An in vivo molecular imaging probe (18)F-Annexin B1 for apoptosis detection by PET/CT: preparation and preliminary evaluation.

There is an increasing need to develop non-invasive molecular imaging strategies for visualizing and quantifying apoptosis status of diseases (especially for cancer) for diagnosis and monitoring treatment response. Since externalization of phosphatidylserine (PS) is one of the early molecular events during apoptosis, Annexin B1 (AnxB1), a member of Annexins family with high affinity toward the head group of PS, could be a potential positron emission tomography (PET) imaging probe for imaging cell death process after labeled by positron-emitting nuclides, such as (18)F. In the present study, we investigated a novel PET probe, (18)F-labeled Annexin B1 ((18)F-AnxB1), for apoptosis imaging. (18)F-AnxB1 was prepared reliably by conjugating AnxB1 with a (18)F-tag, N-succinimidyl 4-[(18)F]fluorobenzoate ([(18)F]SFB), in a radiolabeling yield of about 20 % within 40 min. The in vitro binding of (18)F-AnxB1 with apoptotic cells induced by anti-Fas antibody showed twofold increase compared to those without treatment, confirmed by flow cytometric analysis with AnxV-FITC/PI staining. Stability tests demonstrated (18)F-AnxB1 was rather stable in vitro and in vivo without degradation. The serial (18)F-AnxB1 PET/CT scans in healthy rats outlined its biodistribution and pharmacokinetics, indicating a rapid renal clearance and predominant accumulation into kidney and bladder at 2 h p.i. (18)F-AnxB1 PET/CT imaging was successfully applied to visualize in vivo apoptosis sites in tumor induced by chemotherapy and in kidney simulated by ischemia-reperfusion injury. The high-contrast images were obtained at 2 h p.i. to delineate apoptotic tumor. Apoptotic region could be still identified by (18)F-AnxB1 PET 4 h p.i., despite the high probe retention in kidneys. In summary, we have developed (18)F-AnxB1 as a PS-specific PET probe for the apoptosis detection and quantification which could have broad applications from disease diagnosis to treatment monitoring, especially in the cases of cancer.

Preparation and bioevaluation of (99m)Tc-HYNIC-annexin B1 as a novel radioligand for apoptosis imaging.

Annexin B1, a novel Ca2+-dependent PS-binding protein, has been shown to have a high affinity for PS exposed on the surface of apoptotic cells. To develop and bioevaluate an annexin B1 based PS-targeting radiotracer, annexin B1 was radiolabeled with (99m)Tc using HYNIC as a bifunctional chelator. Binding assays with activated platelets and apoptotic SP2/0 cells were carried out to evaluate the in vitro biological activity of (99m)Tc-HYNIC-annexin B1. Biodistribution of this radioligand was studied in normal mice. Dexamethasone-induced murine thymus apoptosis and fas-mediated murine liver apoptosis models were used to investigate the ability of radiolabeled annexin B1 to detect apoptosis in vivo. The labeling procedure yielded a compound with up to 98% radiochemical purity and good in vitro stability. The in vitro binding assays indicated that (99m)Tc-HYNIC-annexin B1 retain its PS-binding activity. Biodistribution of the compound in mice showed that (99m)Tc-HYNIC-annexin B1 is rapidly cleared from the blood and predominantly accumulates in the kidney. The marked increase in dexamethasone-treated murine thymus uptake and fas-mediated murine liver uptake correlated with histologic evidence of apoptosis. These data suggested that (99m)Tc-HYNIC-annexin B1 retain its in vitro and in vivo biological activities. This radiotracer may therefore be useful as a novel radioligand for the noninvasive detecting of PS externalization associated with apoptosis.