Nuclear medicine imaging of bone infections.

Osteomyelitis is a broad group of infectious diseases that involve the bone and/or bone marrow. It can arise haematogenously, via extension from a contiguous infection, or by direct inoculation during surgery or trauma. The diagnosis is not always obvious and imaging tests are frequently performed as part of the diagnostic work-up. Commonly performed radionuclide tests include technetium-99m ((99m)Tc)-diphosphonate bone scintigraphy (bone), and gallium-67 ((67)Ga) and in vitro labelled leukocyte (white blood cell; WBC) imaging. Although they are useful, each of these tests has limitations. Bone scintigraphy is sensitive but not specific, especially when underlying osseous abnormalities are present. (67)Ga accumulates in tumour, trauma, and in aseptic inflammation; furthermore, there is typically an interval of 1-3 days between radiopharmaceutical injection of and imaging. Currently, this agent is used primarily for spinal infections. Except for the spine, WBC imaging is the nuclear medicine test of choice for diagnosing complicating osteomyelitis. The in vitro leukocyte labelling process requires skilled personnel, is laborious, and is not always available. Complementary marrow imaging is usually required to maximise accuracy. Not surprisingly, alternative radiopharmaceuticals are continuously being investigated. Radiolabelled anti-granulocyte antibodies and antibody fragments, investigated as in vivo leukocyte labelling agents, have their own limitations and are not widely available. (111)In-biotin is useful for diagnosing spinal infections. Radiolabelled synthetic fragments of ubiquicidin, a naturally occurring human antimicrobial peptide that targets bacteria, have shown promise as infection specific radiopharmaceuticals. 2-[(18)F]-fluoro-2-deoxy-d-glucose (FDG) positron-emission tomography (PET) with or without computed tomography (CT) is very useful in musculoskeletal infection. Sensitivities of more than 95% and specificities ranging from 75-99% have been reported in acute and subacute bone and soft-tissue infection. FDG is the radionuclide test of choice for spinal infection. It is sensitive, has a high negative predictive value, and can differentiate degenerative from infectious vertebral body end-plate abnormalities. Data on the accuracy of FDG for diagnosing diabetic pedal osteomyelitis and prosthetic joint infection are inconclusive and its role for these indications remains to be determined. Other PET radiopharmaceuticals that are under investigation as infection imaging agents include gallium-68 citrate ((68)Ga) and iodine-124 fialuridine ((124)I -FIAU).

Labeled leukocyte imaging: current status and future directions.

The ability to radiolabel inflammatory cells that migrate to foci of infection was a significant milestone in the evolution of infection imaging. More than 20 years after being approved for clinical use in the United States, labeled leukocyte imaging using cells labeled with [(99m)Tc]exametazime or [(111)In]oxine remains the radionuclide procedure of choice for diagnosing most infections in the immunocompetent population. In the central nervous system, labeled leukocyte imaging is useful for differentiating infection from tumor; in the postoperative setting, this test facilitates the differentiation of infection from normal postoperative changes. Labeled leukocyte imaging accurately diagnoses mycotic aneurysms and infected prosthetic vascular grafts. In patients with fever of unknown origin, a negative study excludes, with a high degree of certainty, infection as the source of fever. Labeled leukocyte imaging accurately diagnoses pedal osteomyelitis and is useful for distinguishing infection from the neuropathic joint in this population. Together with bone marrow imaging, the labeled leukocyte study is the imaging procedure of choice for diagnosing prosthetic joint infection. There are limitations to the test. Most of the leukocytes labeled are neutrophils, and the procedure is most useful for detecting neutrophil-mediated inflammatory processes, i.e., bacterial infections. It is less useful for illnesses in which the predominant cellular response is other than neutrophilic, such as most opportunistic infections and spinal osteomyelitis. The in vitro labeling procedure is time consuming and is not routinely available. Results of in vivo leukocyte labeling methods have been variable; none are available in the United States. Labeled leukocyte imaging suffers from inherently poor quality images. Single photon emission compute tomography/computed tomography improves lesion localization, and will undoubtedly improve the accuracy of the test. Efforts to develop methods of labeling white cells with positron emitting compounds are underway and, if successful, should further strengthen the role of nuclear medicine in infection imaging.

Imaging of infection and inflammation with 99mTc-Fanolesomab.

