New Imide 5-ht 1a Receptor Ligands – Modification of Terminal Fragment Geometry

Two sets of new o-methoxyphenylpiperazine (MPP; series a) and 1,2,3,4-tetrahydroisoquinoline (THIQ; series b) derivatives, containing various imide moieties derived from NAN190, were synthesized and evaluated in vitro for their ability to bind to the serotonin 5-HT 1A and 5-HT 2A receptors. All new derivatives from series a demonstrated high 5-HT 1A affinities, whereas THIQ analogues were much less active. With respect to 5-HT 2A receptors, three MPP derivatives presented moderate activity but the rest of the investigated compounds were practically inactive. The influence of changes in terminus geometry on 5-HT 1A receptor affinity was analyzed in regard to model compounds NAN190 and MM199.


Introduction
During the last decade a large number of structurally different compounds have been proposed as 5-HT 1A receptor ligands.Among these, 4-substituted long chain 1-arylpiperazines (LCAPs) have been extensively studied.In particular, much effort has been devoted to the role of the terminal part in a ligand-receptor interaction, and in consequence, a great many different fragments were used [1].The imide containing fragments constituted one of the most thoroughly investigated termini type and among others, compounds like buspirone (anxiolytic drug; 5-HT 1A partial agonist [2]) or NAN190 (postsynaptic antagonist [3,4]) were found (Figure 1).

Results and Discussion
The structures of NAN190 and its simplified analog MM77 were chosen as lead molecules for modification of imide terminus.All the new compounds were examined in vitro for their ability to displace [ 3 H]-OH-DPAT and [ 3 H]-ketanserine binding to rat 5-HT 1A and 5-HT 2A receptors respectively, as described in the Experimental Section.The results are presented in Table 1, where additionally, the affinities of parent compounds and previously reported derivatives 1b, 2a, 2b and 4b are also included [9,11].All four newly synthesized MPP derivatives (3a and 5a-7a) showed a high affinity for 5-HT 1A receptors (K i = 1.7-46 nM), however, lower than that of NAN190.This is thus another confirmation that the flat phthalimide terminus optimally binds to the respective receptor region.All modifications of this fragment, namely saturation (2a and 3a), removal of the aromatic ring (4a), changes in its position (5a and 6a) or enlargement (7a) decreased the observed 5-HT 1A receptor affinity.Compound 7a, the least active at the 5-HT 1A sites, was found to be the most potent 5-HT 2A ligand.All the other MPP derivatives displayed a moderate to low affinity for 5-HT 2A receptors (K i = 109-1510 nM), and except for 2a, were at the same time at least 100-fold selective 5-HT 1A agents.This is not unexpected, since it is known that the MPP system prefers 5-HT 1A binding sites.
The THIQ moiety has often been used as a replacement for the arylpiperazine fragment in our studies on the role of individual pharmacophoric groups of LCAPs.Although THIQ itself does not show any affinity for the 5-HT 1A receptors (K i > 50 000 nM) [12] its substitution with butylazaspiro[4.5]decane-7,9-dione(MM199, Figure 1) increased the affinity to nanomolar level [9].The same was observed in the case of the 1-adamantoyloaminobutyl THIQ derivative, for which K i = 0.95 nM was determined [13].It was then concluded that bulky alicyclic termini should be especially well accommodated by the 5-HT 1A binding site.This finding is additionally confirmed by the results obtained for the second group of imide derivatives b.Indeed, the highest 5-HT 1A receptor affinity was observed for the alicyclic systems (2b and 3b), whereas aromatic groups were less favorable, although they were better than the small succinimide alone (4b, K i = 2920 nM).Thus, the replacement of MPP by a THIQ fragment caused a decrease of 5-HT 1A affinity and we did not obtain any more active compounds than the previously examined 2b [11].A more detailed comparison of terminal fragments of MM199, 2b and 3b reveals that their geometry may play a role, and that a bent configuration of the cycloalkyl part (Figure 2) is preferred.With regard to 5-HT 2A receptors, we did not detect a significant activity in the investigated THIQ derivatives.As a result of present investigation three new highly active 5-HT 1A ligands were found (3a, 5a and 6a), which displayed a 100-fold selectivity over 5-HT 2A receptors.During the preparation of this manuscript, Hackling and coworkers published a paper in which compound 7a was investigated as a dopamine D 2 and D 3 receptor ligand [14].In our hands this derivative was a dual 5-HT 1A /5-HT 2A ligand and based on the data obtained by Hackling, it is also a potent but unselective D 2 /D 3 agent (K i = 40 and 29 nM, respectively).Pharmacological work is currently in progress to evaluate the 5-HT 1A functional profile of these four compounds.
Equimolar amounts (1 mmol) of the respective, commercially available acid anhydride and 4-(4aminobutyl)-1-(2-methoxyphenyl)piperazine or 2-(4-aminobutyl)-1,2,3,4-tetrahydroisoquinoline were refluxed in xylene (20 mL) for 3 h.After the mixture was allowed to cool to room temperature, the xylene was evaporated under reduced pressure and the products were isolated on a silica gel column.Free bases were converted into the hydrochloride salts in CHCl 3 or acetone solution by treatment with excess of Et 2 O saturated with gaseous HCl.

Figure 1 .
Figure 1.Structure and binding constants (K i ) of the selected 5-HT 1A receptor ligands.

a
K i value according to Glennon et al. was 0.58 nM [3].b K i value from ref 11. c ref 5. d ref 9.