Pharmacological characterization of the cloned human 5-hydroxytryptamine transporter

Possibility of Predicting Serotonin Transporter Occupancy From the In Vitro Inhibition Constant for Serotonin Transporter, the Clinically Relevant Plasma Concentration of Unbound Drugs, and Their Profiles for Substrates of Transporters

Accurate prediction of target occupancy facilitates central nervous system drug development. In this review, we discuss the predictability of serotonin transporter (SERT) occupancy in human brain estimated from in vitro K_i values for human SERT and plasma concentrations of unbound drug (C_u,plasma), as well as the impact of drug transporters in the blood-brain barrier. First, the geometric means of in vitro K_i values were compared with the means of in vivo K_i values (K_i,u,plasma) which were calculated as C_u,plasma values at 50% occupancy of SERT obtained from previous clinical positron emission tomography/single photon emission computed tomography imaging studies for 6 selective serotonin transporter reuptake inhibitors and 3 serotonin norepinephrine reuptake inhibitors. The in vitro K_i values for 7 drugs were comparable to their in vivo K_i,u,plasma values within 3-fold difference. SERT occupancy was overestimated for 5 drugs (P-glycoprotein substrates) and underestimated for 2 drugs (presumably uptake transporter substrates, although no evidence exists as yet). In conclusion, prediction of human SERT occupancy from in vitro K_i values and C_u,plasma was successful for drugs that are not transporter substrates and will become possible in future even for transporter substrates, once the transporter activities will be accurately estimated from in vitro experiments.

Pharmacologically active drug metabolites: impact on drug discovery and pharmacotherapy

Metabolism represents the most prevalent mechanism for drug clearance. Many drugs are converted to metabolites that can retain the intrinsic affinity of the parent drug for the pharmacological target. Drug metabolism redox reactions such as heteroatom dealkylations, hydroxylations, heteroatom oxygenations, reductions, and dehydrogenations can yield active metabolites, and in rare cases even conjugation reactions can yield an active metabolite. To understand the contribution of an active metabolite to efficacy relative to the contribution of the parent drug, the target affinity, functional activity, plasma protein binding, membrane permeability, and pharmacokinetics of the active metabolite and parent drug must be known. Underlying pharmacokinetic principles and clearance concepts are used to describe the dispositional behavior of metabolites in vivo. A method to rapidly identify active metabolites in drug research is described. Finally, over 100 examples of drugs with active metabolites are discussed with regard to the importance of the metabolite(s) in efficacy and safety.

Tramadol and flurbiprofen depress the cytotoxicity of cisplatin via their effects on gap junctions

PURPOSE

Cancer patients are often concurrently treated with analgesics and antineoplastic drugs, yet the influence of analgesic agents on therapeutic activity of antineoplastic drugs is largely unexplored. This study investigates the effects of three commonly used analgesics, which produce analgesia by different mechanisms, on cytotoxicity induced by cisplatin, a widely used antitumor agent, and the relation between those effects and modulation of gap junction function by the analgesics.

EXPERIMENTAL DESIGN

The role of gap junctions in the modulation of cisplatin toxicity is explored by manipulation of connexin expression, and gap junction presence and function, using clinically relevant concentrations of the analgesics and cisplatin.

RESULTS

Short-term exposure of transformed cells to cisplatin reduced the clonogenic survival in low-density cultures (without gap junction formation) and in high density (with gap junction formation), but the toxic effect was greater at high density. In the absence of connexin expression or with block of connexin channels, cell density had no effect on cisplatin toxicity. Tramadol and flurbiprofen, but not morphine, significantly reduced cisplatin cytotoxicity, but this effect required functional gap junctions between the cells. Tramadol and flurbiprofen inhibited dye-coupling through gap junctions, but morphine did not.

CONCLUSIONS

The results suggest that the density dependence of cisplatin toxicity is mediated by gap junctions. They further indicate that tramadol and flurbiprofen depress cisplatin cytotoxicity through inhibition of gap junction activity, and more generally, that agents that depress junctional communication can counteract the effects of antitumor agents.

