The CRTH inhibitory activities of the synthesized
The CRTH2 inhibitory activities of the synthesized compounds are listed in , . At first we introduced halogen or other substituents at the 4,4′-position of phenyl rings in the benzhydryl moiety in order to obtain SAR and to improve the metabolic stability at this moiety (–). It is well-known that introduction of halogen atom at the -position of the phenyl ring can protect from metabolism. All four -substituted analogs – displayed 4–7-fold more potent activity against human CRTH2 than , but these modifications did not contribute to enhancing activity against guinea pig CRTH2. Specifically, the fluoro () and methoxy () derivatives were comparable to , but introduction of the chloro () and methyl () groups led to a threefold or more loss. These data suggest that only the hydrogen bond acceptor is tolerable to guinea pig CRTH2 whereas a variety of substituents may be acceptable to the human receptor, and accordingly that the introduction of substituents at this position might enhance activity against human CRTH2 but would not improve the species difference between humans and guinea pigs. To facilitate synthesis and block the metabolically labile benzyl position, we converted the methylene moiety of into oxygen (). However, showed 2–3-fold less potent inhibitory activity both to human and guinea pig CRTH2. We did not perform further optimization with this linker. In consideration of planarity around the nitrogen atom, we converted the pyridone scaffold of to benzamides. The benzamides and showed highly potent (nM order) activity with IC values to human CRTH2 of 9.7 and 5.5nM, respectively. In addition, these benzamides also displayed a greater than 10-fold improvement in affinity for guinea pig CRTH2. These data suggest that the lipophilicity of the scaffold may cause the enhancement of the CRTH2 inhibitory activity. Finally, we converted the terminal acetic Melatonin moiety to explore the capability (–). Introduction of only one methyl group to the α-position of the carboxyl group enhanced CRTH2 inhibitory activity (), whereas the activity was drastically diminished in the case of the ethyl () or dimethyl () groups. Meanwhile, elongation of the methylene chain contributed to the enhancement of activity (). These data suggest that bulky substituents around the acid moiety are not acceptable. Although the CRTH2 inhibitory activity of and are comparable, is favorable because it is free from the problem of chirality. Given that showed the most potent activity among the pyridone series so far, we accordingly modified the pyridone scaffold of into pyridazinone . Although the inhibitory activity of against human CRTH2 was comparable to that of , showed twofold more potent activity in guinea pigs than . Compounds , and , with high potency compared to and were subjected to pharmacokinetic experiments in guinea pigs as advanced candidates (). The resulting most potent antagonist showed poor oral availability. We consider that the poor PK profile of was due to greater metabolic lability to in vitro clearance in liver microsomes in guinea pigs (766mL/min/kg) than (120mL/min/kg). The chain elongated compound showed poorer oral availability than the corresponding lead . In contrast, pyridazinone displayed excellent oral availability, and dosing (10mg/kg) to guinea pigs led to an approximately threefold increase in (2.86μg/mL) and comparable AUC (4.85μg·h/mL) to . Evaluating the inhibitory activity of against human DP1 proved that this compound was a selective CRTH2 antagonist (IC=5200nM to human DP1). We next evaluated the in vivo anti-asthmatic activity of compounds and in a guinea pig model, and found that both showed in vivo efficacy orally in a guinea pig model of airway hyperresponsiveness, with ED values of 4.7 and 0.05mg/kg u.i.d., respectively. The in vivo efficacy of was thus drastically improved than that of . In summary, we discovered a novel and selective CRTH2 antagonist from HTS of our chemical library. Initial optimization based on resulted in the discovery of the novel, potent and orally bioavailable CRTH2 antagonist . We achieved not only a improvement in in vitro CRTH2 antagonistic activity against both human and guinea pig but also a drastic improvement in in vivo efficacy compared to that of . Further optimization of this series aimed at improving activities and pharmacokinetic properties will be reported later.