Can NMR diffusion-weighted imaging provide quantitative information on tumor interstital pO2?

Quantitative MRI to Characterize Hypoxic Tumors in Comparison to FMISO PET/CT for Radiotherapy in Oropharynx Cancers

Intratumoral hypoxia is associated with a poor prognosis and poor response to treatment in head and neck cancers. Its identification would allow for increasing the radiation dose to hypoxic tumor subvolumes. 18F-FMISO PET imaging is the gold standard; however, quantitative multiparametric MRI could show the presence of intratumoral hypoxia. Thus, 16 patients were prospectively included and underwent 18F-FDG PET/CT, 18F-FMISO PET/CT, and multiparametric quantitative MRI (DCE, diffusion and relaxometry T1 and T2 techniques) in the same position before treatment. PET and MRI sub-volumes were segmented and classified as hypoxic or non-hypoxic volumes to compare quantitative MRI parameters between normoxic and hypoxic volumes. In total, 13 patients had hypoxic lesions. The Dice, Jaccard, and overlap fraction similarity indices were 0.43, 0.28, and 0.71, respectively, between the FDG PET and MRI-measured lesion volumes, showing that the FDG PET tumor volume is partially contained within the MRI tumor volume. The results showed significant differences in the parameters of SUV in FDG and FMISO PET between patients with and without measurable hypoxic lesions. The quantitative MRI parameters of ADC, T1 max mapping and T2 max mapping were different between hypoxic and normoxic subvolumes. Quantitative MRI, based on free water diffusion and T1 and T2 mapping, seems to be able to identify intra-tumoral hypoxic sub-volumes for additional radiotherapy doses.

Advances in PET and MRI imaging of tumor hypoxia

Tumor hypoxia is a complex and evolving phenomenon both in time and space. Molecular imaging allows to approach these variations, but the tracers used have their own limitations. PET imaging has the disadvantage of low resolution and must take into account molecular biodistribution, but has the advantage of high targeting accuracy. The relationship between the signal in MRI imaging and oxygen is complex but hopefully it would lead to the detection of truly oxygen-depleted tissue. Different ways of imaging hypoxia are discussed in this review, with nuclear medicine tracers such as [¹⁸F]-FMISO, [¹⁸F]-FAZA, or [⁶⁴Cu]-ATSM but also with MRI techniques such as perfusion imaging, diffusion MRI or oxygen-enhanced MRI. Hypoxia is a pejorative factor regarding aggressiveness, tumor dissemination and resistance to treatments. Therefore, having accurate tools is particularly important.

PO2-based biodosimetry evaluation using an EPR technique acts as a sensitive index for chemotherapy

The partial pressure of oxygen (PO₂) in the tumor microenvironment directly affects tumor sensitivity to chemotherapy. In the present study, a lithium phthalocyanine probe was implanted into MCF-7 human breast cancer cells, followed by transplant of the cells into nude mice. The present study used an electron paramagnetic resonance (EPR) oximetry measuring technique to dynamically monitor PO₂ in the tumor microenvironment prior to and following chemotherapy, and aimed to determine the precise time window in which the microenvironmental PO₂ peaked following chemotherapy. The results indicated that PO₂ was significantly higher in breast cancer compared with control (P<0.05). Following four cycles of chemotherapy, the activity of NADH dehydrogenase, succinate-cytochrome c reductase and cytochrome c oxidase in the mitochondria of cells was significantly reduced when compared with their activity prior to chemotherapy (P<0.05). Regional blood flow in tumor tissues undergoing chemotherapy was significantly lower than that prior to chemotherapy (P<0.05). The rate of cellular apoptosis in the PO₂ peak-based chemotherapy group was significantly greater than that in the conventional chemotherapy group after two and four cycles of chemotherapy (P<0.05). Tumor volume in the PO₂ peak-based chemotherapy group was significantly reduced compared with that in the 0.9% NaCl solution control and the conventional chemotherapy groups after four cycles of chemotherapy (P<0.05). The tumor inhibitory rate of the experimental group was significantly higher than that of the conventional chemotherapy group (P<0.01). In conclusion, the present study may provide guidance for the development of effective strategies depending on tumor-maximal response to chemotherapy in an oxygen-rich environment. Additionally, the present study aimed to establish a foundation for a clinical noninvasive assessment intended to guide treatment and formulate individual regimens, in order to improve cancer therapeutics, sensitivity monitoring and curative effect estimation.

Hypoxia reduces adenoviral replication in cancer cells by downregulation of viral protein expression

Successful cancer therapy using replicating viral vectors relies on the spread of virus from infected to uninfected cells. To date, there has been limited clinical success in the use of replicating adenoviruses. In animal models, established xenograft tumors are rarely eliminated despite the persistence of high viral titers within the tumor. Hypoxia is a prevalent characteristic of solid tumors, whereas adenovirus naturally infects tissues exposed to ambient oxygen concentrations. Here, we report that hypoxia (1% oxygen) reduces adenoviral replication in H1299 and A549 lung cancer cells, BxPC-3 pancreatic cancer cells, LNCaP prostate cancer cells and HCT116 colon cancer cells. However, hypoxia does not reduce cell viability or restrict S-phase entry. Importantly, the production of E1a and fiber proteins under hypoxic conditions was substantially decreased at 24 and 48 h compared to room air controls. In contrast, Northern analysis showed similar levels of E1a mRNA in room air and hypoxic conditions. In conclusion, a level of hypoxia similar to that found within solid tumors reduces the replication of adenoviral vectors by reduction of viral protein expression without a reduction in mRNA levels. To further improve oncolytic therapy using a replicating adenovirus, it is important to understand the mechanism through which hypoxia and the virus interact to control expression of viral and cellular proteins, and consequently to develop means to overcome decreased viral production in hypoxic conditions.