Original article Salicylanilides as inhibitors of the protein tyrosine kinase epidermal growth factor receptor Christoph Liechti a , Urs Séquin a, *, Guido Bold b , Pascal Furet b , Thomas Meyer b , Peter Traxler b a Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland b Oncology Research Novartis Pharma AG, CH-4002 Basel, Switzerland Received 10 June 2003; accepted 9 September 2003 Abstract A pharmacophore model for ATP-competitive inhibitors interacting with the active site of the EGFR protein tyrosine kinase and a putative binding mode of 4-anilinoquinazoline suggest that a salicylic acid function could serve as the pharmacophore replacement of a pyrimidine ring. Superpositions by CAMM of salicylanilides with the potent EGFR tyrosine kinase inhibitor 4-[(3′-chlorophenyl)amino]-6,7- dimethoxyquinazoline showed that salicylanilides should act as tyrosine kinase inhibitors. A series of salicylanilides was synthesized and their inhibitory activity against tyrosine kinases determined. Some of them indeed proved to be potent and selective EGFR tyrosine kinase inhibitors. The most potent ones being 28, 16, 20, 6, and 15, with IC 50 in the 23–71 nM range. © 2003 Elsevier SAS. All rights reserved. Keywords: Salicylanilides; Protein tyrosine kinase inhibitors; EGFR PTK 1. Introduction The actions of protein tyrosine kinases (PTK) are funda- mental to signal transduction pathways. In many prolifera- tive diseases (e.g. cancer, psoriasis, restenosis, etc.), deregu- lated PTK activity has been observed [1]. A number of tumor types have dysfunctional growth factor receptor PTKs, which result in inappropriate mitogenic signaling. Thus, ty- rosine kinases are attractive targets for the design of new therapeutic agents against cancer [2,3]. The inhibition of these enzymes might stop the growth of the tumor and can, therefore, be of great therapeutical value. The family of epidermal growth factor receptor (EGFR) PTKs belongs to the large class of the trans-membrane growth factor receptor PTKs and contains four members: EGFR kinase, p185 erbB2 , and the gene products of c-erbB3 and c-erbB4. The EGFR and its ligands (EGF, TGF-a) have been implicated in many different tumors of epithelial origin (e.g. squamous cell carcinoma; breast, ovarian, and NSC lung cancer) [1,4]. Out of the numerous inhibitors of EGFR PTK reported in the last few years, compounds competing with ATP for bind- ing at the catalytic domain are of special interest. Because of the lack of crystal structure data of the kinase, Furet et al. [5] and Palmer et al. [6] postulated hypothetical models of the ATP-binding site of the enzyme based on molecular model- ing. These models were successfully used for the develop- ment of someATP competitive inhibitors of EGFR PTK such as the 4-(phenylamino)pyrrolo[2,3-d]pyrimidines 1 [7], the 4-(phenylamino)pyrazolo[3,4-d]pyrimidines 2 [8], and the 4-(phenylamino)quinazolines 3 [9] (see Fig. 1). From structure activity relationship (SAR) studies, Palmer et al. [6] postulated a binding mode for 4-anilinoquinazolines (3) to the EGFR PTK. In this model, the inhibitor binds to the enzyme via a pyrimidine nitrogen atom through a hydrogen bond to the backbone NH of amino acid Met 769 of EGFR PTK. According to this model the aniline substituent is occu- pying a hydrophobic region located at the bottom of the ATP binding pocket of EGFR PTK (hydrophobic region I in Fig. 5). Substituents in positions 6 and 7 of the quinazoline are directed to another hydrophobic region of the enzyme open to the solvent (hydrophobic region II in Fig. 5). Quinazolines such as 4 with electron-donating groups in these positions (Fig. 2) are highly potent EGFR PTK inhibi- * Corresponding author. E-mail address: urs.sequin@unibas.ch (U. Séquin). European Journal of Medicinal Chemistry 39 (2004) 11–26 www.elsevier.com/locate/ejmech © 2003 Elsevier SAS. All rights reserved. doi:10.1016/j.ejmech.2003.09.010