Suppression of mutagenic activity of a series of 5HT2C receptor agonists by the incorporation of a gem-dimethyl group: SAR using the Ames test and a DNA unwinding assay

A positive result in the Ames test is generally taken as a strong indication for a genotoxic (i.e. DNA damaging) property of the test compound, often sufficient to cause termination of its development as a new therapeutic agent. A number of serotonin receptor ligands have been tested for their mutagenic potential in the Ames assay at an early stage of development. For several compounds increases in the number of revertant colonies were observed in strain TA1537. Consequently, structure-activity relationship investigations were undertaken to search for compounds without mutagenic liability. All compounds are three ringed heterocyclic structures consisting of a benzene ring, a central (generally non-aromatic) 5- or 6-membered ring and a pyrrole or pyrazole ring. Using a gel shift assay we provide evidence that the observed genotoxic effects are strongly influenced by the intercalating properties of the compounds. The highest mutagenic response was seen with a compound possessing a central aromatic ring. The mutagenic activity of the naphthaleno derivatives appears to be stronger when compared with the indeno compounds, probably because of the less curved structure. Dimethyl substitution of the indeno substructure reduces the intercalating ability of the compounds and leads to loss of mutagenic activity. Pyrazole analogues of both indeno and naphthaleno structures appear to produce stronger mutagenic responses than the pyrrole derivatives.