Interventional nuclear medicine: "click" chemistry as an in vivo targeting strategy for imaging microspheres and bacteria

AIM

Pre-targeting is a proven strategy for in vivo delivery of a diagnostic or therapeutic payload. The pre-targeting concept can be realized through various conjugation strategies, one of which is based on copper-free "click" chemistry. Copper-free click reactions have shown in vivo potential for imaging and radionuclide therapy, but this conjugation strategy has not yet been explored in combination with microspheres or unicellular organisms. This study aims to evaluate the in vivo efficacy of strain-promoted azide-alkyne cycloaddition (SPAAC) reactions to achieve imaging and targeting of azide-functionalized macro-aggregated albumin (MAA) microspheres and Staphylococcus aureus bacteria.

METHODS

MAA microspheres (diameter 10-90 μm) were functionalized with a biorthogonal Cy5 fluorophore, bearing an azide functionality (N3), to generate MAA-Cy5-N3. S. aureus (diameter ∼1 μm) were functionalized with 99mTc-UBI29-41-Cy5-N3, generating S. aureus-99mTc-UBI29-41-Cy5-N3. In situ and in vitro click conjugation on the -N3 moieties was studied for 20 h using a radioactivity-based assay and fluorescence microscopy. For in vivo validation, both primary entities, radiolabeled with 99mTc, were deposited into the microvasculature of the liver via intrasplenic injections. Secondary targeting was realized following the intravenous administration of indium-111-radiolabeled diethylenetriaminepentaacetic acid-dibenzocyclooctyne (111In-DTPA-DBCO). To assess click reaction efficiency in vivo, 99mTc and 111In-biodistributions were measured (SPECT and %ID g-1). Use of 111In-DTPA-DBCO in mice without MAA deposits or mice infected with non-functionalized S. aureus served as controls. Ex vivo confocal fluorescence imaging was carried out in excised tissues to confirm the presence of functionalized MAA and bacteria.

RESULTS

In vitro data confirmed effective click reactions on both the MAA particles and the bacterial membrane. SPECT imaging and biodistribution studies revealed significantly (p < 0.05) increased accumulation of 111In-DTPA-DBCO at the sites where MAA-Cy5-N3 (7.5 ± 1.5%ID g-1vs. 3.5 ± 0.5%ID g-1 in control mice) and S. aureus-99mTc-UBI29-41-Cy5-N3 (9.3 ± 1.3%ID g-1vs. 6.0 ± 0.5%ID g-1 in control mice) resided. Ex vivo fluorescence imaging confirmed the presence of either functionalized MAA or S. aureus in excised spleens and livers of mice.

CONCLUSION

Copper-free click chemistry between a DBCO moiety and Cy5-N3-functionalized microspheres or bacterial entities in the liver can be used to realize in vivo imaging and targeting.

