Direct visualization of heterogeneous extravascular distribution of trastuzumab in human epidermal growth factor receptor type 2 overexpressing xenografts

PURPOSE

The high molecular weight and binding affinity of trastuzumab, a monoclonal antibody in use for treatment of breast cancers overexpressing human epidermal growth factor receptor type 2 (HER2), in combination with microenvironmental factors, may limit its distribution and efficacy. We assessed and mapped the distribution of systemically given, unlabeled trastuzumab at micrometer resolution in tumor xenografts using immunohistochemistry.

EXPERIMENTAL DESIGN

Mice bearing MDA-435/LCC6(HER2) xenografts were given single doses of 4 or 20 mg/kg unlabeled trastuzumab with tumor harvest at various time points thereafter; bound trastuzumab was imaged directly in tumor cryosections using fluorescently tagged antihuman secondary antibodies. Combinations of additional markers, including HER2, 5-bromo-2-deoxyuridine, CD31, DioC(7)(3), desmin, and collagen IV were also mapped on the same tumor sections.

RESULTS

Distribution of trastuzumab in MDA-435/LCC6(HER2) tumors is found to be heterogeneous, with tumor margins saturating more thoroughly in doses and times analyzed. Considerable intervessel heterogeneity is also seen. For example, in unsaturated tissues, there remain perfused vessels without any trastuzumab in addition to vessels with a few layers of positively stained perivascular cells, in addition to vessels with bound drug up to 150 microm away. This heterogeneity is independent of HER2 expression, microvessel density, and perfusion. A slightly greater proportion of vessels were associated with pericytes in sections with greater trastuzumab saturation, but this would not adequately account for observed heterogeneous trastuzumab distribution.

CONCLUSIONS

Complete penetration of trastuzumab in tumor tissue was not seen in our study, leaving the possibility that inadequate distribution may represent a mechanism for resistance to trastuzumab.

Suppression of VEGF secretion and changes in glioblastoma multiforme microenvironment by inhibition of integrin-linked kinase (ILK)

Integrin-linked kinase (ILK) was assesed as a therapeutic target in glioblastoma xenograft models through multiple endpoints including treatment related changes in the tumor microenvironment. Glioblastoma cell lines were tested in vitro for sensitivity toward the small-molecule inhibitors QLT0254 and QLT0267. Cell viability, cell cycle, and apoptosis were evaluated using MTT assay, flow cytometry, caspase activation, and DAPI staining. Western blotting and ELISA were used for protein analysis (ILK, PKB/Akt, VEGF, and HIF-1alpha). In vivo assessment of growth rate, cell proliferation, BrdUrd, blood vessel mass (CD31 labeling), vessel perfusion (Hoechst 33342), and hypoxia (EF-5) was done using U87MG glioblastoma xenografts in RAG2-M mice treated orally with QLT0267 (200 mg/kg q.d.). ILK inhibition in vitro with QLT0254 and QLT0267 resulted in decreased levels of phospho-PKB/Akt (Ser473), secreted VEGF, G2-M block, and apoptosis induction. Mice treated with QLT0267 exhibited significant delays in tumor growth (treated 213 mm3 versus control 549 mm3). In situ analysis of U87MG tumor cell proliferation from QLT0267-treated mice was significantly lower relative to untreated mice. Importantly, VEGF and HIF-1alpha expression decreased in QLT0267-treated tumors as did the percentage of blood vessel mass and numbers of Hoechst 33342 perfused tumor vessels compared with control tumors (35% versus 83%). ILK inhibition with novel small-molecule inhibitors leads to treatment-associated delays in tumor growth, decreased tumor angiogenesis, and functionality of tumor vasculature. The therapeutic effects of a selected ILK inhibitor (QLT0267) should be determined in the clinic in cancers that exhibit dysregulated ILK, such as PTEN-null glioblastomas.

Limited tissue penetration of taxanes: a mechanism for resistance in solid tumors

PURPOSE

Limited drug penetration in solid tumors is a potential mechanism of resistance for many anticancer drugs. Taxanes represent a class of drugs that are currently undergoing a new round of development, but with little known of their ability to penetrate and distribute relative to blood vessels within solid tumors.

EXPERIMENTAL DESIGN

We assessed the tissue penetration of paclitaxel and docetaxel in HCT-116 tumor xenografts and in multilayered cell culture (MCC), a three-dimensional cell culture model of the tumor extravascular compartment. In xenografts, taxanes were mapped relative to blood vessels to obtain drug profiles as a function of distance from vasculature. For MCC, cultures were exposed to stirred drug reservoirs and taxanes measured as a function of depth into tissue.

