The present invention addresses the problem of providing a compound that is useful as a therapeutic or prophylactic for diseases related to the immune system, particularly autoimmune diseases such as systemic lupus erythematosus (SLE) and lupus hepatitis in SLE patients, the compound being useful as a result of inhibiting the function of immune cells by inhibiting activated T-cell proliferation or by inhibiting the production of interferon alpha (IFN-[alpha]) by activated plasmacytoid dendritic cells (pDC). The present invention provides a compound represented by general formula (I) [in the formula, X, R1, R2, R3, R4, R5, and R6 are as defined in the specification] or a pharmaceutically acceptable salt thereof.本發明之課題在於提供一種有用的化合物，其係藉由抑制活化T細胞的增生、或抑制因經活化之漿細胞樣樹突細胞(plasmacytoid dendritic cell)(pDC)所致之干擾素α(interferon alpha)(IFN-α)的產生，而抑制免疫細胞的功能，以作為免疫系統相關之疾病，特別是作為全身性紅斑狼瘡(systemic lupus erythematosus)(SLE)及在SLE患者中之狼瘡性腎炎(lupus nephritis)等自體免疫疾病的治療或預防藥。本發明提供一種通式(I)所示之化合物或其藥學上可容許的鹽：[式中，X、R1、R2、R3、R4、R5及R6如說明書所載]。
