Development of radioligands for the serotonergic neurotransmission system for use in positron emission tomography

Abstract

The serotonergic system is implicated in several neuropsychiatric disorders, such as depression, anxiety and schizophrenia. Several hypotheses on the pathophysiology of these disorders are based on the pharmacology of serotonergic drugs. Selective serotonin reuptake inhibitors (SSRIs) are widely used for the treatment of depression and anxiety. Serotonin 5HT1A receptor agonists and antagonists are currently explored as potential anxiolytics and antidepressants. The brain imaging technique Positron Emission Tomography (PET) allows quantitative studies of neuroreceptor binding and receptor occupancy in relation to clinical drug treatment and drug development. A prerequisite is the availability of suitable radioligands.

The main objective of this thesis was to develop new radioligands for PET examinations of the 5-HT1A receptor and the serotonin transporter (5-HTT) in the human brain.

Radiolabeling was performed with the radionuclides carbon-11, fluorine-18, iodine-125 and tritium. The radioligands were characterized in vitro on rat brain homogenates or with autoradiography on post-mortem human brain cryosections. The in vivo brain distribution of radioactivity was measured in the cynomolgus monkey brain with PET after i.v. injection of the radioligands.

[Carbonyl-11C]WAY-100635 is established as a reference radioligand for in vivo imaging of the 5-HT1A receptor and is thus a suitable lead compound for the development of new radioligands with improved properties such as being less prone to metabolism and to be labeled with alternative radionuclides. Halogenated analogues of WAY-100635 were examined with autoradiography and PET. After injection of [11C]6FPWAY, the target to nontarget ratios were high. The specific binding could be inhibited by pretreatment with unlabeled WAY-100635. The early peak equilibrium and the high brain uptake makes [11C]6FPWAY a potential PET radioligand for the 5-HT1A receptor in man.

NAD-299, a recently developed selective 5-HT1A receptor antagonist was labeled with carbon11. Autoradiography demonstrated accumulation of radioactivity in accordance with the known distribution of 5-HT1A receptors. The accumulation of radioactivity in 5-HT1A rich regions was completely inhibited by coincubation with known 5-HT1A receptor antagonists and agonists, thus demonstrating the specificity and selectivity of [11C]NAD-299 binding to 5 HT1A receptors. Furthermore, high target to non-target ratios and early peak equilibrium was recorded with PET. The results motivate further investigation of [11C]NAD-299 as a potential PET radioligand for quantitative studies of 5-HT1A receptors in man.

A series of six phenyltropane analogues with various affinity for the 5-HTT were labeled with carbon-11. One of these, [11C]RTI-357, accumulated in 5-HTT rich regions of the monkey brain. Pretreatment with the selective 5-HTT inhibitor citalopram. indicated that [1 1C]RTI-357 binds specifically to the 5-HTT.

5-methyl-6-nitroquipazine (MNQP), a methylated analogue of the potent 5-HTT inhibitor 6- nitroquipazine was synthesized and radiolabeled with tritium or carbon-11. [3H]MNQP was found to have subnanomolar affinity for the 5-HTT in the rodent brain. High accumulation of radioactivity in 5-HTT rich regions was recorded with PET after injection of [11C]MNQP. The uptake could be reduced by pretreatment with citalopram. Although a promising radioligand for studies of the 5-HTT in vitro, the slow kinetics and moderate ratios recorded in vivo, may limit its use for quantitative studies of the 5-HTT with PET.