A series of 3-(3',4',5'-trimethoxyphenyl)-4-substituted 1H-pyrazole and their related 3-aryl-4-(3',4',5'-trimethoxyphenyl)-1-H-pyrazole regioisomeric derivatives, prepared as cis-rigidified
combretastatin A-4 (CA-4) analogues, were synthesized and evaluated for their in vitro antiproliferative against six different
cancer cell lines and, for selected highly active compounds, inhibitory effects on
tubulin polymerization, cell cycle effects and in vivo potency. We retained the 3',4',5'-trimethoxyphenyl moiety as ring A throughout the present investigation, and a structure-activity relationship (SAR) information was obtained by adding electron-withdrawing (OCF3, CF3) or electron-releasing (alkyl and
alkoxy) groups on the second aryl ring, corresponding to the B-ring of CA-4, either at the 3- or 4-position of the
pyrazole nucleus. In addition, the B-ring was replaced with a benzo[b]thien-2-yl moiety. For many of the compounds, their activity was greater than, or comparable with, that of CA-4. Maximal activity was observed with the two regioisomeric derivatives characterized by the presence of a 4-ethoxyphenyl and a 3',4',5'-trimethoxyphenyl group at the C-3 and C-4 positions, and vice versa, of the 1H-pyrazole ring. The data showed that the 3',4',5'-trimethoxyphenyl moiety can be moved from the 3- to the 4-position of the 1H-pyrazole ring without significantly affecting antiproliferative activity. The most active derivatives bound to the
colchicine site of
tubulin and inhibited
tubulin polymerization at submicromolar concentrations. In vivo experiments, on an orthotopic murine mammary
tumor, revealed that 4c inhibited
tumor growth even at low concentrations (5 mg/kg) compared to CA-4P (30 mg/kg).