The invention provides a compound, useful as an optical probe or sensor of the activity of at least one cytochrome P450 enzyme, and methods of using the compound to screen candidate drugs, and candidate drugs identified by these methods. The optical probe of the invention is a compound having the generic structure Y-L-Q, wherein Q is a chemical moiety that gives rise to optical properties in its hydroxy or hyrdoxylate, phenol or phenoxide form that are different from the optical properties that arise from its ether form.