Yohimbine pharmacokinetics and interaction with the sympathetic nervous system in normal volunteers

Summary

The pharmacokinetics of yohimbine and its effects on sympathoadrenal function were studied in 13 young, healthy, male volunteers after an IV bolus dose of 0.25 or 0.5 mg · kg⁻¹.

Pharmacokinetic analysis showed that distribution was rapid, with a half life between 0.4 and IS min, and the elimination half life ranged between 0.25 and 2.5 h. The volume of distribution (V_ss) was 741, (range 26 to 1271). Only 0.5 to 1 % of unchanged yohimbine was found in the urine, indicating that the major part of the drug was eliminated by hepatic clearance. Total plasma clearance was 1171. h⁻¹, which exceeds the hepatic plasma flow. This means that yohimbine is a high extraction drug with considerable extra-hepatic metabolism. Fractional urine sampling revealed that 0.5-1 % of unchanged yohimbine was excreted in urine in a biphasic manner. The data also suggested the existence of a slower elimination phase, with a half life of 13 h. The venous plasma concentration of noradrenaline (NA) increased 3-fold within 15 min after the yohimbine injection while plasma adrenaline (A) and neuropeptide Y-like immunoreactivity (NPY LI) remained unchanged. The plasma concentration-effect relationship of the changes in circulating NA followed counter-clockwise hysteresis. The results show that the hyperadrenergic state elicited by therapeutic doses of theα ₂-adrenergic autoreceptor antagonist, yohimbine, is due to an interaction with NA but not to release of A or NPY in man.