The invention provides novel compounds having the general formula:wherein X1 is N or N+O-, and one of X2, X3 and X4 is N or N+-O- and the remainder of X2, X3 and X4 is C. R2, R3, R4, R5, R6. A, B and Y are as described herein. Additionally compositions compounds of Formula I and methods of use are further described herein.本發明提供具有以下通式之新穎化合物:其中X1係N或N+O-,且X2、X3及X4中之一者係N或N+-O-且X2、X3及X4中之其餘二者係C。R2、R3、R4、R5、R6、A、B及Y係如本文所述。另外,本文進一步闡述式I化合物之組合物及使用方法。