Abstract
Gemcitabine is the standard care chemotherapeutic agent to treat pancreatic cancer. Previously we demonstrated that calcitriol (1, 25-dihydroxycholecalciferol) has significant anti-proliferative effects in vitro and in vivo in multiple tumor models and enhances the activity of a variety of chemotherapeutic agents. We therefore investigated whether calcitriol could potentiate the cytotoxic activity of gemcitabine in the human pancreatic cancer Capan-1 model system. Isobologram analysis revealed that calcitriol and gemcitabine had synergistic antiproliferative effect over a wide range of drug concentrations. Calcitriol did not reduce the CDDase activity in Capan-1 tumors nor in the livers of Capan-1 tumor bearing mice. Calcitriol and gemcitabine combination promoted apoptosis in Capan-1 cells compared with either agent alone. The combination treatment also increased the activation of caspases-8, -9, -6, and -3 in Capan-1 cells. This result was confirmed by substrate-based caspase activity assay. Akt phosphorylation was reduced by calcitriol and gemcitabine combination treatment compared to single agent treatment. However, ERK1/2 phosphorylation was not modulated by either agent alone or by the combination. Tumor regrowth delay studies showed that calcitriol in combination with gemcitabine resulted in a significant reduction of Capan-1 tumor volume compared to single agent treatment. Our study suggests that calcitriol and gemcitabine in combination promotes caspase-dependent apoptosis, which may contribute to increased anti-tumor activity compared to either agent alone.
Introduction
Calcitriol (1, 25-dihydroxycholecalciferol) is the active metabolite of the secosteroid hormone vitamin D and is well-known for its important role in bone and mineral metabolism. Vitamin D can be synthesized in skin or obtained from the diet.Citation1,Citation2 Calcitriol causes anti-proliferative effects through multiple mechanisms including the induction of cell cycle arrest, apoptosis and differentiation in vitro and in vivo in a variety of cancer cell types including prostate, breast, colon, skin and leukemic cells.Citation3–Citation14 Calcitriol has also been shown to reduce angiogenesis in a number of cancer models, which also contributes to the antitumor effect of calcitriol in vivo.Citation2 Clinical trials have demonstrated that sufficient doses can be administered in patients without the development of hypercalcemia.Citation15–Citation17
Pancreatic cancer is the fourth leading cause of cancer death in the United States even though it only accounts for 2% of all new cancers diagnosed. The overall five year survival rate is 4% for advanced or metastatic carcinoma and 17% for localized, resectable tumors of the pancreas.Citation18 Gemcitabine (difluorodeoxycytidine, dFdC, Gemzar®) is a nucleoside analogue that exhibits antitumor activity and is the standard of care to treat locally advanced and metastatic pancreatic carcinoma. Gemcitabine exhibits cell phase specificity, primarily killing cells undergoing DNA synthesis (S-phase) and also blocking the progression of cells through the cell cycle, specifically from G1 to S-phase. Gemcitabine also induces apoptosis.Citation19 Cytidine deaminase (CDDase) can deaminate gemcitabine which results in its inactive metabolite dFd-U.Citation20 Gemcitabine can also be inactivated by dephosphorylation of dFd-CMP by 59-NU.Citation20
It has been reported that calcitriol works synergistically with chemotherapeutic drugs such as cisplatin and paclitaxel.Citation6,Citation13,Citation21,Citation22 In this study, we investigated the effects of calcitriol alone and in combination with gemcitabine on the Capan-1 pancreatic carcinoma model system in vivo and in vitro. We observed a significant antitumor effect in vivo following the combination treatment which was greater than either agent alone. The interaction of calcitriol and gemcitabine is synergistic over a range of concentrations as assessed by the cytotoxicity assay. The in vitro studies postulate that calcitriol and gemcitabine when used in combination increase apoptosis in Capan-1 cells which might be mediated via inhibition of the Akt survival signaling pathway and the activation of several key members of the caspase family.
Results
Calcitriol promotes gemcitabine antiproliferative effect.
