Drug Distribution

Volume of distribution [Ref. K.D.T. 5^1h/e p 16, 17]

• After the drug reaches the blood, it may be distributed to various tissues.
• This is detennined by a hypothetical parameter, volume of distribution.
• It relates to the amount of drug in the body to the concentration of drug (c) in the blood or plasma

amount of drug in the body  V —

concentration of drug in the plasma

• This volume does not necessarily refer to an identifiable physiological volume but rather to the fluid volume that would he required to contain all the drug in the body at the same concentration.
• if volume of distribution is more, it means more amount of drug is in the tissues and less is in the plasma.
• Thus a drug with high volume of distribution will require higher dose to achieve a particular plasma concentration. Loading dose
• If the time required to attain a steady state concentration of a drug is long relative to the temporal demands of the condition, a loading dose is required.
• The loading dose is one or a series of dose that may be given at the onset of therapy with the aim of achieving the target concentration rapidly.
• Loading dose is given to saturate the tissue stores so it is mainly dependent on volume of distribution.

Also know

• Maintenance dose depends on the clearance.

The volume of distribution of a drug can be determined using the following equation:

Ans. is ‘a’ i.e., 5 It.

Ans is ‘a’ i.e., Extracellular fluid

o A drug with high volume of distribution has more concentration in tissue and less in plasma.

o A drug with low volume of distribution has concentration only in plasma (intravascular)

Relationship between volume of distribution and extent of distribution

Ans. is ‘a’ i.e., Sequestrated in body tissues

o Drug may have volume of distribution for in excess of total body water if it is sequestered in other tissues because most of the drug is present in tissues and plasma concentration is low.

Ans. is ‘None’

o Loading dose is governed by volume of distribution and volume of distribution is affected by lipid solubility. o Maintenance dose is governed by clearance (excretion) of drug and half life.

o For one option go ahead for option b ie half life because :

“Clearance is the most important factor for maintenance dose”.

Ans. is ‘a’ i.e., Volume of distribution

Loading dose is governed by volume of distribution and volume of distribution is affected by lipid solubility.

Maintenance dose is governed by clearance (excretion) of drug and half life.

Ans. is’d’ Excretion of drug

• Loading dose is governed by volume of distribution and volume of distribution is affected by lipid solubility.
• Maintenance dose is governed by clearance (excretion) of drug and half life.
• For one option go ahead for option b ie half life because :
• “Clearance is the most important factor for maintenance dose”.– Katzung 10^th/e – 44