New 5-substituted 5-hydrazine-3-methylisothiazole-4-carboxylic acid derivatives with antitumor activity and a method for their preparation have been disclosed. The disclosed compounds inhibit the proliferation of MV 4-11 human bifenotypic leukemia cells, Lovo colorectal cancer cells, and LoVo/DX doxorubicin resistant colorectal cancer cells. Also disclosed is 5-hydrazine-3-methylisothiazole-4-carboxylic acid intended for preparation 5- substituted 5-hydrazine-3-methylisothiazole-4-carboxylic acid derivatives with anti-tumor activity.