In vivo multiple-mouse imaging at 1.5 T

A multiple-mouse solenoidal MR coil was developed for in vivo imaging of up to 13 mice simultaneously to screen for tumors on a 1.5 T clinical scanner. For the coil to be effective as a screening tool, it should permit acquisition of MRIs in which orthotopic tumors with diameters >2 mm are detectable in a reasonable period of time (<1 hr magnet time) and their sizes accurately measured. Using a spin echo sequence, we demonstrated that this coil provides sufficient sensitivity for moderately high resolution images (156-176 microm in plane-resolution, 1.5 mm slice thickness). This spatial resolution permitted detection of primary brain tumors in transgenic/knockout mice and orthotopic xenografts. Brain tumor size as measured by MRI was correlated with size measured by histopathology (P < 0.001). Metastatic tumors in the mouse lung were also successfully imaged in a screening setting. The multiple mouse coil is simple in construction and may be implemented without any significant modification to the hardware or software on a clinical scanner.

Salmonella-based tumor-targeted cancer therapy: tumor amplified protein expression therapy (TAPET) for diagnostic imaging

In preclinical studies, genetically engineered Salmonella have the ability to localize, selectively accumulate, and persist within transplantable murine tumors, spontaneous murine tumors and human tumor xenographs, and can express therapeutic proteins at high levels. These strains of engineered non-virulent Salmonella typhimurium display the capacity to accumulate and grow selectively in a variety of tumor types and to inhibit the growth of primary and metastatic tumors following intravenous injection into tumor-bearing mice. One strain of the bacteria (VNP20009) which has endogenous antitumor activity is currently in Phase I clinical trials. The bacteria are highly attenuated and genetically stable. The combination of the lipid mutation and the purine auxotrophy attenuate the virulence of the bacteria by greater than 10000-fold and enhance the specificity of the bacteria for tumor tissue. These bacteria have been found to be safe in mice, pigs and monkeys when administered intravenously. Second-generation Salmonella vectors will be developed to include transgenes that will express therapeutic agents and reporter transgenes for non-invasive imaging. We have performed a preliminary study to demonstrate localization of [(14)C]FIAU in tumored mice pretreated with Salmonella expressing HSV1-TK. The [(14)C]FIAU radioactivity and bacterial count data strongly support a Salmonella(TK)-dependent [(14)C]FIAU accumulation of at least 30-fold higher in tumor tissue compared to muscle tissue. These data warrant further investigation on the use of genetically engineered Salmonella as a systemically administered tumor-specific agents for tumor therapy and delivery of diagnostic imaging markers.

Positron emission tomography imaging for herpes virus infection: Implications for oncolytic viral treatments of cancer

Molecular therapy using viruses would benefit greatly from a non-invasive modality for assessing dissemination of viruses. Here we investigated whether positron emission tomography (PET) scanning using [(124)I]-5-iodo-2'-fluoro-1-beta-d-arabinofuranosyl-uracil (FIAU) could image cells infected with herpes simplex viruses (HSV). Using replication-competent HSV-1 oncolytic viruses with thymidine kinase (TK) under control of different promoters, we demonstrate that viral infection, proliferation and promoter characteristics all interact to influence FIAU accumulation and imaging. In vivo, as few as 1 x 107 viral particles injected into a 0.5-cm human colorectal tumor can be detected by [(124)I]FIAU PET imaging. PET signal intensity is significantly greater at 48 hours compared with that at 8 hours after viral injection, demonstrating that PET scanning can detect changes in TK activity resulting from local viral proliferation. We also show the ability of FIAU-PET scanning to detect differences in viral infectivity at 0.5 log increments. Non-invasive imaging might be useful in assessing biologically relevant distribution of virus in therapies using replication-competent HSV.

Imaging transgene expression with radionuclide imaging technologies

A variety of imaging technologies are being investigated as tools for studying gene expression in living subjects. Noninvasive, repetitive and quantitative imaging of gene expression will help both to facilitate human gene therapy trials and to allow for the study of animal models of molecular and cellular therapy. Radionuclide approaches using single photon emission computed tomography (SPECT) and positron emission tomography (PET) are the most mature of the current imaging technologies and offer many advantages for imaging gene expression compared to optical and magnetic resonance imaging (MRI)-based approaches. These advantages include relatively high sensitivity, full quantitative capability (for PET), and the ability to extend small animal assays directly into clinical human applications. We describe a PET scanner (microPET) designed specifically for studies of small animals. We review "marker/reporter gene" imaging approaches using the herpes simplex type 1 virus thymidine kinase (HSV1-tk) and the dopamine type 2 receptor (D2R) genes. We describe and contrast several radiolabeled probes that can be used with the HSV1-tk reporter gene both for SPECT and for PET imaging. We also describe the advantages/disadvantages of each of the assays developed and discuss future animal and human applications.

