Low-molecular contrast agent dynamic contrast-enhanced (DCE)-MRI and diffusion-weighted (DW)-MRI in early assessment of bevacizumab treatment in breast cancer xenografts.

PURPOSE: To investigate the effect of bevacizumab treatment on vascular architecture and function in two xenograft models with different angiogenic properties using diffusion-weighted magnetic resonance imaging (DW-MRI) and dynamic contrast-enhanced MRI (DCE-MRI). MATERIALS AND METHODS: Mice carrying basal-like (MAS98.12) or luminal-like (MAS98.06) orthotopic breast cancer xenografts were treated with bevacizumab (5 mg/kg), doxorubicin (8 mg/kg), or both drugs in combination. DW-MRI and DCE-MRI were performed before and 3 days after treatment using a Bruker 7T preclinical scanner. Mean microvessel density (MVD) and proliferating microvessel density (pMVD) in the tumors were determined for evaluation of vascular response to bevacizumab treatment. RESULTS: No changes in DCE-MRI or DW-MRI parameters were observed in untreated controls during the experiment period. DW-MRI showed increased apparent diffusion coefficient (ADC) values in all treatment groups in both basal-like and luminal-like xenografts. DCE-MRI showed increased contrast agent uptake, particularly in central regions of the tumors, after bevacizumab/combination treatment in both xenograft models. This was accompanied by decreased MVD and pMVD in basal-like xenografts. Doxorubicin treatment had no effect on DCE-MRI parameters in any of the xenograft models. CONCLUSION: Both DW-MRI and DCE-MRI demonstrated an early response to bevacizumab treatment in the xenograft tumors. Increased contrast agent uptake and reduced MVD/pMVD is consistent with a normalization of vascular function.

In vivo MRI and histopathological assessment of tumor microenvironment in luminal-like and basal-like breast cancer xenografts.

PURPOSE: To explore tumor pathophysiology with special attention to the microenvironment in two molecular subtypes of human breast cancer using in vivo magnetic resonance imaging (MRI) and histopathology. The impact of tumor growth, size, and the influence of estradiol were also investigated. MATERIALS AND METHODS: Two orthotopic and directly transplanted human breast cancer models representing luminal-like and basal-like molecular subtypes were characterized by dynamic contrast-enhanced MRI and diffusion-weighted MRI. Ex vivo measurements of vascularization, hypoxia, mitoses, and the level of VEGF activations were associated with the calculated in vivo MRI parameters of the tumors. RESULTS: The vascular permeability and perfusion (K(trans) ) was significantly higher in basal-like compared to luminal-like tumors. These findings were confirmed by a 4-fold higher proliferating microvessel density (pMVD) in basal-like tumors, reflecting the difference in aggressiveness between the subtypes. No effect of tumor growth was observed during 6 days of growth in any of the models; however, large tumors had lower K(trans) , higher extracellular extravascular volume fraction (v(e) ), and more hypoxia than medium-sized tumors. Estradiol withdrawal induced increased K(trans) , v(e) , and tumor water diffusion (ADC) in luminal-like tumors, corresponding to increased VEGFR2 activation, which is likely to cause increased tumor vessel permeability. CONCLUSION: These novel data confirm the potential of functional MRI methods to map histopathologically proven changes in breast tumor vasculature and microenvironment in vivo.

Distinct choline metabolic profiles are associated with differences in gene expression for basal-like and luminal-like breast cancer xenograft models.

BACKGROUND: Increased concentrations of choline-containing compounds are frequently observed in breast carcinomas, and may serve as biomarkers for both diagnostic and treatment monitoring purposes. However, underlying mechanisms for the abnormal choline metabolism are poorly understood. METHODS: The concentrations of choline-derived metabolites were determined in xenografted primary human breast carcinomas, representing basal-like and luminal-like subtypes. Quantification of metabolites in fresh frozen tissue was performed using high-resolution magic angle spinning magnetic resonance spectroscopy (HR MAS MRS). The expression of genes involved in phosphatidylcholine (PtdCho) metabolism was retrieved from whole genome expression microarray analyses. The metabolite profiles from xenografts were compared with profiles from human breast cancer, sampled from patients with estrogen/progesterone receptor positive (ER+/PgR+) or triple negative (ER-/PgR-/HER2-) breast cancer. RESULTS: In basal-like xenografts, glycerophosphocholine (GPC) concentrations were higher than phosphocholine (PCho) concentrations, whereas this pattern was reversed in luminal-like xenografts. These differences may be explained by lower choline kinase (CHKA, CHKB) expression as well as higher PtdCho degradation mediated by higher expression of phospholipase A2 group 4A (PLA2G4A) and phospholipase B1 (PLB1) in the basal-like model. The glycine concentration was higher in the basal-like model. Although glycine could be derived from energy metabolism pathways, the gene expression data suggested a metabolic shift from PtdCho synthesis to glycine formation in basal-like xenografts. In agreement with results from the xenograft models, tissue samples from triple negative breast carcinomas had higher GPC/PCho ratio than samples from ER+/PgR+ carcinomas, suggesting that the choline metabolism in the experimental models is representative for luminal-like and basal-like human breast cancer. CONCLUSIONS: The differences in choline metabolite concentrations corresponded well with differences in gene expression, demonstrating distinct metabolic profiles in the xenograft models representing basal-like and luminal-like breast cancer. The same characteristics of choline metabolite profiles were also observed in patient material from ER+/PgR+ and triple-negative breast cancer, suggesting that the xenografts are relevant model systems for studies of choline metabolism in luminal-like and basal-like breast cancer.

