Pharmacodynamics 0423FA Flashcards
Km
affinity of enz for its substrate.
Km = [s] at 1/2 Vmax
lower Km, higher affinity.
vol of distribution (Vd)
Vd = amt of drug in body / plasma drug conc
low Vd
distribute in BLOOD.
large or charged molecules.
4-8L.
medium Vd
distribute in EC space or body water.
small hydrophilic molecs that do not bind plasma prots.
high Vd
distribute into all tissues.
small lipophilic molecs that bind strongly to extravascular prots
clearance
CL = rate of elimination of drug / plasma drug conc = Vd x Ke (elimination constant)
how many half lives does it take for a drug infused at constant rate to reach STEADY STATE?
4-5 half lives
half life
t1/2 = (0.7Vd) / CL
loading dose
LD = Cp x Vd / F
Cp is target plasma conc.
F is bioavailability (%).
maintenance dose
MD = Cp x CL / F
zero order elimination
same AMOUNT of drug eliminated per unit time. LINEAR decrease in plasma conc.
first order elimination
same FRACTION of drug eliminated per unit time. EXPONENTIAL decrease in plasma conc. related to half lives.
weak acid drugs
phenobarbital, MTX, ASA.
ionized form trapped (excreted) in basic urine.
treat OD w/bicarb.
weak base drugs
amphetamines.
ionized form trapped in acidic urine.
treat OD with ammonium chloride.
phase I metabolism
reduction, oxidation, hydrolysis.
cytochrome P450.
slightly polar, water-soluble metabolites.
