Sr16388 Inhibits Non-Small Cell Lung Tumor Growth And Angiogenesis

3820 We had previously determined that the steroidal antitumor agent, SR16388, inhibited matrix development and reduced microvessel density in a wound healing model, and further that the compound inhibited tumor growth and angiogenesis in a PC3 mouse xenograft model of human prostate cancer. In the present study we proposed to: (1) further characterize the effects of the drug on microvessel development; then (2) to determine if the tumor growth inhibitory activity of SR16388 extends to androgen receptor-positive but androgen-independent prostate cancer and to other invasive carcinomas, in particular non-small cell lung adenocarcinoma. Finally,oral bioavailability and pharmacokinetics would be determined in the rat and the dog. The effects of SR16388 and of a panel of closely related steroids on growth of human dermal microvascular endothelial cells (Clonetics) were compared. SR16388 was significantly more potent (IC50 = 80nM) than the analogs (IC50 = 0.6 - 10 μM). SR16388 (2 μM) prevented formation of tube-like structures in Matrigel after HMVECs were treated with the drug for 8h. In a modified Boyden chamber assay, SR16388 inhibited the migration of HMVECs through collagen membrane by 80% at 500 nM. SR16388 potently inhibited the growth (IC50 =100 and 120 nM) of both A549 non-small cell lung adenocarcinoma cells, and metastatic androgen receptor-positive, but androgen-independent, LNCaP C4-2B prostate cancer cells in culture. The transcription factor Stat3 (Signal Transducers and Activators of Transcription-3) was determined by Western blot to be tyrosine phosphorylated in A549 cells, and SR16388 significantly attenuated Stat3 phosphorylation. Stat3 activation may contribute to carcinogenesis by regulation of expression of several proangiogenic and antiapoptotic factors in human tumors. However, Stat3 was not constitutively phosphorylated in LNCaP C4-2B cells or in PC3 androgen-independent prostate cancer cells. SR16388 was evaluated using a xenograft nude mouse model for its in vivo antitumor activity against A549 tumors. In this study, SR16388 inhibited tumor growth in a dose- response manner at a single daily oral dose of 10, 30, and 100 mg/kg. A dose-response effect was observed for oral dosing of SR16388 in two mouse xenograft studies. This preliminary finding of the drugs efficacy by oral administration was confirmed on determination of favorable oral bioavailabilty and t1/2 of elimination in both male and female Sprague-Dawley rats and Beagle dogs. These data support the further development of orally administered SR16388 for the treatment of advanced neoplasms, including non-small cell lung cancers. Direct effects of the drug on microvascular endothelial cells contribute to its angiogenesis inhibitory activity. Inhibition of Stat3 activation may be a component of the effects of SR16388 on tumors that express constitutively activated Stat3.