Synthesis and Antiproliferative Activity of 2,4,6,7-Tetrasubstituted-2-pyrazolo[4,3-]pyridines.

A library of 2,4,6,7-tetrasubstituted-2-pyrazolo[4,3-]pyridines was prepared from easily accessible 1-phenyl-3-(2-phenylethynyl)-1-pyrazole-4-carbaldehyde via an iodine-mediated electrophilic cyclization of intermediate 4-(azidomethyl)-1-phenyl-3-(phenylethynyl)-1-pyrazoles to 7-iodo-2,6-diphenyl-2-pyrazolo[4,3-]pyridines followed by Suzuki cross-couplings with various boronic acids and alkylation reactions. The compounds were evaluated for their antiproliferative activity against K562, MV4-11, and MCF-7 cancer cell lines. The most potent compounds displayed low micromolar GI values. 4-(2,6-Diphenyl-2-pyrazolo[4,3-]pyridin-7-yl)phenol proved to be the most active induced poly(ADP-ribose) polymerase 1 (PARP-1) cleavage, activated the initiator enzyme of apoptotic cascade caspase 9, induced a fragmentation of microtubule-associated protein 1-light chain 3 (LC3), and reduced the expression levels of proliferating cell nuclear antigen (PCNA). The obtained results suggest a complex action of 4-(2,6-diphenyl-2-pyrazolo[4,3-]pyridin-7-yl)phenol that combines antiproliferative effects with the induction of cell death. Moreover, investigations of the fluorescence properties of the final compounds revealed 7-(4-methoxyphenyl)-2,6-diphenyl-2-pyrazolo[4,3-]pyridine as the most potent pH indicator that enables both fluorescence intensity-based and ratiometric pH sensing.