In vivo degradation forms, anti-degradation strategies, and clinical applications of therapeutic peptides in non-infectious chronic diseases
Introduction
Whilst most common diseases pose no risk to life and manifest only temporary symptoms, a significant fraction of individuals suffer chronic and life-altering pathologies (Vos et al., 2015). For example, diabetes is one of the most prevalent lifelong diseases, affecting 26.9 million adult Americans in 2018, with 1.5 million new cases appearing every year [Centers for Disease Control and Prevention, CDC (CDC, 2020)]. In 2017, the total costs ascribed to diabetes reached $327 billion in the USA (Petersen, 2018). Over 90–95% of these cases are type 2 diabetes (also known as insulin-resistant diabetes), which is traditionally managed by limiting ingestion of carbohydrates, introducing exercise into daily routine, and taking anti-diabetic medications to reduce blood glucose levels (Chatterjee et al., 2017). Although relatively effective in mitigating disease, symptom management does not represent a permanent cure (Genovese et al., 2017). On the other hand, type 1 diabetes (also known as “juvenile diabetes” or “insulin-dependent diabetes”) afflicts over 1.4 million Americans (CDC, 2020). Peptidic insulin peptide is the most well-developed treatment for type 1 diabetes (Atkinson et al., 2014); however, it still poses pitfalls of inconvenience and economic burden for most patients, as insulin has to be injected daily to control blood glucose level.
Autoimmune disorders represent another example of major, life-altering diseases - producing a significant economic, physical, and emotional burden to millions of people worldwide (Kirsch-Volders et al., 2020). Multiple sclerosis (MS) (Picard et al., 2015) is one of the more common auto-immune disorders afflicting over 2.3 million people worldwide and over 1 million adults in the USA (McGinley et al., 2021; Wallin et al., 2019). Current treatment is to slow down disease progression and manage symptoms with corticosteroids, physical therapy, and other medications (McGinley et al., 2021); however, no curative treatment is available for MS.
Cardiovascular disease (CVDs) is the number one cause of death worldwide, affecting over 100 million people in the US annually. In 2018 alone, 121.5 million Americans were living with one form of cardiovascular disease or another (Benjamin et al., 2019). Each year, 1.5 million Americans suffer from heart attacks and strokes, accounting for $320 billion in healthcare costs and lost productivity (Giedrimiene and King, 2017). By 2030, it is expected that the costs for CVDs will rise to $818 billion (Giedrimiene and King, 2017). Unfortunately, treatments available for CVDs are mainly to prevent or delay disease progression before it becomes life threatening and no permanent curative options are accessible for treatments of CVDs (Aguilar-Ballester et al., 2021; Wintrich et al., 2020).
Small therapeutic peptides are emerging as promising therapy avenues for chronic diseases, with potential to address shortcomings of current standard of care (Lau and Dunn, 2018) (also see Fig. 1). Traditionally, therapeutics can be classified into two groups based on their molecular weights: a). Small chemical molecules [<500 dalton (Da)] that are typically given via oral administration, and b) Peptide and protein-based biologics (>5,000 dalton (Da)) that are usually administrated intravenously. However, in recent years, a large number of new peptidic therapeutics with a wide range of molecular sizes from 500 to 5,000 dalton (Da) have also been reported (Craik et al., 2013). These peptides are usually simple linear chains of amino acids (AAs) with some secondary but no tertiary structures (Marqus et al., 2017; Sato et al., 2006). Typically, therapeutic peptides are designed and synthesized as mimetics of AA sequences found in naturally occurring polypeptides that correspond to specific binding sites for enzymes, receptors and other protein domains. These sequences often show highly target specificity and tend to have reduced potential for immunogenicity if sufficiently short (<6 AAs) (Marqus et al., 2017; McGregor, 2008). Even if a specific functional sequence is not known, peptides with sequence overlapping the putative target site can be readily designed and produced for further screening to identify peptides with optimized targeting affinity (Marqus et al., 2017). By specifically and efficiently disrupting protein-protein interactions, peptides have emerged as promising candidate molecules for clinical therapy (Lau and Dunn, 2018; McGregor, 2008).
In 2015, there were over 140 therapeutic peptides under clinical trials and with over 500 in preclinical studies (Fosgerau and Hoffmann, 2015). Therapeutic peptides have been used in clinic to treat various diseases, including type 1 and 2 diabetes, cancer, and MS (Craik et al., 2013; Genovese et al., 2017; Han and Youn, 2019). For example, Exenatide (i.e., Bydureon® or Byetta®) [a glucagon-like-peptide-1 (GLP-1) agonist] (Genovese et al., 2017) and lixisenatide (i.e., Adlyxin® or Lyxumia®) [an analogue of the human GLP-1] (Nauck et al., 2021; Trujillo et al., 2021) are currently used for patients with type 2 diabetes; while Abarelix (i.e., Plenaxis®) [a gonadotropin-releasing hormone antagonist composed of 10 synthetic AAs that inhibit testosterone production in males] is applied to prostate cancer patients (Garnick and Mottet, 2012; Moul, 2014). Interestingly, Bortezomib (i.e., Velcade®) peptide, which contains only 2 AAs, is the first proteasome inhibitor that has been approved for treatment of multiple myeloma and mantle cell lymphoma (Chen et al., 2011). In the field of cardiovascular diseases, Nesiritide (i.e., Natrecor®) [a recombinant 32-AA peptide of human B-type natriuretic peptide]) is probably the first peptide used for patients with heart failure (Elkayam et al., 2002) (also see Table 2).
