A triple suicide gene strategy that improves therapeutic effects and incorporates multimodality molecular imaging for monitoring gene functions.

Gene-directed enzyme prodrug therapy (GDEPT), or suicide gene therapy, has shown promise in clinical trials. In this preclinical study using stable cell lines and xenograft tumor models, we show that a triple-suicide-gene GDEPT approach produce enhanced therapeutic efficacy over previous methods. Importantly, all the three genes (thymidine kinase, cytosine deaminase and uracil phosphoribosyltransferase) function simultaneously as effectors for GDEPT and markers for multimodality molecular imaging (MMI), using positron emission tomography, magnetic resonance spectroscopy and optical (fluorescent and bioluminescent) techniques. It was demonstrated that MMI can evaluate the distribution and function/activity of the triple suicide gene. The concomitant expression of these genes significantly enhances prodrug cytotoxicity and radiosensitivity in vitro and in vivo.

Broad-spectrum multi-modality image registration: from PET, CT, and MRI to autoradiography, microscopy, and beyond.

Image registration and fusion are increasingly important components of both clinical and small-animal imaging and have lead to the development of a variety of pertinent hardware and software tools, including multi-modality, e.g. PET-CT, devices. At the same time, advances in microscopic imaging, including phosphor-plate digital autoradiography and immunohistochemistry, now allow ultra-high (sub-100 microm)-resolution molecular characterization of tissue sections. To date, however, in vivo imaging of intact subjects and ex vivo imaging of harvested tissues sections have remained separate and distinct, making it difficult to reliably inter-compare the former and the latter. The Department of Medical Physics and the Radiation Biophysics Laboratory at Memorial Sloan-Kettering Cancer Center, under the direction of Dr. Clifton Ling, has now designed, fabricated, and tested a stereotactic imaging system for so-called "broad-spectrum" image registration, from coarser-resolution in vivo imaging modalities such as PET, CT, and MRI to ultra-high-resolution ex vivo imaging techniques such as histology, autoradiography, and immunohistochemistry.

A generalized algorithm for determining the time of release and the duration of post-release radiation precautions following radionuclide therapy.

The Nuclear Regulatory Commission has recently amended its regulation concerning patients who have received therapeutic amounts of radioactivity. The amended regulation allows patient release based on a total effective dose equivalent (TEDE) limit of 5 mSv (500 mrem) instead of the activity administered or retained [1,110 MBq (30 mCi)] or the dose rate [0.05 mSv h(-1) (5 mrem h(-1)) at 1 m]. Record-keeping and written post-release radiation safety precautions are required, however. A general algorithm, combining patient-specific kinetics and dose rate measurements, has been developed to systematically determine the actual duration of post-release radiation precautions as well as the time of release post-treatment. This algorithm is based on the maximum permissible effective dose equivalents (MPEDEs) of the respective cohorts exposed, 5 mSv (500 mrem) to non-pregnant adult family members and 1 mSv (100 mrem) to pregnant women, children, and members of the general public. Operational equations to determine the times post-radionuclide treatment of release from medical confinement, of not working, of avoiding pregnant women and children, of limiting holding of children, and of sleeping partners not sleeping together have been derived and illustrated with a hypothetical example. TEDE-based release criteria should be less restrictive than the previous activity-based or dose rate-based release criteria. However, post-release radiation precautions may be more intrusive and longer in duration than those to which most practitioners have grown accustomed. Up to now, however, the duration (typically 1-2 d) of advised post-release precautions had not been rigorously derived from MPEDEs and were generally inappropriately short. Even so, dose-based release criteria should prove more cost-effective overall than hospitalization of patients commonly imposed by activity-based and dose rate-based release criteria.

Effects of time of administration and dietary iodine levels on potassium iodide (KI) blockade of thyroid irradiation by 131I from radioactive fallout.

