Barrier Principles, Membrane Permeability and Topical Deposition

File

Intratympanic Diffusion Kinetics

Intratympanic administration of drugs (corticosteroids or aminoglycosides) depends on passive diffusion through the round window membrane (RVM). This process can be modeled by an adaptation of Fick's diffusion law coupled to the perilymph clearance rate:

dC_peri(t)dt = D · A · (K · C_ear\medio(t) - C_peri(t))h · V_peri - k_e C_peri(t)

Where D is the molecular diffusion coefficient of the drug in the MVR, A is the area of the membrane, h is its thickness, K represents the partition coefficient of the substance, V_peri is the volume of perilymph in the scala tympani, and k_e is the first-order elimination constant of the drug into the systemic circulation or cerebrospinal fluid. Understanding these parameters allows predicting the local therapeutic window in auditory and vestibular pathologies.

Likewise, rhinosinusal mucociliary clearance imposes a severe temporal limitation on the contact of intranasal topical drugs. This compendium analyzes in detail the available pharmacological tools, providing physiological foundations for clinical decision making.