Drug Distribution
DRUG DISTRIBUTION:
- Mainly determined by Volume of distribution –
- Volume necessary to contain the total amount of an administered drug at the same concentration that it is observed in the blood plasma.
- Used as a measure for drug distribution.
CALCULATION:
- Calculated by dividng plasma concentration attained for a particular drug administered I.V. & Initial plasma concentration.
- Vd – Dose administrated I.V. / Plasma concentration (C0).
- Higher volume of distribution means more drug distribution into the tissues & vice-versa.
- Eg: Cholorquine – Highest Vd – 1300 L/Kg
DRUGS WITH HIGHER VOLUME OF DISTRIBUTION:
1. Lipid-soluble drugs:
- Likely to cross blood vessels – Hence, higher volume of distribution.
2. Drugs with high plasma-protein binding:
- Increased duration of stay in plasma —> Low volume of distribution –> Decreased metabolism —> Longer-acting drugs.
Drugs included:
- Benzodiazepines (Diazepam, Chlordiazepoxide, Midazolam)
- Warfarin
- Digoxin
- Calcium channel blockers – Verapamil
- Furosemide
- Imipramine
- Cyclosporin
- Tolbutamide.
DRUGS WITH LOW VOLUME OF DISTRIBUTION:
- Drugs with low Vd are restricted to plasma.
- Hence, their poisoning can be benefited by dialysis.
LOADING DOSE:
- Drugs with higher Vd needs to be administered with higher dosage to attain plasma concentration for its therapeutic response than a low Vd drug.
- This particular dosage is “Loading dose”.
- Volume of distribution is main determinant of loading dose.
Exam Important
DRUG DISTRIBUTION
- Loading dose depends on volume of distribution.
- Apparent volume of distribution of a drug exceeds total body fluid volume if a drug is sequestrated in body tissues.
- Dosage of drug is determined by volume of distribution, half-life & lipid solubility.
Don’t Forget to Solve all the previous Year Question asked on Drug Distribution
