Lecture notes from Imperial College London, Medical Biosciences BSc, 2nd year, pharmacology module.
Phar 2: introduction to pharmacology with pharmacokinetics.
learning outcomes =
- LO1 Administration: List the major routes by which drugs may be administered and their advantages and disadva...
Intro to Pharmacokinetics
- amount that reaches tissues is dependent on different pharmacokinetic factors:
Absorption
- passage of a drug from the site of administration into plasma (systemic circulation)
- bioavailability (systemic) = fraction of the initial dose that gains access to the systemic circulation
- drug move by: - bulk flow transfer (bloodstream): intra-venous administration 100 % bioavailability
- diffusional transfer (to reach bloodstream)
=> cross lipid membrane by - pinocytosis (vesicle containing drug)
- diffusion through aqueous pores (small)
- diffusion through lipids (lipid soluble)
- carrier proteins (with transmembrane p)
spid)
- lipid membranes crossed: - oral = small intestine microvilli, bvw x2 (stomach, small int, liver,systemic)
- inhalational = alveoli/ bronchi, blood vessel wall x2
- intra-nasal = mucous membranes of nasal sinus, blood vessel wall x2
1
- through aqueous pores (gaps bw epithelial/ endothelial cells): < 0.5nm diameter
2
- through lipid: with lipid soluble drugs (drugs are weak acids/ weak bases => ionised & unionised)
=> weak acids ( - , donate H+ , COOH...) more unionised in acidic
·
=> weak bases (+, accept H+... ) more unionised in alkaline
- when pH=pka => drug is 50% dissociated ionised (=charged) form is more polar
- B +
H
+
BH+ HA L
- A
-
+
H + => less lipid soluble
conjugated and
conjugate
-
base
-
.
a
- Henderson-Hasselbalch equation: pka = pH + log([BH+]/[B]) and pka = pH + log([AH]/[A-])
[AH]/[A-] = [BH+]/[B] = 10^(pka - pH)
- pH partition hypothesis: proportion of drug in any body compartment is dependent on pH
=> ion trapping if high or low pH
3 - with carrier transporters: physiologically important molecules in renal tubule, biliary tract, blood brain
barrier, gastrointestinal tract
=> BUT drugs that resemble endogenous m may interact
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