Synthesis and interest in medicinal chemistry of ß-phenylalanine derivatives (ß-PAD): an update (2010-2022).

ß-Phenylalanine derivatives (ß-PAD) represent a structural family of therapeutic interest, either as components of drugs or as starting materials for access to key compounds. As scaffolds for medicinal chemistry work, ß-PAD offer the advantage of great diversity and modularity, a chiral pseudopeptidic character that opens up the capacity to be recognized by natural systems, and greater stability than natural α-amino acids. Nevertheless, their synthesis remains a challenge in drug discovery and numerous methods have been devoted to their preparation. This review is an update of the access routes to ß-PAD and their various therapeutic applications.

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Nα-Aroyl-N-Aryl-Phenylalanine Amides: A Promising Class of Antimycobacterial Agents Targeting the RNA Polymerase.

Tuberculosis (TB), caused by Mycobacterium tuberculosis, remains the leading cause of death from a bacterium in the world. The global prevalence of clinically relevant infections with opportunistically pathogenic non-tuberculous mycobacteria (NTM) has also been on the rise. Pharmacological treatment of both TB and NTM infections usually requires prolonged regimens of drug combinations, and is often challenging because of developed or inherent resistance to common antibiotic drugs. Medicinal chemistry efforts are thus needed to improve treatment options and therapeutic outcomes. Nα-aroyl-N-aryl-phenylalanine amides (AAPs) have been identified as potent antimycobacterial agents that target the RNA polymerase with a low probability of cross resistance to rifamycins, the clinically most important class of antibiotics known to inhibit the bacterial RNA polymerase. In this review, we describe recent developments in the field of AAPs, including synthesis, structural characterization, in vitro microbiological profiling, structure-activity relationships, physicochemical properties, pharmacokinetics and early cytotoxicity assessment.

Discovery of selective monosaccharide receptors via dynamic combinatorial chemistry.

The molecular recognition of saccharides by synthetic hosts has become an appealing but elusive task in the last decades. Herein, we combine Dynamic Combinatorial Chemistry (DCC) for the rapid self-assembly and screening of virtual libraries of receptors, with the use of ITC and NMR to validate the hits and molecular modelling to understand the binding mechanisms. We discovered a minimalistic receptor, 1F (N-benzyl-L-phenylalanine), with considerable affinity for fructose (Ka = 1762 M-1) and remarkable selectivity (>50-fold) over other common monosaccharides. The approach accelerates the discovery process of receptors for saccharides.

Phenylalanine-Based AMPA Receptor Antagonist as the Anticonvulsant Agent with Neuroprotective Activity-In Vitro and In Vivo Studies.

Trying to meet the multitarget-directed ligands strategy, a series of previously described aryl-substituted phenylalanine derivatives, reported as competitive antagonists of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, were screened in vitro for their free-radical scavenging and antioxidant capacity in two different assays: ferric reducing antioxidant power (FRAP) and oxygen radical absorbance capacity fluorescent (ORAC-FL) assays. The most active antioxidants 1 and 8 were further examined to evaluate their neuroprotective properties in vitro. In this study, compound 1 showed a significant neuroprotective effect against the neurotoxin 6-hydroxydopamine in neuroblastoma SH-SY5Y and IMR-32 cell lines. Both compounds also showed prevention from high levels of reactive oxygen species (ROS) in SH-SY5Y cells. Furthermore, the desired monoamine oxidase B (MAO-B) inhibition effect (IC50 = 278 ± 29 nM) for 1 was determined. No toxic effects up to 100 µM of 1 and 8 against neuroblastoma cells were observed. Furthermore, in vivo studies showed that compound 1 demonstrated significant anticonvulsant potential in 6-Hz test, but in neuropathic pain models its antiallodynic and antihyperalgesic properties were not observed. Concluding, the compound 1 seems to be of higher importance as a new phenylalanine-based lead candidate due to its confirmed promise in in vitro and in vivo anticonvulsant activity.

One-Step and Facile Synthesis of Poly(phenylalanine) as a Robust Drug Carrier for Enhanced Cancer Therapy.

