Carrie Cao, PhD

Peptide and protein therapeutics represent an increasingly significant category of biologics used to treat illnesses such as cancer, autoimmune, neurological, and endocrine disorders. Their high target specificity generally makes them a more effective and safer choice of treatment than small-molecule drugs. Currently, there are more than 200 approved therapeutic proteins and over 100 peptides on the market; this figure accounts for 10% of the pharmaceutical market at a value of $40 billion per year (1, 2). With hundreds of protein and peptide drugs in clinical trials and many more in preclinical development, this market is expected to continue grow over the next 5-10 years (3).

	Despite their market growth, delivering protein and peptide drugs to the targeted site within the therapeutic range remains a significant challenge to the biopharmaceutical industry. Protein and peptide therapeutics have much higher molecular weights (MWs) than small-molecule drugs, which hinders their absorption through epithelial cells. Their therapeutic function depends on a properly folded conformation that is susceptible to chemical and physical condition changes. An effective delivery system is fundamental to enable proteins and peptides to overcome their inherent structural instability, diffuse across physical barriers, and achieve the desired bioavailability. Research efforts for optimal protein administration methods are mainly focused on two areas: bioavailability enhancement and noninvasive delivery.

	One commonly used strategy to boost the bioavailability of protein and peptide drugs is to employ the usage of direct structural modifications, which include cyclization (e.g., cyclosporine), amino acid substitution, conjugation to polyethylene glycol (PEG) polymer chains (i.e., pegylation), oligosaccharides (i.e., glycosylation), and fatty acids (i.e., lipidization) (2). Among these, pegylation and glycosylation are proven methods to improve absorption through biological membranes, structural stability, and bioavailability of proteins and peptides. Other benefits conferred by these technologies include improved solubility, decreased dosing frequency due to increased systemic stability, increased efficacy, improved safety profile, and reduced immunogenicity (4). Commercial successes have been seen in several approved pegylated interferon products including peginterferon alfa-2a (Pegasys, Genetech) and peginterferon alfa-2b (PEG-Intron, Merck), both indicated for hepatitis C, and peginterferon beta-1a (Plegridy, Biogen Idec) for multiple sclerosis (4, 5).

	Pegylation can also be used to develop prodrugs. Prodrugs are pharmacologically inactive and are metabolized into active drugs under suitable physiological conditions after administration. Prodrugs provide an alternative strategy to circumvent poor solubility, improving pharmacokinetics and minimizing toxicity of protein and peptide-based drugs (6). Advanced pegylation technologies allow PEG polymers to be readily engineered in various shapes and sizes, with high site-specificity and purity, thus further reducing immunogenicity and protein deactivation during conjugation. In addition, peptide stapling has shown advantages in increasing a peptide’s stability, cellular penetration, and binding affinity by locking the conformation of the peptide through multiple, synthetic, hydrocarbon backbones (2).

	Encapsulating proteins and peptides into drug carrier systems provides another strategy to improve their bioavailability. The carrier provides a shield for the proteins and peptides from both proteolytic enzymes and physical condition changes, also acting as a delivery vehicle across biological membranes. A wide range of materials can be used for carrier construction including microparticles, nanoparticles (NPs), liposomes, solid lipid NPs, and a variety of polymers (e.g., hydrophilic mucoadhesive polymers, thiolated polymers, and hydrogels) (2, 7).

	In the past few decades, NPs have been an extremely active area for developing novel drug-delivery carriers with demonstrated advantages over other systems. NPs are solid colloidal particles with sizes ranging from 10 nm to 1000 nm. They can be made from natural (e.g., chitosan and gelatin), semi-synthetic (e.g., cellulose derivatives), or synthetic (e.g., poly-lactic acid [PLA]) polymers (7). These polymers are biodegradable, nontoxic, non-inflammatory, and non-immunogenic (2). Polymeric NPs also exhibit high stability in biological fluids compared to other carriers.

