PET imaging of (86)Y-labeled anti-Lewis Y monoclonal antibodies in a nude mouse model: comparison between (86)Y and (111)In radiolabels.

UNLABELLED: Absorbed doses in (90)Y radioimmunotherapy are usually estimated by extrapolating from (111)In imaging data. PET using (86)Y (beta(+) 33%; half-life, 14.7 h) as a surrogate radiolabel could be a more accurate alternative. The aim of this study was to evaluate an (86)Y-labeled monoclonal antibody (mAb) as a PET imaging agent and to compare the biodistribution of (86)Y- and (111)In-labeled mAb. METHODS: The humanized anti-Lewis Y mAb hu3S193 was labeled with (111)In or (86)Y through CHX-A"-diethylenetriaminepentaacetic acid chelation. In vitro cell binding and cellular retention of radiolabeled hu3S193 were evaluated using HCT-15 colon carcinoma cells, a cell line expressing Lewis Y. Nude mice bearing HCT-15 xenografts were injected with (86)Y-hu3S193 or (111)In-hu3S193. The biodistribution was studied by measurements of dissected tissues as well as by PET and planar imaging. RESULTS: The overall radiochemical yield in hu3S193 labeling and purification was 42% +/- 2% (n = 2) and 76% +/- 3% (n = 6) for (86)Y and (111)In, respectively. Both radioimmunoconjugates specifically bound to HCT-15 cells. When cellular retention of hu3S193 was studied using (111)In-hu3S193, 80% of initially cell-bound (111)In activity was released into the medium as high-molecular-weight compounds within 8 h. When coadministered, in vivo tumor uptake of (86)Y-hu3S193 and (111)In-hu3S193 reached maximum values of 30 +/- 6 and 29 +/- 6 percentage injected dose per gram and tumor sites were easily identifiable by PET and planar imaging, respectively. CONCLUSION: At 2 d after injection of (111)In-hu3S193 and (86)Y-hu3S193 radioimmunoconjugates, the uptake of (111)In and (86)Y activity was generally similar in most tissues. After 4 d, however, the concentration of (86)Y activity was significantly higher in several tissues, including tumor and bone tissue. Accordingly, the quantitative information offered by PET, combined with the presumably identical biodistribution of (86)Y and (90)Y radiolabels, should enable more accurate absorbed dose estimates in (90)Y radioimmunotherapy.

An investigation of the physical characteristics of 66Ga as an isotope for PET imaging and quantification.

Isotopes commonly used for PET imaging and quantification have a straightforward decay scheme involving "pure" positron (beta +) emission, i.e., 95%-100% beta + abundance, with no additional gamma rays. 66Ga (Emax = 4.2 MeV, T1/2 = 9.5 h) is a member of a category of isotopes with a lower abundance of beta +'s (57%) and a more complicated spectrum involving combinations of gamma rays that are emitted in cascade. These additional gamma rays tend to cause a higher singles rate, resulting in more random coincidence events. The most abundant positron (51.5%) in the spectrum has one of the highest energies considered for PET imaging. For the purpose of monoclonal antibody dosimetry using 66Ga, it is important to verify the quantification in phantoms prior to initiating human studies. A series of quantitative phantom measurements were performed on the PC4600, a head-optimized BGO based scanner with multiple detector rings. Count rate linearity was verified over concentrations ranging from 4.0 kBq/cc to 37 kBq/cc (0.11-1.0 microCi/cc); resolution averaged 16 mm full width half-maximum in the x and y directions in both the direct and cross planes. Axial resolution was 14 mm. The range of the energetic positrons (up to 4.153 MeV, range 7.6 mm in tissue) was verified as a primary source of resolution degradation. Within the limits outlined above, 66Ga is a suitable isotope for use as 66Ga citrate or with monoclonal antibodies in the detection and staging of tumors and other lesions. In addition, the energetic positrons have possible therapeutic applications when used as a monoclonal antibody label.

Quantitative imaging of iodine-124 with PET.

