Recent gene expression profiling has identified five breast cancer subtypes, of which the basal-like breast cancers are the most aggressive and possess serious clinical challenges as there are currently no targeted therapies available. Although there is increasing evidence that these tumors confer specific sensitivity to cisplatin, its success is often compromised due to its dose-limiting nephrotoxicity and development of drug resistance. To overcome this limitation, our focus is to maximize the benefits associated with cisplatin therapy through drug combination strategies. Using a well-validated kinase inhibitors library, we showed that inhibition of mTOR, TGFï¢RI, NFï«B, PI3K/AKT and MAPK pathway sensitized the basal-like MDA-MB-468 cells to cisplatin treatment. Further evaluation demonstrated that combination of mTOR inhibitor, rapamycin, and cisplatin generated significant drug synergism specifically in basal-like cells (MDA-MB-468, MDA-MB-231 and HCC1937). These synergistic effects were not observed in the luminal-like T47D and MCF-7 cells. We further showed that the synergistic effects of rapamycin and cisplatin is mediated through p73. Treatment of rapamycin induced p73 upregulation and synergized cisplatin activity through activation of the p73 pathway. Depletion of endogenous p73 in basal-like cells abolished these synergistic effects suggesting that p73 is required for the rapamycin and cisplatin synergism. In conclusion, combination of mTOR inhibitors and cisplatin may be a useful therapeutic strategy in basal-like breast cancers.
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Recent identification of novel breast cancer subtypes with distinct biological features promises a more specific, effective and less toxic therapies to the patients. Through gene profiling, breast cancer can be categorized into five different subtypes with distinct clinical outcome. The five major subtypes of breast cancer are luminal A, luminal B, human epidermal growth factor receptor-2 (HER2) overexpressing, normal-like and basal-like breast cancer [1, 2]. Of particular important is the basal-like breast cancer which accounts for 15-20% of breast cancers overall and confers a remarkably poor prognosis compared to other subtypes. Majority of basal-like breast cancers exhibit a 'triple-negative' phenotype, characterized by the lack of expression of estrogen receptor (ER), progesterone receptor (PR) or HER2 amplification, and often have high frequency of p53 mutation [3, 4]. Due to the lack of expression of these receptors (ER, PR and HER2), patients with basal-like breast cancers usually do not response to hormonal therapy, Herceptin or chemotherapy [5, 6]. As a consequence, the mortality rate of basal-like breast cancer is relatively high in comparison with the non-basal subtype .
Numerous clinical studies are currently ongoing to identify novel therapy for treatment of basal-like breast cancers. These include the use of specific targeted therapeutic agents (e.g. Cetuximab, Dasatinib, Bevacizumab, Abraxane and Erlotinib) or conventional chemotherapeutics agents (e.g. cisplatin, doxorubicin, and paclitaxel), either as single agent or in combination, as first line therapy for basal-like breast cancers [7-9].
Cisplatin, a chemotherapeutic agent not commonly used for breast cancer, come to light in the management of basal-like breast cancer on account of evidence that breast cancer cells with basal-like phenotype confer a selective sensitivity towards cisplatin as compared to other chemotherapeutic agents. A variety of evidence suggests that basal-like breast cancers may share defects in BRCA1-associated pathways, of which DNA repair mechanism has been compromised . Indeed, recent clinical studies have demonstrated the clear advantage of cisplatin in treatment of basal-like breast cancer compared to other chemotherapeutic agents [11, 12]. Nevertheless, dose-limiting toxicity including nephrotoxicity, neurotoxicity and ototoxicity have withold the wide-spread use of cisplatin in treating breast cancers in the clinic.
To address this problem, we developed a high-throughput screening assay to rapidly identify new therapeutic agents that could synergize the antitumor effects of cisplatin in basal-like breast cancers. Through the use of a small chemical library that targets some of the most relevant oncogenic pathways in basal-like breast cancer, we show that inhibition of mTOR by rapamycin incurred a specific synergistic effect with cisplatin in basal-like breast cancer cells. This synergistic effect is mediated in part through the induction and activation of p73 in the presence of rapamycin and cisplatin, respectively. Together, our findings demonstrate evidence of a synergistic relation between rapamycin and cisplatin in both inhibition of cell growth and induction of apoptosis. This suggests that rapamycin and cisplatin may be a rational combination of a targeted therapy for the refractory basal-like breast cancers.
