Detection of FLASH-radiotherapy tissue sparing in a 3D-spheroid model using DNA damage response markers

PURPOSE

The oxygen depletion hypothesis has been proposed as a rationale to explain the observed phenomenon of FLASH-radiotherapy (FLASH-RT) sparing normal tissues while simultaneously maintaining tumor control. In this study we examined the distribution of DNA Damage Response (DDR) markers in irradiated 3D multicellular spheroids to explore the relationship between FLASH-RT protection and radiolytic-oxygen-consumption (ROC) in tissues.

METHODS

Studies were performed using a Varian Truebeam linear accelerator delivering 10 MeV electrons with an average dose rate above 50 Gy/s. Irradiations were carried out on 3D spheroids maintained under a range of O2 and temperature conditions to control O2 consumption and create gradients representative of in vivo tissues.

RESULTS

Staining for pDNA-PK (Ser2056) produced a linear radiation dose response whereas γH2AX (Ser139) showed saturation with increasing dose. Using the pDNA-PK staining, radiation response was then characterised for FLASH compared to standard-dose-rates as a function of depth into the spheroids. At 4 °C, chosen to minimize the development of metabolic oxygen gradients within the tissues, FLASH protection could be observed at all distances under oxygen conditions of 0.3-1 % O2. Whereas at 37 °C a FLASH-protective effect was limited to the outer cell layers of tissues, an effect only observed at 3 % O2. Modelling of changes in the pDNA-PK-based oxygen enhancement ratio (OER) yielded a tissue ROC g0-value estimate of 0.73 ± 0.25 µM/Gy with a km of 5.4 µM at FLASH dose rates.

CONCLUSIONS

DNA damage response markers are sensitive to the effects of transient oxygen depletion during FLASH radiotherapy. Findings support the rationale that well-oxygenated tissues would benefit more from FLASH-dose-rate protection relative to poorly-oxygenated tissues.

DNA-PK inhibition extends the therapeutic effects of Top2 poisoning to non-proliferating cells, increasing activity at a cost

Type II topoisomerase (Top2) poisoning therapy is used to treat a broad range of cancers via induction of double strand breaks (DSBs) in cells undergoing replication and transcription. Preventing the repair of DSBs via inhibition of DNA-PK, an inhibitor of non-homologous end-joining (NHEJ), increases cell kill with Top2 poisons and has led to the initiation of several clinical trials. To elucidate the cellular mechanisms leading to synergistic activity of dual DNA-PK/Top2 inhibition we looked at their effects in cycling versus non-cycling cells, in 3D spheroids and in xenograft models. Combined DNA-PK/Top2 inhibition was found to not only increase the cell kill in proliferating cells, the cell population that is typically most vulnerable to Top2 poisoning, but also in non-proliferative but transcriptionally active cells. This effect was observed in both cancer and normal tissue models, killing more cells than high concentrations of etoposide alone. The combination treatment delayed tumor growth in mice compared to Top2 poisoning alone, but also led to increased toxicity. These findings demonstrate sensitization of Top2β-expressing, non-cycling cells to Top2 poisoning by DNA-PK inhibition. Expansion of the target cell population of Top2 poison treatment to include non-proliferating cells via combination with DNA damage repair inhibitors has implications for efficacy and toxicity of these combinations, including for inhibitors of DNA-PK currently in clinical trial.

Targeting Hypoxia-Induced Carbonic Anhydrase IX Enhances Immune-Checkpoint Blockade Locally and Systemically

Treatment strategies involving immune-checkpoint blockade (ICB) have significantly improved survival for a subset of patients across a broad spectrum of advanced solid cancers. Despite this, considerable room for improving response rates remains. The tumor microenvironment (TME) is a hurdle to immune function, as the altered metabolism-related acidic microenvironment of solid tumors decreases immune activity. Here, we determined that expression of the hypoxia-induced, cell-surface pH regulatory enzyme carbonic anhydrase IX (CAIX) is associated with worse overall survival in a cohort of 449 patients with melanoma. We found that targeting CAIX with the small-molecule SLC-0111 reduced glycolytic metabolism of tumor cells and extracellular acidification, resulting in increased immune cell killing. SLC-0111 treatment in combination with immune-checkpoint inhibitors led to the sensitization of tumors to ICB, which led to an enhanced Th1 response, decreased tumor growth, and reduced metastasis. We identified that increased expression of CA9 is associated with a reduced Th1 response in metastatic melanoma and basal-like breast cancer TCGA cohorts. These data suggest that targeting CAIX in the TME in combination with ICB is a potential therapeutic strategy for enhancing response and survival in patients with hypoxic solid malignancies.

