In Vitro Skin Penetration of 5α-Reductase Inhibitors from Tectona grandis L.f. Leaf Extracts

The leaf extract of Tectona grandis L.f. has shown potential as a 5α-reductase inhibitor, with two bioactive markers, namely (+)-eperua-8,13-dien-15-oic acid (1) and (+)-eperua-7,13-dien-15-oic acid (2), used for extract standardization. The purpose of this research was to investigate the in vitro skin penetration behavior of 1 and 2 in T. grandis leaf ethanolic extract solution and ready-to-use extract in propylene glycol (PG), and secondly, to determine their physicochemical properties, including partition coefficients and solubility. The appropriate vehicle for the in vitro skin penetration study was evaluated using the shake-flask method. The in vitro skin penetration study was conducted using the Franz diffusion cell model, and the amounts of the two active compounds in the extracts were analyzed using the HPLC method. Compounds 1 and 2 showed poor solubility in distilled water, whereas their solubility in HEPES buffer with 2% w/v of Tween 20 was significantly greater. The partition coefficient (log Po/w) value for 1 was 5.77 ± 0.07, and for 2, it was 5.66 ± 0.02, indicating that both compounds are hydrophobic. After 24 h of an in vitro skin penetration study, 1 in both extracts showed significantly higher cumulative amounts (%) compared to 2. These findings suggest that 1 is more hydrophobic and readily penetrates the stratum corneum. When a PG enhancer was added, high cumulative amount trends of 1 and 2 in the ethanolic extract and extract in PG in the receiver compartment were detected after 24 h. These studies provide important insights that will guide the further development of products with T. grandis extracts for treating hair loss.