Preclinical efficacy of a carboxylesterase 2-activated prodrug of doxazolidine.

Doxazolidine (Doxaz) is a functionally distinct formaldehyde conjugate of doxorubicin (Dox) that induces cancer cell death in Dox-sensitive and resistant cells. Pentyl PABC-Doxaz (PPD) is a prodrug of Doxaz that is activated by carboxylesterase 2 (CES2), which is expressed by liver, non-small-cell lung, colon, pancreatic, renal, and thyroid cancer cells. Here, we demonstrate that in two murine models, PPD was effective at slowing tumor growth and demonstrated markedly reduced cardiotoxic and nephrotoxic effects, as well as better tolerance, relative to Dox. Hepatotoxicity, consistent with liver expression of the murine CES2 homologue, was induced by PPD. Unlike irinotecan, a clinical CES2-activated prodrug, PPD produced no visible gastrointestinal damage. Finally, we demonstrate that cellular response to PPD may be predicted with good accuracy using CES2 expression and Doxaz sensitivity, suggesting that these metrics may be useful as clinical biomarkers for sensitivity of a specific tumor to PPD treatment.

Oral silibinin inhibits lung tumor growth in athymic nude mice and forms a novel chemocombination with doxorubicin targeting nuclear factor kappaB-mediated inducible chemoresistance.

The acute and cumulative dose-related toxicity and drug resistance, mediated via nuclear factor kappaB (NFkappaB), of anthracycline anticancer drugs pose a major problem in cancer chemotherapy. Here, we report that oral silibinin (a flavanone) suppresses human non-small-cell lung carcinoma A549 xenograft growth (P = 0.003) and enhances the therapeutic response (P < 0.05) of doxorubicin in athymic BALB/c nu/nu mice together with a strong prevention of doxorubicin-caused adverse health effects. Immunohistochemical analyses of tumors showed that silibinin and doxorubicin decrease (P < 0.001) proliferation index and vasculature and increase (P < 0.001) apoptosis; these effects were further enhanced (P < 0.001) in combination treatment. Pharmacologic dose of silibinin (60 mumol/L) achieved in animal study was biologically effective (P < 0.01 to 0.001, growth inhibition and apoptosis) in vitro in A549 cell culture together with an increased efficacy (P < 0.05 to 0.001) in doxorubicin (25 nmol/L) combination. Furthermore, doxorubicin increased NFkappaB DNA binding activity as one of the possible mechanisms for chemoresistance in A549 cells, which was inhibited by silibinin in combination treatment. Consistent with this, silibinin inhibited doxorubicin-caused increased translocation of p65 and p50 from cytosol to nucleus. Silibinin also inhibited cyclooxygenase-2, an NFkappaB target, in doxorubicin combination. These findings suggest that silibinin inhibits in vivo lung tumor growth and reduces systemic toxicity of doxorubicin with an enhanced therapeutic efficacy most likely via an inhibition of doxorubicin-induced chemoresistance involving NFkappaB signaling.

Synergistic anti-cancer effects of grape seed extract and conventional cytotoxic agent doxorubicin against human breast carcinoma cells.

With an approach to enhance the efficacy of chemotherapy agents against breast cancer treatment, here, we investigated the anti-cancer effects of grape seed extract (GSE) and doxorubicin (Dox), either alone or in combination, in estrogen receptor-positive MCF-7 and receptor-negative MDA-MB468 human breast carcinoma cells. GSE (25-200 micro g/ml) treatment of cells resulted in 16-72% growth inhibition and 9-33% cell death, in a dose- and a time-dependent manner. In other studies, Dox (10-100 nM) treatment showed 23-96% growth inhibition and 10-55% cell death. Based on these results, several combinations of GSE (25-100 micro g/ml) with Dox (10-75 nM) were next assessed for their synergistic, additive and/or antagonistic efficacy towards cell growth inhibition and death. In both MCF-7 and MDA-MB468 cells, a combination of 100 micro g/ml GSE with 25-75 nM Dox treatment for 48 h showed a strong synergistic effect [combination index (CI) < 0.5] in cell growth inhibition, but mostly an additive effect (CI approximately 1) in cell death. In cell-cycle progression studies, GSE plus Dox combination resulted in a moderate increase in G1 arrest in MCF-7 cells compared to each agent alone. GSE plus Dox combination showed a very strong and significant G1 arrest in MDA-MB468 cells when compared with Dox alone, however, it was less than that observed with GSE alone. In quantitative apoptosis studies, GSE and Dox alone and in combination showed comparable apoptotic death of MCF-7 cells, however, a combination of the two was inhibitory to Dox induced apoptosis in MDA-MB468 cells. This was further confirmed in another estrogen receptor-negative MDA-MB231 cell line, in which GSE and Dox combination strongly inhibited cell growth but did not show any increase in apoptotic cell death caused by Dox. Together, these results suggest a strong possibility of synergistic efficacy of GSE and Dox combination for breast cancer treatment, independent of estrogen receptor status of the cancer cell.

Synergistic anti-cancer effects of silibinin with conventional cytotoxic agents doxorubicin, cisplatin and carboplatin against human breast carcinoma MCF-7 and MDA-MB468 cells.

