Searching for new cell-penetrating agents: hybrid cyclobutane-proline γ,γ-peptides

A non-hydrolysable peptidomimetic for mitochondrial targeting

Peptidomimetics, molecules that mimic the activity of natural peptides with improved stability or bioavailability, have emerged as interesting materials with applications in biomedicine. In this study, we describe a hybrid γ,γ-peptidomimetic that efficiently aims at mitochondria, a key therapeutic target associated with several disorders, in living cells. Peptide backbones with a component of cationic and hydrophobic amino acids have been shown to preferentially target mitochondria due to their high negative membrane potential and hydrophobic character of the membranous invaginations of these key organelles. We here exploit the advantageous bioorthogonal properties of a peptidomimetic scaffold that consists of an alternation of (1S,3R)-3-amino-2,2-dimethylcyclobutane-1-carboxylic acid and an Nα-functionalised cis-γ-amino-L-proline derivative. This peptidomimetic exhibited excellent membrane translocation efficiency, mitochondrial targeting ability, and biocompatibility. Mitochondrial targeting was confirmed to be dependent on the electrochemical potential generated by the electron transport chain. The presence of non-natural amino acids rendered the compound exceptionally stable in the presence of proteases, maintaining its integrity and functionality for targeting the organelle even after 1 week of incubation in serum. This stability, coupled with its targeting abilities and the low cytosolic/endosomal residual signal, facilitated the tracking of relevant mitochondrial dynamics, including fission events and intracellular movement. Additionally, this peptidomimetic scaffold allowed the sustained and precise mitochondrial targeting of a pH sensitive ratiometric probe, 5(6)-carboxy-SNARF-1, which enabled mitochondrial pH monitoring. In summary, our study introduces a biomimetic peptide with exceptional mitochondria-targeting properties, ensuring stability in biological media and offering insights into crucial mitochondrial processes.

Panchromatic Absorbers Tethered for Bioconjugation or Surface Attachment

The syntheses of two triads are reported. Each triad is composed of two perylene-monoimides linked to a porphyrin via an ethyne unit, which bridges the perylene 9-position and a porphyrin 5- or 15-position. Each triad also contains a single tether composed of an alkynoic acid or an isophthalate unit. Each triad provides panchromatic absorption (350–700 nm) with fluorescence emission in the near-infrared region (733 or 743 nm; fluorescence quantum yield ~0.2). The syntheses rely on the preparation of trans-AB-porphyrins bearing one site for tether attachment (A), an aryl group (B), and two open meso-positions. The AB-porphyrins were prepared by the condensation of a 1,9-diformyldipyrromethane and a dipyrromethane. The installation of the two perylene-monoimide groups was achieved upon the 5,15-dibromination of the porphyrin and the subsequent copper-free Sonogashira coupling, which was accomplished before or after the attachment of the tether. The syntheses provide relatively straightforward access to a panchromatic absorber for use in bioconjugation or surface-attachment processes.

Guanidinium-Functionalized Flexible Azaproline Transporter for Efficient Intracellular Delivery of Proapoptotic Peptide and PDL1 Antisense Morpholino Oligo in Human Carcinoma Cells In Vitro

Cell-penetrating peptides (CPPs) are structurally diverse sophisticated tools endowed with high arginine content, amphipathicity, and well-adopted suitable secondary structures. Despite its capability of breaching the lipid barriers, CPP has major limitations such as in vivo metabolic instability, poor bioavailability, and reduced endosomal escape tendency, which are yet to be improved. In this context, we first have introduced a new class of cellular transporter having a guanidinium-functionalized δ-azaproline (δ-azp)-containing peptide where the δ-azp structurally resembles the "proline" amino acid having an additional "N" at the δ-position. This non-natural peptidic backbone was found to impart proteolytic stability, as reported earlier by our group. Herein, we report the synthesis of a flexible azaproline-tetraguanidinium transporter named FAT along with a revised scalable methodology for δ-azp compared to our previously reported procedure. FAT shows a random-coil-like structure as determined by CD spectroscopy, and is hence structurally different from the polyproline PPII helix. Direct translocation is predicted to be the possible mode of the cellular entrance of FAT into CHO cells when the "Bodipy" fluorophore is covalently attached as the cargo. Simultaneously, two other macromolecular therapeutics, e.g., proapoptotic domain peptide (PAD, a 14-mer peptide) and programmed death ligand 1 (PDL1) morpholino (a 25-mer antisense oligo), were successfully conjugated with FAT and delivered into human carcinoma cells, and their efficacy was analyzed by MTT assay and western blot technique, respectively. Having obtained promising results in internalizing different types of cargos, FAT could be envisaged as a potential drug delivery agent as an alternative to natural CPPs for future application.

Hybrid Cyclobutane/Proline-Containing Peptidomimetics: The Conformational Constraint Influences Their Cell-Penetration Ability

A new family of hybrid β,γ-peptidomimetics consisting of a repetitive unit formed by a chiral cyclobutane-containing trans-β-amino acid plus a N^α-functionalized trans-γ-amino-l-proline joined in alternation were synthesized and evaluated as cell penetrating peptides (CPP). They lack toxicity on the human tumoral cell line HeLa, with an almost negligible cell uptake. The dodecapeptide showed a substantial microbicidal activity on Leishmania parasites at 50 µM but with a modest intracellular accumulation. Their previously published γ,γ-homologues, with a cyclobutane γ-amino acid, showed a well-defined secondary structure with an average inter-guanidinium distance of 8–10 Å, a higher leishmanicidal activity as well as a significant intracellular accumulation. The presence of a very rigid cyclobutane β-amino acid in the peptide backbone precludes the acquisition of a defined conformation suitable for their cell uptake ability. Our results unveiled the preorganized charge-display as a relevant parameter, additional to the separation among the charged groups as previously described. The data herein reinforce the relevance of these descriptors in the design of CPPs with improved properties.