To examine whether calcitriol and gemcitabine combination treatment affects cell growth in the human pancreatic cancer model system Capan-1 and the nature of the interaction, their effects on cell growth were examined by MTT assay. Capan-1 cells were pretreated with various doses of calcitriol for 24 h followed by various doses of gemcitabine for 48 h. The results showed that pretreatment with calcitriol enhanced the gemcitabine-mediated growth inhibition in a dose-dependent manner (). Standard median-dose effect isobologram analysis revealed that the interaction between calcitriol and gemcitabine was synergistic (CI < 1) over the combination of calcitriol (0.093–6 µm) with gemcitabine (1.6–200 nM) (). These data indicate that calcitriol promotes the antiproliferative effect of gemcitabine in vitro.
Modulation of CDDase activity in tumor and liver tissues by calcitriol.
Calcitriol modulates the expression and activity of CDDase, the key gemcitabine degrading enzymes in both normal and tumor tissues. Therefore, we investigated if calcitriol-induced changes in CDDase activity contribute to the enhanced antitumor activity of the calcitriol + gemcitabine combination. Time course of the changes in CDDase activity in tumor and liver tissues obtained from Capan-1 tumor-bearing nude mice after a single dose 0.75 µg/mouse calcitriol were shown (). The results show significant (p < 0.05, ANOVA) transient increase in tumor CDDase 24 h post treatment; and no significant (p > 0.632, ANOVA) changes in liver CDDase activity.
Calcitriol promotes gemcitabine-mediated apoptosis.
Calcitriol induces apoptosis in a number of cancer cells. To examine whether apoptosis was involved in calcitriol and gemcitabine induced growth inhibition in pancreatic cancer cells, Capan-1 cells were pretreated with calcitriol for 24 h followed by gemcitabine for 24 h and apoptosis was assessed by annexin V staining. Calcitriol or gemcitabine alone induced similar level of apoptosis in Capan-1 cells ( and B). The pretreatment with calcitriol markedly enhanced gemcitabine-induced apoptosis ( and B), suggesting that calcitriol and gemcitabine may inhibit Capan-1 cell growth, at least in part, through the induction of apoptosis.
Calcitriol and gemcitabine induce the activation of caspases.
To examine the involvement of caspases in calcitriol and gemcitabine-induced apoptosis, the cleavage of caspases was assessed by Western blot analysis. Calcitriol induced modest activation of caspase-9 but not that of caspases-8, -6 or -3 in Capan-1 cells (). In contrast, gemcitabine induced the activation of all these caspases (). The pretreatment with calcitriol enhanced the activation of caspases-8, -6, -9 and -3 (). These results were confirmed using a substrate-based caspase activity assay, which showed that calcitriol alone had minimal effect of caspase activation; however, it enhanced the caspase activity when used in combination with gemcitabine (), supporting the observations that calcitriol promoted gemcitabine-mediated apoptosis induction.
Calcitriol and gemcitabine inhibit Akt activation.
We next investigated the effect of calcitriol and gemcitabine on prosurvival molecules Akt and ERK1/2. Calcitriol slightly decreased the level of phosphorylated Akt, which was further reduced following the addition of gemcitabine (). Gemcitabine alone did not suppress the activation of Akt (). In contrast, ERK1/2 phosphorylation was not reduced by either agent alone or in combination (). These results suggest that Akt may be involved in calcitriol and gemcitabine-mediated growth inhibitory effects in Capan-1 cells.
Calcitriol enhances gemcitabine antitumor activity in vivo.
To investigate whether calcitriol enhances the antitumor activity of gemcitabine in vivo, Capan-1 pancreatic tumor model system was employed. Capan-1 tumor-bearing nude mice were treated in 6 groups for 4 weeks: saline, calcitriol alone (2.5 µg/mouse once or twice a week), gemcitabine alone (6 mg/mouse once a week) or the combination of calcitriol (2.5 µg/mouse once or twice a week) and gemcitabine. Calcitriol or gemcitabine alone resulted in tumor regrowth delay compared with saline control (). Calcitriol administered twice a week reduced tumor growth even further than administered once a week (). The combination treatment with calcitriol and gemcitabine further inhibited tumor growth with a 10-fold reduction in tumor size (p < 0.01) (). The gross images of the tumors removed were shown in . These data indicate that the combined administration of calcitriol and gemcitabine resulted in a greater antitumor effect than either agent alone in the Capan-1 pancreatic cancer model system.
