Rational Design of a Highly Potent and Selective Peptide Inhibitor of PACE4 by Salt Bridge Interaction with D160 at Position P3

Enhancing the Drug-Like Profile of a Potent Peptide PACE4 Inhibitor by the Formation of a Host-Guest Inclusion Complex with β-Cyclodextrin

The enzyme PACE4 has been validated as a promising therapeutic target to expand the range of prostate cancer (PCa) treatments. In recent years, we have developed a potent peptidomimetic inhibitor, namely, compound C23 (Ac-(DLeu)LLLRVK-4-amidinobenzylamide). Like many peptides, C23 suffers from an unfavorable drug-like profile which, despite our efforts, has not yet benefited from the usual SAR studies. Hence, we turned our attention toward a novel formulation strategy, i.e., the use of cyclodextrins (CDs). CDs can benefit compounds through the formation of "host-guest" complexes, shielding the guest from degradation and enhancing biological survival. In this study, a series of βCD-C23 complexes have been generated and their properties evaluated, including potency toward the enzyme in vitro, a cell-based proliferation assay, and stability in plasma. As a result, a new βCD-formulated lead compound has been identified, which, in addition to being more soluble and more potent, also showed an improved stability profile.

Understanding the Role of Intramolecular Ion-Pair Interactions in Conformational Stability Using an Ab Initio Thermodynamic Cycle

Intramolecular ion-pair interactions yield shape and functionality to many molecules. With proper orientation, these interactions overcome steric factors and are responsible for the compact structures of several peptides. In this study, we present a thermodynamic cycle based on isoelectronic and alchemical mutation to estimate the intramolecular ion-pair interaction energy. We determine these energies for 26 benchmark molecules with common ion-pair combinations and compare them with results obtained using intramolecular symmetry-adapted perturbation theory. For systems with long linkers, the ion-pair energies evaluated using both approaches deviate by less than 2.5% in the vacuum phase. The thermodynamic cycle based on density functional theory facilitates calculations of salt-bridge interactions in model tripeptides with continuum/microsolvation modeling and four large peptides: 1EJG (crambin), 1BDK (bradykinin), 1L2Y (a mini-protein with a tryptophan cage), and 1SCO (a toxin from the scorpion venom).

Interleukin-1 Receptor Modulation Using β-Substituted α-Amino-γ-Lactam Peptides From Solid-Phase Synthesis and Diversification

As a key cytokine mediator of inflammation, interleukin-1β (IL-1β) binds to the IL-1 receptor (IL-1R) and activates various downstream signaling mediators, including NF-κB, which is required for immune vigilance and cellular protection. Toward the development of IL-1-targeting therapeutics which exhibit functional selectivity, the all-D-amino acid peptide 1 (101.10, H-D-Arg-D-Tyr-D-Thr-D-Val-D-Glu-D-Leu-D-Ala-NH₂) was conceived as an allosteric IL-1R modulator that conserves NF-κB signaling while inhibiting other IL-1-activated pathways. Employing β-hydroxy-α-amino-γ-lactam (Hgl) stereoisomers to study the conformation about the Thr³ residue in 1, [(3R,4S)-Hgl³]-1 (2b), among all possible diastereomers, was found to exhibit identical in vitro and in vivo activity as the parent peptide and superior activity to the α-amino-γ-lactam (Agl) counterpart. Noting the relevance of the β-hydroxyl substituent and configuration for the activity of (3R,4S)-2b, fifteen different β-substituted-Agl³ analogs of 1 (e.g., 2c-q) have now been synthesized by a combination of solution- and solid-phase methods employing N-Fmoc-β-substituted-Agl³-Val-OH dipeptide building blocks. Introduction of a β-azido-Agl³ residue into the resin bound peptide and subsequent reduction and CuAAC chemistry gave access to a series of amine and triazole derivatives (e.g., 2h-q). β-Substituted-[Agl³]-1 analogs 2c-q exhibited generally similar circular dichroism (CD) spectra as that of Hgl analog 2b in water, presenting curve shapes indicative of β-turn structures. The relevance of the β-substituent was indicated in rodent models of preterm labor and retinopathy of prematurity (ROP), in which certain analogs inhibited preterm birth and vaso-obliteration, respectively, with activity similar to 1 and 2b. The β-substituted-[Agl³]-1 analogs exhibited functional selectivity on IL-1-induced signaling pathways. The described solid-phase method has provided discerning probes for exploring peptide structure-activity relationships and valuable leads for developing prototypes to treat inflammatory events leading to prematurity and retinopathy of prematurity, which are leading causes of infant morbidity and blindness respectively.