UNLABELLED: PET is potentially very useful for the accurate in vivo quantitation of time-varying biological distributions of radiolabeled antibodies over several days. The short half-lives of most commonly used positron-emitting nuclides make them unsuitable for this purpose. Iodine-124 is a positron emitter with a half-life of 4.2 days and appropriate chemical properties. It has not been widely used because of a complex decay scheme including several high energy gamma rays. However, measurements made under realistic conditions on several different PET scanners have shown that satisfactory imaging and quantitation can be achieved. METHODS: Whole-body and head-optimized scanners with different detectors (discrete BGO, block BGO and BaF2 time-of-flight), different septa and different correction schemes were used. Measurements of resolution, quantitative linearity and the ability to quantitatively image spheres of different sizes and activities in different background activities were made using phantoms. RESULTS: Compared with conventional PET nuclides, resolution and quantitation were only slightly degraded. Sphere detectability was also only slightly worse if imaging time was increased to compensate for the lower positron abundance. CONCLUSION: Quantitative imaging with 124I appears to be possible under realistic conditions with various PET scanners.

Quantitative studies of monoclonal antibody targeting to disialoganglioside GD2 in human brain tumors.

Iodine-131 3F8, a murine IgG3 monoclonal antibody that targets to GD2-bearing tumors, was administered intravenously to 12 patients with brain tumors. Six patients received 2 mCi (0.74 Bq) of 131I-3F8, five patients 10 mCi (3.7 Bq)/1.73 m2 of 131I-3F8, and one patient 2.6 mCi (0.96 Bq) of 124I-3F8, with no side-effects. Nine of 11 malignant gliomas and the single metastatic melanoma showed antibody localization, with the best tumor delineation on single-photon emission tomography (SPET) following 10 mCi (3.7 Bq)/1.73 m2 dose. No nonspecific uptake in the normal craniospinal axis was detected. There was no difference in the pharmacokinetics of low-dose versus the higher-dose antibody groups; plasma and total-body half-lives were 18 h and 49 h, respectively. Surgical sampling and time-activity curves based on quantitative imaging showed peak uptake in high-grade glioma at 39 h, with a half-life of 62 h. Tumor uptake at time of surgery averaged 3.5 x 10(-3) %ID/g and peak activity by the conjugate view method averaged 9.2 x 10(-3) %ID/g (3.5-17.8). Mean radiation absorption dose was 3.9 rad per mCi injected (range 0.7-9.6) or 10.5 cGy/Bq (range 1.9-26). There was agreement on positive sites when immunoscintigraphy was compared with technetium-99m glucoheptonate/diethylene triamine penta-acetic acid planar imaging, thallium-201 SPET, and fluorine-18 fluorodeoxyglucose positron emission tomography. Taken together, these data suggest that quantitative estimates of antibody targeting to intracranial tumors can be made using the modified conjugate view method.

Clinical validation of SPECT and CT/MRI image registration in radiolabeled monoclonal antibody studies of colorectal carcinoma.

UNLABELLED: Registration methods combine the anatomic localizing ability of CT or MRI with SPECT images of radiolabeled monoclonal antibodies (Mabs), allowing the accurate staging of patients prior to surgery or following treatment. METHODS: Twenty-four patients (15 males and 9 females, mean age 55 yr, range 29-70 yr) were studied with this technique. Ten patients had suspected colorectal cancer recurrence and were infused with 10 mCi of 131I-CC49 prior to staging laparotomy. Fourteen patients treated in a Phase I radioimmunotherapy study with 131I-CC49 were also studied. All patients underwent SPECT imaging of the abdomen and pelvis 5-7 days following infusion of Mab. RESULTS: Phantom studies demonstrated a 3.6-mm surface fitting mean accuracy of datasets for the liver and 1.8 mm for an intrahepatic tumor. In the presurgical group, SPECT and CT/MRI registration allowed more accurate identification of uptake abnormal sites. Areas of metastatic disease > 1 cm confirmed at surgery were found in six of nine patients with liver lesions and in two patients with extrahepatic (including one patient with pelvic) disease. In patients imaged following radioimmunotherapy, all lesions > 1.5 cm seen on CT/MRI were identified, and activity distribution in tumor and normal tissue could be more accurately assessed. CONCLUSIONS: Routine registration of SPECT and CT/MRI images is feasible and allows more accurate anatomic assessment of sites of abnormal uptake in radiolabeled Mab studies.

Three-dimensional dosimetry for radioimmunotherapy treatment planning.

