Bioavailability

Bioavailability is the fraction of an administered dose that reaches systemic circulation unchanged, available to exert its biological activity. Expressed as a percentage (denoted F), it is a pivotal indicator of peptide efficacy and fundamentally depends on the chosen administration route. For peptides, this parameter is all the more critical as they are fragile molecules, rapidly degraded by digestive enzymes and poorly permeable across mucosal barriers.

The intravenous (IV) route is by definition 100% bioavailable: the entire dose enters circulation directly. It serves as the reference for calculating absolute bioavailability of other routes. The subcutaneous (SC) route, standard for most research and therapeutic peptides (semaglutide, tirzepatide, GHRP, BPC-157), typically reaches 70-90%. Slow absorption via the subcutaneous capillary bed avoids first-pass hepatic effect and offers a more gradual kinetic than IV — ideal for 24-hour or longer effects.

The intramuscular (IM) route shows bioavailability close to SC (80-95%) with slightly faster absorption. It suits oily peptides or large volumes. The oral route, very limited for peptides (often <2%), faces two massive obstacles: proteolytic degradation in the stomach and intestine (pepsin, trypsin, chymotrypsin, brush-border peptidases) and low intestinal barrier permeability to hydrophilic molecules >500 Da. Oral semaglutide (Rybelsus) partially circumvents this with a permeation enhancer (SNAC) that transiently opens tight junctions, reaching 0.4-1% bioavailability but with massive doses (3 to 14 mg versus 0.25 mg SC).

Intermediate mucosal routes offer interesting alternatives. The nasal route enables rapid absorption via the highly vascularized respiratory mucosa (5-40% bioavailability depending on peptide, e.g., nasal oxytocin, PT-141). The sublingual or buccal route, used for Selank, Semax, and some TRH analogs, leverages oral mucosa vascular richness to reach 10-30%. The transdermal route remains marginal for peptides >500 Da without specific carriers (liposomes, nanoparticles, microneedles).

Beyond route, bioavailability depends on formulation. Excipients (phosphate buffers, sodium chloride, benzyl alcohol as bacteriostatic preservative), pH of the reconstituted solution, and peptide solubility at the chosen concentration all influence absorption. Lyophilized peptides reconstituted in standardized bacteriostatic water (0.9% BAC) ensure optimal reproducibility across batches and experiments. In a research protocol, documenting estimated bioavailability percentage and inter-individual variability (data may fluctuate +/- 30% across subjects) is essential for interpreting dose-response measurements.