Cyclic peptides have chemical and biological stabilities and potent binding affinity for their target molecules due to their conformational restriction. Therefore, these are major molecules in mid-size drugs researche. To date, we have developed a cyclic peptide-based chemokine receptor CXCR4 antagonist, which has high anti-HIV activity and is now in Phase III clinical trial as a drug candidate for cancer and leukemia. In addition, we have developed binding ligands for epidermal growth factor receptor(EGFR), which is a receptor overexpressing on the surface of several cancer cells. Bivalent ligands, which contain two CXCR4 ligands or two EGFR ligands, have also been synthesized to have selective recognition for dimeric CXCR4 or EGFR, respectively. These cyclic peptides might be useful as drug delivery molecules targeting cancer cells, thus conjugate molecules, containing these cyclic peptides and therapeutic agents, were synthesized. As a result, targeted delivery of the agents were successful due to specific affinity of these cyclic peptides.
Collagen, characterized by the unique triple helical structure, is a main component of the extracellular matrix, and exists in almost all parts of the body such as skin and bone. It is considered that denatured collagen, which has unfolded portions in the triple helical molecule, is produced and accumulated during the degradation process in the tissues surrounding malignant tumors. Collagen-mimetic peptide (CMP) is a generic name for chemically synthesized peptides that mimic the triple helical structure of collagen. A single-stranded CMP can hybridize to the loose triple helical parts of collagen by its inherent triple helix-forming propensity. In this review, we summarize studies on the collagen-hybridizing CMPs, and introduce a new DDS concept that targets changes in the structure of extracellular matrices. Although the relationship between collagen denaturation and diseases is not well understood, the collagen-hybridizing CMP would be useful for understanding of pathological conditions, and development of new diagnostic and therapeutic strategies.
Nowadays, the development of research tools and pharmaceuticals utilizing various functions of antibodies is receiving much attention, and there is also an interest in IgG-binding peptides(IgBP) having an affinity to the Fc region of antibodies. Selective chemical modification at the Fc region that does not affect its antigen recognition is beneficial not only for improving the quality of the antibody-drug conjugate(ADC) but also for creating a new functional antibody. On the other hand, IgBP is also expected to be applied to the development of antibody-dependent DDS. In this review, we firstly introduce the development of IgBP-mediated non-covalent-type ADC. As a payload, plinabulin, an anticancer and vascular disrupting drug now in Phase III, created by our group, was used. The IgBP-Plinabulin conjugate showed a reasonable binding affinity to the antibody, Herceptin. And, Herceptin-dependent cytotoxic effect of Plinabulin, via the cell internalization of noncovalent Herceptin-Plinabulin complex, was proved in vitro cell evaluation. Secondly, we introduce the development of a new small monocyclic IgBP(Lys8Leu/His17(2-Pya)-OH) with the strongest affinity(Kd = 2.48 nM) and slow-releasing property to the Fc region of IgG based on the structure-activity relationship study from a 17-residue IgBP 2 discovered by the phage-display screening. This potent IgBP would become a promising tag for antibody-dependent DDS in future.
Stapled peptides with covalently cross-linked side chains have been put to ever-increasing use for the development of peptide-based drugs, especially derived from α-helical peptide sequences. Nowadays, a variety of stapling technologies have been developed and allows for the stabilization of helical conformation responsible for the activity of biological helical peptides. Our group found that brefeldin A-inhibited guanine nucleotide-exchange protein 3 (BIG3), over-expressed in breast cancer cell, inhibits the tumor-suppressive effect of prohibitin 2 (PHB2) through the BIG3-PHB2 proteins interaction. Requiring a short α-helical segment of BIG3, the protein-protein interaction was selected as a target of great versatility for growth inhibition of the cancer cells. Therefore, the α-helical segment of BIG3 was subjected to the stapling strategy to develop anti-breast cancer peptides. Glutamine (Gln 165), aspartic acid (Asp 169), and Gln 173 residues in the α-helical region, indispensable for the PPI with PHB2, lie on the same face of the α-helix with i and i + 4 spacing. Several thirteen-residue stapled peptides derived from BIG3 were synthesized and examined their biological activity using culture cell lines and mouse xenograft model. Among the synthetic stapled peptides, the peptides, with the pivotal residues in the same helical face fixed by incorporating the staple on the opposite face with i and i + 4 spacing, showed anti-tumor effects superior to original 28-residue estrogen receptor-α (ERα) activity-regulator synthetic peptide (ERAP) with a oligoArg cell-penetrating peptide (CPP). Stapled version of ERAP (stERAP) more efficiently inhibited the estrogen-induced cancer cell growth than the original ERAP. Furthermore, the long-lasting in vivo anti-tumor effect of the stERAPs in breast cancer xenografts in mice was observed.
