Enfeksiyonun Pozitron Emisyon Tomografi ile Moleküler Görüntülemesi: Florodeoksiglikoz Disindaki Ajanlar

FDG PET’in enfeksiyon ve enflamasyon tanisindaki degeri iyi bilinmektedir. Bu radyofarmasötigin en önemli limitasyonu spesifitesinin düsük olmasidir. Arastirmacilar PET’in tek foton emisyonu yapan radyofarmasötiklere göre avantajindan da yararlanarak enfeksiyon için spesifitesi yüksek PET ajanlari gelistirmeye çalismislardir. Bu amaçla yapilan ilk çalismalarda lökositlerin PET radyonüklidleri ile in vitro isaretlenmesi üzerine odaklanilmistir. Her ne kadar bu konuda yapilan çalismalarin sonuçlari umut verici olsa da pek çok nedenden dolayi kullanimi yayginlasmamistir ve yayginlasacak gibi de görünmemektedir. Dahasi, isaretli lökositler lökosit migrasyonunu göstermede spesifik olsalar da enfeksiyon için gerçek anlamda spesifik degillerdir. I-124 FIAU ile yapilan ilk çalismalarin sonuçlari bu ajanin enfeksiyona spesifik oldugunu düsündürecek sekilde umut verici olsa da son veriler bu ajan için duyulan heyecani azaltir niteliktedir. Ga-68 ile isaretli sideroforlar ve antimikrobiyal peptidler ile yapilan son çalismalarin ön verileri heyecan uyandirmakla birlikte bu verileri destekleyecek, kesinlestirecek klinik çalismalar gerekmektedir.

Ga-68

For nearly fifty years, Ga-67 citrate has been used for imaging infection. There are, however, significant disadvantages to this radiopharmaceutical, which include nonspecific uptake in aseptic inflammation, tumors, and trauma. Image quality, even when performed as a single-photon emission computed tomography/CT study is suboptimal and the interval between administration and imaging usually is 48 to 72 hours. Now that the positron emitter Ga-68 is more widely available, investigators are studying the role of this radionuclide for diagnosing infection. Kumar et al. studied the role of Ga-68 citrate in the diagnosis of a Staphylococcus aureus infection in rodents (18). Moderate radiopharmaceutical uptake was present within five minutes after injection. There was intense focal uptake from thirty minutes to six hours post injection. They also studied one patient with a postoperative intraabdominal infection and observed that the test was positive within thirty minutes after injection. In a pilot study of patients with tuberculosis, Ga-68 accumulated in pulmonary and extra-pulmonary tuberculous lesions and was superior to CT for detecting extra-pulmonary disease. The authors concluded that Ga-68 might be useful for differentiating active from inactive disease and for monitoring treatment response (19).

Nanni et al. performed 40 Ga-68 citrate PET/CT on 31 patients suspected of having musculoskeletal infection (20). Imaging was performed approximately one hour after radiopharmaceutical administration. All 23 cases of musculoskeletal infection were positive. Four of 17 cases without infection also were positive. All four false positive results were due to tumor. Sensitivity, specificity, accuracy, positive predictive and negative predictive value were 100%, 76%, 90%, 85%, and 100%, respectively.

Although Ga-68 citrate reduces or eliminates some of the disadvantages of Ga-67 citrate, such as poor image quality and the long interval between administration and imaging, it still suffers from limited specificity (20,21). As a result, other investigators have turned their attention to complexing Ga-68 with peptides in attempts to develop more specific imaging agents (22). Vascular adhesion protein 1 (VAP-1) is a human endothelial protein that is expressed on cell surfaces under inflammatory conditions. Lankinen et al. postulated that Ga-68 labeled 1,4,7,10-tetraazacyclododecane-N′, N″,N′′′,N″″-tetraacetic acid-peptide targeted to VAP-1 (Ga-68 DOTAVAP-P1) could be useful for imaging early inflammation and infection in healing bones (23). These investigators compared Ga-68 DOTAVAP-P1 PET in 68 rats, including 34 with uninfected healing cortical bone defects and 34 with Staphylococcus aureus osteomyelitis. There was no significant difference in the mean grade of VAP-1 expression at 24 hours and seven days (1.10±0.39 and 1.00±0.47, respectively) in animals with healing cortical bone defects. The mean grade of VAP-1 expression in osteomyelitic animals was significantly higher (p=0.0330) at seven days (2.85±0.34) than at 24 hours (2.39±0.49). VAP-1 expression levels were significantly higher (p<0.0001) in infected animals than in the animals with healing bone defects at both time points. Ga-68 DOTAVAP-P1 uptake was similar in osteomyelitis and the healing cortical bone defects during the first 36 hours after surgery. Beyond 36 hours, uptake was observed only in infected bones. Ga-68 DOTAVAP-P1 uptake was significantly higher (p<0.0001) in the infected bones at 7 days. The authors concluded that Ga-68 DOTAVAP-P1 accurately demonstrates the inflammatory phase in healing bones and the progress of infection in osteomyelitic bones, facilitating the differentiation of bone infection due to Staphylococcus aureus and normal bone healing within seven days after onset. Ujula et al. performed Ga-68 DOTAVAP-P1 PET on healthy rats and rats with Staphylococcus aureus osteomyelitis of the tibia (24). Radiopharmaceutical uptake in osteomyelitis was compared with negative control peptide and competition with an excess of unlabeled DOTAVAP-P1. Ga-68 DOTAVAP-P1 was more efficiently bound to VAP-1-transfected cells than to controls. The agent was rapidly cleared from the circulation, excreted quickly in urine and had an in vivo half-life of 26±2.3 minute. Infected bones demonstrated a modest target-to-background ratio. Compared to Ga-68 DOTAVAP-P1, studies with the negative control peptide and competitors revealed significantly lower uptake at the site of infection. The authors concluded that the results of this investigation represented a proof-of-concept that infection-induced VAP-1 can be targeted by Ga-68 DOTA peptide.