Multi-modal magnetic resonance imaging and histology of vascular function in xenografts using macromolecular contrast agent hyperbranched polyglycerol (HPG-GdF)

Macromolecular gadolinium (Gd)-based contrast agents are in development as blood pool markers for MRI. HPG-GdF is a 583 kDa hyperbranched polyglycerol doubly tagged with Gd and Alexa 647 nm dye, making it both MR and histologically visible. In this study we examined the location of HPG-GdF in whole-tumor xenograft sections matched to in vivo DCE-MR images of both HPG-GdF and Gadovist. Despite its large size, we have shown that HPG-GdF extravasates from some tumor vessels and accumulates over time, but does not distribute beyond a few cell diameters from vessels. Fractional plasma volume (fPV) and apparent permeability-surface area product (aPS) parameters were derived from the MR concentration-time curves of HPG-GdF. Non-viable necrotic tumor tissue was excluded from the analysis by applying a novel bolus arrival time (BAT) algorithm to all voxels. aPS derived from HPG-GdF was the only MR parameter to identify a difference in vascular function between HCT116 and HT29 colorectal tumors. This study is the first to relate low and high molecular weight contrast agents with matched whole-tumor histological sections. These detailed comparisons identified tumor regions that appear distinct from each other using the HPG-GdF biomarkers related to perfusion and vessel leakiness, while Gadovist-imaged parameter measures in the same regions were unable to detect variation in vascular function. We have established HPG-GdF as a biocompatible multi-modal high molecular weight contrast agent with application for examining vascular function in both MR and histological modalities.

Loss of the Notch effector RBPJ promotes tumorigenesis

Kulic et al. show that RBPJ, a transcriptional repressor of Notch, is frequently deleted in human cancers and can function as a tumor suppressor. Loss of RBPJ acts to derepress target gene promoters, allowing Notch-independent activation by alternate transcription factors that promote tumor growth.

Aberrant Notch activity is oncogenic in several malignancies, but it is unclear how expression or function of downstream elements in the Notch pathway affects tumor growth. Transcriptional regulation by Notch is dependent on interaction with the DNA-binding transcriptional repressor, RBPJ, and consequent derepression or activation of associated gene promoters. We show here that RBPJ is frequently depleted in human tumors. Depletion of RBPJ in human cancer cell lines xenografted into immunodeficient mice resulted in activation of canonical Notch target genes, and accelerated tumor growth secondary to reduced cell death. Global analysis of activated regions of the genome, as defined by differential acetylation of histone H4 (H4ac), revealed that the cell death pathway was significantly dysregulated in RBPJ-depleted tumors. Analysis of transcription factor binding data identified several transcriptional activators that bind promoters with differential H4ac in RBPJ-depleted cells. Functional studies demonstrated that NF-κB and MYC were essential for survival of RBPJ-depleted cells. Thus, loss of RBPJ derepresses target gene promoters, allowing Notch-independent activation by alternate transcription factors that promote tumorigenesis.

Abstract 2988: Microenvironmental distribution of trastuzumab is heterogeneous and decreases sharply when administered following a single dose of bevacizumab in Her2+ve xenografts and metastases models

We have previously shown that trastuzumab distributes heterogeneously within MDA-435-Her2 xenografts. Though peak accumulation occurs 24h after administration & trastuzumab was detected greater than 150 µm from many vessels, there remained substantial Her2+ve areas that were negative for bound drug.

Methods: Her2+ve human carcinoma xenografts were grown in NOD/SCID mice as solid tumors or as metastatic models following ip or iv injection. Tumors were MR-imaged (T1-weighted) with a 500kDa Gd-labeled hyperbranched polyglycerol (HPG) contrast agent (CA). Treatments included bevacizumab (BvMAb, 10 mg/kg), trastuzumab (TsMab, 10 mg/kg) and human IgG (10 mg/kg). Multiplexed staining and imaging of whole tumor sections was used to evaluate the distribution of TsMAb in the context of the tumor microenvironment.