RESULTS

Both taxanes exhibited limited penetration, with little drug reaching further than 100 microm into the tissue. Of the two, paclitaxel exhibited up to 2-fold greater penetration than docetaxel. Mapping tumor cell proliferation following treatment allowed the consequences of limited drug penetration to be assessed. In tumor xenografts where reduced drug exposure to cells far from vasculature is one of several factors influencing response to treatment, up to a 75% reduction in S-phase cells was achieved in cells nearest the vessels, but only 50% reduction was observed in the tissue 150 microm away. In MCC-based data, where the influence of reduced cell proliferation with depth into tissue was circumvented, a 5-fold (paclitaxel) and 10-fold (docetaxel) increase in reservoir drug concentration was required to produce a response in cells 150 microm into the tissue equivalent to that seen in cells directly exposed to the drug.

CONCLUSION

These results indicate that limited distribution is an important mechanism of tumor resistance to taxanes.

Decreased levels of hypoxic cells in gefitinib treated ER+ HER-2 overexpressing MCF-7 breast cancer tumors are associated with hyperactivation of the mTOR pathway: therapeutic implications for combination therapy with rapamycin

Developing novel synergistic and more effective combination treatments is necessary for better management of breast cancer in the clinic. It is established that HER-2 overexpressing breast cancers are sensitive to the HER-1 (epidermal growth factor receptor (EGFR)) inhibitor gefitinib, but that this targeted agent produces only moderate therapeutic effects in vivo. Here, we use a model of ER(+) HER-2 overexpressing MCF-7 breast cancer (MCF-7(HER-2)) to identify, as broadly as possible, the in vivo microenvironmental and molecular therapeutic responses to gefitinib to predict a therapeutically viable target for gefitinib-based combination treatment. Our data show a link between in vivo reductions in tumor hypoxia (3-fold decrease, P = 0.002) and elevated activity of the mTOR pathway (3.8-fold increase in phospho-p70-S6K protein, P = 0.006) in gefitinib treated MCF-7(HER-2) tumors. Despite decreased levels of phosphorylated EGFR, HER-2 and Erk1/2 (P = 0.081, 0.005 and 0.034, respectively) the expression of phospho-AKT was not reduced in MCF-7(HER-2) tumors after gefitinib treatment. Levels of ERalpha receptor were, however, 1.8-fold higher in gefitinib treated compared to control tumors (P = 0.008). Based on these results we predict that gefitinib activity against ER(+) HER-2 overexpressing EGFR co-expressing breast cancers should be enhanced if used with agents that target the mTOR pathway. In vitro studies using MCF-7(HER-2) and BT474 breast cancer cells exposed to gefitinib and rapamycin in combination show that this combination produced significantly greater growth inhibitory effects than either of the drugs alone. Chou and Talalay analysis of the data suggested that combination of gefitinib and rapamycin was synergistic (CI < 1) at a number of selected drug ratios and over a broad range of effective doses.

Vascular-specific quantification in an in vivo Matrigel chamber angiogenesis assay

The study of angiogenesis as a therapeutic target requires reliable in vivo assays that can provide physiologically relevant data. A murine in vivo Matrigel-based angiogenesis assay is presented here which includes the quantitative assessment of vascular-specific indicators of neovascularization. Matrigel containing 175 ng/ml bFGF is encapsulated in synthetic chambers which are implanted subcutaneously in C57/B16J mice. Ex vivo implants can be imaged to qualitatively view perfused vasculature within the chambers, or histologically processed to confirm the presence of vascular-specific tissue within the Matrigel. Viable cells are recovered from the excised chambers and quantified cytometrically using endothelial cell-specific markers CD34 and CD144, and for a marker of nucleated cells, Hoechst 33342. Thalidomide, 200 mg/kg/day, was tested using the assay and was found to inhibit angiogenesis by 46%. Angiogenesis inhibitors secreted by LL/M27 tumors were also characterized, where tumor-bearing mice showed a 73% inhibition of angiogenesis compared to tumor-free controls. Analysis of the number of nucleated cells in these samples failed to show a strong correlation with the number of endothelial cells, indicating that quantification of nonvascular-specific tissue in in vivo angiogenesis assays may not be sufficient. This new assay provides an objective, comprehensive determination of the vasculature-specific response of both endogenous and exogenous angiogenesis inhibitors in vivo, and also creates new opportunities for obtaining primary murine endothelial cells.

Tirapazamine causes vascular dysfunction in HCT-116 tumour xenografts

BACKGROUND AND PURPOSE

Tirapazamine is a hypoxic cytotoxin currently undergoing Phase II/III clinical evaluation in combination with radiation and chemotherapeutics for the treatment of non-hematological cancers. Tissue penetration studies using multicellular models have suggested that tirapazamine exposure may be limited to cells close to blood vessels. However, animal studies show tirapazamine enhances the anti-tumour activity of radiation and chemotherapy and clinical studies with tirapazamine, so far, are promising. To investigate this apparent paradox we examined the microregional effects of tirapazamine in vivo by mapping drug effects with respect to the position of blood vessels in tumour cryosections.