Materials and Methods
Cell lines and cell culture
The human breast carcinoma cell lines MCF-7, T47D, MDA-MB-231, MDA-MB-468 and HCC1937 were obtained from the American Type Culture Collection (Manassas, VA) and maintained in RPMI 1640 medium containing 10% fetal bovine serum (FBS), 100 IU/ml penicillin and 100 Î¼g/ml streptomycin (Sigma-Aldrich, St. Louis, MO, USA) at 37°C with 5% CO2.
MTT cell proliferation assay
Dose-response curves and IC50 values were determined using the methyl thiazolyl tetrazolium (MTT) cell viability assay as described previously [4, 13]. Cells were seeded into 96-well plates for 24 hours at a density of 5 Ã- 103 cells/well. Serial drug dilutions were prepared in medium immediately before each assay, and viable cell masses following 3 days of drug exposure were determined by cell-mediated MTT reduction. Cell growth as well as drug activity was determined by measuring absorbance at 550 nm using an Anthos systems plate reader.
Construction of IC50 mean graph
The IC50 mean graph was constructed as defined by the Developmental Therapeutics Program of the National Cancer Institute (http://dtp.nci.nih.gov). The mean graph consists of positive (more sensitive) and negative (less sensitive) "delta" values, generated from a set of IC50 values by using a three-step calculation. The IC50 values for each of cell line against the tested compound were converted to log(IC50) values. For each tested compound, the log(IC50) values are averaged. Finally, the individual IC50 value is then subtracted from the average to generate the delta value. Positive delta values project to the right of the vertical line and represent cellular sensitivities to the test agent that exceed the mean. Negative values project to the left and represent cell line sensitivities to the test agent that are less than the average value.
The Inhibitor Select„¢ chemical library which consists of 160 well-characterized, cell-permeable inhibitors was purchased from EMD Chemicals, USA. MDA-MB-468 cells at the logarithmic phase of growth were seeded into 96-well plate at a density of 5 Ã- 103 cells/well. Each compound was added to a final concentration of 10 µM in the absence or presence of 1 µM cisplatin. Plates were incubated for 72h at 37°C. Cell proliferation was examined using MTT assay as described previously. Combination treatments that induce growth inhibition higher than those of the same doses used alone (p < 0.05; Student's t test) were considered as hit. These compounds were subsequently tested at different concentrations to determine the mode of interaction by isobologram analysis.
Drug interaction analysis
Drug combination analysis was performed by using the method as described by Chou and Talalay . Briefly, cells were seeded at 5 Ã- 103 cells/well in 96-well plates and treated with various concentrations of cisplatin and compound alone or in combination for 72h. Cell proliferation was measured in each well by MTT assay. Multiple drug dose-effect calculations and the combination index plots were generated using Calcusyn software (Biosoft, Cambridge, UK). Combination index, CI < 1, = 1, and >1 indicate synergism, additive effect and antagonism, respectively.
Quantitation of apoptosis by annexin V/PI staining was performed as described previously [3, 4]. Briefly, both floating and attached cells were collected 72h after drug treatments. Apoptotic cell death was determined using the BD ApoAlert annexin V-FITC Apoptosis Kit (BD Biosciences, USA) according to the manufacturer's instructions, and cells were analyzed on a FACSCalibur flow cytometer using CellQuest Pro software (version 5.1.1; BD Biosciences, USA).
Quantitative PCR (qPCR) analysis
Total RNA from cells was extracted using Qiagen RNA isolation kit (Qiagen, Valencia, CA, USA) according to the manufacturer's protocol. First-strand cDNA was synthesized from total RNA using random hexamer primers and the SuperScript II system for RT-PCR (Invitrogen, Carlsbad, USA). Gene expression levels were measured by qPCR using the iQ SYBR Green Supermix reagent and an Biorad iQ5 real-time PCR detector system (Bio-Rad, Richmond, CA, USA). Data analysis was performed using Opticon Monitor Analysis Software V1.08. The expression of each gene was normalized to Î²2M as a reference. The relative copy numbers were calculated from an 8-point standard curve generated from a 10-fold serial dilution of full-length cDNA constructs as described previously [3, 4]. Specific forward and reverse primer sequences are as follows : TAp73fwd, 5'-GCACCACGTTTGAGCACCTCT-3'; TAp73rev, 5'- GCAGATTGAACTGGGCCATGA-3'; Î²2Mfwd, 5'-AGCTGTGCTCGCGCTACTCTC-3'; Î²2Mrev, 5'-CACACGGCAGGCATACTCATC-3'; PUMAfwd PUMArev NOXAfwd NOXArev. The conditions for all QRT-PCR reactions were as follows: 3 minutes at 94°C followed by 40 seconds at 94°C, 40 seconds at 60°C, and 25 seconds at 72°C for 40 cycles. All PCR products were confirmed by the presence of a single peak upon melting curve analysis and by gel electrophoresis. No-template (water) reaction mixtures and no-RT mixtures were performed on all samples as negative controls. All experiments were performed in duplicate.