The uptake mechanisms of Ga-67 in infection are complex and not well understood. It is believed that siderophores, which are gallium avid low molecular weight chelating agents produced by bacteria, are involved (25). The potential of Ga-68 labeled siderophores for imaging infection has been investigated (26,27,28). Petrik et al. conducted in vitro and in vivo investigations of several Ga-68 labeled siderophores using Aspergillus fumigatus cultures (26). Uptake in the cultures depended on iron load and siderophore type. In mice, there was rapid blood clearance and renal excretion of Ga-68 triacetylfusarinine C (TAFC) and Ga-68 ferrioxamine E (FOXE). Furthermore these two agents bound Ga-68 with high affinity and stability, low protein binding and high affinity and specificity for Aspergillus fumigatus. These results suggest that these two agents may be useful for imaging Aspergillus infection. In a rat model of Aspergillus lung infection, Petrik et al. demonstrated highly selective uptake of both Ga-68 TAFC and Ga-68 FOXE in infected lung tissue and good correlation with disease severity (27).

Petrik et al. investigated the specificity of Ga-68 TAFC and Ga-68 FOXE. In vitro uptake of these two radiopharmaceuticals was examined in various fungal, bacterial and yeast cultures and human lung cancer (H1299) cells (28). In vivo imaging was performed in fungal and bacterial rat infection and inflammation models. There was rapid accumulation of Ga-68 TAFC and Ga-68 FOXE in A. fumigatus cultures, with significantly less uptake in other fungal species and minimal uptake in other microorganisms and H1299 cells, with the exception of Ga-68-FOXE uptake in Staphylococcus aureus. There was rapid uptake of both agents in the lungs of A. fumigatus-infected rats, low accumulation in sterile inflammation and no uptake in bacterial abscesses. Ga-68 FOXE was more sensitive, while Ga-68 TAFC was more specific.

Antimicrobial peptides are an important component of the natural defenses of most living organisms. Most are small, cationic and amphipathic (hydrophilic and hydrophobic) and attack pathogens by multiple mechanisms. They exhibit broad-spectrum activity against Gram-positive and Gram-negative bacteria, yeasts, fungi and enveloped viruses. They also are involved in apoptosis, wound healing, and immune modulation (29). While most investigations of antimicrobial peptides have used Tc-99m as the radiolabel, the potential of Ga-68 labeled antimicrobial peptides as infection specific imaging compounds is now being investigated (30,31,32). Ebenhan et al. labeled the UBI29-41 fragment of ubiquicidin with Ga-68, and compared uptake of this agent in staph  aureus muscle infection, turpentine induced muscle inflammation, and ovalbumin induced lung inflammation in rabbits (31). PET/CT was performed at intervals up to two hours post injection. Infected thigh muscle could be clearly differentiated from sterile inflamed thigh muscle. Uptake in the ovalbumin-induced inflammation of the lungs was insignificant. These results suggest that Ga-68 labeled UBI29-41 potentially could differentiate infection from sterile inflammation.

Recently, Mokaleng et al. investigated the potential of an antimicrobial peptide derivative, TBIA101 that was conjugated to DOTA and radiolabeled with Ga-68 for imaging infection (32). Ga-68 DOTA-TBIA101 micro-PET/CT was performed on E. coli muscle infection in mice. Radiopharmaceutical uptake was significantly higher (p=0.333) in infected thigh muscle compared to uninfected thigh muscle, forearm muscle (p=0.092), and background (p=0.021). Ga-68 DOTA-TBIA101 clearly localized the infection site, with no notable uptake in the contralateral muscle.

F-18 flurodeoxyglucose (FDG) işaretli pozitron emisyon tomografi (PET), pek çok enflamatuvar ve enfeksiyöz hastalığın tanısında kullanılan çok değerli bir yöntem haline gelmiştir. FDG yararlı olmakla birlikte, malign ve benign neoplaziler de dahil çok çesitli nonenfeksiyöz enflamatuvar durumda tutulum gösterebildiğinden nonspesifik bir ajandır. PET ile yapılan çalışmaların, tek foton emisyonunu yapan radyofarmasötiklerle yapılanlara oranla pek çok avantajı vardır. PET’in rezolüsyonu daha yüksektir, böylece izlenen tutulumlar net bir şekilde lokalize edilebilir. Ayrıca PET ile semikantitatif analiz yapılabilmesi, enfeksiyöz/nonenfeksiyöz proseslerin ayrımına ve tedavi yanıtının değerlendirilmesine olanak sağlar. Bu nedenlerden dolayı, enfeksiyon görüntülemede spesifik ya da en azından FDG’den daha spesifik PET radyofarmasötikleri geliştirmek için yoğun çabalar sarf edilmesi şaşırtıcı değildir.