Radioimmunoimaging with longer-lived positron-emitting radionuclides: potentials and challenges

Radioimmunoimaging and therapy has been an area of interest for several decades. Steady progress has been made toward clinical translation of radiolabeled monoclonal antibodies for diagnosis and treatment of diseases. Tremendous advances have been made in imaging technologies such as positron emission tomography (PET). However, these advances have so far eluded routine translation into clinical radioimmunoimaging applications due to the mismatch between the short half-lives of routinely used positron-emitting radionuclides such as (18)F versus the pharmacokinetics of most intact monoclonal antibodies of interest. The lack of suitable positron-emitting radionuclides that match the pharmacokinetics of intact antibodies has generated interest in exploring the use of longer-lived positron emitters that are more suitable for radioimmunoimaging and dosimetry applications with intact monoclonal antibodies. In this review, we examine the opportunities and challenges of radioimmunoimaging with select longer-lived positron-emitting radionuclides such as (124)I, (89)Zr, and (86)Y with respect to radionuclide production, ease of radiolabeling intact antibodies, imaging characteristics, radiation dosimetry, and clinical translation potential.

Development of radioimmunotherapeutic and diagnostic antibodies: an inside-out view

Only a handful of radiolabeled antibodies (Abs) have gained US Food and Drug Administration (FDA) approval for use in clinical oncology, including four immunodiagnostic agents and two targeted radioimmunotherapeutic agents. Despite the advent of nonimmunogenic Abs and the availability of a diverse library of radionuclides, progress beyond early Phase II radioimmunotherapy (RIT) studies in solid tumors has been marginal. Furthermore, [18F]fluorodeoxyglucose continues to dominate the molecular imaging domain, underscored by a decade-long absence of any newly approved Ab-based imaging agent (none since 1996). Why has the development of clinically successful Abs for RIT been limited to lymphoma? What obstacles must be overcome to allow the FDA approval of immuno-positron emission tomography (immuno-PET) imaging agents? How can we address the unique challenges that have thus far prevented the introduction of Ab-based imaging agents and therapeutics for solid tumors? Many poor decisions have been made regarding radiolabeled Abs, but useful insight can be gained from these mistakes. The following review addresses the physical, chemical, biological, clinical, regulatory and financial limitations that impede the progress of this increasingly important class of drugs.

Single photon emission computed tomography of pulmonary emboli and venous thrombi using anti-D-dimer

Previous attempts to diagnose thromboemboli using radiolabeled antibodies and nuclear medicine imaging have been disappointing. We present the results of experiments with intravenous technetium-99m-labeled deimmunized antifibrin Fab' fragments to diagnose thromboemboli using single photon emission computed tomography (SPECT), a highly sensitive scintigraphic imaging technique. Pulmonary emboli (PEs) and lower extremity deep vein thrombi (DVTs) were formed in five dogs, then technetium-99m-labeled Fab' ( approximately 400 mg, approximately 260 MBq) were injected via forelimb veins. Thoracic and lower extremity SPECT scans were performed at 2-hour intervals after antibody infusion to visualize the thromboemboli. Four hours after antibody infusion, all PEs and DVTs of mass 0.4 g or greater were clearly visualized on SPECT scans as 'hot spots' within the lungs and legs, respectively. PEs (0.48 +/- 0.09 g) were intensely radiolabeled, yielding clot/blood radioactivity ratios of 22.8 +/- 5.6. DVTs (0.45 +/- 0.31 g) also had high clot/blood ratios (11.7 +/- 2.6). Infusion of these radiolabeled antibody fragments, combined with SPECT, produces clear images of PEs and DVTs within a clinically feasible time frame. The technique reliably identified even peripheral thromboemboli of relatively small size, which are difficult to diagnose with currently available imaging techniques, and may enable imaging of PEs, DVTs, or both in the same patient.

