Changes in microregional perfusion, oxygenation, ATP and lactate distribution in subcutaneous rat tumours upon water-filtered IR-A hyperthermia

Accurate Three-Dimensional Thermal Dosimetry and Assessment of Physiologic Response Are Essential for Optimizing Thermoradiotherapy

Numerous randomized trials have revealed that hyperthermia (HT) + radiotherapy or chemotherapy improves local tumor control, progression free and overall survival vs. radiotherapy or chemotherapy alone. Despite these successes, however, some individuals fail combination therapy; not every patient will obtain maximal benefit from HT. There are many potential reasons for failure. In this paper, we focus on how HT influences tumor hypoxia, since hypoxia negatively influences radiotherapy and chemotherapy response as well as immune surveillance. Pre-clinically, it is well established that reoxygenation of tumors in response to HT is related to the time and temperature of exposure. In most pre-clinical studies, reoxygenation occurs only during or shortly after a HT treatment. If this were the case clinically, then it would be challenging to take advantage of HT induced reoxygenation. An important question, therefore, is whether HT induced reoxygenation occurs in the clinic that is of radiobiological significance. In this review, we will discuss the influence of thermal history on reoxygenation in both human and canine cancers treated with thermoradiotherapy. Results of several clinical series show that reoxygenation is observed and persists for 24-48 h after HT. Further, reoxygenation is associated with treatment outcome in thermoradiotherapy trials as assessed by: (1) a doubling of pathologic complete response (pCR) in human soft tissue sarcomas, (2) a 14 mmHg increase in pO2 of locally advanced breast cancers achieving a clinical response vs. a 9 mmHg decrease in pO2 of locally advanced breast cancers that did not respond and (3) a significant correlation between extent of reoxygenation (as assessed by pO2 probes and hypoxia marker drug immunohistochemistry) and duration of local tumor control in canine soft tissue sarcomas. The persistence of reoxygenation out to 24-48 h post HT is distinctly different from most reported rodent studies. In these clinical series, comparison of thermal data with physiologic response shows that within the same tumor, temperatures at the higher end of the temperature distribution likely kill cells, resulting in reduced oxygen consumption rate, while lower temperatures in the same tumor improve perfusion. However, reoxygenation does not occur in all subjects, leading to significant uncertainty about the thermal-physiologic relationship. This uncertainty stems from limited knowledge about the spatiotemporal characteristics of temperature and physiologic response. We conclude with recommendations for future research with emphasis on retrieving co-registered thermal and physiologic data before and after HT in order to begin to unravel complex thermophysiologic interactions that appear to occur with thermoradiotherapy.

Targeting Hypoxia: Revival of Old Remedies

Tumour hypoxia is significantly correlated with patient survival and treatment outcomes. At the molecular level, hypoxia is a major driving factor for tumour progression and aggressiveness. Despite the accumulative scientific and clinical efforts to target hypoxia, there is still a need to find specific treatments for tumour hypoxia. In this review, we discuss a variety of approaches to alter the low oxygen tumour microenvironment or hypoxia pathways including carbogen breathing, hyperthermia, hypoxia-activated prodrugs, tumour metabolism and hypoxia-inducible factor (HIF) inhibitors. The recent advances in technology and biological understanding reveal the importance of revisiting old therapeutic regimens and repurposing their uses clinically.

External Basic Hyperthermia Devices for Preclinical Studies in Small Animals

Simple Summary

The application of mild hyperthermia can be beneficial for solid tumor treatment by induction of sublethal effects on a tissue- and cellular level. When designing a hyperthermia experiment, several factors should be taken into consideration. In this review, multiple elementary hyperthermia devices are described in detail to aid standardization of treatment design.

Abstract

Preclinical studies have shown that application of mild hyperthermia (40–43 °C) is a promising adjuvant to solid tumor treatment. To improve preclinical testing, enhance reproducibility, and allow comparison of the obtained results, it is crucial to have standardization of the available methods. Reproducibility of methods in and between research groups on the same techniques is crucial to have a better prediction of the clinical outcome and to improve new treatment strategies (for instance with heat-sensitive nanoparticles). Here we provide a preclinically oriented review on the use and applicability of basic hyperthermia systems available for solid tumor thermal treatment in small animals. The complexity of these techniques ranges from a simple, low-cost water bath approach, irradiation with light or lasers, to advanced ultrasound and capacitive heating devices.

Multicomponent thermosensitive lipid complexes enhance desmoplastic tumor therapy through boosting anti-angiogenesis and synergistic strategy

Currently, the chemotherapy drugs-loaded thermosensitive liposomes have not shown an over standard of clinical effects compared to preclinical trials. In addition to the limiting factors of clinical trial design and heating device, abnormal angiogenesis in desmoplastic tumor is a key factor for unexpected clinical efficacy. Malformed tumor vasculature may result in reduced vascular transport and the heterogeneous distribution of thermosensitive liposomes in tumor. Here, we report an anti-angiogenesis strategy through hypoxia-inducible factors (HIF)-1α-vascular endothelial growth factor (VEGF) axis based on icaritin and coix seed oil dual loaded multicomponent thermosensitive lipid complexes (IC-ML). IC-ML could downregulate the HIF-1α expression in HepG2 cells with a synergetic antitumor effect. In addition, HepG2 + LX-2 cells co-cultured 3D tumor spheres administered IC-ML showed the strongest penetration and inhibition of growth. Accordingly, IC-ML displayed improved tumor penetration and superior synergistic antitumor efficacy with HIF-1α-VEGF downregulation in vivo under mild hyperthermia. The improvement of antitumor efficacy of IC-ML comes from the anti-angiogenesis strategy and comprehensive tumor microenvironment remodeling, including depletion of cancer-associated fibroblasts as well as inhibition of M2-type tumor associated macrophage infiltration in desmoplastic tumor. This study proposes a novel multicomponent synergistic antitumor strategy to improve the therapeutic potential of thermosensitive lipid complexes for hepatocellular carcinoma.

The interplay of blood flow and temperature in regional hyperthermia: a mathematical approach

In recent decades, hyperthermia has been used to raise oxygenation levels in tumours undergoing other therapeutic modalities, of which radiotherapy is the most prominent one. It has been hypothesized that oxygenation increases would come from improved blood flow associated with vasodilation. However, no test has determined whether this is a relevant assumption or other mechanisms might be acting. Additionally, since hyperthermia and radiotherapy are not usually co-administered, the crucial question arises as to how temperature and perfusion in tumours will change during and after hyperthermia. Overall, it would seem necessary to find a research framework that clarifies the current knowledge, delimits the scope of the different effects and guides future research. Here, we propose a simple mathematical model to account for temperature and perfusion dynamics in brain tumours subjected to regional hyperthermia. Our results indicate that tumours in well-perfused organs like the brain might only reach therapeutic temperatures if their vasculature is highly disrupted. Furthermore, the characteristic times of return to normal temperature levels are markedly shorter than those required to deliver adjuvant radiotherapy. According to this, a mechanistic coupling of perfusion and temperature would not explain any major oxygenation boost in brain tumours immediately after hyperthermia.

wIRA-heating of piglet skin and subcutis in vivo: proof of accordance with ESHO criteria for superficial hyperthermia

PURPOSE

The quality assurance guidelines of the European Society for Hyperthermic Oncology (ESHO) specify the requirements for appropriate superficial heating using phantoms. In this current piglet study, we have examined these requirements under in vivo conditions.