Being said above, short therapeutic peptides also come with drawbacks (Fig. 1), including susceptibility to breakdown by degradative enzymes such as endogenous proteases and phosphatases in vivo (McGregor, 2008). Even though peptides can be chemically modified to include stable building blocks, several chemical reactions that occur in vivo are still able to rapidly degrade peptides: hydrolysis, deamidation, isomerization, diketopiperazine formation, oxidation, and disulfide exchange being among such reactions (Furman et al., 2015; Xu et al., 2017). Furthermore, when introduced into the circulatory system, small peptides are subject to renal clearance and thus rapidly cleared from the bloodstream through the glomeruli with minimal retention (Di, 2015; Wu and Huang, 2018). Via chemical degradation and physical elimination, only a small percentage of peptides typically remain bioavailable following intravenous injection, and thus the level of peptidic drug administrated may not be high enough to be effective against the disease process targeted.
Regardless of these barriers, small therapeutic peptides have demonstrated promise in basic research studies using animals and in human clinical trials. In this review, we provide an overview of the major forms of in vivo degradation of small therapeutic peptides in the plasma and anti-degradation strategies. We also address the progress of small therapeutic peptides that are either undergoing clinical trials, or have been successfully used as clinical treatments, for patients with non-infectious diseases, such as diabetes, MS, and cancer. Whilst therapeutic peptides based on viral sequences have also been tested as treatments for infectious diseases, including human immunodeficiency virus (HIV)-induced acquired immunodeficiency (AIDS) and hepatitis, these are not addressed in this review. For readers interested in this area the following references provide further information (Caillat et al., 2020; McKinnell and Saag, 2009; Skwarecki et al., 2021).
Section snippets
Half-life and therapeutic efficacy of small therapeutic peptides
Half-life (i.e., elimination half-life) in pharmacokinetics refers to the time elapsed for the concentration of a given substance to decrease to half of its starting concentration in the plasma or blood (Toutain and Bousquet-Melou, 2004); while the therapeutic efficacy denotes the ability of a given drug to produce desired beneficial effects (Adkins and Noble, 1998). Both are critical parameters and often used in basic research and clinical studies, as well as by the pharmaceutical industry to
In vivo forms of chemical degradation of small therapeutic peptides
Chemically-associated degradations are denoted as alternation of peptide structure and/or its molecular composition, which in turn lead to changes in peptide properties (Furman et al., 2015). Seven primary types of chemically-associated degradation have been found to alter peptide structure, each of which is directly or indirectly associated with shorter half-life in vivo, peptide degradation, and reduction of the final concentration of a peptide. These include: 1) disulfide bond formation, 2)
Physical elimination and molecular incompatibility of small therapeutic peptides
Physical elimination (i.e., physical clearance) refers to bodily activities that simply eliminate (e.g., filtered through the glomeruli of the kidney) peptides from the bloodstream without altering peptide's physical or chemical properties (Good et al., 2010). Renal clearance from the circulatory system is a major elimination route to reduce drug concentration in the plasma, thus decreasing half-life and therapeutic efficacy of drugs (including peptides), which should be considered if and when
Current methods
A significant amount of work has been conducted with an intention of increasing in vivo stability and half-life of small therapeutic peptides in animal models and human patients. Four approaches that are commonly used in the field are summarized below (also see Table 1):
Examples of major small therapeutic peptides that are currently used in clinical therapy for major non-infection diseases
To date, around 360 clinical trials have tested therapeutic peptides (https://clinicaltrials.gov/ct2/home. keywords: “therapeutic peptides”). The following sections provide examples of several therapeutic peptidic drugs that are being used in the clinic or may soon advance to this point (also see Table 2).
Conclusion
Small therapeutic peptides are typically linear sequences of less than 50 AAs. As such molecules are often synthesized to correspond to native endogenous peptides, they inherently tend to exhibit low toxicity, together with high target specificity and biocompatibility. Whilst there have been significant advances in the preclinical development, many small peptide-based therapies face obstacles – most especially related to their instability and short half-life once introduced into the body. Due
Funding support
This work was supported by the NIH grants (1R15HL140528-01 for JQH, 1R35 HL161237–01, R01HL056728-19, and 5R01HL141855-04 for RGG), and the Turkish Fulbright Commission Scholarship (2019–2021 for YT). The funders had no roles in the manuscript preparation, information collection, and decision to publish.
Author contributions
YT: original manuscript draft; JQH: conceptualization and revision; RGG: reviewing and editing.
Declaration of competing interest
RGG holds stock in and is a company officer at the Tiny Cargo Company Inc., which has licensed exosomal technology from Virginia Tech. RGG is also a non-remunerated scientific advisory board member and stockholder of FirstString Research Inc, which licensed αCT1 peptide from the Medical University of South Carolina.
Acknowledgement
We thank Ms Linda Collins of the Fralin Life Sciences Institute at Virginia Tech for kindly providing English editing.
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