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Kinetics of ^99mTc-Labeled Pyrophosphate and Polyphosphate in Man

VA Wadsworth Hospital Center, UCLA School of Medicine, and USC School of Pharmacy, Los Angeles, California

Correspondence: For reprints contact: G. T. Krishnamurthy, Nuclear Medicine Service (691/172A), VA Wadsworth Hospital Center, Los Angeles, Calif. 90073.

ABSTRACT

The kinetics of ^99mTc-labeled pyrophosphate were compared with those of polyphosphate in ten patients in a combined study. Both agents cleared from the blood in a biexponential fashion. The clearance half-time of Exponent I was the same for both and was shorter than the clearance half-time of Exponent II. Urinary excretion of both agents was the same during the first hour but during the next 3 hr Tc-pyrophosphate cleared at a slightly more rapid rate, resulting in lower blood background radioactivity. Both agents were bound loosely to plasma proteins, mainly to globulin fractions. The sensitivity of lesion detection was similar for both. Excellent bone images were obtained with both agents. The images with Tc-pyrophosphate were consistently superior owing to the low blood background and they took less time to accumulate an identical number of counts from identical regions. With the amount of ^99mTc-complex used, no hpyocalcemia or tetany was noted, nor was there any significant effect on 1-hr serum levels of inorganic phosphorus and alkaline phosphatase. Four hours after injection, 9.5% of the dose of Tc-pyrophosphate was circulating in blood, 31.7% was excreted in urine, and the remaining 58.8% was taken up by bone and other tissues. The corresponding values with Tc-polyphosphate were 12.5% in blood, 29.0% in urine, and 58.5% in bone and other tissues. Among the soft tissues, the genito-urinary system is most consistently visualized. It is concluded that both Tc-pyrophosphate and Tc-polyphosphate are excellent skeletal-imaging agents and that Tc-pyrophosphate appears slightly superior to Tc-polyphosphate.