Synthesis and screening of aplysinopsin analogs as possible serotonin receptor ligands

Serotonin (5-hydroxytryptamine, 5-HT) is a monoamine neurotransmitter that modulates a number of functions such as mood, sleep, and sexuality. Because the 5-HT2A and 5-HT2C receptor subtypes are closely related, it has been difficult to develop drugs selective for one or the other subtype. Indole alkaloids (aplysinopsins) isolated from a marine sponge showed a low affinity pharmacophore with selectivity for the 5-HT2C subtype. Our goal for the current work was to exploit this natural product pharmacophore in an effort to create derivative compounds that retain their selectivity but that have improved affinity for the 5-HT2C receptor. Our working hypothesis was that halogenating the indole ring of aplysinopsins would result in derivatives with enhanced affinity and selectivity for the human 5-HT2C receptor subtype.

A series of twenty-two aplysinopsin derivatives were synthesized by carrying out an aldol condensation with creatinine, creatinine derivatives, and indole 3-carboxaldehydes, and tested for their biological activity. All the compounds synthesized were characterized by spectroscopic methods and in some cases by X-ray structural analysis. Compounds were screened for their ability to interact with cloned serotonin receptors, 5-HT2A and 5-HT2C, stably expressed in cell lines. Six compounds of the UNT-TWU series: (E)-5-((5-bromo-1 H-indol-3-yl)methylene)-2-imino-1,3-dimethylimidazolidin-4-one ( UNT-TWU-8); (E)-5-((6-fluoro-1H-indol-3-yl)methylene)-2-imino-1,3-dimethylimidazolidin-4-one (UNT-TWU-13); (E)-5-((6-chloro-1H-indol-3-yl)methylene)-2-imino-1,3-dimethylimidazolidin-4-one (UNT-TWU-15); (E)-5-((6-bromo-1H-indol-3-yl)methylene)-2-imino-1,3-dimethylimidazolidin-4-one (UNT-TWU-16); (Z)-3-ethyl-5-((fluoro-1H-indol-3-y1)methylene)-2-imino-1-methylimidazolidin-4-one (UNT-TWU-20); and (E)-5-((5,6-dichloro-1H-indol-3-yl)methylene)-2-imino-1,3-dimethylimidazolidin-4-one (UNT-TWU-22) showed selectivity for cloned human serotonin 5-HT2A or 5-HT2C receptors, while the other compounds tested did not. UNT-TWU-13 and UNT-TWU-20 displayed high affinity for the cloned human serotonin 5-HT2A receptor subtype, while UNT-TWU-15, UNT-TWU-16, the synthetic version of a naturally occurring aplysinopsin (known as NP7), and UNT-TWU-22 displayed affinity for the cloned serotonin 5-HT2C receptor subtype.

Based on these findings, the type and position of halogenation on the indole ring determines selectivity for 5-HT2A and 5-HT2C receptor subtypes. Appropriately halogenated rings can result in high affinity derivatives with selectivities greater than 100-fold.