The present invention relates to compounds of formula (I) that have CYP450 inhibiting properties and are useful as boosting agents when used with HIV treatments. These compounds are represented by formula including the salts and stereoisomeric forms thereof, wherein R1 is 5-thiazolyl or 3-pyridinyl; R2 is iso-butyl, 2,2-dimethylpropyl; 2-hydroxy-2-methyl-propyl or cyclohexylmethyl; R3 is phenyl optionally substituted with one or more halogens, trifluoromethyl, C1-6-alkyl or C1-6-alkoxy wherein optionally two of said alkoxy groups are be linked to each other to form a 5 or 6-membered ring; heteroaryl; C3-7cycloalkyl optionally substituted with one or more halogens; C1-6alkyl optionally substituted with heteroaryl; —O—CH2— (heteroaryl). As boosting agents they are able to increase at least one of the pharmacokinetic variables of certain drugs when co-administered, or improve the bioavailability of certain drugs. Methods for the preparation of the compounds of the invention and pharmaceutical compositions comprising these compounds are also provided.