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

Abstract

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.

Technical Advances in Current PET and Hybrid Imaging Systems

“…In addition, increased patient throughput due to reduced scan times, and reduced radiation exposure achieved by reducing radiopharmaceutical doses are important benefits of TOF measurements from the practical and ethical viewpoints. The potential usefulness of TOF measurement in I-124 PET, which has attracted much interest for dose monitoring in radio-immunotherapy because of its long half-life (4.2 days) [19], is also evident, because the improved SNR in TOF PET can compensate for reduced I-124 PET sensitivity caused by its low branching ratio for positron emission (0.23) [20].…”

Radiosynthesis of cis‐4‐[124I]iodo‐L‐proline as a prototype probe for imaging anterograde axoplasmic transport systems using positron emission tomography (PET)

“…Iodine-124 has not found widespread application in PET imaging because of its lower positron abundance (23%) and a complex decay scheme which affects the resolution of PET images. 2 However, when delivered directly into brain tissue, iodine-124 yields a 'point source' imaging in customcalibrated microPET cameras. We report the synthesis and evaluation of cis-4-[ 124 I]iodo-l-proline 2 as a prototype probe for the in vivo imaging of anterograde axoplasmic transport system in the rodent using microPET (Scheme 1).…”

Preparation of the iodine‐124 derivative of the Bolton–Hunter reagent ([124I]I‐SHPP) and its use for labelling a VEGF antibody as a PET tracer

“…Although the decay of iodine-124 produces only 23% positrons, this isotope is still regarded as useful for positron emission tomography. 9 So far, investigators have used [ 124 I]iodine labelled antibodies to study malignancies such as breast cancer, 10,11 carcinoembryonic antigen-producing tumors, 12 colorectal cancer, 13 ovarian cancer, 14 and neuroblastoma. 15,16 It should be noted that all these papers are based on direct methods for labelling tyrosine residues of antibodies with iodine-124.…”