I'm wondering if someone out there has more information than me. Retinoids have well known metabolic pathways in vivo, and it's usually something like:
Retinyl Palmitate --> Retinol --> Retinaldehyde (Retinal) --> Retinoic Acid (Tretinoin) which is the biologically active form of the Retinoids.
Retinoids are used in cosmetics all the time, and while Retinol is available OTC, Retinoic Acid (Tretinoin - a.k.a. "Retin-A" by brand) is available by prescription only since it is a 'stronger' version of Retinol. It's not difficult to find 1% Retinol creams or higher, but Retinoic Acid treatments top out at 0.1%.
However, I can't help but wonder 'why'? One study I was able to dig up was here: Retinoic Acid Biosynthesis and Metabolism PDF. The table on page 5 shows Physiological Concentrations - starting at 50 microM for Retinol and ending at "<50nM?" suggesting an ultimate conversion rate of a little less than 10% which would make the 1% Retinol treatments roughly equivalent to a 0.1% Retinoic Acid treatment (and a 2% Retinol treatment to a 0.2% Retinoic Acid treatment - double what's available by prescription).
Do other studies/other evidence support the rate of conversion of Retinol to Retinoic Acid to be roughly 10% or suggest than a 1% Retinol topical treatment is equivalent to a 0.1% Retinoic Acid topical treatment? Or are there some other interfering factors that reduce the utilization of Retinol to Retinoic Acid that I'm missing?