Pharmacokinetics - Introduction Flashcards
- PR_BK_03 Drug chemistry: solubility, partition coefficients and movement of drugs through membranes: Lipid solubility; influence of pKa and pH; partition coefficients. Passive and active transport mechanisms - PR_BK_17 Pharmacokinetics: general principles: absorption, distribution and redistribution; elimination, excretion. Chemical properties of drugs and their pharmacokinetics: blood-brain-barrier and placental barrier. Protein binding: plasma and tissue. Body compartments; adipose and vesse (2 cards)
What is the Henderson-Hasselbalch equation?
Relates to the concentrations of dissociated and undissociated substances, for both acids and bases, in context of pH and the dissociation constant for that substance
pH = pKa + log₁₀([Acceptor][Donor])
Acceptor means base (which will accept a proton), and Donor means acid (which will donate a proton)
What is pKa
The pH at which a weak acid or base is 50% ionised, and is unique to each molecule.
Examples:
Aspirin is a weak acid with pKa 3.5 - so is almost completely ionised at physiological pH
Lidocaine is a weak base with pKa 7.9, so is only 25% ionised at physiological pH
Propofol is a weak acid with pKa 11, so is almost entirely unionised at physiological pH
pKa is determined from the molecular structure, and calculated using the Henderson-Hasselbalch equation
