal in medicinal chemistry. We can tackle this problem using Molecular Docking or Machine Learning (ML) models for one specific protein. Unfortunately, these models fail to account for large and complex big data sets of preclinical assays reported in public databases. This includes multiple conditions of assays, such as different experimental parameters, biological assays, target proteins, cell lines, organism of the target, or organism of assay. On the other hand, perturbation theory (PT) models allow us to predict the properties of a query compound or molecular system in experimental assays with multiple boundary conditions based on a previously known case of reference. In this work, we report the first PTML (PT + ML) study of a large ChEMBL data set of preclinical assays of compounds targeting dopamine pathway proteins. The best PTML model found predicts 50000 cases with accuracy of 70-91% in training and external validation series. We also compared the linear PTML model with alternative PTML models trained with multiple nonlinear methods (artificial neural network (ANN), Random Forest, Deep Learning, etc.). Some of the nonlinear methods outperform the linear model but at the cost of a notable increment of the complexity of the model. We illustrated the practical use of the new model with a proof-of-concept theoretical-experimental study. We reported for the first time the organic synthesis, chemical characterization, and pharmacological assay of a new series of l-prolyl-l-leucyl-glycinamide (PLG) peptidomimetic compounds. In addition, we performed a molecular docking study for some of these compounds with the software Vina AutoDock. The work ends with a PTML model predictive study of the outcomes of the new compounds in a large number of assays. Therefore, this study offers a new computational methodology for predicting the outcome for any compound in new assays. This PTML method focuses on the prediction with a simple linear model of multiple pharmacological parameters (IC₅₀, EC₅₀, K_i, etc.) for compounds in assays involving different cell lines used, organisms of the protein target, or organism of assay for proteins in the dopamine pathway.

Trifluoromethylated proline analogues as efficient tools to enhance the hydrophobicity and to promote passive diffusion transport of the l-prolyl-l-leucyl glycinamide (PLG) tripeptide

The synthesis of four CF3-proline analogues of the PLG peptide is reported. Our results show that the incorporation of trifluoromethylated amino acids (Tfm-AAs) at the N-terminal position of a peptide significantly increases its hydrophobicity. In addition, depending on the relative configuration and the position of the CF3 group, Tfm-AAs can also promote passive diffusion transport.

Highly efficient one-pot assembly of peptides by double chemoselective coupling

An efficient and faster methodology for the assembly of oligopeptides by a three-step one-pot protocol with high yields and virtually no epimerization.

Novel l-prolyl-l-leucylglycinamide (PLG) tripeptidomimetics based on a 2-azanorbornane scaffold as positive allosteric modulators of the D2R

Replacement of l-prolyl residue in the PLG sequence by an enantiopure (1R,3S,4S)-2-azanorbornane scaffold afforded active peptidomimetics compatible with suppression of the C-terminal carboxamide pharmacophore.