Brain-Targeted Delivery of a Leucine-enkephalin Analogue by Retrometabolic Design^†

A brain-targeted chemical delivery system (CDS) based on retrometabolic drug design was applied to a Leu-enkephalin analogue, Tyr-d-Ala-Gly-Phe-d-Leu (DADLE). The molecular architecture of the peptide CDS disguises its peptide nature from neuropeptide-degrading enzymes and provides lipophilic, bioreversible functions for the penetration through the blood−brain barrier. These functions were provided by a targetor, a 1,4-dihydrotrigonellyl group, on the N-terminus and a bulky, lipophilic ester group on the C-terminus. A spacer amino acid residue was also inserted between the targetor and the parent peptide to assure the release of DADLE by specific enzymes. Four CDSs were synthesized by segment-coupling method that proved to be superior to sequential elongation in obtaining this type of peptide conjugates. Intravenous injection of the compounds produced a significant and long-lasting response in rats monitored by the tail-flick latency measurements. CDSs having the bulkier cholesteryl group showed a better efficacy than those having the smaller 1-adamantaneethyl ester. The spacer was the most important factor to manipulate the rate of DADLE release and, thus, the pharmacological activity; proline as a spacer produced more potent analgesia than alanine. The antinociceptive effect of the CDSs was naloxone-reversible and methylnaloxonium-irreversible, confirming that central opiate receptors were solely responsible for mediating analgesia induced by the peptide CDS that delivered, retained, and then released the peptide in the brain

10β,17α-Dihydroxyestra-1,4-dien-3-one: A Bioprecursor Prodrug Preferentially Producing 17α-Estradiol in the Brain for Targeted Neurotherapy

Uterotrophic effect of 17α-estradiol, the C17 epimer of the main human estrogen 17β-estradiol, was shown to manifest in animal models at doses lower than those necessary for central outcome raising concerns about its potential to treat maladies of the central nervous system. We introduce here 10β,17α-dihydroxyestra-1,4-dien-3-one (α-DHED) that acts as a bioprecursor prodrug producing 17α-estradiol with remarkable selectivity to the brain and, therefore, without appreciable exposure of the periphery to the parent steroid. This distinguishing feature of α-DHED is shown by using an estrogen-responsive mouse model with complementary LC-MS/MS measurement of drug contents in target tissues. Our data warrant further research to fully establish the potential of α-DHED for a safe and efficacious 17α-estradiol-based neurotherapy

Application of Screening Experimental Designs to Assess Chromatographic Isotope Effect upon Isotope-Coded Derivatization for Quantitative Liquid Chromatography–Mass Spectrometry

Isotope effect may cause partial chromatographic separation of labeled (heavy) and unlabeled (light) isotopologue pairs. Together with a simultaneous matrix effect, this could lead to unacceptable accuracy in quantitative liquid chromatography–mass spectrometry assays, especially when electrospray ionization is used. Four biologically relevant reactive aldehydes (acrolein, malondialdehyde, 4-hydroxy-2-nonenal, and 4-oxo-2-nonenal) were derivatized with light or heavy (<i>d</i>₃-, ¹³C₆-, ¹⁵N₂-, or ¹⁵N₄-labeled) 2,4-dinitrophenylhydrazine and used as model compounds to evaluate chromatographic isotope effects. For comprehensive assessment of retention time differences between light/heavy pairs under various gradient reversed-phase liquid chromatography conditions, major chromatographic parameters (stationary phase, mobile phase pH, temperature, organic solvent, and gradient slope) and different isotope labelings were addressed by multiple-factor screening using experimental designs that included both asymmetrical (Addelman) and Plackett–Burman schemes followed by statistical evaluations. Results confirmed that the most effective approach to avoid chromatographic isotope effect is the use of ¹⁵N or ¹³C labeling instead of deuterium labeling, while chromatographic parameters had no general influence. Comparison of the alternate isotope-coded derivatization assay (AIDA) using deuterium versus ¹⁵N labeling gave unacceptable differences (>15%) upon quantifying some of the model aldehydes from biological matrixes. On the basis of our results, we recommend the modification of the AIDA protocol by replacing <i>d</i>₃-2,4-dinitrophenylhydrazine with ¹⁵N- or ¹³C-labeled derivatizing reagent to avoid possible unfavorable consequences of chromatographic isotope effects

