CSF/plasma ratio of 10-hydroxynortriptyline is influenced by sex and body height.

The CSF/plasma ratios of nortriptyline (NT) and its major metabolite 10-hydroxy-NT (10-OH-NT) were investigated retrospectively in 25 depressed patients. For 10-OH-NT (but not NT), a significant influence of sex and body height was found, most conspicuously in males, in whom the ratio related to body height curvilinearly (N = 8; R = 0.93; P < 0.01). In males, the NT/10-OH-NT ratio in plasma correlated with body height (N = 8; r = 0.80; P < 0.05). Hypothetically, CSF circulation is partly influenced by body height, which accounts for a steeper gradient of 10-OH-NT across the blood-brain barrier in taller persons. From the lumbar site, the more polar 10-OH-NT is assumed to be eliminated by bulk flow via the villi, while the less polar NT exits by diffusion in the choroid plexus. Prospective studies are urgently needed to further evaluate the distribution of antidepressants in the CSF.