How does drug partitioning into milk during lactation occur, and what PK considerations does it create for nursing infants?

How does drug partitioning into milk during lactation occur, and what PK considerations does it create for nursing infants?

• A. Drugs pass into milk by passive diffusion; milk-to-plasma ratio and drug's PK properties influence infant exposure and safety
• B. Active transport only determines milk partitioning
• C. Milk partitioning is determined by protein binding in plasma only
• D. It is not a concern for nursing infants

Answer: (A)

Milk partitioning during lactation happens mainly through passive diffusion from the mother’s plasma into the milk. The extent of transfer is captured by the milk-to-plasma concentration ratio, and it is driven by the drug’s physicochemical and pharmacokinetic properties. Lipid-soluble, small molecules cross membranes more readily, so they tend to appear in milk more than large or highly polar ones. Whether the drug is ionized at the pH of plasma and milk also matters because ionization affects membrane permeability; the two compartments have slightly different pH, which can influence how much exists in the diffusion-ready form. Importantly, only the unbound fraction of drug in plasma can diffuse into milk, so drugs that are highly protein-bound in plasma transfer less readily. For nursing infants, this transfer means exposure that depends on the milk-to-plasma ratio and the drug’s PK traits, and it raises safety considerations because infants have immature hepatic and renal function, which can slow clearance and increase the impact of any drug they ingest through milk. Active transport into milk is not the main driver for most drugs, and ignoring lactation exposure can miss potential risks.

Milk partitioning during lactation happens mainly through passive diffusion from the mother’s plasma into the milk. The extent of transfer is captured by the milk-to-plasma concentration ratio, and it is driven by the drug’s physicochemical and pharmacokinetic properties. Lipid-soluble, small molecules cross membranes more readily, so they tend to appear in milk more than large or highly polar ones. Whether the drug is ionized at the pH of plasma and milk also matters because ionization affects membrane permeability; the two compartments have slightly different pH, which can influence how much exists in the diffusion-ready form. Importantly, only the unbound fraction of drug in plasma can diffuse into milk, so drugs that are highly protein-bound in plasma transfer less readily.

For nursing infants, this transfer means exposure that depends on the milk-to-plasma ratio and the drug’s PK traits, and it raises safety considerations because infants have immature hepatic and renal function, which can slow clearance and increase the impact of any drug they ingest through milk. Active transport into milk is not the main driver for most drugs, and ignoring lactation exposure can miss potential risks.