Oral Peptides, or Orally Delivered Peptides

The ability to oral delivery of therapeutic peptides and proteins (PPs) will greatly benefit peptide base therapeutics. Orally delivered therapeutic peptides that are taken through the mouth are short amino acid chain peptides that can be taken by mouth, usually as a pill or liquid, rather than via injection.

Historically, commercially available protein formulations employing non-invasive delivery routes such as pulmonary, ophthalmic, nasal, rectal, buccal, vaginal and other routes have been investigated. However, oral administration is the most convenient and preferred mode of therapeutic delivery, with the highest patient compliance, therapeutic simplicity, and low cost of production.

Oral delivery of PPs is difficult to achieve due to significant barriers in the gastrointestinal tract (GIT). Barriers to circumvent include acid-catalyzed hydrolysis, proteolytic degradation by enzymes, inability to cross the membrane, first-pass metabolism during transfer across the absorption barrier, high molecular weight ( > 700 Da), and hydrophilicity.

While most peptides have historically required needles because the acids and enzymes in the stomach destroy them, recent breakthroughs in drug delivery have made "peptide pills" a reality for several major health conditions. However, oral delivery is challenging since regular peptides face a "hostile environment" in the human digestive system, which is why oral versions are very difficult to engineer. The stomach and intestines produce proteases designed specifically to break down proteins into food. These enzymes don't distinguish between a steak and a life-saving medicine. The high acidity of the stomach can cause peptides to unfold and denature or lose their shape, rendering them inactive. Peptides are often large molecules that struggle to pass through the tight junctions of the intestinal lining to reach the bloodstream.

The following challenges for oral delivery must be solved for the production of a successful peptide drug:

• Enzymatic degradation by proteases in the stomach and small intestine that renter peptide-based drugs useless.
• Extreme acidity in the stomach can denature peptide drugs.
• Poor permeability of hydrophilic peptides. 

To survive the gut and reach the blood, scientists use several "cloaking" and "shielding" techniques:

• Permeation Enhancers (PEs) such as SNAC, used in oral Ozempic, temporarily loosen the intestinal lining or protect the peptide locally allowing it to slip through into the blood. To produce this type of peptide, the peptide is co-formulated with the carrier molecule.
• Cyclic & Double-Bridged Peptides make peptides too rigid for digestive enzymes to break them down. In this case, the peptide is produced in ring shape.
• Enteric Coatings in tablets only dissolve in the alkaline environment of the small intestine, bypassing the harsh stomach acid entirely. In this case, tablets are sprayed with pH-sensitive polymers that remain solid in the stomach but dissolve in the small intestine. The polymers are stable at pH 1.5 to 3.5 but dissolve at pH 6 to 7.
• Liposomes & Nanoparticles allow wrapping peptides in a tiny "fat bubble" or lipid that can protect it from enzymes and help it merge with cell membranes for better absorption. Nanoparticles utilize biocompatible plastics such as PLGA for the creation of a slow-release shell.

Current Examples of Oral Peptides

While many are still in clinical trials, several are already FDA-approved and are used.

Table 1: FDA-approved oral peptides.

Since some of these PEs including SNAC, C8, C10 and acyl carnitines are currently in advanced clinical trials. Since several enhancer-based formulations maybe soon be available as pharmaceuticals for selected highly potent oral peptides with high therapeutic index post-marketing observations will most likely decipher the true toxicological effects of repeat dosing of selected PEs in high doses. However, as a cautionary approach, PE-containing formulations may not be the choice for patients with inflammatory bowel disease, irritable bowel syndrome or celiac disease.

Oral Peptides may have a higher patient compliance, avoiding "needle phobia", are easier to store since no refrigeration is needed, and gut-specific diseases can be treated directly. However, the low bioavailability of some oral peptides, often only 1 to 2% of the drug reach the blood, the high production costs, and strict dosing requirements, may limit their use. Also, oral peptides must be taken on an empty stomach with a specific amount of water.

Absorption enhancers for protein and peptide drugs (PPDs)

Absorption enhancers improve drug absorption by facilitating intestinal cells permeation by changing membrane fluidity or mucus viscosity, and/or opening tight junctions, generally governed by passive diffusion and modeled by Fick's first law of diffusion. Commonly used absorption enhancers are surfactants, fatty acids, chelators, glycerides, bile salts, salicylates, chitosan and cholesterol.

Sodium N-[8-(2-hydroxybenzoyl)amino]caprylate (SNAC) is a promising absorption enhancer can enhance passive permeation of polar charged drug molecules through the intestinal epithelium. SNAC has not been reported to be associated with significant disruption of the tight junctions, change in membrane fluidity, thus the low toxicity is beneficial for later clinical studies.

Another effective permeation enhancer, 8-(N-2-hydroxy-5-chloro-benzoyl)-amino-caprylic acid) (5-CNAC) is the leading examples of Eligen® technology from Emisphere. It was reported that 5-CNAC can deliver macromolecules (> 150 kDa), enhances transcellular absorption without disrupting intestinal integrity. Karsdal et al. (Osteoarthritis and Cartilage) incorporated 5-CNAC with calcitonin for oral administration. 5-CNAC interacts with calcitonin forming an insoluble entity at low pH in stomach, once it reaches small intestine at higher pH, the complex dissolves and facilitates intestinal drug uptake, resulting in systemic exposure of intact peptide (Liu et al. 2015)

Mucolytic agents

Mucolytic agents are nanoparticles composed of poly(acrylic acid) and papain were prepared and characterized regarding particle size and zeta potential. Analysis of nanoparticles showed mean diameters sub-200 nm (162.8–198.5 nm) and negative zeta potentials advancing the mobility in mucus gel. 

