Demonstration of tumor-selective retention of fluorinated nitroimidazole probes by 19F magnetic resonance spectroscopy in vivo

Synthesis and characterization of a hypoxia-sensitive MRI probe

Tissue hypoxia occurs in pathologic conditions, such as cancer, ischemic heart disease and stroke when oxygen demand is greater than oxygen supply. An imaging method that can differentiate hypoxic versus normoxic tissue could have an immediate impact on therapy choices. In this work, the gadolinium(III) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) with a 2-nitroimidazole attached to one carboxyl group via an amide linkage was prepared, characterized and tested as a hypoxia-sensitive MRI agent. A control complex, Gd(DO3A-monobutylamide), was also prepared in order to test whether the nitroimidazole side-chain alters either the water proton T(1) relaxivity or the thermodynamic stability of the complex. The stabilities of these complexes were lower than that of Gd(DOTA)(-) as expected for mono-amide derivatives. The water proton T(1) relaxivity (r(1)), bound water residence lifetime (τ(M)) and rotational correlation time (τ(R)) of both complexes was determined by relaxivity measurements, variable temperature (17) O NMR spectroscopy and proton nuclear magnetic relaxation dispersion (NMRD) studies. The resulting parameters (r(1) =6.38 mM(-1)  s(-1) at 20 MHz, τ(M) =0.71 μs, τ(R) =141 ps) determined for the nitroimidazole derivative closely parallel to those of other Gd(DO3A-monoamide) complexes of similar molecular size. In vitro MR imaging experiments with 9L rat glioma cells maintained under nitrogen (hypoxic) versus oxygen (normoxic) gas showed that both agents enter cells but only the nitroimidazole derivative was trapped in cells maintained under N(2) as evidenced by an approximately twofold decrease in T(1) measured for hypoxic cells versus normoxic cells exposed to this agent. These results suggest that the nitroimidazole derivative might serve as a molecular reporter for discriminating hypoxic versus normoxic tissues by MRI.

MRI & MRS assessment of the role of the tumour microenvironment in response to therapy

MRI and MRS techniques are being applied to the characterisation of various aspects of the tumour microenvironment and to the assessment of tumour response to therapy. For example, kinetic parameters describing tumour blood vessel flow and permeability can be derived from dynamic contrast-enhanced MRI data and have been correlated with a positive tumour response to antivascular therapies. The ongoing development and validation of noninvasive, high-resolution anatomical/molecular MR techniques will equip us with the means to detect specific tumour biomarkers early on, and then to monitor the efficacy of cancer treatments efficiently and reliably, all within a clinically relevant time frame. Reliable tumour microenvironment imaging biomarkers will provide obvious advantages by enabling tumour-specific treatment tailoring and potentially improving patient outcome. However, for routine clinical application across many disease types, such imaging biomarkers must be quantitative, robust, reproducible, sufficiently sensitive and cost-effective. These characteristics are all difficult to achieve in practice, but image biomarker development and validation have been greatly facilitated by an increasing number of pertinent preclinical in vivo cancer models. Emphasis must now be placed on discovering whether the preclinical results translate into an improvement in patient care and, therefore, overall survival.

Imaging tumor hypoxia by magnetic resonance methods

Tumor hypoxia results from the negative balance between the oxygen demands of the tissue and the capacity of the neovasculature to deliver sufficient oxygen. The resulting oxygen deficit has important consequences with regard to the aggressiveness and malignancy of tumors, as well as their resistance to therapy, endowing the imaging of hypoxia with vital repercussions in tumor prognosis and therapy design. The molecular and cellular events underlying hypoxia are mediated mainly through hypoxia-inducible factor, a transcription factor with pleiotropic effects over a variety of cellular processes, including oncologic transformation, invasion and metastasis. However, few methodologies have been able to monitor noninvasively the oxygen tensions in vivo. MRI and MRS are often used for this purpose. Most MRI approaches are based on the effects of the local oxygen tension on: (i) the relaxation times of (19)F or (1)H indicators, such as perfluorocarbons or their (1)H analogs; (ii) the hemodynamics and magnetic susceptibility effects of oxy- and deoxyhemoglobin; and (iii) the effects of paramagnetic oxygen on the relaxation times of tissue water. (19)F MRS approaches monitor tumor hypoxia through the selective accumulation of reduced nitroimidazole derivatives in hypoxic zones, whereas electron spin resonance methods determine the oxygen level through its influence on the linewidths of appropriate paramagnetic probes in vivo. Finally, Overhauser-enhanced MRI combines the sensitivity of EPR methodology with the resolution of MRI, providing a window into the future use of hyperpolarized oxygen probes.

In vivo19F Magnetic Resonance Spectroscopy and Chemical Shift Imaging of Tri-Fluoro-Nitroimidazole as a Potential Hypoxia Reporter in Solid Tumors

PURPOSE

2-Nitro-alpha-[(2,2,2-trifluoroethoxy)methyl]-imidazole-1-ethanol (TF-MISO) was investigated as a potential noninvasive marker of tissue oxygen levels in tumors using (19)F magnetic resonance spectroscopy (MRS) and (19)F chemical shift imaging.

EXPERIMENTAL DESIGNS

In vitro data were obtained using high-performance liquid chromatography on tumor cells incubated under varying oxygen conditions to determine the oxygen-binding characteristics. In vivo data were obtained using a well-characterized hypoxic murine breast tumor (MCa), in addition to studies on a rat prostate tumor model (R3327-AT) implanted in nude mice. Detection of intratumor (19)F signal from TF-MISO was done using MRS for up to 10 h following a 75 mg/kg i.v. injection. Localized distribution of the compound in the implanted MCa tumor has been imaged using slice-selective two-dimensional chemical shift imaging 6 h after injection.

