MDMA (3,4-methylenedioxy methamphetamine) is a psychoactive drug which is thought to act via stimulation of secretion as well as inhibition of re-uptake of large amounts of serotonin, noradrenaline and dopamine in the brain. MDMA also acts directly on a number of receptors, including 5-HT_2A receptors. There is considerable evidence that 5-HT_2A antagonists can modulate behavioral and physiological effects of MDMA in animals.

Nantenine an aporphine alkaloid ex Nandina domestica has been reported to block and reverse a range of behavioral and physiological effects of MDMA in mice. It is known that nantenine has moderate 5-HT _2A antagonist activity. However, very little structure-activity relationship (SAR) studies have been performed on nantenine with regards to its activity at the 5-HT_2A receptor.

As part of our research focus to develop aporphine-based 5-HT_2A antagonists as potential MDMA antagonists, we have prepared a library of novel analogs to investigate the structural requirements for nantenine’s 5-HT_2A activity. Our studies demonstrate that the N-methyl group, methylenedioxy ring and structural rigidity of the aporphine nucleus are important for activity, and that appropriate substitutions on the aromatic aporphine core can improve 5-HT_2A antagonist activity.

To elucidate possible binding modes of these compounds and to determine the correlation with our binding data, we built a 5-HT_2A homology model based on a bovine rhodopsin template and then performed docking/scoring experiments.

Our results suggest that members of the C1 series of nantenine analogs bind to the 5-HT_2A receptor in the same orientation but differently than nantenine. In addition to known and important interaction between the protonated nitrogen of the ligands and Asp155 of the receptor, some of these analogs established a hydrogen bond with Ser242 as well as hydrophobic interactions with Phe234 and Gly238. These latter interactions may account for their enhanced activity as compared to nantenine.

Our findings will be useful in the future design of high affinity 5-HT _2A ligands based on the nantenine aporphine core structure.

For the second part of the project, nantenine as well as a number of flexible analogs were evaluated for acetylcholinesterase (AChE) inhibitory activity in a microplate spectrophotometric assays based on Ellman’s method. It was found that the rigid aporphine core of nantenine and N-methyl substituent are important structural requirements for its anticholinesterase activity. Nantenine showed mixed inhibition kinetics in enzyme assays. Molecular docking experiments suggest that nantenine binds preferentially to the catalytic site of AChE but is also capable of interacting with the peripheral anionic site (PAS) of the enzyme thus accounting for its mixed inhibition profile. The aporphine core of nantenine may thus be a useful template for the design of novel PAS or dual-site AChE inhibitors; inhibiting the PAS is desirable for prevention of aggregation of the amyloid peptide Aβ a major causative factor in the progression of Alzheimer’s disease (AD).