SB-649915, a novel, potent 5-HT1A and 5-HT1B autoreceptor antagonist and 5-HT re-uptake inhibitor in native tissue

An increase in brain 5-HT levels is thought to be the key mechanism of action which results in an antidepressant response. It has been proven that selective serotonin re-uptake inhibitors are effective antidepressants but the delay to therapeutic onset of these agents is thought to be due to the time required for 5-HT1A, and possibly 5-HT1B, autoreceptor desensitisation. Therefore an agent incorporating 5-HT re-uptake inhibition coupled with 5-HT1A and/or 5-HT1B autoreceptor antagonism may provide a fast acting clinical agent. The current studies describe the in vitro profile of SB-649915 (6-[(1-{2-[(2-methylquinolin-5-yl)oxy]ethyl}piperidin-4-yl)methyl]-2H-1,4-benzoxazin-3(4H)-one), a novel compound which has high affinity for human recombinant 5-HT1A, 5-HT1B and 5-HT1D receptors (pKi values of 8.6, 8.0, 8.8, respectively) and the human recombinant 5-HT transporter (pKi value of 9.3). SB-649915 also displays high affinity for rat, guinea pig, mouse and marmoset native tissue 5-HT1A, 5-HT1B and 5-HT1D receptors and rat native tissue 5-HT transporters (pKi values>or=7.5). In functional [35S]GTPgammaS binding studies, SB-649915 (up to 1 microM) does not display intrinsic activity in HEK293 cells expressing human recombinant 5-HT1A receptors but acts as a partial agonist at human recombinant 5-HT1B and 5-HT1D receptors with intrinsic activity values of 0.3 and 0.7, respectively, as compared to the full agonist 5-HT. From Schild analysis, SB-649915 caused a concentration-dependent, rightward shift of 5-HT-induced stimulation of basal [35S]GTPgammaS binding in cells expressing human recombinant 5-HT1A or 5-HT1B receptors to yield pA2 values of 9.0 and 7.9, respectively. In electrophysiological studies in rat dorsal raphe nucleus, SB-649915 did not affect the cell firing rate up to 1 microM but attenuated (+)8-hydroxy-2-(di-n-propylamino) tetralin-induced inhibition of cell firing with an apparent pKb value of 9.5. SB-649915 (1 microM) significantly attenuated exogenous 5-HT-induced inhibition of electrically-stimulated [3H]5-HT release from guinea pig cortex. In studies designed to enhance endogenous 5-HT levels, and therefore increase tone at 5-HT1B autoreceptors, SB-649915 significantly potentiated [3H]5-HT release at 100 and 1000 nM. In LLCPK cells expressing human recombinant 5-HT transporters and in rat cortical synaptosomes, SB-649915 inhibited [3H]5-HT re-uptake with pIC50 values of 7.9 and 9.7, respectively. In summary, SB-649915 is a novel, potent 5-HT1A/1B autoreceptor antagonist and 5-HT re-uptake inhibitor in native tissue systems and represents a novel mechanism that could offer fast acting antidepressant action.

Potential of [11C]DASB for measuring endogenous serotonin with PET: binding studies

The serotonin transporter radioligand [11C]-3-amino-4-(2-dimethylaminomethylphenylsulfanyl)-benzonitrile, or [11C]DASB, was examined in order to assess its potential for measuring fluctuations in endogenous serotonin concentrations with positron emission tomography. Binding characteristics of [11C]DASB and the propensity for serotonin to displace the tracer were explored in rat brain homogenates. Experiments showed that serotonin displaced [11C]DASB in vitro. Ex vivo experiments performed after tranylcypromine injection (3 or 15 mg/kg) showed a dose-dependent trend in radioactivity uptake and suggested that serotonin may compete with [11C]DASB for transporter binding.

Sertraline: a review of its use in the management of major depressive disorder in elderly patients

Sertraline is a selective serotonin reuptake inhibitor (SSRI) with well established antidepressant and anxiolytic activity. Results from several well designed trials show that sertraline (50 to 200 mg/day) is effective in the treatment of major depressive disorder in elderly patients (> or =60 years of age). Primary endpoints in most studies included the Hamilton Depression Rating Scale (HDRS), Clinical Global Impression (CGI) score and the Montgomery-Asberg Depression Rating Scale (MADRS). Sertraline was significantly more effective than placebo, and was as effective as fluoxetine, nortriptyline and imipramine in elderly patients. During one trial, amitriptyline was significantly more effective than sertraline [mean reduction from baseline on one of six primary outcomes (HDRS)], although no quantitative data were provided. Subgroup analysis of data from a randomised, double-blind trial in elderly patients with major depressive disorder suggests that vascular morbidity, diabetes mellitus or arthritis does not affect the antidepressant effect of sertraline. Secondary endpoints from these clinical trials suggest that sertraline has significant benefits over nortriptyline in terms of quality of life. In addition, significant differences favouring sertraline in comparison with nortriptyline and fluoxetine have been recorded for a number of cognitive functioning parameters. Sertraline is generally well tolerated in elderly patients with major depressive disorder, and lacks the marked anticholinergic effects that characterise the adverse event profiles of tricyclic antidepressants (TCAs). The most frequently reported adverse events in patients aged > or =60 years with major depressive disorder receiving sertraline 50 to 150 mg/day were dry mouth, headache, diarrhoea, nausea, insomnia, somnolence, constipation, dizziness, sweating and taste abnormalities. The tolerability profile of sertraline is generally similar in younger and elderly patients. Sertraline has a low potential for drug interactions at the level of the cytochrome P450 enzyme system. In addition, no dosage adjustments are warranted for elderly patients solely based on age.

