Whereas drugs are intended to be selective, at least some bind to several physiologic targets, explaining both side effects and efficacy. As many drug-target combinations exist, it would be useful to explore possible interactions computationally. Here, we compared 3,665 FDA-approved and investigational drugs against hundreds of targets, defining each target by its ligands. Chemical similarities between drugs and ligand sets predicted thousands of unanticipated associations. Thirty were tested experimentally, including the antagonism of the β1 receptor by the transporter inhibitor Prozac, the inhibition of the 5-HT transporter by the ion channel drug Vadilex, and antagonism of the histamine H4 receptor by the enzyme inhibitor Rescriptor. Overall, 23 new drug-target associations were confirmed, five of which were potent (< 100 nM). The physiological relevance of one such, the drug DMT on serotonergic receptors, was confirmed in a knock-out mouse. The chemical similarity approach is systematic and comprehensive, and may suggest side-effects and new indications for many drugs.

As a result of superior efficacy and overall tolerability, atypical antipsychotic drugs have become the treatment of choice for schizophrenia and related disorders, despite their side effects. Weight gain is a common and potentially serious complication of some antipsychotic drug therapy, and may be accompanied by hyperlipidemia, hypertension and hyperglycemia and, in some extreme cases, diabetic ketoacidosis. The molecular mechanism(s) responsible for antipsychotic drug-induced weight gain are unknown, but have been hypothesized to be because of interactions of antipsychotic drugs with several neurotransmitter receptors, including 5-HT 2A and 5-HT 2C serotonin receptors, H 1 -histamine receptors, a 1 -and a 2 -adrenergic receptors, and m3-muscarinic receptors. To determine the receptor(s) likely to be responsible for antipsychotic-drug-induced weight gain, we screened 17 typical and atypical antipsychotic drugs for binding to 12 neurotransmitter receptors. H 1 -histamine receptor affinities for this group of typical and atypical antipsychotic drugs were significantly correlated with weight gain (Spearman r ¼ À0.72; po0.01), as were affinities for a 1A adrenergic (r ¼ À0.54; po0.05), 5-HT 2C (r ¼ À0.49; po0.05) and 5-HT 6 receptors (r ¼ À0.54; po0.05), whereas eight other receptors' affinities were not. A principal components analysis showed that affinities at the H 1 , a 2A , a 2B , 5-HT 2A , 5-HT 2C , and 5-HT 6 receptors were most highly correlated with the first principal component, and affinities for the D 2 , 5-HT 1A , and 5-HT 7 receptors were most highly correlated with the second principal component. A discriminant functions analysis showed that affinities for the H 1 and a 1A receptors were most highly correlated with the discriminant function axis. The discriminant function analysis, as well as the affinity for the H 1 -histamine receptor alone, correctly classified 15 of the 17 drugs into two groups; those that induce weight gain and those that do not. Because centrally acting H 1 -histamine receptor antagonists are known to induce weight gain with chronic use, and because H 1 -histamine receptor affinities are positively correlated with weight gain among typical and atypical antipsychotic drugs, it is recommended that the next generation of atypical antipsychotic drugs be screened to avoid H 1 -histamine receptors.

Our data imply that activation of 5-HT(2B) receptors is necessary to produce VHD and that serotonergic medications that do not activate 5-HT(2B) receptors are unlikely to produce VHD. We suggest that all clinically available medications with serotonergic activity and their active metabolites be screened for agonist activity at 5-HT(2B) receptors and that clinicians should consider suspending their use of medications with significant activity at 5-HT(2B) receptors.

The diterpene salvinorin A from Salvia divinorum has recently been reported to be a high-affinity and selective -opioid receptor agonist (Roth et al., 2002). Salvinorin A and selected derivatives were found to be potent and efficacious agonists in several measures of agonist activity using cloned human -opioid receptors expressed in human embryonic kidney-293 cells. Thus, salvinorin A, salvinorinyl-2-propionate, and salvinorinyl-2-heptanoate were found to be either full (salvinorin A) or partial (2-propionate, 2-heptanoate) agonists for inhibition of forskolinstimulated cAMP production. Additional studies of agonist potency and efficacy of salvinorin A, performed by cotransfecting either the chimeric G proteins Gaq-i5 or the universal G protein Ga16 and quantification of agonist-evoked intracellular calcium mobilization, affirmed that salvinorin A was a potent and effective -opioid agonist. Results from structure-function studies suggested that the nature of the substituent at the 2-position of salvinorin A was critical for -opioid receptor binding and activation. Because issues of receptor reserve complicate estimates of agonist efficacy and potency, we also examined the agonist actions of salvinorin A by measuring potassium conductance through G protein-gated K ϩ channels coexpressed in Xenopus oocytes, a system in which receptor reserve is minimal. Salvinorin A was found to be a full agonist, being significantly more efficacious than (trans)-