RESULTS

The in vitro results showed that TF-MISO preferentially accumulates in cells incubated under anoxic conditions. The in vivo (19)F MR spectral features (line width and chemical shift) were recorded as a function of time after injection, and the results indicate that the fluorine atoms are indeed sensitive to changes in the local environment while still providing a detectable MR signal. Ex vivo spectra were collected and established the visibility of the (19)F signal under conditions of maximum hypoxia. Late time point (>6 h) tumor tissue concentrations, as obtained from (19)F MRS, suggest that TF-MISO is reduced and retained in hypoxic tumor. The feasibility of obtaining TF-MISO tumor distribution maps in a reasonable time frame was established.

CONCLUSIONS

Based on the results presented herein, it is suggested that TF-MISO has the potential to be a valid magnetic resonance hypoxia imaging reporter for both preclinical hypoxia studies and hypoxia-directed clinical therapy.

Prospects for bioreductive drug development

Bioreductive drugs are inactive prodrugs that are converted into potent cytotoxins under conditions of either low oxygen tension or in the presence of high levels of specific reductases. The biochemical basis for selectivity relies on the ability of oxygen to reverse the activation process and the presence of elevated reductase levels in some tumour types. Key criteria for an ideal bioreductive drug should include poor activity against aerobic cells, activation over a broad range of oxygen tensions and, penetration through the aerobic fraction of cells. In addition, the active drug should be capable of killing non-proliferating cells. Numerous compounds are currently at various stages of drug development but Mitomycin C, which is generally considered to be the prototype bioreductive drug, is the only one in clinical use today. Of the drugs currently being evaluated clinically, tirapazamine has definite clinical activity against a variety of solid tumours when used in combination with cisplatin. Other drugs, such as EO9 and various nitroimidazoles, have not been impressive in the clinic and further development is required to improve properties such as drug delivery in the case of indoloquinones. A novel approach to exploiting tumour hypoxia is the development of a gene-directed enzyme prodrug therapy (GDEPT) strategy, where a gene encoding for a prodrug activating enzyme has been placed under the control of a hypoxia responsive promoter sequence. It is generally recognised that bioreductive drugs must be directed towards patients whose tumours have hypoxic regions or have appropriate enzymological characteristics. In terms of identifying tumour hypoxia, there has been considerable progress in the development of nitroimidazole based hypoxia markers that can be detected either via non-invasive or invasive procedures. Another strategy currently undergoing preclinical evaluation is the use of agents that modulate tumour blood flow and synergistic effects have been reported between bioreductive drugs and photodynamic therapy or inhibitors of nitric oxide synthase for example. The development of clinically useful bioreductive drugs depends therefore on the expertise of scientists and clinicians with varying backgrounds. The purpose of this review is to describe and critically assess recent developments in this field, with particular emphasis being placed on drug development and strategies aimed at optimising bioreductive drug activity.

Utility of 19F MRS detection of the hypoxic cell marker EF5 to assess cellular hypoxia in solid tumors

BACKGROUND AND PURPOSE

The present studies were undertaken to determine whether 19F MRS could be used to quantify the binding of the pentafluorinated derivative of etanidazole (EF5) in hypoxic cells of solid tumors.

MATERIALS AND METHODS

A 4.7 T imaging magnet was used for the in situ and in vitro evaluation of EF5 signals. In order to develop a better understanding of these NMR measurements the characteristics of parent, reduced unbound, and reduced bound EF5 signals were examined in vitro using a 12 T spectrometer.

RESULTS

In situ data acquired using a 4.7 T imaging magnet, showed retention of EF5 signals in KHT sarcomas that was absent in muscles for 6 h after EF5 injection. In vitro studies showed no difference in the NMR detectable signal of parent and reduced unbound EF5. T2 values determined using parent EF5 samples revealed a T2 time of 675 ms. In contrast, EF5 bound to KHT tumor cells gave rise to signals of low intensity, broad line widths, and T2 relaxation times of less than 30 ms. When the same samples were analyzed using the 4.7 T imaging magnet, the CF3 and CF2 fluorine peaks were readily identifiable in the parent EF5 sample but no fluorine signal could be detected from EF5 bound to KHT tumor cells.

CONCLUSION

The inability to resolve bound EF5 metabolites even at high field strengths (12 T), coupled with the short T2 relaxation times of the bound EF5, and the limits of detection of the in situ applied imaging magnet (4.7 T), meant that hypoxic cells could not be quantified in tumors using the 19F MRS technique. In situ 19F MRS measurements of EF5 signals (parent/reduced unbound) may reflect conditions of tumor physiology and thus indicate the extent of tumor hypoxia but they are not capable of resolving the cellular oxygenation status of the tumor cells.

Current issues in the utility of 19F nuclear magnetic resonance methodologies for the assessment of tumour hypoxia

It is now well established that uncontrolled proliferation of tumour cells together with the chaotic and poorly regulated blood supply of solid tumours result in tissue hypoxia, and that hypoxic regions of tumours are resistant to radiotherapy and chemotherapy. The development and application of non-invasive methods to rapidly determine the degree and extent of tumour hypoxia in an individual tumour would clearly enhance cancer treatment strategies. This review describes the current status of two (19)F nuclear magnetic resonance (NMR) methodologies that have been exploited to investigate tumour hypoxia, namely: (i) (19)F NMR oximetry following administration of perfluorocarbons, from which tumour p(O)(2) measurements can be made; and (ii) (19)F NMR measurements of the tumour retention of fluorinated 2-nitroimidazoles.

