1. Gilead Sciences & IOCB Research Centre, Centre for New Antivirals and Antineoplastics (IOCB), Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, CZ-166 10, Prague 6, Czech Republic, and Rega Institute for Medical Research (REGA), Katholieke Universiteit Leuven, Minderbroedersstraat 10, B-3000 Leuven, Belgium