For probing the importance of planarity of imidazolidinone motif of 4-phenyl-1-(benzenesulfonyl)imidazolidinones 1 for their cytotoxicity, 4-phenyl-2-(benzoyl)[1,2,5]thiadiazolidine-1,1-dioxide (2a), 4-phenyl-2-(p-toluoyl)[1,2,5]thiadiazolidine-1,1-dioxide (2b), 4-phenyl-2-(phenylcarbamoyl)[1,2,5]thiadiazolidine-1,1-dioxide (3a), and 4-phenyl-2-(p-tolylcarbamoyl)[1,2,5]thiadiazolidine-1,1-dioxide (3b) were prepared along with their regioisomers (5a, 5b, 9a, 9b) and their cytotoxicity were measured against human lung carcinoma (A549), human colon carcinoma (COLO205), human ovarian cancer (SK-OV-3), human leukemic cancer (K562), and murine colon adenocarcinoma (Colon26) cell lines in vitro. All compounds prepared do not show any activity against all five cancer cell lines unlike 1. Compounds 1 possess planarity of imidazolidinone, especially in sulfonylurea moiety ($-SO_2$NHCONH-). However compounds 2 and 3 have nonplanar 5-membered ring, [1,2,5]thiadiazolidine-1,1-dioxides. Such structural differentiation might result in the loss of activity. Therefore the inactivity of 2 and 3 could also be an indication for the necessity of planarity of imidazolidinone ring of 1 for their cytotoxic activity.