Protein isolation and Western blot analysis
Protein lysates from cells were extracted in ice-cold lysis buffer (0.75% NP-40, 1 mM DTT, and protease inhibitors in PBS). Total protein (25 Î¼g) was subjected to SDS-PAGE followed by immunoblotting with the following antibodies: p73 (diluted 1:1,000, Ab-2; CalBiochem); pS6K (diluted 1:1,000; Cell Signaling Technology); S6K (diluted 1:1,000, Ab9645; Abcam); and Î²-tubulin (diluted 1:2,500, D-10; Santa Cruz Biotechnology).
Lentiviral production and infection
The shRNA lentiviral constructs were created by transferring the U6 promoter-shRNA cassette into a lentiviral backbone, and high-titre lentiviral stocks were generated by co-transfection with packaging vectors into 293T cells as described previously [3, 4, 13]. The shRNA target sequences for TAp73 was 5'-GGATTCCAGCATGGACGTCTT-3'. The TAp73 targeted sequence is found within p73 exon 3. Therefore, this shRNA does not target Î”Np73 .
Selective sensitivity of basal-like breast cancer toward cisplatin
To gain an overview of the selectivity of chemotherapeutic agents for basal-like breast cancer cells, we compared their antiproliferative properties in a panel of basal-like and luminal-like breast cancer cell lines which has been validated previously through gene profiling . All cells were treated with increasing concentrations of cisplatin, paclitaxel or doxorubicin for 72 hours and growth measured using the MTT assay. Figure 1A and B summarizes the results from these breast cancer cell lines in which basal-like breast cancer cells demonstrated selective sensitivity to cisplatin. This selectivity was absence in cells treated with paclitaxel or doxorubicin suggesting that basal-like breast cancer cells confer selective sensitivity towards cisplatin (Figure 1A, B and Supplement Table 1).
Small chemical library screening identify rapamycin as synergistic agents for cisplatin
Although cisplatin is currently one of the most used agents in the treatment of cancer, the use of cisplatin is hampered by its side effects, especially neurotoxicity, nephrotoxicity and rug resistance . Hence, the present study was aimed to identify chemosensitizers that could synergize the effects of cisplatin for treatment of basal-like breast cancers.
To identify small molecules that enhance sensitivity of basal-like breast cancer cells to cisplatin, a cell-based high-throughput screen was performed using MDA-MB-468 cell line and a small chemical library consisting of 160 well validated specific inhibitors. The screens were done in 96-well plates to which compounds were added at 10 µM, followed by cisplatin at 1 µM. Cell viability was measured 72 hours later by MTT assay. Each plate included controls of untreated cells, cells treated with compounds or cisplatin only, and cells treated with a combination of both agents. Combinations of the treatments that induced growth inhibition higher than those of the same doses used alone (p < 0.05; Student's t-test) was used as a cutoff for scoring hits.
The molecules identified in this screen includes rapamycin, [3-(Pyridin-2-yl)-4-(4-quinonyl)]-1H-pyrazole (LY364947), 4-(3-Chloroanilino)-6,7-dimethoxyquinazoline (AG1478), (E)3-[(4-Methylphenyl)sulfonyl]-2-propenenitrile (BAY11-7082), 2-(4-Morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) and 4-(4-Fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole (SB203580). Structures of these compounds and their growth inhibitory effects were shown in Figure 2. The molecular target of these compounds was listed in Table 1.