Radioiodines, particularly 131I, may be released into the environment in breach-of-containment nuclear reactor accidents and localize in and irradiate the thyroid with an attendant risk of neoplastic growth and other adverse health effects. Pharmacologic thyroid blockade by oral potassium iodide (KI) (50-100 mg in adults) can substantially reduce thyroid uptake of and irradiation by internalized radioiodine. In the current analysis, computer modeling of iodine metabolism has been used to systematically elucidate the effects of two practically important but highly variable factors on the radioprotective effect of KI: the time of administration relative to exposure to radioiodine and the dietary level of iodine. In euthyroid adults receiving iodine-sufficient diets (250 microg d(-1) in the current analysis), KI administered up to 48 h before 131I exposure can almost completely block thyroid uptake and therefore greatly reduce the thyroid absorbed dose. However, KI administration 96 h or more before 131I exposure has no significant protective effect. In contrast, KI administration after exposure to radioiodine induces a smaller and rapidly decreasing blockade effect. KI administration 16 h or later after 131I exposure will have little effect on thyroid uptake and absorbed dose and therefore little or no protective effect. The 131I thyroid absorbed dose is two-fold greater with insufficient levels of dietary iodine, 2,900 cGy/37 MBq, than with sufficient levels of dietary iodine, 1,500 cGy/37 MBq. When KI is administered 48 h or less before 131I intake, the thyroid absorbed doses (in cGy/37 MBq) are comparably low with both sufficient and insufficient dietary iodine levels. When KI is administered after 131I intake, however, the protective effect of KI is less and decreases more rapidly with insufficient than with sufficient dietary iodine. For example, KI administration 2 and 8 h after 131I intake yields protective effects of 80 and 40%, respectively, with iodine-sufficient diets, but only 65 and 15% with iodine-deficient diets. In conclusion, whether exposed populations receive sufficient or insufficient dietary iodine, oral KI is an effective means of reducing thyroid irradiation from environmentally dispersed radioiodine but is effective only when administered within 2 d before to approximately 8 h after radioiodine intake.

Internal radionuclide radiation dosimetry: a review of basic concepts and recent developments.

Internal dosimetry deals with the determination of the amount and the spatial and temporal distribution of radiation energy deposited in tissue by radionuclides within the body. Nuclear medicine has been largely a diagnostic specialty, and model-derived average organ dose estimates for risk assessment, the traditional application of the MIRD schema, have proven entirely adequate. However, to the extent that specific patients deviate kinetically and anatomically from the model used, such dose estimates will be inaccurate. With the increasing therapeutic application of internal radionuclides and the need for greater accuracy, radiation dosimetry in nuclear medicine is evolving from population- and organ-average to patient- and position-specific dose estimation. Beginning with the relevant quantities and units, this article reviews the historical methods and newly developed concepts and techniques to characterize radionuclide radiation doses. The latter include the 3 principal approaches to the calculation of macroscopic nonuniform dose distributions: dose point-kernel convolution, Monte Carlo simulation, and voxel S factors. Radiation dosimetry in "sensitive" populations, including pregnant women, nursing mothers, and children, also will be reviewed.

Age-dependent thyroid absorbed doses for radiobiologically significant radioisotopes of iodine.