Significant emphasis is being placed on combination chemotherapy of cancer using cytotoxic agents and naturally occurring chemopreventive agents, having different mechanisms of action with non-overlapping toxicity. In this regard, here we assessed whether a cancer preventive agent silibinin synergizes the therapeutic potential of doxorubicin (Dox), cisplatin or carboplatin, the chemotherapeutic drugs, in both estrogen-dependent and -independent human breast carcinoma, MCF-7 and MDA-MB468 cells, respectively. When tested alone, each of the four agents showed growth inhibition in both the cell lines in a dose- and a time-dependent manner. Based on their growth inhibitory effects, several combinations of silibinin (25-100 microM) with Dox (10-75 nM), cisplatin (0.2-2 microg/ml) or carboplatin (2-20 microg/ml) were next assessed for their synergistic, additive and/or antagonistic efficacy towards cell growth inhibition and apoptotic death. The strongest synergistic effects for cell growth inhibition [combination index (CI) 0.35 for MCF-7 and 0.45 for MDA-MB468 cells] were evident at a silibinin dose of 100 microM plus 25 nM Dox, in both the cell lines. Most of the CIs for other combinations of these three drugs with silibinin also suggested strong synergistic effects for cell growth inhibition in both MCF-7 and MDA-MB468 cells. In quantitative apoptosis studies, combination of silibinin with Dox resulted in much stronger apoptotic death compared to each agent alone in both cell lines. In case of silibinin combination with cisplatin, it showed no additional apoptotic effect in either cell line. Similarly, silibinin plus carboplatin combination showed stronger apoptotic effect only in MCF-7 cells. Together, these results suggest a possible synergism between silibinin and conventional cytotoxic agents for breast cancer treatment, and warrant further in vivo studies in pre-clinical breast cancer models.

Silibinin strongly synergizes human prostate carcinoma DU145 cells to doxorubicin-induced growth Inhibition, G2-M arrest, and apoptosis.

PURPOSE: We recently demonstrated the strong anticancer efficacy of silibinin,an active constituent of a widely consumed dietary supplement milk thistle extract, against human prostate cancer cells in culture and nude mice xenografts. We also observed that pharmacologically achievable concentrations of silibinin in animal studies were in the range of 25-100 microM, depending on the dose regimen, which did not show any apparent toxicity to the animals. In this study, we assessed whether silibinin synergizes the therapeutic potential of the chemotherapeutic drug doxorubicin against prostate cancer, the effectiveness of which is limited because of high systemic toxicity. EXPERIMENTAL DESIGN: Prostate cancer cells were treated with silibinin and doxorubicin, either alone or in combination, and cell growth was determined by manual cell counting. Cell cycle progression was assessed by saponin/propidium iodide staining and fluorescence-activated cell sorter analysis. Protein levels of cell cycle regulators were determined by Western blotting, and cdc2/p34 kinase activity was analyzed by in-beads kinase assay. Apoptosis was quantified by annexin V/propidium iodide staining and fluorescence-activated cell sorter analysis. RESULTS: Silibinin strongly synergized the growth-inhibitory effect of doxorubicin in prostate carcinoma DU145 cells (combination index, 0.235-0.587), which was associated with a strong G(2)-M arrest in cell cycle progression, showing 88% cells in G2-M phase by this combination compared with 19 and 41% of cells in silibinin and doxorubicin treatment alone, respectively. The underlying mechanism of G2-M arrest showed a strong inhibitory effect of combination on cdc25C, cdc2/p34, and cyclin B1 protein expression and cdc2/p34 kinase activity. More importantly, this combination caused 41% apoptotic cell death compared with 15% by either agent alone. Silibinin and doxorubicin alone as well as in combination were also effective in inhibiting the growth of androgen-dependent prostate carcinoma LNCaP cells. CONCLUSION: These findings suggest a need for in vivo studies with this combination in preclinical prostate cancer models. Positive outcomes might be relevant for a clinical application in prostate cancer patients.

Dietary feeding of silibinin inhibits advance human prostate carcinoma growth in athymic nude mice and increases plasma insulin-like growth factor-binding protein-3 levels.

We have reported recently the anticancer effect of flavonoid antioxidant silymarin, the major part of milk thistle extract, against advanced human prostate carcinoma DU145 cells (X. Zi et al., Cancer Res., 58: 1920-1929, 1998) and later identified that silibinin is the main active component in silymarin responsible for its effect in cell culture studies. On the basis of these observations, here we assessed in vivo growth inhibitory potential of silibinin against advanced human prostate cancer (PCA). Dietary feeding of silibinin at 0.05 and 0.1% doses (w/w) for 60 days, 24 h after s.c. DU145 tumor xenograft implantation in athymic male nude mice, significantly inhibited tumor volume by 35 and 58% (P < 0.05), and wet weight of tumor by 29 and 40% (P < 0.05), respectively. In a second experiment where mice were fed with these test diets for 3 weeks before tumor xenograft implantation and continued on these diets for a total of 63 days, tumor volume and wet weight of tumor were reduced by 53-64% (P < 0.001-0.05) and 31-52% (P < 0.05), respectively. In both studies, animals did not show weight loss or reduced food consumption. These in vivo anticancer effects of silibinin were associated with an increased accumulation (up to 5.8 fold; P < 0.05) of human insulin-like growth factor-binding protein-3 in mouse plasma. In additional studies assessing biological availability of silibinin in nude mice and its antiproliferative activity at such doses in DU145 cells in culture, silibinin levels in plasma and prostate were found to be in the range of 7-13 microg/ml and 3.7-4.6 microg/g, respectively. At these biologically achievable silibinin concentrations, increased IGFBP-3 level in DU145 cell culture medium and a strong DU145 cell growth inhibition were observed that were irreversible in the absence of silibinin in culture medium. These findings extend and translate our observations on in vitro anticancer effect of silibinin/silymarin to an in vivo preclinical PCA model, which may form the basis for a Phase I clinical trial in PCA patients.