Peptide-Drug Conjugates: An Emerging Direction for the Next Generation of Peptide Therapeutics

Abstract

Building on recent advances in peptide science, medicinal chemists have developed a hybrid class of bioconjugates, called peptide-drug conjugates, that demonstrate improved efficacy compared to peptides and small molecules independently. In this Perspective, we discuss how the conjugation of synergistic peptides and small molecules can be used to overcome complex disease states and resistance mechanisms that have eluded contemporary therapies because of their multi-component activity. We highlight how peptide-drug conjugates display a multi-factor therapeutic mechanism similar to that of antibody-drug conjugates but also demonstrate improved therapeutic properties such as less-severe off-target effects and conjugation strategies with greater site-specificity. The many considerations that go into peptide-drug conjugate design and optimization, such as peptide/small-molecule pairing and chemo-selective chemistries, are discussed. We also examine several peptide-drug conjugate series that demonstrate notable activity toward complex disease states such as neurodegenerative disorders and inflammation, as well as viral and bacterial targets with established resistance mechanisms.

Figure 1:. Chemical Structures of Human Immunodeficiency Virus Cell Entry Inhibitors.

Peptide-drug conjugate structures are displayed such that peptides are in blue, linkers in black and small molecules in red. Cartoon representations of protein targets are displayed on the right with target names below. (A) HIV entry inhibitor peptide P26 is N-terminally conjugated to the N-(carboxyphenyl)pyrrole, Aoc, through a β-alanine linker. (B) P26 is conjugated to the antiviral triterpene sapogenin through a chemical linker that contains a triazole moiety covalently attached to the side-chain of a C-terminal lysine and an additional β-alanine residue. (C) HIV entry inhibitor peptide SP22 is conjugated to the CCR5 inhibitor, TAK, via a PEG-12 linker covalently attached to the C-terminal lysine side-chain.

Figure 2:. Chemical Structures of Dengue Virus Protease Inhibitors.

Peptide-drug conjugate structures are displayed such that peptides are in blue and small molecules in red. Cartoon representations of protein targets are displayed on the right with target names below. (A) A tribasic tetrapeptide complementary to the DENV protease recognition motif is covalently linked to a C-terminal boronic acid moiety. (B) A dibasic retro-tripeptide complementary to the DENV protease recognition motif was covalently attached to an arylcyanoacrylamide at the N-terminus. (C) The N-benzoyl capped dibasic tripeptide contains an unnatural 4-benzyloxy-D-phenylglycine residue at the C-terminus with a β-lactam ring covalently attached to the terminal nitrogen in the S configuration.

Figure 3:. Chemical Structures of Vancomycin Peptide-Drug Conjugates.

Peptide-drug conjugate structures are displayed such that non-vancomycin peptides are in blue, linkers in black, small molecules in red, and additional chemical matter in green. Cartoon representations of protein and membrane targets are displayed on the right with target names below. (A) A compound from the vancapticin PDC series. A four-lysine peptide was covalently linked to vancomycin through the C-terminal lysine side chain and vancomycin carboxylic acid. These PDCspossess hydrophobic moieties at the peptide N-terminus. (B) A hexa-arginine peptide was covalently attached to vancomycin through a C-terminal cysteine sidechain linked to the vancomycin secondary amine through a cyclohexane and pyrrolidine-2,5-dione linker. (C) A lipopolysaccharide-binding peptide was conjugated to the free carboxylic acid of vancomycin via a linker that contains a pyrrolidine-2,5-dione, 2 PEG units and 2 amide moieties.

Figure 4:. Chemical Structures of Galantamine-Peptide Conjugates.

Peptide-drug conjugate structures are displayed such that peptides are in blue, linkers in black and small molecules in red. Cartoon representations of protein targets are displayed on the right with target names below. (A) Example of a galantamine-peptide drug conjugate where galantamine (shown in red) is covalently linked through an ester bond to a tripeptide comprising N-(3,4-dichlorophenyl)-d,l-Ala-OH (shown in blue) at position 6. (B) Example of a galantamine-peptide drug conjugate with a shortened sequence of OM 99–2 covalently linked at position 11 through an amide bond to an Asp residue, which is also linked to a nicotinic residue. (C) Example of a galantamine-peptide drug conjugate with an analog of Leu-Val-Phe-Phe (Aβ17-Aβ20) covalently linked through an ester bond to position 6 of galantamine.

Figure 5:. Chemical Structure of Anti-Neurodegenerative Peptide-Drug Conjugates.

Peptide-drug conjugate structures are displayed such that peptides are in blue, linkers in black, small molecules in red, and additional chemical matter in green. Cartoon representations of protein targets are displayed on the right with target names below. (A) A Trolox-peptide conjugate where trolox is linked through an amide bond to Aβ peptide variant Aβ_36–42. (B) General structure and an example of a tripartite structure consisting of membrane anchor (raftophile), spacer and pharmacophore (inhibitor). The tripartite structure shown features GL 189 as the BACE1 inhibitor and dihydrocholesterol as the membrane anchor linked through a H-3Gl-OH spacer.^, (C) Example of a shuttle-cargo fusion molecule from Hybrid Molecule Subset I. Peptidic carrier TfR-P is conjugated to fallypride directly through an amide bond. (D) Example of a shuttle-cargo fusion molecule from Hybrid Molecule Subset II. TfR-P is conjugated to fallypride through a PEG-2 linker. (E) Example of a shuttle-cargo fusion molecule from Hybrid Molecule Subset III. Angiopep-2 was conjugated to fallypride through a PEG₄ linker. (F) Example of of a shuttle-cargo fusion molecule from Hybrid Molecule Subset IV. Carrier peptide TfR-P is conjugated to fallypride through a PEG-3 linker and derivatized with a radiolabeling moiety (SiFA, shown in green), which is linked to the carrier peptide of the compound.

Figure 6:. Chemical Structure of Analgesic and Anti-inflammatory Peptide-drug conjugates.

Peptide-drug conjugate structures are displayed such that peptides are in blue, linkers in black, small molecules in red, and additional chemical matter in green. Cartoon representations of protein targets are displayed on the right with target names below. (A) The cyclic peptide DNCP was linked to β-NalA through an l-Thr-d-Phe linker. (B) Example of a betulinic acid peptide conjugate where betulinic acid is linked to an azido peptide directly through a C-C bond. (C) Example of a Methotrexate-Hyaluronic acid conjugate where methotrexate is linked to hyaluronic acid through a cleavage susceptible peptide and a PEG-3 linker. (D) Methotrexate-cIBR conjugate where methotrexate is linked by an amide bond to the N-terminus of cIBR peptide. (E) β-carboline peptide conjugate where a Β-carboline alkaloid is linked to ARPAK pentapeptide through an amide bond.