Findings: BT474, MDA361 and SKOV3 primary tumor xenografts all show highly heterogeneous distribution of TsMAb at 24h, where some areas are highly saturated with TsMAb and others contain unbound Her2+ tissue. Small metastases in the peritoneal cavity and lung also exhibit substantial proportions of tissue that are negative for TsMAb. CD31-stained vessels dual-labeled for a perfusion marker are often seen to have little or no bound TsMAb on perivascular tumor cells. Pretreatment with BvMAb for 24-72h results in significant reductions in tumor accumulation of TsMAb, with tumors showing 60-90% of bound TsMAb relative to IgG-treated controls (p<0.05). BvMAb -treated xenografts had reduced fractions of perfused vessels and reduced measures of vascular function assessed using DCE-MRI (49% reduction in CA, p<0.05). Corresponding parameter maps of MR-imaged high vascular function and histological images of bound TsMAb do not consistently correlate. Detailed maps of the tumor microenvironment show that even with VEGF blockade, the distribution of TsMAb still does not correspond to vessels that are perfused. Significant numbers of perfused vessels persist with no perivascular bound TsMAb despite the presence of Her2+ cells; saturation with bound TsMAb is also not consistently correlated with the presence or absence of smooth muscle, collagen or ZO-1 stained tight junctions, examined using immunohistochemistry.

Summary: The extravascular distribution of TsMAb in Her2-overexpressing xenografts, including micrometastases, is heterogeneous and is not explained simply by the presence or absence of vascular function. Pre-treatment with BvMAb for even short exposures of 24-72h worsens the distribution of TsMAb within the tumor microenvironment. The persistence of tissues that are unbound for trastuzumab suggests the possibility of inadequate drug exposure as a mechanism for drug resistance that may be exacerbated by pre-treatment with a VEGF blockade.

Citation Format: Jennifer H. E. Baker, Alastair H. Kyle, Stefan A. Reinsberg, Firas Moosvi, Jordan Cran, Urs Hafeli, Katayoun Saatchi, Andrew I. Minchinton. Microenvironmental distribution of trastuzumab is heterogeneous and decreases sharply when administered following a single dose of bevacizumab in Her2+ve xenografts and metastases models. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2988. doi:10.1158/1538-7445.AM2014-2988

Endothelial-specific Notch blockade inhibits vascular function and tumor growth through an eNOS-dependent mechanism

Notch signaling is important for tumor angiogenesis induced by vascular endothelial growth factor A. Blockade of the Notch ligand Dll4 inhibits tumor growth in a paradoxical way. Dll4 inhibition increases endothelial cell sprouting, but vessels show reduced perfusion. The reason for this lack of perfusion is not currently understood. Here we report that inhibition of Notch signaling in endothelial cell using an inducible binary transgenic system limits VEGFA-driven tumor growth and causes endothelial dysfunction. Neither excessive endothelial cell sprouting nor defects of pericyte abundance accompanied the inhibition of tumor growth and functional vasculature. However, biochemical and functional analysis revealed that endothelial nitric oxide production is decreased by Notch inhibition. Treatment with the soluble guanylate cyclase activator BAY41-2272, a vasorelaxing agent that acts downstream of endothelial nitric oxide synthase (eNOS) by directly activating its soluble guanylyl cyclase receptor, rescued blood vessel function and tumor growth. We show that reduction in nitric oxide signaling is an early alteration induced by Notch inhibition and suggest that lack of functional vessels observed with Notch inhibition is secondary to inhibition of nitric oxide signaling. Coculture and tumor growth assays reveal that Notch-mediated nitric oxide production in endothelial cell requires VEGFA signaling. Together, our data support that eNOS inhibition is responsible for the tumor growth and vascular function defects induced by endothelial Notch inhibition. This study uncovers a novel mechanism of nitric oxide production in endothelial cells in tumors, with implications for understanding the peculiar character of tumor blood vessels.

Targeting the tumour vasculature: exploitation of low oxygenation and sensitivity to NOS inhibition by treatment with a hypoxic cytotoxin

