Further Studies on Imidazo[4,5-b]pyridine AT 1 Angiotensin II Receptor Antagonists. Effects of the Transformation of the 4-Phenylquinoline Backbone into 4-Phenylisoquinolinone or 1-Phenylindene Scaffolds Andrea Cappelli,* ,† Gal.la Pericot Mohr, †,# Germano Giuliani, † Simone Galeazzi, † Maurizio Anzini, † Laura Mennuni, ‡ Flora Ferrari, ‡ Francesco Makovec, ‡ Eva M. Kleinrath, § Thierry Langer, § Massimo Valoti, | Gianluca Giorgi, ⊥ and Salvatore Vomero † Dipartimento Farmaco Chimico Tecnologico and European Research Centre for Drug DiscoVery and DeVelopment, UniVersita ` di Siena, Via A. Moro, 53100 Siena, Italy, Rotta Research Laboratorium S.p.A., Via Valosa di Sopra 7, 20052 Monza, Italy, Department of Pharmaceutical Chemistry, Institute of Pharmacy, UniVersity of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria, Dipartimento di Scienze Biomediche, UniVersita ` di Siena, Via A. Moro, 53100 Siena, Italy, and Dipartimento di Chimica, UniVersita ` di Siena, Via A. Moro, 53100 Siena, Italy ReceiVed March 20, 2006 The 4-phenylquinoline fragment of novel AT 1 receptor antagonists 4 based on imidazo[4,5-b]pyridine moiety was replaced by 4-phenylisoquinolinone (compounds 5) or 1-phenylindene (compounds 6) scaffolds to investigate the structure-activity relationships. Binding studies showed that most of the synthesized compounds display high affinity for the AT 1 receptor. Because of the in vitro high potency of carboxylic acids 5b,f, they were evaluated in permeability (in Caco-2 cells) and in pharmacokinetic studies in comparison with quinoline derivatives 4b,i,j,k. The studies showed that these compounds are characterized by rapid excretion, low membrane permeability, and low oral bioavailability. The structure optimization of the indene derivatives led to compounds 6e,f possessing interesting AT 1 receptor affinities. Optimization produced polymerizing AT 1 receptor ligand 6c, which forms a thermoreversible polymer (poly-6c) and is released from the latter by a temperature-dependent kinetics. The results suggest the possibility of developing novel polymeric prodrugs based on a new release mechanism. Finally, a set of 34 AT 1 receptor antagonists was used as a new test for the evaluation of the predictive capability of the previously published qualitative and quantitative pharmacophore models. Introduction Angiotensin II (Ang II) is an octapeptide produced by the renin-angiotensin system (RAS), which plays a key role in the pathophysiology of hypertension. In humans, Ang II interacts with two main receptor subtypes: AT 1 and AT 2 . 1 The AT 1 receptor subtype mediates virtually all the known physiological actions of Ang II in cardiovascular, neuronal, endocrine, and hepatic cells as well as in other ones. This receptor belongs to the G-protein-coupled receptor (GPCR) superfamily and shows the seven hydrophobic transmembrane domains forming R- helices in the lipid bilayer of the cell membrane. The interaction of Ang II with the AT 1 receptor induces a conformational change, which promotes the coupling with the G protein(s) and leads to the signal transduction via several effector systems (phospholipases C, D, A2, adenyl cyclase etc.). 1,2 The parallel discovery of losartan and eprosartan, potent and orally active nonpeptide Ang II antagonists, has stimulated the design of a large number of congeners. 3 Among them, irbesartan, candesartan, valsartan, telmisartan, and olmesartan are on the market and some 20 other compounds are being developed. The biphenyl fragment bearing an acidic moiety (tetrazole ring, -COOH, -SO 2 -NH-CO-) is common to most of these compounds, which differ in the nature of the pendent hetero- cyclic system (valsartan lacks the heterocyclic moiety) connected to the para position of the distal phenyl by means of a methylene group. In fact, in the design of new nonpeptide Ang II antagonists, the strategy followed by most medicinal chemists was concerned with the molecular modification of the imidazole moiety of losartan (1). Among the large variety of the heterocyclic systems developed, an outstanding position is occupied by the imidazo[4,5-b]pyridine moiety of compound 2a (L-158,809). 4 This congener of losartan has been reported to show a subnanomolar AT 1 receptor affinity about 1 order of magnitude higher than that of losartan and represents one of the most potent nonpeptide Ang II antagonists so far developed. This suggests that the stereoelectronic characteristics of the imidazo[4,5-b]pyridine moiety can be considered optimal for the interaction with the receptor. However, relatively little information is available on the effects of the molecular modification of the phenyl group bearing the acidic moiety (distal phenyl ring). Moreover, 4-phenyl-3-tetrazolylpyridyl derivative 2d was reported to be a very potent Ang II antagonist showing, however, a somewhat poorer oral bioavailability than 2c (L-158,338). The authors suggested elsewhere that the decrease in lipophilicity has a negative effect on the oral potency of compound 2d with respect to 2c. 5 These observations led to some molecular modifications involving the distal phenyl group of compounds 2 and ultimately to the development of compounds 4. 6 Some members of this series of 4-phenylquinoline derivatives showed in vitro proper- ties comparable to those shown by losartan. However, phar- macokinetic studies performed with the selected candidate for further preclinical studies (CR3210, 4b) showed a relatively low oral bioavailability and a rapid excretion. 7,8 Owing to the very interesting in vitro properties of quinoline derivatives 4, the program of structural modification was pursued with the synthesis of both isoquinolinone derivatives 5 and indene * To whom correspondence should be addressed. Tel: +39 0577 234320. Fax: +39 0577 234333. E-mail: cappelli@unisi.it. † Dipartimento Farmaco Chimico Tecnologico and European Research Centre for Drug Discovery and Development, Universita ` di Siena. ‡ Rotta Research Laboratorium S.p.A.. § University of Innsbruck. | Dipartimento di Scienze Biomediche, Universita ` di Siena. ⊥ Dipartimento di Chimica, Universita ` di Siena. # Present address: Medicinal Chemistry, Siena Biotech, Via Torre Fiorentina 1, 53100 Siena, Italy. 6451 J. Med. Chem. 2006, 49, 6451-6464 10.1021/jm0603163 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/06/2006