PATIENTS AND METHODS

Tirapazamine was administered i.p. to mice bearing HCT-116 tumours, which were excised at various times after treatment. Images of multiple-stained cryosections were overlaid to provide microregional information on the relative position of proliferating cells, hypoxia, perfusion and vasculature.

RESULTS

We observed extensive and permanent vascular dysfunction in a large proportion of tumours from mice treated with tirapazamine. In the affected tumours, blood flow ceased in the centrally located tumour vessels, leaving a rim of functional vessels around the periphery of the tumour. This vascular dysfunction commenced within 24 h after tirapazamine administration and the areas affected appeared to be replaced by necrosis over the following 24-48 h.

CONCLUSIONS

Because the majority of hypoxic cells are located in the center of tumours we propose that the activity of tirapazamine in vivo may be related to its effects on tumour vasculature and that its activity against hypoxic cells located distal to functional blood vessels may not be as important as previously believed.

Microregional effects of gemcitabine in HCT-116 xenografts

To examine the tumor microregional effects after gemcitabine administration to mice, we mapped the location of proliferating and hypoxic cells relative to vasculature in human colon cancer xenografts. The S-phase marker bromodeoxyuridine was used as a surrogate of drug effect and administered 2 hours before tumor excision, whereas vessel position and perfusion were assessed via staining for CD31 and intravenous injection of carbocyanine, respectively. Hypoxia was detected using pimonidazole. Images of the four markers were overlaid to reveal the spatial relationship between proliferation, vasculature, and hypoxia and to examine the microregional effects. Within 1 day after administration of 240 mg/kg of gemcitabine, proliferation throughout the tumor was completely inhibited. Over time, a reemergence of dividing cells occurred in relation to the distance from vasculature. Microregional analysis revealed that cells located distal to vasculature commenced cycling sooner than cells located proximal to vasculature. A similar trend was seen after multiple doses of gemcitabine (40 mg/kg on days 1, 4, 7, and 10). The possibility that the effect of gemcitabine could be attributed to changes in oxygenation was discounted after examining the vessel perfusion and patterns of hypoxia. The effect of gemcitabine was examined in multilayered cell culture, and at doses <30 micromol/L, a gradient in proliferation between the exposed and unexposed sides was observed. We show a differential effect on cell proliferation in relation to vasculature and conclude that cells distal to blood vessels are less affected by gemcitabine probably because of limited penetration.

HER-2/neu Overexpression Increases the Viable Hypoxic Cell Population within Solid Tumors without Causing Changes in Tumor Vascularization

The effects of HER-2/neu overexpression on the tumor microenvironment in an aggressive breast cancer xenograft model were investigated. These studies focused on tumors derived following the subcutaneous injection of MDA-MB-435/LCC6 cells transfected with human c-erbB2 (LCC6HER-2) into SCID-Rag2M mice. LCC6HER-2 tumors were more viable (H&E-stained tumor sections) than isogenic vector control tumors (LCC6Vector). Correspondingly, a 2.7-fold increase in trypan blue–excluding cells (P = 0.00056) and a 4.8-fold increase in clonogenic cells (P = 0.00146) were noted in cell suspensions derived from disaggregated LCC6HER-2 versus LCC6Vector tumors. Tumor sections stained with the antibody detecting 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide (EF5), a marker of hypoxia, showed a greater fraction of hypoxic tissue in LCC6HER-2 tumors compared with control tumors. Flow cytometric analyses based on viable tumor cells (DNA content ≥ 2N) in cell suspensions from disaggregated tumors confirmed that there were significantly more EF5-positive cells (i.e., hypoxic) in LCC6HER-2 than in LCC6Vector tumors (16.41 ± 8.1% and 5.96 ± 4.1%, respectively; P = 0.0015). Protein levels of phosphorylated (Ser536) nuclear factor-κB p65 were significantly elevated in LCC6HER-2 tumors (P = 0.00048), and a trend in increased hypoxia-inducible factor-1α protein levels was observed in LCC6HER-2 compared with LCC6Vector tumors. Despite the substantial viable hypoxic cell fraction and a 1.7-fold increase of vascular endothelial growth factor protein (P = 0.05) in LCC6HER-2 tumors, no significant differences were found (P &gt; 0.05) between LCC6HER-2 and LCC6Vector vasculature (CD31 staining and Hoechst 33342 perfusion). These results suggest that HER-2/neu overexpression may be linked with overall increased tumor viability and a significant increase in the population of viable hypoxic cells, which is not due to differences in tumor vascularization.