Many cancer research efforts focus on exploiting genetic-level features that may be targeted for therapy. Tissue-level features of the tumour microenvironment also represent useful therapeutic targets. Here we investigate the presence of low oxygen tension and sensitivity to NOS inhibition of tumour vasculature as potential tumour-specific features that may be targeted by hypoxic cytotoxins, a class of therapeutics currently under investigation. We have previously demonstrated that tirapazamine (TPZ) mediates central vascular dysfunction in tumours. TPZ is a hypoxic cytotoxin that is also a competitive inhibitor of NOS. Here we further investigated the vascular-targeting activity of TPZ by combining it with NOS inhibitor L-NNA, or with low oxygen content gas breathing. Tumours were analyzed via multiplex immunohistochemical staining that revealed irreversible loss of perfusion and enhanced tumour cell death when TPZ was combined with either low oxygen or a NOS inhibitor. Tumour growth rate was reduced by TPZ + NOS inhibition, and tumours previously resistant to TPZ-mediated vascular dysfunction were sensitized by low oxygen breathing. Additional mapping analysis suggests that tumours with reduced vascular-associated stroma may have greater sensitivity to these effects. These results indicate that poorly oxygenated tumour vessels, also being abnormally organized and with inadequate smooth muscle, may be successfully targeted for significant anti-cancer effects by inhibition of NOS and hypoxia-activated prodrug toxicity. This strategy illustrates a novel use of hypoxia-activated cytotoxic prodrugs as vascular targeting agents, and also represents a novel mechanism for targeting tumour vessels.

The combination of gefitinib and RAD001 inhibits growth of HER2 overexpressing breast cancer cells and tumors irrespective of trastuzumab sensitivity

Background

HER2-positive breast cancers exhibit high rates of innate and acquired resistance to trastuzumab (TZ), a HER2-directed antibody used as a first line treatment for this disease. TZ resistance may in part be mediated by frequent co-expression of EGFR and by sustained activation of the mammalian target of rapamycin (mTOR) pathway. Here, we assessed feasibility of combining the EGFR inhibitor gefitinib and the mTOR inhibitor everolimus (RAD001) for treating HER2 overexpressing breast cancers with different sensitivity to TZ.

Methods

The gefitinib and RAD001 combination was broadly evaluated in TZ sensitive (SKBR3 and MCF7-HER2) and TZ resistant (JIMT-1) breast cancer models. The effects on cell growth were measured in cell based assays using the fixed molar ratio design and the median effect principle. In vivo studies were performed in Rag2M mice bearing established tumors. Analysis of cell cycle, changes in targeted signaling pathways and tumor characteristics were conducted to assess gefitinib and RAD001 interactions.

Results

The gefitinib and RAD001 combination inhibited cell growth in vitro in a synergistic fashion as defined by the Chou and Talalay median effect principle and increased tumor xenograft growth delay. The improvement in therapeutic efficacy by the combination was associated in vitro with cell line dependent increases in cytotoxicity and cytostasis while treatment in vivo promoted cytostasis. The most striking and consistent therapeutic effect of the combination was increased inhibition of the mTOR pathway (in vitro and in vivo) and EGFR signaling in vivo relative to the single drugs.

Conclusions

The gefitinib and RAD001 combination provides effective control over growth of HER2 overexpressing cells and tumors irrespective of the TZ sensitivity status.

Opposing roles for CD34 in B16 melanoma tumor growth alter early stage vasculature and late stage immune cell infiltration

Tumor growth and metastasis are determined by the complex interplay of factors, including those intrinsic to tumor cells and extrinsic factors associated with the tumor microenvironment. Our previous work demonstrated key roles for CD34 in the maintenance of vascular integrity and eosinophil and mast cell homing. Since both of these functions affect tumor development, we characterized the effect of CD34 ablation on tumor growth using the B16F1 melanoma model. Intriguingly, we found that CD34 plays a biphasic role in tumor progression. In early growth, both subcutaneous-injected tumors and intravenous-injected lung metastases grew more slowly in Cd34^−/− mice. This correlated with abnormal vessel morphology and increased vascular permeability in these mice. Bone marrow transplantation experiments confirmed that this reflects a non-hematopoietic function of CD34. At later stages, subcutaneous tumor growth was accelerated in Cd34^−/− mice and surpassed growth in wildtype mice. Bone marrow chimera experiments demonstrated this difference was due to a hematopoietic function for CD34 and, correspondingly we found reduced intra-tumor mast cell numbers in Cd34^−/− mice. In aggregate, our analysis reveals a novel role for CD34 in both early and late tumor growth and provides novel insights into the role of the tumor microenvironment in tumor progression.

Abstract C248: Targeting the tumor microenvironment: Vascular-specific damage by tirapazamine influenced by hypoxia and vascular stability

Background: Tirapazamine (TPZ; SR 4233; 3-amino-1,2,4-benzotriazine-1,4-di-N-oxide) is a Phase II/III bioreductive anti-cancer agent with greater toxicity to hypoxic vs oxygenated cells in vitro. We have previously reported that TPZ is able to mediate dose-dependent, irreversible central vascular dysfunction in vivo, leaving a hallmark viable rim of surviving peripheral vessels as seen with other vascular targeting agents (VTAs).