Absorbed-dose calculations for radioimmunotherapy are generally based on tracer imaging studies of the labeled antibody. Such calculations yield estimates of the average dose to normal and target tissues assuming idealized geometries for both the radioactivity source volume and the target volume. This work describes a methodology that integrates functional information obtained from SPECT or PET with anatomical information from CT or MRI. These imaging modalities are used to define the actual shape and position of the radioactivity source volume relative to the patient's anatomy. This information is then used to calculate the spatially varying absorbed dose, depicted in "colorwash" superimposed on the anatomical imaging study. By accounting for individual uptake characteristics of a particular tumor and/or normal tissue volume and superimposing resulting absorbed-dose distribution over patient anatomy, this approach provides a patient-specific assessment of the target-to-surrounding normal tissue absorbed-dose ratio. Such information is particularly important in a treatment planning approach to radioimmunotherapy, wherein a therapeutic administration of antibody is preceded by a tracer imaging study to assess therapeutic benefit.

Development of a method to measure kinetics of radiolabelled monoclonal antibody in human tumour with applications to microdosimetry: positron emission tomography studies of iodine-124 labelled 3F8 monoclonal antibody in glioma.

We present a method to assess quantitatively the immunological characteristics of tumours using radiolabelled monoclonal antibody and positron emission tomography (PET) to improve dosimetry for radioimmunotherapy. This method is illustrated with a glioma patient who was injected with 96.2 MBq of iodine-124 labelled 3F8, a murine antibody (IgG3) specific against the ganglioside GD2. Serial PET scans and plasma samples were taken over 11 days. A three-compartment model was used to estimate the plasma to tumour transfer constant (K1), the tumour to plasma transfer constant k2, the association and dissociation constants (k3, k4) of antibody binding, and the binding potential. Tumour radioactivity peaked at 18 h at 0.0045% ID/g. The kinetic parameters were estimated to be: K1 = 0.048 ml h-1 g-1, k2 = 0.16 h-1, k3 = 0.03 h-1, k4 = 0.015 h-1 and BP = 2.25. Based on these kinetic parameters, the amount of tumour-bound radiolabelled monoclonal antibody was calculated. This method permits estimates of both macrodosimetry and microdosimetry at the cellular level based on in vivo non-invasive measurement.

High-resolution positron emission tomography of human ovarian cancer in nude rats using 124I-labeled monoclonal antibodies.

PET has inherently high resolution and excellent contrast imaging and accurately measures radioactivity concentrations in vivo. When combined with specific immunological targeting it might provide a highly specific and sensitive radioimmunoscintigraphic tool. To investigate this we injected 124I-labeled MAb MX35 or MAb MH99 monoclonal antibodies (doses 200-400 mu Ci) intravenously into nude rats bearing subcutaneous human ovarian cancer xenografts (SK-OV-7 and SK-OV-3 cell lines). A melanoma cell line (SK-MEL-30) was used as a control tumor. These murine monoclonal antibodies react with cell-surface antigens expressed by most ovarian cancer cells, including the ovarian cell line used. Imaging was performed at 1-6 days using a high-resolution positron emission tomograph (PCR-I) with a spatial resolution of 4.5 mm. The slice thicknesses were 0.5 and 1.0 cm. Forty to seventy thousand coincident pulses were obtained per frame. The PET results were compared with those of autopsy and histology. Samples of blood, tumor, and normal tissues were obtained at various time points. PET calculation of isotope uptake ratios demonstrated specific localization of the antibodies in tumor, with ratios of tumor to normal tissue uptake as high as 6:1. Subcutaneous ovarian cancer nodules as small as 7 mm were identified with PET imaging. The results corresponded well with tissue sampling. Our findings suggest that PET imaging of tumors with 124I-labeled monoclonal antibodies may be useful in human diagnostic and therapeutic applications in ovarian cancer as well as other diseases.

Radiation dose in CT.

