In vivo

Definition

In vivo refers to biological research conducted within a living organism, including animals (mouse, rat, dog, monkey) or humans. The term comes from Latin meaning 'in life.' In vivo studies investigate how a compound is absorbed, distributed, metabolized, and excreted in a living system; how it binds to target and off-target molecules; and what biological effects it produces. In vivo research is essential because living organisms have complex interactions between tissues and systems (nervous system, endocrine system, immune system, kidney and liver function) that are absent in simplified in vitro models. A peptide that shows promising effects in cell cultures may fail in animal studies due to poor absorption, rapid degradation, off-target effects, or toxicity. Conversely, in vivo studies can reveal beneficial effects not apparent in cell cultures because of systemic interactions. In vivo studies in animals are required before human clinical trials. In vivo studies in humans (clinical trials) provide the most direct evidence of safety and efficacy but are expensive, time-consuming, and ethically restricted to compounds showing sufficient promise.

Animal models are chosen based on relevance to the condition being investigated. Small rodent models (mice, rats) are used for initial toxicity screening, mechanistic studies, and disease modeling because they are inexpensive and share substantial genetic and physiological similarities with humans. Larger animals (dogs, monkeys) are sometimes used for toxicity studies and bioavailability studies because their physiology more closely resembles humans. Disease-specific models (transgenic mice with genetic mutations causing disease-like phenotypes) are valuable for studying compounds in disease contexts.

Human in vivo studies include clinical trials, which are the gold standard for evaluating peptide safety and efficacy. Clinical trials are structured with specific phases: Phase 1 focuses on safety and pharmacokinetics in small groups of healthy volunteers; Phase 2 evaluates efficacy in small groups of affected patients; Phase 3 confirms efficacy and monitors adverse events in larger patient populations; Phase 4 is post-marketing surveillance. Each phase must demonstrate acceptable safety before progression to the next phase. Regulatory approval is based primarily on data from in vivo human clinical trials.