Recently, it has been reported that compounds bearing a sulfonamide moiety posses many types of biological activities, including anticancer activity. There are a variety of mechanisms for their anticancer activity, and the most prominent mechanism is the inhibition of carbonic anhydrase (CA) isozymes. The present work reports the synthesis of some new thiazolo[4,5-b]pyrane, thiazolo[4,5-b]pyrano[2,3-d]pyrimidine derivatives bearing a sulfonamide moiety. The design of the structures of these compounds complies with the general pharmacophoric requirements for CA inhibiting anticancer drugs. The newly synthesized compounds were evaluated for their in vitro anticancer activity against human breast cancer cell line (MCF7). Most of the screened compounds showed interesting cytotoxic activities compared to doxorubicin as a reference drug. Compounds 5, 6, 10 and 12 (IC(50) values 39.4 μM, 41.6 μM, 35.72 μM and 34.64 μM, respectively) exhibited higher cytotoxic activities than the reference drug doxorubicin (IC(50) = 71.8 μM). Additionally, the previously mentioned compounds were evaluated again for their ability to enhance the cell killing effect of γ-radiation.