Pharmacokinetics — GI Absorption

Drug Factors

1. Lipid solubility
   ◦ ↑ lipid sol → ↑ absorption
   ◦ Must still have some water soluble component
2. Molecular size
   ◦ ↓ size → ↑ rate of absorption
   ◦ Larger molecules require processes other than simple diffusion
3. Degree of ionisation
   ◦ pH-pKa gives the degree of ionisation
   ◦ ↓’d ionisation favours absorption
4. Physical forms
   ◦ Gases faster than liquids faster than solids
5. Dosage forms
   ◦ Capsules more rapidly absorbed than tablets
6. Formulation
   ◦ The addition of Na to the formulation for stability can reduce the bioabalability via the oral route
7. Concentration
   ◦ From Fick

Patient factors

1. Area of absorptive surface
   ◦ intestinal resection or coeliac → ↓ absorptive surface → ↓ absorption from Fic
2. Vascularity
   ◦ Shock → ↓ absorption
3. pH
   ◦ effect degree of ionisation
   ▪ ↓ pH favours absorption of acidic drugs
   ▪ ↑ pH favours absorption of basic drugs
4. Presence of other substances
   ◦ Statins should be taken with food
   ◦ Milk ↓’s and Vit C ↑’s absorption of iron
5. GI motility
   ◦ ↑ motility → ↓ absorption
6. Functional Integrity of the absorptive surface
   ◦ Coeliac disease → flattened mucosa → ↓ absorption
7. Coexisting disease state
   ◦ diarrhoea → ↓ absorption

Gladwin 2016

Answer by Gladwin