Abstract
1033
Objectives: The primary objective of this study is to develop a PET probe that is easy to synthesize and to assess the selectivity and specificity of this PET probe towards targeting melanoma tumors.
Methods: 4-F-18 Fluoroethoxy (2-diethylaminoethyl) benzamide (F-18 FE-BENZ) was obtained by reacting F-18 Fluoroethyltosylate with 4-hydroxy-N-(2-diethylaminoethyl) benzamide followed by HPLC purification. In vitro binding and binding affinity of F-18 FE-BENZ was performed using B16F1 cells. The selectivity and tumor targeting ability of F-18 FE-BENZ was assessed using microPET imaging studies in mice bearing B16F1 melanoma tumor xenografts and HT-29 human colorectal adenocarcinoma tumor xenografts. The B16F1 and HT-29 tumor xenografts were established in C57BL mice in athymic nu/nu mice, respectively. The imaging studies were performed when the tumor diameter reached >5 mm in diameter. After anesthetizing mice with 2% isoflurane in 1L/min oxygen, the mice were placed in prone position in the center of the Field of View of the microPET scanner. The mice were injected with 3.0 - 10.5 MBq (81 - 285 µCi) of F-18 FEBENZ via the tail vein injection. The microPET images were acquired starting at injection time. The images were reconstructed and subsequently analyzed using ASIPro (Siemens). Regions of interests (ROIs) were drawn and the time activity curves were generated for several of the key organs. The uptake in various organs is expressed as %ID/g and SUV.
Results: F-18 FE-BENZ was synthesized in ~30% radiochemical yields and in >99% radiochemical purity using a simple and facile radiochemical synthesis procedure. The IC50 for the F-18 FE-BENZ was 0.13 nM. The in vitro cell binding to B16F1 cells was rapid and reached at 24% levels by 45 min and plateaued thereafter. The microPET images show an early distribution of radioactivity in the tissues followed by a steady clearance of radioactivity from most normal tissues. The radioactivity rapidly accumulated in B16F1 tumor and reached its maximum value by 30 min post injection. The tumor was clearly visible on microPET scans by 10 min post injection. In contrast, while the colorectal tumor HT-29 was also visible on microPET scans, the uptake levels were >4-fold lower than that for the B16F1 melanoma tumors. For the B16F1 tumor bearing mice, the radioactivity levels expressed as %ID/g of tissue was 1.52±0.14, 1.11±0.01, 1.21±0.18, 1.70±0.39, 1.21±0.77, and 7.20±5.61 in the brain, muscle, lung, liver, kidney, and tumor, respectively. A high uptake in B16F1 tumor and a low retention of radioactivity in normal tissues provided a high tumor to tissue ratios for the B16F1 tumor group. In contrast, the tumor to tissue ratio were significantly low for the HT-29 group.
Conclusion: We developed a melanoma imaging agent that is easy to synthesize in high radiochemical yields and purity using a facile one-step radiochemistry procedure. This PET probe shows a high binding affinity to melanoma in vitro. The microPET studies show a distinct and selective uptake in melanoma tumors. A low retention in normal tissues in conjunction with a high uptake in melanoma tumors helped clearly delineate B16F1 melanoma tumors as early as 10 min post injection. More studies are warranted to further assess the clinical potential of this promising radiotracer. Research Support: We would like to thank Kimberly Black, Holly Smith, Leah Rutkowski, and Li Wu of Wake Forest University Health Sciences for their excellent technical help. The strong support from the Center for Biomolecular Imaging program at WFUHS is greatly appreciated and acknowledged.