Bicyclic heterocyclic derivatives of formula I useful in inhibiting PDGF receptor mediated biological activity. wherein A is and R1, R1a, R2, R3, R4, R5, R6 and X are as defined herein.本發明係關於適用於抑制PDGF受體介導之生物活性的式I之雙環雜環衍生物,其中A為且R1、R1a、R2、R3、R4、R5、R6及X如本文中所定義。