Discussion
Pancreatic cancer is one of the most deadly solid malignancies worldwide. Gemcitabine is used as the first line therapy in most cases but is only moderately efficacious. The overall 5-year survival rate is <5% even with the treatment. Research is ongoing in the hope of improving the antitumor effect of gemcitabine without increasing the toxicity. Several clinical trials have been conducted using gemcitabine in combination with a number of cytotoxic agents; most of which had disappointing results. Using innovative targeted therapies in conjunction with gemcitabine may be more effective.Citation23,Citation24 Pancreatic carcinoma cells pre-treated with the soy isoflavone genistein before treatment with gemcitabine were more susceptible to apoptosis than either agent alone. It was noted that genistein enhanced the effects of gemcitabine through the downregulation of NFκB and Akt. The antitumor effects of the combination treatment were also observed in vivo with inactivation of NFκB in the tumor.Citation25
Targeting the epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGFR) and platelet-derived growth factor receptor (PDGFR) via inhibition of phosphorylation along with gemcitabine treatment had a significant antitumor effect in vivo, with greater than 80% reduction in tumor size and improved survival rates. Increased apoptosis was observed in tumor cells and tumor-associated endothelial cells, along with decreased proliferation. There was also a decrease in mean vessel density observed in the treated tumor samples. Gemcitabine inhibits angiogenesis observed in several animal models of pancreatic carcinoma.Citation26,Citation27 Gemcitabine in combination with antiangiogenic agents is under investigation for potential therapeutic intervention in pancreatic carcinoma. Matrix-metalloproteinase inhibitors (MMPIs), anti-VEGF, COX-2 inhibitors and thalidomide are some of those agents being studied.Citation28
Calcitriol has antitumor effects over a wide range of tumors.Citation2 Preclinical studies have shown that calcitriol has additive or synergistical effects when used in combination with chemotherapeutic agents including cisplatin and paclitaxel. Calcitriol enhances doxorubicin-caused oxidative damage by reducing the expression and activity of cytoplasmic antioxidant enzyme Cu/Zn superoxide dismutase, and thus sensitizes breast cancer cells to doxorubicin treatment.Citation29 Calcitriol and cisplatin combination treatment enhances the antitumor activity of cisplatin in squamous cell carcinoma (SCC) cells, possibly through enhancing the expression of mitogen-activated protein kinase kinase kinase and induces caspase 3 activation.Citation21 Calcitriol promotes the cisplatin antiproliferative effects in SCC cells by the induction of p73 expression and enhanced apoptosis.Citation22 Calcitriol has also been shown to promote the growth inhibitory effects of paclitaxel, and the mechanisms may include enhanced Bcl-2 phosphorylation in breast cancer cells and reduced expression of p21 in prostate cancer cells.Citation6,Citation30 Clinical trials demonstrated that calcitriol can be safely administered to achieve sufficient doses in patients as compared to the doses given in the in vitro model systems.Citation15–Citation17,Citation31 Therefore, we investigated whether calcitriol would enhance the antitumor activity of gemcitabine in a pancreatic adenocarcinoma model system Capan-1.
Calcitriol treatment upregulates CDDase expression in some cells but not others.Citation32 In our previous study, we observed decrease in CDDase activity in cancer patient peripheral blood monocytes during calcitriol treatment.Citation33 Therefore, we suggested a pharmacokinetic approach for enhancing antitumor activity of drugs such as gemcitabine that are catabolized by CDDase if used in combination with calcitriol. The results of present study show no calcitriol-mediated decrease in CDDase activity and thus the modulation of gemcitabine PK plays no role in the antitumor activity synergy of the calcitriol + gemcitabine in the Capan-1 pancreatic tumor model system.
Treatment of Capan-1 cells with calcitriol and gemcitabine resulted in increased apoptosis as assessed by annexin V staining, which is associated with an increase in caspase-8 cleavage and enhanced activation of caspase-6. This suggests that enhanced cytotoxicity may be mediated by increased activation of the caspase-8/caspase-6/nuclear lamin pathway.
Caspase-9 was modestly increased in Capan-1 cells after treatment with calcitriol or gemcitabine alone and strongly increased by the combination. Caspase-3 was not activated by calcitriol but by gemcitabine and the combination treatment. The pro-survival signaling molecule Akt phosphorylation was strongly decreased by the combination of gemcitabine and calcitriol, but the phosphorylation of ERK1/2 was not markedly affected by the two agent combination. These data indicate that gemcitabine and calcitriol-enhanced cytotoxicity may result, at least in part, from the inhibition of the activation of Akt survival signaling pathway.
