DR Medhat MRCP

Drug administration & absorption :
Adminstration : oral (GIT), parentral (I.V,I.M,S.C,Intradermal,into body cavity),topical
Absorption (oral route) :
- GIT drug absorption occurs by diffusion.
- Drugs must be dissolved to bring the drug molecules in contact with gut epith.
- Drugs must be lipid soluble so it can cross phospholipid part of cell membrane.
Drug availability :
- i.e percentage of concentration of administrated drug which reaches the systemic
circulation & gain an access for tissue receptors to act.
- I.V drugs have 100% availability
- Oral drugs have < 100% availability due to :
Incomplete absorption from gut.
Pre-systemic metabolism e.g in gut ,enterohepatic circulation,liver
- Foods decrease absorption & availability of lipid soluble drugs e.g atenolol
- Foods e.g grapefruit juice - - pre-systemic metabolizing enzymes (e.g in gut) thus
increase absorption & availability of drugs that undergo pre-systemic metabolism
e.g metoprolol , verpamil, salbutamol.
Drug distribution :
- i.e penetration of the drug into different body compartments e.g intracellular
,interstitial fluid & plasma.
- The following factors affect drug distribution :
1) Lipid solubility :
o Lipid insoluble drugs remain largely in extracellular fluid → ↓
distribution
o Lipid soluble drugs distribute freely throughout the body &cross into
transcellular compartment .
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