In recent decades, many poly(amino acid)s have been successfully prepared for various biomedical applications. To date, the synthesis and purification procedures used to generate these poly(amino acid)s have generally been complicated and costly. Here, a one-step synthesis strategy was developed and optimized via direct polymerization using thionyl chloride to easily and economically obtain poly(amino acid)s. Phenylalanine (Phe) was selected as a model amino acid to construct a family of biodegradable and biocompatible poly(phenylalanine) (PPhe) molecules with a tunable molecular weight. The prepared PPhe can self-assemble into nanoparticles (PP-NPs) through nanoprecipitation with a particle size of approximately 100 nm. PP-NPs exhibit a high drug-loading capacity (>12 wt %) of paclitaxel (PTX, a commercial antitumor drug) and good therapeutic effects in CT26 cells. The in vivo evaluation of PTX@PP-NPs indicates that it has a prolonged blood circulation time and high tumor aggregation after intravenous injection, resulting in significant antitumor effects in CT26 tumor-bearing mice with minimal toxicity to normal organs. Overall, this study provides a facile and simple strategy for synthesizing poly(amino acids) and a PPhe-based nanoparticle platform for effectively delivering various small-molecule drugs.

Design, synthesis, and mechanism study of dimerized phenylalanine derivatives as novel HIV-1 capsid inhibitors.

HIV-1 capsid (CA) plays indispensable and multiple roles in the life cycle of HIV-1, become an attractive target in antiviral therapy. Herein, we report the design, synthesis, and mechanism study of a novel series of dimerized phenylalanine derivatives as HIV-1 capsid inhibitors using 2-piperazineone or 2,5-piperazinedione as a linker. The structure-activity relationship (SAR) indicated that dimerized phenylalanines were more potent than monomers of the same chemotype. Further, the inclusion of fluorine substituted phenylalanine and methoxyl substituted aniline was found to be beneficial for antiviral activity. From the synthesized series, Q-c4 was found to be the most potent compound with an EC50 value of 0.57 µM, comparable to PF74. Interestingly, Q-c4 demonstrated a slightly higher affinity to the CA monomer than the CA hexamer, commensurate with its more significant effect in the late-stage of the HIV-1 lifecycle. Competitive SPR experiments with peptides from CPSF6 and NUP153 revealed that Q-c4 binds to the interprotomer pocket of hexameric CA as designed. Single-round infection assays showed that Q-c4 interferes with the HIV-1 life cycle in a dual-stage manner, affecting both pre-and post-integration. Stability assays in human plasma and human liver microsomes indicated that although Q-c4 has improved stability over PF74, this kind of inhibitor still requires further optimization. And the results of the online molinspiration software predicted that Q-c4 has desirable physicochemical properties but some properties still have some violation from the Lipinski rule of five. Overall, the dimerized phenylalanines are promising novel platforms for developing future HIV-1 CA inhibitors with considerable potential for optimization.

Design, synthesis, and antiviral activity of phenylalanine derivatives as HIV-1 capsid inhibitors.

The HIV-1 Capsid (CA) is considered as a promising target for the development of potent antiviral drugs, due to its multiple roles during the viral life cycle. Herein, we report the design, synthesis, and antiviral activity evaluation of series of novel phenylalanine derivatives as HIV-1 CA protein inhibitors. Among them, 4-methoxy-N-methylaniline substituted phenylalanine (II-13c) and indolin-5-amine substituted phenylalanine (V-25i) displayed exceptional anti-HIV-1 activity with the EC50 value of 5.14 and 2.57 µM respectively, which is slightly weaker than that of lead compound PF-74 (EC50 = 0.42 µM). Besides, surface plasmon resonance (SPR) binding assay demonstrated II-13c and V-25i prefer to combine with CA hexamer rather than monomer, which is similar to PF-74. Subsequently, molecular dynamics simulation (MD) revealed potential interactions between representative compounds with HIV-1 CA hexamer. Overall, this work laid a solid foundation for further structural optimization to discover novel promising HIV-1 CA inhibitors.

Nucleophilic radiosynthesis of boron neutron capture therapy-oriented PET probe [18F]FBPA using aryldiboron precursors.

A reliable copper-mediated nucleophilic radiosynthesis of the PET imaging probe [18F]FBPA was developed using novel aryldiboron precursors. The carrier-free [18F]FBPA with radiochemical purity >99% was prepared routinely via the two-step synthesis with an automatic module and can be used for clinical PET imaging of tumours.

Synthesis, Enantiomeric Resolution and Biological Evaluation of HIV Capsid Inhibition Activity for Racemic, (S)- and (R)-PF74.