Median dose effect and isobologram analyses reveal that the combination of calcitriol with gemcitabine is strongly synergistic in vitro. Results from tumor regrowth delay experiments showed that calcitriol alone had antitumor effect in pancreatic cancer in vivo, in a dose-dependent manner. When it was administered twice a week, it delayed tumor growth to a similar extent as gemcitabine. The combination therapy with calcitriol and gemcitabine for 4 weeks significantly inhibited tumor growth as compared to either agent alone.
In summary, we investigated the in vitro and in vivo effects of treatment with gemcitabine in combination with calcitriol. Our results show that the combination treatment of calcitriol and gemcitabine in human pancreatic cancer model system Capan-1 is synergistic and preferable to either agent alone. Induction of apoptosis mediated via the activation of key caspase family members and inhibition of Akt survival signaling pathway are vital to the success of the combination treatment. It is also possible that the combination treatment is targeting the tumor vasculature since previous studies have shown the anti-proliferative effects of calcitriol on tumor-derived endothelial cells.Citation34 Understanding the effects of calcitriol in enhancing gemcitabine treatment of pancreatic carcinoma cells in vitro and in vivo is essential in designing more effective targeted therapies for pancreatic cancer using gemcitabine.
Material and Methods
Materials.
Calcitriol was from Hoffmann-LaRoche (Nutley, NJ). Gemcitabine (Gemzar®) was from Eli Lilly and Company (Indianapolis, IN). Anti-caspase 3 (9662), anti-caspase 8 (4927), anti-caspase 9 (9504), anti-caspase 6 (9762) and anti-phospho-Akt (Ser473, 9271) were from Cell Signaling Technology (Beverly, MA). Anti-phospho-ERK1/2 (sc-7383) was from Santa Cruz (Santa Cruz, CA). Anti-actin (CP-01) was from Calbiochem (San Diego, CA).
Cell culture and tumor model system.
Capan-1 cells (human pancreas, adenocarcinoma, ATCC) were cultured in Iscove's Dulbecco medium/20% FBS/penicillin and streptomycin (100 U/ml). Capan-1 tumors were routinely produced by s.c. inoculation of 3 × 106 log-phase tissue culture cells in the right rear flank of nude mice. The mice protocols used in tumor regrowth delay were approved by the Institutional Animal Care and Use Committee at Roswell Park Cancer Institute.
Tumor growth assay.
Capan-1 tumor cells (3 × 106 cells) were inoculated s.c. into nude mice. At day 8–9 post implantation, when the tumors were palpable (6.5 × 5 mm), animals were treated with i.p. calcitriol or gemcitabine alone or in combination. Tumor growth was assessed by measuring tumor size with calipers three times/week. Tumor volumes were calculated by (length × width2)/2 and expressed as a fraction of pre-treatment size at the time of the first treatment.
In vitro cytotoxicity assay/CI index determination.
Capan-1 cells were suspended at 0.15 × 105 cells/ml and 100 µl/well dispensed into 96-well microtiter plates. The following day, various concentrations of each agent were added. Calcitriol was reconstituted in 100% ethanol (ETOH) and stored protected from light under a layer of nitrogen gas at −70°C. Dilutions of calcitriol were made in medium just prior to use. Gemcitabine was also diluted in medium just prior to use. The cells were harvested 48 h after treatment by adding 20 µl of a stock solution of 0.5% MTT (5 mg/ml) to each well. The plates were incubated for an additional 3–4 h at 37°C. Formazan crystals were dissolved with 100 µl of 10% SDS/10 mM HCl solution overnight at 37°C. The absorbance at 590 nm was measured using a SPECTRAmax340pc microplate reader. The CalcuSyn program (T.C. Chou and M.P. Hayball, Biosoft) was used to analyze the drug combinations. Constant ratios of drug concentrations were used in these studies, and mutually exclusive equations were used to determine the combination index (CI). CI < 1, = 1, > 1 indicates synergistic, additive and antagonistic effects, respectively.
CDDase activity assay.
