Conversion rate of topical Retinol to Retinoic Acid (Tretinoin)?

Question

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?

Answer 1

My guess is that the effect you describe is due to compartmentalisation: both retinol and retinoate are practically lipophils and so will localize in membranes and other lipid "pools", while both human retinal dehydrogenase enzymes are cytosolic. Probably the only occasion they have to get at their substrate is when it's bound to RBP. This however introduces another factor complicating the simple model.