PF74 is a capsid-targeting inhibitor of HIV replication that effectively perturbs the highly sensitive viral uncoating process. A lack of information regarding the optical purity (enantiomeric excess) of the single stereogenic centre of PF74 has resulted in ambiguity as to the potency of different samples of this compound. Herein is described the synthesis of enantiomerically enriched (S)- and (R)-PF74 and further enrichment of the samples (≥98%) using chiral HPLC resolution. The biological activities of each enantiomer were then evaluated, which determined (S)-PF74 (IC50 1.5 µM) to be significantly more active than (R)-PF74 (IC50 19 µM). Computational docking studies were then conducted to rationalise this large discrepancy in activity, which indicated different binding conformations for each enantiomer. The binding energy of the conformation adopted by the more active (S)-PF74 (ΔG = -73.8 kcal/mol) was calculated to be more favourable than the conformation adopted by the less active (R)-enantiomer (ΔG = -55.8 kcal/mol) in agreement with experimental observations.

Generation of Fluorescent Versions of Saccharomyces cerevisiae RPA to Study the Conformational Dynamics of Its ssDNA-Binding Domains.

Replication protein A (RPA) is an essential single-stranded DNA (ssDNA)-binding protein that sequesters ssDNA and protects it from nucleolytic degradation. The RPA-ssDNA nucleoprotein acts as a hub to recruit over two dozen DNA metabolic enzymes onto ssDNA to coordinate DNA replication, repair, and recombination. RPA functions as a heterotrimer composed of RPA70, RPA32, and RPA14 subunits and has multiple DNA-binding and protein-interaction domains. Several of these domains are connected by disordered linkers allowing RPA to adopt a wide variety of conformations on ssDNA. Here we describe a fluorescence-based tool to monitor the dynamics of select DNA-binding domains of RPA. Noncanonical amino acids are utilized to site-specifically engineer fluorescent probes in Saccharomyces cerevisiae RPA heterologously expressed in BL21 (DE3) and its derivatives. A procedure to synthesize 4-azido-L-phenylalanine (4AZP), a noncanonical amino acid, is also described. Sites for fluorophore positioning that produce a measurable change in fluorescence upon binding to ssDNA are detailed. This fluorescence enhancement through noncanonical amino acid (FEncAA) approach can also be applied to other DNA-binding proteins to investigate the dynamics of protein-nucleic acid interactions.

A periodic development of BPA and BSH based derivatives in boron neutron capture therapy (BNCT).

Boron neutron capture therapy (BNCT) is a particular type of radiotherapy that requires a selective and high concentration of boron accumulation in neoplastic cells. To distinguish the distribution of boron compounds between tumour and normal cells, multiple research groups have been involved and successively innovated a wide variety of boron-based compounds. Despite the development of numerous boron compounds, only boronophenylalanine (BPA) and sodium mercaptoundecahydro-closo-dodecaborate (BSH) have emerged as effective in clinical trials. Here, we highlight the detailed progress in the molecular design of BPA and BSH derivatives from the historical perspective to the latest advances in light of the widely accepted performance required for effective BNCT. In this report, we have provided an overview of a variety of derivatives of BPA and BSH, including amino acids, peptides, polymers, monoclonal antibodies and chelated complexes, and it is observed that such derivatives of BPA and BSH are judicious choices for BNCT. Finally, we have summarised the critical issues for BPA and BSH that must be addressed if BNCT is to become a more widely accepted clinical modality.

Rational Design of 2-Substituted DMAP-N-oxides as Acyl Transfer Catalysts: Dynamic Kinetic Resolution of Azlactones.

A novel concept that conversion of chiral 2-substituted DMAP into its DMAP-N-oxide could significantly enhance the catalytic activity and still be used as an acyl transfer catalyst is presented. A new type of chiral 2-substituted DMAP-N-oxides, derived from l-prolinamides, has been rationally designed, facilely synthesized, and applied in the dynamic kinetic resolution of azlactones. Using simple MeOH as the nucleophile, various l-amino acid derivatives were produced in high yields (up to 98% yield) and enantioselectivities (up to 96% ee). Furthermore, α-deuterium labeled l-phenylalanine derivative was also obtained. Experiments and DFT calculations revealed that in 2-substituted DMAP-N-oxide, the oxygen atom acted as the nucleophilic site and the N-H bond functioned as the H-bond donor. High enantioselectivity of the reaction was governed by steric factors, and the addition of benzoic acid reduced the activation energy by participating in the construction of a H-bond bridge. The theoretical chemical study indicated that only when attack directions of the chiral catalyst were fully considered could the correct calculation results be obtained. This work paves the way for the utilization of the C2 position of the pyridine ring and the development of chiral 2-substituted DMAP-N-oxides as efficient acyl transfer catalysts.

Peptidyl Fluoromethyl Ketones and Their Applications in Medicinal Chemistry.

