5-Hydroxytryptamine2 (5-HT2) structure-function relationships of the nitro and amino phenylpiperazines on intact...

Nitro and amino phenylpiperazines were synthesized to study the agonist and antagonist activities of the phenylpiperazines at the human platelet 5-hydroxytryptamine2 (5-HT2) receptor. Amplification of ADP-induced aggregation and binding competition experiments with [3H]ketanserin were used to evaluate receptor interactions in this system. All the monosubstituted phenylpiperazines were antagonists despite the wide variation in electronic and hydrophobic properties. The parent compound, unsubstituted phenylpiperazine (PP), had the lowest affinity for the [3H]ketanserin binding site. The intensely electron-withdrawing NO2 substituent increased binding affinity at all ring positions and this activity correlated with antagonist potency in platelet aggregation studies (rank order: 4-NO2-PP greater than 3-NO2-PP greater than 2-NO2-PP). NH2 substitution decreased binding affinity at the 4- and 2-positions compared with the analogous NO2 substituted compounds; however, evaluation of NH2-PP antagonist potency in aggregation studies was complicated due to slow association with the receptor. To compare the activities of the phenylpiperazines at other 5-HT sites, binding competition experiments were performed using [3H]5-HT in bovine brain membranes. The rank order of the affinities for the NO2 substituted compounds was distinctly different from that determined with platelets, reflecting the heterogeneous composition of 5-HT receptor subtypes in the brain membrane preparations. The platelet aggregation experiments demonstrated that marked alteration of the electronic and hydrophobic properties of phenylpiperazine by ring substitution did not impart 5-HT2 agonist activity. By contrast, 5-HT2 antagonist activity appeared to be enhanced markedly by electron-withdrawing resonance effects which decreased the electron density at the 1'-piperazine nitrogen. This enhancement appeared to be specific for the 5-HT2 receptor subtype.