Antibodies are indisputably the most successful reagents in molecular-targeting therapy. However, use of antibodies has been limited due to the biophysical properties and the cost to manufacture. To enable new applications where antibodies show some limitations, we have developed an alternative-binding molecule with non-immunoglobulin domain. The molecule is a conformationally constrained peptide with helix-loop-helix structure, termed “molecular-targeting HLH peptides”. Here, we introduce the molecular-targeting peptides that show antibody-like functions, high affinity and high specificity for the targeted proteins. Based on our technology of phage-displayed libraries for antibodies, we constructed a phage-displayed library of “molecular-targeting HLH peptides”, which was screened against a variety of disease-related proteins. The screened HLH peptides showed a strong binding affinity(KD of 4 nM) to the targeted protein and a long half-life(>2 weeks) in mouse sera, proving an enzyme-resistant property. Furthermore, immunization of the peptide to mice showed no induction of the antibody titer(non-immunogenic). The molecular-targeting HLH peptide is expected to be a new class of modalities of med-size medicines.
Present cancer therapeutics has remarkably made progress by clinical application of various kinds of molecular target agents including small molecular compounds and antibody drugs such as Gefitinib and Rituximab over two decades. Clinical achievement brought by these agents has been also changing the concept of drug development for cancer therapeutics. We have focused on developing the Peptide-Drug Conjugate (PDC) based on our novel tumor-homing peptides as a drug carrier, which may have potential for innovating current cancer therapeutics.
New therapeutic modalities are needed to address the problem of pathological but undruggable proteins. One emerging technology is small molecules that shorten lifespan of disease-related proteins. This article reviews induction of proteasomal degradation of the target proteins by hybrid small molecules composed of a ubiquitin ligase ligand coupled to a ligand for the target protein. This article also describes recent efforts towards undruggable proteins such as substrate binding proteins and aggregation-prone proteins, and the challenges for drug discovery.
Many of the glycans exist as glycoconjugates such as glycoproteins, glycolipids, and glycosaminoglycans, which have a variety of functions based on the vast structural diversity. The cell surface glycoconjugates form a glycocalyx to cover and protect the cell from the environments. Consequently, cell surface glycans play important roles in the various recognition events. Glycans are involved in various life phenomena such as immunity, infection, inflammation, cancer, aging and etc. Aberrant glycosylation is linked to various diseases such as type 2 diabetes, emphysema, neurodegenerative diseases, cancer, and etc. The pathogenesis of glycan-related diseases such as congenital disorders of glycosylation (CDGs), lysosomal diseases, some forms of congenital muscular dystrophy has been clarified. Diagnostic agents based on aberrant glycosylation on cell surface and glycoproteins in cancer have been developed as cancer biomarkers (e.g., pancreatic, ovarian, liver, and prostate cancers). Cell surface glycans are the targets for viral and bacterial infections. Influenza viruses use sialic acid on the cell surface for infection. Sialidase inhibitors such as Tamiflu and Relenza have been developed as anti-influenza drugs. Most of the protein biologics are glycoproteins, and the roles of protein glycans have been revealed in regulation of the functions and glycan-dependent dynamics. Bacterial-derived glycans have been developed as glycan vaccines and vaccine adjuvants. Lipopolysaccharide and the active entity lipid A from Gram negative bacteria activate toll-like receptor 4 (TLR4) to induce strong immune and inflammatory responses. Structural modification of lipid A can alter the activity from high-inflammatory to low-inflammatory. Lipid A derivative 3D-MPL, that moderately activates TLR4 and induces anti-viral responses was developed as an adjuvant for virus vaccines. Self-adjuvanting strategy which uses vaccines conjugated with antigen and adjuvant is also reviewed. A self-adjuvanting vaccine can elicit specific immune response by simultaneous interaction of both an antigen and an adjuvant with the same immune cell. The adjuvant part can recruit the conjugated vaccine to immune cells, activates target immune cells via innate immune receptors, and then facilitates the uptake of the antigen into cells. Application of glycan related drugs is expected for new immunotherapies, anti-inflammatory treatments, regenerative medicine, and diagnostic glycan microarrays, and etc in the near future.