|
|
||||||||
Basic Science Investigations |
1 Department of Nuclear Medicine, University Medical Centre Nijmegen, Nijmegen, The Netherlands
2 Discovery Research, Bristol-Meyers Squibb Medical Imaging, North Billerica, Massachusetts
| ABSTRACT |
|---|
|
|
|---|
-half-life = 30 ± 6 min, 85%; ß-half-life = 25.7 ± 0.8 h, 15%). Abscess-to-background ratios, as derived from the region-of-interest analysis, increased to 34 ± 7 at 24 h after injection. The images of both groups showed moderate uptake in the liver, spleen, kidneys, and bone marrow. No activity was seen in the bladder, indicating almost complete retention in the kidneys. The uptake in the abscess could be blocked completely by injection of an excess of nonradioactive agent, indicating a specific receptor-ligand interaction of the radiolabeled agent in the infected tissue. Biodistribution data showed that after saturation of the LTB4 receptor, the abscess uptake, in percentage injected dose per gram, was significantly reduced (0.03 ± 0.02 vs. 0.24 ± 0.06, P = 0.008). Conclusion: The modified LTB4 antagonist showed infectious foci rapidly after injection because of specific receptor-ligand interaction. Because of the high abscess-to-background ratios that were obtained and the fact that no accumulation of radioactivity was observed in the gastrointestinal tract, this compound has excellent characteristics for revealing infectious and inflammatory foci.
Key Words: leukotriene B4 antagonist infection imaging scintigraphy
| INTRODUCTION |
|---|
|
|
|---|
The use of radiolabeled chemotactic peptides as an infection-imaging agent is based on the enhanced expression of high-affinity receptors on infiltrating granulocytes activated after an inflammatory response. Several of these peptides (chemokines, platelet factor 4, N-formyl-methionyl-leucyl-phenylalanine, and other chemotactic compounds) have been studied for the detection of infection or inflammation (5).
In this study, we examined the characteristics of an 111In-labeled leukotriene B4 (LTB4) antagonist for its potential to reveal infectious foci scintigraphically. LTB4 is a potent chemoattractant that activates granulocytes and macrophages as a reaction to an inflammatory response (6,7). At the site of the inflammation, enhanced LTB4 synthesis occurs mainly by leukocytes from arachidonic acid through the 5-lipoxygenase pathway (8). LTB4 responses are receptor mediated, and 2 classes of stereospecific binding sites for LTB4 have been identified. The first type of receptor, BLT1, is a high-affinity receptor (dissociation constant for LTB4 is 1 nmol/L) mainly expressed on human neutrophils (9). The second receptor type, BLT2, is a low-affinity receptor (dissociation constant for LTB4 is 23 nmol/L) that is more ubiquitously expressed (6,10). Binding of LTB4 to BLT1 and BLT2 receptors promotes chemotaxis and chemokinesis. Therefore, LTB4 is considered an important mediator in both acute and chronic inflammatory diseases (9,11).
In a previous study, we showed the potential of a 99mTc-labeled LTB4 antagonist, RP517, to visualize experimental infection (1214). The disadvantage of 99mTc-RP517, however, was its hepatobiliary clearance, causing high uptake in the organs of the digestive tract relatively early after injection. This gastrointestinal uptake of 99mTc-RP517 limits its applicability as an infection-imaging agent (12).
Because the radiolabeled LTB4 antagonist showed rapid and high accumulation in the infected tissue, we produced a hydrophilic bivalent LTB4 antagonist conjugated with a diethylenetriaminepentaacetic acid (DTPA) moiety to allow radiolabeling with 111In. In the present study, the leukocyte-binding characteristics of this modified bivalent LTB4 antagonist were studied both in vitro and in vivo.
| MATERIALS AND METHODS |
|---|
|
|
|---|
|
Radiolabeling of DPC11870-11
Labeling of DPC11870-11 with 111In was performed in metal-free 0.25 mol/L ammonium acetate buffer, pH 5.5, for 30 min at room temperature. Labeling experiments were performed to determine the maximum specific activity of the radiolabeled agent. Radiochemical purity was checked by instant thin-layer chromatography on silica gel strips (Gelman Sciences, Inc.) in 0.1 mol/L sodium citrate buffer, pH 6.0. Strips were analyzed in a well-type
-counter (Wizard; Pharmacia-LKB). In addition, RP-HPLC was performed on a C18 column (Rx-C18, 4.6 mm x 25 cm; Zorbax) on an Agilent 1100 system equipped with an in-line radiodetector (Canberra Packard). During analysis, a gradient was used from 100% solvent NH4Ac, pH 7.0, to 100% acetonitrile in 50 min, at a flow rate of 1 mL/min.
