Professional question - drug levels in breastmilk

[Dotans]

Hello InfantRisk team,

I hope this is the right place for this type of question.
My name is Don and I am a medical consultant on, among other things, medications and breastfeeding.
I am using Dr. Hale's excellent book on a daily basis and I have a question that needs clarification: In many monographs, drugs' milk levels are given as measured in pharmacokinetic studies. My question is how should I use this bit of data?
I know how to use the table of pharmacokinetic data given in each monograph in order to understand a drug's behavior in the body and in breastmilk, but most of the times no reference range is given for a drug's milk level with which I can compare the milk level that was found in a certain study.
Just as an example, for Bupropion it is said that: "In a study of 10 breastfeeding patients who received 150 mg bupropion SR daily for 3 days and then 300 mg bupropion SR daily thereafter for 4 more days, milk concentrations of bupropion averaged 45 µg/L". What conclusion can I draw from this sentence? Should this data be taken into consideration along with the M/P ratio?

I will be very grateful for a short explanation on how to correctly interpret the data of drug's level in milk.

Thank you!

[admin]

Dotans:

The amount of drug listed is only important if you know the amount of milk the infant ingests. Unfortunately we seldom know how much milk the baby gets from the breast each day. Thus we commonly use the Relative Infant Dose which gives you a feeling for just how much of the mothers dose, actually gets to the infant each day. While not perfect, it is a better estimate of the exposure of the infant to the drug.

As for the Milk/Plasma ratio, just forget it. We rarely if ever use it, and it often imparts the wrong impression about some drugs. For instance, the milk/plasma ratio of Ranitidine is 6, but the clinical dose the infant actually gets is really quite low...because the plasma levels of ranitidine is quite low. So 6 times something very low is even lower.

So, stick with the Relative Infant Dose, its your best indication of how much drug the infant actually gets per day. Usually, anything less than 10%, is OK.

Tom Hale Ph.D.