Characterization of 4-Hydroxy-2-nonenal-Modified Peptides by Liquid Chromatography−Tandem Mass Spectrometry Using Data-Dependent Acquisition: Neutral Loss-Driven MS³ versus Neutral Loss-Driven Electron Capture Dissociation

Reactive oxygen species generated during oxidative stress can lead to unfavorable cellular consequences, predominantly due to formation of 4-hydroxy-2-nonenal (HNE) during lipid peroxidation. Data-dependent and neutral loss (NL)-driven MS3 acquisition have been reported for the identification of HNE adducts by mass spectrometry-based proteomics. However, the limitation associated with this method is the ambiguity in correct assignment of the HNE modification site when more than one candidate site is present as MS3 is triggered on the neutral loss ion. We introduce NL-triggered electron capture dissociation tandem mass spectrometry (NL-ECD-MS/MS) for the characterization of HNE-modification sites in peptides. With this method performed using a hybrid linear ion trap-Fourier transform ion cyclotron resonance (FTICR) mass spectrometer, ECD in the FTICR unit of the instrument is initiated on precursor ions of peptides showing the neutral loss of 156 Da corresponding to an HNE molecule in the prescan acquired via collision-induced dissociation tandem mass spectrometry in the linear ion trap. In addition to manifold advantages associated with the ECD method of backbone fragmentation, including extensive sequence fragments, ECD tends to retain the HNE group during MS/MS of the precursor ion, facilitating the correct localization of the modification site. The results also suggest that predisposition of a peptide molecular ion to lose HNE during collision-induced dissociation-based fragmentation is independent of its charge state (2+ or 3+). In addition, we have demonstrated that coupling of solid-phase enrichment of HNE-modified peptides facilitates the detection of this posttranslational modification by NL-driven strategies for low-abundance proteins that are susceptible to substoichiometric carbonylation during oxidative stress

Rapid Label-Free Identification of Estrogen-Induced Differential Protein Expression <i>In Vivo</i> from Mouse Brain and Uterine Tissue

Protein abundance profiling from tissue using liquid chromatography−tandem mass spectrometry-based “shotgun” proteomics and label-free relative quantitation was evaluated for the investigation of estrogen-regulated protein expression in the mouse brain and uterus. Sample preparation involved a 30-min protein extraction in 8 M aqueous urea solution, followed by disulfide reduction, thiol alkylation, and trypsin digestion of the extracted proteins, and was performed on 3−4 mg of tissue to evaluate the suitability of this methodology to expedite the survey of cellular pathways that are affected in vivo by an experimental therapeutic intervention in an animal model. The label-free proteomic approach (spectral counting) was suitable to identify even subtle changes in cortical protein levels and revealed significant estrogen-induced upregulation of ATP synthase (both α- and β-isoforms), aspartate aminotransferase 2, and mitochondrial malate dehydrogenase without any prior subcellular fractionation of the tissue or the use of multidimensional chromatographic separation. The methodology was also suitable to observe various up- and downregulated proteins in the uterine tissue of ovariectomized mice upon treatment with 17β-estradiol. In addition to confirming a very significant decrease in the abundance of glutathione S-transferase recognized as a marker of estrogen’s impact, our studies have also revealed potential new protein markers such as desmin and lumican that are critical components of cytoskeletal arrangement and, hence, regulation of their abundance could contribute to major morphological changes in the uterus occurring upon estrogenic stimulation

Combinatorial Lead Optimization of a Neuropeptide FF Antagonist

The tripeptide Pro-Gln-Arg-NH2, derivatized at the secondary amino group of the proline residue with 5-(dimethylamino)-1-naphthalenesulfonyl (dansyl-PQR-NH2), antagonizes the central anti-opioid action of neuropeptide FF in animals after systemic administration and, therefore, is a therapeutic lead to treat opiate withdrawal. For a combinatorial optimization to improve potency, libraries focused on the possible replacement of the proline and glutamine residues of this lead compound were obtained by a solid-phase split-and-mix method using coded amino acids (excluding cysteine) as building blocks. After screening for competitive binding against a radioiodinated neuropeptide FF analogue, 5-(dimethylamino)-1-naphthalenesulfonyl-Gly-Ser-Arg-NH2 (dansyl-GSR-NH2) has emerged as one of the compounds in the library with high affinity to the NPFF receptor and even with a moderate increase compared to dansyl-PQR-NH2 in its predicted ability to penetrate the central nervous system

