How deep into the skin do topical acne medications penetrate?

Question

Given a topical gel that is a mixture of

• dapsone (6%), an antibiotic
• spironolactone (5%), an androgen blocker
• niacinamide (2%), a form of Vitamin B3 and skin brightener

How deep into the skin do topical acne medications penetrate? Are they able to reach the infection inside the skin? Does increasing the strength/dosage of a product increase how deep the product can penetrate? Does skin type (dry, oily, etc) have any effect on how well products absorb?

Obviously this is a very broad question as there at least three major categories of dermatology topicals, being:

• Prescription topicals (such as those mentioned above or adapalene)
• Drug store products (benzoyl peroxide, salicyclic acid, etc)
• Home remedies (aloe vera gel, honey, tea tree oil, etc)

How deep into the skin do topical acne medications penetrate?

And, if I may, a final bonus question:
Are any of the solutions listed above any good at fighting deep cystic acne?

Answer 1

Cystic acne are often called "severe acne."

Treatment of cystic acne usually involves prescribed drugs, so it's a doctor (a dermatologist) who prescribes them. These drugs can have serious side effects, so it's important to consider all the precautions.

According to American Academy of Dermatology:

Antibiotic + medicine you apply to the acne: This is often the first treatment recommended for severe acne. Taking an antibiotic can reduce the redness and swelling of acne. The medicine you apply to your skin works on reducing bacteria and clogged pores.

This means an oral antibiotic + some acne ointment.

Isotretinoin: This is a potent medicine that attacks all four causes of acne—bacteria, clogged pores, excess oil, and inflammation (redness and swelling). About 85% of patients see permanent clearing after one course of isotretinoin.

Due to possible side effects, you will need to carefully consider whether you want to take this medicine. If you decide to take isotretinoin, you must enroll in a monitoring program.

So, they are antibiotics and isotretinoin, which are effective against infection.

In general, greater medication doses have greater effects but also side effects, and there is usually some dose above which the effect does not increase with the dose.

Absorption vs penetration, according to Transdermal Drug Delivery: Innovative Pharmaceutical Developments Based on Disruption of the Barrier Properties of the stratum corneum (Pharmaceutics, 2015):

Percutaneous absorption of molecules is a step wise process involving:

• Penetration: The entry of a substance into a particular layer of the skin
• Partitioning from the stratum corneum into the aqueous viable epidermis
• Diffusion through the viable epidermis and into the upper dermis
• Permeation: The penetration of molecules from one layer into another, which is different both functionally and structurally from the first layer;
• Absorption: The uptake of a substance into the systemic circulation.

So, penetration is the first step of drug absorption, which may or may not result in the absorption of a drug into circulation.

For a potential user, discussing exact doses and depths of dermal drug absorption my be more misleading than helpful. There are some more details about dermal drug absorption here.