SR16388 inhibits STAT3 activation and angiogenesis in lung cancer model

4621

In earlier studies, the orally administered steroidal antitumor agent SR16388, in combination treatments with either paclitaxel or the epidermal growth factor receptor (EGFR) inhibitor erlotinib hydrochloride, strongly inhibited the growth of A549 non-small cell lung cancer (NSCLC) xenografts in nude mice. We have now examined the anti-tumor SR16388’s activities in more detail in the same model. In the present study SR16388 at 20 mg/kg significantly (P = 0.0029) decreased A549 tumor microvessel density (MVD) by 25%, as compared with the untreated controls when determined by CD31 immunohistochemistry. Erlotinib hydrochloride (po, 20 mg/kg) and paclitaxel (iv, 8 mg/kg, once weekly) alone had no significant effect on MVD over 30 days. Combination treatments of either drug together with SR16388 at the same dose schedules as for the single-agent regimens significantly decreased the tumor MVDs by 22% and 30%, respectively. Evaluating the effect of SR16388 on in vivo A549 tumor proliferation using bromodeoxyuridine immunohistochemistry, we found that it significantly enhanced the antiproliferative effects of either paclitaxel or erlotinib in the drug combination studies. Combinations of different concentrations of SR16388 with erlotinib or with paclitaxel were examined systematically for their growth inhibitory effects on A549 cells to determine optimum drug concentrations for additional in vivo drug combination studies, and to investigate possible synergistic effects of two drug combinations using Chou-Talaley analysis. Lysates from same individual tumors were examined by Western blot for total STAT3 and for pTyr705 STAT3 staining. SR16388 was found to consistently inhibit activating STAT3 tyrosine phosphorylation in vivo, confirming the effects of the drug found previously in cell culture. STAT3 is highly activated in many human tumors including NSCLC, and constitutively activated STAT3 has been implicated in tumor angiogenesis as well as tumor cell survival and growth. An inhibitor of STAT3 activation may exert antitumor effects both by direct effects on tumor cells and indirectly by angiogenesis inhibition, as found for SR16388. Mutations of the EGFR that activate STAT pathways may be significant for lung cancer cell survival mechanisms. The EGFR expressed by A549 cells is reported to be the wild-type receptor. It is possible that SR16388 would more effectively inhibit growth of NSCLC that depend on constitutively activated EGFR for survival. This determination is in progress.

An expanded study of structure-activity relationships of analogs of SR16388 will be presented. The novel compounds were examined for their effects on growth of microvascular endothelial cells and on A549 lung cells. A preliminary report of the effects of SR16388 on cancer gene expression will relate the effects to the proposed contributing mechanisms to the drugs actions.