	NPs are versatile in terms of application. Their physiochemical properties can be adjusted to fit their desired use by altering their sizes, surface charges, and hydrophobicity. Moreover, NPs can be engineered to encapsulate a broad range of molecules--from small chemical entities to macromolecules--by direct encapsulation, absorption, or chemical linkage. NPs also have the ability to accommodate a variety of functions, such as targeted delivery and sustained release through surface modifications. These surface modifications include conjugating NPs with a number of materials such as PEG polymers, absorption enhancers (e.g., chitosan, lectin), and targeting ligands (e.g., antibodies and cell-penetrating ligands) (7).

	Designing target-specific protein carriers often requires applying a combination of the aforementioned strategies. For example, a group of researchers from Massachusetts Institute of Technology (MIT) and Brigham and Women’s Hospital have developed targeted NPs for oral delivery of biologics such as insulin. These NPs are coated with standard PLA-PEG, which are conjugated to the Fc portion of immunoglobulin G (IgG) (8). The Fc portion targets the neonatal Fc fragment of IgG receptor transporter-α (FCGRT; FCRN), which is expressed in the adult intestines, and enables the FCRN-targeting nanoparticles to be effectively transported across intestinal epithelium. In this respect, tissue-, organ-, and disease-specific receptors provide opportunities for targeted delivery design.

	With respect to the delivery route, parenteral administration, such as intravenous, subcutaneous, and intramuscular, is the primary method for therapeutic proteins and peptides. This invasive process can cause pain, discomfort, or other reactions at the injection site, which often results in poor patient compliance--particularly in long-term use. Improved parenteral formulations (e.g., prefilled syringes and autoinjectors) and noninvasive delivery methods are much preferred by healthcare givers and patients alike. In addition, because many best-selling biologics--mainly the monoclonal antibodies--go off patent in 2020, competition within the protein therapeutic market is heating up. An efficacious delivery system with better patient compliance over the existing treatments could be the key to extend patent protection, as well as develop differentiated products.

	Among noninvasive routes, transdermal delivery has advanced considerably in recent years. This system provides fast onset of action, avoidance of first-pass metabolism, needle-free administration, and versatility in delivering both small and macromolecule drugs. In addition, due to its ease of use and painless delivery, the transdermal route leads to better patient compliance. Also, prolonged, continuous, and controlled-release can be easily achieved through this method.

	The goal of transdermal delivery for proteins and peptides is enabling them to penetrate through the outmost layer of the skin, which is the stratum corneum. Due to their large molecular size, protein and peptide therapeutics usually require chemical or physical interventions to increase skin permeability transiently--for example, forming of small pores so that these large molecules can pass through the stratum corneum. Among the many technologies in development to achieve this goal, the microneedle system is most promising. Microneedles are designed to puncture holes only in the stratum corneum, avoiding capillaries and nerve endings at the viable epidermis (the skin layer beneath stratum corneum). Proteins and peptides can be delivered in several ways: topical administration after microneedle treatment, infusion through hollow microneedles, or the slow release of encapsulated drugs into biodegradable microneedles (2). Additionally, microneedles can be introduced through injection on the skin or in the form of a patch.

	Other commonly investigated noninvasive routes for protein and peptide delivery include pulmonary and nasal administration. Both pulmonary and nasal delivery offer large surface areas for absorption, low enzymatic activity compared to the gastrointestinal tract, direct systemic delivery, and avoidance of first-pass metabolism. The advancement in delivery devices such as dry power inhalation (DPI) and pressurized metered dose inhalation (pMDI) systems allows proteins to overcome physical barriers and reach the desired site. In addition, nasal formulations offer a pathway to enter the central nervous system (CNS), thus overcoming the blood-brain barrier. This feature offers a promising route for direct brain delivery. Another strategy to improve brain delivery is to design systems targeting specific receptors in the CNS, such as the lactoferrin receptor, which is highly expressed in neurodegenerative diseases such as Alzhemimer’s and Parkinson’s disease (7). Overall, achieving effective delivery of peptide and protein therapeutics through noninvasive routes requires a combination of technologies, including chemical modifications, targeted drug carrier systems, and possibly novel delivery devices.

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	When referring to this article, please cite it as C. Cao, "Advances in Delivering Protein and Peptide Therapeutics,” 

Articles

A variety of strategies have been employed to deliver proteins and peptides in improved parenteral formulations and/or via noninvasive routes.