The present application describes compounds according to Formula I, wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, X and Y are described herein. Additionally, the present application describes pharmaceutical compositions comprising at least one compound according to Formula I and optionally one or more additional therapeutic agents. Finally, the present application describes methods of treatment using the compounds according to Formula I both alone and in combination with one or more additional therapeutic agents.