Enteric Coatings for Peptides: Surviving Acid

Oral Peptide Drugs

pH 1-2 in the stomach

Stomach acid destroys most peptide drugs within minutes. Enteric coatings are polymers that remain intact at acidic pH but dissolve in the alkaline environment of the small intestine, protecting peptides during gastric transit.

Verma et al., Drug Development Research, 2021

Verma et al., Drug Development Research, 2021

View as image

Peptide drugs face a gauntlet when swallowed. The stomach's pH of 1-2 denatures their three-dimensional structure within minutes, and pepsin, the stomach's primary protease, cleaves peptide bonds indiscriminately.^[1] Even peptides that survive acid exposure then encounter pancreatic proteases, the mucus layer, and the intestinal epithelium. Enteric coatings address the first and most destructive of these barriers: stomach acid itself. For the full picture of why oral peptide delivery is so difficult, see our guide to the future of oral peptide drugs.

Why stomach acid destroys peptides

The problem is structural, not just chemical. Peptide drugs depend on precise three-dimensional folding to bind their target receptors. At pH 1-2, protonation disrupts the hydrogen bonds and electrostatic interactions that maintain this structure. The peptide unfolds, exposing its backbone to enzymatic attack.^[1]

A 2026 review catalogued the four sequential barriers oral peptides face: the acidic pH environment, enzymatic degradation by gastric and pancreatic proteases, the mucus gel layer lining the intestinal wall, and the tight junctions of the intestinal epithelial barrier.^[2] Enteric coatings address the first barrier. The remaining three require additional strategies, including permeation enhancers and enzyme inhibitors.

The magnitude of the challenge is reflected in bioavailability numbers. Most orally administered peptides achieve less than 1% bioavailability, meaning more than 99% of the swallowed dose never reaches the bloodstream in active form. Even the two FDA-approved oral peptide drugs, Rybelsus (semaglutide) and Mycapssa (octreotide), achieve approximately 1% oral bioavailability with their respective delivery technologies.^[3]

How enteric coatings work

Enteric coatings are polymers applied to tablets or capsules that exploit the pH gradient of the gastrointestinal tract. The stomach maintains a pH of 1-3 during digestion, while the duodenum and jejunum operate at pH 6-7.5. Enteric polymers contain carboxylic acid groups that remain protonated (and therefore insoluble) at low pH but ionize and dissolve at higher pH.

The polymer chemistry

The most widely used enteric coating polymers fall into three categories:

Methacrylic acid copolymers (Eudragit series). Eudragit L100 dissolves at pH 6.0 and above, while Eudragit S100 dissolves at pH 7.0 and above. The difference is the ratio of methacrylic acid to methyl methacrylate, which determines the density of ionizable carboxylic acid groups. Higher carboxylic acid content means dissolution at lower pH.

Cellulose-based polymers. Cellulose acetate phthalate (CAP) dissolves at pH 6.0, while hydroxypropyl methylcellulose phthalate (HPMCP) dissolves at pH 5.0-5.5 depending on the degree of phthalyl substitution. Hydroxypropyl methylcellulose acetate succinate (HPMCAS) has a pKa of approximately 5, making it suitable for duodenal-targeted release.

Polyvinyl derivatives. Polyvinyl acetate phthalate (PVAP) provides gastric resistance with dissolution at pH 5.0, offering an alternative when cellulose-based polymers are unsuitable due to formulation incompatibilities.

The choice of polymer determines where in the intestine the coating dissolves. This matters because different regions of the small intestine have different protease concentrations, surface areas, and transporter densities. For peptide drugs, the goal is typically rapid dissolution in the proximal small intestine, where the epithelial surface area is largest and enzymatic activity, while still present, is lower than in the stomach.

What enteric coatings protect against

Enteric coatings shield peptides from three gastric threats simultaneously. First, they prevent acid-mediated denaturation by keeping the peptide in a sealed compartment at neutral pH. Second, they block access by pepsin, the primary gastric protease that is active only at acidic pH. Third, they prevent mechanical disruption from gastric motility, which can expose peptides to larger surface areas of acidic fluid.^[4]

What enteric coatings do not protect against is equally important. Once the coating dissolves in the intestine, the peptide faces pancreatic proteases (trypsin, chymotrypsin, elastase), the mucus barrier that limits diffusion to the epithelium, and the tight junctions between epithelial cells that block paracellular transport. This is why enteric coatings alone are insufficient for oral peptide delivery and are typically combined with other strategies.

Two approaches to oral peptide delivery: stomach vs. intestine

The two FDA-approved oral peptide drugs illustrate fundamentally different strategies for dealing with stomach acid.