Peptidyl fluoromethyl ketones occupy a pivotal role in the current scenario of synthetic chemistry, thanks to their numerous applications as inhibitors of hydrolytic enzymes. The insertion of one or more fluorine atoms adjacent to a C-terminal ketone moiety greatly modifies the physicochemical properties of the overall substrate, especially by increasing the reactivity of this functionalized carbonyl group toward nucleophiles. The main application of these peptidyl α-fluorinated ketones in medicinal chemistry relies in their ability to strongly and selectively inhibit serine and cysteine proteases. These compounds can be used as probes to study the proteolytic activity of the aforementioned proteases and to elucidate their role in the insurgence and progress on several diseases. Likewise, if the fluorinated methyl ketone moiety is suitably connected to a peptidic backbone, it may confer to the resulting structure an excellent substrate peculiarity and the possibility of being recognized by a specific subclass of human or pathogenic proteases. Therefore, peptidyl fluoromethyl ketones are also currently highly exploited for the target-based design of compounds for the treatment of topical diseases such as various types of cancer and viral infections.

Facile synthesis of a novel genetically encodable fluorescent α-amino acid emitting greenish blue light.

We report the facile synthesis and characterization of a novel fluorescent α-amino acid 4-phenanthracen-9-yl-l-phenylalanine (Phen-AA) (5) that emits greenish blue light in the visible region. This genetically encodable l-α-amino acid has excellent photostability with a 75% quantum yield. It readily gets into human cells, being clearly imaged upon 405 nm laser excitation. The synthetic procedure is resistant to racemization and only involves three simple steps which use mild conditions and generate the Phen-AA in reasonably good yield. It may find broad applications in research, biotechnology, and the pharmaceutical industry.

Synthesis and Inhibitory Studies of Phosphonic Acid Analogues of Homophenylalanine and Phenylalanine towards Alanyl Aminopeptidases.

A library of novel phosphonic acid analogues of homophenylalanine and phenylalanine, containing fluorine and bromine atoms in the phenyl ring, have been synthesized. Their inhibitory properties against two important alanine aminopeptidases, of human (hAPN, CD13) and porcine (pAPN) origin, were evaluated. Enzymatic studies and comparison with literature data indicated the higher inhibitory potential of the homophenylalanine over phenylalanine derivatives towards both enzymes. Their inhibition constants were in the submicromolar range for hAPN and the micromolar range for pAPN, with 1-amino-3-(3-fluorophenyl) propylphosphonic acid (compound 15c) being one of the best low-molecular inhibitors of both enzymes. To the best of our knowledge, P1 homophenylalanine analogues are the most active inhibitors of the APN among phosphonic and phosphinic derivatives described in the literature. Therefore, they constitute interesting building blocks for the further design of chemically more complex inhibitors. Based on molecular modeling simulations and SAR (structure-activity relationship) analysis, the optimal architecture of enzyme-inhibitor complexes for hAPN and pAPN were determined.

Synthesis and anticancer activity of open-resorcinarene conjugates.

The first example of conjugation of open-resorcinarenes with chlorambucil, ibuprofen, naproxen and indomethacin are presented. The cytotoxic properties of the obtained conjugates were tested against the cancer cell lines U-251, PC-3, K-562, HCT-15, MCF-7 and SKLU-1. It was found that the conjugate with chlorambucil, naproxen or indomethacin (having 8 moieties) was toxic towards cancer cell lines U-251 and K-562, with no activity against non-cancerous COS-7 cells. The conjugates with naproxen and indomethacin showed high selectivity towards U-251 tumor cells.

Selective synthesis of L-2-[18 F]fluoro-alpha-methylphenylalanine via copper-mediated 18 F-fluorination of (mesityl)(aryl)iodonium salt.

L-2-[18 F]fluoro-alpha-methylphenylalanine (2-[18 F]FAMP) is a promising amino acid tracer for positron emission tomography (PET) imaging, yet the low production yield of direct electrophilic radiofluorination with [18 F]F2 necessitates further optimization of the radiolabeling process. This paper describes a two-step preparation method for L-2-[18 F]fluoro-alpha-methylphenylalanine (2-[18 F]FAMP) starting from [18 F]fluoride. The (Mesityl)(L-alpha-methylphenylalanine)-2-iodonium tetrafluoroborate precursors with various protecting groups were prepared. The copper-mediated 18 F-fluorination of the iodonium salt precursors successfully produced 2-[18 F]FAMP. The highest radio chemical conversion of 57.6% was noted with N-Piv-protected (mesityl)(aryl)iodonium salt in the presence of 5 equivalent of Cu (OTf)2 . Subsequent deprotection with 57% hydrogen iodide produced 2-[18 F]FAMP within 120 min in 21.4 ± 11.7% overall radiochemical yield with >95% radiochemical purity and an enantiomeric excess >99%. The obtained 2-[18 F]FAMP showed comparable biodistribution profiles in normal mice with that of the carrier-added 2-[18 F]FAMP. These results indicate that usefulness of copper mediated 18 F-fluorination for the production of 2-[18 F]FAMP, which would facilitate clinical translation of the promising tumor specific amino acid tracer. Individual facilities could adopt either production method based on radioactivity demand and equipment availability.