Noninvasive Measurements of Capecitabine Metabolism in Bladder Tumors Overexpressing Thymidine Phosphorylase by Fluorine-19 Magnetic Resonance Spectroscopy

PURPOSE

Previous studies have shown that tumor response to capecitabine strongly correlates with tumor thymidine phosphorylase (TP). The aims of our study were to (a). investigate the pharmacological role of TP by measuring the pharmacokinetics (PK) of capecitabine in a human bladder tumor model that was characterized by the overexpression of TP and (b). develop the use of PK measurements for capecitabine by fluorine-19 magnetic resonance spectroscopy as a noninvasive surrogate marker for determining TP levels in tumors and for predicting tumor response to capecitabine in patients.

EXPERIMENTAL DESIGN

TP overexpressing (2T10) and control tumors were grown s.c. in nude mice. Mice were given a dose of capecitabine or 5'-deoxy-5-fluorouridine (5'DFUR). (19)F tumor spectra were acquired for determination of rate constants of capecitabine breakdown and buildup and subsequent breakdown of intermediates, 5'-deoxy-5-fluorocytidine (5'DFCR) and 5'DFUR. The rate constant of 5'DFUR breakdown was also evaluated.

RESULTS

The rate constant of breakdown of intermediates was significantly faster in 2T10 tumors than controls (P < 0.003). No significant differences in the rate of capecitabine breakdown or intermediate buildup were observed. The rate constant of 5'DFUR breakdown in the 2T10 tumors was doubled compared with controls (P < 0.001).

CONCLUSIONS

This study confirmed the expected pathway of capecitabine metabolism and showed that the level of TP was related to the rate of 5'DFUR conversion. Using in vivo fluorine-19 magnetic resonance spectroscopy to mea-sure the PK of capecitabine and its intermediate metabolites in tumors may provide a noninvasive surrogate method for determining TP levels in tumors and for predicting tumor response to capecitabine in patients.

GLUT-1 and CAIX as intrinsic markers of hypoxia in carcinoma of the cervix: relationship to pimonidazole binding

The presence of hypoxia in tumours results in the overexpression of certain genes, which are controlled via the transcription factor HIF-1. Hypoxic cells are known to be radioresistant and chemoresistant, thus, a reliable surrogate marker of hypoxia is desirable to ensure that treatment may be rationally applied. Recently, the HIF-1-regulated proteins Glut-1 and CAIX were validated as intrinsic markers of hypoxia by comparison with pO(2) measured using oxygen electrodes. We compare the expression of Glut-1 and CAIX with the binding of the bioreductive drug hypoxia marker pimonidazole. Pimonidazole was administered to 42 patients with advanced carcinoma of the cervix, 16 hr before biopsy. Sections of single or multiple biopsies were then immunostained for Glut-1 and CAIX, and the area of staining scored by eye, using a "field-by-field" semi-quantitative averaging system. Using 1 biopsy only, Glut-1 (r = 0.54, p = <0.001) correlated with the level of pimonidazole binding, and Glut-1 and CAIX expression also correlated significantly (r = 0.40, p = <0.009). Thus, our study has shown that HIF-1 regulated genes have potential for future use as predictors of the malignant changes mediated by hypoxia, and warrant further investigation as indicators of response to cancer therapy.

MRI of the tumor microenvironment

The microenvironment within tumors is significantly different from that in normal tissues. A major difference is seen in the chaotic vasculature of tumors, which results in unbalanced blood supply and significant perfusion heterogeneities. As a consequence, many regions within tumors are transiently or chronically hypoxic. This exacerbates tumor cells' natural tendency to overproduce acids, resulting in very acidic pH values. The hypoxia and acidity of tumors have important consequences for antitumor therapy and can contribute to the progression of tumors to a more aggressive metastatic phenotype. Over the past decade, techniques have emerged that allow the interrogation of the tumor microenvironment with high resolution and molecularly specific probes. Techniques are available to interrogate perfusion, vascular distribution, pH, and pO(2) nondestructively in living tissues with relatively high precision. Studies employing these methods have provided new insights into the causes and consequences of the hostile tumor microenvironment. Furthermore, it is quite exciting that there are emerging techniques that generate tumor image contrast via ill-defined mechanisms. Elucidation of these mechanisms will yield further insights into the tumor microenvironment. This review attempts to identify techniques and their application to tumor biology, with an emphasis on nuclear magnetic resonance (NMR) approaches. Examples are also discussed using electron MR, optical, and radionuclear imaging techniques.

Monitoring pharmacokinetics of anticancer drugs: non-invasive investigation using magnetic resonance spectroscopy

Magnetic resonance spectroscopy (MRS) offers the unique advantage of detecting, identifying and quantifying chemicals deep within the living body in a totally non-invasive manner. In studies on pharmacology and toxicology of anticancer drugs, MRS and the closely related technique magnetic resonance imaging (MRI) have many uses. MRS in particular, despite its low sensitivity, offers unique insights into pharmacokinetics (the changing concentration of the drug at its site of action) which can be monitored, and metabolism (both activation and detoxification) can be detected in real time. This review considers some recent work on (19)F, (31)P, (1)H and (13)C MRS of anticancer drugs. Future possibilities for (13)C MRS and (1)H MRS studies of drugs and their metabolites are considered in detail.

2-nitroimidazol-5-ylmethyl as a potential bioreductively activated prodrug system: reductively triggered release of the PARP inhibitor 5-bromoisoquinolinone

5-Chloromethyl-1-methyl-2-nitroimidazole reacted efficiently with the anion derived from 5-bromoisoquinolin-1-one to give 5-bromo-2-((1-methyl-2-nitroimidazol-5-yl)methyl)isoquinolin -1-one. Biomimetic reduction effected release of the 5-bromoisoquinolin-1-one. The 2-nitroimidazol-5-ylmethyl unit thus has potential for development as a general prodrug system for selective drug delivery to hypoxic tissues.