MYCAPSSA: bypass the stomach entirely

Mycapssa (oral octreotide), approved by the FDA in 2020, uses enteric-coated capsules containing Transient Permeation Enhancer (TPE) technology. The enteric coating protects the capsule through the stomach. Once in the small intestine, the coating dissolves, releasing sodium caprylate (C8) along with octreotide. The C8 transiently opens tight junctions between intestinal epithelial cells, allowing octreotide to pass through.^[5]

The clinical reality illustrates the efficiency gap. Mycapssa requires a 20 mg oral dose to achieve plasma levels equivalent to a 0.1 mg subcutaneous injection, reflecting a relative oral bioavailability of approximately 0.7%.^[5] Phase III trials demonstrated that this was sufficient for therapeutic efficacy in acromegaly, but the 200-fold dose differential highlights how much drug is lost in transit.

This bypass strategy is conceptually straightforward: protect the peptide from the stomach, release it in the intestine, and enhance absorption at the intestinal wall. Enteric coating handles step one. Permeation enhancers handle step three. The mucus barrier and residual enzymatic degradation account for most of the loss.

Rybelsus: protect the peptide in the stomach

Rybelsus (oral semaglutide), approved in 2019, takes the opposite approach. Instead of enteric-coating the tablet to bypass the stomach, it uses sodium salcaprozate (SNAC) to create a protective microenvironment within the stomach itself.^[6]

SNAC works through three simultaneous mechanisms: it raises local pH in the immediate vicinity of the dissolving tablet, shielding semaglutide from acid denaturation; it forms a complex with semaglutide that protects against pepsin; and it enhances transcellular absorption directly across stomach epithelial cells.^[7] This means Rybelsus achieves absorption in the stomach rather than the intestine, a fundamentally different pharmacokinetic pathway.

The Rybelsus approach avoids enteric coatings entirely. Its oral bioavailability is also approximately 1%, similar to Mycapssa, but through stomach absorption rather than intestinal absorption.^[3] The development of how SNAC makes oral semaglutide possible is covered in detail in our dedicated article.

What this comparison reveals

The fact that both approaches achieve roughly 1% bioavailability through entirely different anatomical routes suggests the bottleneck is not stomach acid alone. Acid is one barrier among several, and overcoming it, whether by enteric coating or pH buffering, does not solve the downstream challenges of enzymatic degradation, mucus penetration, and epithelial permeability.

A 2021 review of formulation strategies confirmed this: enteric coatings, mucoadhesive systems, nanoparticle co-encapsulation, and timed-release formulations each address different aspects of the delivery problem, but no single strategy achieves high oral bioavailability for peptides.^[4]

The next generation: combining approaches

The trend in oral peptide formulation is toward multi-barrier strategies that layer enteric coatings with other technologies.

Enteric coating plus permeation enhancers

MYCAPSSA already combines enteric coating with C8 permeation enhancement. A 2025 preclinical and clinical study tested whether combining SNAC and C10 (sodium caprate) in erodible tablets could improve gastric peptide delivery beyond either enhancer alone.^[8] This hybrid approach attempts to get the best of both worlds: local pH protection from SNAC plus tight junction opening from C10.

A 2024 preclinical study demonstrated that an oral peptide delivery formulation called Axcess achieved biopotencies of 9% for exendin-4 and 14.8% for semaglutide in animal models, substantially higher than the ~1% seen with current FDA-approved formulations.^[9] These numbers, if they translate to human clinical trials, would represent a significant improvement in oral peptide delivery efficiency.

pH-responsive nanoparticles

Beyond traditional polymer coatings, researchers are developing pH-responsive nanoparticle systems that encapsulate peptides in particles designed to swell and release their payload at intestinal pH. These systems can also be engineered with mucus-penetrating surface modifications and protease inhibitor co-payloads, addressing multiple barriers simultaneously.

A 2026 review of strategies for overcoming oral peptide delivery barriers highlighted that current approaches (nanoparticles, lipid-based systems, permeation enhancers, enzyme inhibitors, and targeted release formulations) have pushed bioavailability to the ~10% range in preclinical models, but translating these results to humans remains the central challenge.^[10]

The small molecule alternative

One response to the enteric coating challenge is to bypass peptide chemistry entirely. Danuglipron, Pfizer's small molecule GLP-1 receptor agonist, was designed to activate the same receptor as semaglutide without being a peptide at all. Small molecules are inherently resistant to stomach acid and proteases and can be absorbed through standard oral drug pathways. The trade-off is that small molecules often achieve lower receptor selectivity and may have different side effect profiles.