Prosthetic joint infections: radionuclide state-of-the-art imaging

Prosthetic joint replacement surgery is performed with increasing frequency. Overall the incidence of prosthetic joint infection (PJI) and subsequently prosthesis revision failure is estimated to be between 1 and 3%. Differentiating infection from aseptic mechanical loosening, which is the most common cause of prosthetic failure, is especially important because of different types of therapeutic management. Despite a thorough patient history, physical examination, multiple diagnostic tests and complex algorithms, differentiating PJI from aseptic loosening remains challenging. Among imaging modalities, radiographs are neither sensitive nor specific and cross-sectional imaging techniques, such as computed tomography and magnetic resonance imaging, are limited by hardware-induced artefacts. Radionuclide imaging reflects functional rather than anatomical changes and is not hampered by the presence of a metallic joint prosthesis. As a result scintigraphy is currently the modality of choice in the investigation of suspected PJI. Unfortunately, there is no true consensus about the gold standard technique since there are several drawbacks and limitations inherent to each modality. Bone scintigraphy (BS) is sensitive for identifying the failed joint replacement, but cannot differentiate between infection and aseptic loosening. Combined bone/gallium scintigraphy (BS/GS) offers modest improvement over BS alone for diagnosing PJI. However, due to a number of drawbacks, BS/GS has generally been superseded by other techniques but it still may have a role in neutropenic patients. Radiolabelled leucocyte scintigraphy remains the gold standard technique for diagnosing neutrophil-mediated processes. It seems to be that combined in vitro labelled leucocyte/bone marrow scintigraphy (LS/BMS), with an accuracy of about 90%, is currently the imaging modality of choice for diagnosing PJI. There are, however, significant limitations using in vitro labelled leucocytes and considerable effort has been devoted to developing alternative radiotracers, such as radiolabelled HIGs, liposomes, antigranulocyte antibodies and fragments, as well as more investigational tracers such as radiolabelled antibiotics, antimicrobial peptides, bacteriophages and thymidine kinase. On the other hand, positron emission tomography (PET) is still growing in the field of PJI imaging with radiotracers such as (18)F-fluorodeoxyglucose (FDG), (18)F-FDG white blood cells and (18)F-fluoride. But unfortunately this superb tomographic technique will only receive full acceptance when specific PET uptake patterns can be successfully developed. The emergence of hybrid modality imaging using integrated single photon emission computed tomography (SPECT) and PET with computed tomography (SPECT/CT and PET/CT) may also have a contributing role for more accurate assessment of joint replacement complications, especially combined with new radiotracers such as (68)Ga and (64)Cu. Finally, in searching for infection-specific tracers, currently there is no such diagnostic agent available.

Evaluation of 99mTc-UBI 29-41 scintigraphy for specific detection of experimental multidrug-resistant Staphylococcus aureus bacterial endocarditis

AIM

The aim of this paper was to test the ability of technetium-99m labelled synthetic peptide UBI 29-41 scintigraphy (99mTc-UBI 29-41), composed of the antimicrobial peptide ubiquicidin, specifically targets microorganisms in to discriminate between infected and uninfected endocarditis using a rat model previously validated.

METHODS

99mTc-UBI 29-41 scintigraphy was evaluated for its accumulation in infective endocarditis (IE) with multidrug resistant Staphylococcus aureus (MRSA) performed in an experimental rat model, resembling early endocarditis in humans. Serial planar scintigraphic and biodistribution analysis of infected vegetations are compared to rats with sterile vegetations. Heart-to-lung uptake ratios (T/NT ratios) were calculated in both with in-vivo scintigraphy and in ex vivo tissue samples.

RESULTS

Bacterially infected vegetations were already observed at 15 min after injection of 99mTc-UBI 29-41 while no significant uptake was observed in sterile vegetations. Moreover, a good correlation (R2=0.819) was calculated between T/NT ratios of 99mTc-UBI 29-41 and the number of viable MRSA present in the infected vegetation. There was no correlation between the 99mTc-UBI 29-41 uptake and the weight of the vegetations in either case.

CONCLUSION

In this experimental study in rats, planar 99mTc-UBI 29-41 scintigraphy permitted early and specific detection of MRSA induced endocarditis. Furthermore, accumulation of the tracer depends on the number of viable MRSA and not on the weight of the vegetation. This proof of principle offers much promise that 99mTc-UBI 29-41 scintigraphy can be a dedicated non-invasive imaging tool for the early detection of infective endocarditis. Finally, this model has to be further evaluated with state-of-the art small imaging modalities.

