Comparisons of labeling efficiency, biological activity and biodistribution among 125I-, 67Ga-DTPA-and 67Ga-DFO-lectins

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Abstract
The labeling efficiency, biological activity and biodistribution of 125I labeled and 67Ga chelating agent conjugated lectins were investigated. Pisum sativum agglutinin (PSA) and Lens culinaris agglutinin (LCH) were efficiently labeled with 67Ga using bifunctional chelating agents such as diethylenetriaminepentaacetic acid (DTPA) and deferoxamine (DFO), whereas labeling with 125I was significantly less efficient. The agglutinating activity of these lectins towards Ehrlich ascites tumor (EAT) cells was retained on conjugation with DFO, but not with DTPA. The in vitro binding ratio of 67Ga-DFO-lectins for EAT cells was almost the same as that of 125I-lectins. However, the value was significantly decreased in the case of 67Ga-DTPA-lectins. In the biodistribution study of radiolabeled lectins in Ehrlich solid tumor (EST) bearing mice, the accumulation of radioactivity in tumor tissue was very much less with 67Ga-DTPA-lectins than with 125I-lectins. However, the concentration was significantly elevated in the case of 67Ga-DFO-lectins. While, these lectins accumulated in liver, spleen, lung, and kidney to a greater extent than 67Ga citrate, the tumor to organ ratios became very low. These low tumor to organ ratios, in contrast to 67Ga citrate, will certainly inhibit the tumor delineation, and therefore it seems that in spite of a high accumulation ratio of 67Ga-DFO-lectins in tumor tissue, these agents are not useful in tumor detection.