MATERIALS AND METHODS

The evaluation is based on simultaneous, invasive temperature measurements at 8 different depths between 2 and 20 mm in the thigh of anesthetized piglets during irradiation with water-filtered infrared radiation (wIRA). Temperature probes were equally distributed in an area of 10 cm diameter of homogeneously irradiated skin. Piglets were irradiated to 126.5 mW cm^-2 in the spectral range of IR-A.

RESULTS

Heating rates and specific absorption rates were in full accordance with the ESHO standards. Due to early onset of thermoregulation, the desired temperature rise of 6 K at a depth of 5 mm was achieved after about 10 min of exposure, i.e. 4 min later than required for phantoms. After reaching thermal steady state, on average T ₉₀ ≥ 40 °C occurred in tissue depths up to 20 mm, T ₅₀ ≥ 41 °C up to 16 mm, and a mean CEM₄₃ T ₉₀ ≈ 1 min was calculated for depths up to 8 mm.

CONCLUSIONS

Piglet data are comparable with preliminary literature data assessed in vivo in the abdominal wall and in recurrent breast cancer of humans. The potential of wIRA-HT for adequate treatment of superficial tissues/cancers in the clinical setting thus is confirmed. To ensure therapeutically needed doses of wIRA-HT, irradiation times should be extended.

Thermal field formation during wIRA-hyperthermia: temperature measurements in skin and subcutis of piglets as a basis for thermotherapy of superficial tumors and local skin infections caused by thermosensitive microbial pathogens

Purpose: The temporal and spatial formation of the temperature field and its changes during/upon water-filtered infrared-A (wIRA)-irradiation in porcine skin and subcutis were investigated in vivo in order to get a detailed physical basis for thermotherapy of superficial tumors and infections caused by thermosensitive microbial pathogens (e.g., Mycobacterium ulcerans causing Buruli ulcer). Methods: Local wIRA-hyperthermia was performed in 11 anesthetized piglets using 85.0 mW cm^-2, 103.2 mW cm^-2 and 126.5 mW cm^-2, respectively. Invasive temperature measurements were carried out simultaneously in 1-min intervals using eight fiber-optical probes at different tissue depths between 2 and 20 mm, and by an IR thermometer at the skin surface. Results: Tissue temperature distribution depended on incident irradiance, exposure time, tissue depths and individual 'physiologies' of the animals. Temperature maxima were found at depths between 4 and 7 mm, exceeding skin surface temperatures by about 1-2 K. Tissue temperatures above 37 °C, necessary to eradicate M. ulcerans at depths <20 mm, were reached reliably. Conclusions: wIRA-hyperthermia may be considered as a novel therapeutic option for treatment of local skin infections caused by thermosensitive pathogens (e.g., in Buruli ulcer). To ensure temperatures required for heat treatment of superficial tumors deeper than 4 mm, the incident irradiance needed can be controlled either by (a) invasive temperature measurements or (b) control of skin surface temperature and considering possible temperature increases up to 1-2 K in underlying tissue.

Biophysical and photobiological basics of water-filtered infrared-A hyperthermia of superficial tumors

Thermography-controlled, water-filtered infrared-A (wIRA) is a novel, effective and approved heating technique listed in the ESHO quality assurance guidelines for superficial hyperthermia clinical trials (2017). In order to assess the special features and the potential of wIRA-hyperthermia (wIRA-HT), detailed and updated information about its physical and photobiological background is presented. wIRA allows for (a) application of high irradiances without skin pain and acute grade 2-4 skin toxicities, (b) prolonged, therapeutically relevant exposure times using high irradiances (150-200 mW/cm²) and (c) faster and deeper heat extension within tissues. The deeper radiative penetration depth is mainly caused by forward Mie-scattering. At skin surface temperatures of 42-43 °C, the effective heating depth is 15 mm (T ≥ 40 °C) and 20 mm (T ≥ 39.5 °C). Advantages of wIRA include its contact-free energy input, easy power steering by a feed-back loop, extendable treatment fields, real-time and noninvasive surface temperature monitoring with observation of dynamic changes during HT, and - if necessary - rapid protection of temperature-sensitive structures. wIRA makes the compliant heating of ulcerated and/or bleeding tumors possible, allows for HT of irregularly shaped and diffusely spreading tumors, is independent of individual body contours, allows for very short 'transits' between HT and RT (1-4 min) or continuous heating between both therapeutic interventions. New treatment options for wIRA-HT may include malignant melanoma, vulvar carcinoma, skin metastases of different primary tumors, cutaneous T-and B-cell lymphoma, large-area hemangiomatosis, inoperable squamous cell, basal cell and eccrine carcinoma of the skin with depth extensions ≤20 mm.

Temporal changes in tumor oxygenation and perfusion upon normo- and hyperbaric inspiratory hyperoxia

BACKGROUND

Inspiratory hyperoxia under hyperbaric conditions has been shown to effectively reduce tumor hypoxia and to improve radiosensitivity. However, applying irradiation (RT) under hyperbaric conditions is technically difficult in the clinical setting since RT after decompression may be effective only if tumor pO2 remains elevated for a certain period of time. The aim of the present study was to analyze the time course of tumor oxygenation and perfusion during and after hyperbaric hyperoxia.

MATERIALS AND METHODS

Tumor oxygenation, red blood cell (RBC) flux for perfusion monitoring, and vascular resistance were assessed continuously in experimental rat DS-sarcomas by polarographic catheter electrodes and laser Doppler flowmetry at 1 and 2 atm (bar) of environmental pressure during breathing of pure O2 or carbogen (95 % O2 + 5 % CO2).

RESULTS

During room air breathing, the tumor pO2 followed very rapidly within a few minutes the change of the ambient pressure during compression or decompression. With O2 breathing under hyperbaric conditions, the tumor pO2 increased more than expected based on the rise of the environmental pressure, although the time course was comparably rapid. Breathing carbogen, the tumor pO2 followed with a slight delay of the pressure change, and within 10 min after decompression the baseline values were reached again. RBC flux increased during carbogen breathing but remained almost constant with pure O2, indicating a vasodilation (decrease in vascular resistance) with carbogen but a vasoconstriction (increase in vascular resistance) with O2 during hyperbaric conditions.

CONCLUSION

Since the tumor pO2 directly followed the environmental pressure, teletherapy after hyperbaric conditions does not seem to be promising as the pO2 reaches baseline values again within 5-10 min after decompression.

Thermoresponsive fluorinated small-molecule drugs: a new concept for efficient localized chemotherapy

We review the drugs used in combination with hyperthermia for cancer therapy and recent advances on small thermoresponsive molecules.