The 6 compounds identified are specific inhibitors of the mTOR, TGF-ï¢, EGFR, NFï«B PI3K/AKT and MAPK pathways which have been previously reported to be upregulated specifically in basal-like breast cancers [7, 17-20]. However, when tested for synergism with cisplatin at fixed concentration ratio, only rapamycin showed synergism when combined with cisplatin in MDA-MB-468 cells using the isobologram method that simulate the median-dose combination index (CI) . The mean combination index at ED50, ED75, and ED90 of rapamycin (10:1 cisplatin:rapamycin) when combined with cisplatin was 0.52 ï‚± 0.06, where combination index < 1 denotes synergy. The remaining compounds (LY364947, AG1478, BAY11-7082, LY294002 and SB203580) show mainly additive effects (CI close to 1) with cisplatin in MDA-MB-468 cells, reflecting the narrow range of concentrations over which they enhanced cisplatin killing.
Specific synergistic effects of rapamycin and cisplatin in basal-like breast cancer cells
Next, we sought to investigate if combination of cisplatin with rapamycin exhibit specific synergism in basal-like breast cancers by comparing their effects in a panel of breast cancer cell lines. The antiproliferative effect of treatment was evaluated using MTT assays and drug interaction was assessed by the isobologram method as described in the previous section. The results demonstrated that combination of rapamysin and cisplatin exhibited selective synergistic effects only in the basal-like MDA-MB-468, MDA-MB-231 and HCC1937 cells, but not in the luminal-like T47D or MCF-7 cells (Figure 3A and Table 2).
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To ensure that the lack of synergistic effects of rapamycin and cisplatin observed in luminal-like cells is not due the general lack of sensitivity of luminal-like cells to cisplatin per se, we compared the apoptotic effects of cisplatin and rapamycin alone or in combination in MDA-MB-231 and T47D cells using an equal potent dose of cisplatin (IC50 dose). Both cell lines exhibited similar amount of apoptosis following treatment with their respective IC50 dose of cisplatin. Interestingly, when both cell lines were treated with combination of cisplatin and rapamycin, synergistic effects were evidenced only in MDA-MB-231 cells but not in T47D cells. The potentiation effects of rapamycin observed in MDA-MB-231 cells were further enhanced by sequential treatment with rapamycin for 6 hours followed by cisplatin (data not shown). These results demonstrated that rapamycin is capable of sensitizing basal-like breast cancer cells to cisplatin, suggesting that the synergistic effects of rapamycin and cisplatin may be mediated through a common pathway.
Rapamycin sensitizes basal-like cells to cisplatin through p73 up regulation
Previous study has shown that inhibition of mTOR by rapamycin up regulate p73 in breast cancer cells . We and others have also demonstrated that p73 is overexpressed in a subset of triple-negative breast tumors and that p73 is required for cisplatin sensitivity in basal-like breast cancer cells . On the basis of these findings, we ask if the synergistic effects of rapamycin and cisplatin combination could be explained by the activation of the p73 pathway.
To test this notion, MDA-MB-231 cells were treated with 10 µM of cisplatin and 100 nM rapamycin alone or in combination for 48 hours. mTOR inhibition was confirmed using phospho-S6K as a marker of mTOR activity. Quantitation of the mRNA and protein expression of the TAp73 was performed using qPCR and immunoblotting, respectively. The results showed that treatment of rapamycin or cisplatin alone did not change the expression of TAp73 mRNA but induced significant up regulation of TAp73 protein expression in MDA-MB-231 cells (Figure 4A and B).
Next, we evaluated the expression of the two potent pro-apoptotic BH3 only proteins, PUMA and NOXA, which has been identified as specific p73 target genes, using qPCR [22, 23]. As expected, treatment of cells with cisplatin for 48 hours induced mRNA expression of PUMA and NOXA in both MDA-MB-231 and MDA-MB-468 cells. Significant induction of PUMA and NOXA were also observed in MDA-MB-231 cells but not in MDA-MB-468 cells following treatment with rapamycin alone. When cells were treated with combination of cisplatin and rapamycin, the expression of PUMA and NOXA was further enhanced, corroborated with the massive induction of apoptosis as shown in Figure 3B. Together, these results suggest that rapamycin synergize cisplatin activity in basal-like cells through induction of p73 pathway.
p73 is required for the synergistic effects of rapamycin and cisplatin in basal-like breast cancer cells
To further evaluate whether p73 is the effector of the synergistic effects of rapamycin and cisplatin in basal-like breast cancer cells, we generated a series of isogenic cell lines that have been depleted for TAp73 by stably expressing a shRNA species that target specifically human TAp73. Unlike MDA-MB-231 cells which express one predominant TAp73 isoform (TAp73ï¢) isoform, MDA-MB-468 cells express high levels of two p73 isoforms, TAp73ï¡ and TAp73ï¢ . Figure 5A showed efficient knock-down of TAp73 isoforms in MDA-MB-231 and MDA-MB-468 cells.