(99m)Tc-fanolesomab, a murine M class antigranulocyte antibody, is injected directly into patients, avoiding in vitro leukocyte labeling. Normal distribution includes reticuloendothelial system, genitourinary tract, and blood pool. Small bowel activity appears within 4 h, colonic activity by 24 h. Accumulation in infection is via two mechanisms: binding to circulating neutrophils that migrate to the infection and binding to neutrophils and neutrophil debris containing CD-15 receptors already sequestered in the infection. (99m)Tc-fanolesomab is valuable in atypical appendicitis. Its sensitivity, specificity, and accuracy, in 200 patients were 91%, 86%, and 87%, respectively. This agent is comparable to (111)In- labeled leukocytes for diagnosing osteomyelitis in the appendicular skeleton in general and in diabetic patients with pedal ulcers. Preliminary experience suggests (99m)Tc-fanolesomab might replace in vitro labeled leukocytes for other indications as well. Initial clinical investigations found the agent was safe. A transient decrease in circulating leukocytes within 20 min after injection occurred, but with no associated clinical complaints. Recovery averaged about 20 min. One study found no statistically significant HAMA titer elevation and no adverse reactions following injection. In another investigation 5 out of 30 subjects who received two separate antibody injections, exhibited HAMA induction with no serious or severe adverse events. Forty-nine adverse events, including 4 severe ones, were reported among 523 subjects in clinical trials. In 2004, (99m)Tc-falosomab was approved in the United States for use in patients with equivocal presentation of appendicitis. However, following postmarketing reports of serious adverse events, including two fatalities, the agent was withdrawn in late 2005, and its future is uncertain.

Imaging of infection and inflammation with 18F-FDG-labeled leukocytes.

Conventional scintigraphic imaging with (67)Ga citrate and in vitro labeled leukocytes is routinely used to evaluate infectious and inflammatory conditions. Recent studies suggest that positron emission tomography (PET) with [(18)F]fluorodeoxyglucose (FDG) also may be useful in this setting. Both (67)Ga citrate and [(18)F]FDG are highly sensitive tracers, but their specificity for detecting infection is lower than that of in vitro labeled leukocytes, which is the radionuclide gold standard for imaging most infections. PET has several advantages over conventional scintigraphic techniques, including higher spatial resolution and faster imaging times. In vitro [(18)F]FDG labeled leukocytes represent an initial attempt to develop an infection-specific, positron-emitting tracer. The experience to date with PET imaging of [(18)F]FDG labeled leukocytes is reviewed in this article.

[Correlation of hematologic parameters with bone marrow and spleen uptake in FDG PET]. / Correlación de los parámetros hematológicos con la captación de FDG en médula ósea y bazo en la PET.

OBJECTIVE: To assess the relationship between various hematologic parameters and bone marrow (BM) and splenic uptake of FDG in PET imaging. MATERIAL AND METHODS: 29 patients with Hodgkin's disease (HD) referred for baseline FDG PET imaging before treatment and without evidence of bone marrow (BM) involvement were included in the study. Splenic uptake also was analyzed in 18 patients without splenic involvement. BM and splenic activity were visually graded on a 3 point scale. Activity pattern was classified as homogeneous or heterogeneous. Semi-quantitative analysis was also performed by drawing regions of interest (ROI) over the spine and spleen. ROIs also were drawn over right lung and liver. FDG uptake ratios for the spine and spleen in comparison with the lung and liver were generated. Visual scoring of marrow and splenic uptake, and the various ratios were correlated with hemoglobin (Hb), white blood cell (WBC), and platelet counts, and correlation coefficients were calculated. RESULTS: In 27/29 patients (93 %) BM and in 18/18 patients (100 %) spleen uptake was diffuse. There was a direct correlation between BM and spleen uptake of FDG with increasing WBC, which was stronger than the inverse correlation seen with Hb (the lower the Hb the greater the uptake). Correlation with platelet counts was weaker. CONCLUSION: There is a correlation between hematologic parameters such as Hb, WBC and platelet counts and the uptake of FDG in BM and spleen in PET imaging. Knowledge of this correlation should help to better interpret and understand PET imaging.

Pulmonary activity on labelled leukocyte images: patterns of uptake and their significance.

