How exquisitely designed is the human body? Despite our efforts to occasionally derail our health and well-being, our bodies come with helpful built-in protective functional barriers. The blood-brain barrier and the placenta are two examples. In basic terms, both restrict the free flow of substances from the systemic circulation and help prevent harmful substances from reaching the brain and the fetus, respectively. The placenta is unique in that it develops along with the fetus and, at delivery, is expelled after having done its work. But what happens when a disease or treatment alters the ability of the placenta to operate as a control gate for the fetus?
Dr. Janet R. Hardy
In keeping with this column’s title, let’s start with bugs. Based on the 2021 World Malaria Report, malaria continues to strike hardest against pregnant women and children in Africa.1 In 2020 in 33 moderate- and high-transmission African countries, 34% of pregnancies (11.6 million of 33.8 million) were exposed to malaria infection. Malaria infection during pregnancy is associated with adverse birth outcomes, including small for gestational age and preterm birth, which in turn increase the risk for neonatal and childhood mortality.
Dr. Melissa S. Tassinari
Malaria is caused by the parasite of the genus Plasmodium and is transmitted by infective female Anopheles mosquitoes. The predominant parasite in sub-Saharan Africa is Plasmodium falciparum. Pregnant women are particularly vulnerable. Once a subject is bitten, the P. falciparum parasite is injected into the human blood stream where it is taken up initially by the liver and subsequently by the erythrocytes of the host which adhere to placental receptors, triggering placental inflammation and subsequent damage. This leads to impaired placental development and function, placental insufficiency, and the adverse birth outcomes identified above.2 In targeting the placenta, this parasite can cause structural and functional placental alterations through infection and inflammation. A recent review by McColl et al. has shown that placental inflammation with or without infection affects the normal function of placental amino acid transporters, leading to similar adverse pregnancy outcomes.3
Moving on to drugs and drug safety in pregnancy, concern generally focuses on exposure during pregnancy that might directly affect the fetus at critical time windows during growth and development. There is a need to understand not only the size of the drug molecules and the degree to which they cross the placenta, but also how those medications may affect the development and function of the placenta itself. New research methods such as the “placenta-on-a-chip” that models the transport of nutrients and drugs allow direct evaluation of placental function.4 Assessing placental function using such tools during drug development will contribute to a better understanding of the safety and efficacy of new medications for use in pregnancy, providing important information at the preclinical phases.5
The placenta is a dynamic organ with metabolic, endocrine, immunologic, and transport functions. Most importantly, it protects a healthy pregnancy. It also provides the advantage of immunologic protection to the fetus when maternal antibodies cross the placenta and provide initial protection until the newborn’s own immune system matures. Using our knowledge of placental alteration models and new research methods such as “placenta-on-a-chip” can help expand our understanding of the role of the placenta in medication safety in pregnancy.
Dr. Hardy is executive director, head of pharmacoepidemiology, at Biohaven Pharmaceuticals. She serves as a member of Council for the Society for Birth Defects Research and Prevention, represents the BDRP on the Coalition to Advance Maternal Therapeutics, and is a member of the North American Board for Amandla Development, South Africa. Dr. Tassinari is a consultant and was formerly employed by Pfizer and the Food and Drug Administration. Dr. Tassinari is a past president of BDRP (formerly the Teratology Society) and currently serves as a member of the External Science Advisory Committee for The Medicines for Malaria Venture and is a member of the Science Advisory Committee for the COVID-19 Vaccines International Pregnancy Exposure Registry.
References
1. World malaria report 2021. Geneva: World Health Organization; 2021.
2. Chua CLL et al. Front Immunol. 2021;12:621382.
3. McColl ER et al. Drug Metab Dispos. May 2022.
4. Blundeli C et al. Adv Healthc Mater. 2018. January;7(2).
5. David AL et al. Ther Innov Regul Sci. 2022.