Viability and biodistribution of 68Ga MPO-labelled human platelets.

The viability and biodistribution of 68Ga-mercaptopyridine-N-oxide (MPO)-labelled autologous platelets was studied in 10 patients. The average platelet labelling yield was 36 +/- 12% and injected activity was 2.0 +/- 0.9 mCi 68Ga. The % activity in platelets per ml whole blood was 64 +/- 20% at 15 min-1.0 h postinjection and 76 +/- 14% at 2-4h. The average recovery of platelets (% injected platelets circulating in peripheral blood) was 31 +/- 21% at 15 min-1 h and 39 +/- 20% at 2-4 h. The positron emission tomographic (PET) images showed high circulating vascular background. Two patients had technically inadequate scans, and six were false negative due to high blood background. One patient with a massive pulmonary embolus occurring 24 h prior to scanning had marked uptake of 68Ga platelets in a large clot in the superior branch of the right main pulmonary artery. A second patient, with 68Ga platelets circulating during angioplasty of a left posterior tibial artery stenosis, had intense uptake in the lesion shown on the PET scan obtained 4 h following the procedure. These results indicate good viability of 68Ga-MPO-labelled autologous human platelets, but poor visualization of clots by PET imaging, due to the high blood background at early times.

Pretargeted immunoscintigraphy: effect of hapten valency on murine tumor uptake.

A method of radioimmunoscintigraphy using bivalent "Janus" haptens with an apparent enhanced affinity ("avidity") for the antibody is described. Janus with 50 micrograms pretargeted Mab WC3A11 resulted in significantly higher murine tumor concentrations (approximately 7%/g) compared to monovalent haptens (approximately 1.4%/g, p < 0.001), and the same high tumor-to-background ratios (approximately 3/1). Janus was synthesized by coupling two molecules of BABE together with a 1,4 butanedithiol linker. Janus itself was rapidly excreted (T1/2b = 42 min) by the kidneys and did not concentrate in any other organs or tissues. Three-step pretargeted immunoscintigraphy (binder, chaser, tracer) with 111In- or 67Ga-Co(III) Janus produced excellent mouse tumor images in 3 hr with high tumor-to-background ratios. The use of short-lived tracers, such as 99mTc and 68Ga, with a T1/2p of hours to image antibodies that localize slowly over several days in vivo is accessible with this new technology.

Chelate conjugates of monoclonal antibodies for imaging lymphoid structures in the mouse.

Radiolabeling of a mouse monoclonal antibody (MoAb) specific for the mouse histocompatibility alloantigen IAk expressed by the B lymphocytes of BALB/k and C3H mice but not BALB/c mice was performed by mixing the chelate-labeled anti (alpha) IAk MoAb with purified, no-carrier-added 111In citrate. Labeling efficiency was 85-95%, and the labeled alpha IAk MoAb retained its antigen binding properties in vitro and in vivo. The organ, spleen, and lymph node distribution of intravenously and subcutaneously administered 111In alpha IAk MoAb was compared in mice, two IAk positive and one IAk negative strains, and to 125I alpha IAk MoAb in one IAk positive strain. The 111In alpha IAk MoAb was more stable in vivo compared to 125I alpha IAk MoAb, as shown by a much slower excretion and a higher absolute uptake in lymph nodes and spleen. Lymph node to blood ratio was increased twofold by intravenous anti-EDTA MoAb. Subcutaneous injection permitted clear images of the tiny lymph nodes in the mouse. Potential clinical applications of 111In alpha lymphocyte MoAb include localization of normal lymph nodes and T & B cell leukemias and lymphomas, as well as detecting lymphatic metastases of other cancers. Therapy may also be possible using MoAbs labeled with beta-emitting metal ions such as yttrium-90.

Conjugation of antibodies with bifunctional chelating agents: isothiocyanate and bromoacetamide reagents, methods of analysis, and subsequent addition of metal ions.

