Why Is Rapamycin Not a Rapalog?

Rapamycin (sirolimus) is an immunosuppressive drug approved by the Food and Drug Administration (FDA). It is also a leading candidate for targeting aging. Rapamycin and its analogs (everolimus, temsirolimus, ridaforolimus) inhibit the mammalian target of rapamycin (mTOR) kinase by binding to FK506-binding proteins (FKBP) and have a similar chemical structure that only differs in the functional group present at carbon-40. Analogs of rapamycin were developed to improve its pharmacological properties, such as low oral bioavailability and a long half-life. The analogs of rapamycin are referred to as "rapalogs." Rapamycin is the parent compound and should therewith not be called a "rapalog."