111Indium Labelling of Recombinant Activated Coagulation Factor VII: In Vitro and Preliminary In Vivo Studies in Healthy Rats

The aim of this study is to investigate whether 111Indium-labelled recombinant FVIIa (rFVIIa) could be a potential radiopharmaceutical for localization of bleeding sources. DTPA-conjugated rFVIIa was radiolabelled with 111In chloride. In vitro binding efficiency of 111In-DTPA-rFVIIa to F1A2-Mab-sepharose was 99% in buffer, while it was 88–82% in serum. The binding efficiency of 111In-DTPA-rFVIIa to TF (1–209)-sepharose was 48% in buffer whereas 39%–36% in serum, respectively. In vivo experiment was conducted in healthy rats, and gamma camera images were taken immediately after iv. administration of 1.6–1.8 MBq 111In-DTPA-rFVIIa up to 120–130 min. Five min after administration of 111In-DTPA-rFVIIa, percentage of 111In activity was 6.0% in the cardiac region and 24.5% in the liver region. After 2 hours activity was decreased to 3.3% in heart while it had increased to 42.0% in the liver. The 111In-DTPA-rFVIIa might be a potential radiopharmaceutical for visualisation of tissues with significant TF expression such as acute bleeding lesions in the gastrointestinal tract

Importance of Attenuation Correction (AC) for Small Animal PET Imaging

The purpose of this study was to investigate whether a correction for annihilation photon attenuation in small objects such as mice is necessary. The attenuation recovery for specific organs and subcutaneous tumors was investigated. A comparison between different attenuation correction methods was performed. Methods: Ten NMRI nude mice with subcutaneous implantation of human breast cancer cells (MCF-7) were scanned consecutively in small animal PET and CT scanners (MicroPETTM Focus 120 and ImTek’s MicroCATTM II). CT-based AC, PET-based AC and uniform AC methods were compared. Results: The activity concentration in the same organ with and without AC revealed an overall attenuation recovery of 9–21% for MAP reconstructed images, i.e., SUV without AC could underestimate the true activity at this level. For subcutaneous tumors, the attenuation was 13 ± 4% (9–17%), for kidneys 20 ± 1% (19–21%), and for bladder 18 ± 3% (15–21%). The FBP reconstructed images showed almost the same attenuation levels as the MAP reconstructed images for all organs. Conclusions: The annihilation photons are suffering attenuation even in small subjects. Both PET-based and CT-based are adequate as AC methods. The amplitude of the AC recovery could be overestimated using the uniform map. Therefore, application of a global attenuation factor on PET data might not be accurate for attenuation correction