D. Martinez et al., Positron emission tomography study of pindolol occupancy of 5-HT1A receptors in humans: Preliminary analyses, NUCL MED BI, 27(5), 2000, pp. 523-527
Preclinical studies in rodents suggest that augmentation of serotonin reupt
ake inhibitors (SSRIs) therapy by the 5-hydroxytryptamine(1A) (5-HT1A) rece
ptor agent pindolol might reduce the delay between initiation of treatment
and antidepressant response. This hypothesis is based on the ability of pin
dolol to potentiate the increase in serotonin (5-HT) transmission induced b
y SSRIs, an effect achieved by blockade of the 5-HT1A autoreceptors in the
dorsal raphe nuclei (DRN). However, placebo-controlled clinical studies of
pindolol augmentation of antidepressant therapy have reported inconsistent
results. Here, we evaluated the occupancy of 5-HT1A receptors following tre
atment with controlled release pindolol in nine healthy volunteers with pos
itron-emission tomography (PET). Each subject was studied four times: at ba
seline (scan 1), following 1 week of oral administration of pindolol CR (7.
5 mg/day) at peak level, 4 h after the dose (scan 2), and at 10 h following
the dose (scan 3), and following one dose of pindolol CR (30 mg) (at peak
level, 4 h) (scan 4). Pindolol occupancy of 5-HT1A receptors was evaluated
in the DRN and cortical regions as the decrease in binding potential (BP) o
f the radiolabelled selective 5-HT1A antagonist [carbonyl-C-11]WAY-100635 o
r [carbonyl-C-11] N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)-N-(2-pyridyl)cyc
lohexanecarboxamide abbreviated as [C-11]WAY-100635. Pindolol dose-dependen
tly decreased [C-11]WAY-100635 BP. Combining all the regions, occupancy was
20 +/- 8% at scan 2, 14 +/- 8% at scan 3, and 44 +/- 8% at scan 4 The resu
lts of this study suggest that at doses used in clinical studies of augment
ation of the SSRI effect by pindolol (2.5 mg t.i.d.), the occupancy of 5-HT
1A receptors is moderate and highly variable between subjects. This factor
might explain the variable results obtained in clinical studies. On the oth
er hand, at each dose tested, pindolol occupancy of 5-HT1A receptors was hi
gher in the DRN compared to cortical regions, demonstrating a significant i
n vivo selectivity for DRN 5-HT1A autoreceptors relative to cortico-limbic
postsynaptic receptors. This selectivity is necessary for the potentiation
of 5-HT transmission, and this finding represents an important proof of con
cept in the development of 5-HT1A agents for this application. Early evalua
tion of new drugs with PET imaging will enable rapid screening of compounds
based on DRN selectivity and more appropriate determination of doses for c
linical trials. NUCL MED BIOL 27;5: 523-527, 2000. (C) 2000 Elsevier Scienc
e Inc. All rights reserved.