The energy deposited in the patient by the rotating x-ray beam in computed tomography produces more uniform absorbed dose values within the section of imaged tissue than those produced in conventional radiologic procedures. The dose values within a specific section are determined by factors such as voltage, current, scan time, scan field, rotation angle, filtration, collimation, and section thickness and spacing. For routine dose determinations, a pencil ionization chamber is usually employed with a plastic phantom. Dose for a specific patient can be determined with thermoluminescent dosimeters placed on the patient. Multiple-scan procedures normally increase the dose in a specific section by less than a factor of two. Typical multiple-scan average doses are in the range of 40-60 mGy for head scans and 10-40 mGy for body scans. Integral dose, however, is directly proportional to the number of sections in an examination. When examination factors are changed to reduce dose, the image noise increases. An optimum protocol is one that results in a balance between dose and image quality.

PET scanning of iodine-124-3F9 as an approach to tumor dosimetry during treatment planning for radioimmunotherapy in a child with neuroblastoma.

A patient with advanced neuroblastoma who had failed chemotherapy presented with a large abdominal mass and virtually total bone marrow replacement by tumor on repeated marrow biopsies. She was considered a candidate for a Phase I 131I-3F8 radioimmunotherapy trial, (MSKCC 89-141A). As a potential aid to treatment planning, a test dose of 124I-3F8 was injected and the patient was imaged over the 72 hr postinjection using two BGO based PET scanners of different designs. Time activity curves were obtained, and the cumulated activity concentration of radiolabeled 3F8 in tumor was determined. Based on MIRD, an estimated radiation absorbed dose for 131I-3F8 was 7.55 rad/mCi, in the most antibody avid lesions. Because of low uptake and unfavorable dosimetry in some bulky tumor sites, it was decided not to treat the patient with radiolabeled antibody. Positron emission tomography of 124I-labeled antibodies can be used to measure cumulated activity or residence time in tumor for more accurate estimates of radiation absorbed tumor dose from radioiodinated antibodies and can help guide management decisions in patients who are candidates for radioimmunotherapy.

Quantitative imaging of I-124 using positron emission tomography with applications to radioimmunodiagnosis and radioimmunotherapy.

Positron emission tomography (PET) is potentially useful for the quantitative imaging of radiolabeled antibodies, leading in turn to improved dosimetry in radioimmunotherapy. Iodine-124 is a positron-emitting nuclide with appropriate chemical properties and half-life (4.2 days) for such studies since the radiolabeling of antibodies with iodine is well understood and the half-life permits measurements over several days. Unfortunately, I-124 has a complex decay scheme with many high-energy gamma rays and a positron abundance of only 25%. It has therefore been largely ignored as a PET-imaging nuclide. However, measurements made with phantoms and animals under realistic conditions using a BGO-based PET scanner have shown that satisfactory imaging and quantitation can be achieved. Investigations of spatial resolution, the linearity of regional observed count rate versus activity in the presence of other activity, and the visualization and quantitation of activity in spheres with different surrounding background activities were carried out with phantoms up to 22 cm in diameter. Compared with F-18, spatial resolution was only slightly degraded (13.5 mm FWHM vs 12 mm FWHM) while linearity was the same over a 10:1 activity range (0.015 to 0.15 MBq/ml for I-124). The visualization and quantitation of spheres was also slightly degraded when using similar imaging times. Increasing the imaging time for I-124 reduced the difference. To verify that the technique would work in vivo, measurements were made of human neuroblastoma tumors in rats which had been injected with I-124 labeled 3F8 antibody. Although the number of samples was small, good agreement was achieved between image-based measurements and direct measurements of excised 4-g tumors. Thus quantitative imaging of I-124 labeled antibodies appears to be possible under realistic conditions.

A phase I trial of monoclonal antibody M195 in acute myelogenous leukemia: specific bone marrow targeting and internalization of radionuclide.