Capan-1 tumor-bearing nude mice were treated with a single dose 0.75 µg/mouse calcitriol for 4 to 48 h. Tumors and liver tissues were harvested and rinsed twice with 5 ml of cold normal saline, 5 mM Tris-HCL buffer, pH 7.4 and stored at −80°C until enzyme activity assay. Tissues were sonicated in 0.1 ml of cold 5 mM Tris-HCL buffer, pH 7.4 containing 5 mM DTT and centrifuged at 14,000 rpm for 10 min at 4oC. The clear supernatant was assayed for CDDase activity as previously described.Citation32,Citation33 Briefly, CDDase activity was measured spectrophotometrically by monitoring the rate of cytidine deamination to uridine at 286 nm. The assay contained 50–100 µg of protein of tumor or liver tissue in 1 ml of 20 mM Tris HCL buffer pH 7.4 containing 100 mM KCL. Absorbance change at 286 nm was recorded for 3 min prior to and after the addition of 100 µM cytidine. The difference in absorbance change/min the presence and absence of cytidine was used to calculate CDDase activity. CDDase activity was expressed as nmol uridine formed/min/mg protein. (uridine extinction coefficient = 3,000 cm2/mmol).
Substrate-based caspase activity assay.
Caspase-3, 6, 8 and 9 activity was measured using the caspase-family Colorimetric Assay kit from BioVision Research Products (Mountain View, CA) according to the manufacturer's protocol. Briefly, the cells were pre-treated for 24 h with calcitriol (0.75 µM) and/or gemcitabine (12.5 µM) was added the following day for another 24 h. The cells were trypsinized and centrifuged at 1,000 rpm for 10 min. The cell pellets (1–5 × 106 cells) were resuspended in 50 µl of ice cold cell lysis buffer and placed on ice for 10 min. The lysates were centrifuged for 1 min at 11,000 rpm and the supernatant collected to assay protein concentration. 50–200 µg protein was diluted in 50 µl cell lysis buffer for each assay. 50 µl of 2x reaction buffer containing 10 mM DTT was added to each sample and 5 µl of 4 mM pNA conjugated substrate was added to each sample and incubated at 37°C for 1–2 h. The samples were read using a spectrophotometer (Absorbance 400 nm). Caspase activity was expressed as absorbance (O.D.) per milligram of protein per reaction.
Apoptosis analysis—annexin V staining.
Capan-1 cells were pre-treated for 24 h with calcitriol (0.75 µM) and/or gemcitabine (6.25 µM or 12.5 µM) was added the following day for a further 24 h. The cells were harvested by trypsinization following treatment. Cells were stained with Annexin V-PE and 7-AAD according to the manufacturer's instructions (BD Pharmingen).Citation14 The data was analysed using Winlist™ program (Verity House, Topsham, ME).
Western blot analysis.
Following pre-treatment for 24 h with calcitriol, the cells were treated with varying concentrations of gemcitabine for a further 24 h. Cell lysates were prepared and Western blot analysis performed as described previously.Citation14
Statistical analysis.
The data for calcitriol-induced changes in tissue CDDase activity was analyzed by One-way ANOVA using GraphPad Prism 5 software (La Jolla, CA). A p-value of <0.05 was considered significant. In all other analyses, statistical significances between groups were determined by two-tailed Student's t-test.
Grant support
This study was supported by NIH/NCI grants CA067267 and CA085142 to Dr. Candace S. Johnson and CA095045 to Dr. Donald L. Trump. It was also supported, in part, by the NCI Cancer Center Support Grant to the Roswell Park Cancer Institute (CA016056).
