Glossary
In Vivo
In vivo means inside a living organism. What animal studies add beyond cell work, how to judge model and species claims, and where translation tends to fail.
In vivo translates as "within the living" and refers to experiments in a whole organism instead of in culture. In peptide research it nearly always means rodent studies — mice and rats — while zebrafish, rabbit, dog and non-human primate models enter at later phases.
What animal work adds, and its remaining limits
An in vivo experiment reintroduces, all at the same time, every process that a cell assay strips out: uptake from the site of administration, binding to plasma proteins, distribution into tissues, clearance by liver and kidney, contact with peptidases, immune recognition and homeostatic feedback. A peptide with nanomolar receptor affinity in vitro might never get to that receptor in an animal — and many peptide programmes end at exactly this point.
Reading the results still demands caution. The validity of the model is the typical weakness: an induced rodent model only partly mirrors the human disease whose name it carries. Differences between species in receptor sequence, metabolic rate and peptidase profile all alter outcomes, and scaling from rodent to human is not a simple ratio. Quality also varies: randomisation, blinding and justification of sample size are reported unevenly across the peptide literature.
Remember the framing rule used on this site: in vivo results are stated by species and model and are never used to support statements about people.
Related terms and material
preclinical · phase 1/2/3 trial · species reactivity. Reference material: all research products.