In Vivo

The term in vivo, literally "in the living", designates any experiment conducted within an intact organism, most often an animal model and, strictly within clinical trial frameworks, a human. It is the step that validates the physiological, pharmacokinetic, and toxicological relevance of a molecule before any possible development in humans.

Common animal models in peptide research span several scales. Rodents (mice, rats) dominate through accessible cost, fast reproductive cycle, availability of genetically modified lines (knock-in, knock-out, conditionals, humanized), and an enormous reference database enabling cross-study comparison. Disease-specific models include ob/ob and db/db diabetic mice, ZDF rats, tendon/ligament regeneration models, cardiovascular ischemia-reperfusion models, and xenografted or genetically induced tumor models. The zebrafish (Danio rerio) has emerged for fast embryonic screening (transparent, external development, 3-5 days to phenotype). Non-human primates remain used in late pharmaceutical development when human pharmacology must be finely modeled (complex therapeutic peptides, antibodies).

Classical in vivo peptide pharmacology studies include: pharmacokinetics (PK, plasma curves after IV, SC, IP, oral administration, calculation of half-life, clearance, volume of distribution, bioavailability), pharmacodynamics (PD, measurement of biological effect over time), dose-effect studies (NOAEL, LOAEL, ED50 determination), single-dose and repeated-dose toxicity (14-28 days), genotoxicity and carcinogenesis studies, reproduction and teratology, and efficacy studies on disease models.

The regulatory and ethical framework is strict. The 3R principle (Reduction of animal numbers, Refinement of procedures to minimize pain, Replacement by alternative methods when possible), formalized by Russell and Burch in 1959, structures every experimental approach. In Europe, Directive 2010/63/EU and its national transpositions (in France: Code Rural R. 214-87 et seq.) require project authorizations validated by ethics committees, mandatory training of experimenters, approved facilities, and solid scientific justification for each experiment. In the United States, the IACUC (Institutional Animal Care and Use Committee) plays an equivalent role, anchored in the Animal Welfare Act.

Complementarity with in vitro is essential. In vitro allows fast, ethical, low-cost screening but lacks integrated physiology. In vivo validates biological relevance but is expensive, time-consuming, and raises ethical questions. A serious research program combines both: in vitro validation followed by in vivo confirmation on a minimal relevant model, then deeper study on more complex models. Human-translatable data are reinforced by consistency between in vitro (human cells), in vivo (animal models), and ex vivo (human tissues, patient-derived organoids) results.

For peptides sold as Research Use Only (RUO), these products are not authorized for human administration; published studies referencing them have all been conducted within in vivo animal or in vitro research frameworks.