Detection of fungal infections using radiolabeled antifungal agents

The outcome of antifungal therapy depends on the progression of the infection at the start of therapy. Unfortunately, most patients are diagnosed once the fungal infection has progressed considerably as a result of the non-specific clinical signs of fungal infections in immunocompromised patients and the poor sensitivity of current mycological diagnostic tests. This review will highlight current fungal diagnostic techniques and will focus on scintigraphic methods for the specific detection of fungal infections in mice. For this purpose, antifungal components (e.g. fluconazole and antifungal peptides) are radiolabeled e.g. with technetium-99m ((99m)Tc) and their in vivo distribution is monitored in infected mice. It has been demonstrated that (99m)Tc-fluconazole is an excellent tracer to detect Candida albicans infections in mice as it distinguishes these infections from bacterial infections and sterile inflammations. However, this radiopharmaceutical only poorly detects infections with Aspergillus fumigatus in mice. (99m)Tc-peptides derived from antifungal peptides/proteins, such as human ubiquicidin and lactoferrin, can distinguish C. albicans and A. fumigatus infections from sterile inflammations, but not from bacterial infections, in mice. Furthermore, the efficacy of fluconazole in C. albicans-infected mice could be successfully monitored using (99m)Tc-ubiquicidin. In conclusion, neither (99m)Tc-fluconazole nor the (99m)Tc-peptides tested are optimal tracers for fungal infections. Nonetheless, since early initiation of antifungal therapy for candidemia reduces its high mortality rate, a positive result with (99m)Tc-fluconazole scintigraphy is of clinical relevance. Finally, the possibility that other (radiolabeled) antifungal agents, e.g. voriconazole, caspofungin, antifungal plant or insect defensins, can be useful for detection of fungal infections should be considered.

Outcome of intensive immunosuppression and autologous stem cell transplantation in patients with severe rheumatoid arthritis is associated with the composition of synovial T cell infiltration

OBJECTIVE

To determine clinical and immunological correlates of high dose chemotherapy (HDC) + autologous stem cell transplantation (ASCT) in patients with severe rheumatoid arthritis (RA), refractory to conventional treatment.

METHODS

Serial samples of peripheral blood and synovial tissue were obtained from seven patients with RA treated with HDC and autologous peripheral blood grafts enriched for CD34+ cells. Disease activity was assessed with the Disease Activity Score (DAS), serum concentrations of C reactive protein (CRP), and human immunoglobulin (HIg) scans, and the extent of immunoablation was determined by immunophenotyping of peripheral blood mononuclear cells, and immunohistochemistry and double immunofluorescence of synovium.

RESULTS

Clinical responders (n = 5) had a larger number of cells at baseline expressing CD3, CD4, CD27, CD45RA, CD45RB, and CD45RO in synovium (p < 0.05), higher activity on HIg scans (p = 0.08), and a trend towards higher concentrations of CRP in serum than non-responders (n = 2). Subsequent remissions and relapses in responders paralleled reduction and re-expression, respectively, of T cell markers. A relatively increased expression of CD45RB and CD45RO on synovial CD3+ T cells was seen after HDC + ASCT. No correlations were found between DAS and changes in B cells or macrophage infiltration or synoviocytes.

CONCLUSIONS

HDC + ASCT results in profound but incomplete immunoablation of both the memory and naïve T cell compartment, which is associated with longlasting clinical responses in most patients. The findings provide strong circumstantial evidence for a role of T cells in established RA, and demonstrate a role for the synovium in post-transplantation T cell reconstitution.

99mTc-antimicrobial peptides: promising candidates for infection imaging

This review presents the state of the art of imaging of bacterial and fungal infections in laboratory animals using antimicrobial peptides labelled with technetium-99m ((99m)Tc). The mechanistic basis of this approach is that these peptides accumulate at sites of infection, but not in sterile inflammatory lesions, because of their preferential binding to bacteria and fungi over mammalian cells. For practical reasons, such as production of large amounts of peptides under good laboratory practice conditions and favourable pharmacokinetics, synthetic peptides representing such binding domains of natural antimicrobial peptides are preferred. On the basis of their preferential in vitro and in vivo binding to microorganisms over human cells, fast and easy penetration into the target area, and rapid clearance from the circulation via the urinary tract, various (99m)Tc-antimicrobial peptides were identified. Next, it was determined whether these radiopharmaceuticals distinguish infectious foci from sites of sterile inflammation. Further experiments with (99m)Tc-ubiquicidin-derived peptides in infected laboratory animals have revealed that the radioactivity at the infectious site correlated well with the number of viable bacteria present, indicating that these (99m)Tc-labelled peptides may enable the monitoring of the efficacy of antimicrobial therapy. Together, (99m)Tc-labelled synthetic peptides derived from human ubiquicidin are promising candidates for imaging of bacterial and fungal infections in nuclear medicine.

