Permeability refers to passage of solutes such as ions and small molecules through biological membranes.
The permeability and thus the bioavailability of chemicals into the systemic circulation and various tissues after application play a pivotal role in their therapeutic and toxic effects.
The penetration of topically applied compounds into and through skin can be measured using intact human and pig skin mounted into flow-through and static systems. The advantage of intact native skin is that it retains its stratum corneum (the main barrier layer) and therefore reflects in-vivo penetration of chemicals better than reconstructed skin models.
Once in the systemic circulation, a chemical may pass into the brain via the blood-brain-barrier (BBB). Efflux transporters located in the BBB play a protective role against the entry of potentially neurotoxic chemicals into the CNS. Passage of chemicals across the BBB can be measured in-vitro using microvascular endothelial cells and primary rat glial cells cultured in a trans-well format.