Characterization and validation of noninvasive oxygen tension measurements in human glioma xenografts by 19F-MR relaxometry

PURPOSE

The aim of this study was to characterize and to validate noninvasive 19F-magnetic resonance relaxometry for the measurement of oxygen tensions in human glioma xenografts in nude mice. The following three questions were addressed: 1. When perfluorocarbon compounds (PFCs) are administrated intravenously, which tumor regions are assessed by 19F-MR relaxometry? 2. Are oxygen tension as detected by 19F-MR relaxometry (pO2/relaxo) comparable to Eppendorf O2-electrode measurements (pO2/electrode)? 3. Can 19F-MR relaxometry be used to detect oxygen tension changes in tumor tissue during carbogen breathing?

METHODS AND MATERIALS

Slice-selective 19F-MR relaxometry was carried out with perfluoro-15-crown-5-ether as oxygen sensor. The PFC was injected i.v. 3 days before the 19F-MR experiments. Two datasets were acquired before and two after the start of carbogen breathing. The distribution of PFCs and necrotic areas were analyzed in 19F-Spin Echo (SE) density MR images and T2-weighted 1H-SE MR images, respectively. One day after the MR investigations, oxygen tensions were measured by oxygen electrodes in the same slice along two perpendicular tracks. These measurements were followed by (immuno)histochemical analysis of the 2D distribution of perfused microvessels, hypoxic cells, necrotic areas, and macrophages.

RESULTS

The PFCs mainly became sequestered in perfused regions at the tumor periphery; thus, 19F-MR relaxometry probed mean oxygen tensions in these regions throughout the selected MR slice. In perfused regions of the tumor, mean PO2/relaxo values were comparable to mean PO2/electrode values, and varied from 0.03 to 9 mmHg. Median pO2/electrode values of both tracks were lower than mean pO2/relaxo values, because low pO2 electrode values that originate from hypoxic and necrotic areas were also included in calculations of median pO2/electrode values. After 8-min carbogen breathing, the average PO2/relaxo increase was 3.3 +/- 0.8 (SEM) mmHg and 2.1 +/- 0.6 (SEM) after 14 min breathing.

CONCLUSIONS

We have demonstrated that PFCs mainly became sequestered in perfused regions of the tumor. Here, mean PO2/relaxo values were comparable to mean PO2electrode values. In these areas, carbogen breathing was found to increase the PO2/relaxo values significantly.

31P-magnetic resonance spectroscopy and 2H-magnetic resonance imaging studies of a panel of early-generation transplanted murine tumour models

The objective of this study was first to determine whether three slowly growing early-generation murine transplantable tumours, the T40 fibrosarcoma, T115 mammary carcinoma and T237 lung carcinoma, exhibit patterns of energetics and blood flow during growth that are different from those of the faster growing RIF-1 fibrosarcoma. Serial measurements were made with 31P-magnetic resonance spectroscopy (MRS), relating to nutritive blood flow and 2H-magnetic resonance imaging (MRI), which is sensitive to both nutritive and large-vessel (non-nutritive) flow. All four tumour lines showed a decrease in betaNTP/Pi and pH with growth; however, each line showed a different pattern of blood flow that did not correlate with the decrease in energetics. Qualitative histological analysis strongly correlated with the 2H-MRI. Second, their response to 5 mg kg(-1) hydralazine i.v. was monitored by 31P-MRS. A marked decrease in betaNTP/Pi and pH was observed in both the RIF-1 fibrosarcoma and the third-generation T115 mammary carcinoma after hydralazine challenge. In contrast, the fourth generation T40 fibrosarcoma and T237 lung carcinoma showed no change in 31P-MRS parameters. However, a fifth-generation T237 cohort, which grew approximately three times faster than fourth-generation T237 cohorts, exhibited a significant deterioration in betaNTP/Pi and pH in response to hydralazine. These data are consistent with a decoupling between large-vessel and nutritive blood flow and indicate that early-generation transplants that have a slow growth rate and vascular tone are more appropriate models of human tumour vasculature than more rapidly growing, repeatedly transplanted tumours.

The relationship between tumour oxygenation determined by oxygen electrode measurements and magnetic resonance spectroscopy of the fluorinated 2-nitroimidazole SR-4554

The relationship between two methods of assessing tumour oxygenation in vivo, namely oxygen electrode measurement and magnetic resonance spectroscopy (MRS) of the fluorinated 2-nitroimidazole SR-4554, was investigated. Using three tumour models (two sites), no linear correlation was observed between 19F retention index and pO2 parameters (r < or = 0.3). Substantial retention of SR-4554 (19F retention index > 0.5) was, however, associated with low tumour pO2 (% pO2 < or = 5 mmHg = 60%). Depending on the pO2 parameters used, SR-4554 administration was shown to produce either a significant or a non-significant increase in tumour oxygenation. We conclude that measurement of SR-4554-related compound(s) by 19F-MRS has the potential to detect clinically relevant levels of tumour hypoxia.

