Structure and Basic Functions
The morphological basis of the barrier is formed by a monolayer of cells known as the syncytiotrophoblast (syncytiotrophoblastus). These cells exhibit strict polarity: their apical membrane contacts maternal blood, whereas the basal membrane faces the fetal circulation.
The primary function of the placenta (placenta) in the context of pharmacokinetics is the regulation of compound exchange. It operates selectively:
- To the fetus: Endogenous compounds essential for development (glucose, amino acids) are delivered.
- From the fetus: Metabolic waste products are eliminated into the maternal circulation.
- Xenobiotics (including exogenous pharmacological agents) encounter a selective filter that limits their penetration.
How Drugs Cross the Placenta
The passage of substances through the trophoblast (trophoblastus) membrane obeys classical laws of biological membranes. Two fundamentally different mechanisms exist depending on molecular properties.
Passive Diffusion of Lipophilic Compounds This is the primary pathway for most fat-soluble substances. The success of diffusion depends directly on two factors:
- Molecular Weight (MW):
- $\le$ 500 Daltons — molecules easily cross the barrier.
- 500 to 1,000 Daltons — penetrate slowly and incompletely.
- Greater than 1,000 Daltons — practically do not cross membranes.
- pH Gradient and the "Ion Trapping" Phenomenon:
Maternal blood has a pH of approximately 7.4, whereas fetal plasma is more acidic (pH 7.0–7.2). If a mother takes weakly basic drugs, they easily cross to the fetus but become ionized in the acidic environment. The ionized form loses lipophilicity, cannot return across the membrane, and begins to accumulate in fetal tissues.
Transport of Hydrophilic Substances Polar hydrophilic molecules require carrier proteins (pinocytosis is sometimes utilized, but rarely).
- Facilitated diffusion mainly transports endogenous substrates.
- Active transport is necessary for drugs structurally similar to natural metabolites. It can be primary (ATP-dependent) or secondary (dependent on a cation gradient, such as sodium). SLC family transporters, which operate in both directions, play an important role here.
Mechanisms of Active Fetal Protection
The placenta not only filters substances but also actively extrudes those that manage to penetrate inside. This process is called efflux. This protective system is localized on the apical membrane of the syncytiotrophoblast.
ATP-dependent ABC transporters serve as key protective elements. The most important of these is P-glycoprotein (P-glycoprotein). It operates as a unidirectional pump, pumping foreign lipophilic substances out of placental cells back into the maternal blood.
Substrates Removed by P-glycoprotein:
- Rifampin (antibiotic)
- HIV protease inhibitors
- Verapamil (calcium channel blocker)
- Cyclosporine (immunosuppressant)
Also standing guard is the breast cancer resistance protein (BCRP), which prevents anti-cancer drugs (e.g., Doxorubicinum) from reaching the fetus. The clinical challenge lies in drug interactions. If a patient takes P-glycoprotein inhibitors, the protective pump is blocked. Co-administration of such inhibitors with cyclosporine or saquinavir sharply increases toxic effects on the fetus.
Barrier Dynamics and Critical Periods
The placental barrier changes its properties over time. At 32–35 weeks of gestation, physiological thinning occurs, causing drug permeability to increase significantly.
Periods of Maximum Risk:
- Period of Organogenesis: The time of organ system formation. Complete avoidance of pharmacotherapy is required due to a high risk of teratogenicity. For example, lithium salts (Lithium) during this period disrupt cardiac tube formation.
- Pre-pregnancy Period (Before Conception): Drugs capable of tissue accumulation pose a danger. Retinoids (Retinoids) taken before pregnancy can persist in the body for a long time and negatively affect the fetus after conception.
Any medication use during pregnancy requires strict medical supervision, as complications can occur at any gestational age. Besides the placenta, other blood-tissue barriers exist in the body whose primary goal is to protect specific tissues from polar hydrophilic compounds.