The objective of this study was to characterize, and determine the significance of, pulmonary activity on labelled leukocyte images. This retrospective review included 137 immunocompetent patients who had undergone 111In labelled autologous leukocyte chest imaging and chest X-ray within 7 days. Pulmonary activity was classified as normal, focally increased, or diffusely increased. Images were correlated with chest X-rays and final diagnoses. One hundred and twelve patients (82%) had normal pulmonary activity. Seventy-six had normal chest X-rays; none had pulmonary infection. Thirty-six patients had chest X-ray abnormalities; only one had pulmonary infection. Twenty-five patients had abnormal pulmonary activity. In 13 patients it was segmental or lobar in distribution. The chest X-ray was abnormal in 12: pneumonia (11) and cystic fibrosis (one). The chest X-ray was normal in one patient with pneumonia. Two patients with non-segmental pulmonary activity did not have pulmonary infection. The chest X-ray was abnormal in one (pulmonary edema) and normal in one (sepsis). Ten patients had diffuse pulmonary activity. Chest X-ray was abnormal in two patients: adult respiratory distress syndrome (ARDS) (one) and drug toxicity (one). No patient with diffuse pulmonary activity had pulmonary infection. In summary, negative labelled leukocyte imaging excludes pulmonary infection with a high degree of certainty (the negative predictive value was 99% in this series), and can exclude pneumonia as the cause of a chest X-ray abnormality. Focal pulmonary activity strongly suggests pneumonia, while diffuse pulmonary activity is unlikely to indicate infection.

Role of nuclear medicine in diagnosis of the infected joint replacement.

Some complications of joint replacement surgery are easily diagnosed; however, differentiating infection from aseptic loosening is difficult because these entities are remarkably similar at clinical and histopathologic examination. Clinical signs and symptoms, laboratory tests, radiography, and joint aspiration are insensitive, nonspecific, or both. Cross-sectional imaging modalities are hampered by artifacts produced by the prosthetic devices themselves. Radionuclide imaging is not affected by the presence of metallic hardware and is therefore useful for evaluating the painful prosthesis. Bone scintigraphy is useful as a screening test, despite an accuracy of only 50%-70%, because normal results essentially exclude a prosthetic complication. The addition of gallium-67, a nonspecific inflammation-imaging agent, improves the accuracy of bone scintigraphy to 70%-80%. The accuracy of combined leukocyte-marrow imaging, 90%, is the highest among available radionuclide studies. Its success is due to the fact that leukocyte imaging is most sensitive for detection of neutrophil-mediated inflammation (ie, infection). The success of leukocyte-marrow imaging is tempered by the limitations of in vitro labeling. In vivo labeling has been investigated, and a murine monoclonal antigranulocyte antibody appears promising. Some investigations have focused on fluorodeoxyglucose imaging. Although this method is sensitive, specificity is a concern.

Localizing infection with a technetium-99m-labeled peptide: initial results.

In vitro-labeled leukocyte imaging is useful for the detection of infection, but an in vivo labeling method is preferable. This study sought to evaluate the safety and efficacy of a leukocyte-avid peptide for the detection of infection, to determine the effects of peptide dose on performance and to compare the peptide with in vitro-labeled leukocytes. A 23-amino acid peptide, P483, containing the platelet factor-4 heparin-binding sequence, was labeled with 99mTc and complexed with heparin (P483H). Thirty patients were injected with 29 microg (n = 11), 145 microg (n = 10) or 290 microg (n = 9) of labeled peptide, and imaged 15 min and 90-120 min later. Early and late images were interpreted individually and jointly. Twenty patients underwent (111)In-labeled leukocyte scintigraphy. Fourteen patients had infection: osteomyelitis (n = 7), vascular graft (n = 2), abscess (n = 2), joint replacement (n = 1), surgical wound (n = 1) and pneumonia (n = 1). There were 10 adverse events in six patients; all were mild and resolved spontaneously, and without any intervention. The sensitivity, specificity and accuracy were the same for both early and late imaging: 0.86, 0.81 and 0.83, respectively. Interpreting early and late images together did not improve the results. No relationship between peptide dose and study accuracy was found. In patients undergoing both examinations, the accuracies of the peptide and in vitro-labeled leukocyte imaging were identical: 0.80. In summary, 99mTc-P483H safely, rapidly and accurately detected focal infection, was comparable with in vitro-labeled leukocyte imaging and therefore merits further investigation.