"Facilitated" amino acid transport is upregulated in brain tumors

The goal of this study was to determine the magnitude of "facilitated" amino acid transport across tumor and brain capillaries and to evaluate whether amino acid transporter expression is "upregulated" in tumor vessels compared to capillaries in contralateral brain tissue. Aminocyclopentane carboxylic acid (ACPC), a non-metabolized [14C]-labeled amino acid, and a reference molecule for passive vascular permeability, [67Ga]-gallium-diethylenetriaminepentaacetic acid (Ga-DTPA), were used in these studies. Two experimental rat gliomas were studied (C6 and RG2). Brain tissue was rapidly processed for double label quantitative autoradiography 10 minutes after intravenous injection of ACPC and Ga-DTPA. Parametric images of blood-to-brain transport (K1ACPC and K1Ga-DTPA, microL/min/g) produced from the autoradiograms and the histology were obtained from the same tissue section. These three images were registered in an image array processor; regions of interest in tumor and contralateral brain were defined on morphologic criteria (histology) and were transferred to the autoradiographic images to obtain mean values. The facilitated component of ACPC transport (deltaK1ACPC) was calculated from the K1ACPC and K1Ga-DTPA data, and paired comparisons between tumor and contralateral brain were performed. ACPC flux, K1ACPC, across normal brain capillaries (22.6 +/- 8.1 microL/g/min) was >200-fold greater than that of Ga-DTPA (0.09 +/- 0.04 microL/g/min), and this difference was largely (approximately 90%) due to facilitated ACPC transport. Substantially higher K1ACPC values compared to corresponding K1DTPA values were also measured in C6 and RG2 gliomas. The deltaK1ACPC values for C6 glioma were more than twice that of contralateral brain cortex. K1ACPC and deltaK1ACPC values for RG2 gliomas was not significantly higher than that of contralateral cortex, although a approximately 2-fold difference in facilitated transport is obtained after normalization for differences in capillary surface area between RG2 tumors and contralateral cortex. K1ACPC, deltaK1ACPC, and K DTPA were directly related to tumor cell density, were higher in regions of "impending" necrosis, and the tumor/contralateral brain ACPC radio-activity ratios (0 to 10 minutes) were very similar to that obtained with 0 to 60 minutes experiments. These results indicate that facilitated transport of ACPC is upregulated across C6 and RG2 glioma capillaries, and that tumors can induce upregulation of amino acid transporter expression in their supporting vasculature. They also suggest that early imaging (e.g., 0 to 20 minutes) with radiolabeled amino acids in a clinical setting may be optimal for defining brain tumors.

Anti-neoplastic properties of human corticotropin releasing factor: involvement of the nitric oxide pathway

We report a series of the in vivo and in vitro studies that evaluate the anti-neoplastic potential of hCRF in W256 rat mammary carcinoma. Using magnetic resonance imaging (MRI) and direct measurements of tumor and peritumoral brain water content we found that hCRF treatment (100 micrograms/kg subcutaneously twice a day for 3 days) caused significant inhibition of growth and vascular permeability of the i.c. W256 tumors. hCRF also exhibited antiproliferative and differentiation-inducing effects in W256 cells in vitro. The calculated IC50 values were 70 nM and 100 nM of hCRF, as measured by digital videomicroscopic quantitation of tumor cell population growth rate and by [3H]-thymidine incorporation assay, respectively. The observed effects in W256 cells were CRF receptor mediated. This was shown in two ways: by the presence of relatively high levels of CRF1 receptor mRNA in W256 cells, and by the fact that the tumor growth inhibitory and differentiation inducing effects of hCRF in vitro were abolished by the CRF receptor antagonist a-helical CRF (9-41). Antiproliferative and differentiation inducing effects of hCRF in W256 cells involve activation of nitric oxide synthase (NOS) and L-arginine-NO pathway. This was shown by using the inhibitor of NOS, the L-nitro-arginine methyl esther (L-NAME), which prevented the antiproliferative and differentiation inducing effects of hCRF in vitro. The cytotoxicity of NO in W256 cells was assessed by the addition of sodium nitroprusside (SNP) to the media. SPN exhibited dose-dependent cytotoxicity in W256 cells with IC50 of 100 muM SNP as measured by [3H]-thymidine incorporation assay. We conclude, that hCRF has substantial anti-neoplastic effects which include inhibition of proliferation and induction of differentiation of the tumor cells in vitro, and a decrease in tumor vascular permeability (and possibly neo-angiogenesis) in vivo.