Bioreductive metabolism of the novel fluorinated 2-nitroimidazole hypoxia probe N-(2-hydroxy-3,3,3-trifluoropropyl)-2-(2-nitroimidazolyl) acetamide (SR-4554)

The aim of this work was to study the metabolic characteristics of the novel fluorinated 2-nitroimidazole hypoxia probe N-(2-hydroxy-3,3,3-trifluoropropyl)-2-(2-nitroimidazolyl) acetamide (SR-4554). HPLC and 19F NMR methods were employed to evaluate the rate of reductive metabolism of SR-4554 and the nature of the resulting metabolites, respectively. SR-4554 was enzymatically reduced by mouse liver microsomes (1.1 +/- 0.1 nmol of SR-4554 reduced/min/mg protein), purified rat and human NADPH:cytochrome P450 reductase (17.8 +/- 0.4 and 5.0 +/- 0.5 nmol of SR-4554 reduced/min/mg protein, respectively), and SCCVII tumour homogenates (2.3 +/- 0.3 nmol of SR-4554 reduced/min/g tumour) under nitrogen. NADPH:cytochrome P450 reductase was a major microsomal enzyme involved in the bioreduction of SR-4554 by liver microsomes. In a panel of murine and human tumour xenografts, cytochrome P450 reductase activities were found to be low and only varied by 3-fold between different tumour types, suggesting that enzyme activities within the tumours are unlikely to influence markedly in vivo reductive metabolism. Reduction of SR-4554 by mouse liver microsomes showed a characteristic oxygen dependence with a half-maximal inhibition of 0.48 +/- 0.06%. Thus, the reductive metabolism of SR-4554 can be employed to detect the low oxygen tensions that occur within both murine and human tumours. Soluble, low molecular weight reductive metabolites of SR-4554 were identified by 19F NMR. These metabolite peaks appeared (up to 0.12 ppm) downfield of the parent drug peak. In conclusion, SR-4554 undergoes an oxygen-dependent metabolism that involves NADPH:cytochrome P450 reductase. 19F NMR is capable of identifying reduced metabolites that are undetectable by HPLC.

Nuclear magnetic resonance spectroscopy of cancer

Nuclear magnetic resonance spectroscopy (MRS) offers a non-invasive approach for studying tumour biochemistry and physiology. This review highlights NMR nuclei (31P, 1H, 19F, 13C, 2H) that have been observed in both pre-clinical and clinical spectroscopic studies of cancer.

Development and validation of a solid-phase extraction and high-performance liquid chromatographic assay for a novel fluorinated 2-nitroimidazole hypoxia probe (SR-4554) in Balb/c mouse plasma

N-(2-Hydroxy-3,3,3-trifluoropropyl)-2-(2-nitro-1-imidazolyl) acetamide, a novel 2-nitroimidazole, is currently being developed as a non-invasive probe for tumour hypoxia. A sensitive (minimum quantifiable level = 25 ng/ml; C.V. = 6.01%) and selective assay has, therefore, been developed for the analysis of this compound in mouse plasma. The assay employed a solid-phase extraction followed by a rapid (10 min) HPLC analysis with UV-photodiode-array detection. No drug-related metabolites were observed in plasma when mice were treated with 180 mg/kg of the drug. The assay has proved to be suitable for studying the plasma pharmacokinetics of this fluorinated 2-nitroimidazole in mice.

The novel fluorinated 2-nitroimidazole hypoxia probe SR-4554: reductive metabolism and semiquantitative localisation in human ovarian cancer multicellular spheroids as measured by electron energy loss spectroscopic analysis

The novel fluorinated 2-nitroimidazole SR-4554 is undergoing preclinical development as a magnetic resonance spectroscopy and imaging probe for hypoxic tumour cells. We have used electron energy loss spectroscopic analysis (EELS) to show selective reduction and differential subcellular localisation of SR-4554 in human ovarian multicellular spheroids. SR-4554 was demonstrated to be metabolised by these A2780 cells under hypoxic but not under normal aerobic cell culture conditions. The EELS technique illustrated that the relative amount of drug within the cytoplasm of cells from both the inner region (150-160 microns from edge) and outer edge of the spheroid did not differ significantly after an initial 3 h incubation with drug. In contrast, an 8-fold differential between the amount of drug retained in the cytoplasm (primarily ribosomes and endoplasmic reticulum) of cells from the inner vs outer regions of the spheroids was observed following a subsequent 2 h 'chase' culture in drug-free medium. Within cells from the hypoxic region of the spheroid, SR-4554 was mainly associated with the endoplasmic reticulum, nucleus and the cytoplasmic side of intracellular vesicles and also to a lesser extent with the nuclear periphery. Interestingly, the drug was only weakly associated with the mitochondria and plasma membrane of the cells. The characteristics of cellular and subcellular distribution of SR-4554 are consistent with the hypothesis that 2-nitroimidazole compounds undergo hypoxia-mediated enzymatic reduction to reactive species. These reactive species are selectively retained in the cells in which they are metabolised through covalent association with subcellular components. These findings provide additional support for the clinical development of the drug as a non-invasive probe for tumour hypoxia and at the same time illustrate the utility of the EELS technique for examining the heterogeneity of drug distribution both between and within cells.

Nitroimidazoles and imaging hypoxia

Decreased tissue oxygen tension is a component of many diseases. Although hypoxia can be secondary to a low inspired pO2 or a variety of lung disorders, the commonest cause is ischemia due to an oxygen demand greater than the local oxygen supply. In tumors, low tissue pO2 is often observed, most often due to a blood supply inadequate to meet the tumor's demands. Hypoxic tumor tissue is associated with increased resistance to therapy. In the heart tissue hypoxia is often observed in persistent low-flow states, such as hibernating myocardium. In patients with stroke, hypoxia has been associated with the penumbral region, where an intervention could preserve function. Despite the potential importance of oxygen levels in tissue, difficulty in making this measurement in vivo has limited its role in clinical decision making. A class of compounds known to undergo different intracellular metabolism depending on the availability of oxygen in tissue, the nitroimidazoles, have been advocated for imaging hypoxic tissue. When a nitroimidazole enters a viable cell the molecule undergoes a single electron reduction, to form a potentially reactive species. In the presence of normal oxygen levels the molecule is immediately reoxidized. This futile shuttling takes place for some time, before the molecule diffuses out of the cell. In hypoxic tissue the low oxygen concentration is not able to effectively compete to reoxidize the molecule and further reduction appears to take place, culminating in the association of the reduced nitroimidazole with various intracellular components. The association is not irreversible, since these agents clear from hypoxic tissue over time. Initial development of nitroimidazoles for in vivo imaging used radiohalogenated derivatives of misonidazole, such as fluoromisonidazole, some of which have recently been employed in patients. Two major problems with fluoromisonidazole are its relatively low concentration within the lesion and the need to wait several hours to permit clearance of the agent from the normoxic background tissue (contrast between lesion and background typically < 2:1 at about 90 min after injection). Even with high-resolution positron emission tomographic imaging, this combination of circumstances makes successful evaluation of hypoxic lesions a challenge.(ABSTRACT TRUNCATED AT 400 WORDS)

