A compound of formula (I), or a pharmaceutically acceptable salt thereof, where R1 is a (C1-C6)alkyl or (C3-C6)cycloalkyl group; R2 is a (C1-C6)alkyl, (C3-C6)cycloalkyl, (C1-C6)alkenyl, (C1-C6)fluoroalkyl, (C1-C3)fluoroalkoxy, or (C1-C6)alkoxy(C1-C6)alkyl group, substituted: (i) with one to three hydroxyl groups, or (ii) with an NRaRb group, where Ra and Rb are independently a hydrogen atom or a (C1-C3)alkyl group; or a pyrrolidinylmethyl group substituted with one to three hydroxyl groups; R9 is the same as R2 or hydrogen; the R2 and R9 groups independently being substitutable with an —OCOR3 group, where R3 is a natural or unnatural amino acid derivative or a piperidyl group; alternatively, R2 and R9 together form a heterocyclic compound; X and Y are independently a substitutable phenyl or heteroaryl group, the heteroaryl group being a thienyl, pyridyl, pyrimidinyl, thiazolyl, pyrrolyl, or furanyl group; and R6 is a hydrogen or a (C1-C3)alkyl group.