As expected, treatment of cisplatin alone induced significant amount of apoptosis in MDA-MB-468 vector control cells. This apoptotic effects were further enhanced in the presence of rapamycin, consistent with our previous observations (Figure 3B). In stark contrast, depletion of TAp73 not only reduced the amount of apoptosis following treatment of cisplatin alone but also completely abrogated the synergistic effects of rapamycin (Figure). This result is further supported by the isobologram analysis which showed a lack of synergism of rapamycin-cisplatin treatment in the TAp73 depleted cells. Together, these results suggest that TAp73 is required for the synergistic effects of rapamycin and cisplatin in basal-like breast cancers.
By gene profiling, breast cancers can be classified into 5 molecularly distinct subtypes: luminal A, luminal B, HER2+, basal-like and normal breast cancers. The basal-like subtype, which represents 15-20% of breast cancers, has been subjected to extensive investigation in recent years due to its association with poor patient survival [1, 2, 20, 24]. Unlike many breast cancers, patients diagnosed with basal-like breast cancers are not eligible for molecular targeted therapy that target ER (e.g. tamoxifen, aromatase inhibitors) or HER2 (e.g. Herceptin) as they do not express the estrogen receptor (ER) or progesterone receptor (PR), nor do they have amplified HER2 [1, 24]. The treatment option therefore is relied on aggressive conventional chemotherapies which have limited efficacy, many side effects and often high rate of relapse. Hence, development of an effective therapeutic strategy remains an important goal in the management of basal-like breast cancer.
Several lines of evidence has suggested a link between basal-like breast cancers and BRCA1 deficiency [7, 25, 26]. In most cases, the clinical features and outcomes for women with sporadic basal-like breast cancers are broadly similar to those with BRCA1-related cancers including high tendency of developing high grade, high mitotic index tumors, shorter time of relapse, similar pattern of metastatic spread and cytogenetic changes associated with frequent loss of X-chromosome inactivation [7, 27-32]. The majority of BRCA1-associated cancers are also 'triple-negative' (ER, PR and HER2 negative), express basal cytokeratins and other markers commonly seen in basal-like breast cancers (e.g. p53, P-cadherin and EGFR) . Gene expression profiling also demonstrated that BRCA1-associated cancers segregate strongly with basal-like breast cancers [2, 29, 33, 34]. Although BRCA1 somatic gene mutations are uncommon in sporadic basal-like cancers, these tumors have been shown to have a dysfunctional BRCA1 pathway due to BRCA1 gene promoter methylation and/or BRCA1 pathway transcriptional inactivation [7, 25, 26].
The fundamental biological similarities between hereditary BRCA1-related breast cancers and basal-like cancers suggest that strategies targeting the dysfunctional BRCA1 pathway may be effective in basal-like breast cancers. There is increasing evidence that the DNA repair defects characteristic of BRCA1 related cancers, especially defective homologous recombination, confer sensitivity to certain systemic agents, such as platinum-based chemotherapy and poly(ADP-ribose) polymerase (PARP) inhibitors [34-38]. Indeed, recent clinical studies revealed that sporadic basal-like cancers responded to platinum-based chemotherapy and were associated with a high rate of complete pathologic response [9, 11, 38]. Consistent with the clinical data, our in vitro study also reveals that basal-like breast cancer cells confer specific sensitivity to cisplatin as compared to other chemotherapeutic agents (e.g. doxorubicin or paclitaxel) (Figure 1), further support research into the utility of platinum-based agents in basal-like breast cancers.
Given the high specificity and response rate of basal-like breast cancers toward platinum-based therapy, our focus is to maximize the benefits associated with this therapy through drug combination strategies. Using a small chemical library consisted of 160 well-validated and specific inhibitors that target the human kinome, we have identified 6 compounds that significantly potentiate the antiproliferative effects of cisplatin in basal-like breast cancer cells. These compounds include rapamycin, LY364947, AG1478, BAY11-7082, LY294002 and SB203580 which targets the mTOR, TGFï¢RI, NFï«B, PI3K/AKT and MAPK pathway respectively. Of note, these pathways have been reported previously to be over activated in basal-like breast cancers [7, 17, 18, 20].