Non-invasive determination of tumor oxygen tension and local variation with growth

PURPOSE

The objective was to develop and demonstrate a novel noninvasive technique of measuring regional pO2 in tumors. The method is based on measuring 19F nuclear magnetic resonance spin-lattice relaxation rate (R1 = 1/T1) of perfluorocarbon (PFC) emulsion discretely sequestered in a tumor.

METHODS AND MATERIALS

We have examined pO2 in the Dunning prostate tumor R3327-AT1 implanted in a Copenhagen rat. Oxypherol blood substitute emulsion was administered intravenously and became sequestered in tissue. Proton magnetic resonance imaging (MRI) showed tumor anatomy and correlated 19F MRI indicated the distribution of perfluorocarbon. Fluorine-19 spectroscopic relaxometry was used to measure pO2 in the tumor and repeated measurements over a period of 3 weeks showed the variation in local pO2 during tumor growth.

RESULTS

Perfluorocarbon initially resided in the vascularized peripheral region of the tumor: 19F nuclear magnetic resonance R1 indicated pO2 approximately 75 torr in a small tumor (approximately 1 cm) in an anesthetized rat. As the tumor grew, the sequestered PFC retained its original distribution. When the tumor had doubled in size the residual PFC was predominantly in the core of the tumor and the pO2 of this region was approximately 1 torr indicating central tumor hypoxia.

CONCLUSION

We have demonstrated a novel noninvasive approach to monitoring regional tumor pO2. Given the critical role of oxygen tension in tumor response to therapy this may provide new insight into tumor physiology, the efficacy of various therapeutic approaches, and ultimately provide a clinical technique for assessing individual tumor oxygenation.

An enzyme-linked immunosorbent assay for hypoxia marker binding in tumours

An enzyme-linked immunosorbent assay (ELISA) has been developed for measuring the in vivo binding of a hexafluorinated 2-nitroimidazole (CCI-103F) in tumour tissue biopsies. The binding of CCI-103F is believed to reflect the presence of hypoxia in tumours. The ELISA provides a sensitive and convenient method of measuring CCI-103F binding which does not require the injection of radioactive reagents. The ELISA is based on reagents prepared from synthetic antigens formed by the reductive activation and binding of CCI-103F to proteins in novel test tube experiments. Calibration of the ELISA involved comparing the ELISA with the radioactivity contained either in protein-CCI-103F adducts formed in vitro with tritiated CCI-103F or in tissues isolated from a tumour-bearing dog which had been injected with tritium-labelled CCI-103F. The two approaches to calibration are compared. The scope and limitation of the ELISA for measuring the binding of CCI-103F is discussed and an example of the application of the ELISA to measuring changes in tumour hypoxia in canine patients undergoing fractionated radiation therapy is presented.

Development of an ELISA for the detection of 2-nitroimidazole hypoxia markers bound to tumor tissue

Canine and rodent tumors covalently bind the fluorinated 2-nitroimidazole, CCI-103F, in a way that immunohistochemical analysis shows is consistent with the location of tumor hypoxia. We have now developed a rapid, quantitative, and non-radioactive enzyme linked immunosorbent assay for the binding of CCI-103F in biopsy samples of spontaneous canine tumors. Issues of antigen stability during tissue processing, calibration of the ELISA, and the use of biopsy samples for measuring tumor hypoxia by the ELISA approach are addressed.

4-Fluorobenzylamine and phenylalanine methyl ester conjugates of 2-nitroimidazole: evaluation as hypoxic cell radiosensitizers

We have synthesized two 2-nitroimidazole derivatives and evaluated their hypoxic radiosensitization properties. The first, a 4-fluorobenzylamine conjugate of 2-nitroimidazole (PK-110), was designed so that it could also be labeled with the F-18 and used for positron emission tomographic imaging of hypoxia. The second, the L-phenylalanine methyl ester conjugate of 2-nitroimidazole (PK-130), was designed in an attempt to exploit amino acid transport channels to enhance drug transport into the tumor. The effects of these drugs (and SR-2508, for comparison) in vitro on the aerobic and hypoxic radiosensitivity of Chinese hamster V79 cells were evaluated using clonogenic assays. PK-130 and PK-110 at 0.1 and 1.0 mM were more efficient hypoxic cell radiosensitizers than obtained with 1.0 mM SR-2508. Marginal aerobic radiosensitization was observed for 1.0 mM treatment with PK-130 and PK-110, however, no aerobic radiosensitization was observed at 0.1 mM. Glutathione (GSH) depletion (less than 5% of control levels) by L-buthionine sulfoximine (BSO) further enhanced the SER for both PK-130 and PK-110 at 0.1 mM to 3.2 +/- 0.63 and 2.4 +/- 0.16, respectively. The results of this study encourage the in vivo tumor radiosensitization evaluation of PK-130 and PK-110.

