Step 1: Understanding the Concept:
Transdermal permeation needs a drug to pass through two very different environments in the skin, the lipid-rich stratum corneum on top, and the more watery viable epidermis and dermis underneath.
Step 2: Key Concept or Approach:
A workable transdermal drug needs a log P value that lets it dissolve into the skin's lipids without getting stuck there, so it can still move on into the aqueous tissue and reach the bloodstream.
Step 3: Detailed Explanation:
If log P is below 1, the drug is too water-loving and cannot get through the fatty stratum corneum in the first place.
If log P is 5-7 or higher, the drug is so fat-loving that it stays locked inside the stratum corneum and cannot move into the aqueous layers below.
A log P of 1-3 sits between these extremes, giving the drug enough lipophilicity to cross the stratum corneum and enough polarity to keep moving into the deeper, water-rich tissue.
This dual solubility is why formulators treat a log P of 1-3 as the target window for transdermal candidates.
Step 4: Final Answer:
The ideal log P range for a transdermal drug candidate is 1-3.