Table 2. Common absorption/penetration enhancers and their mechanisms of action (Mehrota et al. 2023).

References

Borchard G, Lueβen HL, de Boer AG, Verhoef JC, Lehr CM, Junginger HE. The potential of mucoadhesive polymers in enhancing intestinal peptide drug absorption III: effects of chitosan-glutamate and carbomer on epithelial tight junctions in vitro. J Control Release. 1996;39(2-3):131–8. [ScienceDirect]

Bowe CL, Mokhtarzadeh L, Venkatesan P, Babu S, Axelrod HR, Sofia MJ, et al. Design of compounds that increase the absorption of polar molecules. Proc Natl Acad Sci U S A. 1997;94(22):12218–23. [PMC] [PubMed]

Chen G, Kang W, Li W, Chen S, Gao Y. Oral delivery of protein and peptide drugs: from non-specific formulation approaches to intestinal cell targeting strategies. Theranostics. 2022 Jan 1;12(3):1419-1439. [PMC]

Cox DS, Raje S, Gao H, Salama NN, Eddington ND. Enhanced permeability of molecular weight markers and poorly bioavailable compounds across Caco-2 cell monolayers using the absorption enhancer, zonula occludens toxin. Pharm Res. 2002;19(11): 1680–8. [PubMed]

Gschwind HP, Glaenzel U, Waldmeier F, Wirz B, Sabia HD, Picard F, Weiss HM, Choi L, Swart PJ, Vasudevan A, Azria M. Metabolism and disposition of the oral absorption enhancer 14C-radiolabeled 8-(N-2-hydroxy-5-chlorobenzoyl)-amino-caprylic acid (5-CNAC) in healthy postmenopausal women and supplementary investigations in vitro. Eur J Pharm Sci. 2012 Aug 30;47(1):44-55. [ScienceDirect]

Karsdal MA, Byrjalsen I, Henriksen K, Riis BJ, Lau EM, Arnold M, Christiansen C. The effect of oral salmon calcitonin delivered with 5-CNAC on bone and cartilage degradation in osteoarthritic patients: a 14-day randomized study. Osteoarthritis Cartilage. 2010 Feb;18(2):150-9. [Osteoarthritis and Cartilage]

Larazotide

Liu M., Zhang J., Shan W., and Huang Y., Developments of mucus penetrating nanoparticles. Asian Journal of Pharmaceutical Sciences. Volume 10, Issue 4, 2015, Pages 275-282,

Marinaro M, Di Tommaso A, Uzzau S, Fasano A, De Magistris MT. Zonula occludens toxin is a powerful mucosal adjuvant for intranasally delivered antigens. Infect Immun. 1999 Mar;67(3):1287-91. [PMC]

McCartney F, Gleeson JP, Brayden DJ. Safety concerns over the use of intestinal permeation enhancers: A mini-review. Tissue Barriers. 2016 Apr 12;4(2):e1176822. [PMC]

Mehrotra S, Kalyan Bg P, Nayak PG, Joseph A, Manikkath J. Recent Progress in the Oral Delivery of Therapeutic Peptides and Proteins: Overview of Pharmaceutical Strategies to Overcome Absorption Hurdles. Adv Pharm Bull. 2024 Mar;14(1):11-33. [PMC]

Peng H, Wang J, Chen J, Peng Y, Wang X, Chen Y, Kaplan DL, Wang Q. Challenges and opportunities in delivering oral peptides and proteins. Expert Opin Drug Deliv. 2023 Jul-Dec;20(10):1349-1369. [PMC]

Raiman J, Törmälehto S, Yritys K, Junginger HE, Mönkkönen J. Effects of various absorption enhancers on transport of clodronate through Caco-2 cells. Int J Pharm. 2003;261(1-2):129–36. [PubMed]

Solis-Herrera C, Kane MP, Triplitt C. Current Understanding of Sodium N-(8-[2-Hydroxylbenzoyl] Amino) Caprylate (SNAC) as an Absorption Enhancer: The Oral Semaglutide Experience. Clin Diabetes. 2024 Winter;42(1):74-86. [PMC]

Swenson ES, Milisen WB, Curatolo W. Intestinal permeability enhancement: efficacy, acute local toxicity, and reversibility. Pharm Res. 1994;11(8):1132–42. [PubMed]

Thanou M, Verhoef JC, Junginger HE. Oral drug absorption enhancement by chitosan and its derivatives. Adv Drug Deliv Rev. 2001; 52(2): 117–26. [PubMed]

Wang WL, Lu RL, DiPierro M, Fasano A. Zonula occludin toxin, a microtubule binding protein. World J Gastroenterol. 2000 Jun;6(3):330-334. [PMC]

Yeh PY, Berenson MM, Samowitz WS, Kopečková P, Kopecek J. Site-specific drug delivery and penetration enhancement in the gastrointestinal tract. J Control Release. 1995;36(1-2):109–24. [PubMed]