Targeting tumor perfusion and oxygenation to improve the outcome of anticancer therapy

Radiotherapy and chemotherapy are widespread clinical modalities for cancer treatment. Among other biological influences, hypoxia is a main factor limiting the efficacy of radiotherapy, primarily because oxygen is involved in the stabilization of the DNA damage caused by ionizing radiations. Radiobiological hypoxia is found in regions of rodent and human tumors with a tissue oxygenation level below 10 mmHg at which tumor cells become increasingly resistant to radiation damage. Since hypoxic tumor cells remain clonogenic, their resistance to the treatment strongly influences the therapeutic outcome of radiotherapy. There is therefore an urgent need to identify adjuvant treatment modalities aimed to increase tumor pO(2) at the time of radiotherapy. Since tumor hypoxia fundamentally results from an imbalance between oxygen delivery by poorly efficient blood vessels and oxygen consumption by tumor cells with high metabolic activities, two promising approaches are those targeting vascular reactivity and tumor cell respiration. This review summarizes the current knowledge about the development and use of tumor-selective vasodilators, inhibitors of tumor cell respiration, and drugs and treatments combining both activities in the context of tumor sensitization to X-ray radiotherapy. Tumor-selective vasodilation may also be used to improve the delivery of circulating anticancer agents to tumors. Imaging tumor perfusion and oxygenation is of importance not only for the development and validation of such combination treatments, but also to determine which patients could benefit from the therapy. Numerous techniques have been developed in the preclinical setting. Hence, this review also briefly describes both magnetic resonance and non-magnetic resonance in vivo methods and compares them in terms of sensitivity, quantitative or semi-quantitative properties, temporal, and spatial resolutions, as well as translational aspects.

Hyperthermically induced changes in high spectral and spatial resolution MR images of tumor tissue--a pilot study

This pilot study investigated the feasibility of using MRI based on BOLD (blood-oxygen-level-dependent) contrast to detect physiological effects of locally induced hyperthermia in a rodent tumor model. Nude mice bearing AT6.1 rodent prostate tumors inoculated in the hind leg were imaged using a 9.4 T scanner using a multi-gradient echo pulse sequence to acquire high spectral and spatial resolution (HiSS) data. Temperature increases of approximately 6 °C were produced in tumor tissue using fiber-optic-guided light from a 250 W halogen lamp. HiSS data were acquired over three slices through the tumor and leg both prior to and during heating. Water spectra were produced from these datasets for each voxel at each time point. Time-dependent changes in water resonance peak width were measured during 15 min of localized tumor heating. The results demonstrated that hyperthermia produced both significant increases and decreases in water resonance peak width. Average decreases in peak width were significantly larger in the tumor rim than in normal muscle (p = 0.04). The effect of hyperthermia in tumor was spatially heterogeneous, i.e. the standard deviation of the change in peak width was significantly larger in the tumor rim than in normal muscle (p = 0.005). Therefore, mild hyperthermia produces spatially heterogeneous changes in water peak width in both tumor and muscle. This may reflect heterogeneous effects of hyperthermia on local oxygenation. The peak width changes in tumor and muscle were significantly different, perhaps due to abnormal tumor vasculature and metabolism. Response to hyperthermia measured by MRI may be useful for identifying and/or characterizing suspicious lesions as well as guiding the development of new hyperthermia protocols.

Interstitial laser thermotherapy of a rat liver tumour: effect of hepatic inflow occlusion

BACKGROUND AND OBJECTIVE

Interstitial laser thermotherapy was used to treat rat liver tumours. The aim was to investigate the influence of temperature and temporary hepatic inflow occlusion on tumour growth and blood perfusion.

STUDY DESIGN/MATERIALS AND METHODS

Liver tumours were treated at 44°C at the tumour border for 30 minutes, hepatic inflow occlusion only, or a combination of these methods. Interstitial laser Doppler flowmetry was used to measure hepatic perfusion at the tumour border during and after heat treatment, for a total time of 60 minutes. Tumour growth was evaluated 6 days after treatment.

RESULTS

Tumours subjected to the combined treatment of hepatic inflow occlusion and interstitial laser thermotherapy displayed a blood perfusion reduction 30 minutes after treatment to 18 ± 5% of initial perfusion, which was significantly lower than achieved with thermotherapy alone (52 ± 10%, P = 0.02). The combined treatment and treatment with thermotherapy alone resulted in relative tumour growth of 0.3 ± 0.1 and 1.0 ± 0.2, respectively (P = 0.04).

CONCLUSION

Inflow occlusion enhanced the effect of thermotherapy not by augmenting treatment temperatures but by increasing the thermal sensitivity of the tumour, reflected by an immediate effect on tumour blood perfusion.

NADPH oxidase-mediated reactive oxygen species production activates hypoxia-inducible factor-1 (HIF-1) via the ERK pathway after hyperthermia treatment

Hyperthermia (HT) is a strong adjuvant treatment with radiotherapy and chemotherapy because it causes tumor reoxygenation. However, the detailed molecular mechanisms of how HT enhances tumor oxygenation have not been elucidated. Here we report that 1 h of HT activates hypoxia-inducible factor-1 (HIF-1) in tumors and its downstream targets, vascular endothelial growth factor (VEGF) and pyruvate dehydrogenase kinase 1 (PDK1). Consistent with HIF-1 activation and up-regulation of its downstream genes, HT also enhances tumor perfusion/vascularization and decreases oxygen consumption. As a result, tumor hypoxia is reduced after HT, suggesting that these physiological changes contribute to HT-induced tumor reoxygenation. Because HIF-1 is a potent regulator of tumor vascularization and metabolism, our findings suggest that HIF-1 plays a role in HT-induced tumor reoxygenation by transactivating its downstream targets. We demonstrate that NADPH oxidase-mediated reactive oxygen species production, as a mechanism, up-regulates HIF-1 after HT. Furthermore, we determine that this pathway is initiated by increased transcription of NADPH oxidase-1 through the ERK pathway. In conclusion, this study determines that, although HIF-1 is a good therapeutic target, the timing of its inhibition needs to be optimized to achieve the most beneficial outcome when it is combined with other treatments of HT, radiation, and chemotherapy.

Pathophysiological and vascular characteristics of tumours and their importance for hyperthermia: heterogeneity is the key issue

Tumour blood flow before and during clinically relevant mild hyperthermia exhibits pronounced heterogeneity. Flow changes upon heating are not predictable and are both spatially and temporally highly variable. Flow increases may result in improved heat dissipation to the extent that therapeutically relevant tissue temperatures may not be achieved. This holds especially true for tumours or tumour regions in which flow rates are substantially higher than in the surrounding normal tissues. Changes in tumour oxygenation tend to reflect alterations in blood flow upon hyperthermia. An initial improvement in the oxygenation status, followed by a return to baseline levels (or even a drop to below baseline at high thermal doses) has been reported for some tumours, whereas a predictable and universal occurrence of sustained increases in O(2) tensions upon mild hyperthermia is questionable and still needs to be verified in the clinical setting. Clarification of the pathogenetic mechanisms behind possible sustained increases is mandatory. High-dose hyperthermia leads to a decrease in the extracellular and intracellular pH and a deterioration of the energy status, both of which are known to be parameters capable of acting as direct sensitisers and thus pivotal factors in hyperthermia treatment. The role of the tumour microcirculatory function, hypoxia, acidosis and energy status is complex and is further complicated by a pronounced heterogeneity. These latter aspects require additional critical evaluation in clinically relevant tumour models in order for their impact on the response to heat to be clarified.

