Peptide drug development has made great progress in the last decade thanks to new production, modification, and analytic technologies. Peptides have been produced and modified using both chemical and biological methods, together with novel design and delivery strategies, which have helped to overcome the inherent drawbacks of peptides and have allowed the continued advancement of this field. A wide variety of natural and modified peptides have been obtained and studied, covering multiple therapeutic areas. This review summarizes the efforts and achievements in peptide drug discovery, production, and modification, and their current applications. We also discuss the value and challenges associated with future developments in therapeutic peptides.
Therapeutic peptides are a unique class of pharmaceutical agents composed of a series of well-ordered amino acids, usually with molecular weights of 500-5000 Da. Research into therapeutic peptides started with fundamental studies of natural human hormones, including insulin, oxytocin, vasopressin, and gonadotropin-releasing hormone (GnRH), and their specific physiological activities in the human body. Since the synthesis of the first therapeutic peptide, insulin, in 1921, remarkable achievements have been made resulting in the approval of more than 80 peptide drugs worldwide. The development of peptide drugs has thus become one of the hottest topics in pharmaceutical research.
The first half of the 20th century witnessed the discovery of several life-saving bioactive peptides, such as insulin and adrenocorticotrophic hormone, which were initially studied and isolated from natural sources. The discovery and development of insulin, a peptide with 51 amino acids, has been considered as one of the monumental scientific achievements in drug discovery. It was first isolated by Frederick Banting in 1921 and further developed by Frederick and Charles Best, and was already available for patients with diabetes mellitus just a year after its first isolation. In 1923, insulin became the first commercial peptide drug and has since benefited thousands of diabetes patients to date. However, the production of human insulin during the 20th century could not keep up with the high market demand, and animal-derived insulins, such as bovine and porcine insulin, dominated the insulin market for almost 90 years until they were replaced by recombinant insulin.
More peptide hormones and their receptors with therapeutic potential were identified and characterized from the 1950s to the 1990s. Meanwhile, the technologies used for protein purification and synthesis, structure elucidation, and sequencing made substantial progress, thus accelerating the development of peptide drugs, leading to nearly 40 peptide drugs being approved worldwide. Notably, synthetic peptides such as synthetic oxytocin, synthetic vasopressin, and recombinant human insulin began to be developed in addition to natural peptides.
Peptide drug development entered a new era with the advent of the 21st century, since when advances in structural biology, recombinant biologics, and new synthetic and analytic technologies have significantly accelerated the process. A sophisticated system of peptide drug development has been established, including peptide drug discovery, drug design, peptide synthesis, structural modification, and activity evaluation. A total of 33 non-insulin peptide drugs have been approved worldwide since 2000 (Table 1). In addition, these peptide drugs are no longer simply hormone mimics or composed simply of natural amino acids. For example, enfuvirtide is a 36-amino acid biomimetic peptide mimicking human immunodeficiency virus (HIV) proteins used in combination therapy for the treatment of HIV-1; ziconotide is a neurotoxic peptide derived from the cone snail Conus magus, which was approved in 2004 and is used to manage severe chronic pain; teduglutide is a glucagon-like peptide 2 (GLP-2) analogue used to treat short bowel syndrome, and is manufactured using a strain of Escherichia coli modified by recombinant DNA technology; and liraglutide is a chemically synthesized analogue of human glucagon-like peptide 1(GLP-1), made by attaching a C-16 fatty acid (palmitic acid) with a glutamic acid spacer on lysine residue (position 26 in the sequence), which acts as a GLP-1 receptor agonist to manage type 2 diabetes mellitus (T2DM). All these peptide drugs have been used in a wide range of therapeutic areas, such as urology, respiratory, pain, oncology, metabolic, cardiovascular, and antimicrobial applications. To date, more than 170 peptides are in active clinical development (Table 2), with many more in preclinical studies.
Peptide drugs account for a significant proportion of the pharmaceutical market, with worldwide sales of more than $70 billion in 2019, a more than two-fold increase compared with 2013. According to Njardarson et al., the top 200 drug sales in 2019, included 10 non-insulin peptide drugs. Interestingly, the top three sales of peptide drugs were all GLP-1 analogues for treating T2DM, including Trulicity (dulaglutide) ranked at 19 with $4.39 billion retail sales, Victoza (liraglutide), ranked at 32 with $3.29 billion sales, and Rybelsus (semaglutide), ranked at 83 with $1.68 billion sales (Fig. 1).
