Pharmacokinetics

8 important questions on Pharmacokinetics

The degree of dissociation of the drug is dependent on (2 factors):

- pKa: dissociation constant. The intrinsic ability of the drug to split itself
- pH of the environment

What is the partition-coefficient (log P)

Log P = Xoctanol/Xwater
It says something about the lipophilicity of a substance. The higher log P, the more lipophilic the substance.

Phase I reactions: Functional reactions (4 characteristics)

Functional group is built into the drug.
- Oxidation, reduction, hydrolysis
- Metabolites are polar/hydrophilic
- Metabolites can be inactive or active
- Metabolites sometimes participate in phase II reactions
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Phase II reactions: Coupling reactions (4 characteritics)

Conjugation with endogenous hydrophilic compounds
- glucuronidation, acetylation
- Metabolite is always hydrophilic and inactive after the reaction
- Can take place without phase I reaction
- The metabolite is usually excreted by the liver, through the bile

Cytochrome P450 system

Most important system for Phase I reactions.
- Oxidative enzymes (microsomal enzymes)
- Expressed in ER
- Cytochrome P450: iron-containing proteins
- Expressed mostly in liver/intestines
- Broad specificity (chance of competition between drugs)

Called CYP enzymes. The substrate binds the iron part of the enzyme and is then metabolized. 3 different families, 6 subfamilies

Two factors influencing biotransformation

- Enzyme induction
- Enzyme inhibition

Biological availability (F)

Fraction of the dose of the drug that after first passage through the portal vein and the liver enters the systemic circulation in the unchanged form.
Depends on:
- Absorption, good absorption higher F
- Biotransformation, poorly metabolized means higher F
Inducer would mean a lower biological availability

Working dose (formula)

working dose = oral dose x F (bioavailability)

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