HER-2/neu overexpression increases the viable hypoxic cell population within solid tumors without causing changes in tumor vascularization

The effects of HER-2/neu overexpression on the tumor microenvironment in an aggressive breast cancer xenograft model were investigated. These studies focused on tumors derived following the subcutaneous injection of MDA-MB-435/LCC6 cells transfected with human c-erbB2 (LCC6(HER-2)) into SCID-Rag2M mice. LCC6(HER-2) tumors were more viable (H&E-stained tumor sections) than isogenic vector control tumors (LCC6(Vector)). Correspondingly, a 2.7-fold increase in trypan blue-excluding cells (P = 0.00056) and a 4.8-fold increase in clonogenic cells (P = 0.00146) were noted in cell suspensions derived from disaggregated LCC6(HER-2) versus LCC6(Vector) tumors. Tumor sections stained with the antibody detecting 2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)-acetamide (EF5), a marker of hypoxia, showed a greater fraction of hypoxic tissue in LCC6(HER-2) tumors compared with control tumors. Flow cytometric analyses based on viable tumor cells (DNA content >/= 2N) in cell suspensions from disaggregated tumors confirmed that there were significantly more EF5-positive cells (i.e., hypoxic) in LCC6(HER-2) than in LCC6(Vector) tumors (16.41 +/- 8.1% and 5.96 +/- 4.1%, respectively; P = 0.0015). Protein levels of phosphorylated (Ser(536)) nuclear factor-kappaB p65 were significantly elevated in LCC6(HER-2) tumors (P = 0.00048), and a trend in increased hypoxia-inducible factor-1alpha protein levels was observed in LCC6(HER-2) compared with LCC6(Vector) tumors. Despite the substantial viable hypoxic cell fraction and a 1.7-fold increase of vascular endothelial growth factor protein (P = 0.05) in LCC6(HER-2) tumors, no significant differences were found (P > 0.05) between LCC6(HER-2) and LCC6(Vector) vasculature (CD31 staining and Hoechst 33342 perfusion). These results suggest that HER-2/neu overexpression may be linked with overall increased tumor viability and a significant increase in the population of viable hypoxic cells, which is not due to differences in tumor vascularization.

Direct assessment of drug penetration into tissue using a novel application of three-dimensional cell culture

The failure of many anticancer drugs to control growth of solid cancers may stem in part from inadequate delivery to tumor regions distant from vasculature. Although the identification of new anticancer drug targets has led to the development of many new drug candidates, there is a lack of methodology for identifying drugs that adequately penetrate tumor tissue. We have developed a novel multilayered cell culture-based assay, which detects the penetration of anticancer drugs based on their effect within tissue. Drug exposures are made over 1 hour to one side of a disk of tissue approximately 150-microm thick, with the other side temporarily closed off, and penetration is then assessed 1-3 days later via bromodeoxyuridine-based detection of S-phase cells. Using this assay, the tissue distribution of a selection of anthracycline analogues was assessed. At clinically relevant exposures, none of the agents were able to affect cells on the far side of the culture at levels approaching that seen on the near (exposed) side. Doxorubicin and epirubicin exhibited approximately 10-fold decreases in the drug exposure seen by the cells on the far side relative to those on the near side of the cultures, whereas for daunorubicin and mitoxantrone, approximately 30-fold and >30-fold decreases were observed respectively. Results were consistent with the observed gradients in drug-derived fluorescence of doxorubicin, epirubicin, and daunorubicin. This model could be applied as a simple anticancer drug development screen to discover drugs that exhibit desirable penetration properties.

Tumor distribution of bromodeoxyuridine-labeled cells is strongly dose dependent

Bromodeoxyuridine (BrdUrd) is used extensively to measure the fraction of proliferating cells in tumors. Unlike endogenous markers of proliferation such as proliferating cell nuclear antigen (PCNA) and Ki-67, BrdUrd is exogenously administered and reaches the tumor via vasculature where it must then diffuse throughout the tissue to label S-phase cells. In this study, we examine the dose dependence of BrdUrd on the tumor distribution of labeled cells in histological sections. Analysis of the distribution of labeled cells in SiHa tumor xenografts showed that a dose between 400 and 1000 mg/kg was required to label cells 150 micro m from blood vessels, approaching the border of necrosis. Lower doses resulted in only the cells close to blood vessels being labeled. Interestingly, cells residing furthest from blood vessels still labeled albeit at half the level of cells situated proximal to the tumor vasculature. Results were compared with the penetration of BrdUrd seen in vitro using multilayered cell culture (MCC), a three-dimensional tissue culture model of solid tumors. Using MCC, an exposure of 100 micro M BrdUrd for 1 h was required for labeling of S-phase cells 150 micro m into the tissue, whereas cells adjacent to the edge of the tissue could be adequately labeled with just 5 micro M BrdUrd for 1 h. The area under the curve for a 100 mg/kg BrdUrd dose in mice was found to be approximately 30 micro M x h.