Improved radioiodination of biomolecules using exhaustive Chloramine-T oxidation

To improve standardization in analytical reagents we investigated Chloramine-T radioiodination (125I) of several biomolecules based on the use of a single amount of the oxidizing agent Chloramine-T as the limiting reagent being exhausted during the course of the reaction. Whenever the labeling yield resulted in less than one atom 125I/molecule, a second amount of the oxidizing agent was added. Thereafter, the integrity of the various biomolecules was assessed using radioimmunoassays, radioreceptor binding assays, or radioimmunometric assays. Purification yields were done by gel permeation (56% +/- 19%, n=230) or by precipitation with trichloroacetic acid (59% +/- 19%, n=230). Specific activity (117 +/- 61 MBq/nmol) and the degree of iodine incorporation (1.4 +/- 0.8 atoms of 125I/molecule) were achieved after 300 sec of incubation. A second addition of Chloramine-T resulted in an increased labeling yield of all biomolecules tested by a mean factor of 1.8 +/- 0.9. After the second addition of Chloramine-T, we observed for some biomolecules a significant (p<0.001) decreased effect in biological performance. In conclusion, the use of Chloramine-T as a limiting reagent resulted in molecules with appropriate immunological and biological performance. In general, tracers were minimally damaged and assessment of the shelf life as well as storing conditions showed the usefulness of the standardization of biomolecule labeling.

99mTc-labeled antimicrobial peptides for detection of bacterial and Candida albicans infections

This study compared the possibilities and limitations of 99mTc-labeled synthetic peptides derived from two human antimicrobial peptides, namely, ubiquicidin (UBI) and lactoferrin (hLF), for the scintigraphic detection of bacterial and fungal infections in mice and rabbits. The rationale of our approach was that selected peptides accumulate in infected areas but not in sterile inflammatory lesions, because they bind preferentially to microorganisms. 99mTc-labeled human neutrophil peptides (defensins), ciprofloxacin, and human polyclonal IgG were included as control agents.

METHODS

99mTc-labeled peptides and control agents were injected intravenously into animals that had been injected intramuscularly 18 h earlier with multidrug-resistant Staphylococcus aureus, Klebsiella pneumoniae, or fluconazole-resistant Candida albicans. Sterile inflammatory sites were induced by the injection of heat-killed microorganisms or lipopolysaccharide (LPS) into the thigh muscle. Up to 4 h after injection, the accumulation of 99mTc-labeled compounds in the infected/inflamed thigh muscles was determined using scintigraphic techniques and radioactivity counts in dissected tissues.

RESULTS

Scintigraphy revealed that 99mTc-labeled peptides UBI 29-41, UBI 18-35, UBI 31-38, hLF 1-11, and defensins, which showed preferential in vitro binding to microorganisms in a former study, accumulated at a significantly higher rate (P < 0.01) in bacterial and C. albicans infections in mice and rabbits than in inflamed tissues induced by heat-killed microorganisms or by LPS. No significant difference in the accumulation of 99mTc-labeled ciprofloxacin was observed between infected and sterile inflamed thigh muscles in mice.

CONCLUSION

99mTc-labeled antimicrobial peptides UBI 29-41, UBI 18-35, UBI 31-38, hLF 1-11, and defensins accumulate significantly in tissues infected with gram-positive and gram-negative bacteria and C. albicans. Significantly lower (P < 0.01) accumulation of these peptides occurs in sterile inflamed tissues. These data indicate that the peptides preferentially tag microorganisms at the site of infection, which is in agreement with their preferential binding to the microorganisms in vitro and in vivo. 99mTc-labeled ciprofloxacin does not distinguish between infections and sterile inflammatory lesions, which implies that its specificity for the detection of bacterial infections is not warranted.

