GlossaryPharmacokineticsTissue Residency
Pharmacokinetics

Tissue Residency

Also known as: tissue persistence · depot effect

Tissue residency is an informal term for how long a drug stays in a tissue, or bound to its target, after blood levels fall. It is not a standard PK parameter.

Pharmacokinetic studies mostly measure drug in blood plasma, because blood is easy to sample. 'Tissue residency' refers to a different question: how long a drug persists outside the blood, whether in an organ, at the injection site, or bound to its receptor. It has no standard definition or formula, so when you see the phrase, ask which of these the writer means and how it was measured.

Two ideas are often blended. The first is tissue concentration: how much drug is physically present in a tissue over time. Measuring this is harder than it sounds; a review of antibiotic research concluded that comparing drug levels in ground-up tissue samples with blood levels, and drawing conclusions from that, is often misleading. The second is target residence time: how long a drug stays bound to its receptor. Some drugs keep acting long after their blood concentration has dropped to insignificant levels, and the lifetime of the drug-receptor complex is one model pharmacologists use to explain that.

Why it matters

Tissue residency is often invoked to explain why a peptide with a very short plasma half-life might still have lasting effects. That is a hypothesis, not a measurement, unless someone has measured the drug in the tissue, or on its target, at repeated time points.

For research peptides, the evidence is thin. The one published pharmacokinetic and distribution study of BPC-157 was in rats and beagle dogs: intact BPC-157 had an elimination half-life under 30 minutes, the radiolabeled peptide was broken down into small fragments and single amino acids, and tissue levels were reported as total radioactivity, which counts those breakdown products along with intact peptide. Those data cannot tell you how long intact BPC-157 stays in any human tissue. Treat statements that a research peptide 'accumulates at injury sites' as unproven unless they cite a study that measured it.

How to read a tissue-residency claim

  1. Ask what was measured: intact drug, total radioactivity, or only an effect.
  2. Check the species and whether the tissue studied was injured or healthy.
  3. Check the method; tissue-homogenate levels compared with blood levels are often misleading.
  4. Check the timing; persistence needs repeated measurements, not a single time point.
  5. If the claim rests only on an effect outlasting the plasma half-life, treat it as an inference.
Worked example

Claim: 'BPC-157 has a short half-life but stays in tendons for days.' Check: the published pharmacokinetic data are from rats and dogs, the plasma half-life of the intact peptide was under 30 minutes, and tissue data were total radioactivity rather than intact peptide. Verdict: that source provides no measurement behind the 'days in tendons' part.

Common mistakes

  • Treating 'tissue residency' as a measured number when it is usually an inference.
  • Reading radioactivity in a tissue as intact drug; the radioactive tag stays on breakdown fragments too.
  • Extending animal tissue data to people.
  • Assuming an effect that outlasts plasma levels proves the drug is still in the tissue; it may reflect target binding or changes the drug set in motion.
  • Citing a large volume of distribution as proof that a drug resides at a particular site.

Sources

  1. ReviewTissue concentrations: do we ever learn?. J Antimicrob Chemother, 2008. PMID 18065413.
  2. ReviewThe importance of binding kinetics and drug-target residence time in pharmacology. Br J Pharmacol, 2024. PMID 37160660.
  3. Preclinical studyPharmacokinetics, distribution, metabolism, and excretion of body-protective compound 157, a potential drug for treating various wounds, in rats and dogs. Front Pharmacol, 2022. PMID 36588717.

Page updated . How we grade evidence: methodology.

Related Terms

Volume of Distribution

Volume of distribution (Vd) is the volume a drug would have to fill, at its plasma concentration, to account for all the drug in your body.

Half-Life

Half-life (t½) is the time it takes for the concentration of a drug in your blood plasma to fall by half.

Clearance Rate

Clearance (CL) is the volume of plasma your body fully clears of a drug per unit of time, usually given in liters per hour.

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