Chiral Cyclobutane-Containing Cell-Penetrating Peptides as Selective Vectors for Anti-Leishmania Drug Delivery Systems

Two series of new hybrid γ/γ-peptides, γ-CC and γ-CT, formed by (1S,2R)-3-amino-2,2,dimethylcyclobutane-1-carboxylic acid joined in alternation to a Nα-functionalized cis- or trans-γ-amino-l-proline derivative, respectively, have been synthesized and evaluated as cell penetrating peptides (CPP) and as selective vectors for anti-Leishmania drug delivery systems (DDS). They lacked cytotoxicity on the tumoral human cell line HeLa with a moderate cell-uptake on these cells. In contrast, both γ-CC and γ-CT tetradecamers were microbicidal on the protozoan parasite Leishmania beyond 25 μM, with significant intracellular accumulation. They were conjugated to fluorescent doxorubicin (Dox) as a standard drug showing toxicity beyond 1 μM, while free Dox was not toxic. Intracellular accumulation was 2.5 higher than with Dox-TAT conjugate (TAT = transactivator of transcription, taken as a standard CPP). The conformational structure of the conjugates was approached both by circular dichroism spectroscopy and molecular dynamics simulations. Altogether, computational calculations predict that the drug-γ-peptide conjugates adopt conformations that bury the Dox moiety into a cavity of the folded peptide, while the positively charged guanidinium groups face the solvent. The favorable charge/hydrophobicity balance in these CPP improves the solubility of Dox in aqueous media, as well as translocation across cell membranes, making them promising candidates for DDS.

Cyclobutane-Containing Scaffolds as Useful Intermediates in the Stereoselective Synthesis of Suitable Candidates for Biomedical Purposes: Surfactants, Gelators and Metal Cation Ligands

Efficient and versatile synthetic methodologies are reported for the preparation of products that are suitable candidates to be used as surfactants, gelators for hydroxylic solvents or metal cation ligands, with potential use in several fields including biomedical applications. The common structural feature of all the synthesized products is the presence of a cis or trans-1,2- or cis-1,3-difunctionalized cyclobutane ring. In the two first cases, the key intermediates including enantiomerically pure 1,3-diamines and 1,3-amino alcohols have been prepared from β-amino acid derivatives obtained, in turn, from a chiral half-ester. This compound is also precursor of γ-amino esters. Furthermore, two kind of polydentate ligands have also been synthesized from a symmetric 1,5-diamine obtained from norpinic acid, which was easily prepared from commercial verbenone.

Hetero-intermolecular [2+2] photocycloaddition of 1,4-dihydropyridines: a combined experimental and DFT study

A detailed DFT mechanistic study of hetero-intermolecular [2+2] photocycloaddition of 1,4-DHPs was carried out for investigating the regioselectivity of such transformation.

Stereoselectivity of Proline/Cyclobutane Amino Acid-Containing Peptide Organocatalysts for Asymmetric Aldol Additions: A Rationale

Several α,β,α- or α,γ,α-tripeptides, consisting of a central cyclobutane β- or γ-amino acid being flanked by two d- or l-proline residues, have been synthesized and tested as organocatalysts in asymmetric aldol additions. High yields and enantioselectivities have been achieved with α,γ,α-tripeptides, being superior to peptides containing a cyclobutane β-amino acid residue. This is probably due to their high rigidity, which hinders some of the peptide catalysts to adopt the proper active conformation. This reasoning correlates with the major conformation of the peptides in the ground state, as suggested by ¹H NMR and computational calculations. The configuration of the aldol products is controlled by the proline chirality, and consequently, the R/S configuration of aldol products can be tuned by the use of either commercially available d- or l-proline. The enantioselectivity in the aldol reactions is reversed if the reactions are carried out in the presence of water or other protic solvents such as methanol. Spectroscopic and theoretical investigations revealed that this effect is not the consequence of conformational changes in the catalyst but rather caused by the participation of a water molecule in the rate determining transition state, in such a way that the preferential nucleophilic attack is oriented to the opposite enantiotopic aldehyde face.

A convenient solid-phase strategy for the synthesis of antimicrobial cyclic lipopeptides

A concise solid-phase synthesis of cyclic lipopeptides derived from the antimicrobial peptide c(Lys-Lys-Leu-Lys-Lys-Phe-Lys-Lys-Leu-Gln) (BPC194) was accomplished. Three different synthetic routes were explored. Best results were obtained using a protocol that includes as key steps: (i) synthesis of the cyclic peptidyl resin incorporating the Lys residue to be acylated protected at the N(ε)-amino group with an ivDde group, (ii) selective removal of the ivDde group, and (iii) acylation. These compounds were screened for their in vitro growth inhibition of bacterial and fungal phytopathogens and for their cytotoxic effects on eukaryotic cells. A sequence with high antimicrobial activity and low hemolysis was identified, constituting a good candidate for the design of new antimicrobial agents.