Abbreviations
CDDase | = | cytidine deaminase |
CI | = | combination index |
dFdC | = | difluorodeoxycytidine |
EGFR | = | epidermal growth factor receptor |
ERK1/2 | = | extracellular signal regulated kinase 1/2 |
ETOH | = | ethanol |
MMPIs | = | matrix-metalloproteinase inhibitors |
MTT | = | 3-(4,5-Dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide |
NFκB | = | nuclear factor-κB |
VEGFR | = | vascular endothelial growth factor receptor |
PDGFR | = | platelet-derived growth factor receptor |
SCC | = | squamous cell carcinoma |
References
- Reichel H, Koeffler HP, Norman AW. The role of the vitamin D endocrine system in health and disease. N Engl J Med 1989; 320:980 - 991
- Deeb KK, Trump DL, Johnson CS. Vitamin D signalling pathways in cancer: potential for anticancer therapeutics. Nat Rev Cancer 2007; 7:684 - 700
- Getzenberg RH, Light BW, Lapco PE, Konety BR, Nangia AK, Acierno JS, et al. Vitamin D inhibition of prostate adenocarcinoma growth and metastasis in the Dunning rat prostate model system. Urology 1997; 50:999 - 1006
- Peehl DM, Skowronski RJ, Leung GK, Wong ST, Stamey TA, Feldman D. Antiproliferative effects of 1,25-dihydroxyvitamin D3 on primary cultures of human prostatic cells. Cancer Res 1994; 54:805 - 810
- McGuire TF, Trump DL, Johnson CS. Vitamin D(3)-induced apoptosis of murine squamous cell carcinoma cells. Selective induction of caspase-dependent MEK cleavage and upregulation of MEKK-1. J Biol Chem 2001; 276:26365 - 26373
- Hershberger PA, Yu WD, Modzelewski RA, Rueger RM, Johnson CS, Trump DL. Calcitriol (1,25-dihydroxycholecalciferol) enhances paclitaxel antitumor activity in vitro and in vivo and accelerates paclitaxel-induced apoptosis. Clin Cancer Res 2001; 7:1043 - 1051
- Bernardi RJ, Trump DL, Yu WD, McGuire TF, Hershberger PA, Johnson CS. Combination of 1alpha,25-dihydroxyvitamin D(3) with dexamethasone enhances cell cycle arrest and apoptosis: role of nuclear receptor cross-talk and Erk/Akt signaling. Clin Cancer Res 2001; 7:4164 - 4173
- Kumagai T, O'Kelly J, Said JW, Koeffler HP. Vitamin D2 analog 19-nor-1,25-dihydroxyvitamin D2: antitumor activity against leukemia, myeloma and colon cancer cells. J Natl Cancer Inst 2003; 95:896 - 905
- Muto A, Kizaki M, Yamato K, Kawai Y, Kamata-Matsushita M, Ueno H, et al. 1,25-Dihydroxyvitamin D3 induces differentiation of a retinoic acid-resistant acute promyelocytic leukemia cell line (UF-1) associated with expression of p21(WAF1/CIP1) and p27(KIP1). Blood 1999; 93:2225 - 2233
- Munker R, Kobayashi T, Elstner E, Norman AW, Uskokovic M, Zhang W, et al. A new series of vitamin D analogs is highly active for clonal inhibition, differentiation and induction of WAF1 in myeloid leukemia. Blood 1996; 88:2201 - 2209
- Mangelsdorf DJ, Koeffler HP, Donaldson CA, Pike JW, Haussler MR. 1,25-Dihydroxyvitamin D3-induced differentiation in a human promyelocytic leukemia cell line (HL-60): receptor-mediated maturation to macrophage-like cells. J Cell Biol 1984; 98:391 - 398
- Beer TM, Myrthue A. Calcitriol in cancer treatment: from the lab to the clinic. Mol Cancer Ther 2004; 3:373 - 381
- Light BW, Yu WD, McElwain MC, Russell DM, Trump DL, Johnson CS. Potentiation of cisplatin antitumor activity using a vitamin D analogue in a murine squamous cell carcinoma model system. Cancer Res 1997; 57:3759 - 3764
- Ma Y, Yu WD, Kong RX, Trump DL, Johnson CS. Role of Nongenomic Activation of Phosphatidylinositol 3-Kinase/Akt and Mitogen-Activated Protein Kinase/Extracellular Signal-Regulated Kinase Kinase/Extracellular Signal-Regulated Kinase 1/2 Pathways in 1,25D3-Mediated Apoptosis in Squamous Cell Carcinoma Cells. Cancer Res 2006; 66:8131 - 8138
- Smith DC, Johnson CS, Freeman CC, Muindi J, Wilson JW, Trump DL. A Phase I trial of calcitriol (1,25-dihydroxycholecalciferol) in patients with advanced malignancy. Clin Cancer Res 1999; 5:1339 - 1345