Recombinant human thyroid-stimulating hormone-aided scintigraphy: comparison of imaging at multiple times after I-131 administration.

PURPOSE: Recombinant human thyroid-stimulating hormone (rhTSH) may be used in lieu of thyroid hormone withdrawal for the evaluation of thyroid cancer. Scintigraphy using the existing rhTSH protocol is performed 48 hours after I-131 administration. The authors investigated the feasibility of whole-body imaging at 72 hours and evaluated thyroid tissue uptake at 48 to 144 hours. METHODS: Thirty-two patients who previously had thyroidectomy for thyroid cancer were examined. Whole-body imaging was performed routinely at 48 and 72 hours after I-131 administration. Thyroid tissue was visualized in 12 patients, and large foci were imaged for as long as 144 hours. Activity ratios for thyroid tissue to background were determined for 10 patients. RESULTS: Whole-body images at 48 and 72 hours were comparable, generally with lower background activity at 72 hours, and thyroid tissue was well visualized at all times after 48 hours. Thyroid tissue-to-background activity ratios at 72 to 144 hours were equal to or greater than those at 48 hours in 9 of 10 patients. CONCLUSIONS: Recombinant human TSH-aided whole-body scintigraphy is possible 72 hours after I-131 administration, adding flexibility and convenience to the existing protocol and permitting confirmation of findings at 48 hours. Tracer uptake in thyroid tissue persists at later times. Therefore, rapid clearance of background activity appears to be the primary cause of the previously reported decrease in radioiodine retention in euthyroid persons receiving rhTSH.

Role of radionuclide imaging in the diagnosis of postoperative infection.

Postoperative infections are a serious cause of morbidity and mortality and are difficult to diagnose. Signs and symptoms that are generally associated with infection may be masked by, or mistaken for, normal postoperative changes. Anatomic imaging modalities provide high-quality anatomic detail and are the procedures of choice in affected patients because of their availability, ease of performance, accuracy, and value in the selection of treatment options. However, radionuclide studies demonstrate physiologic processes, which often precede anatomic changes, and can help distinguish normal postoperative inflammation from infection. Radionuclide studies are also useful in identifying complicated orthopedic infections, in which the often extensive distortions produced by metallic hardware can confound the interpretation of anatomic images. Of the three agents (gallium-67 citrate, indium-111-labeled leukocytes, technetium-99m-labeled leukocytes) that are currently approved in the United States for imaging of infection, In-111-labeled leukocyte imaging is the procedure of choice for diagnosing postoperative infection. Gallium scintigraphy is best reserved for those situations in which leukocyte imaging is not available or there is concern that the suspected infection may not incite a neutrophil response. In general, the value of radionuclide imaging is maximized when used only in those patients for whom the results of anatomic imaging are negative, nondiagnostic, or at odds with the clinical impression.

Diagnosing spinal osteomyelitis: a comparison of bone and Ga-67 scintigraphy and magnetic resonance imaging.

PURPOSE: The objective of this investigation was to compare the accuracies of bone and Ga-67 scintigraphy and magnetic resonance imaging (MRI) for diagnosing spinal osteomyelitis and to determine the optimal radionuclide approach to this disorder. METHODS: Twenty-two patients, with 24 sites of possible spinal osteomyelitis, who underwent three-phase bone scintigraphy with SPECT, Ga-67 scintigraphy with SPECT, and MRI with and without contrast were included in this retrospective review. Bone scans were interpreted as three-phase studies, delayed planar images alone, delayed planar plus SPECT, and SPECT alone (to identify uptake patterns). Sequential bone/ Ga-67 images were interpreted as planar and as SPECT studies. Planar and SPECT Ga-67 images were also interpreted alone. Precontrast MRI studies were used to identify osteomyelitis, whereas postcontrast images were used to identify soft tissue infection. RESULTS: Eleven sites of spinal osteomyelitis were identified. Tracer uptake in two contiguous vertebrae, as noted on SPECT, was the most accurate bone scan criterion for detecting spinal osteomyelitis (71 %). SPECT bone/Ga-67 was significantly more accurate (92%) than both planar bone/Ga-67 (75%) and bone SPECT (P = 0.15 and P = 0.2, respectively). SPECT Ga-67 was as accurate as SPECT bone/Ga-67 and as sensitive as MRI (91 %); the radionuclide study was slightly but not significantly more specific (92% vs. 77%) than MRI. Of 11 sites of extraosseous infection, 10 were identified on MRI, 9 on SPECT Ga-67, 7 on planar Ga-67, and none on bone scintigraphy. CONCLUSIONS: Spinal osteomyelitis and accompanying soft tissue infection can be diagnosed accurately with a single radionuclide procedure: SPECT Ga-67. This procedure can be used as a reliable alternative when MRI cannot be performed and as an adjunct in patients in whom the diagnosis is uncertain.