Human lactoferrin and peptides derived from its N terminus are highly effective against infections with antibiotic-resistant bacteria

Since human lactoferrin (hLF) binds to bacterial products through its highly positively charged N terminus, we investigated which of the two cationic domains is involved in its bactericidal activity. The results revealed that hLF lacking the first three residues (hLF(-3N)) was less efficient than hLF in killing of antibiotic-resistant Staphylococcus aureus, Listeria monocytogenes, and Klebsiella pneumoniae. Both hLF preparations failed to kill Escherichia coli O54. In addition, hLF(-3N) was less effective than hLF in reducing the number of viable bacteria in mice infected with antibiotic-resistant S. aureus and K. pneumoniae. Studies with synthetic peptides corresponding to the first 11 N-terminal amino acids, designated hLF(1-11), and fragments thereof demonstrated that peptides lacking the first three N-terminal residues are less effective than hLF(1-11) in killing of bacteria. Furthermore, a peptide corresponding to residues 21 to 31, which comprises the second cationic domain, was less effective than hLF(1-11) in killing of bacteria in vitro and in mice having an infection with antibiotic-resistant S. aureus or K. pneumoniae. Using fluorescent probes, we found that bactericidal hLF peptides, but not nonbactericidal peptides, caused an increase of the membrane permeability. In addition, hLF killed the various bacteria, most probably by inducing intracellular changes in these bacteria without affecting the membrane permeability. Together, hLF and peptides derived from its N terminus are highly effective against infections with antibiotic-resistant S. aureus and K. pneumoniae, and the first two arginines play an essential role in this activity.

Candidacidal activities of human lactoferrin peptides derived from the N terminus

In light of the need for new antifungal agents, the candidacidal activities of human lactoferrin (hLF) and synthetic peptides representing the first, hLF(1-11), and second, hLF(21-31), cationic domains of its N terminus were compared. The results revealed that hLF(1-11) was more effective in killing fluconazole-resistant Candida albicans than hLF(21-31) and much more effective than lactoferrin, as determined microbiologically and by propidium iodide (PI) staining. By using hLF(1-11) and various derivatives, it was found that the second and third residues of the N terminus of hLF(1-11) were critical for its candidacidal activity. Detailed investigation to elucidate the mechanism of action of hLF(1-11) revealed a dose-dependent release of ATP by Candida upon exposure to hLF(1-11). Our observations that sodium azide reduced the PI uptake and candidacidal activity of hLF(1-11) and that, upon exposure to hLF(1-11), the fluorescent dye rhodamine 123 first accumulated inside the mitochondria and later was released into the cytoplasm indicate that the peptide triggers the energized mitochondrion. Furthermore, oxidized ATP, which interferes with the interaction of ATP with its extracellular receptors, blocked the candidacidal action of hLF(1-11), as measured microbiologically and by PI staining. Addition of ATP (or analogues) was not a sufficient stimulus to kill C. albicans or to act synergistically with suboptimal concentrations of the peptide. The main conclusions are that the first two arginines at the N terminus of hLF are critical in the candidacidal activity of hLF(1-11) and that extracellular ATP is essential but not sufficient for the peptide to exert its candidacidal activity.

Technetium-99m labelled antimicrobial peptides discriminate between bacterial infections and sterile inflammations

The aim of this study was to select technetium-99m labelled peptides that can discriminate between bacterial infections and sterile inflammations. For this purpose, we first assessed the binding of various 99mTc-labelled natural or synthetic peptides, which are based on the sequence of the human antimicrobial peptide ubiquicidin (UBI) or human lactoferrin (hLF), to bacteria and to leucocytes in vitro. In order to select peptides that preferentially bind to bacteria over host cells, radiolabelled peptides were injected into mice intraperitoneally infected with Klebsiella pneumoniae (K. pneumoniae) and the amount of radioactivity associated with the bacteria and with the leucocytes was quantitated. The next phase focussed on discrimination between bacterial infections and sterile inflammatory processes using 99mTc-labelled peptides in mice intramuscularly infected with various bacteria (e.g. multi-drug-resistant Staphylococcus aureus) and in animals that had been injected with lipopolysaccharides (LPS) of bacterial origin to create a sterile inflammatory process. Also, we studied the distribution of 99mTc-labelled UBI 29-41 and UBI 18-35 in rabbits having an experimental thigh muscle infection with K. pneumoniae and in rabbits injected with LPS. Based on the results of our in vitro and in vivo binding assays, two peptides, i.e. UBI 29-41 and UBI 18-35, were selected as possible candidates for infection imaging. The radiolabelled peptides can detect infections with both gram-positive and gram-negative bacteria in mice as early as 5-30 min after injection, with a target-to-non-target (T/NT) ratio between 2 and 3; maximum T/NT ratios were seen within 1 h after injection. In rabbits, high T/NT ratios (>5) for 99mTc-labelled UBI 29-41 were observed from 1 h after injection. No accumulation of the selected 99mTc-labelled UBI-derived peptides was observed in thighs of mice and rabbits previously injected with LPS. Scintigraphic investigation into the biodistribution of 99mTc-labelled UBI peptides revealed that these peptides were rapidly removed from the circulation by renal excretion. Similar data were observed for 99mTc-labelled defensin 1-3. Our data for 99mTc-labelled hLF and related peptides indicate that these compounds are less favourable for infection detection. Taken together, 99mTc-labelled UBI 18-35 and UBI 29-41 enable discrimination between bacterial infections and sterile inflammatory processes in both mice and rabbits. Based on their characteristics, we consider these peptides the candidates of preference for detection of bacterial infections in man.