Synthesis of Azido Acids and Their Application in the Preparation of Complex Peptides

Azido acids are important synthons for the synthesis of complex peptides. As a protecting group, the azide moiety is atom-efficient, easy to install and can be reduced in the presence of many other protecting groups, making it ideal for the synthesis of branched and/or cyclic peptides. α-Azido acids are less bulky than urethane-protected counterparts and react more effectively in coupling reactions of difficult-to-form peptide and ester bonds. Azido acids can also be used to form azoles on complex intermediates. This review covers the synthesis of azido acids and their application to the total synthesis of complex peptide natural products.

1 Introduction

2 Synthesis of α-Azido Acids

2.1 From α-Amino Acids or Esters

2.2 Via α-Substitution

2.3 Via Electrophilic Azidation

2.4 Via Condensation of N-2-Azidoacetyl-4-Phenylthiazolidin- 2-Thi one Enolates with Aldehydes and Acetals

2.5 Synthesis of α,β-Unsaturated α-Azido Acids and Esters

3 Synthesis of β-Azido Acids

3.1 Preparation of Azidoalanine and 3-Azido-2-aminobutanoic Acids

3.2 General Approaches to Preparing β-Azido Acids Other Than Azi doalanine­ and AABA

4 Azido Acids in Total Synthesis

4.1 α-Azido Acids

4.2 β-Azido Acids and Azido Acids Containing an Azide on the Side Chain

5 Conclusions

Enhanced anti-tumor activity of the Multi-Leu peptide PACE4 inhibitor transformed into an albumin-bound tumor-targeting prodrug

The proprotein convertase PACE4 has been validated as a potential target to develop new therapeutic interventions in prostate cancer (PCa). So far, the most effective compound blocking the activity of this enzyme has been designed based on the structure of a small peptide Ac-LLLLRVKR-NH₂ known as the Multi-Leu (ML) peptide. Optimization of this scaffold led to the synthesis of compound C23 (Ac-[DLeu]LLLRVK-amidinobenzylamide) with a potent in vivo inhibitory effect on the tumor growth. However, further developments of PACE4 inhibitors may require additional improvements to counter their rapid renal clearance and to increase their tumor targeting efficiency. Herein, we explored the transformation of the ML-peptide into an albumin-binding prodrug containing a tumor specific release mechanism based on the prostate-specific antigen. Our data confirms that intravenous treatment using the ML-peptide alone has little effect on tumor growth, whereas by using the ML-prodrug in LNCaP xenograft-bearing mice it was significantly reduced. Additionally, excellent in vivo stability and tumor-targeting efficiency was demonstrated using a radiolabelled version of this compound. Taken together, these results provide a solid foundation for further development of targeted PACE4 inhibition in PCa.

Improving the Selectivity of PACE4 Inhibitors through Modifications of the P1 Residue

Paired basic amino acid cleaving enzyme 4 (PACE4), a serine endoprotease of the proprotein convertases family, has been recognized as a promising target for prostate cancer. We previously reported a selective and potent peptide-based inhibitor for PACE4, named the multi-Leu peptide (Ac-LLLLRVKR-NH₂ sequence), which was then modified into a more potent and stable compound named C23 with the following structure: Ac-dLeu-LLLRVK-Amba (Amba: 4-amidinobenzylamide). Despite improvements in both in vitro and in vivo profiles of C23, its selectivity for PACE4 over furin was significantly reduced. We examined other Arg-mimetics instead of Amba to regain the lost selectivity. Our results indicated that the replacement of Amba with 5-(aminomethyl)picolinimidamide increased affinity for PACE4 and restored selectivity. Our results also provide a better insight on how structural differences between S1 pockets of PACE4 and furin could be employed in the rational design of selective inhibitors.