Recent findings have implicated the 5-hydroxytryptamine 2B (5-HT 2B ) serotonin receptor in mediating the heart valve fibroplasia [valvular heart disease (VHD)] and primary pulmonary hypertension observed in patients taking the now-banned appetite suppressant fenfluramine (Pondimin, Redux). Via large-scale, random screening of a portion of the receptorome, we have discovered that the amphetamine derivative 3,4-methylenedioxymethamphetamine (MDMA, "Ecstasy") and its N-demethylated metabolite 3,4-methylenedioxyamphetamine (MDA) each preferentially bind to and activate human recombinant 5-HT 2B receptors. We also demonstrate that MDMA and MDA, like fenfluramine and its N-deethylated metabolite norfenfluramine, elicit prolonged mitogenic responses in human valvular interstitial cells via activation of 5-HT 2B receptors. We also report that pergolide and dihydroergotamine, two drugs recently demonstrated to induce VHD in humans, potently activate 5-HT 2B receptors, thus validating this assay system for its ability to predict medications that might induce VHD. Our discovery that MDMA and a major metabolite, MDA, induce prolonged mitogenic responses in vitro similar to those induced by fenfluramine and norfenfluramine in vivo (i.e., valvular interstitial cell fibroplasia) predict that long-term MDMA use could lead to the development of fenfluramine-like VHD. Because of the widespread abuse of MDMA, these findings have major public health implications. These findings also underscore the necessity of screening current and future drugs at h5-HT 2B receptors for agonist actions before their use in humans.

Ephedrine is a long-studied stimulant available both as a prescription and over-the-counter medication, as well as an ingredient in widely marketed herbal preparations, and is also used as a precursor for the illicit synthesis of methamphetamine. Ephedrine is related to phenylpropanolamine, a decongestant removed from the market place due to concerns that its use increased the risk of hemorrhagic stroke. Standard pharmacology texts emphasize that ephedrine is both a direct and indirect adrenergic agonist, activating adrenergic receptors both by direct agonist activity as well as by releasing norepinephrine via a carrier-mediated exchange mechanism. Chemically, ephedrine possesses two chiral centers. In the present study, we characterized the stereoisomers of ephedrine and the closely related compounds pseudoephedrine, norephedrine, pseudonorephedrine (cathine), methcathinone, and cathinone at biogenic amine transporters and a large battery of cloned human receptors (e.g., "receptorome"). The most potent actions of ephedrine-type compounds were as substrates of the norepinephrine transporter (EC 50 values of about 50 nM) followed by substrate activity at the dopamine transporter. Screening the receptorome demonstrated weak affinity at ␣ 2 -adrenergic and 5-hydroxytryptamine 7 receptors (K i values 1-10 M) and no significant activity at ␤-adrenergic or ␣ 1 -adrenergic receptors. Viewed collectively, these data indicate that the pharmacological effects of ephedrine-like phenylpropanolamines are likely mediated by norepinephrine release, and although sharing mechanistic similarities with, they differ in important respects from those of the phenylpropanonamines methcathinone and cathinone and the phenyisopropylamines methamphetamine and amphetamine.

The effect of endocytosis inhibitors on 5-hydroxytryptamine(2A) (5-HT(2A)) receptor desensitization and resensitization was examined in transiently transfected human embryonic kidney (HEK) 293 cells and in C6 glioma cells that endogenously express 5-HT(2A) receptors. In HEK-293 cells, 5-HT(2A) receptor desensitization was unaffected by cotransfection with a dominant-negative mutant of dynamin (DynK44A), a truncation mutant of arrestin-2 [Arr2(319-418)], or by two well-characterized chemical inhibitors of endocytosis: concanavalin A (conA) and phenylarsine oxide (PAO). In contrast, beta 2-adrenergic receptor desensitization was significantly potentiated by each of these treatments in HEK-293 cells. In C6 glioma cells, however, DynK44A, Arr2(319-418), conA, and PAO each resulted in the potentiation of 5-HT(2A) and beta-adrenergic receptor desensitization. The cell-type-specific effect of Arr2(319-418) on 5-HT(2A) receptor desensitization was not related to the level of GRK2 or GRK5 expression. Interestingly, although beta 2-adrenergic receptor resensitization was potently blocked by cotransfection with DynK44A, 5-HT(2A) receptor resensitization was enhanced, suggesting the existence of a novel cell-surface mechanism for 5-HT(2A) receptor resensitization in HEK-293 cells. In addition, Arr2(319-418) had no effect on 5-HT(2A) receptor resensitization in HEK-293 cells, although it attenuated the resensitization of the beta 2-adrenergic receptor. However, in C6 glioma cells, both DynK44A and Arr2(319-418) significantly reduced 5-HT(2A) receptor resensitization. Taken together, these results provide the first convincing evidence of cell-type-specific roles for endocytosis inhibitors in regulating GPCR activity. Additionally, these results imply that novel GRK and arrestin-independent mechanisms of 5-HT(2A) receptor desensitization and resensitization exist in HEK-293 cells.

Although M(1) muscarinic receptor-selective partial agonists have shown promise in some preclinical antipsychotic drug models, these studies indicate that it is unlikely that the salutary actions of clozapine and similar atypical antipsychotic drugs are mediated solely by M(1) muscarinic receptor activation. It is possible, however, that the M(1) agonism of N-desmethylclozapine contributes to the uniquely beneficial actions of clozapine. Thus, these results are consistent with the notion that a balanced degree of activity at multiple biogenic amine receptors, including M(1) muscarinic agonism, is responsible for the uniquely beneficial actions of clozapine.