Imaging tumour physiology and vasculature to predict and assess response to heat

The vascular supply of tumours and the tumour microenvironment both play an important role when tumours are treated with hyperthermia. Blood flow is one of the major vehicles by which heat is dissipated thus the vascular supply will influence the ability to heat the tumour. It also influences the type of microenvironment that exists within tumours, and it is now well-established that cells existing in areas of oxygen deficiency, nutrient deprivation and acidic conditions are more sensitive to the effect of hyperthermia. The vascular supply and microenvironment are also affected by hyperthermia. In general, mild heat temperatures transiently improve blood flow and oxygenation, while higher hyperthermia temperatures cause vascular collapse and so increase the adverse microenvironmental conditions. Being able to image these vascular and microenvironmental parameters both before and after heating will help in our ability to predict and assess response. Here we review the various techniques that can be applied to supply this information, especially using non-invasive imaging approaches.

Effect of local hyperthermia on lymphangiogenic factors VEGF-C and -D in a nude mouse xenograft model of tongue squamous cell carcinoma

It is widely accepted that tumor-induced lymphangiogenesis driven by vascular endothelial growth factor (VEGF)-C- and/or VEGF-D-induced activation of VEGF receptor (VEGFR)-3 could promote lymphatic metastasis. In this study, tumor growth and intratumoral expression level of VEGF-C and -D following administration of local hyperthermia was evaluated in nude mice model of tongue squamous cell carcinoma (SCC). The data demonstrated that the size of tumor in local hyperthermia was 26.5% of control group, and that local hyperthermia markedly suppressed the mRNA and protein expression of VEGF-C and -D as determined by quantitative real-time RT-PCR, Western blot and immunohistochemistry (P<0.05). These results suggest that, accompanying with tumor growth inhibition, local hyperthermia may act as an anti-lymphangiogenic role by suppressing the expression of tumor VEGF-C and -D, and thereby inhibiting cancer cell lymphatic metastasis in tongue SCC.

Mechanisms of Focal Heat Destruction of Liver Tumors

BACKGROUND

Focal heat destruction has emerged as an effective treatment strategy in selected patients with malignant liver tumors. Radiofrequency ablation, interstitial laser thermotherapy, and microwave treatment are currently the most widely applied thermal ablative techniques. A major limitation of these therapies is incomplete tumor destruction and overall high recurrences. An understanding of the mechanisms of tissue injury induced by focal hyperthermia is essential to ensure more complete tumor destruction. Here, the currently available scientific literature concerning the underlying mechanisms involved in the destruction of liver tumors by focal hyperthermia is reviewed.

METHODS

Medline was searched from 1960 to 2004 for literature regarding the use of focal hyperthermia for the treatment of liver tumors. All relevant literature was searched for further references.

RESULTS

Experimental evidence suggests that focal hyperthermic injury occurs in two distinct phases. The first phase results in direct heat injury that is determined by the total thermal energy applied, tumor biology, and the tumor microenvironment. Tumors are more susceptible to heat injury than normal cells as the result of specific biological features, reduced heat dissipating ability, and lower interstitial pH. The second phase of hyperthermic injury is indirect tissue damage that produces a progression of tissue injury after the cessation of the initial heat stimulus. This progressive injury may involve a balance of several factors, including apoptosis, microvascular damage, ischemia-reperfusion injury, Kupffer cell activation, altered cytokine expression, and alterations in the immune response. Blood flow modulation and administration of thermosensitizing agents are two methods currently used to increase the extent of direct thermal injury. The processes involved in the progression of thermal injury and therapies that may potentially modulate them remain poorly understood.

CONCLUSION

Focal hyperthermia for the treatment of liver tumors involves complex mechanisms. Evidence suggests that focal hyperthermia produces both direct and indirect tissue injury by differing underlying processes. Methods to enhance the effects of treatment to achieve complete tumor destruction should focus on manipulating these processes.

Focal hyperthermia produces progressive tumor necrosis independent of the initial thermal effects

Focal hyperthermia, produced using laser, radio frequency, and microwave, is used to treat liver tumors. The exact mechanisms of tissue destruction using focal hyperthermia are, however, unknown. Clinical and experimental studies suggest a progression of injury after cessation of the initial heat stimulus. This study investigates the mechanisms and time sequence of progressive tissue necrosis induced using focal hyperthermia in a murine model of colorectal liver metastases. Focal hyperthermia produced using a neodymium-yttrium aluminum garnet (Nd-YAG) laser source was applied to the normal liver and colorectal cancer liver metastases in inbred male CBA strain mice. The extent of direct lethal thermal injury was assessed histochemically using vital stain for nicotinamide adenine dinucleotide (NADH) diaphorase immediately after laser application. Tissue injury at subsequent time points was assessed using both NADH diaphorase staining and routine histology to determine the temporal relationship between tissue necrosis and time. Thermal injury occurring immediately after the application of 100 joules of energy was greater in the tumor tissue than in the normal liver (mean [standard error of the mean (SEM)]), measuring 23.5 (3.4) and 16.3 (2.6) mm(3), respectively (P=0.046), despite similar tissue temperature profiles. There was a significant increase in tissue necrosis after initial injury that was greater in the normal liver than in the tumor tissue. In the normal liver, the peak volume of necrosis was 137.4 (9.8) mm(3) and occurred at 3 days, whereas in the tumor tissue the peak was 49.0 (3.5) mm(3) at 4.5 days (P < 0.001). Focal hyperthermia produces tissue necrosis that occurs in two phases. The first phase is caused by the direct lethal thermal injury followed by a second phase involving a progression of necrosis beyond the initial thermal effects. The normal liver and the tumor tissue responded differently to focal hyperthermia. In the tumor tissue, the direct injury is more pronounced, whereas the progression of injury is more rapid and extensive in the normal liver.

Progressive microvascular injury in liver and colorectal liver metastases following laser induced focal hyperthermia therapy

BACKGROUND AND OBJECTIVES

Focal hyperthermia by laser or radiofrequency is currently the preferred method for local ablation of liver tumors. The underlying mechanism of action of focal hyperthermia, in particular the relationship between the microvascular and tissue effect is uncertain and was investigated in a murine model.

STUDY DESIGN/MATERIALS AND METHODS

Focal hyperthermia produced by a Neodymium-Yttrium-Aluminium-Garnet laser (wavelength 1,064 nm) was applied to the normal liver and colorectal cancer liver metastases in inbred male CBA strain mice at 2 W for 50 seconds (100 J). Tissue injury was assessed at 0, 24, 48, 72, 120, and 168 hours following therapy by measurements of necrosis in tissue sections stained for nicotinamide adenine dinucleotide (NADH) diaphorase activity. Characteristics of microvascular injury were assessed at the various time points by scanning electron microscopy (SEM) of vascular resin casts, Laser Doppler flowmetry, and confocal in vivo microscopy.

