PHAR0005 L3: Basic Principles of Pharmacokinetics

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20 Terms

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Pharmacokinetics

The study of the time-course of drug concentrations throughout the body, including studies of metabolites and the mathematical interpretation of the data.

  • Liberation

  • Absorption

  • Distribution

  • Metabolism

  • Excretion


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Crossing the plasma membrane (methods)

  1. Using specialized pores

  2. Active transport

  3. Pinocytosis

  4. Membrane lipid


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Partition coefficient (P)

A measure of a drug’s lipid solubility relative to its water solubility in its natural state.

  • [drug at organic phase]/[drug at aqueous phase]

  • High value: More drug dissolves in the lipid phase → the drug is more lipophilic.

  • Low value: More drug remains in the water phase → the drug is more hydrophilic.


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Distribution coefficient (D)

Accounts for all forms of a drug present at a particular pH, including ionized and unionized forms.

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Weak acid dissocciation

[HA] → [H+] + [A-], where HA is the unionized form.

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Weak base dissociation

[BH+] → [H+] + [B]

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Unionized drug (significance)

A drug in this form is more permeable.

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Henderson-Hasselbach equation

  • pH = pKa + log [A-]/[HA] = -log [H+]

  • [H+] [OH-] = Kw = 1×10-14


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Acid dissociation constant (Ka)

[H+] [A-] / [HA]

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Drug ionization depends on

  1. The pKa of the drug

  2. The pH of the solution in which the drug is dissolved in


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Ion trapping

A phenomenon in which a drug crosses the membrane in its unionized form , then becomes ionized on the other side, which can happen because of pH differences.

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pKa

A measure of how easily an acid gives up its H+.

  • Value = -log(Ka)

  • Low value: strong acid

  • High value: weak acid


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pH < pKa (weak acid)

More unionized aka more [HA]

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pH < pKa (weak base)

More ionized aka more [BH+]

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pH > pKa (weak acid)

More ionized aka more [A-].

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pH > pKa (weak base)

More unionized aka more [B].

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undo log: c = log(x)

10c = x

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pH-partition hypothesis

  1. The drug (weak acid or base) will ionize to different degrees on respective sides of the membrane.

  2. The total drug concentrations (ionized plus nonionized drug) on either side of the membrane will be unequal.

  3. The compartment in which the drug is more highly ionized will contain the greater total drug concentration. 


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Drug ratio r

[A-]/[HA] = 10(pH - pKa)

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Total drug concentration

[HA] + [A-]