Retention of misonidazole in normal and malignant tissues: interplay of hypoxia and reductases

Tritiated misonidazole (MISO) was injected intravenously (iv) into mice bearing five different tumors. At 24 hr the tumors were removed for analysis of bound MISO, and at the same time three normal tissues were removed (liver, labial gland, and esophagus). The labial gland and esophagus were selected as representatives of sebaceous and stratified squamous tissues, respectively, tissues that in many parts of the body retain high levels of MISO. The tumors used were early transplant generations of spontaneous mouse tumors of mammary gland, lung, and liver. The levels (mean +/- SEM) of MISO at 24 hr for the five tumors and three normal tissues, expressed as percentage of the injected dose per gram of tissue were: A110 (0.03 +/- 0.007), A114 (0.103 +/- 0.04), A150 (0.09 +/- 0.005), A167 (0.037 +/- 0.012), A168 (0.122 +/- 0.0016), esophagus (0.507 +/- 0.09), labial gland (0.125 +/- 0.013), liver (0.11 +/- 0.004). Histochemical examination of the normal tissues showed reductase activity in all three. In the esophagus and labial gland, inhibition of the reaction by dicumarol indicated the likely presence of the reductase DT-diaphorase which, by 2-electron transfer, can be expected to reduce MISO to a reactive, locally-binding metabolite, even in the presence of oxygen.

Development of an in vivo 19F magnetic resonance method for measuring oxygen deficiency in tumors

A 19F magnetic resonance spectroscopy (MRS) approach to measuring hypoxia in experimental tumors in rats at a field strength of 2.35 T has been investigated in a combined study of in situ and excised tumors. The detection of tumor hypoxia is based on the hypoxia marker approach which depends on the selective, covalent binding of a fluorinated 2-nitroimidazole to hypoxic cells. The 19F MRS measurement of in vivo hypoxia marker binding was made at a fixed postinjection time when unbound, circulating marker molecule concentrations had dropped below detectable levels. A correlation between 19F MRS and scintillation counting measures of tumor-bound, tritium-labeled hypoxia marker was observed. There was no correlation between integrated 19F MRS hypoxia marker signals and the in vivo 31P MRS parameters of hypoxia which have been developed to measure normal tissue ischemia. Radiolabeling studies and previous immunohistochemical studies with the fluorinated hypoxia marker support the conclusion that the 19F MRS approach has promise as a physically noninvasive guide to the use of hypoxia-dependent therapies at clinically usable MRS field strengths.

A fluorinated 2-nitroimidazole, KU-2285, as a new hypoxic cell radiosensitizer

To develop new hypoxic cell radiosensitizers, we incorporated fluorine atoms into the side chain of the 2-nitroimidazole. Of the resulting compounds, KU-2285 (a 2-nitroimidazole with an N1-substituent of CH2CF2CONHCH2-CH2OH) was considered the most useful as a hypoxic cell radiosensitizer. In this study, its in vivo radiosensitizing activity and acute toxicity were compared with those of etanidazole. The reduction potentials of KU-2285 and etanidazole were -0.96 V and -1.05 V vs Ag/Ag+ in N,N-dimethylformamide, respectively, and their respective octanol/water partition coefficients were 0.25 and 0.040. The in vivo radiosensitizing activity of KU-2285 was found to be similar to that of etanidazole at the same administration dose when assayed by an in vivo-in vitro assay, a growth delay assay, and a tumor control assay using SCC VII tumor or transplanted mammary tumor in C3H/He mice. Although the radiosensitizing activity of etanidazole was reduced when it was administered orally, there was no significant difference in the radiosensitizing activity of KU-2285 whether it was administered intravenously, intraperitoneally, or orally. The acute toxicity measured as the LD50/7 in 8-week-old female C3H/HeJ mice was found to be 2.4 g/kg (intravenously), 2.1 g/kg (intraperitonealy), and 4.25 g/kg (orally) for KU-2285, whereas it was 4.75 g/kg (intravenously) for etanidazole.

19F nuclear magnetic resonance imaging of drug distribution in vivo: the disposition of an antifolate anticancer drug in mice

The application of 19F nuclear magnetic resonance imaging to the study of drug distribution in vivo is discussed. CB3988 (C2-desamino-C2-methyl-N10-propargyl-2'-trifluoromethyl-5,8-dideazafolic acid) is a fluorinated representative of a class of quinazoline antifolates which act as inhibitors of thymidylate synthase and which are being evaluated for the treatment of human cancer. 19F images were obtained in vivo from the abdomen of mice following intravenous injection of CB3988 (500 mg/kg). Time resolutions of 4 and 20 min were achieved for two- and three-dimensional imaging, respectively. These images were consistent with the presence of high concentrations of drug (up to 26 mg/ml) in the gall bladder, urinary bladder, and small intestine, as confirmed ex vivo by extraction and HPLC analysis. The results indicate the potential value of 19F NMR imaging in pharmacokinetic studies.

Distribution of pimonidazole and RSU 1069 in tumour and normal tissues

The tritium-labelled analogues of pimonidazole and RSU 1069 were injected into mice bearing the KHT murine sarcoma which has a hypoxic cell fraction of approximately 10%. The distribution of activity at 24 h was recorded using autoradiography and measurement of tissue activity. Autoradiographs with both drugs showed high activity in particular cells within tumour, eye (melanin-associated cells), eyelid (Meibomian gland), liver (centrilobular area), skin (sebaceous gland and melanin), stomach (squamous area), footpad, oesophagus, labial gland, Zymbal's gland, preputial gland, parotid gland (intralobular ducts) and airway epithelium. These tissues had previously been identified as sites of binding of misonidazole. The measurement of total tissue radioactivity showed significantly higher activity in liver, eyelid (Meibomian gland), oesophageal lining, kidney and labial gland than was found in the tumour.

