Transmission of HIV-1 in breast milk. Excretion of varicella-herpes zoster virus in breast milk. There seems to be two aspects to your question. In the first instance it is the square root of molecular weight that is usually related to diffusion across membranes. A variety of models have been used in order to predict drug diffusion into human milk.
Protein binding is more important for basic drugs and pKa for acidic drugs.
I hope this helps. My fellow and I are working on this topic for a year now. It is one of the least understood subject even so it is very significant one. We evaluated over 1200 drugs and developed FG-LA, the first clinical lassification of drugs used during pregnancy, the classification describes many parameters including the drug excretion into milk and documented adverse effects on infants and lactatating mothers. Gamal Hussein, Pharm.