The photoprotective capabilities of more than 30 compounds, many of which have not been used previously as ultraviolet light (UVL) protective agents, were studied by in vitro and in vivo testing. No correlation between in vitro and in vivo effectiveness was identified. Lack of effectiveness in one system does not preclude effectiveness in another.
Many agents that imparted protection from x-radiation and UVL damage in vitro lacked topical efficacy in vivo. The in vivo protection from UVL achieved by most of the agents at clinical concentrations is a function of their ultraviolet absorption spectrum.
Topical studies on humans showed that propyl gallate is a more effective protectant than the best known commercial sunscreen. It is nontoxic, almost nonstaining to fabric, and substantive to the epidermis. Unfortunately, it proved to be a strong contact sensitizer.
Many clues to the unknowns in medicine are locked in the library, waiting for someone to open the right book at the right time.
Alphonse Raymond Dochez 1882-1964
Kahn G, Curry MC. Ultraviolet Light Protection by Several New Compounds. Arch Dermatol. 1974;109(4):510-517. doi:10.1001/archderm.1974.01630040020004