Purpose: To explore how this agent, which is a bioreductive cytotoxin in vitro, is able to mediate catastrophic damage to tumor endothelium, likely the most oxygenated cell population in solid tumors, we have investigated the role of the tumor microenvironment including tumor hypoxia and vascular function.

Methods: Vascular dysfunction may be qualitatively and quantitatively assessed using multiplex immunohistochemistry-based staining of frozen tumor sections for vasculature, perfusion, hypoxia, apoptosis and proliferation. Additional features of the microenvironment such as vascular permeability, architecture and support cells were also assessed using this tumor-mapping approach.

Modification of tumor hypoxia was achieved via induction of mild anemia or low oxygen content gas breathing of tumor-bearing mice, and vascular function was increased by over-expression of VEGF in tumor models or systemic administration of nitric oxide (NO), or decreased via inhibition of nitric oxide synthase (NOS).

Results: DCE-MRI studies correlated with tumor mapping data have shown that greater pre-treatment perfusion in HCT116 colorectal xenografts predicts for decreased sensitivity to the anti-vascular effects of TPZ. In contrast to observations using DCE-MRI, increasing tumor perfusion and vascular function caused an increase in tumor vessel sensitivity to TPZ, increasing both the magnitude and frequency of response in tumors. Both NOS inhibition and excess NO availability were able to potentiate anti-cancer effects of TPZ through enhancement of its vascular damage.

Increasing tumor hypoxia by decreasing blood oxygenation was also found to potentiate the anti-vascular effects of TPZ, with otherwise resistant HT29 colorectal xenograft tumors showing a strong vascular dysfunction effect.

Vascular architecture and tumor microenvironmental features were mapped in sensitive HCT116 and resistant HT29 colorectal xenograft models, with vascular maturity, permeability, microregional location of proliferating cells and hypoxia all identified as distinct in the two models.

Summary: This work demonstrates that the vascular targeting effects of TPZ are potentiated by both hypoxia and vascular destabilization, and emphasizes the importance of specifically investigating the activity of anti-cancer agents in the context of the tumor microenvironment in vivo. This work was funded by a grant from CIHR & Jennifer Baker is a recipient of a graduate studentship from Michael Smith Foundation for Health Research.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C248.

Irinophore C, a novel nanoformulation of irinotecan, alters tumor vascular function and enhances the distribution of 5-fluorouracil and doxorubicin

PURPOSE

To examine the antitumor effects of Irinophore C, a nanopharmaceutical formulation of irinotecan, on the tissue morphology and function of tumor vasculature in HT-29 human colorectal tumors.

EXPERIMENTAL DESIGN

Fluorescence microscopy was used to map and quantify changes in tissue density, tumor vasculature, hypoxia, and the distribution of Hoechst 33342, a perfusion marker, and the anticancer drug, doxorubicin. Noninvasive magnetic resonance imaging was used to quantify Ktrans, the volume transfer constant of a solute between the blood vessels and extracellular tissue compartment of the tumor, as a measure of vascular function. Following treatment with Irinophore C, 19F magnetic resonance spectroscopy was used to monitor the delivery of 5-fluorouracil (5-FU) to the tumor tissue, whereas scintigraphy was used to quantify the presence of bound [14C]5-FU.

RESULTS

Irinophore C decreased cell density (P = 8.42 x 10(-5)), the overall number of endothelial cells in the entire section (P = 0.014), tumor hypoxia (P = 5.32 x 10(-9)), and K(trans) (P = 0.050). However, treatment increased the ratio of endothelial cells to cell density (P = 0.00024) and the accumulation of Hoechst 33342 (P = 0.022), doxorubicin (P = 0.243 x 10(-5)), and 5-FU (P = 0.0002) in the tumor. Vascular endothelial growth factor and interleukin-8, two proangiogenic factors, were down-regulated, whereas the antiangiogenic factor TIMP-1 was up-regulated in Irinophore C-treated tumors.

CONCLUSIONS

Irinophore C treatment improves the vascular function of the tumor, thereby reducing tumor hypoxia and increasing the delivery and accumulation of a second drug. Reducing hypoxia would enhance radiotherapy, whereas improving delivery of a second drug to the tumor should result in higher cell kill.

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.

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.

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.