Ten patients with myeloid leukemias were treated in a phase I trial with escalating doses of mouse monoclonal antibody (mAb) M195, reactive with CD33, a glycoprotein found on myeloid leukemia blasts and early hematopoietic progenitor cells but not on normal stem cells. M195 was trace-labeled with iodine-131 (131I) to allow detailed pharmacokinetic and dosimetric studies by serial sampling of blood and bone marrow and whole-body gamma-camera imaging. Total doses up to 76 mg were administered safely without immediate adverse effects. Absorption of M195 onto targets in vivo was demonstrated by biopsy, pharmacology, flow cytometry, and imaging; saturation of available sites occurred at doses greater than or equal to 5 mg/m2. The entire bone marrow was specifically and clearly imaged beginning within hours after injection; optimal imaging occurred at the lowest dose. Bone marrow biopsies demonstrated significant dose-related uptake of M195 as early as 1 hour after infusion in all patients, with the majority of the dose found in the marrow. Tumor regressions were not observed. An estimated 0.33 to 1.0 rad/mCi 131I was delivered to the whole body, 1.1 to 6.1 rad/mCi was delivered to the plasma, and up to 34 rad/mCi was delivered to the red marrow compartment. 131I-M195 was rapidly modulated, with a majority of the bound immunoglobulin G (IgG) being internalized into target cells in vivo. These data indicate that whole bone marrow ablative doses of 131I-M195 can be expected. The rapid, specific, and quantitative delivery to the bone marrow and the efficient internalization of M195 into target cells in vivo also suggest that the delivery of other isotopes such as auger or alpha emitters, toxins, or other biologically important molecules into either leukemia cells or normal hematopoietic progenitor cells may be feasible.

Technical challenges associated with the radiolabeling of monoclonal antibodies utilizing short-lived, positron emitting radionuclides.

Many factors, both physical and chemical, affect the radiolabeling of a chemical compound. These factors add a new dimension of complexity when the labeling of monoclonal antibodies is attempted with short-lived, positron emitting radionuclides. A lack of appreciation for the unique chemical separations associated with the radionuclide incorporated into the synthetic precursor can often lead to unexpected or non-reproducible results.

A phase I toxicity, pharmacology, and dosimetry trial of monoclonal antibody OKB7 in patients with non-Hodgkin's lymphoma: effects of tumor burden and antigen expression.

Eighteen patients with relapsed non-Hodgkin's lymphoma (NHL) were infused with escalating doses of monoclonal antibody (mAb) OKB7, trace-labeled with iodine-131 (131I), in order to study toxicity, pharmacology, antibody localization, and dosimetry of radioiodine. OKB7 is a noncytotoxic mouse immunoglobulin G2b (IgG2b) mAb reactive with B cells and most B-cell NHL. Three patients each were treated at six dose levels ranging from 0.1 mg to 40 mg. All patients had radionuclide imaging and counting daily, had serial blood sampling to study pharmacokinetics, human antimouse antibody (HAMA), and circulating antigen, and had a biopsy of accessible lymphoma to determine delivery of isotope to tumors and assess the effect of tumor antigen expression on mAb delivery. Bone marrow biopsies were also done in the majority of patients. There was no toxicity. Serum clearance showed a median early phase half-life of 1.9 hours and a later phase half-life of 21.7 hours. Median total body clearance half-life was 22 hours. Pharmacokinetics were not dose-related. HAMA was detected in five patients. Circulating blocking antigen was detected in the serum of four patients, but at levels that were of pharmacologic consequence only in one. Biopsied tumor tissue from five patients did not express OKB7 antigen. No significant uptake of antibody was seen in these tumor sites. Mean total uptake of isotope into lymphoma measured in biopsies correlated linearly over the 400-fold increase in injected mAb dose. However, the percent of injected dose found per gram of tumor was unrelated to dose, but correlated inversely with tumor burden. In two patients with minimal tumor burden, 1.0 mg and 5.0 mg doses of OKB7 resulted in tumor to body radioisotope dose ratios of 22 and 7, which would theoretically permit tolerable delivery of 4,400 and 1,400 rads to these tumors, respectively, if OKB7 were conjugated with higher doses of 131I.

Imaging of human tumor xenografts with an indium-111-labeled anti-epidermal growth factor receptor monoclonal antibody.