- Beer TM, Munar M, Henner WD. A Phase I trial of pulse calcitriol in patients with refractory malignancies: pulse dosing permits substantial dose escalation. Cancer 2001; 91:2431 - 2439
- Muindi JR, Peng Y, Potter DM, Hershberger PA, Tauch JS, Capozzoli MJ, et al. Pharmacokinetics of high-dose oral calcitriol: results from a phase 1 trial of calcitriol and paclitaxel. Clin Pharmacol Ther 2002; 72:648 - 659
- Freelove R, Walling AD. Pancreatic cancer: diagnosis and management. Am Fam Physician 2006; 73:485 - 492
- Mini E, Nobili S, Caciagli B, Landini I, Mazzei T. Cellular pharmacology of gemcitabine. Ann Oncol 2006; 17:7 - 12
- Galmarini CM, Mackey JR, Dumontet C. Nucleoside analogues: mechanisms of drug resistance and reversal strategies. Leukemia 2001; 15:875 - 890
- Hershberger PA, McGuire TF, Yu WD, Zuhowski EG, Schellens JH, Egorin MJ, et al. Cisplatin potentiates 1,25-dihydroxyvitamin D3-induced apoptosis in association with increased mitogen-activated protein kinase kinase kinase 1 (MEKK-1) expression. Mol Cancer Ther 2002; 1:821 - 809
- Ma Y, Yu WD, Hershberger PA, Flynn G, Kong RX, Trump DL, et al. 1alpha,25-Dihydroxyvitamin D3 potentiates cisplatin antitumor activity by p73 induction in a squamous cell carcinoma model. Mol Cancer Ther 2008; 7:3047 - 3055
- Toschi L, Finocchiaro G, Bartolini S, Gioia V, Cappuzzo F. Role of gemcitabine in cancer therapy. Future Oncol 2005; 1:7 - 17
- Lidestahl A, Permert J, Linder S, Bylund H, Edsborg N, Lind P. Efficacy of systemic therapy in advanced pancreatic carcinoma. Acta Oncol 2006; 45:136 - 143
- Banerjee S, Zhang Y, Ali S, Bhuiyan M, Wang Z, Chiao PJ, et al. Molecular evidence for increased antitumor activity of gemcitabine by genistein in vitro and in vivo using an orthotopic model of pancreatic cancer. Cancer Res 2005; 65:9064 - 9072
- Yokoi K, Sasaki T, Bucana CD, Fan D, Baker CH, Kitadai Y, et al. Simultaneous inhibition of EGFR, VEGFR and platelet-derived growth factor receptor signaling combined with gemcitabine produces therapy of human pancreatic carcinoma and prolongs survival in an orthotopic nude mouse model. Cancer Res 2005; 65:10371 - 10380
- Amoh Y, Li L, Tsuji K, Moossa AR, Katsuoka K, Hoffman RM, et al. Dual-color imaging of nascent blood vessels vascularizing pancreatic cancer in an orthotopic model demonstrates antiangiogenesis efficacy of gemcitabine. J Surg Res 2006; 132:164 - 169
- Saif MW. Anti-angiogenesis therapy in pancreatic carcinoma. Jop 2006; 7:163 - 173
- Ravid A, Rocker D, Machlenkin A, Rotem C, Hochman A, Kessler-Icekson G, et al. 1,25-Dihydroxyvitamin D3 enhances the susceptibility of breast cancer cells to doxorubicin-induced oxidative damage. Cancer Res 1999; 59:862 - 867
- Wang Q, Yang W, Uytingco MS, Christakos S, Wieder R. 1,25-Dihydroxyvitamin D3 and all-trans-retinoic acid sensitize breast cancer cells to chemotherapy-induced cell death. Cancer Res 2000; 60:2040 - 2048
- Beer TM, Javle MM, Ryan CW, Garzotto M, Lam GN, Wong A, et al. Phase I study of weekly DN-101, a new formulation of calcitriol, in patients with cancer. Cancer Chemother Pharmacol 2007; 59:581 - 587
- Muindi JR, Peng Y, Wilson JW, Johnson CS, Branch RA, Trump DL. Monocyte fructose 1,6-bisphosphatase and cytidine deaminase enzyme activities: potential pharmacodynamic measures of calcitriol effects in cancer patients. Cancer Chemother Pharmacol 2007; 59:97 - 104
- Watanabe S, Uchida T. Expression of cytidine deaminase in human solid tumors and its regulation by 1 alpha,25-dihydroxyvitamin D3. Biochim Biophys Acta 1996; 1312:99 - 104
- Bernardi RJ, Johnson CS, Modzelewski RA, Trump DL. Antiproliferative effects of 1alpha,25-dihydroxyvitamin D(3) and vitamin D analogs on tumor-derived endothelial cells. Endocrinology 2002; 143:2508 - 2514