Labeling with nonradioactive InCl3 (Aldrich) was performed under the same conditions as described above. A 3-fold molar excess of InCl3 was added to DPC11870-11 in ammonium acetate buffer, pH 5.5.
Receptor-Binding Assay
In vitro binding studies were performed on purified human granulocytes. The granulocytes were purified as described previously (16). To 1 x 108 cells, increasing amounts of nonradioactive indium-DPC11870-11 (010 µmol/L) were added in the presence of 10,000 cpm 111In-DPC11870-11 (0.4 nmol/L) in 0.25 mol/L Tris/HCl, pH 7.2. After incubation during 1 h at 37°C, cells were washed twice (5 min, 5,000g), the supernatant was discarded, and the radioactivity in the pellet (total bound activity) was measured in a shielded well-type
-counter (Wizard).
The specifically bound fraction versus the nonradioactive LTB4 antagonist concentration was plotted. The 50% inhibitory concentration (IC50) was determined as being the concentration of nonradioactive indium-DPC11870-11 that caused 50% inhibition of the maximum binding of the 111In-DPC11870-11.
Infection Model
Thirteen female New Zealand White rabbits weighing 2.32.8 kg were kept in cages (1 rabbit per cage) and fed standard laboratory chow and water ad libitum. An Escherichia coli infection was induced in the left thigh muscle by intramuscular injection of 4 x 109 colony-forming units of E. coli. During this procedure, the rabbits were anesthetized by subcutaneous injection of 0.7 mL of a mixture of 0.315 mg/mL fentanyl and 10 mg/mL fluanisone (Hypnorm; Janssen Pharmaceutica). All animal experiments were approved by the local animal welfare committee in accordance with the Dutch legislation and performed in accordance with their guidelines.
Imaging and Biodistribution
Twenty-four hours after induction of the infection, when swelling of the infected muscle was apparent, 13 rabbits were intravenously injected with 11 MBq of 111In-DPC11870-11 (3 µg) in the lateral ear vein. Three rabbits received 2 mg of nonradioactive indium-DPC11870-11 2 min before the injection of the radiolabeled compound. The excess of nonradioactive agent was injected to induce saturation of the LTB4 receptors in vivo. This experiment was performed to determine receptor-specific accumulation of 111In-DPC11870-11 in the abscess.
For scintigraphic imaging, the rabbits were immobilized in a mold and placed prone on a gamma camera (Orbiter; Siemens) using a medium-energy parallel-hole collimator. Images (300,000 counts per image) were obtained up to 24 h after injection and stored digitally in a 256 x 256 matrix. All images were windowed identically, allowing a fair comparison among the various experiments. The scintigraphic results were analyzed by drawing regions of interest over the abscess and the contralateral muscle (background). Abscess-to-contralateral muscle ratios were calculated.
The 3 rabbits that received an excess of nonradioactive agent and the 5 rabbits injected only with radiolabeled antagonist were euthanized at 6 h after injection with a lethal dose of sodium phenobarbital to determine the biodistribution of the agent. At 24 h after injection, the other 5 rabbits were euthanized. A blood sample was taken by cardiac puncture. Tissues were dissected and weighed. The activity in tissues was measured in a shielded well-type
-counter together with the injection standards and was expressed as the percentage injected dose (%ID) per gram.
Pharmacokinetics
The pharmacokinetics of the 111In-labeled agent were studied in detail in 2 rabbits that were injected with the 111In-DPC11870-11 and 2 rabbits that received an excess of nonradioactive DPC11870-11 before the 111In-DPC11870-11. Blood samples were drawn at 1 min before and 3, 15, 30, 90, 120, 240, 360, and 1,440 min after injection. The activity in the samples was determined in a
-counter and expressed as %ID assuming that the total blood weight accounted for 6% of total body weight (17). The
- and ß-half-lives (t
and t
ß, respectively) were calculated assuming a 2-phase linear model for the blood clearance. In addition, blood samples were divided into 2 portions; the first tube was used for white blood cell analysis; the second tube was centrifuged (5 min, 1,500g), and activity in the pellet and plasma was determined. 111In-DPC11870-11 and the plasma samples were analyzed by fast protein liquid chromatography (FPLC) on a Biosep 3000 gel filtration column (Phenomenex), using phosphate-buffered saline, pH 7.2, as eluent, at a flow rate of 1 mL/min.
