Intra- and intertumor heterogeneities in total, chronic, and acute hypoxia in xenografted squamous cell carcinomas. Detection and quantification using (immuno-)fluorescence techniques

Tumor Oxygenation by Myo-Inositol Trispyrophosphate Enhances Radiation Response

PURPOSE

Tumor hypoxia is a major limiting factor for successful radiation therapy outcomes, with hypoxic cells being up to 3-fold more radiation resistant than normoxic cells; tumor hypoxia creates a tumor microenvironment that is hostile to immune response. Thus, pharmaceutical-induced tumor oxygenation before radiation therapy represents an interesting method to enhance the efficacy of radiation therapy. Myo-inositol trispyrophosphate (ITPP) triggers a decrease in the affinity of oxygen to hemoglobin, which leads to an increased release of oxygen upon tissue demand, including in hypoxic tumors.

METHODS AND MATERIALS

The combined treatment modality of high-dose bolus ITPP with a single high-dose fraction of ionizing radiation (IR) was investigated for its mechanics and efficacy in multiple preclinical animal tumor models in immunocompromised and immunocompetent mice. The dynamics of tumor oxygenation were determined by serial hypoxia-oriented bioimaging. Initial and residual DNA damage and the integrity of the tumor vasculature were quantified on the immunohistochemical level in response to the different treatment combinations.

RESULTS

ITPP application did not affect tumor growth as a single treatment modality, but it rapidly induced tumor oxygenation, as demonstrated by in vivo imaging, and significantly reduced tumor growth when combined with IR. An immunohistochemical analysis of γH2AX foci demonstrated increased initial and residual IR-induced DNA damage as the primary mechanism for radiosensitization within initially hypoxic but ITPP-oxygenated tumor regions. Scheduling experiments revealed that ITPP increases the efficacy of ionizing radiation only when applied before radiation therapy. Irradiation alone damaged the tumor vasculature and increased tumor hypoxia, which were both prevented by combined treatment with ITPP. Interestingly, the combined treatment modality also promoted increased immune cell infiltration.

CONCLUSIONS

ITPP-mediated tumor oxygenation and vascular protection triggers immediate and delayed processes to enhance the efficacy of ionizing radiation for successful radiation therapy.

Acute Hypoxia Profile is a Stronger Prognostic Factor than Chronic Hypoxia in Advanced Stage Head and Neck Cancer Patients

Hypoxic head and neck tumors respond poorly to radiotherapy and can be identified using gene expression profiles. However, it is unknown whether treatment outcome is driven by acute or chronic hypoxia. Gene expression data of 398 head and neck cancers was collected. Four clinical hypoxia profiles were compared to in vitro acute and chronic hypoxia profiles. Chronic and acute hypoxia profiles were tested for their association to outcome using Cox proportional hazard analyses. In an initial set of 224 patients, scores of the four clinical hypoxia profiles correlated with each other and with chronic hypoxia. However, the acute hypoxia profile showed a stronger association with local recurrence after chemoradiotherapy (p = 0.02; HR = 3.1) than the four clinical (chronic hypoxia) profiles (p = 0.2; HR = 0.9). An independent set of 174 patients confirmed that acute hypoxia is a stronger prognostic factor than chronic hypoxia for overall survival, progression-free survival, local and locoregional control. Multivariable analyses accounting for known prognostic factors substantiate this finding (p = 0.045; p = 0.042; p = 0.018 and p = 0.003, respectively). In conclusion, the four clinical hypoxia profiles are related to chronic hypoxia and not acute hypoxia. The acute hypoxia profile shows a stronger association with patient outcome and should be incorporated into existing prediction models.

Rationale for Combining Radiotherapy and Immune Checkpoint Inhibition for Patients With Hypoxic Tumors

In order to compensate for the increased oxygen consumption in growing tumors, tumors need angiogenesis and vasculogenesis to increase the supply. Insufficiency in this process or in the microcirculation leads to hypoxic tumor areas with a significantly reduced pO2, which in turn leads to alterations in the biology of cancer cells as well as in the tumor microenvironment. Cancer cells develop more aggressive phenotypes, stem cell features and are more prone to metastasis formation and migration. In addition, intratumoral hypoxia confers therapy resistance, specifically radioresistance. Reactive oxygen species are crucial in fixing DNA breaks after ionizing radiation. Thus, hypoxic tumor cells show a two- to threefold increase in radioresistance. The microenvironment is enriched with chemokines (e.g., SDF-1) and growth factors (e.g., TGFβ) additionally reducing radiosensitivity. During recent years hypoxia has also been identified as a major factor for immune suppression in the tumor microenvironment. Hypoxic tumors show increased numbers of myeloid derived suppressor cells (MDSCs) as well as regulatory T cells (Tregs) and decreased infiltration and activation of cytotoxic T cells. The combination of radiotherapy with immune checkpoint inhibition is on the rise in the treatment of metastatic cancer patients, but is also tested in multiple curative treatment settings. There is a strong rationale for synergistic effects, such as increased T cell infiltration in irradiated tumors and mitigation of radiation-induced immunosuppressive mechanisms such as PD-L1 upregulation by immune checkpoint inhibition. Given the worse prognosis of patients with hypoxic tumors due to local therapy resistance but also increased rate of distant metastases and the strong immune suppression induced by hypoxia, we hypothesize that the subgroup of patients with hypoxic tumors might be of special interest for combining immune checkpoint inhibition with radiotherapy.

