Radiotracers for Bone Marrow Infection Imaging

Introduction: Radiotracers are widely used in medical imaging, using techniques of gamma-camera imaging (scintigraphy and SPECT) or positron emission tomography (PET). In bone marrow infection, there is no single routine test available that can detect infection with sufficiently high diagnostic accuracy. Here, we review radiotracers used for imaging of bone marrow infection, also known as osteomyelitis, with a focus on why these molecules are relevant for the task, based on their physiological uptake mechanisms. The review comprises [(67)Ga]Ga-citrate, radiolabelled leukocytes, radiolabelled nanocolloids (bone marrow) and radiolabelled phosphonates (bone structure), and [(18)F]FDG as established radiotracers for bone marrow infection imaging. Tracers that are under development or testing for this purpose include [(68)Ga]Ga-citrate, [(18)F]FDG, [(18)F]FDS and other non-glucose sugar analogues, [(15)O]water, [(11)C]methionine, [(11)C]donepezil, [(99m)Tc]Tc-IL-8, [(68)Ga]Ga-Siglec-9, phage-display selected peptides, and the antimicrobial peptide [(99m)Tc]Tc-UBI(29-41) or [(68)Ga]Ga-NOTA-UBI(29-41). Conclusion: Molecular radiotracers allow studies of physiological processes such as infection. None of the reviewed molecules are ideal for the imaging of infections, whether bone marrow or otherwise, but each can give information about a separate aspect such as physiology or biochemistry. Knowledge of uptake mechanisms, pitfalls, and challenges is useful in both the use and development of medically relevant radioactive tracers

Post Mortem Leukocyte Scintigraphy in Juvenile Pigs with Experimentally Induced Osteomyelitis

We have previously demonstrated that 111In-labeled autologous leukocyte scintigraphy is able to detect osteomyelitis in living juvenile pigs. In animal research studies, it may well be an advantage if the animals could be scanned after euthanasia. Applying traditional scanning of living animals to euthanized animals will render anaesthesia unnecessary and be ideal for obtaining good and reliable scans for the correct interpretation of imaging afterwards, since the animals do not move. The autologous leukocytes of the pigs were collected, marked with 111In, and reinjected into the pigs and allowed for homing to the site of infections as usual while the pigs were alive. In this study, we demonstrate that it is possible to perform SPECT/CT with 111In-labelled autologous leukocytes almost 24 hrs after euthanasia with the same detectability of osteomyelitic lesions as in living pigs (78% versus 79%). The pigs in this study had exactly the same experimental conditions as the living pigs and were examined in parallel with the living pigs except for euthanasia prior to the leukocyte scan and that no PET/CT scans were performed

The arterial blood sampling associated with PET imaging studies can lead to post-scan complications in Göttingen minipigs

When disease progression or treatment effects are investigated, pigs are frequently used for imaging studies. We recently showed that prolonged anesthesia with intensive blood sampling and road transportation affect internal organs in non-recovery imaging studies in domestic pigs. In the present study, we examined if repeated non-invasive scans per se affect internal organs and post-scan observations. Göttingen minipigs are frequently used for longitudinal imaging studies due to their low adult body weight. Computed tomography scans did not detect significant organ damage in minipigs (N=4) placed in sternal recumbence for a few hours. Upon reviewing medical records of minipigs positioned in sternal recumbence during PET scans (N=40), two cases (5 %) of minor post-scan complications were found during the first two weeks post scanning. Minipigs in dorsal recumbence with the surgical placement of femoral catheters for blood sampling (N=14) were more frequently (7 cases, 50 %) associated with minor-to-moderate post-scan complications. Regardless of whether minipigs were placed in sternal or dorsal recumbency, a non-significant decrease in body weight was observed. The results indicate that the anesthetized Göttingen minipigs are slightly affected by the simple short-term scanning procedures and that the blood sampling procedures should be reduced when possible. Post-operative care should be improved due to the higher incidence of post-scanning complications in minipigs that were femoral artery catheterized

Preclinical Testing of Radiopharmaceuticals for the Detection and Characterization of Osteomyelitis:Experiences from a Porcine Model

The development of new and better radioactive tracers capable of detecting and characterizing osteomyelitis is an ongoing process, mainly because available tracers lack selectivity towards osteomyelitis. An integrated part of developing new tracers is the performance of in vivo tests using appropriate animal models. The available animal models for osteomyelitis are also far from ideal. Therefore, developing improved animal osteomyelitis models is as important as developing new radioactive tracers. We recently published a review on radioactive tracers. In this review, we only present and discuss osteomyelitis models. Three ethical aspects (3R) are essential when exposing experimental animals to infections. Thus, we should perform experiments in vitro rather than in vivo (Replacement), use as few animals as possible (Reduction), and impose as little pain on the animal as possible (Refinement). The gain for humans should by far exceed the disadvantages for the individual experimental animal. To this end, the translational value of animal experiments is crucial. We therefore need a robust and well-characterized animal model to evaluate new osteomyelitis tracers to be sure that unpredicted variation in the animal model does not lead to a misinterpretation of the tracer behavior. In this review, we focus on how the development of radioactive tracers relies heavily on the selection of a reliable animal model, and we base the discussions on our own experience with a porcine model