In light of the post-Chernobyl increase in pediatric thyroid cancer incidence, among other recent events, there is renewed interest in radioiodine thyroid dosimetry and effects. Among the radioiodines produced in fission of 235U, only 131I [(T1/2)p = 8.04 d], 132I (2.3 h), 133I (20.3 h), and 135I (6.7 h) may undergo significant environmental dispersion. Age-dependent thyroid absorbed dose estimates for these radiobiologically significant radioiodines and for the "medical" radioisotopes 123I (13.2 h) and 125I (60 d) have been derived, incorporating the effect of absorption following inhalation or ingestion. This effect has generally been ignored in previously derived estimates of radioiodine absorbed doses to the thyroid. Based on the latest ICRP lung and gut models, inhaled radioiodine is absorbed at a rate of 0.175 h(-1) and exhaled at 0.101 to 0.118 h(-1) (depending on age) and ingested radioiodine is completely absorbed in the stomach at a rate of 1 h(-1). Whole-body compartmental models (SAAM II) were fit to previously published 24-h thyroid uptakes, thyroid half-times, and 48-h plasma concentration of protein-bound iodine. The resulting fitted models were used to calculate thyroid residence times of radioiodine. The mean thyroid absorbed doses [cGy/37 kBq (rad/microCi) injected intravenously] were then calculated using the age-dependent S(thyroid<--thyroid) factors (MIRDOSE III), with the highest doses (from 0.49 for 123I to 36 for 131I) in newborns and the lowest doses (from 0.014 for 123I to 1.4 for 131I) in adults in inverse relation to the thyroid mass. Although the thyroid absorbed dose for inhalation is substantially (30 to 70%) less than that for injection for all radioiodines and at all ages, it is markedly (25%) less for ingestion only for short-lived 132I.

Bremsstrahlung radiation exposure from pure beta-ray emitters.

UNLABELLED: With increasing therapeutic use of radionuclides that emit relatively high-energy (>1 MeV) beta-rays and the production in vivo of bremsstrahlung sufficient for external imaging, the potential external radiation hazard warrants evaluation. METHODS: The exposure from a patient administered beta-ray-emitting radionuclides has been calculated by extending the National Council on Radiation Protection and Measurement model of a point source in air to account for biologic elimination of activity, the probability of bremsstrahlung production in vivo and its mean energy and the absorption by the patient's body of the bremsstrahlung thus produced. To facilitate such calculations, a quantity called the "specific bremsstrahlung constant" (in C/kg-cm2/MBq-h), betaBr, was devised and calculated for several radionuclides. The specific bremsstrahlung constant is the bremsstrahlung exposure rate (in C/kg/h) in air at 1 cm from a 1 MBq beta-ray emitter of a specified maximum beta-ray energy and frequency of emission in a medium of a specified effective atomic number. RESULTS: For pure beta-ray emitters, the retained activities at which patients can be released from medical confinement (i.e., below which the effective dose equivalent at 1 m will not exceed the maximum recommended value of 0.5 cSv for infrequently exposed members of the general public) are extremely large: on the order of hundreds of thousands to millions of megabecquerels. CONCLUSION: Radionuclide therapy with pure beta-ray emitters, even high-energy beta-ray emitters emitted in bone, does not require medical confinement of patients for radiation protection.

Radiation dose to patients and relatives incident to 131I therapy.

Radioiodine long has proven to be a safe and effective treatment for thyroid disease. Nonetheless, persisting concerns regarding radiogenic stochastic risks (e.g., carcinogenesis) to patients, their families, and the general public have led regulators to establish criteria for release of 131I-containing patients from medical confinement, with limits ranging from as low as 2 mCi in some parts of Europe to as high as 30 mCi in the United States. To optimize clinical efficacy and cost-effectiveness of 131I therapy, such regulations should be based on logical dosimetric considerations. The thyroidal absorbed dose, proportional to maximum uptake and effective half-life and inversely proportional to mass, is typically approximately 1,500 rad/mCi of 131I administered to a euthyroid adult (based on a thyroid maximum uptake of 25%, effective half-life equivalent to the physical half-life of 131I (8.04 days), and mass of 20 g). As thyroid uptake increases from 0% to 100%, extrathyroidal absorbed doses range from a minimum of 0.15 to 0.5 rad/mCi for breast and gonads to a maximum of 1.5 to 2 rad/mCi for stomach and salivary glands; the absorbed doses of the urinary bladder wall, in contrast, decrease with increasing thyroid uptake, from 2 to 0.6 rad/mCi. In hyperthyroid patients (approximately 15%) with a small iodine pool (so-called small patients), the short effective half-life of radioiodine in the thyroid and high serum concentrations of long-lived protein-bound 131I result in a standard 7,000-rad absorbed dose for treatment of Graves' disease requiring an administered activity of 28 mCi of 131I and yielding a prohibitively high blood absorbed dose of 150 rad. Importantly, once the fetal thyroid begins to function and accumulate radioiodine at a gestational age of 10-12 weeks, fetal thyroid absorbed doses as large as 5,000 rad/mCi of 131I administered to the mother can result. Thus, pregnancy is an absolute contraindication to administration of 131I because of the risk of radiogenic cretinism. Based on actual measurements of thyroid activity and of external absorbed dose, the total thyroid and mean extrathyroidal absorbed doses to adult family members from immediately released 131I-treated patients are approximately 0.01 and approximately 0.02 rad/mCi administered, respectively, yielding an effective dose of approximately 0.02 rem/mCi. A maximum permissible effective dose of 0.5 rem for adults therefore is consistent with a release criterion of 30 mCi of retained 131I. Lower-activity release criteria therefore may be unnecessarily restrictive.