Limitations of enteric coatings for peptide drugs

Enteric coatings are a mature pharmaceutical technology, having been used for acid-labile small molecule drugs for over seven decades. Their application to peptide drugs, however, exposes several limitations that do not apply to traditional pharmaceuticals.

Coating thickness and dissolution lag. Thicker coatings provide more reliable gastric protection but create a delay between intestinal arrival and drug release. For peptides with narrow absorption windows, this lag can reduce bioavailability by allowing the formulation to transit past the optimal absorption site before the coating fully dissolves.

Gastric emptying variability. Enteric-coated formulations are highly sensitive to gastric emptying patterns, which vary with meal composition, body position, age, and disease state. A high-fat meal can delay gastric emptying by hours, extending the time the coating must resist acid degradation. This variability contributes to the high interpatient and intrapatient pharmacokinetic variability seen with oral peptide drugs. Rybelsus, for instance, must be taken on an empty stomach with no more than 4 ounces of water, and patients must wait at least 30 minutes before eating.^[11]

Incomplete protection. Even well-formulated enteric coatings can develop microcracks or defects during manufacturing, storage, or gastric transit. For small molecule drugs, minor coating failures may be clinically insignificant because the drug is inherently stable. For peptides, even brief acid exposure through a coating defect can irreversibly denature the active molecule.

They solve only one of four barriers. The most fundamental limitation is scope. Enteric coatings protect against stomach acid, but oral peptide bioavailability is limited by at least three additional barriers that operate downstream. Even perfect enteric coating performance cannot deliver oral bioavailability above single-digit percentages without additional formulation strategies for the intestinal barriers.

The Bottom Line

Enteric coatings protect peptide drugs from stomach acid by using pH-responsive polymers that dissolve only in the alkaline environment of the small intestine. While they reliably address gastric destruction, they solve only one of four barriers to oral peptide delivery. The two FDA-approved oral peptide drugs illustrate contrasting approaches: MYCAPSSA uses enteric coating to bypass the stomach, while Rybelsus uses SNAC to protect semaglutide within the stomach. Both achieve approximately 1% oral bioavailability, suggesting that downstream barriers, not stomach acid alone, determine oral peptide efficiency.

Frequently Asked Questions

What is an enteric coating on a peptide drug?

An enteric coating is a polymer layer applied to tablets or capsules that remains intact in stomach acid (pH 1-3) but dissolves in the alkaline environment of the small intestine (pH 5-7). For peptide drugs, this prevents acid-mediated destruction during gastric transit. Common enteric polymers include Eudragit L100, cellulose acetate phthalate, and HPMCAS.

Why can't you just swallow most peptide drugs?

Peptide drugs face four barriers when swallowed: stomach acid denatures their structure, proteases break their bonds, the mucus layer blocks diffusion, and intestinal epithelial tight junctions prevent absorption. Together these barriers destroy more than 99% of most orally administered peptides before they reach the bloodstream.

Does oral semaglutide use an enteric coating?

No. Rybelsus (oral semaglutide) uses a different approach called SNAC (sodium salcaprozate) that raises local pH in the stomach and enhances absorption directly through stomach epithelial cells. This is the opposite of enteric coating, which bypasses the stomach entirely. MYCAPSSA (oral octreotide) does use enteric coating combined with a permeation enhancer.

What is the bioavailability of oral peptide drugs?

Currently approved oral peptide drugs achieve approximately 1% bioavailability (Asano et al., 2023). This means 99% of the swallowed dose is destroyed or not absorbed. MYCAPSSA requires a 20 mg oral dose to match a 0.1 mg subcutaneous injection. Preclinical formulations have achieved 9-15% bioavailability in animal models, but human translation remains challenging.

What pH do enteric coatings dissolve at?

Different enteric polymers dissolve at different pH thresholds. Eudragit L100 dissolves at pH 6.0, Eudragit S100 at pH 7.0, cellulose acetate phthalate at pH 6.0, and HPMCP at pH 5.0-5.5. The choice of polymer determines where in the intestine the coating releases the drug, which affects both absorption efficiency and exposure to intestinal proteases.

Are enteric coatings enough to make peptide pills work?

Enteric coatings alone are insufficient for oral peptide delivery. They protect against stomach acid but not against the three remaining barriers: intestinal proteases, the mucus layer, and epithelial impermeability. Modern oral peptide formulations combine enteric coatings with permeation enhancers, enzyme inhibitors, or nanoparticle systems to address multiple barriers simultaneously.