The diabetic foot.

Foot complications in diabetics often lead to amputation. Ulceration is the most common complication in the diabetic forefoot and underlies more than 90% of cases of pedal osteomyelitis. The diagnosis of osteomyelitis is, nevertheless, difficult, and imaging is an important part of the work-up. Plain radiographs, although useful for anatomical information, are neither sensitive nor specific. Three-phase bone scintigraphy is sensitive but not specific. Labelled leucocyte scintigraphy and MRI are both useful and are complementary to one another. Labelled leucocyte scintigraphy is valuable for diagnosis as well as follow-up of pedal osteomyelitis. MRI offers exquisite anatomical detail, which is invaluable for guiding surgical management. The principal complication in the mid and hind foot is the neuropathic or Charcot joint. Although infection of the neuropathic joint is infrequent, its diagnosis is difficult. The extensive bony changes that accompany this disorder severely diminish the value of radiography and bone scintigraphy. It is not always possible to distinguish the marrow oedema of neuropathy from that of osteomyelitis and the role of MRI in the evaluation of this entity is still uncertain. Uptake of labelled leucocytes in the absence of infection may occur and is owing, at least in part, to haematopoietically active marrow. Combined leucocyte/marrow scintigraphy holds considerable promise for identifying the infected Charcot joint.

Transplanted hepatocytes engraft, survive, and proliferate in the liver of rats with carbon tetrachloride-induced cirrhosis.

Repopulation of the cirrhotic liver with disease-resistant hepatocytes could offer novel therapies, as well as systems for biological studies. Establishing whether transplanted hepatocytes can engraft, survive, and proliferate in the cirrhotic liver is a critical demonstration. Dipeptidyl peptidase IV-deficient F344 rats were used to localize transplanted hepatocytes isolated from the liver of syngeneic normal F344 rats. Cirrhosis was induced by administration of carbon tetrachloride with phenobarbitone and these drugs were withdrawn prior to cell transplantation. Cirrhotic rats showed characteristic hepatic histology, as well as significant portosystemic shunting. When hepatocytes were transplanted via the spleen, cells were distributed immediately in periportal areas, fibrous septa, and regenerative nodules of the cirrhotic liver. Although some transplanted cells translocated into pulmonary capillaries, this was not deleterious. At 1 week, transplanted cells were fully integrated in the liver parenchyma, along with expression of glucose-6-phosphatase and glycogen as reporters of hepatic function. Transplanted cells proliferated in the liver of cirrhotic animals and survived indefinitely. At 1 year, transplanted hepatocytes formed large clusters containing several-fold more cells than normal control animals, which was in agreement with increased cell turnover in the cirrhotic rat liver. The findings indicate that the cirrhotic liver can be repopulated with functionally intact hepatocytes that are capable of proliferating. Liver repopulation using disease-resistant hepatocytes will be applicable in chronic conditions, such as viral hepatitis or Wilson's disease.

Radionuclide analysis of drug-induced blood-pool changes in liver and other organs.

