Background: The purpose of antibiotic treatment in pregnant women is to treat the mother and/or the fetus since it is known that antibiotics administered to the mother cross the placenta and reach the fetus. A comparison of the drug concentration in maternal and fetal plasma gives an indication of the exposure of the fetus to the maternally administered antibiotics. Aim: The aim of this study was to review the literature pertaining to the placental transfer of antibiotics in mail and to classify the antibiotics according to the type of transfer involved. A table has been developed for use by physicians that lists the name of the antibiotic, the drug concentration in the cord and material plasma at delivery and the type of transfer involved. Methods: An initial medline search was performed with the key words "placental transfer of antibiotics" with the limit of "human". A second medline search was performed with the key words "placental transfer of..." followed by the class names of the antibiotic such as penicillins, cephalosporins, aminoglycosides, tetracyclines and macrolides. The bibliographic search on the placental transfer of antibiotics covered the period up to July 2005. Results: 3 types of placental transfers were identified. A few antibiotics cross the placenta rapidly and equilibrate in the maternal and cord plasma; this type of transfer is termed "complete" and include the antibiotics ampicillin, methicillin, cefmenoxime and cefotiam. Antibiotics which show incomplete transfer to the placenta where concentrations are lower in the cord than material plasma are said to have "incomplete" transfer and these include azlocillin, dicloxacillin, piperacillin, sulbenicillin, cefoxitin, amikacin, gentamicin, kanatnycin, streptomycin, fosfornycin, thiamphenicol, griseofulvin, vancomycin and colistimethate. Ceftizoxime is the only antibiotic so far known whose concentrations are higher in the cord than material plasma. This type of transfer is called "exceeding" transfer. Conclusion: All examined antibiotics cross the human placenta including those with a molecular weight greater than 1000 kDa such as vancomycin and colistimethate but there are 3 distinct types of placental transfer: complete, incomplete and exceeding and most antibiotics exhibit incomplete transfer.
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