The mouse monoclonal antibody (mAb) 225 IgG1 against the epidermal growth factor (EGF) receptor has been investigated for its capacity to localize in human tumor xenografts. The EGF receptor is the product of the c-erb-B proto-oncogene (also known as EGFR). Elevated expression of EGF receptors has been demonstrated in many human tumors and tumor cell lines. We studied A431 human vulvar squamous cell carcinoma cells, with 2 X 10(6) receptors per cell; MDA-MB-468 (MDA 468) human breast adenocarcinoma cells, with 3 X 10(5) receptors per cell; and MCF-7 human breast adenocarcinoma cells, with 5 X 10(3) receptors per cell. The 111In-labeled pentetic acid (DTPA), derivative of mAb 225 (111In-DTPA-225) was injected intraperitoneally into nude mice bearing subcutaneous tumor xenografts. We measured uptake by quantifying radioactivity in tumor and normal tissues and by obtaining gamma camera images. Uptake in A431 xenografts was 28% +/- 2.4% of the injected dose per gram of tumor on day 3 and 12.4% +/- 3.0% on day 7. Distribution ratios comparing uptake in the tumor with that in normal tissues were consistently greater than 4. In contrast, there was far less uptake of the control mAb KS1/4S-1 labeled with 111In. This conjugate, 111In-DTPA-KS1/4S-1, has an IgG1 isotype but does not bind to human or murine cells. Imaging of the tumor with mAb 225 was excellent, especially on days 3-7. MDA 468 xenografts exhibited reduced localization of mAb 225 in the tumor. For MCF-7 xenografts, the tumor uptake of mAb 225 after 7 days was only 0.70% +/- 0.10% of the injected dose per gram of tumor, which was comparable to the uptake of the KS1/4S-1 control mAb. The ratio of the concentration of radioactivity in the tumor to that in normal tissue (distribution ratio) showed poor selectivity of uptake, and imaging was not obtained. These observations suggest that labeled mAb can target the product of a proto-oncogene, the EGF receptor, when it is expressed at high levels in human tumor xenografts.

Radiation dosimetry survey of computed tomography systems from ten manufacturers.

Profiles of the absorbed dose delivered throughout cylindrical and anthropomorphic phantoms during single scans by computed tomography systems from ten manufacturers were measured using LiF thermoluminescent dosemeters (TLD) and X-ray therapy verification film. The dose profiles demonstrate that a significant portion of the dose is delivered outside the imaged volume of a single scan. The doses measured in cylindrical Plexiglas phantoms were similar to those measured in anthropomorphic cross sections of tissue substitute materials except near the centre of the thorax section. The film results, obtained using a single calibration curve, agreed with the TLD results to within 25% for most systems.

An evaluation of CT systems from ten manufactures.

During the past three years, 23 computerized tomography (CT) machines from ten different manufacturers located in Europe and the USA have been evaluated. Using the same set of phantoms and measurement techniques, performance data have been derived including noise, uniformity of CT numbers, low contrast detectability, spatial resolution, modulation transfer function, effective photon energy and linearity of CT numbers. Thermoluminescent and film dosimetry were used to determine single-slice dose profiles at the periphery and centre of specialized anthropomorphic phantoms. The results of this study are presented in tabular and graphical form together with some general conclusions on their implications.

A method of correcting for linear drift in computed tomography brain scans.

Linear drift of X-ray attenuation coefficients must be corrected if quantitative comparisons are to be made between computed tomography (CT) brain scans of the same individual performed at different times. Such a correction is accomplished by comparing the low (cerebrospinal fluid) end of the attenuation coefficient frequency histograms using a percentile--percentile plot. A "drift correction" permits serial quantitative assessments of the progression or regression of white matter hypodensity, such as occurs in drug induced leukoencephalopathy.

Partial volume summation: a simple approach to ventricular volume determination from CT.

The accurate determination of ventricular volume from computed tomography (CT) is not a trivial problem. The direct approach of measuring the area within a visually determined boundary or contour level and multiplying by the nominal slice thickness may be subject to large errors because such boundaries are not, in general, well defined. When the ventricles are filled with a high-contrast material such as metrizamide, we may use the technique of 'partial volume summation,' a simple but accurate, clinically applicable method which may be performed on a standard DeltaScan-50 or similar system.

Determination of ventricular volume following metrizamide CT ventriculography.

Volume averaging, relatively slight differences in the mean attenuation coefficients of CSF and white/grey matter, and the irregular contours of the human ventricular system have so far seriously limited the accuracy of CT estimation of ventricular volume. Taking advantage of the high attenuation of metrizamide-containing CSF, we have developed three methods for computing ventricular volume after metrizamide CT ventriculography; these methods depend upon computer analysis of X-ray absorption data obtained from contiguous CT brain slices. All three methods were validated by CT scanning a formalin-fixed cadaver brain containing an apoxy-resin cast of the ventricular system. Calculated ventricular volumes were compared with the actual measured volume of the ventricular cast.