CONCLUSION

Sertraline is an effective and well tolerated antidepressant for the treatment of major depressive disorder in patients aged > or =60 years. Since elderly patients are particularly prone to the anticholinergic effects of TCAs as a class, SSRIs such as sertraline are likely to be a better choice for the treatment of major depressive disorder in this age group. In addition, sertraline may have advantages over the SSRIs paroxetine, fluoxetine and fluvoxamine in elderly patients because of the drug's comparatively low potential for drug interactions, which is of importance in patient groups such as the elderly who are likely to receive more than one drug regimen.

Non-amines, drugs without an amine nitrogen, potently block serotonin transport: novel antidepressant candidates?

The serotonin transporter (SERT) is a principal site of action of therapeutic antidepressants in the brain. Without exception, these inhibitors of serotonin transport contain an amine nitrogen in their structure. We previously demonstrated that novel compounds without an amine nitrogen in their structure (non-amines), blocked dopamine transport in cells transfected with the human dopamine transporter. The present study investigated whether, in the absence of an amine nitrogen, certain non-amines bind selectively to the SERT and block the transport of serotonin. At 10 microM concentration, select non-amines displayed no, or little, affinity for 9 serotonin, 5 dopamine, 7 adrenergic, 5 muscarinic cholinergic, 3 opiate and histamine receptors. The affinities of non-amines for [(3)H]citalopram binding sites on the SERT and their potencies for blocking [(3)H]serotonin transport were measured in cloned human SERT stably or transiently expressed in HEK-293. Whether oxa- or carba-based, non-amines bound to [(3)H]citalopram-labeled sites and blocked [(3)H]serotonin transport in the low nanomolar range, at values equal to or higher than those of some conventional antidepressants. A non-amine, O-1809, was 99-fold more selective for the serotonin over the dopamine transporter. As substituents on the aromatic ring of non-amines confer high affinity for the SERT, we investigated the hypothesis that aromatic-aromatic interactions may contribute significantly to non-amine/transporter association. A SERT mutant was produced in which a highly conserved aromatic amino acid, phenylalanine 548, was replaced by an alanine (F548A). Although the affinities of several non-amines were unchanged in the mutant SERT, the affinity of imipramine was decreased, revealing possible differences in amine and non-amine binding domains on the SERT. The similar affinities of non-amines and conventional antidepressant drugs for the SERT support the view that an amine nitrogen is not essential for drugs to block serotonin transport with high affinity. Non-amines open avenues for developing a new generation of antidepressants.

Substantial loss of substrate by diffusion during uptake in HEK-293 cells expressing neurotransmitter transporters

Human embryonic kidney 293 (HEK-293) cells stably transfected with the human serotonin (5-HT) or dopamine transporter (hSERT, hDAT), or the rat GABA transporter GAT-1 were incubated with saturating concentrations of transporter substrates (hSERT: [(3)H]5-HT, [(3)H]N-methyl-phenyl-pyridinium (MPP+); hDAT: [(3)H]dopamine, [(3)H]MPP(+); rGAT: [(3)H]GABA). Uptake velocities decreased significantly over time for [(3)H]5-HT and [(3)H]dopamine (already visible at 1 min), but not for [(3)H]MPP(+) or [(3)H]GABA. In efflux experiments cells were preloaded and substrate diffusion into the medium was studied following the addition of appropriate uptake inhibitors. Fractional effluxes were (% min(-1)) 1.27, 0.72, 0.27 and 0.08 for [(3)H]5-HT, [(3)H]dopamine, [(3)H]MPP(+) and [(3)H]GABA, respectively. The results suggest that in uptake experiments the more lipophilic substrates [(3)H]5-HT and [(3)H]dopamine leave the cells by diffusion already after a short time (1 min) of accumulation.

