Just as it is possible to label a drug to find out whether it crosses the blood-brain barrier and then to measure the uptake and washout kinetics into the brain, it is also possible to use the labeled drug to measure uptake elsewhere. In this case the question is whether a drug reaches a fetus across the placental barrier. Reasons can include a desire to treat the fetus, or a desire to choose a drug partly on the basis of its exclusion from the fetus to minimize fetal toxicity and teratogenicity. These examples show selected images obtained from a study of live pregnant rats injected with I-124 labeled drug at 8, 11, 12 and 20 days of gestation. Injections were done at varying times, images shown are primarily of the abdomen only.
The "string of pearls" image of the fetuses is visible in the images. The rats had at least nine fetuses. From the whole-animal image at 8 days it is clear that this molecule did NOT accumulate well in the uterus or fetuses. The vast majority of administered dose was excreted. This can be a good result, of course. Further, uptake in this case appears to be in the placental areas and not in the uterine wall or contained within the fetuses.