The bioisosteres of 1,3,4-oxadiazoles and 1,3,4-thiadiazoles are well-known pharmacophores for many medicinally important drugs. Throughout the past 10 years, 1,3,4-oxa-/thiadiazole nuclei have been very attractive to researchers for drug design, synthesis, and the study of their potential activity towards a variety of diseases, including microbial and viral infections, cancer, diabetes, pain, and inflammation. This work is an up-to-date comparative study that identifies the differences between 1,3,4-thiadiazoles and 1,3,4-oxadiazoles concerning their methods of synthesis from different classes of starting compounds under various reaction conditions, as well as their biological activities and structure–activity relationship.

A new series of 5-(4-chlorophenyl)-1,3,4-thiadiazole-based compounds featuring pyridinium (3), substituted piperazines (4a–g), benzyl piperidine (4i), and aryl aminothiazoles (5a–e) heterocycles were synthesized. Evaluation of the cytotoxicity potential of the new compounds against MCF-7 and HepG2 cancer cell lines indicated that compounds 4e and 4i displayed the highest activity toward the tested cancer cells. A selectivity study demonstrated the high selective cytotoxicity of 4e and 4i towards cancerous cells over normal mammalian Vero cells. Cell cycle analysis revealed that treatment with either compound 4e or 4i induced cell cycle arrest at the S and G2/M phases in HepG2 and MCF-7 cells, respectively. Moreover, the significant increase in the Bax/Bcl-2 ratio and caspase 9 levels in HepG2 and MCF-7 cells treated with either 4e or 4i indicated that their cytotoxic effect is attributed to the ability to induce apoptotic cell death. Finally, an in vivo radioactive tracing study of compound 4i proved its targeting ability to sarcoma cells in a tumor-bearing mice model.

Novel 1,3,4-thiadiazole derivatives (3-5) were designed, synthesized, and screened for cytotoxic activity. Compound 3 bearing substituted benzenesulfonamide scaffold exhibited remarkable potency against breast cancer (MCF-7), hepatoma (HepG2), colon cancer (HCT116), and lung cancer (A549) cells and lower potency on the normal cells (WI-38), as well as, it possesses higher anticancer activity than starting 5-(4-chlorophenyl)-1,3,4-thiadiazole-2-amine (2) and the positive control Staurosporine. The anticancer activity of compound 3 is at least partly attributed to the inhibition of the tumor-associated human carbonic anhydrase isoforms IX and XII. The high selectivity of compound 3 for cancer cells over normal cells and for tumor-associated CA isoforms IX and XII over the off-target cytosolic isoform II reflects its safety. Pharmacokinetic study of compound 3 was evaluated in a normal mice model based on a radiopharmaceutical chemistry approach.