Uptake and distribution of fluorescently labeled cobalamin in neoplastic and healthy breast tissue

TLDR: Fluorescent analogs ofcobalamin (vitamin B12) have been developed as diagnostic markers ofcancer cells as discussed by the authors. But the results ofpreliininary studies suggest that fluorescent analogs may be a useful tool in therapeutic breast operations to define tumor margins and to distinguishneoplastic breast tissue from healthy breast tissue.

Abstract: Fluorescent analogs ofcobalamin (vitamin B12)have been developed as diagnostic markers ofcancer cells.These compounds are recognized by transcobalamin, a cobalamin transport protein, with high affinity, asshown by surface plasmon resonance. The cellular sequestration and gross distribution of fluorescentcobalamm bioconjugates in breast tissue is being examined by epifluorescence microscopy. Thedistribution ofeach compound is being evaluated in proliferative and non-proliferative tissue, ie. normaltissue and breast carcinoma. The results ofpreliininary studies suggest that fluorescent analogs ofcobalamin may be a useful tool in therapeutic breast operations to define tumor margins and to distinguishneoplastic breast tissue from healthy breast tissue.Keywords: cobalamin, vitamin B12, breast, neoplastic, fluorescent, transcobalamin, TCII 1. ENTRODUCFION Eukaryotic cells require cobalamin for metabolism and cell replication. Methionine synthase utilizesmethylcobalamin as a cofactor to methylate homocysteine in a coupled reaction with N5methyl-tetrahydrofolate. N5Methyl-tetrahydrofolate serves as a methyl group donor, thereby regeneratingtetrahydrofolate, which ultimately converts deoxyuridine monophosphate to deoxythymidinemonophosphate.' Rapidly proliferating cells have an increased demand for thymidine to support DNAreplication and thus have a need for high levels ofcobalamin. Tumor cells manifest this need forcobalamin by increasing their level ofcobalamin transport and storage.2'3In blood, cobalamin is bound to transcobalamin (TCII). Cobalamin enters cells via receptor-mediatedendocytososis ofthe TCII-cobalamin complex.4 This process allows a tumor to sequester high levels ofradiolabeled cobalamin, as seen in mice that have an implanted fibrosarcoma5 and in companion felineswith a primary mammary tumor.5 The unsaturated serum binding capacity ofvitamin B12 increases 3 to 26fold in patients with acute promyelocytic leukemia.6'7 In addition, elevated levels ofcirculating TCII havebeen observed in patients with breast carcinoma.8Breast cancer is the second most common type of cancer in women.9 Surgical therapy for breast cancerrequires the removal of a fraction of healthy breast tissue to ensure complete excision ofthe tumor. Areliable method for detecting neoplastic margins intraoperatively and differentiating it from healthy tissueis not available. Fluorescent derivatives ofcobalamin may be useful as tumor imaging agents to fill thisvoid in current medical technology. This would ensure complete excision ofneoplastic tissue whileminimizing the amount ofhealthy tissue removed. Cobalamin has been labeled with Oregon Green,naphthofluorescein, and fluorescein fluorophores. The internalization of these cobalamin bioconjugates bytumor (target) tissue is now being evaluated and compared with internalization levels in surroundingnormal (background) tissue in breast carcinoma.