Intraoperative localization of the sentinel node in breast cancer: technical aspects of lymphoscintigraphic methods

Axillary lymph node dissection is an important part of the surgical treatment of breast cancer as a staging procedure. Recent progressive advances in early detection have led to the treatment of small primary carcinomas; thus, a great number of axillary dissections show completely negative lymph nodes. The sentinel node (SN) concept, developed for melanoma patients, seems to be similarly valid in breast cancer and has the potential to change the standard surgical approach in these patients. To verify the accuracy of lymphoscintigraphic method associated with radioguided biopsy of the sentinel node in a large series of patients, we studied 382 patients with operable breast cancer. Lymphoscintigraphy (LS) was performed the day before surgery; three different-sized ranges of 99mTechnetium-labeled colloid particles were injected either by subdermal or peritumoral administration. Planar scans were registered in anterior and oblique projections, and a cutaneous marker was placed over the skin corresponding to the SN as visualized. SNs were localized and removed during surgery, using a gamma-detecting probe (GDP); total axillary dissection was then performed. In 54 patients, blue dye was also administrated in the tumor bed immediately after excision of the primary. LS identified at least one SN in 377 of 382 cases (98.7%). Axillary SN was localized in 371 cases (97.1%). The overall concordance between SN status and other axillary nodes was 96.8% (359 of 371). Localization of the SN was easier when large-size particles of colloidal albumin were injected in a volume of 0.4 ml. GDP successfully localized SN in 54/54 cases (100%), while blue dye identified SN in 37/54 patients (68.5%). In 33 of 37 cases (89%) the dye and LS identified the same node. LS and GDP-guided surgery provide accurate identification and removal of the SN, particularly when large-size radiolabeled colloids are injected in a small volume.

99mTc-IMMU-4 monoclonal antibody scan in colorectal cancer. A prospective study

A blinded prospective study of 34 patients with colorectal adenocarcinoma using the Fab' fragment of the anticarcinoembryonic antigen monoclonal antibody type IMMU-4 labeled with technetium 99m was conducted to compare, on a lesion-by-lesion basis, the findings of radioimmunoscintigraphy, preoperative computed tomography, and exploratory celiotomy. Of 115 lesions detected at surgery, 113 were adenocarcinoma. Radioimmunoscintigraphy detected 59 lesions and computed tomography detected 62; both studies combined detected 72. Twenty-nine (54%) lesions missed by radioimmunoscintigraphy and 24 (45%) missed by computed tomography were 1 cm or smaller. When both studies were combined, the sensitivities were 90%, 24%, and 42%, and the specificities were 52%, 86%, and 61% for hepatic, extrahepatic intra-abdominal, and pelvic lesions, respectively. In 10 patients, additional information obtained with the radioimmunoscintigram could have altered the treatment of these patients. In this study, radioimmunodetection scan was complementary to computed tomographic scan in the examination of patients with colorectal carcinoma.

Immunoimaging of choroidal melanoma: assessment of its diagnostic accuracy and limitations in 101 cases

Immunoscintigraphy (IS) was performed on 101 patients with space occupying intraocular lesions including choroidal melanomas (85), choroidal naevi (11), non-melanoma metastases (three), and other melanoma simulating lesions (two). Scintigraphic images with conventional and emission computer tomography techniques were obtained after the intravenous injection of 99mTc-labelled F(ab')2 fragments of monoclonal antibody (MoAb) 225.28S directed against the high molecular weight-melanoma associated antigen (HMW-MAA). Immunohistochemistry was performed on sections of four out of 10 melanoma-containing eyes to confirm MoAb binding. IS demonstrated positive scans in 66 out of 85 choroidal melanomas, offering a sensitivity of 78%. Sensitivity was dependent on the lesion size. True negative results were obtained in 15 out of 16 non-melanoma lesions (specificity 94%). False positive antibody accumulation was found in one patient with a post-traumatic subretinal haemorrhage. Immunohistochemistry demonstrated positive MoAb 225.28S binding in all melanoma sections. In summary IS offered substantial sensitivity and specificity in the differentiation of intraocular lesions, particularly choroidal melanomas, naevi, and metastases. In combination with other diagnostic procedures such as ultrasound echography and fluorescein angiography IS proved to be a valuable method in the diagnosis of choroidal melanoma.