Placental binding and transfer of radiopharmaceuticals: technetium-99m d, 1-HMPAO

Placental binding and transfer of 99mTc d, 1-hexamethyl propyleneamine oxime (HMPAO) was studied in vitro using human placenta and pregnant guinea pigs.

Five pieces of human placenta were incubated in 50 ml Earle's solution containing 1.85 MBq 99mTc d, 1-HMPAO. The percent binding of the tracer to the placenta per 1 ml standard solution was calculated. Pregnant guinea pigs representing first, second and third trimesters were each injected with 74 MBq 99mTc d, 1-HMPAO through the jugular or femoral vein after sedation was induced with pentabarbital sodium. Static images were obtained, the guinea pigs were killed, and the fetuses were removed, weighed and imaged separately. The placentas, maternal and fetal brains, lungs, livers and kidneys also were removed, and the radioactivity was assayed in a dose calibrator for each organ. The percent radioactivity in each organ was calculated.

The binding of 99mTc d, 1-HMPAO to human placenta ranged from 2.95% +/- 1.5% to 5.82% +/- 0.3% per 1 ml standard solution. Both the binding of 99mTc d, 1-HMPAO to guinea pig placenta and its transfer to the fetus increased with gestational age. The percent binding ranged from 0.09% +/- 0.06% to 0.43% +/- 0.05%, whereas that of transfer to the fetus ranged from 0.05% +/- 0.03% to 2.19% +/- 0.64%. Of the amount transfered to the fetus, the order of accumulation in the fetal organs was liver > blood >> brain > lungs > kidneys > heart.

Technetium-99m d, 1-HMPAO binds to the placenta, and a minimal amount crosses the placental barrier and is transfered into the fetal circulation, mostly in the liver but a measurable amount is found in brain tissue.