Assessment of the Arterial Input Curve for [99mTc]-d,l-HM-PAO by Rapid Octanol Extraction

Abstract

The in vitro conversion of the lipophilic molecule [99mTc]– d,l-hexamethylpropyleneamine oxime ([99mTc]– d,l-HM-PAO) to a hydrophilic form was studied in saline, plasma, and blood at 37°C by paper chromatography and by octanol extraction. The octanol:saline ratio was 79.9. From this value and the corresponding octanol: plasma and octanol:blood partitioning values, an estimate of the transport of the lipophilic compound by various components of blood was made: 20% is carried in hemoglobin, 53% by the plasma proteins and 27% by the water phases of the red blood cell and plasma. Octanol extraction provided a rapid method for measuring the radiochemical purity (RCP) of lipophilic [99mTc]– d,l-HM-PAO. In saline, the RCP declined with a half-life of more than 1 h. In human plasma and whole blood, the conversion of [99mTc]– d,l-HM-PAO was biexponential due to the differences in the conversion rates of the d and l isomeric forms. The initial half-life representing the conversion rate of the l form was 1.7 min in blood and 1.4 min in plasma, while the conversion half-life of the d form was 7.4 and 24.4 min, respectively. In vivo, the RCP of arterial blood sampled after an i.v. bolus injection showed an initial peak value of 75% (68–79%) during the initial, first passage of the bolus. It declined to approximately 35% (29–40%) after 1.5 min and reached very low levels (about 1%) at 6 to 10 min. Quantitative measurements of cerebral blood flow using [99mTc]– d,l-HM-PAO necessitates a rapid method for RCP determination in arterial blood such as the one described here.