Treatment planning for internal radionuclide therapy: three-dimensional dosimetry for nonuniformly distributed radionuclides.

A calculational approach is described that provides the spatially varying radiation absorbed dose, presented as isodose contours superimposed on CT images, from nonuniform and/or irregular cumulated activity distributions. CT images are read from magnetic tape and are displayed on a high-resolution color graphics display monitor. Source tissue geometries are defined on a series of contiguous CT images automatically (by an edge detection algorithm) or manually (using a trackball), thereby obtaining a three-dimensional representation of the various source volumes of activity. Dose calculations are performed using a radionuclide-specific absorbed dose point kernel in the form of a lookup table. The method described yields the spatially varying dose delivered to tumor and normal tissue volumes from a patient-specific cumulated activity distribution in a clinically implementable manner. This level of accuracy in determining normal tissue and tumor doses may prove valuable in the evaluation and implementation of radionuclides and radiolabeled compounds for therapeutic purposes.

Design of a 13C (1H) RF probe for monitoring the in vivo metabolism of [1-13C]glucose in primate brain.

The design of an RF probe suitable for obtaining proton-decoupled 13C spectra from a subhuman primate brain is described. Two orthogonal saddle coils, one tuned to the resonant frequency of 13C and the other to the resonant frequency of 1H, were used to monitor the in vivo metabolism of [1-13C]glucose in rhesus monkey brain at 2.1 T. Difference spectra showed the appearance of 13C-enriched glutamate and glutamine 30 to 40 min after a bolus injection of [1-13C]glucose.

Quantitative SPECT in radiation dosimetry.

Accurate and precise radiation dosimetry is critical for the successful therapeutic application of systemically administered radionuclides, including, of course, radionuclides in the form of radiolabeled antibody. This requires determination, based on discrete serial measurements, of the time-dependent concentrations and/or total amounts of radioactivity in situ in order to calculate source region cumulated activities. Based on extensive studies (with clinically realistic numbers of counts and accuracies of the order of 10%) in simple geometric phantoms, in complex anthropomorphic phantoms, in animal models, and in humans, quantitative rotating scintillation camera-based single-photon emission computed tomography (SPECT) now appears to be a practical approach to such measurements. The basis of the quantitative imaging capability of a three-dimensional imaging modality such as SPECT is the elimination in the reconstructed image of counts emanating from activity surrounding the source region. Subject to considerations such as the reconstruction algorithm, attenuation and scatter corrections, and, most importantly, statistical uncertainty, the counts in a pixel in a reconstructed image are therefore directly proportional to the actual counts emanating from the corresponding voxel in situ. Among intrinsic, pre-processing, and post-processing attenuation corrections, post-processing algorithms, the most widely used approach in current commercial SPECT systems, have proven adequate in uniformly attenuating parts of the body (eg, abdomen, pelvis), subject to accurate delineation of the body contour. Although a number of sophisticated scatter correction methods have been developed, the lack of explicit scatter correction has, in practice, not been a major impediment to reasonably accurate quantitative SPECT imaging, despite scattered radiation representing up to 50% of the counts in a large source region (eg, liver). Because of its mathematical propagation in the image reconstruction process, statistical uncertainty (ie, "noise") in SPECT is far greater than would be expected if it were distributed according to Poisson statistics, as in planar imaging. The low "single slice" sensitivity of rotating scintillation camera-based SPECT is therefore the principal limitation of practical quantitative SPECT. Accordingly, absolute quantitation of count-limited clinical images has been accomplished using a judiciously selected "non-ramp" filter function. In summary, reasonable quantitative SPECT imaging is now feasible clinically, even without sophisticated scatter corrections, at least in uniformly attenuating parts of the body.(ABSTRACT TRUNCATED AT 400 WORDS)

