Pharmacology: Drug Absorption and Distribution Flashcards
absorption
drug absorbed from site of administration into plasma
distribution
drug reversibly leaves blood stream entering IC and interstitial fluids
what 4 ways can small molecules cross the cell membrane
- passive diffusion
- facilitated diffusion
- active transport
- endocytosis
where are the principle sites of carrier mediated drug transport
BBB, placenta, GI tract, renal tubules, biliary tract
what is saturation kinetics
limited or finite amounts of carrier proteins
what does the proportion of ionisation of a drug depend on
drug pKa and local pH
what is pKa
pH when 50% of drug is ionised and the other is un-ionised
what equation is used to determine proportions if un-/ionised drug in a given pH
Hendersoon-Hasselbach
pKa = pH + log (AH/A-) or (BH+/B)
where AH = acid, A- = conjugate base
and BH+ = base and B = conjugate acid
describe where a weak base or weak acid would accumulate in the body
- weak base, compartment with low pH
- weak acid, compartment with high pH
how do low pHs facilitate weak acid absorption (and vice versa)
- an acid wont dissociate in a low pH and is therefore absorbed
- a base won’e become ionised in a high pH and is therefore absorbed
apparent volume of distribution (Vd)
extent of which a drug divides between the plasma and tissue compartments
equation for Vd
Vd = dose/ plasma conc. of drug
a low/high Vd means what
- low = drug retained in vascular compartmetns
- high - drug readily crosses into tissue compartments
what happends with a drug of high Vd
it will cross into tissue compartments easily and will be concentrated in one part of the body rather than spreading
for drugs with = potency what Vd would be preferred
low
