Reduction of serum retinol levels following a single oral dose of all-trans retinoic acid in humans.

Following a single oral dose of all-trans retinoic acid (RA) (0.167 mmole) in corn oil to 6 healthy human subjects, the mean serum retinol (ROL) level fell by approximately 20% within 1 h and remained depressed for 24 h. After dosing, RA appeared in the blood within 30 min, peaked at 0.3-0.5 mumol/l, and then declined to very low concentrations after 7 h. All-trans retinoyl beta-glucuronide (RAG) appeared simultaneously with RA in the plasma, albeit more sporadically, whereas only traces of 4-oxoretinoic acid (4-oxoRA) were detected. Some possible physiologic consequences of therapeutic uses of all-trans RA are discussed.