Drug Absorption and Factors Flashcards
Role of charge
Needs to be soluble in water but also able to pass through the plasma membrane, a balance of charged and hydrophobic groups
Rates of absorption
Intravenous > oral > intramuscular > subcutaneous
Mouth/rectum has a direct route to the vena cava, many routes distribute to the body before getting to the vena cava— first pass effect
Roles of size
If it is small (MW 20000) then it ha to pass to the larger pores of lymph
Bioavailability
Fraction of the dose on the blood after absorption from its site of administration. Cn indicate the extent which it is released, degraded, or lost from first pads effects
IV F=1.0
All other routes F<1.0
F_oral = AUCoral/AUCiv
Passive diffusion
Molecule is soluble in water and can pass through the lipid membrane. All end up with equal amounts overtime.
Driving force is concentration, no energy used
Facilitated diffusion
Involves a carrier protein, the driving force is the electrochemical gradient. Saturable process as there is a limited amount of carrier proteins. No energy use and will eventually have equal amounts on each side
Active transport
Energy is required, driven by ATP pump. Used to drive solute against its concentration gradient. Limited number of pumps so also a saturable process. Will be more amount of drugs on one side
Partion coefficient
K_org / K_aqu = [Drug_org] / [Drug_aqu]
Indicates affinity for fat, as it increases the lipophilicity increases as ell
Distribution law
A substance will distribute itself between two immiscible phases so that at equilibrium, the ratio of the concentration is a constant
- independent of amounts and volumes used
- dependent on temperatures and solvents used
Ion-trapping
pH gradient exists across a membrane. Drugs that are weak acids and bases will ionize to different extents and there will be more of a total drug on that side. There is the SAME concentration of unionized drug on both sides, but unequal of total
Henderson-Hasselbach Equation
Dissociation/ionization of weak acids and bases
Weak acids
HA H+ + A-
pH - pKa = log A-/HA
Weak bases
BH+ B + H+
pH - pKa = log B/BH+