Statistical Analysis
All mean values are presented as mean ± SD. Statistical analysis was performed using the 2-sided Student t test. The level of significance was set at 0.05.
| RESULTS |
|---|
|
|
|---|
|
The IC50 of DPC11870-11 was 10 nmol/L. Nonspecific binding was determined as the binding of 111In-DPC11870-11 to granulocytes in the presence of 10 µmol/L nonradioactive indium-DPC11870-11. The nonspecific binding of 111In-DPC11870-11 to the granulocytes was relatively high (40%). The competitive binding assay was also performed using human lymphocytes and erythrocytes. 111In-DPC11870-11 did not bind specifically to these cell types. The amount of 111In-DPC11870-11 bound to these cells (incubated in the presence and absence of nonradioactive indium-DPC11870-11) did not exceed 5%.
Imaging and Biodistribution
The scintigraphic images after the injection of 111In-DPC11870-11 are shown in Figure 3. Immediately after injection of the radiolabeled LTB4 antagonist, an accumulation of radioactivity was observed in the lung, liver, and kidneys. Subsequently, an accumulation of activity was seen in the spleen and bone marrow.
|
The biodistribution data derived from ex vivo counting of dissected tissues, as summarized in Figure 4, were consistent with the scintigraphic images. Uptake of 111In-DPC11870-11 in the abscess at 6 h after injection was 0.24 ± 0.06 %ID/g and remained constant until 24 h after injection (0.25 ± 0.02 %ID/g). The radioactivity concentrations in the blood at 6 and 24 h after injection were 0.09 ± 0.03 %ID/g and 0.03 ± 0.01 %ID/g, respectively. Uptake in the bone marrow and spleen was relatively high (0.59 ± 0.04 %ID/g and 0.69 ± 0.08 %ID/g, respectively, at 6 h after injection), whereas uptake in the other tissues was relatively low.
|
Pharmacokinetics
Figure 5 depicts the radioactivity concentrations in the blood after injection of 111In-DPC11870-11. In the first hours after injection of the radiolabeled compound, the clearance was fast (t
= 30 ± 6 min, 85%); later, the compound cleared from the blood much more slowly (t
ß = 25.7 ± 0.8 h, 15%). Considering the molecular weight of the agent, the t
ß is remarkably high, suggesting an association with blood cells or serum proteins. Measurement of plasma samples demonstrated that more than 98% of the radioactivity was found in the plasma (Fig. 5). FPLC analysis of plasma samples on a gel filtration column showed 1 radioactive peak at 11 min, coeluting with proteins with a molecular weight of 50100 kDa. Incubation of the 111In-labeled compound in phosphate-buffered saline with 0.5% bovine serum albumin and analysis with FPLC on a gel filtration column resulted in elution patterns with 1 activity peak with a retention time corresponding to bovine serum albumin (11 min). Neither the rabbits injected with 111In-DPC11870-11 nor the animals injected with 111In-DPC11870-11 and an excess of nonradioactive compound showed significant changes in white blood cell counts. As shown in Figure 6, the white blood cell concentration before injection of the 111In-DPC11870-11 was 7.6 ± 3.3 x 109/L, and at all time points after injection the cell concentration was within the normal range (3.213.2 x 109/L). This finding indicated that this LTB4 antagonist did not induce changes in peripheral leukocyte counts.
|
|
| DISCUSSION |
|---|
|
|
|---|
Administration of an excess of nonradioactive indium-labeled compound indicated that the targeting of infectious foci was a result of specific receptor-ligand interaction. The in vivo receptor-blocking experiments also indicated that the targeting of cells in the bone marrow and spleen was dependent on interaction with receptor-positive cells. These findings are in accordance with those of Yokomizo et al. (18), who showed that highest expression of human BLT1 messenger RNA was observed in leukocytes, followed by cells in the spleen and thymus.
Unlike the images obtained with the more lipophilic agent 99mTc-RP517 as reported previously, the images obtained with 111In-DPC11870-11 showed no accumulation in the gastrointestinal tract (12). Abscess uptake for 111In-DPC11870-11 was higher at all times, compared with that for 99mTc-RP517 (0.035 ± 0.007 %ID/g at 4 h after injection; 0.12 ± 0.02 %ID/g at 20 h after injection). In contrast, blood levels of 99mTc-RP517 were lower (0.024 ± 0.004 %ID/g at 4 h after injection; 0.004 ± 0.0004 %ID/g at 20 h after injection), resulting in lower background activity. In this animal model, abscess-to-contralateral muscle ratios were in the same range for both agents.