Preparation of the chelating agent (S)-4-[2,3-bis[bis(carboxymethyl)am ino]propyl]phenyl isothiocyanate is reported. Procedures for conjugation of this and (S)-N-4-[2,3-bis[bis-(carboxymethyl)amino] propyl]phenyl bromoacetamide to monoclonal antibodies and other proteins are described. The conjugates may be purified quickly by centrifugation through Sephadex G-50. The number of protein-bound chelating groups may be measured by titration with standard 57Co2+, using thin-layer chromatography to monitor binding. The labeled products retain their immunoreactivity, as illustrated by experiments in vivo with chelate-conjugated antibody to mouse I-AK antigen.

Clinical studies with In-111 BLEDTA, a tumor-imaging conjugate of bleomycin with a bifunctional chelating agent.

Indium-111 BLEDTA, a bleomycin analog containing an EDTA group, was used for tumor imaging in 110 patients with cancer. Scans with In-111 BLEDTA agreed with biopsy results in 75 of 95 patients (79% accuracy). A positive scan was obtained in 71 of 88 patients with a positive biopsy (81% sensitivity). In 21 of 95 patients (22%), the scan revealed tumor sites that had not been detected. The main limitation of visualization was the size of the tumor (1.5--2.0 cm diameter was the smallest size seen). Background radioactivity in the liver, spleen, and bone marrow also made tumor detection in these areas more difficult. The cause of this background, and of false-positive uptake in sites of inflammation, is correlated with specific radiolabeling of polymorphonuclear leukocytes by In-111 BLEDTA. Means of eliminating this background are discussed.

BLEDTA: tumor localization by a bleomycin analogue containing a metal-chelating group.

Two different Co(III) complexes of the antitumor antibiotic bleomycin have been prepared, and their in vivo distribution in mice has been investigated. The more thermodynamically stable of the Co(III)-bleomycin complexes has been modified by reaction with the bifunctional chelating agent 1-(p-bromoacetamidophenyl)ethylenedinitrilotetraacetic acid, to give a bleomycin derivative (BLEDTA) containing a powerful metal-chelating group. BLEDTA was radiolabeled with 111In(III) and its in vivo distribution in mice was examined. The potential of 111In-labeled BLEDTA as a tumor-visualizing agent was also investigated in humans with biopsy-proven cancers, predominantly (70%) squamous carcinoma of the head and neck. All of the 29 patients studied had at least one clinically proven site of the disease visualized with 111In-BLEDTA. These clinical results are significantly better than results we obtained in a comparable group of patients using directly labeled 111In-bleomycin and are similar to those reported by Nouel for 57Co-bleomycin [GANN Monogr. Cancer Res., 19, 301 (1976)].

Indium-111-labeled autologous platelets for location of vascular thrombi in humans.

Twenty-two patients suspected of having either venous or arterial thrombi were studied with In-111-labeled autologous platelets. Whole-body scans were performed 3, 24, and 48 hr following i.v. injection. Twelve patients studied with saline-washed platelets had unsatisfactory 15-min recovery and biologic half-time. When the labeling was carried out in plasma, these values compared favorably with normal values reported for Cr-51-labeled autologous platelets. Of ten patients studied using platelets labeled in plasma, three had normal scans, six had abnormal scans, and one had an equivocal scan. All six abnormal scans were confirmed with corresponding positive findings in either the venogram, arteriogram, or lung scan. J Nucl Med 19: 626-634, 1978.

Bifunctional chelates for radio-pharmaceutical labeling.

The use of the first of a new class of chelating agents for the binding of metal ions to macromolecules as a novel approach to radiopharmaceutical labeling is described. Advantages are mild reaction conditions, large variety of accessible radionuclides and reactive groups, and separation of synthetic organic chemistry from radiochemistry. Human serum albumin and bovine fibrinogen labeled with 111Indium using this technique were relatively stable in vitro and in vivo, showed little alteration in function, and have potential as tumor localizing agents in humans.