The present invention provides compounds of the formula: wherein R 1 is C 3-6 cycloalkyl optionally substituted by one or more fluorine atoms, or C 1-6 alkyl optionally substituted by one or more fluorine atoms, or C 3-6 cycloalkylmethyl optionally ring-substituted by one or more fluorine atoms； and R 2 is phenyl optionally substituted by one or more fluorine atoms, and pharmaceutically acceptable salts and solvates thereof； in combination with one or more additional therapeutic agents.