Imaging experimental brain tumors with 1-aminocyclopentane carboxylic acid and alpha-aminoisobutyric acid: comparison to fluorodeoxyglucose and diethylenetriaminepentaacetic acid in morphologically defined tumor regions

The goal of this study was to evaluate the differences and define the advantages of imaging experimental brain tumors in rats with two nonmetabolized amino acids, 1-aminocyclopentane carboxylic (ACPC) acid and alpha-aminoisobutyric (AIB) acid compared with imaging with fluorodeoxyglucose (FDG) or the gallium-diethylenetriaminepentaacetic acid chelate (Ga-DTPA). 1-aminocyclopentane carboxylic acid, AIB, and FDG autoradiograms were obtained 60 minutes after intravenous injection to simulate positron emission tomography (PET) imaging, whereas the Ga-DTPA autoradiograms were obtained 5 or 10 minutes after injection to simulate gadolinium (Gd)-DTPA-enhanced magnetic resonance (MR) images. Three experimental tumors were studied (C6, RG2, and Walker 256) to provide a range of tumor types. Triple-label quantitative autoradiography was performed, and parametric images of the apparent distribution volume (Va, mL/g) for ACPC or AIB, relative glucose metabolism (R, micromol/100 g/min), vascular permeability to Ga-DTPA (K1, microL/min/g), and histology were obtained from the same tissue section. The four images were registered in an image array processor, and regions of interest in tumor and contralateral brain were defined on morphologic criteria (histology) and were transferred to the autoradiographic images. A comparative analysis of all measured values was performed. The location and morphologic characteristics of the tumor had an effect on the images and measurements of Va, R, and K1. Meningeal extensions of all three tumors consistently had the highest amino acid uptake (Va) and vascular permeability (K1) values, and subcortical portions of the tumors usually had the lowest values. Va and R (FDG) values generally were higher in tumor regions with high-cell density and lower in regions with low-cell density. Tumor areas identified as "impending" necrosis on morphologic criteria consistently had high R values, but little or no change in Va or K1. Tumor necrosis was seen consistently only in the larger Walker 256 tumors; low values of R and Va for AIB (less for ACPC) were measured in the necrotic-appearing regions, whereas K1 was not different from the mean tumor value. The highest correlations were observed between vascular permeability (K1 for Ga-DTPA) and Va for AIB in all three tumors; little or no correlation between vascular permeability and R was observed. The advantages of ACPC and AIB imaging were most convincingly demonstrated in C6 gliomas and in Walker 256 tumors. 1-aminocyclopentane was substantially better than FDG or Ga-DTPA for identifying tumor infiltration of adjacent brain tissue beyond the macroscopic border of the tumor; ACPC also may be useful for identifying low-grade tumors with an intact blood-brain barrier. Contrast-enhancing regions of the tumors were visualized more clearly with AIB than with FDG or Ga-DTPA; viable and necrotic-appearing tumor regions could be distinguished more readily with AIB than with FDG. [11C]-labeled ACPC and AIB are likely to have similar advantages for imaging human brain tumors with PET.

Regression of C6 rat brain tumors by cells expressing an antisense insulin-like growth factor I receptor RNA

We have previously shown that C6 cells expressing an antisense insulin-like growth factor I receptor (IGF-IR) RNA are no longer tumorigenic in syngeneic rats, protecting them from subsequent subcutaneous tumor challenge and causing regression of established subcutaneous tumors. In the present study, we have investigated the efficacy of this strategy on intracerebrally implanted C6 rat glioblastoma cells. We demonstrate that C6 cells expressing an antisense IGF-IR RNA implanted for 24 h in the subcutaneous tissue of the rats are able to elicit an anti-tumor response in the brain, leading to complete brain tumor regression and long-term survival of the rats. These findings suggest the possibility of therapeutic intervention in human gliomas.

Corticotropin-releasing factor decreases vasogenic brain edema

We report the first series of studies comparing the anti-edematous effects of human corticotropin-releasing factor (hCRF) and dexamethasone in an experimental model of vasogenic peritumoral brain edema. Both hCRF and dexamethasone effectively decreased blood-brain barrier (BBB) permeability of intracerebral RG2 gliomas in rats as observed by contrast-enhanced T(1)-weighted magnetic resonance imaging. A decrease in the water content of tumor and peritumoral brain tissue was observed with proton-density magnetic resonance imaging and confirmed by direct wet/dry tissue measurements. The calculated ED(50) for hCRF was 59 micrograms/kg s.c. twice a day, and that for dexamethasone was 0.61 mg/kg i.m. twice a day; the hCRF:dexamethasone dose-potency ratio was 120:1 on a molar basis. The anti-edematous action of hCRF is not mediated by the release of adrenal corticosteroids. A direct action of hCRF on the tumor microvasculature results in restoration of BBB integrity and up-regulation of BBB-specific protein expression. The average survival time with chronic treatment was prolonged significantly in the hCRF-treated group (35 days) compared with the dexamethasone-treated group (28 days; P < 0.05) and the saline-treated control group (22 days; P < 0.0001). hCRF, as an alternative to corticosteroid therapy, may provide substantial benefits with respect to reducing the major side effects encountered with long-term, high-dose corticosteroid treatment.

