An efficient and simple synthesis of fused pyridazine derivatives has been developed. These fused pyridazines were used as key starting material for the preparation of new heterocyclic compounds. All the structures of the newly synthesized compounds were confirmed on the basis of FT-IR, ¹H and ¹³CNMR, mass spectral techniques. Six of the new synthesized compounds (6_f , 8_c-f  and  8_i) were selected and tested against a panel human tumor cell line HCT-116 (colon cancer) using Imatinib as standard drug. The cytotoxicity data of the tested compounds virus Imatinib in means of IC₅₀ values were (6_f, 8_c-f and 8i) recorded as in (Figure I). The obtained data showed that the compounds 1-(3-(2-methoxyphenyl)-7-methoxyphenylisoxazolo[4,5-d]pyridazin-4-yl)hydrazine (8_i) and 1-(3-(2-methoxyphenyl)-1-phenyl-7-methoxyphenyl-1H-pyrazolo[4,3-d]pyridazin-4-yl)hydrazine (8_f) have high reactivity towards cell line HCT-116 (colon cancer) more than the standard drug Imatinib.