Starting from pyrazolo[3,4-d]pyrimidine ethyl ester 4 and its corresponding acid hydrazide 5, several new compounds were synthesized such as schiff bases 6a-e, acetyl azide derivative 7, phthalimido derivatives 8, compounds containing oxadiazole ring 9,10a-c, triazole ring system 11, 14,15, thiadiazole moiety 13 and phenyl thiosemicarbazide part 12. Some of the synthesized compounds were screened for their antitumor activity against human breast adenocarcinoma cell line (MCF-7) using doxorubicin as a positive control. Compounds 6d, 10b, 12 were found to exhibit good cytotoxic activity with IC50 equal to (4.6, 4.6, 4.8 μg/mL), respectively.