Imaging of bacterial infections with 99mTc-labeled human neutrophil peptide-1

This study was undertaken to evaluate whether 99mTc-labeled human neutrophil peptide (HNP)-1 can be used as a tracer for rapid visualization of bacterial infections.

METHODS

Mice were injected intramuscularly with 1 million Staphylococcus aureus or Klebsiella pneumoniae organisms and 5 min later were injected intravenously with 0.4 microg (0.8 MBq) 99mTc-HNP-1. At various intervals, detailed information about clearance and accumulation of this tracer at sites of infection and in various organs was obtained by scintigraphy. 99mTc-labeled immunoglobulin G (IgG), an established marker of infection and inflammation, was used for comparison.

RESULTS

After injection into S. aureus- or K. pneumoniae-injected mice, 99mTC-HNP-1 was rapidly removed from the circulation, mainly through the kidneys and bladder, with half-lives of 170 and 55 min, respectively. Similar half-lives were observed for 99mTc-IgG in these animals. Visualization of foci with S. aureus or K. pneumoniae, as indicated by a ratio of 1.3 or higher between the targeted thigh muscle (containing bacteria) and the nontargeted (contralateral) thigh muscle (T/NT), was already achieved 5 min after injection of 99mTc-HNP-1. Similar T/NTs for 99mTc-IgG were obtained 4 h after injection of the tracer, indicating that imaging of foci of bacteria with 99mTc-HNP-1 is much faster than with 99mTc-IgG. To obtain insight into factors that contribute to accumulation of 99mTc-HNP-1 at sites of infection, the binding of this tracer to bacteria and leukocytes was assessed using a peritoneal infection model. Binding of 99mTC-HNP-1 to bacteria was approximately 1000 times higher than binding to leukocytes. Although the number of bacteria in the peritoneum was 1000-fold lower than the number of leukocytes, a significant correlation between binding of 99mTc-HNP-1 to bacteria on the one hand and accumulation of tracer on the other was still found, in contrast to 99mTc-IgG.

CONCLUSION

99mTc-HNP-1 allows rapid visualization of bacterial infections. Binding of this tracer to bacteria most likely contributes significantly to the accumulation of 99mTc-HNP-1 at sites of infection.

Interaction of a monoclonal antibody against hEGF with a receptor site for EGF

Epidermal growth factor (EGF) has been detected by radioimmunoassay (RIA) in different body fluids such as serum, amniotic fluid, and urine. Human tumor tissues with EGF receptors (EGF-Rc) may be saturated with EGF, which may be of prognostic value. An RIA was envisaged to measure human epidermal growth factor (hEGF) levels using EGF-Rc as capture agent and a monoclonal antibody anti-hEGF (MAb anti-hEGF) labeled with 125Iodine as a marker for this binding. The purpose of this work was to study the feasibility of MAb anti-hEGF to detect the receptor binding sites and to investigate the interaction between MAb anti-hEGF and the EGF-Rc. Various binding experiments were performed to study possible interference and interactions in the complex MAb anti-hEGF and the receptor. Affinity constants were determined by means of Scatchard plot analysis to interpret the complex stability challenged with other compounds for a better understanding of the interaction process. Binding constants were of the same order for all the ligands tested separately involving the EGF-Rc, but were significantly higher (t = 15.7, p < 0.05) for hEGF in its binding to MAb anti-hEGF. It was possible with equilibrium studies and competition experiments to evaluate the interaction of EGF and MAb anti-hEGF with the EGF receptor. This observation makes the MAb anti-hEGF a potential tracer for the quantitation of receptors in vitro, and possibly for the detection of membrane receptors on tumor cells in vivo.