Immunohistochemical detection of a hypoxia marker in spontaneous canine tumours

An immunoperoxidase technique has been used to detect the in vivo binding of a 2-nitroimidazole hypoxia marker in histochemical sections of a variety of excised canine tumours. The binding occurred 10-12 cell diameters away from tumour blood vessels, consistent with the expected location of hypoxic cells in tissues in which oxygen concentration gradients are established by diffusion. Hypoxic fractions ranging from 4 to 13% have been estimated on the basis of morphometric analysis of multiple tumour sections. The binding of the marker was restricted to the cytoplasm of the cells. The marker appeared in regions adjacent to necrosis but also in regions free of necrosis. As in earlier autoradiography studies, binding was occasionally observed in cells adjacent to tumour blood vessels. Generally, binding to normal tissues was not observed. However, binding to smooth muscle cells surrounding arterioles in some sections of normal tissue and tumour tissue was observed.

Dynamic measurements of hexafluoromisonidazole (CCI-103F) retention in mouse tumours by 1H/19F magnetic resonance spectroscopy

Selective retention of hexafluoromisonidazole, CCI-103F, in RIF-1 and SCCVII tumours of C3H/Km mice has been measured by 1H/19F magnetic resonance spectroscopy (MRS) on a Bruker AM-400 multinuclear spectrometer. CCI-103F concentrations in tumours and in normal tissues were measured using an MRS technique in which the water component in the tissues serves as an internal concentration reference. The biodistribution and elimination half-life of the drug in the tissues after i.p. injections were determined. The plasma half-life of the drug (41 min) was measured by high-pressure liquid chromatography. The two tumour lines and liver have longer retention times with half-lives of 47, 129 and 81 min, respectively, while normal tissues, muscle and brain have little retention of CCI-103F and clear the drug very quickly. Dynamic measurements of CCI-103F retention in tumours by MRS may provide a non-invasive probe for assessing tumour hypoxia.

Pharmacokinetic studies with the antifolate C2-desamino-C2-methyl-N10-propargyl-2'-trifluoromethyl-5,8-dideazafolic acid (CB3988) in mice and rats using in vivo 19F-NMR spectroscopy

In vivo 19F-NMR spectroscopy has been used to study the pharmacokinetics of the experimental antifolate drug CB3988 (C2-desamino-C2-methyl-N10-propargyl-2'trifluoromethyl-5,8-dideazafolic acid) in mice and rats. NMR results have been compared to those obtained by HPLC and the effect of the inclusion of the CF3 group evaluated by comparing the pharmacokinetics of CB3988 and ICI 198583 (C2-desamino-C2-methyl-N10-propargyl-5,8-dideazafolic acid) in rats. In mice, following the administration of CB3988 (500 mg kg-1 i.v.), drug could be detected in both the upper and the lower abdomen. NMR signal from the upper abdomen reached maximum intensity 10-40 min after administration, declining thereafter with a half life of 28 min. Signal detected in the lower abdomen reached maximum intensity 60-90 min after treatment. HPLC analyses indicated that CB3988 was present at appreciable concentrations (about 20-30 mg ml-1) in both bile and urine which is consistent with the signal from the upper and lower abdomen being derived from the gall bladder and urinary bladder, respectively. Studies in rats also indicated that CB3988 (100 mg kg-1 i.v.) rapidly entered and was cleared from the upper abdomen. Comparison of data from rats with intact and cannulated bile ducts suggested that 19F-NMR could detect CB3988 undergoing enterohepatic circulation. Furthermore, comparison of the plasma half life of CB3988 with the half life for the decline of the NMR signal from the upper abdomen suggested that NMR measurements may reflect the plasma clearance of CB3988. When the pharmacokinetics of CB3988 and ICI 198583 were compared the only significant difference was in the alpha phase half life which was 2-fold faster for CB3988. These data demonstrate that CB3988 is cleared rapidly by both biliary and urinary excretion. This is in contrast to N10-propargyl-5,8-dideazafolic acid, where delayed excretion is associated with hepatic and renal toxicities. The ability to study CB3988 pharmacokinetics non-invasively by 19F-NMR spectroscopy confirms the utility of the technique and, since 19F-NMR can be applied directly to clinical investigations, it may be possible to obtain similar information in humans.

Radiation and chemotherapy sensitizers and protectors

Radiosensitizers and radioprotectors are part of the chemical modifier approach to cancer therapy whereby the state of the tumor cells and/or normal tissues are modified such that a therapeutic gain is achieved using conventional radiation or chemotherapy. Radiosensitization can be achieved by the use of oxygen-mimetic compounds, agents that alter DNA sensitivity to irradiation, maneuvers that alter DNA repair processes, and manipulation of tissue oxygenation. Standard chemotherapeutic agents such as cisplatin can be utilized in a manner that optimizes the radiosensitization properties. Protection and sensitization can occur by altering the thiol status of the cell. The chemical modifiers field is both developing novel approaches to cancer treatment and increasing the understanding of basic cancer biology.

Micropharmacokinetics of chemical modifiers

Classical pharmacokinetic analysis of plasma, urine and tissue specimens continues to be of major value to the rational development of chemical modifiers of cancer treatment. However, in addition, increasingly sophisticated analytical techniques are becoming available which allow the pathways of microdistribution and micrometabolism of drugs to be traced down to the cellular and molecular level. New developments described here include flow cytometry, magnetic resonance spectroscopy, and molecular enzymology. These are predicted to have a major impact on the optimization of chemical modification.