Transfer of aciclovir from plasma to human breast milk

Aciclovir (CAS 59277-89-3) is frequently used in herpes simplex virus disea ses, but administration to lactating women occurs only rarely. Therefore, i nformation about the pharmacokinetics of aciclovir in human breast milk is limited. The concentration in breast milk is 2 to 3 fold increased compared to plasma. The reason for this increase is unknown until now. An active tr ansport mechanism has been assumed. The aim of this study was to prove whet her the higher concentration of aciclovir in human breast milk is due to on ly a passive transfer. Two chambers separated by a semipermeable membrane w ere used. The first chamber contained plasma with aciclovir, the second cha mber breast milk without aciclovir. The increase in aciclovir concentration in the second chamber was determined. The concentration of aciclovir in br east milk exceeded that of plasma after 2 h and reached a higher concentrat ion. Thus, the higher aciclovir concentration in human breast milk is due t o passive diffusion. No active transport mechanism is needed.