Question

What should be the log P value for an ideal drug candidate for transdermal permeation:

• A. Below 1
• B. 1-3
• C. 5-7
• D. Above 7

Hint:

For transdermal drugs, a log P value of 1-3 ensures optimal permeation through the lipid-rich skin barrier while maintaining systemic bioavailability.

Answer 1

The Correct Option is B

Step 1: Understanding log P value. The log P value is a measure of a drug's lipophilicity, representing its distribution between a lipid (oil) phase and an aqueous phase. It determines how easily a drug can permeate lipid membranes, such as the skin. 

Step 2: Log P value range for transdermal permeation. For transdermal delivery, the drug must have balanced hydrophilic and lipophilic properties: - A log P value of 1-3 is ideal, ensuring sufficient lipid solubility for permeating the skin's lipid barrier while maintaining water solubility for systemic absorption. 

Step 3: Why other options are incorrect. 
- (A) Below 1: Too hydrophilic, making it difficult to cross the lipid-rich stratum corneum. 
- (C) 5-7: Too lipophilic, leading to accumulation in the skin and poor systemic absorption. 
- (D) Above 7: Extremely lipophilic, making it almost impossible to achieve systemic delivery.

Answer 2

The question asks what log P range makes a drug a good candidate for crossing the skin in transdermal delivery. Log P measures how a drug splits between an oily (lipid) phase and a watery phase, so let's weigh each range against what the skin barrier actually needs.

1. Below 1: A drug this hydrophilic struggles to cross the lipid-rich stratum corneum, the skin's outer barrier layer, because it does not partition well into fatty material at all.
2. 1-3: This range gives a drug enough lipid character to slip through the fatty stratum corneum, while still keeping enough water solubility to move on into the aqueous, blood-supplied dermis underneath and reach systemic circulation. This dual solubility is exactly what transdermal delivery needs.
3. 5-7: A drug this lipophilic gets trapped inside the fatty stratum corneum layer itself, building up there instead of moving through it, which limits how much actually reaches the bloodstream.
4. Above 7: An even higher log P value makes the drug almost completely insoluble in water, so even if it enters the skin's lipid layer, it cannot cross into the aqueous dermis to reach systemic circulation at all.

Only the 1-3 range balances lipid solubility for crossing the stratum corneum with enough water solubility for the drug to move on into the bloodstream, which is exactly what an ideal transdermal candidate needs.

So the correct answer is 1-3.