Peptide Bioavailability: How Route of Administration Changes Absorption

There is a relationship between the method of peptide delivery and the amount of peptide this way. A pair of vials of the same substance, despite being treated the same way, can behave very differently in terms of the concentration of peptides in blood capillaries of a human being. The post offers a description of the different ways the bioavailability can be measured and a short description of the science behind these measurements.

What Bioavailability Means for Peptides

Definition and Why It Matters

Bioavailability is the fraction of an administered compound that reaches systemic circulation in unchanged form. An intravenous injection delivers 100% bioavailability by definition, the entire dose enters the bloodstream directly. Every other route loses some fraction to degradation, incomplete absorption or metabolism before the compound reaches circulation. For peptides, bioavailability is the core variable that separates route choices: the same milligram amount administered orally vs subcutaneously can produce a 20-fold difference in circulating levels.

First-Pass Metabolism

Compounds taken by mouth go through the gut wall and into the liver before entering the bloodstream. This process is called first-pass metabolism and it kills most peptides before they work. Peptic bonds are broken by stomach acid. Proteins in the intestine further break down the products. The enzymes in the liver also change whatever is left after absorption. The end result is that oral bioavailability for unmodified peptides is often in the single digits. Sources of administration that do not go through the GI system like injections intranasal spray and topical delivery avoid some or all of the obstacles.

Bioavailability by Route

Subcutaneous Injection

The peptide is injected subcutaneously and travels to the layer of fat under the skin, before being absorbed through the blood capillaries and hence into the general circulation. Since the injection route avoids the GI tract and first-pass metabolism, the bioavailability of the peptide is almost 100%. The only loss incurred is due to the local breakdown of the peptide at the injection site and slow absorption from the fat tissue. The SC injection route is standard for most research peptides including BPC-157, TB-500, Ipamorelin and MOTS-c.

Oral Administration

Most peptides have low oral bioavailability of under 5%. This is because gastric acid (pH 1.5-3.5) and digestive enzymes destroy the peptide chain before it is absorbed. Enteric coatings help capsules survive in the stomach, but do not stop enzyme action in the small intestine. BPC-157 is an exception, because of its originating from gastric juice, it is particularly stable in acid compared to most peptides, and given results from animal studies, it has a biological effect when given orally that can be compared to that of injected peptides. See our oral peptides guide for more details.

Nasal Delivery

Nasal mucosa has a moist and vascularized surface that allows for easy absorption. Administration into the nasal cavity avoids GI tract and partially avoids the blood-brain barrier due to the use of the olfactory nerve pathway. So nasal delivery is the most suitable method for the administration of peptides that are used for targeting the CNS such as Semax and Selank. According to published figures, the nasal bioavailability of peptides is estimated at 10–30% which is less than that of injections but more than that of oral administration. The constraining factor to nasal delivery is the volume for administration because a limited amount of solution can be used at a time.

Topical Application

Topical peptides like GHK-Cu face the skin barrier, the stratum corneum is designed to keep molecules out. Systemic bioavailability from topical application is typically below 5% for peptides. However, for skincare applications the target tissue is the skin itself, not the bloodstream. A copper peptide cream does not need systemic bioavailability, it needs to penetrate the epidermis and reach the dermis, which is a different and shorter delivery challenge.

Peptide-Specific Bioavailability Data

BPC-157 Oral Stability Exception

BPC-157's origin in human gastric juice gives it unusual stability in acidic conditions. Published animal studies report biological effects (gastrointestinal protection, liver protection, and even tendon healing markers) with oral BPC-157 at doses comparable to injectable protocols. A 2018 study in the Journal of Physiology and Pharmacology demonstrated oral BPC-157 efficacy in gastric lesion models. This gastric stability is the reason oral BPC-157 capsules exist as a product category, and why BPC-157 is the only research peptide commonly sold in both injectable and oral forms.

Semax Nasal Absorption

Although the absolute bioavailability of Semax is low, the drug reaches the requisite functional concentrations in the CNS via a nasal spray. The drug operates through two main mechanisms: first, the drug is released through the nasal mucosa and absorbed into systemic circulation; second, Semax crosses directly into the brain via the olfactory nerve. The second mechanism is true only in the case of the nasal route and makes the use of nasal verses injection more favorable for the formulation of Semax and Selank. According to the data presented in scientific literature form Russia, an effective dose of nasal Semax is from 200 to 600 mcg per day.