Antimitotic agents are widely used in cancer chemotherapy but the numerous side effects and the onset of resistance limit their clinical efficacy. Therefore, with the purpose of discovering more selective and efficient anticancer agents to be administered alone or in combination with traditional drugs, we synthesized a large library of 1,3,4-thiadiazoline analogues, maintaining the pharmacophoric structure of an antiproliferative compound known as K858: this is a new inhibitor of kinesin Eg5, able to induce the mitotic arrest in colorectal cancer cells and in xenograft ovarian cancer cells. We screened 103 compounds to assess their antiproliferative activity on PC3 prostate cancer cell line. Two derivatives, compounds 32 (corresponding to K858) and 33, have shown to be the most effective against prostate tumor cells and also towards two melanoma cell lines (SK-MEL-5 and SK-MEL-28) at low micromolar concentrations, confirming the pharmacological activity of this scaffold and revealing the potential role of 1,3,4-thiadiazolines in the management of cancer.