Quantitative Analysis of [18F]FMISO PET for Tumor Hypoxia: Correlation of Modeling Results with Immunohistochemistry

PURPOSE

Quantitative evaluation of tumor hypoxia based on H-1-(3-[¹⁸F]fluoro-2-hydroxypropyl)-2-nitroimidazole ([¹⁸F]FMISO) positron emission tomography (PET) can deliver important information for treatment planning in radiotherapy. However, the merits and limitations of different analysis methods in revealing the underlying physiological feature are not clear. This study aimed to assess these quantitative analysis methods with the support of immunohistological data.

PROCEDURES

Sixteen nude mice bearing xenografted human squamous cell carcinomas (FaDu or CAL-33) were scanned using 2-h dynamic [¹⁸F]FMISO PET. Tumors were resected and sliced, and the hypoxia marker pimonidazole was immunostained followed by H&E staining. The pimonidazole signal was segmented using a k-means clustering algorithm, and the hypoxic fraction (HF) was calculated as the hypoxic area/viable tumor-tissue-area ratio pooled over three tissue slices from the apical, center, and basal layers. PET images were analyzed using various methods including static analysis [standard uptake value (SUV), tumor-to-blood ratio (T/B), tumor-to-muscle ratio (T/M)] and kinetic modeling (Casciari αk _A , irreversible and reversible two-tissue compartment k ₃, Thorwarth w _A k ₃, Patlak K _i , Logan V _d , Cho K), and correlated with HF.

RESULTS

No significant correlation was found for static analysis. A significant correlation between k ₃ of the irreversible two-tissue compartment model and HF was observed (r = 0.61, p = 0.01). The correlation between HF and αk _A of the Casciari model could be improved through reducing local minima by testing more sets of initial values (r = 0.59, p = 0.02) or by reducing the model complexity by fixing three parameters (r = 0.63, p = 0.0008).

CONCLUSIONS

With support of immunohistochemistry data, this study shows that various analysis methods for [¹⁸F]FMISO PET perform differently for assessment of tumor hypoxia. A better fitting quality does not necessarily mean a higher physiological correlation. Hypoxia PET analysis needs to consider both the mathematical stability and physiological fidelity. Based on the results of this study, preference should be given to the irreversible two-tissue compartment model as well as the Casciari model with reduced parameters.

Identification of hypoxia-regulated angiogenic genes in colorectal cancer

The tumour hypoxia would trigger the angiogenesis switch for survival, and increase the ability of cancer cells to invade and metastasis. However, hypoxia regulated genes that invovled in angiogenesis in colorectal cancer (CRC) have not been explored in detail. The aim of this study was to explore angiogenic genes under hypoxia condition in CRC. Here, we found that endothelial cells tube formation and cancer cells invasion ability were promoted even under chronic hypoxia condition (72 h) in colon adenocarcinoma HCT-116 cells. Then, we explored the differentially expressed genes (DEGs) under chronic hypoxia condition by microarray from Gene Expression Omnibus (GEO) database. Subsequent bioinformatic analysis identified 17 genes that invovled in angiogenesis, blood vessel development, blood vessel morphgensis, vascular development. of these genes, VEGF-A, Smad7, Jun, IL-8, CXCR-4, PDGF-A, TGF-A, ANGPTL-4 expression levels up-regulated under hypoxia condition. Additionally, the gene expression level in acute hypoxia (24 h) was significantly higher than chronic condition (72 h). Finally, knockdown of hypoxia inducible factor (HIF-1α) by shRNA reversed the role of Smad7, CXCR-4, PDGF-A, TGF-A and ANGPTL-4 overexpression in HCT-116 cells, these findings provide the potential angiogenic targets for the treatment of colorectal cancer.

Preclinical evaluation of parametric image reconstruction of [18F]FMISO PET: correlation with ex vivo immunohistochemistry

Compared to indirect methods, direct parametric image reconstruction (PIR) has the advantage of high quality and low statistical errors. However, it is not yet clear if this improvement in quality is beneficial for physiological quantification. This study aimed to evaluate direct PIR for the quantification of tumor hypoxia using the hypoxic fraction (HF) assessed from immunohistological data as a physiological reference. Sixteen mice with xenografted human squamous cell carcinomas were scanned with dynamic [18F]FMISO PET. Afterward, tumors were sliced and stained with H&E and the hypoxia marker pimonidazole. The hypoxic signal was segmented using k-means clustering and HF was specified as the ratio of the hypoxic area over the viable tumor area. The parametric Patlak slope images were obtained by indirect voxel-wise modeling on reconstructed images using filtered back projection and ordered-subset expectation maximization (OSEM) and by direct PIR (e.g., parametric-OSEM, POSEM). The mean and maximum Patlak slopes of the tumor area were investigated and compared with HF. POSEM resulted in generally higher correlations between slope and HF among the investigated methods. A strategy for the delineation of the hypoxic tumor volume based on thresholding parametric images at half maximum of the slope is recommended based on the results of this study.