Monoclonal antibody to an intracellular antigen images human melanoma transplants in nu/nu mice.

Mouse monoclonal antibody TA99 detects a 70-kDa pigmentation-associated glycoprotein in human melanoma cell lines. The antigen cannot be detected on the cell surface by sensitive rosetting techniques or absorption studies, nor can it be detected as a secreted product in culture fluids. Contrary to expectation, 125I-labeled TA99 specifically localized to pigmented human melanoma transplants in nu/nu mice; no localization to nonpigmented melanoma or control tumors was found. Tumor imaging was initially obscured by circulating 125I-labeled TA99 during the first 6 days after antibody injection. With clearance of 125I-labeled TA99 from the blood (half-life, 4-7 days), specific tumor images could be clearly defined by day 13. Due to the persistence of 125I-labeled TA99 at the tumor site (8.9% of the injected dose at 1 week and 4.6% at 8-10 weeks), images were obtainable for up to 10 weeks. At 8-10 weeks, the tumor/blood ratio was 10(4)-10(5), and the tumor/normal tissue ratio ranged from 10(2) to 10(5). In view of these findings, antibodies detecting intracellular antigens may have a role in tumor imaging and therapy.

Imaging studies of patients with malignant fibrous histiocytoma using C-11-alpha-aminoisobutyric acid (AIB).

Alpha-aminoisobutyric acid (AIB), a synthetic, nonmetabolized amino acid which is rapidly transported into viable cells by the A-type or alanine-preferring amino acid transport system, has been labeled with the short-lived, positron-emitting radionuclide carbon-11. Carbon-11 labeled AIB is currently being evaluated as a tumor imaging agent for in vivo amino acid transport studies in patients with cancer. In this study, C-11 AIB was used to image two patients with malignant fibrous histiocytoma (MFH), a pleomorphic sarcoma. Following intravenous administration of C-11 AIB, tumors in the distal femur of one patient and in the anterior chest wall of another patient were well visualized using high energy gamma scintigraphy. Since therapy may alter the accumulation of amino acids in tumor tissue, studies using C-11 AIB in patients with MFH before and after chemotherapy are in progress.

Evaluation of [1-11C]-alpha-aminoisobutyric acid for tumor detection and amino acid transport measurement: spontaneous canine tumor studies.

Alpha-aminoisobutyric acid (AIB), or alpha-methyl alanine, is a nonmetabolized amino acid transported into cells, particularly malignant cells, predominantly by the 'A' amino acid transport system. Since it is not metabolized, [1-11C]-AIB can be used to quantify A-type amino acid transport into cells using a relatively simple compartmental model and quantitative imaging procedures (e.g. positron tomography). The tissue distribution of [1-11C]-AIB was determined in six dogs bearing spontaneous tumors, including lymphosarcoma, osteogenic sarcoma, mammary carcinoma, and adenocarcinoma. Quantitative imaging with tissue radioassay confirmation at necropsy showed poor to excellent tumor localization. However, in all cases the concentrations achieved appear adequate for amino acid transport measurement at known tumor locations. The observed low normal brain (due to blood-brain barrier exclusion) and high (relative to brain) tumor concentrations of [1-11C]-AIB suggest that this agent may prove effective for the early detection of human brain tumors.

