Molecular mechanism of action of 5,6-dihydroxytryptamine. Synthesis and biological evaluation of 4-methyl-, 7-methyl-, and 4,7-dimethyl-5,6-dihydroxytryptamines.

The major mechanism by which the serotonin neurotoxin 5,6-dihydroxytryptamine (5,6-DHT) expresses its neurodegenerative action may involve alkylation of biological nucleophiles by the electrophilic quinoid autoxidation products. To determine the relative importance of various sites on these autoxidation products toward alkylation we have rationally designed and synthesized 4-Me-5,6-DHT (16a), 7-Me-5,6-DHT (16b), and 4,7-Me2-5,6-DHT (16c). The indole nucleus of these analogues was constructed by the reductive cyclization of the corresponding 2, beta-dinitrostyrenes, and the aminoethyl side chain was introduced via gramine methiodides. Redox data showed that all the analogues are more readily oxidized compared to 5,6-DHT. The biological activity was evaluated in differentiated neuroblastoma N-2a cells in culture. The order of inhibitory potency, as determined by measuring the inhibition of incorporation of [3H]thymidine into DNA, was 16c much greater than 16a greater than 5,6-DHT approximately equal to 16b. The order of affinity (expressed as IC50 values in microM) for serotonergic uptake as determined by measuring their inhibition of [3H]-5-HT uptake was 5,6-DHT (4) greater than 16c (20) greater than 16a (23) greater than 16b (52). The results of these studies established that these rationally designed C-methylated analogues of 5,6-DHT are suitable probes for elucidating the molecular mechanism of action of 5,6-DHT.