UNLABELLED: Although it is possible to repopulate the animal liver with transplanted hepatocytes, the success of such transplants depends, in part, on the number of transplanted cells that enter the hepatic sinusoids. Pharmacologic alteration of hepatic vascular tone, and hence blood volume, can increase the number of cells that are successfully transplanted. Although analysis of changes in vascular beds is helpful for developing strategies for cell transplantation, convenient methods to analyze such changes are lacking. The objective of this study was to determine whether 99mTc-labeled red blood cells could be used to reveal pharmacologically induced blood pool changes in various organs. METHODS: F344 rats were injected with syngeneic labeled red blood cells and subjected to blood pool analysis with gamma camera imaging. Animals were treated with phenylephrine, phentolamine, labetalol, and nitroglycerine. To correlate hepatic blood pool changes with structural alterations at the vascular level, microspheres were injected into the portal circulation of these animals. RESULTS: Phenylephrine significantly increased cardiac and pulmonary blood pools, findings in agreement with its alpha-adrenergic effects. Phentolamine increased the hepatic, splenic, and pulmonary blood pools, whereas labetalol increased only the pulmonary blood pool. Nitroglycerine increased both hepatic and splenic blood pools. Prior administration of phentolamine, labetalol, and nitroglycerine prevented the phenylephrine-induced changes. When microspheres were injected into the portal circulation after nitroglycerine administration, they penetrated more distal locations in the liver lobule. CONCLUSION: These data indicate that it is possible, using radionuclide methods, to noninvasively show pharmacologically induced hemodynamic changes. This finding is potentially useful for studying hepatic physiology and may also have applications for cell therapy.

Integration and proliferation of transplanted cells in hepatic parenchyma following D-galactosamine-induced acute injury in F344 rats.

To determine whether liver repopulation with cell transplantation could be of therapeutic value in acute hepatic failure, it is necessary to establish the fate of transplanted hepatocytes. This study used dipeptidyl peptidase IV-deficient F344 rats as recipients to analyse the engraftment and proliferation of transplanted hepatocytes. Syngeneic hepatocytes were transplanted intrasplenically 24-30 h after induction of liver injury by D-galactosamine (GalN). Portosystemic shunting was analysed with 99m-Tc-labelled albumin microspheres. GalN-treated rats showed characteristic hepatic necrosis, inflammation, gamma-glutamyl transpeptidase activation, and regenerative activity, without increased portosystemic shunting (>99% 99m-Tc activity was in the liver in normal and GalN-treated rats). Transplanted cells entered hepatic sinusoids promptly and were observed in liver plates at 48 h. The number of transplanted cells increased in GalN-treated rats by approximately seven-fold (range two- to 12-fold), along with evidence for DNA synthesis between 3 and 14 days after cell transplantation and greater prevalence of larger transplanted cell clusters. These findings indicate that the liver can be safely repopulated in animals with acute liver failure, although the time required for regenesis of plasma membrane structures and proliferation in transplanted hepatocytes will need to be considered in developing therapeutic strategies.

Radionuclide imaging of nonosseous infection.

Nuclear medicine is an important tool in the diagnostic evaluation of patients with a variety of nonosseous infections. In the immunocompetent population labeled leukocyte imaging is the radionuclide procedure of choice, with Gallium imaging reserved for those situations in which the leukocyte study is nondiagnostic or cannot be performed. Fever of unknown origin is caused by infection in less than one-third of cases, and therefore the number of positive leukocyte studies will be relatively low. The negative leukocyte study is also useful, however, as it has been demonstrated that a negative study excludes, with a high degree of certainty, focal infection as the cause of an FUO. In the cardiovascular system, labeled leukocyte scintigraphy is very useful for diagnosing mycotic aneurysms and infected prosthetic vascular grafts, with a sensitivity of about 90%. The specificity of the study is somewhat more variable--false positive results have been described in perigraft hematomas, graft thrombosis, bleeding, and pseudoaneurysms. In the central nervous system, labeled leukocyte imaging can provide important information about the etiology of contrast enhancing brain lesions identified on computed tomography, i.e., distinguishing between neoplasm and infection. In the immunocompromised population, typified by the AIDS patient, Gallium scintigraphy is the radionuclide procedure of choice for diagnosing opportunistic diseases. In the thorax, a normal Gallium scan, in the setting of a negative chest X-ray, virtually excludes pulmonary disease. A negative Gallium scan in a patient with an abnormal chest X-ray and Kaposi's sarcoma study suggests that the patient's respiratory problems are related to Kaposi's sarcoma. Focal pulmonary parenchymal uptake is most often associated with bacterial pneumonia, although Pneumocystis carinii pneumonia can occasionally present in this fashion. Diffuse pulmonary parenchymal uptake of Gallium can be due to numerous causes, but in general, the more intense the uptake, the greater the likelihood that the patient has P. carinii pneumonia. Lymph node uptake is most often due to lymphoma or mycobacterial disease. In the abdomen, Gallium is also useful for detecting nodal disease. but is not reliable for detecting large bowel disease. Labeled leukocyte imaging should be performed when colitis is a concern. Both 18FDG PET and 201Tl SPECT imaging of the brain are useful for distinguishing between central nervous system lymphoma and toxoplasmosis in the HIV (+) patient. On both studies, lymphoma manifests as a focus of increased tracer uptake, whereas toxoplasmosis shows little or no uptake of either tracer.