Proteomic characterization of breast cancer xenografts identifies early and late bevacizumab-induced responses and predicts effective drug combinations.

PURPOSE: Neoangiogenesis is an important feature in tumor growth and progression, and combining chemotherapy and antiangiogenic drugs have shown clinical efficacy. However, as treatment-induced resistance often develops, our goal was to identify pathways indicating response and/or evolving resistance to treatment and inhibit these pathways to optimize the treatment strategies. EXPERIMENTAL DESIGN: To identify markers of response and/or resistance, reverse-phase protein array (RPPA) was used to characterize treatment-induced changes in a bevacizumab-responsive and a nonresponsive human breast cancer xenograft. Results were combined with bioinformatic modeling to predict druggable targets for optimization of the treatment. RESULTS: RPPA analysis showed that both tumor models responded to bevacizumab with an early (day 3) upregulation of growth factor receptors and downstream signaling pathways, with persistent mTOR signaling until the end of the in vivo experiment. Adding doxorubicin to bevacizumab showed significant and superior growth inhibition of basal-like tumors, whereas no additive effect was seen in the luminal-like model. The combination treatment corresponded to a continuous late attenuation of mTOR signaling in the basal-like model, whereas the inhibition was temporary in the luminal-like model. Integrating the bevacizumab-induced dynamic changes in protein levels with bioinformatic modeling predicted inhibition of phosphoinositide 3-kinase (PI3K) pathway to increase the efficacy of bevacizumab monotherapy. In vivo experiments combining bevacizumab and the PI3K/mTOR inhibitor BEZ235 confirmed their significant and additive growth-inhibitory effect in the basal-like model. CONCLUSIONS: Treatment with bevacizumab caused compensatory upregulation of several signaling pathways. Targeting such pathways increased the efficacy of antiangiogenic therapy.

Subtype-specific response to bevacizumab is reflected in the metabolome and transcriptome of breast cancer xenografts.

Antiangiogenic therapy with bevacizumab has shown varying results in breast cancer clinical trials. Identifying robust biomarkers for selecting patients who may benefit from such treatment and for monitoring response is important for the future use of bevacizumab. Two established xenograft models representing basal-like and luminal-like breast cancer were used to study bevacizumab treatment response on the metabolic and gene expression levels. Tumor samples were obtained from mice treated with bevacizumab, doxorubicin or a combination of the two drugs, and high resolution magic angle spinning magnetic resonance spectroscopy and gene expression microarray analysis was performed. Combination treatment with bevacizumab showed the strongest growth inhibiting effect in basal-like tumors, and this was reflected by a significant change in the metabolomic and transcriptomic profiles. In the luminal-like xenografts, addition of bevacizumab did not improve the effect of doxorubicin. On the global transcriptomic level, the largest gene expression changes were observed for the most efficient treatment in both models. Glycerophosphocholine showed opposite response in the treated xenografts compared with untreated controls; lower in basal-like and higher in luminal-like tumors. Comparing combination therapy with doxorubicin monotherapy in basal-like xenografts, 14 genes showed significant differential expression, including very low density lipoprotein receptor (VLDLR) and hemoglobin, theta 1 (HBQ1). Bevacizumab-treated tumors were associated with a more hypoxic phenotype, while no evidence was found for associations between bevacizumab treatment and vascular invasion or tumor grade. This study underlines the importance of characterizing biological differences between subtypes of breast cancer to identify personalized biomarkers for improved patient stratification and evaluation of response to therapy.

Effect of antiangiogenic therapy on tumor growth, vasculature and kinase activity in basal- and luminal-like breast cancer xenografts.

Several clinical trials have investigated the efficacy of bevacizumab in breast cancer, and even if growth inhibiting effects have been registered when antiangiogenic treatment is given in combination with chemotherapy no gain in overall survival has been observed. One reason for the lack of overall survival benefit might be that appropriate criteria for selection of patients likely to respond to antiangiogenic therapy in combination with chemotherapy, are not available. To determine factors of importance for antiangiogenic treatment response and/or resistance, two representative human basal- and luminal-like breast cancer xenografts were treated with bevacizumab and doxorubicin alone or in combination. In vivo growth inhibition, microvessel density (MVD) and proliferating tumor vessels (pMVD = proliferative microvessel density) were analysed, while kinase activity was determined using the PamChip Tyrosine kinase microarray system. Results showed that both doxorubicin and bevacizumab inhibited basal-like tumor growth significantly, but with a superior effect when given in combination. In contrast, doxorubicin inhibited luminal-like tumor growth most effectively, and with no additional benefit of adding antiangiogenic therapy. In agreement with the growth inhibition data, vascular characterization verified a more pronounced effect of the antiangiogenic treatment in the basal-like compared to the luminal-like tumors, demonstrating total inhibition of pMVD and a significant reduction in MVD at early time points (three days after treatment) and sustained inhibitory effects until the end of the experiment (day 18). In contrast, luminal-like tumors only showed significant effect on the vasculature at day 10 in the tumors having received both doxorubicin and bevacizumab. Kinase activity profiling in both tumor models demonstrated that the most effective treatment in vivo was accompanied with increased phosphorylation of kinase substrates of growth control and angiogenesis, like EGFR, VEGFR2 and PLCγ1. This may be a result of regulatory feedback mechanisms contributing to treatment resistance, and may suggest response markers of value for the prediction of antiangiogenic treatment efficacy.