Synthesis of 99mTc-pefloxacin: A new targeting agent for infectious foci

Summary This investigation focused on the labeling of pefloxacin, a fluoroquinolone antibacterial agent, with ^99mTc to form ^99mTc-pefloxacin complex. The labeling process was done by direct addition of pertechnetate in isotonic solution to Sn-pefloxacin solution. The labeling technique is effective, as a high labeling yield (98%) was obtained after 30-minute reaction time. Different factors were found that influenced this labeling reaction: 0.5 mg pefloxacin or more must be used to prevent the formation of colloids in the reaction medium. Fifty micrograms of stannous chloride dihydrate were found to be sufficient to reduce all pertechnetate with activity ranging from 37 to 3700 MBq without the detection of free pertechnetate or colloids in the reaction mixture. The pH of the reaction medium was found to play an important role in the labeling process. The labeling reaction proceeds well at neutral pH (pH 6) but at acidic pH value (pH 4 or below) the yield of ^99mTc-pefloxacin complex decreased markedly to a labeling yield of 5%. The reaction mixture must be heated to 100 °C in an oil bath to enhance the formation of the ^99mTc-pefloxacin complex. The biodistribution data show that ^99mTc labeled pefloxacin was retained in infectious focus. The retention was specific since the abscess uptake of ^99mTc-pefloxacin remained high as compared to the uptake of aseptic foci at 24-hour post injection. Also, the clearance of the tracer from other tissues is rapid on the contrary to its clearance from the septic focus. This supports the hypothesis that ^99mTc-pefloxacin is retained at the infectious site because of its specific binding to the gyrase enzymes of bacterial cells.