Synthesis and quality assurance of [11C]alpha-aminoisobutyric acid (AIB), a potential radiotracer for imaging and amino acid transport studies in normal and malignant tissues.

Carbon-11 labeled alpha-aminoisobutyric acid (AIB), a synthetic amino acid, was prepared by the modified Bucherer-Strecker amino acid synthesis from acetone, ammonium carbonate and [11C]KCN in the presence of carrier KCN. This method results in the labeling of AIB in the carboxyl group. The label is stable in this position because AIB is not a metabolized after cellular uptake. AIB is rapidly accumulated in viable cells including malignant cells. Since it is a non-metabolized amino acid, AIB offers the possibility of studying amino acid transport in vivo without interference by radiolabeled metabolic products. Radiochemical yields of [11C]AIB of 35-60% have been obtained in 70-80 min with radiopurities greater than 99%. Carrier added syntheses gave 15-25 mCi of [11C]AIB with specific activities of 0.3 Ci/mmol. Our quality control program which insures that [11C]AIB is suitable for imaging studies in patients with cancer includes HPLC analyses of product identity and purity, apyrogenecity and isotonicity assays, and a sensitive test for cyanide.

Neuroleptic binding sites: specific labeling in mice with [18F]haloperidol, a potential tracer for positron emission tomography.

Haloperidol labeled with fluorine-18 (T 1/2 = 110 min, positron emission 97%), prepared yielding .04 Ci/millimole by the Balz-Schiemann reaction, was evaluated in a murine model as a potential radiotracer for noninvasive determination, by positron-emission tomography, of regional concentrations of brain dopamine receptors in patients. As the haloperidol dose in mice was increased from 0.01 to 1000 micrograms/kg, the relative concentration of [18F]haloperidol (microCi per g specimen/microCi per g of body mass), at one hour after injection decreased from 30 to 1.0 in the striatum and from 8.0 to 1.0 in the cerebellum. The striatal radioactivity, plotted as relative concentration against log of dose, decreased sigmoidally, presumably reflecting competition between labeled and unlabeled haloperidol for a single class of accessible binding sites. Because the cerebellum is relatively deficient in dopamine receptors, the observed decrease in cerebellar radioactivity may reflect a saturable component of haloperidol transport into brain. The high brain concentrations and the unexpectedly high striatum-to-cerebellum concentration ratios (greater than 4 at haloperidol doses less than or equal to 1 microgram/kg) suggest that [18F]haloperidol warrants further investigation as a potential radiotracer for dopamine receptors.

Evaluation of the response to xenon-133 radiations by thermoluminescent dosimeters used during the accident at Three Mile Island.

An evaluation is presented of the accuracy and sensitivity of three types of TLD's used during the accident at the Three Mile Island Nuclear Station. This evaluation indicated that, due to the method of calibration, all the dosimeters over-responded to 133Xe radiations. The response ranged from slightly above unity to almost two. Exposures of the TLD's were of two types, namely, the characteristic X-rays either were or were not filtered from the beam. The angular sensitivity of the dosimeters is also reported.

Synthesis and body distribution of alpha-aminoisobutyric acid-L-11C in normal and prostate cancer-bearing rat after chemotherapy.

This paper describes the synthesis of alpha-aminoisobutyric acid-11C (11C-AIB) and studies its body distribution in healthy and tumor-bearing animals. High tissue levels of 11C-AIB were found in organs with high metabolic activity (pancreas, liver, kidney). The prostate tumor 11C-AIB concentrations are significantly lower than that of pancreas, liver, and kidney, but increased when compared with normal prostate levels. Effective chemotherapy, which reduces tumor growth of two different prostate adenocarcinoma cell lines (R-3327-H, R-3327-G) also decreases 11C-AIB levels and the 11C-AIB prostate to tumor ratio.