The conjugation of maleimide-functionalized closo-dodecaborate (MID) to transferrin (TF), which has no free SH cysteine residue, proceeded at room temperature for 12 h in PBS buffer (pH = 7.4). MID-TF conjugates were accumulated into the cells in the time- and concentration-dependent manners through the TF receptor mediated mechanism. In vivo biodistribution study of MID-TF conjugates in colon 26-bearing mice revealed the time-dependent selective accumulation of MID-TF conjugates into tumor very similar to that of MID-BSA. The tumor boron concentration of MID-BSA was higher than that of MID-TF conjugates (62 ppm vs. 20 ppm at a dose of 30 mg [B]/kg) 12 h after administration, suggesting that the EPR effect is not remarkably observed in the case of MID-TF conjugates because TF has smaller molecular weight than BSA (48 kDa vs. 68 kDa).
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