The behavior of99mTc-hexamethylpropyleneamineoxime (99mTc-HMPAO) in blood and brain

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Abstract
99mTc-hexamethylpropyleneamineoxime (99mTc-HMPAO) is a reagent for scanning cerebral blood flow. We investigated how99mTc-HMPAO changed in the blood and brain. The99mTc-HMPAO, which was prepared by adding of99mTcO - 4 to HMPAO and Sn(II), consisted of primary and secondary complexes, reduced hydrolyzed99mTc, and99mTc0pertechnetate. The percentage of the primary complex in99mTc-HMPAO decreased with time after preparation. The primary complex converted to the secondary one very rapidly in the presence of plasma. When99mTc-HMPAO was injected into patients,99mTc activity was immediately partitioned in the plasma fraction, with approximately 60% in whole blood. In plasma,99mTc was found to be associated with proteins such as albumin and globulin.99mTc trapped in red cells was not washed out with either plasma or saline. Biodistribution studies showed that the less lipophilic compounds of99mTc-HMPAO could not pass through the blood brain barrier (BBB), and therefore did not accumulate in the brain. The results of gel chromatography and equilibrium dialysis indicated that no specific99mTc binding protein was present in the brain. Considering the instability of99mTc-HMPAO in vivo, we proposed that the speed at which the primary complex converted to the less lipophilic compounds was important in allowing99mTc-HMPAO to pass through the BBB and to be fixed in the brain.