Injection Site Radioactivity of ^99mTc-Labeled Mannosylated Dextran for Sentinel Lymph Node Mapping

Abstract

The high and persistent radioactivity at the injection site hinders the accuracy and expansion of sentinel lymph node (SLN) mapping. We investigated the mechanism underlying the undesirable radioactivity after subcutaneous injection of (99m)Tc-labeled mannosylated dextran ((99m)Tc(CO)3-DCM20), a SLN mapping agent targeting mannose receptors on macrophages and dendritic cells, in a mouse model. Biodistribution studies were performed 1 h after subcutaneous injection of (99m)Tc(CO)3-DCM20 from the rear footpad of mice in the presence of varying molar amounts of DCM20 or DC15, a modified dextran without mannose. Biodistribution studies were also conducted after subcutaneous injection of [(125)I]radioiodinated mannosyl-neoglycoalbumin ((125)I-NMA) from the rear footpad. The distribution of fluorescence-labeled DCM20 and DC15 at the injection site was also compared 1 h after subcutaneous injection by immunofluorescent histochemistry. The radioactivity levels of (99m)Tc(CO)3-DCM20 at the injection site and popliteal lymph node, a SLN in this model, decreased with an increase in the molar amounts of DCM20, whereas no significant changes in biodistribution were observed after injection of (99m)Tc(CO)3-DCM20 with varying molar amounts of DC15. (125)I-NMA exhibited rapid elimination of radioactivity from both the popliteal lymph node and the injection site. The fluorescence-labeled DCM20 colocalized well with CD68-positive cells such as macrophages and dendritic cells at the injection site. While partial colocalization was observed between DC15 and CD68-positive cells, the signal intensity was very weak. These findings suggest that specific binding of (99m)Tc(CO)3-DCM20 to the mannose receptor on macrophages and dendritic cells would be responsible for the sustained radioactivity levels at the injection site. These results also imply that discriminated blockage of (99m)Tc(CO)3-DCM20 binding to mannose receptors at the injection sites would reduce the radioactivity at the injection site.

Preparation of ^99mTc-Labeled Mannan-<i>S</i>-Cysteine and Effect of Molecular Size of Mannan on Its Biodistribution

“…Synthesis of Allyl Mannan Addition of allyl group to the mannan backbone was performed following the procedure outlined by Pirmettis et al 14) with minor modifications. Mannan (50 mg) was dissolved in 250 µL of distilled water, followed by the addition of 125 µL of 2.5 M NaOH, 1 mg of sodium borohydride and 15.2 µL of allyl bromide.…”

“…Synthesis of Mannan-S-Cysteine (MSC) L-Cysteine derivatization of the prepared allyl mannan was performed according to the Pirmettis et al 14) with minor modifications. To allyl mannan (20 mg), dissolved in 100 µL of distilled water, was added 14.3 mg of L-cysteine hydrochloride monohydrate and 1.2 mg of ammonium persulfate.…”

“…For example, 99m Tc(CO) 3 -DCM20 probe comprises an dextran backbone that has S-derivatized cysteine as a chelator for labeling with the [ 99m Tc(OH 2 ) 3 (CO) 3 ] + precursor and mannose for targeting the MMR. 14) However, undesirable radioactivity retention of 99m Tc (CO) 3 -DCM20 at the injection site was observed in vivo biodistribution study. 14,15) Considering the decreased accumulation of 99m Tc(CO) 3 -DCM20 at the injection site in the presence of DCM20, development of co-injection of competitive inhibitor could be a rational strategy to reduce the undesirable radioactivity retention.…”

“…14) However, undesirable radioactivity retention of 99m Tc (CO) 3 -DCM20 at the injection site was observed in vivo biodistribution study. 14,15) Considering the decreased accumulation of 99m Tc(CO) 3 -DCM20 at the injection site in the presence of DCM20, development of co-injection of competitive inhibitor could be a rational strategy to reduce the undesirable radioactivity retention. 15) Mannan (N-linked) has been known as a natural polysaccharide linked to asparagine of cell wall proteins in yeast.…”

“…14,15) Considering the decreased accumulation of 99m Tc(CO) 3 -DCM20 at the injection site in the presence of DCM20, development of co-injection of competitive inhibitor could be a rational strategy to reduce the undesirable radioactivity retention. 15) Mannan (N-linked) has been known as a natural polysaccharide linked to asparagine of cell wall proteins in yeast. Unlike dextran, mannan consists of conserved core structure (Man 8 GlcNAc 2 ) and more diverse outer chains.…”