Effect of ring fluorination on the pharmacology of hallucinogenic tryptamines

A series of fluorinated analogues of the hallucinogenic tryptamines N,N-diethyltryptamine (DET), 4-hydroxy-N,N-dimethyltryptamine (4-OH-DMT, psilocin), and 5-methoxy-DMT was synthesized to investigate possible explanations for the inactivity of 6-fluoro-DET as a hallucinogen and to determine the effects of fluorination on the molecular recognition and activation of these compounds at serotonin receptor subtypes. The target compounds were evaluated using in vivo behavioral assays for hallucinogen-like and 6-HT1A agonist activity and in vitro radioligand competition assays for their affinity at 5-HT2A, 5-HT2C, and 5-HT1A receptor sites. Functional activity at the 5-HT2A receptor was determined for all compounds. In addition, for some compounds functional activity was determined at the 5-HT1A receptor. Hallucinogen-like activity, evaluated in the two-lever drug discrimination paradigm using LSD-trained rats, was attenuated or abolished for all of the fluorinated analogues. One of the tryptamines, 4-fluoro-5-methoxy-DMT (6), displayed high 5-HT1A agonist activity, with potency greater than that of the 5-HT1A agonist 8-OH-DPAT. The ED50 Of 6 in the two-lever drug discrimination paradigm using rats trained to discriminate the 5-HT1A agonist LY293284 was 0.17 mu mol/kg, and the K-i at [H-3] 8-OH-DPAT-labeled 5-HT1A receptors was 0.23 nM. The results indicate that fluorination of hallucinogenic tryptamines generally has little effect on 5-HT2A/2C receptor affinity or intrinsic activity. Affinity at the 5-HT1A receptor was reduced, however, in all but one example, and all of the compounds tested were full agonists but with reduced functional potency at this serotonin receptor subtype. The one notable exception was 4-fluoro-5-methoxy-DMT (6), which had markedly enhanced 5-HT1A receptor affinity and functional potency. Although it is generally considered that hallucinogenic activity results from 5-HT2A receptor activation, the present results suggest a possible role for involvement of the 5-HT1A receptor with tryptamines.