Bioavailability
Definition
The proportion of an administered drug that reaches the systemic circulation in an unchanged, pharmacologically active form, expressed as a percentage of the total dose given, which determines the drug's therapeutic effectiveness.
Bioavailability
Bioavailability is a fundamental pharmacokinetic parameter that describes the fraction of an administered dose of a drug that successfully reaches the systemic circulation in its active form. For drugs administered intravenously, bioavailability is by definition 100%, as the entire dose enters the bloodstream directly. For all other routes of administration — including oral, subcutaneous, intramuscular, and transdermal — bioavailability is typically less than 100% due to incomplete absorption, first-pass metabolism in the liver, or degradation at the site of administration.
For peptide-based therapeutics like retatrutide, bioavailability is a critical consideration in formulation and route selection. Peptides are rapidly degraded by proteolytic enzymes in the gastrointestinal tract, which is why most GLP-1 receptor agonists, including retatrutide, are administered via subcutaneous injection rather than orally. The subcutaneous route provides relatively high and consistent bioavailability for peptide drugs, as absorption from the subcutaneous tissue depot into the bloodstream is predictable and avoids first-pass hepatic metabolism. The bioavailability achieved through subcutaneous injection directly influences the dosing regimen, as it determines what administered dose is needed to achieve the target circulating drug concentrations.
Understanding bioavailability is also essential when comparing different formulations or routes of administration for the same drug. For instance, oral semaglutide achieves a bioavailability of only approximately 1% due to gastrointestinal degradation, requiring substantially higher oral doses compared to the subcutaneous formulation to achieve equivalent plasma levels. This contrast illustrates why the subcutaneous route remains the preferred delivery method for most peptide-based metabolic therapies, including retatrutide, where reliable and efficient drug delivery is essential for maintaining the steady-state concentrations needed for once-weekly dosing.