Structure and basic functions
The morphological basis of the barrier is formed by a monolayer of cells called the syncytiotrophoblast (syncytiotrophoblastus). These cells exhibit strict polarity: their apical membrane is in contact with maternal blood, while the basal membrane is directed toward the fetal circulation.
The primary function of the placenta (placenta) in 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 medicinal substances) encounter a selective filter that restricts their penetration.
How drugs cross the placenta
The passage of substances through the trophoblast membrane (trophoblastus) obeys the classical laws of biological membranes. Two fundamentally different mechanisms are distinguished depending on the properties of the molecules.
Passive diffusion of lipophilic compounds This is the primary pathway for most lipid-soluble substances. The success of diffusion depends directly on two factors:
- Molecular weight (MW):
- $\le$ 500 Daltons — molecules easily cross the barrier.
- 500 to 1000 Daltons — penetrate slowly and incompletely.
- Greater than 1000 Daltons — practically do not cross the 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 woman takes weakly basic drugs, they readily cross to the fetus but become ionized in the acidic environment. The ionized form loses lipophilicity, cannot return, and begins to accumulate in fetal tissues.
Transport of hydrophilic substances Polar hydrophilic molecules require the involvement of carrier proteins (pinocytosis is sometimes used, but rarely).
- Facilitated diffusion primarily 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). Members of the SLC transporter family, which operate in both directions, play an important role here.
Mechanisms of active fetal defense
The placenta not only filters substances but also actively expels those that have managed to penetrate inside. This process is called efflux. This defense system is localized on the apical membrane of the syncytiotrophoblast.
The key defensive elements are ATP-dependent ABC transporters. The most important of these is P-glycoprotein (P-glycoprotein). It acts as a unidirectional pump, actively pumping foreign lipophilic substances out of the placental cells and back into the maternal blood.
Substrates eliminated by P-glycoprotein:
- Rifampicin (antibiotic)
- HIV protease inhibitors
- Verapamil (calcium channel blocker)
- Cyclosporine (immunosuppressant)
Also standing guard is the breast cancer resistance protein (BCRP), which prevents anti-tumor agents (e.g., Doxorubicinum) from reaching the fetus. The clinical issue lies in drug interactions. If a patient takes P-glycoprotein inhibitors, the protective pump is blocked. The co-administration of such inhibitors with cyclosporine or saquinavir dramatically increases toxic effects on the fetus.
Barrier dynamics and critical periods
The blood-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 when organ systems are formed. Complete avoidance of pharmacotherapy is required due to the high risk of teratogenicity. For example, lithium salts (Lithium) during this period disrupt the formation of the cardiac tube.
- Pregravid period (before conception): drugs capable of accumulating in tissues pose a hazard. Retinoids (Retinoids) taken before pregnancy can persist in the body for a long time and exert adverse effects on the fetus after conception.
Any medication use during pregnancy requires strict medical supervision, as complications can arise at any gestational age. Besides the placenta, the body has other histohematic barriers whose main purpose is to protect specific tissues from polar hydrophilic compounds.
Briefly: answers to topical questions
Which SLC family transporters provide active transport of hydrophilic substances across the placenta?
Active transport of hydrophilic substances across the placenta is mediated by transporters of the SLC (Solute Carrier) family. They are capable of transporting substances in both directions — from mother to fetus and vice versa.
Key mechanisms of active transport:
- Primary transport — is ATP-dependent.
- Secondary transport — depends on the transmembrane cation gradient (e.g., sodium ions).
This mechanism is crucial for drugs that share structural similarity with endogenous molecules (e.g., amino acids). Specific names of SLC family transporters in the placenta are not specified in the sources.
Which ABC transporters mediate the efflux of xenobiotics from the syncytiotrophoblast?
The efflux of xenobiotics from placental cells is carried out by ATP-binding cassette (ABC) transporters localized on the apical membrane of the trophoblast.
The main proteins of this group include:
- P-glycoprotein (P-glycoprotein) — a key protein that ensures the unidirectional expulsion of substances into maternal blood to prevent them from reaching the fetus.
- Breast cancer resistance protein (BCRP) — a specific protein that prevents the penetration of anti-tumor agents by pumping them back into the maternal blood.
Which medications act as substrates for P-glycoprotein?
Substrates for P-glycoprotein include various groups of medicinal products that are actively eliminated from tissues by this protein.
These include:
- Rifampicin — an antibiotic.
- HIV protease inhibitors — antiviral agents.
- Verapamil — a calcium channel blocker.
- Cyclosporine — an immunosuppressant.
- Domperidone (Domperidonum) — a prokinetic and antiemetic agent ($D_2$-receptor blocker) that is actively cleared from the brain, thereby avoiding central side effects.
Which anti-tumor agents are blocked by the breast cancer resistance protein (BCRP)?
The breast cancer resistance protein (BCRP) blocks the transplacental passage of the anti-tumor drug doxorubicin.
- Doxorubicin (Doxorubicinum) — an anti-tumor agent that undergoes efflux (elimination) back into the maternal blood due to the action of this specific transporter.
Other specific drugs blocked by BCRP are not mentioned in the sources.
Which drugs have a teratogenic effect when taken during the period of organogenesis?
According to sources, drugs and substances with teratogenic effects in early pregnancy include:
- Lithium salts (Lithium) — an example of teratogenicity during the period of cardiac tube formation.
- Thalidomide — intake in early pregnancy is associated with phocomelia.
- Methotrexate — an antineoplastic agent classified in the risk group.
- Most narcotic substances — exhibit pronounced teratogenic effects, especially in early pregnancy.
Ethanol is described in sources as having embryopathic and fetopathic effects, and therefore is not classified here among agents with explicitly stated teratogenic effects.
Which drugs are dangerous during the pre-conception period due to their ability to accumulate in tissues?
During the pre-conception period (before conception), retinoids pose a particular danger.
- Retinoids (Retinoids) — have the ability to accumulate in the body.
They can persist in maternal tissues for a long time and exert a negative impact on the fetus even after pregnancy has occurred.
To remember the "ion trap" phenomenon, imagine the fetal plasma as an acidic trap. Weakly basic molecules easily flow into it, but due to the acidic environment, they acquire a charge (ionize). Once charged, they lose the ability to "seep" back through the fatty barrier to the mother and remain locked inside the child's body.
Class and exam questions
Why large molecules do not harm the fetus?
Molecules with a mass greater than 1000 Daltons are too large to pass through the trophoblast membranes by passive diffusion. Therefore, they practically do not cross the blood-placental barrier.
What happens if P-glycoprotein is blocked?
Inhibition of P-glycoprotein turns off the "pump" that expels toxins back into the maternal blood. As a result, placental permeability to hazardous xenobiotics increases, which can harm the child.
At what gestational age does the barrier become most vulnerable?
At approximately 32–35 weeks of gestation. At this time, the placenta thins out, which physiologically increases its permeability to any medicinal substances.
Is it safe to take medications prone to accumulation if pregnancy is only being planned?
This is very dangerous. Drugs like retinoids accumulate in the body. Even if they are discontinued before conception, they can be released from tissues and exert a teratogenic effect on the fetus.
What else to study on this topic
- Mechanism of action of the SLC transporter family
- Specifics of ABC transport proteins (P-glycoprotein and BCRP)
- The "ion trap" phenomenon and calculation of the degree of ionization
- Effect of lithium salts on cardiac tube development
- Pharmacokinetics of retinoids during pre-conception preparation
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