Synthesis of aminobenzyltriethylenetetraaminohexaacetic acid: conjugation of the chelator to protein by an alkylamine linkage.

The conjugation of a chelating agent to an antibody as an anchoring site for a radionuclide is the first step in the successful preparation of a radiolabeled antibody for a diagnostic and therapeutic application. The high affinity of the protein bound chelator towards radionuclide ensures a higher selectivity in the delivery of the radionuclide to the targeted tissue. 4-Aminobenzylderivativetriethlenetetraaminohexaacetic acid (TTHA), a hexadentate chelating agent has been now prepared for conjugation with proteins in view of the higher affinity of TTHA metal ions as compared to DTPA. The latent crosslinking potential of alpha-hydroxy aldehydes has been used to conjugate the new chelating agent to proteins through an alkylamine linkage. On incubation of amino benzyl TTHA with glycoladehyde at neutral pH and room temperature, the reagent is converted to oxo ethyl amino benzyl TTHA. On addition of albumin to this reaction mixture, the oxo ethylamino benzyl TTHA generates reversible schiff base adducts with the amino groups of albumin. The reduction of the Schiff base adducts of the chelator with the protein by sodium cyanoborohydride stabilizes the schiff base adducts as stable alkylamine linkages. 4-Thiocyanatobenzyl TTHA has also been prepared and conjugated to albumin through a thiocarbamoyl linkage. Both preparations of TTHA conjugated albumin complexed with 99mTc and 111In, with high affinity and no decomposition of the complex was noticed for at least up to 6 hrs after the preparation. The radiolabels complexed with these TTHA -albumin conjugates could not be 'chased' out by free DTPA. A comparison of the biodistribution of 111In, bound to the TTHA conjugated through an alkylamine and a thiocarbamoyl linkage showed that 111In complexed with alkylamine linked TTHA was retained in blood to a level nearly 17% higher compared to that seen with thicarbamoyl linked TTHA, one hr after the injection into mice. Thus, the alkylamine linkage appears to be more stable under the in vivo conditions. The glycolaldehyde mediated alkylation procedure offers a mild, simple and rapid method for preparation of drug-protein (antibody) conjugates.

Improved tumor targeting of rhenium-186-labeled anti-human high-m.w. melanoma-associated antigen monoclonal antibody 763.74 following purification with anti-idiotypic monoclonal antibody MK2-23.

Because of its high-energy beta emissions and imageable gamma emissions, 186Re represents an attractive isotope to radiolabel monoclonal antibodies (MAbs) recognizing human tumor-associated antigens (TAAs) for radioimmunoscintigraphy (RIS) and radioimmunotherapy (RIT) of patients with malignant diseases. Application of 186Re, however, is hindered by the frequent denaturation of MAbs following exposure to the strong reducing conditions employed in the labeling procedures. To overcome this problem, we have utilized a direct labeling procedure and combined it with affinity chromatography over columns of immobilized anti-idiotypic (anti-id) MAbs which recognize idiotopes in the antigen-combining site of the radiolabeled MAb. The validity of the procedure was demonstrated with the anti-high-m.w. melanoma-associated antigen (HMW-MAA) MAb 763.74 and its corresponding anti-id MAb MK2-23. Utilizing this approach, MAb 763.74 was labeled to specific activities of 2.8 +/- 0.6 mCi/mg with 186Re. Furthermore, its immunoreactivity, which was reduced to about 30% following labeling with 186Re, was improved to about 50% by affinity chromatography over columns of anti-id MAb MK2-23. The improvement in immunoreactivity of 186Re MAb 763.74 resulted in a significant (p < 0.05) increase in targeting to human melanoma tumors grown in nude mice and an increase in sensitivity of RIS. Our results suggest that direct labeling of anti-TAA MAb with 186Re coupled with purification over affinity columns of anti-id MAb may facilitate the application of 186Re to RIS and RIT.