Kinetics, brain uptake, and receptor binding of tandospirone and its metabolite 1-(2-pyrimidinyl)-piperazine

Tandospirone is an azaspirodecanedione derivative under investigation as an antidepressant. Metabolism of tandospirone in humans and rodents leads to 1-(2-pyrimidinyl)-piperazine (1-PP), presumed to have pharmacologic activity. To determine the relative contributions of tandospirone and 1-PP after tandospirone administration, we evaluated open-field activity, pharmacokinetics, and receptor binding of tandospirone and 1-PP in a mouse model. Tandospirone significantly reduced open-field activity during 30 minutes at doses of 1-20 mg/kg. 1-PP had no significant effect on activity except for a trend toward reduction at 20 mg/kg. At 30 minutes after administration, plasma and cortex concentrations of tandospirone and 1-PP increased in proportion to dose. Plasma protein binding (free fraction) for tandospirone was 30.4%, and for 1-PP, 87.5%. Receptor binding studies indicated that tandospirone bound with high affinity to serotonin1A sites, and with low affinity to serotonin2, alpha 1, alpha 2, and benzodiazepine sites. 1-PP bound with high affinity to alpha 2 sites and with low affinity to the other sites evaluated. A "receptor occupancy index" of tandospirone cortex concentrations divided by receptor affinity suggests that after acute administration of tandospirone, effects are likely to be due to the parent compound rather than to the metabolite. Similar conclusions are likely to be correct for other azaspirodecanediones, including buspirone.