There are no useful CYP3A probes that quantitatively predict the in vivo kinetics of other CYP3A substrates and no expectation that one will be found.

The search for a substrate that may serve as a probe to quantitatively predict the in vivo kinetics of cytochrome P450 3A (CYP3A) drugs has been of particular interest because more than half of all human drugs appear to be substrates for this enzyme. Even three closely related 1,4-benzodiazepines-alprazolam, midazolam, and triazolam-are inadequate probes to predict the pharmacokinetics of each other in an individual. If these drugs--all metabolized through the same CYP3A pathways in humans, all FDA Biopharmaceutical Classification System Class 1 compounds exhibiting high solubility and high permeability and thus unaffected by transporter differences--cannot quantitatively predict the pharmacokinetics of their closely related congeners, there is little hope that any quantitative CYP3A probe will ever be found.