Abstract: The present invention relates to a compound of formula I: wherein: R1 is C1-6 alkyl, C3-8 cycloalkyl, aryl, heterocyclyl, or —COR1x, where the C1-6 alkyl, C3-8 cycloalkyl, aryl, and heterocyclyl may be substituted; and R1x is C3-8 cycloalkyl, aryl, or heterocyclyl, any of which may be substituted; R2, R3, R4, R5, R6, and R7 are each independently hydrogen, halogen, C1-6 alkyl, or aryl, where the C1-6 alkyl or aryl may be substituted; R8 is hydrogen, C1-6 alkyl, aryl, or heterocyclyl, any of which may be substituted; or a pharmaceutically acceptable salt or ester thereof.