Question

how to compare between two drugs regarding their polarity , what are the factors ? what should i look for in their structures ?

Answer 1

To reach its target a drug must pass through several membranes. To get across membranes the drug must be relatively non polar. To be soluble in water a drug must be soluble.

Hence polarity is important parameter for drugs and different drugs differ in their polarity.

Polarity, P is given by P= amount of drug dissolved in octanol/ amount of drug dissolved in water

logP is used to describe this ratio.

Now how to find one drug is polar or other is not:

Some drugs change their ionic form depending on the pH of the medium. Ionised(charged) state of molecules are always more polar than uncharged forms.

Two such class of drugs are amines and carboxylic acids drugs.

At approximately pH 12

The equilibrium below is evenly distributed between ammonium salt and amine.

Thus polarity of a drug molecule depends on the nature of functional groups attached.

Hope this helps you.

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