Cell Pathways, Inc.

Purpose: We studied the safety and clinical activity of exisulind in combination with capecitabine in 35 patients with metastatic breast cancer (MBC). Patients and Methods: All patients had received previous anthracycline and taxane chemotherapies. Two dose levels of exisulind were explored, 125 and 250 mg orally bid as continuous daily therapy, concomitant with capecitabine 2,000 mg/m2 for 14 days in 21-day cycles. In the phase I study, the dose-limiting toxicities were hand-foot syndrome and diarrhea. The 125-mg bid dose was selected for phase II testing. Results: The most common nonhematologic grade 2 to 3 adverse events were hand-foot syndrome (57%) and fatigue (48%). The most frequent grade 2 to 3 laboratory abnormality was granulocytopenia. No death, unexpected adverse events, or cumulative toxicity were encountered. One complete and four partial responses were achieved (objective response rate, 16%) in the 31 patients assessable for response. The median duration of response was 31 weeks; three patients experienced stable disease longer than 26 weeks. Overall clinical benefit (complete response, partial response, or stable disease > 26 weeks) was 23%. Fourteen specimens were available for immunohistochemical assessment of phosphodiesterase-5 isoenzyme (PDE-5) and PDE-2 expression, which are the targets of exisulind. Eighty percent of tumors showed some expression of PDE-5 in the invasive cancer cells including 35% that showed moderate or strong staining. PDE-2 showed moderate or strong staining in 78% of tumors. There was no apparent association between tumor response and staining intensity. Conclusion: Exisulind (125 mg orally bid) in combination with capecitabine is well tolerated and the combination has anticancer activity similar to that of capecitabine alone in heavily pretreated patients with MBC.

We reported previously a significant increase in survival of nude rats harboring orthotopic A549 human non-small cell lung cancer tumors after treatment with a combination of exisulind (Sulindac Sulfone) and docetaxel (D. C. Chan, Clin. Cancer Res., 8: 904-912, 2002). The purpose of the current study was to determine the biochemical mechanisms responsible for the increased survival by an analysis of the effects of both drugs on A549 orthotopic lung tumors and A549 cells in culture. Orthotopic A549 rat lung tissue sections from drug-treated rats and A549 cell culture responses to exisulind and docetaxel were compared using multiple apoptosis and proliferation analyses [i.e., terminal deoxynucleotidyl transferase-mediated nick end labeling, active caspase 3, the caspase cleavage products cytokeratin 18 and p85 poly(ADP-ribose) polymerase, and Ki-67]. Immunohistochemistry was used to determine cyclic GMP (cGMP) phosphodiesterase (PDE) expression in tumors. The cGMP PDE composition of cultured A549 cells was resolved by DEAE-Trisacryl M chromatography and the pharmacological sensitivity to exisulind, and additional known PDE inhibitors were determined by enzyme activity assays. Exisulind inhibited A549 cell cGMP hydrolysis and induced apoptosis of A549 cells grown in culture. PDE5 and 1 cGMP PDE gene family isoforms identified in cultured cells were highly expressed in orthotopic tumors. The in vivo apoptosis rates within the orthotopic tumors increased 7-8-fold in animals treated with the combination of exisulind and docetaxel. Exisulind increased the in vivo apoptosis rates as a single agent. Docetaxel, but not exisulind, decreased proliferative rates within the tumors. The data indicate that exisulind-induced apoptosis contributed significantly to the increased survival in rats treated with exisulind/docetaxel. The mechanism of exisulind-induced apoptosis involves inhibition of cGMP PDEs, and these results are consistent with a cGMP-regulated apoptosis pathway.

In platelets, the nitric oxide (NO)–induced cGMP response is indicative of a highly regulated interplay of cGMP formation and cGMP degradation. Recently, we showed that within the NO-induced cGMP response in human platelets, activation and phosphorylation of phosphodiesterase type 5 (PDE5) occurred. Here, we identify cyclic GMP-dependent protein kinase I as the kinase responsible for the NO-induced PDE5 phosphorylation. However, we demonstrate that cGMP can directly activate PDE5 without phosphorylation in platelet cytosol, most likely via binding to the regulatory GAF domains. The reversal of activation was slow, and was not completed after 60 min. Phosphorylation enhanced the cGMP-induced activation, allowing it to occur at lower cGMP concentrations. Also, in intact platelets, a sustained NO-induced activation of PDE5 for as long as 60 min was detected. Finally, the long-term desensitization of the cGMP response induced by a low NO concentration reveals the physiological relevance of the PDE5 activation within NO/cGMP signaling. In sum, we suggest NO-induced activation and phosphorylation of PDE5 as the mechanism for a long-lasting negative feedback loop shaping the cGMP response in human platelets in order to adapt to the amount of NO available.