DR Medhat MRCP

2) Binding to plasma & tissue proteins :
o Drugs bounded to plasma proteins will require large (loading) dose to
saturate the drug-binding sites rapidly then rapid ↑ plasma level e.g
digoxin.
- Once the distribution sites of lipid-soluble or protein-bound drugs have been
saturated, the plasma concentration of the drug reaches a steady state & when
the drug is stopped and its plasma concentration ↓ it redistributes into the
plasma from its reservoirs (lipid ,proteins ,tissues) → maintain plasma
concentration so the effect of the drug may take longer time to wean off e.g
amiodarone which takes around 50 days after DC for plasma concentration to ↓
by ½ (wide distribution ,tissue reservoir).
Volume of distribution :
• Def :
- It`s a property of a drug which shows how much drug needs to be in the body to
get a certain concentration in the plasma.
- V D =
- It is constant (not variable) for each drug & it`s used together with measurement
of plasma concentration in calculation of dose needed to be given to certain lean
(not average) body weight as shown in the example of morphine below.
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