RG-2 glioma growth attenuation and severe brain edema caused by local production of interleukin-2 and interferon-gamma

Two aspects of cytokine therapy of intracerebral tumors are considered in this study: modulation of tumor growth in vivo and central nervous system toxicity. Coimplantation of RG-2 glioma cells and retroviral vector producer cell lines was performed to provide a local source of interleukin-2 (IL-2) or IFN-gamma within the tumor and coinitiate an antitumor immune response. We demonstrated that local intratumoral production of IL-2 and IFN-gamma generates a cell-mediated antitumor response in vivo. This response was manifest as a diffuse infiltration of monocytes/macrophages, CD4+ and CD8+ T cells, and activation of microglial OX42+ cells in intracerebral RG2 tumors. The cell-mediated antitumor immune response resulted in the early suppression of intracranial and subcutaneous tumor growth, but the effect was not sustained and there were no tumor regressions. The absence of increased survival of animals with intracranial tumors is explained in part by the severe central nervous system toxicity caused by local production of IL-2 and IFN-gamma. Central nervous system toxicity induced blood-brain barrier disruption, vasogenic brain edema, and dislocation of the brain midline structures, as observed by dynamic magnetic resonance imaging and direct measurements of tissue water content. The clinical application of IL-2 and IFN-gamma gene transfer therapy for intracerebral tumors must consider the potential for severe vasogenic brain edema associated with intracerebral production of these cytokines.

Iododeoxyuridine uptake and retention as a measure of tumor growth

Iodine-131-iododeoxyuridine (IUdR) uptake and retention was measured in two C6 glioma cell lines (C6m and C6a) with different growth characteristics. Animals with intracerebral (i.c.) C6a tumors had a mean survival of 16 days, whereas only 1 of 20 animals with i.c. C6m tumors died during an 8-wk period of observation. The growth of i.c. C6m tumors could be described by the Gompertz equation; tumor doubling time increased from 1.9 to 5.2 days between Days 8 and 16 after tumor inoculation. Corresponding measurements of 131I-IUdR uptake and retention (24 hr after IUdR administration) by i.c. C6m tumors were also time-dependent and decreased from 0.075 to 0.027 to 0.011 %dose/g in 8-, 10- and 16-day-old tumors, respectively. Iodine-131-IUdR uptake in "rapidly growing" i.c. C6a tumors was substantially higher (0.30 %dose/g at 24 hr) than that in "slowly growing" i.c. C6m tumors and corresponded with differences in the survival data. Subcutaneous C6a tumors had comparable high uptake values (0.49 %dose/g at 24 hr), and 93% of total tumor radioactivity was recovered in DNA 24 hr after IUdR administration. Clearance of radioactivity was rapid in nonproliferative tissues; more than 80% of plasma radioactivity was cleared in 24 hr. Tumor-to-cortex radioactivity ratios ranged from 100/1 to 120/1 and 150/1 between 24, 48 and 96 hr after IUdR injection respectively. A "washout strategy," which reduces tissue background activity and increases specificity for PET and SPECT imaging of IUdR-DNA incorporation, is possible with longer-lived radioisotopes of iodine.

Correlations between brain oedema volume on CT and CSF dynamics in severely head injured patients

The CSF dynamics were studied in 18 patients with severe head injury who remained comatose over 6 hours after trauma (GCS less than 8). Amount of brain oedema was estimated by CT tomodensitometry. In addition, CSF parameters of PVI. Elastance (E), compliance (C) and resorption resistance (R) were calculated from serial bolus infusion tests. We observed a decrease in viscoelastic parameters as indexed by PVI, however, no increase in resistance to CSF outflow. It was noted that patients with lower PVI developed more severe brain oedema during the 3-5 day post traumatic period. From these data, we conclude that buffering capacity in severe head injury is mainly affected by the volume of brain oedema and not by the haematoma volume.

Quantitative analysis of CT visualized periventricular and perifocal brain oedema by means of CSF dynamics infusion test

Brain oedema of different pathogenesis cannot be differentiated with CSF infusion tests. In cases with periventricular oedema the CSS viscoelastic parameters are changed more towards the decompensated state than in cases with perifocal oedema.