Placenta detachment: unexpected high concentrations of 5-hydroxytryptamine (serotonin) in fetal blood and its mitogenic effect on placental cells in bovine

The purpose of this study was twofold: (1) to determine whether there is a profile of 5-hydroxytryptamine (5-HT, serotonin) concentrations in fetal bovine blood and tissues during pregnancy, parturition and the early neonatal period and (2) to determine whether 5-HT has a 'mitogenic' effect on cultured placentome cells in bovine. Results revealed a 5-HT concentration profile in fetal blood. Overall concentrations of 5-HT in fetal blood were 6.6 times (P< 0.001) that of adult cows and 2.8 times (P< 0.001) that of blood collected during caesarean section (from umbilical veins) and from 18-72 h old calves. Mid-term and full-term pregnancy fetuses were not statistically different from each other. Overall concentrations of 5-HT in the intestinal wall of the fetus were 4.4 times higher (P< 0.05) than in the 24 h calf. Concentration of 5-HT in full-term muscle was 3.5 times higher than in mid-term muscle and 2.8 times higher than in 24 h old calf muscle (P< 0.05). Concentrations of 5-HT in mid-term and full-term cotyledon were 4.4 times higher (P< 0.05) than in post partum cotyledon. Characterized trophoblast cells and a heterogeneous population of bovine cotyledon cells treated with 5-HT (2.5, 5.0 and 10.0 microm) incorporated between 2.0 and 3.0 times more(3)[H]-thymidine than untreated controls, indicating a dose-dependent (r=0.94) positive mitogenic effect of 5-HT. Both groups of cultured cells responded equally. Five-HT treatment did not affect either cell number or cell size. It was concluded that a 5-HT concentration profile exists in fetal bovine blood and tissues and that 5-HT has the ability to act as a mitogen in bovine placental cells.

Ligand interaction with the purified serotonin transporter in solution and at the air/water interface

The purified serotonin transporter (SERT) was spread at the air/water interface and the effects both of its surface density and of the temperature on its interfacial behavior were studied. The recorded isotherms evidenced the existence of a stable monolayer undergoing a lengthy rearrangement. SERT/ligand interactions appeared to be dependent on the nature of the studied molecules. Whereas an unrelated drug (chlorcyclizine) did not bind to the spread SERT, it interacted with its specific ligands. Compared to heterocyclic drugs, for which binding appeared to be concentration-dependent, a 'two-site' mechanism was evidenced for pinoline and imipramine.

Parallel synthesis of 3-aryloxy-2-propanolamines and evaluation as dual affinity 5-HT(1A) and 5-HT re-uptake ligands

A solution phase synthesis for the preparation of 3-aryloxy-2-propanolamine libraries has been developed. This resulted in the identification of 5 as a ligand with dual affinity for 5-HT1A and serotonin reuptake receptors which shows excellent pharmacokinetic parameters.

Induction of serotonin transporter by hypoxia in pulmonary vascular smooth muscle cells. Relationship with the mitogenic action of serotonin

-The increased delivery of serotonin (5-hydroxytryptamine, 5-HT) to the lung aggravates the development of hypoxia-induced pulmonary hypertension in rats, possibly through stimulation of the proliferation of pulmonary artery smooth muscle cells (PA-SMCs). In cultured rat PA-SMCs, 5-HT (10(-8) to 10(-6) mol/L) induced DNA synthesis and potentiated the mitogenic effect of platelet-derived growth factor-BB (10 ng/mL). This effect was dependent on the 5-HT transporter (5-HTT), since it was prevented by the 5-HTT inhibitors fluoxetine (10(-6) mol/L) and paroxetine (10(-7) mol/L), but it was unaltered by ketanserin (10(-6) mol/L), a 5-HT2A receptor antagonist. In PA-SMCs exposed to hypoxia, the levels of 5-HTT mRNA (measured by competitive reverse transcriptase-polymerase chain reaction) increased by 240% within 2 hours, followed by a 3-fold increase in the uptake of [3H]5-HT at 24 hours. Cotransfection of the cells with a construct of human 5-HTT promoter-luciferase gene reporter and of pCMV-beta-galactosidase gene allowed the demonstration that exposure of cells to hypoxia produced a 5.5-fold increase in luciferase activity, with no change in beta-galactosidase activity. The increased expression of 5-HTT in hypoxic cells was associated with a greater mitogenic response to 5-HT (10(-8) to 10(-6) mol/L) in the absence as well as in the presence of platelet-derived growth factor-BB. 5-HTT expression assessed by quantitative reverse transcriptase-polymerase chain reaction and in situ hybridization in the lungs was found to predominate in the media of pulmonary artery, in which a marked increase was noted in rats that had been exposed to hypoxia for 15 days. These data show that in vitro and in vivo exposure to hypoxia induces, via a transcriptional mechanism, 5-HTT expression in PA-SMCs, and that this effect contributes to the stimulatory action of 5-HT on PA-SMC proliferation. In vivo expression of 5-HTT by PA-SMC may play a key role in serotonin-mediated pulmonary vascular remodeling.