Attempts to Target <i>Staphylococcus aureus</i> Induced Osteomyelitis Bone Lesions in a Juvenile Pig Model by Using Radiotracers

Background [18F]FDG Positron Emission Tomography cannot differentiate between sterile inflammation and infection. Therefore, we, aimed to develop more specific radiotracers fitted for differentiation between sterile and septic infection to improve the diagnostic accuracy. Consequently, the clinicians can refine the treatment of, for example, prosthesis-related infection. Methods: We examined different target points; Staphylococcus aureus biofilm (68Ga-labeled DOTA-K-A9 and DOTA-GSGK-A11), bone remodeling ([18F]NaF), bacterial cell membranes ([68Ga]Ga-Ubiquicidin), and leukocyte trafficking ([68Ga]Ga-DOTA-Siglec-9). We compared them to the well-known glucose metabolism marker [18F]FDG, in a well-established juvenile S. aureus induced osteomyelitis (OM) pig model. Results: [18F]FDG accumulated in the OM lesions seven days after bacterial inoculation, but disappointingly we were not able to identify any tracer accumulation in OM with any of the supposedly more specific tracers. Conclusion: These negative results are, however, relevant to report as they may save other research groups from conducting the same animal experiments and provide a platform for developing and evaluating other new potential tracers or protocol instead

Lymph Nodes Draining Infections Investigated by PET and Immunohistochemistry in a Juvenile Porcine Model

Background: [(18)F]FDG and [(11)C]methionine accumulate in lymph nodes draining S. aureus -infected foci. The lymph nodes were characterized by weight, [(11)C]methionine- and [(18)F]FDG-positron emissions tomography (PET)/computed tomography (CT), and immunohistochemical (IHC)-staining. Methods: 20 pigs inoculated with S. aureus into the right femoral artery were PET/CT-scanned with [(18)F]FDG, and nine of the pigs were additionally scanned with [(11)C]methionine. Mammary, medial iliac, and popliteal lymph nodes from the left and right hind limbs were weighed. IHC-staining for calculations of area fractions of Ki-67, L1, and IL-8 positive cells was done in mammary and popliteal lymph nodes from the nine pigs. Results: The pigs developed one to six osteomyelitis foci. Some pigs developed contiguous infections of peri-osseous tissue and inoculation-site abscesses. Weights of mammary and medial iliac lymph nodes and their [(18)F]FDG maximum Standardized Uptake Values (SUV(FDGmax)) showed a significant increase in the inoculated limb compared to the left limb. Popliteal lymph node weight and their FDG uptake did not differ significantly between hind limbs. Area fractions of Ki-67 and IL-8 in the right mammary lymph nodes and SUV(Metmax) in the right popliteal lymph nodes were significantly increased compared with the left side. Conclusion: The PET-tracers [(18)F]FDG and [(11)C]methionine, and the IHC- markers Ki-67 and IL-8, but not L1, showed increased values in lymph nodes draining soft tissues infected with S. aureus. The increase in [(11)C]methionine may indicate a more acute lymph node response, whereas an increase in [(18)F]FDG may indicate a more chronic response

Utility of (11)C-methionine and (11)C-donepezil for imaging of Staphylococcus aureus induced osteomyelitis in a juvenile porcine model:comparison to autologous (111)In-labelled leukocytes, (99m) Tc-DPD, and (18)F-FDG

The aim of this study was to compare (11)C-methionine and (11)C-donepezil positron emission tomography (PET) with (111)In-labeled leukocyte and (99m)Tc-DPD (Tc-99m 3,3-diphosphono-1,2-propanedicarboxylic acid) single-photon emission computed tomography (SPECT), and (18)F-fluorodeoxyglucose ((18)F-FDG) PET to improve detection of osteomyelitis. The tracers’ diagnostic utility where tested in a juvenile porcine hematogenously induced osteomyelitis model comparable to osteomyelitis in children. Five 8-9 weeks old female domestic pigs were scanned seven days after intra-arterial inoculation in the right femoral artery with a porcine strain of Staphylococcus aureus. The sequential scan protocol included Computed Tomography, (11)C-methionine and (11)C-donepezil PET, (99m)Tc-DPD and (111)In-labelled leukocytes scintigraphy, and (18)F-FDG PET. This was followed by necropsy of the pigs and gross pathology, histopathology, and microbial examination. The pigs developed a total of 24 osteomyelitic lesions, 4 lesions characterized as contiguous abscesses and pulmonary abscesses (in two pigs). By comparing the 24 osteomyelitic lesions, (18)F-FDG accumulated in 100%, (111)In-leukocytes in 79%, (11)C-methionine in 79%, (11)C-donepezil in 58%, and (99m)Tc-DPD in none. Overall, (18)F-FDG PET was superior to (111)In-leukocyte SPECT and (11)C-methionine in marking infectious lesions