The present invention relates to a compound having the following formula (I): (I) wherein: - R is a (C1-C6)alkyl group;- R'' is preferably H;- Ar is a (C5-C12)arylene radical;- X1 is -C(=O)- or -SO2-; and- R' is chosen from the group consisting of possibly substituted (C1-C6)alkyl, heteroaryl, (C5-C12)aryl, and (hetero)cycloalkyl groups, or a pharmaceutically acceptable salt and/or tautomeric form thereof, or its racemates, diastereomers or enantiomers.