6-Arylamino-5,8-quinazolinediones were synthesized and evaluated for in vitro cytotoxic activity against human solid tumor cell lines A549, SK-OV-3, SK-MEL-2, XF498 and HCT-15. Most of the 6-arylamino-5,8-quinazolinediones exhibited potent activity against the cell lines HCT-15 and SK-MEL-2. Streptonigrin (1) is a heterocyclic quinone that exhibits potent antitumor effects. The 7-amino-5,8-quinolinedione moiety of streptonigrin has been proposed to be important in determining their antitumor activity.1 Many structural variants of 2 showed that the 5,8-quinolinedione ring seems to be required for antitumor activity of 12. The presence of substituents such as substituted amino groups and chlorine of the quinones improves their cytotoxicity against tumor cell lines.1,3-5 Studies on the activity of heterocyclic quinones, as an analog of 5,8-quinolinedione, showed that the number and position of nitrogen played an important role for the cytotoxicity.6 The activity of the heterocyclic quinones in cleaving the Hella cell DNA follows the order of 5,8-quinazolinediones > 5,8-isoquinolinediones > 5,8-quinolinediones.6 We previously reported that 7-arylamino-6-chloro-5,8-isoquinolinediones 3 and 6-arylamino-7-chloro-5,8-quinolinediones 4 showed potent cytotoxic activity.5,7 Based on this information, 5,8-quinazolinedione derivatives 5-6, bioisosteres of 3 and 4, were synthesized and their cytotoxicity was evaluated. There have been only a few reports on 5,8-quinazolinedione compounds showing cytotoxicities against tumor cell lines.3,8-10 The cytotoxic activities of the 6-arylamino-5,8-quinazolinediones 5-6 against tumor cell lines have not been found in the literature. Therefore, we describe herein our preliminary results on the cytotoxicity of 5,8-quinazolinediones 5-6 against human tumor cell lines (Table 1).