RESULTS

Focal hyperthermia produced progressive tissue and vascular injury. There was an initial reduction in blood flow and increased vascular permeability in the microcirculation of both tumor and liver tissue immediately following heat application as assessed by laser Doppler flowmetry and confocal in vivo microscopy, respectively. SEM of vascular casts showed heterogeneous regions of microvascular injury immediately following heat application. The extent of initial vascular disruption or occlusion on SEM of vascular resin casts (mean+/-SE) was significantly less than the tissue necrosis in liver (1.9+/-0.1 mm vs. 3.0 mm+/-0.2 mm P<0.001), but was equivalent to the tissue injury in tumor tissue (3.5 mm+/-0.2 mm vs. 3.6 mm+/-0.1 mm P = 0.907). This was followed by a progressive increase in both microvascular and tissue injury in liver and tumor that peaked by 72 hours following treatment. The peak microvascular injury and tissue damage in liver (6.6 mm+/-0.2 and 6.3 mm+/-0.2 mm, respectively) was significantly greater than the extent of microvascular and tissue damage in tumors (4.8 mm+/-0.2 mm and 4.5 mm+/-0.2 mm, respectively) (P<0.001). The progression of microvascular injury in liver and tumor at times preceded the tissue injury.

CONCLUSION

Focal hyperthermia produces both progressive microvascular and tissue damage in liver and colorectal liver metastases. An increase in tissue injury following focal hyperthermia may be a direct result of progressive microvascular damage. Tumor vessels appear more susceptible to direct focal hyperthermia destruction than liver sinusoids. The liver sinusoids are however more susceptible to progressive damage or occlusion following the initial laser thermal stimulus.

Microenvironmental adaptation of experimental tumours to chronic vs acute hypoxia

This study investigated long-term microenvironmental responses (oxygenation, perfusion, metabolic status, proliferation, vascular endothelial growth factor (VEGF) expression and vascularisation) to chronic hypoxia in experimental tumours. Experiments were performed using s.c.-implanted DS-sarcomas in rats. In order to induce more pronounced tumour hypoxia, one group of animals was housed in a hypoxic atmosphere (8% O₂) for the whole period of tumour growth (chronic hypoxia). A second group was acutely exposed to inspiratory hypoxia for only 20 min prior to the measurements (acute hypoxia), whereas animals housed under normal atmospheric conditions served as controls. Acute hypoxia reduced the median oxygen partial pressure (pO₂) dramatically (1 vs 10 mmHg in controls), whereas in chronically hypoxic tumours the pO₂ was significantly improved (median pO₂=4 mmHg), however not reaching the control level. These findings reflect the changes in tumour perfusion where acutely hypoxic tumours show a dramatic reduction of perfused tumour vessels (maybe the result of a simultaneous reduction in arterial blood pressure). In animals under chronic inspiratory hypoxia, the number of perfused vessels increased (compared to acute hypoxia), although the perfusion pattern found in control tumours was not reached. In the chronically hypoxic animals, tumour cell proliferation and tumour growth were significantly reduced, whereas no differences in VEGF expression and vascular density between these groups were observed. These results suggest that long-term adaptation of tumours to chronic hypoxia in vivo, while not affecting vascularity, does influence the functional status of the microvessels in favour of a more homogeneous perfusion.

Combined hyperthermia and chlorophyll-based photodynamic therapy: tumour growth and metabolic microenvironment

The effects of combined and simultaneously applied localised 43 degrees C hyperthermia (HT) and an antivascular bacteriochlorophyll-serine-based photodynamic therapy (Bchl-ser-PDT) on tumour growth and several microenvironmental parameters were examined. Rats bearing DS-sarcomas were allocated to treatment groups: (i) sham-treatment (control), (ii) Bchl-ser-PDT (20 mg kg(-1) i.v.), (iii) localised HT, (iv) Bchl-ser-PDT+HT. The light source used was an infrared-A irradiator, which, by use of appropriate filters, delivered the different ranges of wavelengths required. Following treatment, tumour volume was monitored. The greatest tumour growth inhibition was seen with Bchl-ser-PDT+HT, and subsequent experiments identified the pathophysiological basis for this effect. Red blood cell flux in tumour microvessels declined rapidly upon Bchl-ser-PDT+HT, reaching approximately 10% of initial values by the end of treatment. Similarly, tumour oxygenation worsened, reaching almost anoxic levels by the end of the treatment period. Assessment of metabolic parameters showed a pronounced increase in lactate levels and a decrease in ATP concentrations after combined treatment. The results presented suggest that vascular collapse and flow stasis resulting in a deterioration of tumour oxygenation and a switch from oxidative to glycolytic glucose turnover are key elements in the tumour eradication seen with this novel approach in which an antivascular PDT and HT are combined and simultaneously applied.

Microcirculatory function, tissue oxygenation, microregional redox status and ATP distribution in tumors upon localized infrared-A-hyperthermia at 42 degrees C

Since local hyperthermia (HT) affects microenvironmental parameters, the aim of the study was to analyze the impact of 42 degrees C-HT on microcirculatory function, tumor pO2, microregional redox status and ATP distribution in experimental rat tumors. Subcutaneously growing DS-sarcomas were treated with localized HT using infrared-A radiation resulting in a mean tumor temperature of 42 degrees C. The relative red blood cell (RBC) flux in the tumor was assessed using the laser Doppler technique and the mean tumor pO2 measured continuously using O2-sensitive catheter electrodes. In a second series of experiments, the microregional distribution of the mitochondrial redox status and ATP concentration was measured. Although the average RBC flux increased by 63%, tumor pO2 rose only by approx. 6%. No distinct changes were seen in the mitochondrial redox status. The microregional distribution of the redox status as well as of the ATP concentration showed considerable heterogeneity. In conclusion, although 42 degrees C-HT leads to a distinct improvement in tumor perfusion, there is practically no change in the oxygenation status. The latter finding can be explained by an equivalent increase in the oxygen consumption rate of the cells which increases by approx. 58% at 42 degrees C compared to normothermia.

