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Transcript of Introduction
Properties of drugs
and membrane permeation
Drug membrane permeation
Goal: prevent, cure, control diseases
: what body does to drug (quantitative description of drug concentrations in body over time)
Specific routes of administration affect optimum dose.
: what drug does to body
Effects of drugs:
related to dose
better predicted by concentration achieved in blood/target tissue (than dose)
related to length of time in body
Drug disposition processes (ADME) and terminology
Site of administration
Volume of distribution (Vd)
Organic - natural or synthetic
tens of amino acids (small peptidic hormones)
Enzymes, hormones, antibodies
*All ADME processes depend on drug movement across membrane
Skin, corneal epithelia
Cells of the organs
Water pores of 8 A allow also passage of:
Small (< 150kDa) hydrophilic molecules
Membrane plays a passive role; no energy is required beyond that necessary to maintain its integrity.
Rate of drug transport is proportional to Concentration gradient.
Frequently follows first-order (linear) kinetics:
Rate of transport = k[Conc]
where k is a first-order constant.
Process is independent of the presence of other compounds.
Transport determined by physicochemical properties of the drug:
lipid solubility and degree of drug ionization
Rate diffusion =
P * A * C
P = permeability (drug's physicochemical properties)
A = surface area
C = concentration gradient
d = membrane thickness
Oxygen (alcohols, aldehydes, carboxylic acids)
Higher partition coefficient = more dissolved in Octanol = higher lipophilicity
Measured by Partition Coefficient
Drug Ionization, pH, and Partition
Rate of passive diffusion depends on concentration -
of diffusable form of drug.
May be different from "total" concentration of drug
Un-ionized form is more diffusable: Ionization depends on pH and pKa of drug.
HA <-> A-
B <-> BH+
Very different from diffusion!
Physicochemical properties for facilitated transport
Metabolized into conjugate (phase II)
to possess these properties to be a substrate