In this article, we review the historical development of peptide drugs and current advances in peptide drug discovery. We focus on the pharmaceutical characteristics of therapeutic peptides and highlight new technologies that have improved the design, synthesis, modification, and evaluation of peptide drugs, and provide new perspectives in the applications of peptide drugs. We also refer readers to several recent reviews for further reading.
Therapeutic peptides commonly act as hormones, growth factors, neurotransmitters, ion channel ligands, or anti-infective agents. They bind to cell surface receptors and trigger intracellular effects with high affinity and specificity, with a similar mode of action to biologics, including therapeutic proteins and antibodies. However, compared with biologics, therapeutic peptides show less immunogenicity and have lower production costs.
Small molecule drugs are known to have an extended therapeutic history with inherent advantages, including low production costs and sale prices, oral administration, and good membrane penetration ability. Both naturally extracted and chemically synthesized small molecules show competitive price advantages compared with peptides and biologics (proteins or antibodies). Oral administration of small molecules has the benefits of better safety and improved patient compliance, while their small size also enables them to penetrate the cell membrane to target intracellular molecules. However, their small size also means that it is difficult for them to inhibit large surface interactions, such as protein-protein interactions (PPIs), effectively. PPIs usually occupy a contact area of 1500–3000 A2, while small molecules only cover 300–1000 A2 of the protein surface, due to their limited molecular size. By contrast, the unique physiochemical properties of peptide drugs, including their larger size and more flexible backbone, enable them to act as potent inhibitors of PPIs. The clinical use of small molecules is also limited by their low specificity compared with peptide drugs. For example, sorafenib and sunitinib are tyrosine kinase inhibitors that inhibit the tyrosine kinase domain activity of vascular endothelial growth factor (VEGF) receptors, resulting in anti-angiogenic effects that are used to treat cancer patients; however, they also target other kinase receptors such as serine/threonine kinase receptors, leading to cytotoxicity.
As natural amino acid-based therapeutics, therapeutic peptides have two intrinsic drawbacks (Fig. 2): membrane impermeability and poor in vivo stability, which represent major stumbling blocks for peptide drug development.
These intrinsic advantages and disadvantages of peptides present both challenges in peptide drug development and also opportunities and directions for peptide drug design and optimization.
The history of peptide drug discovery started by exploiting natural hormones and peptides with well-studied physiological functions for treating diseases caused by hormone deficiencies, such as a lack of insulin required to regulate blood glucose levels in patients with T1DM or T2DM. Diabetes is treated either by insulin injection or by stimulating insulin secretion-related targets such as GLP-1 receptor, to produce insulin. Searching for natural peptides and hormones or replace them by animal homologues, such as insulin, GLP-1, somatostatin, GnRH, 8-Arg-Vasopressin, and oxytocin, were the initial strategies used for peptide drug discovery and development (Fig. 3). However, the drawbacks associated with these natural peptides aroused interest in optimizing their natural sequences, leading to a series of natural hormone-mimetic peptide drugs.
GLP-1 derived peptide drugs (Fig. 4a): GLP-1 is a 37-amino acid peptide that regulates insulin production and secretion, with a very short half-life in vivo. Extensive efforts have been made to modify its sequence to enhance the stability of this hormone, while maintaining its potency and pharmacological effect, leading to the development of the three top-selling anti-T2DM peptide drugs: Trulicity (dulaglutide), Victoza (liraglutide), and Ozempic (semaglutide).
Gonadotropin-releasing hormone (GnRH) derived peptide drugs (Fig. 4b): GnRH is a peptide containing 10 amino acids that is produced by GnRH neurons in the hypothalamus. Modification of the native sequence of GnRH has led to the development of several peptide drugs, such as leuprolide and degarelix. Leuprolide has the same biological activity as GnRH by activating GnRH receptors, and is used as a GnRH receptor agonist for treating hormone-responsive prostate cancer, endometriosis, uterine fibroids, and precocious puberty. While the sequence of degarelix is optimized from GnRH, it acts as a GnRH antagonist by competitively binding to the GnRH receptor and is used to treat terminal prostate cancer.
Many other approved peptide drugs are also derived from natural hormones, including octreotide, a somatostatin mimic peptide drug, used for the treatment of growth hormone producing tumors and pituitary tumors; desmopressin, an 8-Arg-vasopressin mimicking peptide drug, used for diabetes insipidus and nocturia; carbetocin, an oxytocin homologue used to treat amenorrhea and atosiban, an oxytocin antagonist for suppressing premature labor.