Intensified oxidative and nitrosative stress following combined ALA-based photodynamic therapy and local hyperthermia in rat tumors

Oxidative stress-related changes in tumors upon localized hyperthermia (HT), 5-aminolevulinic acid-based photodynamic therapy (ALA-PDT) and their combination (ALA+HT) were examined after the observation that the antitumor effects of ALA-PDT could be significantly enhanced upon simultaneous application of HT. Rats bearing s.c. DS-sarcomas (0.6-1.0 ml) on the hind foot dorsum were anesthetized and underwent one of the following treatments: (i) ALA-PDT (375 mg/kg 5-ALA i.v.); (ii) localized HT, 43 degrees C for 60 min; (iii) combined ALA-PDT and HT [=ALA+HT]. Appropriate control experiments were also performed. After treatment, tumors were excised and rapidly frozen for later analysis of nitrosative stress (protein nitration), apoptotic events (TUNEL, caspase activation, DNA and RNA fragmentation), expression of heat shock proteins (hsp70 and HO-1), glutathione (GSH) levels and glutathione peroxidase (GPx) activity. Protein nitration was found to increase upon treatment, being especially pronounced in the ALA+HT group, and could partially be related to areas surrounding microvessels. The extent of nitrosative stress also correlated well with the appearance of the markers of apoptosis and the inhibition of in vivo tumor growth as seen in a previous study. GSH levels decreased upon treatment, the reduction being most prominent in the ALA-PDT and ALA+HT groups. GPx activity, however, showed a significant decrease only in the ALA-PDT group. Whereas hsp70 expression increased upon HT, ALA-PDT caused a decrease, and these opposing effects were nullified with ALA+HT. The results obtained point to a number of cellular mechanisms-including effects on cellular defense mechanisms and an abrogation of the heat shock defense mechanism-that may interact to achieve the potentiated tumor response rate seen in vivo upon combined treatment.

Enhanced effects of aminolaevulinic acid-based photodynamic therapy through local hyperthermia in rat tumours

The possibility of enhancing aminolaevulinic acid (ALA)-based photodynamic therapy (PDT) by simultaneous application of localised hyperthermia (HT) was evaluated. Treatments of rat DS-sarcomas included: (i) control, (ii) ALA administration (375 mg kg(-1), i.p.), no illumination, (iii) 'nonthermal' illumination, (iv) ALA-PDT: that is, ALA administration, 'nonthermal' illumination, (v) localised HT, 43 degrees C, 60 min (vi) ALA-PDT+HT: ALA administration with full spectrum irradiation resulting in ALA-PDT and HT. Tumour volume was monitored for 90 days or until a target volume (3.5 ml) was reached. No differences were seen between the first three groups, with all tumours reaching the target volume by 8-11 days. A total of 13 and 15% of tumours did not reach the target volume by day 90 following HT or ALA-PDT treatment, respectively. ALA-PDT+HT showed the greatest antitumour effect (P=0.0001), with 61% of the tumours not reaching the target volume. Viability and in vitro growth were also assessed in cells from tumours excised after treatment. ALA-PDT+HT reduced the fraction of viable tumour cells by 85%, and in vitro culture showed pronounced growth delay compared to control cells. These results demonstrate an enhanced antitumour effect upon ALA+HT, which appears to involve direct cell toxicity rather than solely vascular damage.

Tumour tissue monitoring during photodynamic and hyperthermic treatment using bioimpedance spectroscopy

Electrical bioimpedance spectroscopy is a fast and relatively easily applicable method for tissue characterization. In the frequency range up to 10 MHz, current conduction through tissue is mainly determined by tissue structure, i.e. the extra- and intra-cellular compartments and the insulating cell membranes. Therefore, changes in the extra- and intra-cellular fluid volumes are reflected in the impedance spectra. Investigations of tumours (DS sarcoma, implanted on the hind foot dorsum of rats) during treatment with localized hyperthermia (HT), photodynamic therapy (PDT) and the combination of these two components were carried out using impedance spectroscopy in the frequency range of 37 Hz to 3.7 MHz. Data collected reveal totally different, but characteristic, behaviour patterns for the three treatments. HT caused a slow increase in conductance at high frequencies (G(HF)) and in the extracellular space index (ECSI), indicating an increase in extracellular fluid volume and in total fluid content. With PDT, G(HF) increased immediately upon commencement of irradiation and was accompanied by a distinct decrease in ECSI, indicating the development of a pronounced intracellular oedema.

Nifedipine improves blood flow and oxygen supply, but not steady-state oxygenation of tumours in perfusion pressure-controlled isolated limb perfusion

Isolated limb perfusion allows the direct application of therapeutic agents to a tumour-bearing extremity. The present study investigated whether the dihydropyridine-type Ca(2+)-channel blocker nifedipine could improve blood flow and oxygenation status of experimental tumours during isolated limb perfusion. Perfusion was performed by cannulation of the femoral artery and vein in rats bearing DS-sarcoma on the hind foot dorsum. Perfusion rate was adjusted to maintain a perfusion pressure of 100-140 mmHg throughout the experiment. Following equilibration, nifedipine was continuously infused for 30 min (8.3 microg min(-1) kg(-1) BW). During constant-pressure isolated limb perfusion, nifedipine can significantly increase perfusion rate (+100%) and RBC flux (+60%) through experimental leg tumours. "Steal phenomena" in favour of the surrounding normal tissue and oedema formation were not observed. Despite the increased oxygen availability (+63%) seen upon application of this calcium channel blocker, nifedipine does not result in a substantial reduction of tumour hypoxia, most probably due to an increase in O(2) uptake with rising O(2) supply to the tumour-bearing hind limb. Nifedipine application during isolated limb perfusion can enhance tumour microcirculation and may therefore promote the delivery (pharmacokinetics) of anti-cancer drugs to the tumour and by this improve the efficacy of pressure-controlled isolated limb perfusion.

Dynamics of tumor oxygenation and red blood cell flux in response to inspiratory hyperoxia combined with different levels of inspiratory hypercapnia

BACKGROUND AND PURPOSE

Increasing arterial oxygen partial pressure (pO2) by breathing hyperoxic gases is an effective means of improving tumor oxygenation, although the efficacy of adding CO2 to the inspiratory gas has been discussed controversially. This study aimed at analyzing the impact of different inspiratory CO2 fractions on the time course of oxygenation and perfusion changes in experimental tumors during and after inspiratory hyperoxia.

MATERIAL AND METHODS

Perfusion and oxygenation of rat DS-sarcomas were studied during spontaneous breathing of pure oxygen or hyperoxic gas mixtures containing different CO2 fractions (1, 2.5 or 5%). Red blood cell (RBC) flux was assessed as a measure of tumor perfusion using the laser Doppler technique and temporal changes in mean tumor pO2 were measured polarographically.

RESULTS

Mean tumor pO2 increased 3.6-fold with pure oxygen, approx. 3.3-fold when 1 or 2.5% CO2 was added and 2.7-fold during carbogen breathing. RBC flux also increased by 25-30% with all gases. With pure oxygen and with 1% CO2 (+99% O2), perfusion changes paralleled those of the mean arterial blood pressure whereas with higher CO2 fractions, a decrease in resistance to flow was observed. The differences found with the various gas mixtures were more pronounced after the end of hyperoxia. With pure oxygen, perfusion immediately returned to pretreatment values whereas with higher CO2 fractions perfusion remained elevated for at least 30 min.

CONCLUSIONS

Higher inspiratory CO2 fractions (2.5 or 5%) lead to a prolonged improvement of tumor perfusion after the end of inspiratory hyperoxia when compared with pure oxygen breathing. Since no principal differences in oxygenation and perfusion were seen between the gases containing 2.5 and 5% CO2, the former may be preferable for inspiratory hyperoxia.