Radiolabelled antimicrobial peptides for imaging of infections: a review

At present, it is difficult to distinguish between bacterial infections and sterile inflammatory processes using radiopharmaceuticals. This is so for a variety of reasons, including binding to bacteria with low affinity (e.g. infection) and binding to a specific micro-organism (e.g. radiolabelled monoclonal antibodies or F(ab)2 fragments thereof against micro-organisms). In this review, we propose that radiolabelled antimicrobial peptides should be the first choice in the development of new radiopharmaceuticals for imaging of bacterial infections. Antimicrobial peptides are a recently discovered component of the innate defence system of plants, animals and humans. These peptides, which now number more than 100, with proven microbicidal activity against a variety of micro-organisms, share certain properties, such as their small size and cationic charge. The latter allows them to bind preferentially to a broad spectrum of micro-organisms. We have recently demonstrated that radiolabelled human defensins allow the rapid visualization of bacterial infections in mice. Furthermore, binding of this antimicrobial peptide to bacteria is the major factor contributing to the accumulation of this tracer in bacterial infections. Based on these considerations, we believe that radiolabelled antimicrobial peptides will be an important asset in the imaging of infections in patients.

Antibacterial activity of human neutrophil defensins in experimental infections in mice is accompanied by increased leukocyte accumulation

Neutrophil defensins (or human neutrophil peptides-HNP) are major constituents of the azurophilic granules of human neutrophils and have been shown to display broad-spectrum antimicrobial activity. Other activities of these defensins, which are released from stimulated neutrophils, include cytotoxic, stimulatory, and chemotactic activities toward a variety of target cells. We studied the potential use of HNP-1 for antibacterial therapy of experimental bacterial infections in mice. In experimental peritoneal Klebsiella pneumoniae infections in mice, HNP-1 injection was shown to markedly reduce bacterial numbers in the infected peritoneal cavity 24 h after infection. This antibacterial effect was found to be associated with an increased influx of macrophages, granulocytes, and lymphocytes into the peritoneal cavity. These leukocytes appeared to be a requirement for the antibacterial effect, since in leukocytopenic mice administration of HNP-1 did not display antibacterial activity. HNP-1 treatment also reduced bacterial numbers in experimental K. pneumoniae or Staphylococcus aureus thigh muscle infections. In this model, radiolabeled HNP-1 was found to accumulate at the site of infection, whereas most of the injected HNP-1 was rapidly removed from the circulation via renal excretion. These results demonstrate that neutrophil defensins display marked in vivo antibacterial activity in experimental infections in mice and that this activity appears to be mediated, at least in part, by local leukocyte accumulation.

The labeling of proteins and LDL with 99mTc: a new direct method employing KBH4 and stannous chloride

A new direct labeling technique for proteins with technetium-99m (99mTc) has been developed and makes use of borohydride and stannous chloride. The method is simple and reproducible and gives a high labeling efficiency and high retention of biological activity for proteins, including polyclonal immunoglobulin (Ig), antifibrin monoclonal antibody, tissue type plasminogen activator, fibrinogen and low density lipoprotein (LDL). This method can be used in kit-format and takes about 20 min preparation time at room temperature. Both in vitro and in vivo studies indicate good stability of the label. In vivo, mice and rabbit images show significant accumulation of 99mTc-Ig at an inflammation area, 99mTc-antifibrin at a thrombus site and 99mTc-LDL in atherosclerotic lesions. The method is attractive for routine research and clinical purposes.