To further investigate the mode of interaction between these compounds and cisplatin, we performed a drug combination study using the isobologram approach as described previously (Ref). Out of the 6 compounds identified, rapamycin showed the strongest synergistic effects with cisplatin while others (LY364947, AG 1478, BAY11-7082, LY294002 and SB203580) showed mainly additive effects. This result is consistent with other studies which show that inhibition of mTOR by RNAi or small molecules (e.g. rapamycin, CCI-779, RAD001) enhances cisplatin chemosensitivity in ovarian [39-41], endometrial , head and neck [43, 44], lung , skin [46, 47] and liver  cancers.
We next compared the synergistic effects of rapamycin in combination with cisplatin in a panel of luminal-like and basal-like breast cancer cell lines that has been previously validated by gene profiling . Intriguingly, the synergistic effects were observed only in MDA-MB-468, MDA-MB-231 and HCC1937 basal-like cells, but not in MCF-7 or T47D luminal-like cells.
Several models have been proposed to explain the synergistic effects of rapamycin and cisplatin in cancer cells. Beuvnk et al., 2005 showed that RAD001 (Everolimus), a rapamycin derivative, dramatically enhances cisplatin-induced apoptosis in wild-type p53 but not mutant p53 tumor cells by inhibiting p53-induced p21 expression . Wangpaichitr et al., 2008 demonstrated that inhibition of mTOR by CCI-779 decreased levels of the anti-apoptotic proteins, BCL2/BCLxL, and increasing apoptosis in lung cancer cells that is resistance to cisplatin . Although these models provide important evidence for mTOR inhibition and cisplatin synergism in cancer cells, it fails to explain the specific synergism we observed in basal-like breast cancer cells, as the basal-like cells that we tested are p53 mutated and do not express high level of BCL2/BCLxL (data not shown). This led us to postulate that a common signal transduction pathway inhibited by rapamycin may be an important component that synergizes cisplatin sensitivity in basal-like cells. Since p73 has been reported to mediate cisplatin sensitivity in a subset of triple-negative breast cancer cells  and that inhibition of mTOR by rapamycin or RNAi lead to upregulation of p73 , we postulated that activation of the p73 pathways might be important for the synergistic effects of rapamycin.
To test the role of p73 in rapamycin and cisplatin synergism, we first evaluated the expression of p73 mRNA and protein levels following treatment with cisplatin or rapamycin alone or in combination in MDA-MB-231 cells. Consistent with previous studies, treatment of cells with cisplatin or rapamycin alone induces p73 protein expression followed by transcriptional activation of the 2 potent pro-apoptotic p73 target genes, PUMA and NOXA. When MDA-MB-231 cells were co-treated with rapamycin and cisplatin, the elevation of p73 and its pro-apoptotic target genes were synergistically enhanced. The observed changes in p73 protein in MDA-MB-231 cells, however, were not due to parallel changes in p73 RNA levels, suggesting that inhibition of mTOR might lead to inactivation of a yet unknown p73 specific protein degradation pathway.
To validate that the rapamycin and cisplatin synergism is mediated by p73, we generated isogenic MDA-MB-231 and MDA-MB-468 cells that were depleted for p73 using a lentiviral-shRNA that target specifically the transactivating isoform of p73 (TAp73). Indeed, depletion of TAp73 in MDA-MB-231 and MDA-MB-468 cells completely abrogated the synergistic effects of rapamycin suggesting that the synergism between rapamycin and cisplatin required p73 function.
Although the combination of cisplatin and rapamycin has not been previously investigated in clinical study, it is worth noting that a phase II neo-adjuvant clinical trial of cisplatin and RAD001 (Everolimus), in patients with triple-negative breast cancers has recently open for recruitment (ClinicalTrials.gov Identifier: NCT00930930), and will be able to address the potential of cisplatin and mTOR inhibitors combination therapy directly. It would be equally intriguing to determine the role of p73 related pathway as potential biomarkers that might predict response to treatment given the pivotal role of p73 in the synergistic effects of mTOR inhibition and cisplatin sensitivity. In conclusion, combination of mTOR inhibitors and cisplatin may be a useful therapeutic strategy in basal-like breast cancers.
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