Intratumor heterogeneity in perfusion in human melanoma xenografts measured by contrast-enhanced magnetic resonance imaging

The perfusion in tumors shows substantial spatial heterogeneity compared to that in normal tissues. The aim of the present study was to evaluate the intratumor heterogeneity in perfusion in tumors of two amelanotic human melanoma xenograft lines, A-07 and R-18, grown intradermally in Balb/c nu/nu mice. A non-invasive contrast-enhanced magnetic resonance imaging method yielding results in absolute values was applied. The perfusion was determined in manually defined regions of interest, corresponding to a whole tumor or to subregions of a tumor. The mean perfusion and the intertumor heterogeneity in perfusion were similar for the two tumor lines. For whole A-07 tumors, the perfusion ranged from 0.089 mL/(g . min) to 0.20 mL/(g . min) [mean: 0.15 mL/(g . min)], and for whole R-18 tumors, from 0.030 mL/(g . min) to 0.17 mL/(g . min) [mean: 0.13 mL/(g . min)]. The intratumor heterogeneity, on the other hand, was estimated to be 6.4 times larger in A-07 tumors than in R-18 tumors. The highest perfusion values, up to 0.69 mL/(g . min), were found in subregions of A-07 tumors. The intratumor heterogeneity was substantially larger than the intertumor heterogeneity in A-07 tumors, whereas in R-18 tumors, the intratumor heterogeneity was similar to the intertumor heterogeneity. These observations imply that measurements of mean tumor perfusion may have limited value as a predictive assay for outcome of treatment.

Disparate responses of tumour vessels to angiotensin II: tumour volume-dependent effects on perfusion and oxygenation

Perfusion and oxygenation of experimental tumours were studied during angiotensin II (AT II) administration whereby the rate of the continuous AT II infusion was chosen to increase the mean arterial blood pressure (MABP) by 50-70 mmHg. In subcutaneous DS-sarcomas the red blood cell (RBC) flux was assessed using the laser Doppler technique and the mean tumour oxygen partial pressure (pO2) was measured polarographically using O2-sensitive catheter and needle electrodes. Changes in RBC flux with increasing MABP depended mainly on tumour size. In small tumours, RBC flux decreased with rising MABP whereas in larger tumours RBC flux increased parallel to the MABP. As a result of these volume-dependent effects on tumour blood flow, the impact of AT II on tumour pO2 was also mainly tumour volume-related. In small tumours oxygenation decreased with increasing MABP during AT II infusion, whereas in large tumours a positive relationship between blood pressure and O2 status was found. This disparate behaviour might be the result of the co-existence of two functionally distinct populations of tumour vessels. In small tumours, perfusion decreases presumably due to vasoconstriction of pre-existing host vessels feeding the tumour. In larger malignancies, newly formed tumour vessels predominate and seem not to have this vasoresponsive capability (lack of smooth muscle cells and/or AT receptors), resulting in an improvement of perfusion which is not tumour-related per se, but is due to the increased perfusion pressure.

Quantitative changes of metabolic and bioenergetic parameters in experimental tumors during fractionated irradiation

PURPOSE

Previous studies with rat rhabdomyosarcomas indicate that during fractionated irradiation profound alterations of the tumor microvasculature and the oxygenation status occur when the total dose exceeds 45 Gy. At this dose a destruction which included all structures of the vessels and a significant worsening in tumor oxygenation were found. The aim of the present study was to analyze whether these effects of fractionated irradiation on the microvasculature and on tumor oxygenation also induce changes in the bioenergetic and metabolic status in the tumors during radiation treatment.

METHODS AND MATERIALS

R1H rhabdomyosarcomas of the rat implanted into the flank were irradiated with 60Co-gamma-rays using 5 fractions of 3 Gy per week over 5 weeks. During this irradiation schedule, tumors were investigated each week for the microregional distributions of glucose, lactate, and ATP concentrations. For this, tumors were rapidly excised, shock-frozen and quantitative bioluminescence measurements were performed on tumor tissue sections.

RESULTS

ATP concentrations remained unchanged during fractionated irradiation up to a total dose of 45 Gy. Above this dose, a significant decrease in ATP levels was observed. Lactate concentrations changed only slightly during irradiation whereas glucose levels increased continuously over the whole irradiation period.

CONCLUSIONS

During fractionated irradiation of R1H tumors with a total dose of 75 Gy, the bioenergetic and metabolic status of the tumors changed considerably. This became most obvious once a dose of 45 Gy had been achieved. The severe energy depletion and worsening of tumor oxygenation might be the result of destruction of tumor blood vessels as has been described previously in the same tumor model. The modification of the tumor micromilieu appears to be an important parameter in the responsiveness of tumor cells to radiation and for local tumor control.

Water-filtered infrared-A radiation: a novel technique for localized hyperthermia in combination with bacteriochlorophyll-based photodynamic therapy

A novel application of an infrared-A (IR-A) radiation source equipped with a water-filter in the radiation path is described, which allows for tumour treatment with a simultaneous combination of localized hyperthermia (HT) and bacteriochlorophyll-serine (Bchl-ser) based photodynamic therapy (PDT). Using this system, the IR-A radiation was used to heat tumours to 43 degrees C for 60 min, while at the same time activating the Bchl-ser which was injected i.v. at a dose of 20 mg/kg, 10 min following commencement of HT. The growth of tumours undergoing this combined therapy was compared to that of tumours undergoing HT alone or sham-treated controls. Within the 90 day observation period, 100% of tumours in sham-treated animals, 80% in HT-treated animals and only 17% in HT + Bchlser-treated animals reached the end point target volume of 3.5 ml. Thus, the tumour growth inhibition effect of HT can be substantially enhanced by combination with Bchl-ser-PDT. This novel technique has proved to be well-tolerated, easy to apply and should be suitable for treatment of superficial malignancies, especially where hypoxic tumour areas are present.

Regional perfusion and oxygenation of tumors upon methylxanthine derivative administration

PURPOSE

The use of methylxanthine derivatives has been postulated as a means of increasing tumor perfusion and thus ameliorating tumor hypoxia. The aim of this study was to quantify and compare the effects of three methylxanthine derivatives: pentoxifylline (PX), torbafylline (TB), and HWA 138 (HW) on tumor perfusion and oxygenation.

METHODS AND MATERIALS

Anesthetized Sprague Dawley rats with DS-sarcomas implanted subcutaneously onto the hind foot dorsum were used in this study. Mean arterial blood pressure (MABP) was measured throughout experiments. Regional red blood cell (RBC) flux was monitored using a multichannel laser Doppler device and tumor oxygenation on a more global level was assessed polarographically using an O2-sensitive catheter electrode. The methylxanthine derivatives were administered as a single dose intraperitoneally (for PX 50 mg/kg; for TB and HW 75 mg/kg).

