Conformationally restricted macrocyclic analogues of combretastatins Carmen Mateo, Raquel A ´ lvarez, Concepcio ´n Pe ´rez-Melero, Rafael Pela ´ez * and Manuel Medarde * Laboratorio de Quı ´mica Orga ´ nica y Farmace ´utica, Facultad de Farmacia, Universidad de Salamanca, Campus Miguel de Unamuno, E-37007 Salamanca, Spain Received 19 July 2007; revised 30 August 2007; accepted 31 August 2007 Available online 4 September 2007 Abstract—New analogues of combretastatins have been evaluated as inhibitors of tubulin polymerization. These compounds pres- ent a macrocyclic structure, in which the para positions of the aromatic moieties have been linked by a 5- or 6-atoms chain, in order to produce a conformational restriction. This could contribute to determine the active conformation for these ligands. Such a con- formational restriction and/or the steric hindrance makes them less potent inhibitors than the model compound CA-4. Ó 2007 Elsevier Ltd. All rights reserved. Microtubules are structures formed by polymerization of ab-tubulin dimers. They are essential to many cellular processes, such as maintenance of cellular shape, intra- cellular transport, or mitotic spindle assembly during cell division. 1 Inhibition of microtubule formation leads to mitotic arrest and promotes vascular disruption in angiogenic vessels through activation of the RhoA and/or the VE-cadherin/b-catenin pathways. 2 Tubulin- binding agents such as colchicine or vinblastine display both effects, but in these two cases the vascular shut- down occurs at concentrations close to the maximum tolerated dose. 3 Combretastatin A-4 (CA-4, Fig. 1) is one of the most potent inhibitors of tubulin polymerization, binding at the colchicine site on the protein. It is currently in phase I/II clinical trials as its phosphate prodrug 4 due to its antimitotic and antiangiogenic properties. Other mem- bers of the combretastatin family also inhibit tubulin polymerization, such as deoxycombretastatin, 5 combre- tastatin, 6 and related dihydroxy and dioxolane deriva- tives (Fig. 1), 7 among others. In the two latter, the configuration of the stereogenic carbons has proved to be essential for antitubulin activity. The structural requirements for a potent antitubulin activity are those present in CA-4. 8 A 3,4,5-trimethoxy- phenyl and a 4-methoxy-3-X-phenyl (being X = H, OH, NH 2 and their aminoacyl, phosphate, or other solubiliz- ing derivatives) separated by a 2-atoms bridge with cis disposition are the structural features for the most po- tent compounds. The aromatic rings show a non-copla- nar disposition. However, the conformation of CA-4 bound to tubulin is not known. Colchicine and podo- phyllotoxin show different spatial arrangement of the aromatic rings when bound to tubulin 9 and, despite this, both of them are potent inhibitors of polymerization (IC 50 = 0.8–3.3 and 0.3–3 lM, respectively). This fact has been attributed to the flexibility of the binding site, which can accommodate ligands bearing different struc- tural scaffolds and spatial dispositions of the aromatic rings. Many combretastatin analogues have been prepared in which the phenyl rings are locked in the cis orientation by the formation of small cycles on the bridge, in an at- tempt to prevent them from isomerizing. 8,10 As part of our research directed at the synthesis and evaluation of new antimitotic agents, we have designed new analogues of combretastatins in which a macrocycle is formed by linking the para positions of both aromatic rings, through the corresponding oxygen or nitrogen atoms. This restriction generates new cis-locked ana- logues, irrespective of the bridge structure. Conforma- tional restriction by macrocycle formation on bioactive 0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2007.08.075 Keywords: Combretastatins; Conformational restriction; Macrocycles; Tubulin polymerization inhibition. * Corresponding authors. Tel.: +34 923 294528; fax: +34 923 294515; e-mail addresses: pelaez@usal.es; medarde@usal.es Available online at www.sciencedirect.com Bioorganic & Medicinal Chemistry Letters 17 (2007) 6316–6320