Pharmacokinetic analysis showed that the compound cleared from the blood rapidly in the first hours after administration. The distribution half-life (t
) was in accordance with the low molecular weight of the compound. A fraction of the radiolabeled agent (15%), however, seemed to clear more slowly from the blood (t
ß = 25.7 h), which is unusual for a low-molecular-weight compound (molecular weight, 3,127 Da). This slow clearance may be due to interaction of the radiolabeled agent with serum proteins (19). The FPLC analysis of plasma samples indeed showed that 111In-DPC11870-11 interacted with albumin.
Radiolabeled chemotactic peptides have been intensively studied for their applicability to the imaging of infectious and inflammatory foci. Their theoretically high affinity for blood cell receptors, good penetrating ability, and rapid background clearance suggest that they are ideal candidates for this application. A main disadvantage of some chemotactic and chemoattractive compounds is the induction of transient leukopenia after intravenous injection (4). The data in this study show that indium-DPC11870-11 does not provoke any of these changes in rabbits, even at very high doses (2 mg). Therefore, it seems unlikely that the LTB4 antagonist used in this study will provoke side effects after intravenous administration in patients.
| CONCLUSION |
|---|
|
|
|---|
Because of these findings, further in vitro and in vivo studies are warranted, preferably with an analog that can be labeled with 99mTc. Replacement of the DTPA moiety with a 99mTc-chelating moiety will be attempted.
| ACKNOWLEDGMENTS |
|---|
| FOOTNOTES |
|---|
For correspondence or reprints contact: Julliëtte E.M. van Eerd, BSc, Department of Nuclear Medicine, University Medical Center Nijmegen, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands.
E-mail: J.vaneerd{at}nucmed.umcn.nl
| REFERENCES |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
N. Dumarey, D. Egrise, D. Blocklet, B. Stallenberg, M. Remmelink, V. del Marmol, G. Van Simaeys, F. Jacobs, and S. Goldman Imaging Infection with 18F-FDG-Labeled Leukocyte PET/CT: Initial Experience in 21 Patients J. Nucl. Med., April 1, 2006; 47(4): 625 - 632. [Abstract] [Full Text] [PDF] |
||||
![]() |
J. E.M. van Eerd, M. Broekema, T. D. Harris, D. S. Edwards, W. J.G. Oyen, F. H.M. Corstens, and O. C. Boerman Imaging of Infection and Inflammation with an Improved 99mTc-Labeled LTB4 Antagonist J. Nucl. Med., September 1, 2005; 46(9): 1546 - 1551. [Abstract] [Full Text] [PDF] |
||||
![]() |
M. Tulchinsky and A. M. Peters Leukocyte Receptor-Binding Radiopharmaceuticals for Infection and Inflammation Scintigraphy J. Nucl. Med., May 1, 2005; 46(5): 718 - 721. [Full Text] [PDF] |
||||
![]() |
J. E.M. van Eerd, W. J.G. Oyen, T. D. Harris, H. J.J.M. Rennen, D. S. Edwards, F. H.M. Corstens, and O. C. Boerman Scintigraphic Imaging of Infectious Foci with an 111In-LTB4 Antagonist Is Based on In Vivo Labeling of Granulocytes J. Nucl. Med., May 1, 2005; 46(5): 786 - 793. [Abstract] [Full Text] [PDF] |
||||
![]() |
J. E.M. van Eerd, H. J.J.M. Rennen, W. J.G. Oyen, T. D. Harris, D. S. Edwards, F. H.M. Corstens, and O. C. Boerman Scintigraphic Detection of Pulmonary Aspergillosis in Rabbits with a Radiolabeled Leukotriene B4 Antagonist J. Nucl. Med., October 1, 2004; 45(10): 1747 - 1753. [Abstract] [Full Text] [PDF] |
||||
![]() |
J. E.M. van Eerd, P. Laverman, W. J.G. Oyen, T. D. Harris, D. S. Edwards, C. E. Ellars, F. H.M. Corstens, and O. C. Boerman Imaging of Experimental Colitis with a Radiolabeled Leukotriene B4 Antagonist J. Nucl. Med., January 1, 2004; 45(1): 89 - 93. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY | THE JOURNAL OF NUCLEAR MEDICINE |