RESULTS

Following drug administration, initial decreases in MABP down to 75% of baseline values were observed for all three substances. PX, HW, and TB caused initial transient reductions in mean RBC flux followed by gradual increases to values of 137 +/- 27%, 139 +/- 14%, and 122 + 14% respectively at t = 60 min. Following a small initial decrease upon drug administration, O2 partial pressure (pO2) rose to 160 +/- 31%, 153 +/- 34%, and 121 +/- 11% for PX, HW, and TB, respectively at t = 60 min. At the end of the observation period (t = 90 min), increases in RBC flux and pO2 were still evident. When individual tumors were considered, a variety of patterns (including opposing effects) for changes in RBC flux were seen, not necessarily reflected in the mean values. Thus, while the methylxanthine derivatives caused an increased average tumor perfusion, there is evidence suggesting that a redistribution of tumor blood flow occurs which may amplify preexisting heterogeneity.

CONCLUSIONS

Substantial improvements in tumor oxygenation and perfusion were observed after administration of the methylxanthine derivatives. These substances may therefore be of use during tumor therapies in which the outcome may be detrimentally affected by the presence of hypoxia.

Modulation of tumor oxygenation

There is a large body of evidence suggesting that deficiencies in the O2 supply of tumors exist due to restrictions (i) in the O2 delivery by perfusion and/or diffusion, and (ii) in the O2 transport capacity. Whereas the former are mostly based on inadequate and heterogeneous microcirculatory functions, the latter are predominantly due to tumor-associated anemia. Possible uses and limitations of measures are discussed which can increase the microvascular O2 content and thus may preferentially serve to enhance diffusion-limited O2 availability. In addition, means are described for improving and increasing the uniformity of microcirculation thus possibly enhancing perfusion-limited O2 delivery. Reducing cellular respiration rate should be of benefit in both pathophysiological conditions. Because both types of O2 limitation coexist in solid tumors, appropriate combinations should be aimed at eradicating tumor hypoxia which is present in at least one third of cancers in the clinical setting.

Bioluminescence imaging of metabolites in a human tumour xenograft after treatment with hyperthermia and/or the radiosensitizer pimonidazole

The levels and distribution of ATP, glucose and lactate in human tumour xenografts following either single or combined treatment with hyperthermia (43 degrees C for 30 min) and/or pimonidazole (1 mg/g b.w) were determined by bioluminescence and compared with the mean 'global' levels obtained from the same tumours using conventional biochemical analysis. In general, the levels of ATP, glucose and lactate measured with both methods were in good agreement although the latter was consistently lower after bioluminescence determination. Compared with controls, neither the levels of ATP nor glucose were greatly affected in this tumour following treatment with the various modalities, whereas those of lactate were considerably increased as determined by both methods. The spatial distribution of ATP and glucose from controls and treated tumours was largely confined to the periphery and generally remained unchanged irrespective of treatment without any apparent alterations in the shape of the distribution curve. However, the increased lactate levels tended to accumulate towards the central region of the tumour after hyperthermia and/or sensitizer, showing an almost Gaussian-like distribution compared with controls. These results are in agreement with previous studies of global tumour metabolism showing an enhanced glycolytic activity with increased lactate levels after single or combined modality treatment.

Changes in tumor oxygenation during a combined treatment with fractionated irradiation and hyperthermia: an experimental study

PURPOSE

To determine the influence of adjuvant hyperthermia on the oxygenation status of fractionated irradiated tumors.

METHODS AND MATERIALS

Oxygen partial pressure (pO2) in rat rhabdomyosarcomas (R1H) was measured sequentially at weekly intervals during a fractionated irradiation with 60Co-gamma-rays (60 Gy/20f/4 weeks) in combination with local hyperthermia (8 f(HT) at 43 degrees C, 1 h/4 weeks). Tumors were heated twice weekly with a 2450 MHz microwave device at 43 degrees C, 1 h starting 10 min after irradiation. The pO2 measurements (pO2-histograph, Eppendorf, Germany) were performed in anesthetized animals during mechanical ventilation and in hemodynamic steady state. All tumor pO2 measurements were correlated to measurements of the arterial oxygen partial pressure (paO2) determined by a blood gas analyzer.

RESULTS

The oxygenation status of R1H tumors decreased continuously from the start of the combined treatment, with increasing radiation dose and number of heat fractions. In untreated controls a median tumor pO2 of 23 +/- 2 mmHg (mean +/- SEM) was measured. Tumor pO2 decreased to 11 +/- 2 mmHg after 30 Gy + 4 HT (2 weeks), and to 6 +/- 2 mmHg after 60 Gy + 8HT (4 weeks). The increase in the frequency of pO2-values below 5 mmHg and the decrease in the range of the pO2 histograms [delta p(10/90)] further indicated that tumor hypoxia increased relatively rapidly from the start of combined treatment. After 60 Gy + 8HT 48 +/- 5% (mean +/- SEM) of the pO2-values recorded were below 5 mmHg.

CONCLUSIONS

These findings suggest that adjuvant hyperthermia to radiotherapy induces greater changes in tumor oxygenation than radiation alone [cf. (39)]. This might be of importance for the temporary application of hyperthermia in the course of a conventional radiation treatment.

Divergent changes of flow through individual blood vessels upon localized heating

The changes of microregional perfusion in a hamster cheek pouch membrane were investigated. The vessel network of the membrane was visualized by preparing a transparent chamber, which was heated with circulating water at 42 degree C. Blood perfusion was monitored by using a laser Doppler flowmeter (LDF), which was used either in a conventional way by positioning the probe stationary or in a novel way by constantly moving the probe over the surface of the chamber (scanning). When a segment of tissue was subjected to the LDF scanning, the profile of scanned LDF values was well correlated with the distribution of vessels. Therefore, this scanning technique was useful in localizing the probe in tissues with respect to vessels. Since the scanning can be repeated every other minute, this technique also offered continuous monitoring of tissue blood flow at multiple sites. Upon heating, different vessels individually responded to the first and second heatings followed by coolings, suggesting a heterogeneous heat response in the connective tissue of the hamster cheek pouch membrane. This scanning technique proved very useful in collecting information for the study of the heterogeneous nature of blood flow in normal and tumour tissues.

Accumulation of purine catabolites in solid tumors exposed to therapeutic hyperthermia

Intensified adenosine triphosphate (ATP) degradation following therapeutic hyperthermia is often observed in solid tumors. As a result, accumulation of purine catabolites can be expected together with formation of protons at several stages during degradation to the final product, uric acid. Proton formation in turn can contribute to the development of heat-induced acidosis. Furthermore, oxidation of hypoxanthine and xanthine may result in generation of reactive oxygen species, which may lead to DNA damage, lipid peroxidation and protein denaturation, thus also contributing to heat-induced cytotoxicity. In hyperthermia experiments a tumor-size-dependent, significant increase in the levels of the following catabolites has been demonstrated: [symbol: see text] [IMP + GMP] (sum of guanosine and inosine monophosphate levels), inosine, hypoxanthine, xanthine and uric acid, along with a drop in ATP and guanosine triphosphate (GTP) levels. These data suggest that formation of reactive oxygen species and protons during purine degradation may indeed play a significant role in the antitumor effect of hyperthermia.