Design, synthesis and structure-activity relationships of 4-phenyl-1H-1,2,3-triazole phenylalanine derivatives as novel HIV-1 capsid inhibitors with promising antiviral activities.

HIV-1 CA is involved in different stages of the viral replication cycle, performing essential roles in both early (uncoating, reverse transcription, nuclear import, integration) and late events (assembly). Recent efforts have demonstrated HIV-1 CA protein as a prospective therapeutic target for the development of new antivirals. The most extensively studied CA inhibitor, PF-3450074 (PF-74, discovered by Pfizer), that targets an inter-protomer pocket within the CA hexamer. Herein we reported the design, synthesis, and biological evaluation of a series of 4-phenyl-1H-1,2,3-triazole phenylalanine derivatives as HIV-1 CA inhibitors based on PF-74 scaffold. Most of the analogues demonstrated potent antiviral activities, among them, the anti-HIV-1 activity of 6a-9 (EC50 = 3.13 µM) is particularly prominent. The SPR binding assay of selected compounds (6a-9, 6a-10, 5b) suggested direct and effective interaction with recombinant CA proteins. The mechanism of action studies also demonstrated that 6a-9 displays the effects in both the early and late stages of HIV-1 replication. To explore the potential binding mode of the here presented analogues, 6a-9 was analyzed by MD simulation to predict its binding to the active site of HIV-1 CA monomer. In conclusion, this novel series of antivirals can serve as a starting point for the development of a new generation of HIV-1 treatment regimen and highlights the potentiality of CA as a therapeutic target.

Solid-state synthesis of cyclo LD-diphenylalanine: A chiral phase built from achiral subunits.

The solid-state structure of LL/DD or LD/DL diphenylalanine diluted in KBr pellets is studied by infrared (IR) absorption and vibrational circular dichroism (VCD) spectroscopy. The structure depends on the absolute configuration of the residues. The natural LL diphenylalanine exists as a mixture of neutral and zwitterionic structures, depending on the humidity of the sample, while mostly the zwitterion is observed for LD diphenylalanine whatever the experimental conditions. The system undergoes spontaneous cyclization upon heating at 125°C, resulting to the formation of a diketopiperazine (DKP) dipeptide as the only product. The reaction is faster for LD than for LL diphenylalanine. As expected, LL and DD diphenylalanine react to form the LL and DD enantiomers of cyclo diphenylalanine. Interestingly, the DKP dipeptides formed from the LD or DL diphenylalanine show unexpected optical activity, with opposite VCD spectra for the products formed from the LD and DL reagents. This is explained in terms of chirality synchronization between the monomers within the crystal, which retain the symmetry of the reagent, resulting to the formation of a new chiral phase made from transiently chiral molecules.

Self-Assembly of Aromatic Amino Acid Enantiomers into Supramolecular Materials of High Rigidity.

Most natural biomolecules may exist in either of two enantiomeric forms. Although in nature, amino acid biopolymers are characterized by l-type homochirality, incorporation of d-amino acids in the design of self-assembling peptide motifs has been shown to significantly alter enzyme stability, conformation, self-assembly behavior, cytotoxicity, and even therapeutic activity. However, while functional metabolite assemblies are ubiquitous throughout nature and play numerous important roles including physiological, structural, or catalytic functions, the effect of chirality on the self-assembly nature and function of single amino acids is not yet explored. Herein, we investigated the self-assembly mechanism of amyloid-like structure formation by two aromatic amino acids, phenylalanine (Phe) and tryptophan (Trp), both previously found as extremely important for the nucleation and self-assembly of aggregation-prone peptide regions into functional structures. Employing d-enantiomers, we demonstrate the critical role that amino acid chirality plays in their self-assembly process. The kinetics and morphology of pure enantiomers is completely altered upon their coassembly, allowing to fabricate different nanostructures that are mechanically more robust. Using diverse experimental techniques, we reveal the different molecular arrangement and self-assembly mechanism of the dl-racemic mixtures that resulted in the formation of advanced supramolecular materials. This study provides a simple yet sophisticated engineering model for the fabrication of attractive materials with bionanotechnological applications.