Potential of [18F]-fluoromisonidazole positron-emission tomography for radiotherapy planning in head and neck squamous cell carcinomas

BACKGROUND AND PURPOSE

Positron-emission tomography (PET) with [(18)F]-fluoromisonidazole (FMISO) permits consideration of radiotherapy dose escalation to hypoxic volumes in head and neck cancers (HNC). However, the definition of FMISO volumes remains problematic. The aims of this study are to confirm that delayed acquisition at 4 h is most appropriate for FMISO-PET imaging and to assess different methods of volume segmentation.

PATIENTS AND METHODS

A total of 15 HNC patients underwent several FMISO-PET/computed tomography (CT) acquisitions 2, 3 and 4 h after FMISO injection. Three automatic methods of PET image segmentation were tested: fixed threshold, adaptive threshold based on the ratio between tumour-derived and background activities (R(T/B)) and the fuzzy locally adaptive Bayesian (FLAB) method. The hypoxic fraction (HF), which is defined as the ratio between the FMISO and CT volumes, was also calculated.

RESULTS

The R(T/B) for images acquired at 2, 3 and 4 h differed significantly, with mean values of 2.5 (1.7-2.9), 3 (2-4.5) and 3.4 (2.3-6.1), respectively. The mean tumour volume, as defined manually using CT images, was 39.1 ml (1.2-116 ml). After 4 h, the mean FMISO volumes were 18.9 (0.1-81), 9.5 (0.9-33.1) and 12.5 ml (0.9-38.4 ml) with fixed threshold, adaptive threshold and the FLAB method, respectively; median HF values were 0.47 (0.1-1.93), 0.25 (0.11-0.75) and 0.35 (0.14-1.05), respectively. FMISO volumes were significantly different.

CONCLUSION

The best contrast is obtained at the 4-hour acquisition time. Large discrepancies were found between the three tested methods of volume segmentation.

In vitro hypoxic cytotoxicity and hypoxic radiosensitization. Efficacy of the novel 2-nitroimidazole N,N,N-tris[2-(2-nitro-1H-imidazol-1-yl)ethyl]amine

BACKGROUND AND PURPOSE

Tumor hypoxia is a major problem in radiation therapy of solid tumors because of the radiosensitizing effect of oxygen. Nitroimidazole-containing compounds are oxygen mimetics accumulating in hypoxic tumor areas. However, the broad use of 2-nitroimidazoles as a hypoxic radiosensitizer is limited by their partially low efficacy and/or high neurotoxicity.

MATERIALS AND METHODS

Here, we characterized the in vitro hypoxic cytotoxicity and hypoxic radiosensitizing efficacy of N,N,N-tris [2-(2-nitro-1H-imidazol-1-yl)ethyl]amine (PRC) in a hypoxia-sensitive lymphoma and a hypoxia-resistant glioblastoma cell line by colony formation assay and flow cytometry.

RESULTS

PRC exerted high hypoxic cytotoxic and radiosensitizing action on both cell lines at almost absent toxicity under normoxic conditions. In particular, under hypoxia, but not normoxia, PRC targeted the mitochondria resulting in oxidative stress, G(2)/M cell cycle arrest, and triggering of the intrinsic apoptosis pathway.

CONCLUSION

Our in vitro findings suggest that PRC might be a promising new 2-nitroimidazole for improving radiation therapy of hypoxic tumors in vivo.

Tumor oxygenation: an appraisal of past and present concepts and a look into the future : Arisztid G. B. Kovách Lecture

Since 1970, the multifactorial pathogenesis of the deficient and heterogeneous oxygenation of transplanted murine tumors and of human cancers (including parameters determining oxygen delivery, e.g., blood flow, diffusion geometry, oxygen transport capacity of the blood) has been investigated in vivo. Hypoxia and/or anoxia was quantitatively assessed and characterized using microtechniques and special preclinical tumor models. Hypoxia subtypes were identified, and critical supply conditions were theoretically analyzed. In the 1980s, first experiments on humans were carried out in cancers of the rectum and of the oral cavity. In the 1990s, the clinical investigations were carried out on cancers of the breast and of the uterine cervix, clearly showing that hypoxia is a hallmark of locally advanced human tumors. In multivariate analysis, hypoxia was found to be an independent, adverse prognostic factor for patient survival due to hypoxia-driven malignant progression and hypoxia-associated resistance to anticancer therapy.