The Road from Host-Defense Peptides to a New Generation of Antimicrobial Drugs

Abstract

Host-defense peptides, also called antimicrobial peptides (AMPs), whose protective action has been used by animals for millions of years, fulfill many requirements of the pharmaceutical industry, such as: (1) broad spectrum of activity; (2) unlike classic antibiotics, they induce very little resistance; (3) they act synergically with conventional antibiotics; (4) they neutralize endotoxins and are active in animal models. However, it is considered that many natural peptides are not suitable for drug development due to stability and biodisponibility problems, or high production costs. This review describes the efforts to overcome these problems and develop new antimicrobial drugs from these peptides or inspired by them. The discovery process of natural AMPs is discussed, as well as the development of synthetic analogs with improved pharmacological properties. The production of these compounds at acceptable costs, using different chemical and biotechnological methods, is also commented. Once these challenges are overcome, a new generation of versatile, potent and long-lasting antimicrobial drugs is expected.

Keywords: AMP; antibiotic; antimicrobial; cathelicidins; customizable units; gene mining; peptide modification; peptides; site-selective; tag-and-modify.

Scheme 1

Bioguided search of AMPs.

Scheme 2

Identification of ‘cryptic’ or ‘silent’ products by bioinformatics techniques.

Scheme 3

Identification of ‘cryptic’ or ‘silent’ products from animal sources by bioinformatics techniques, and production in microalgae.

Figure 1

(Left) Transformed S. elongatus growing in BG-11 agar plates containing spectinomycin (5 µg/mL). Negative control with no insert (Left) and containing the bird cathelicidin peptide Mm_KR-26; (Right) Comparison of the microalga vectors pChlamy_4 and pSyn_6 (ThermoFisher) used for the recombinant expression of cathelicidin antimicrobial peptides into the microalga C. reindhartii and S. elontgatus, respectively.

Scheme 4

SPPS in the preparation of peptides with non-proteinogenic units, and cyclization of a BPC precursor.

Scheme 5

Ligation of peptides in the synthesis of a precursor of polymyxin E. Reaction conditions: (a) Fmoc-SPPS; (b) HATU, DIEA, DMF, 45 min.

Scheme 6

Synthesis of a precursor of snaking-1. Reaction conditions: (i) Boc-Ala-Pam, DIC, CH₂Cl₂, 1 h; (ii) TFA; (iii) TrtSCH₂CH₂CO₂H, HBTU, DIPEA, DMF, 16 min; (iv) TFA, TIPS, H₂O; (v) Boc-Cys-(4-MeBn)-OH, HBTU, DIPEA, DMF, 1 h; (vi) Boc SSP; (vii) Boc-Pro-Pam, DIC, CH₂Cl₂, 1 h; (viii) Boc SSPS. TIPS = triisopropylsilane; DIC = diisopropylcarbodiimide; DIPEA = diisopropylethylamine; DMF = N,N-dimethylformamide; HBTU = O-benzotriazole-N,N,N′,N′-tetramethyluronium hexafluorophosphate; Pam = phenylacetamidomethyl; Trt = triphenylmethyl.

Scheme 7

Peptide ligation using click’ chemistry.

Scheme 8

Site-selective modification of peptides in the discovery of new AMPs.

Figure 2

Structures of colistin A, its analog CB-182,804, the Cantab analogs CA-824 and CA-1049, and the Barcelona disulfide analogs. The positions were changes were made are displayed in colors.

Figure 3

Structures of AMP analogues: (Left) the NApFFKK peptide; (Center) Synthetic AMPs with ferrocene or rutacene groups; (Right) Unnatural tryptophan units in a ferritin-inspired peptide, LTX-109.

Figure 4

(Left) Murepavadin (31), an anti-pseudomonas peptide in Phase II trials, with a l-Pro-d-Pro unit that induces a β-turn in the β-hairpin structure; (Center) The 12-residue β-hairpin mimetic L27-11 (32) with a potent and specific antipseudomonas activity; optimization of this hit generated POL7001 (33) (Dab = L-2,4-diaminobutyric acid); (Right) Another synthetic peptidomimetic antibiotic JB-95 (34), active against E. coli.

Figure 5

Cyclic peptides with alternating d, l-units self-assemble and form pores in the bacterial membrane.

Figure 6

AMP analogs; acyllysine oligomers (OAKs) with alternating acyl (A) and lysine (K) units.

Scheme 9

Peptoid analogs, alternating cationic and hydrophobic chains. Cycle 41 gave the best results. Reaction conditions (above): (a) R-NH₂, base; (b) BrCH₂COOH; (c) 20% HFIP, CH₂Cl₂; (d) PyBOP, DIEA, DMF. Conversion yields >95% were observed in the synthesis of cyclic peptoid analogues.

Figure 7

AMP analogs: (Left) α- and β-peptoids 44 and 45; (Center) substituted PrAMPs 46–48; (Right) introduction of cationic chains in analogues 49 and capped-termini derivatives 50.

Figure 8

β-peptides with alternating hydrophobic and cationic units and different folding patterns.