Targeting cancer cells with vitamin B12 (cobalamin) is hampered by unwanted physiologic tissue uptake mediated by transcobalamin. Adhering to good manufacturing practice, we have developed a new (99m)Tc-cobalamin derivative ((99m)Tc(CO)3-[(4-amido-butyl)-pyridin-2-yl-methyl-amino-acetato] cobalamin, (99m)Tc-PAMA-cobalamin). The derivative shows no binding to transcobalamin but is recognized by haptocorrin, a protein present in the circulation and notably expressed in many tumor cells. In this prospective study, we investigated cancer-specific uptake of (99m)Tc-PAMA-cobalamin in 10 patients with various metastatic tumors.
Methods: Ten patients with biopsy-proven metastatic cancer were included. Dynamic imaging was started immediately after injection of 300-500 MBq of (99m)Tc-PAMA-cobalamin, and whole-body scintigrams were obtained at 10, 30, 60, 120, and 240 min and after 24 h. The relative tumor activity using SPECT/CT over the tumor region after 4 h was measured in comparison to disease-free lung parenchyma. Patients 3-10 received between 20 and 1,000 μg of cobalamin intravenously before injection of (99m)Tc-PAMA-cobalamin. The study population comprised 4 patients with adenocarcinomas of the lung, 3 with squamous cell carcinomas of the hypopharyngeal region, 1 with prostate adenocarcinoma, 1 with breast, and 1 with colon adenocarcinoma.
Results: The median age of the study group was 61 ± 11 y. Six of 10 patients showed positive tumor uptake on (99m)Tc-PAMA-cobalamin whole-body scintigraphy. The scan was positive in 1 patient with colon adenocarcinoma, in 3 of 4 lung adenocarcinomas, in 1 of 3 hypopharyngeal squamous cell carcinomas, and in 1 breast adenocarcinoma. Renal uptake was between 1% and 3% for the left kidney. Predosing with cobalamin increased the tumor uptake and improved blood-pool clearance. The best image quality was achieved with a predose of 20-100 ug of cold cobalamin. The mean patient dose was 2.7 ± 0.9 mSv/patient.
Conclusion: To our knowledge, we report for the first time on (99m)Tc-PAMA-cobalamin imaging in patients with metastatic cancer disease and show that tumor targeting is feasible.
Keywords: cancer; cobalamin; haptocorrin; transcobalamin receptors; vitamin B12.