Pharmacological characterization and determination of pharmacokinetic and pharmacodynamic relationship of PF-00885706, a novel partial agonist selective for the 5-HT(4) receptor.

The pharmacological profile of PF-00885706, a selective 5-HT(4) receptor partial agonist, was investigated. PF-00885706 displayed a high binding affinity for the human 5-HT(4d) receptor with a K(i) of 3.7 nM that translates to functional agonist activity in vitro with EC(50) values of 4.0 nM and 6.6 nM in cell-based assays of human recombinant 5-HT(4d) receptors and rat tunica muscularis mucosae tissues, respectively. In both assays, partial agonism was confirmed with E(max) values of 84% and 78%, respectively. Notably, PF-00885706 was highly selective, displaying >1000-fold higher affinity for 5-HT(4d) receptors compared to 5-HT(1A), 5-HT(1B), 5-HT(1D), 5-HT(2A), 5-HT(2B), 5-HT(2C), 5-HT(3), 5-HT(7), and D(2long) receptors. Furthermore, in vitro binding assays demonstrated that PF-00885706 had no biologically significant interaction with physiologically important enzymes, ion channels including hERG channel, or receptors at concentrations up to 10 microM except for binding to the sigma(2) receptor. PF-00885706 exhibited weak binding affinity for the sigma(2) receptor yielding a K(i) value of 3 microM, which is more than 800-fold weaker than that for the 5-HT(4d) receptor. Oral administration of PF-00885706 to dogs resulted in marked and long-lasting stimulation of gastric motility with a minimum effective dose of 0.001 mg/kg. Pharmacokinetic analysis revealed that PF-00885706 has a low to moderate volume of distribution and the complete absorption in dogs. Pharmacokinetic and pharmacodynamic analysis of PF-00885706 in the dog gastric motility model showed a correlation between plasma concentrations and enhancement of gastric motility. Thus, PF-00885706 is an orally active, highly selective partial agonist for 5-HT(4) receptors that is expected to be effective for the treatment with gastrointestinal dysmotility disorders with reduced adverse effects mediated by other related receptors.

In vitro and in vivo pharmacological characterization of PF-01354082, a novel partial agonist selective for the 5-HT(4) receptor.

The pharmacological profile of PF-01354082, a selective 5-HT(4) receptor partial agonist, was investigated. PF-01354082 displayed high affinity for human 5-HT(4d) and dog 5-HT(4h) receptors in binding studies, having Ki values of 2.0 nM and 4.2 nM, respectively. By contrast, PF-01354082 did not show significant affinity for several other 5-HT receptors (5-HT(1A), 5-HT(1B), 5-HT(1D), 5-HT(2A), 5-HT(2B), 5-HT(2C), 5-HT(3A), and 5-HT(7)) or the dopamine D(2long) receptor. Functional assays using either cells expressing human recombinant 5-HT(4d) receptors or rat tunica muscularis mucosae demonstrated that PF-01354082 exhibited partial agonist activity at the 5-HT(4) receptor. The effects of PF-01354082 on in vitro receptor binding, ion channel activity, and sites of uptake were further investigated. PF-01354082 did not show biologically relevant binding activity at concentrations up to 10 microM except for binding to the 5-HT(4e) receptor. Furthermore, PF-01354082 decreased I(HERG) current by only 11% at a concentration of 300 microM, indicating that the compound had greater than 150,000-fold selectivity for the human 5-HT(4d) receptor over hERG channels. An in vivo study using a gastric motility model in conscious dogs demonstrated that oral administration of PF-01354082 resulted in marked and sustained stimulation of gastric motility in a dose-dependent manner. These results indicate that PF-01354082 is an orally active, highly selective, partial agonist of the human 5-HT(4) receptor that is expected to exert a favorable effect on gastrointestinal motor disorders with reduced adverse effects mediated by other related receptors.

Up-regulation of P-glycoprotein expression in small intestine under chronic serotonin-depleted conditions in rats.

To investigate the role of serotonin (5-HT), an important neurotransmitter and hormone/paracrine agent in the small intestine, in the transport activity of P-glycoprotein (P-gp), the intestinal transport of quinidine, a P-gp substrate, was examined in 5-HT-depleted rats prepared by intraperitoneal administration of p-chlorophenylalanine, a specific inhibitor of tryptophan hydroxylase in 5-HT biosynthesis. In the in vitro transport study, quinidine transport across rat jejunum was significantly enhanced in both the secretory and absorptive directions under 5-HT-depleted conditions, although the secretory transport was still predominant. The electrophysiological study suggested that the quinidine transport via passive diffusion was enhanced presumably through a paracellular route. This might be due to looser tight junctions under 5-HT-depleted conditions. The voltage-clamp technique clearly indicated that the secretory transport of quinidine through the transcellular pathway was also enhanced by the depletion of 5-HT. Furthermore, 5-HT depletion increased verapamil-sensitive secretory transport of quinidine in rat jejunum. These results indicate that the secretory transport of quinidine via P-gp was significantly enhanced under 5-HT-depleted conditions. The level of ATP, an energy source for functioning P-gp, wet weight of jejunum, and total protein level in rat jejunal mucosa were not changed by 5-HT depletion, but the expression of P-gp in the brush-border membrane of rat jejunum was significantly induced, which is partly responsible for the enhancement of P-gp activity under the 5-HT-depleted condition.