Proteomic Analysis of Mouse Brain Microsomes: Identification and Bioinformatic Characterization of Endoplasmic Reticulum Proteins in the Mammalian Central Nervous System

The endoplasmic reticulum (ER) is the main source for the storage and release of intracellular calcium in neurons and, thus, contributes to the functionality of a diverse set of pathways that control critical aspects of central nervous system function including but not limited to gene expression, neurotransmission, learning, and memory. ER-derived proteins obtained after subcellular fractionation of mouse brain homogenate were digested with trypsin and the corresponding peptides fractionated by strong cation exchange chromatography followed by LC-MS/MS analysis on a hybrid linear ion trap−Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. A comprehensive catalogue representing 1914 proteins was generated from this particular proteomic analysis using identification criteria that corresponded to a false positive identification rate of 0.4%. Various molecular functions and biological processes relevant to the ER were identified upon gene ontology (GO)-based analysis including pathways associated with molecular transport, protein trafficking and localization, and cell signaling. Comparison of the 2D-LC-MS/MS results with those obtained from shotgun LC-MS/MS analyses demonstrated that most molecular functions and biological processes were represented via GO analysis using either methodology. Results from this comparison as well as a focused investigation into components of calcium-mediated signaling in the mouse brain ER are also presented

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

Synthesis and Biological Evaluation of 17β-Alkoxyestra-1,3,5(10)-trienes as Potential Neuroprotectants Against Oxidative Stress

17β-O-Alkyl ethers (methyl, ethyl, propyl, butyl, hexyl, and octyl) of estradiol were obtained from 3-O-benzyl-17β-estradiol with sodium hydride/alkyl halide, followed by the removal of the O-benzyl protecting group via catalytic transfer hydrogenation. An increase compared to estradiol in the protection of neural (HT-22) cells against oxidative stress due to exposure of glutamate was furnished by higher (C-3 to C-8) alkyl ethers, while methyl and ethyl ethers decreased the neuroprotective effect significantly. Lipophilic (butyl and octyl) ethers blocking the phenolic hydroxyl (3-OH) of A-ring were inactive

Metabolism-Based Brain-Targeting System for a Thyrotropin-Releasing Hormone Analogue^†

Gln-Leu-Pro-Gly, a progenitor sequence for the thyrotropin-releasing hormone (TRH) analogue [Leu2]TRH (pGlu-Leu-Pro-NH2), was covalently and bioreversibly modified on its N- and C-termini (by a 1,4-dihydrotrigonellyl and a cholesteryl group, respectively) to create lipoidal brain-targeting systems for the TRH analogue. The mechanism of targeting and the recovery of the parent peptide at the target site involve several enzymatic steps, including the oxidation of the 1,4-dihydropyridine moiety. Due to the lipid insolublity of the peptide pyridinium conjugate obtained after this reaction, one of the rudimentary steps of brain targeting (i.e., trapping in the central nervous system) can be accomplished. Our design also included spacer amino acid(s) inserted between the N-terminal residue of the progenitor sequence and the dihydrotrigonellyl group to facilitate the posttargeting removal of the attached modification. The release of the TRH analogue in the brain is orchestrated by a sequential metabolism utilizing esterase/lipase, peptidyl glycine α-amidating monooxygenase (PAM), peptidase cleavage, and glutaminyl cyclase. In addition to in vitro experiments to prove the designed mechanism of action, the efficacy of brain targeting for [Leu2]TRH administered in the form of chemical-targeting systems containing the embedded progenitor sequence was monitored by the antagonistic effect of the peptide on the barbiturate-induced anesthesia (measure of the activational effect on cholinergic neurons) in mice, and considerable improvement was achieved over the efficacy of the parent peptide upon using this paradigm