Several 2-alkyl-5-methoxytryptamine analogues were designed and prepared as potential 5-HT(6) serotonin agonists. It was found that 5-HT(6) receptors accommodate small alkyl substituents at the indole 2-position and that the resulting compounds can bind with affinities comparable to that of serotonin. In particular, 2-ethyl-5-methoxy-N, N-dimethyltryptamine (8) binds with high affinity at human 5-HT(6) receptors (K(i) = 16 nM) relative to 5-HT (K(i) = 75 nM) and was a full agonist, at least as potent (8: K(act) = 3.6 nM) as serotonin (K(act) = 5.0 nM), in activating adenylate cyclase. Compound 8 displays modest affinity for several other populations of 5-HT receptors, notably h5-HT(1A) (K(i) = 170 nM), h5-HT(1D) (K(i) = 290 nM), and h5-HT(7) (K(i) = 300 nM) receptors, but is otherwise quite selective. Compound 8 represents the first and most selective 5-HT(6) agonist reported to date. Replacing the 2-ethyl substituent with a phenyl group results in a compound that retains 5-HT(6) receptor affinity (i.e., 10: K(i) = 20 nM) but lacks agonist character. 2-Substituted tryptamines, then, might allow entry to a novel class of 5-HT(6) agonists and antagonists.

2-Arylaminoquinoxalines were prepared by the condensation of 2-chloroquinoxaline with the appropriate Mannich bases in the presence of HCl. To synthesize the Mannich bases, 4-acetamidophenol was reacted with formaldehyde and dialkylamine to yield 3-[(dialkylamino) methyl]-4-hydroxyacetanilide, followed by hydrolysis. Antimalarial activities of the new arylaminoquinoxalines were evaluated against the rodent malaria parasite Plasmodium yoelii at a dose of 75 mg kg(-1). Three compounds synthesized (2-[3-[(diethylamino) methyl]-4-hydroxyanilino]-quinoxaline dihydrochloride (2b), 2-[3-[(pyrrolidinyl) methyl]-4-hydroxyanilino]-quinoxaline dihydrochloride (2f), and 2-[3-[(piperidinyl) methyl]-4-hydroxyanilino]-quinoxaline dihydrochloride (2g)) showed moderate antimalarial activity.

Various sulfonyl-containing compounds (e.g. sulfonamides, sulfones) bind at human 5-HT6 serotonin receptors, but it has been difficult relating the binding mode(s) of such agents to one another, even though many possess a common SO2 moiety, to identify a common pharmacophore model(s). On the basis of the hypothesis that an ergoline-type conformation might be important for the binding of some sulfonamide-containing arylalkylamines, we prepared for examination at h5-HT6 receptors a series of compounds, including phenylethylamines 6, pyrroloethylamine 7, and phenylpiperazines 9. The results (with Ki values ranging from about 1 nM to >1000 nM) suggest that many of these agents likely bind in a related fashion, and structure-affinity studies indicate that the benzenesulfonamide portion of the phenylethylamine and phenylpiperazine analogues can be "reversed", abbreviated to a sulfone, and moved to an adjacent position with relatively little impact on affinity. Although a benzenesulfonamide (or related arylsulfonamide) group might be common to various 5-HT6 ligands, there appears to be some latitude with regard to the specific constitution and location of the sulfonamide moiety even within the same arylalkylamine structural framework. A pharmacophore model is presented to account for some of the current findings.