1. Dipartimento Farmaco Chimico Tecnologico and European Research Centre for Drug Discovery and Development, Università di Siena, Via A. Moro, 53100 Siena, Italy, Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Rottapharm S.p.A., Via Valosa di Sopra 7, 20052 Monza, Italy