Transporters in the Mammary Gland-Contribution to Presence of Nutrients and Drugs into Milk

A large number of nutrients and bioactive ingredients found in milk play an important role in the nourishment of breast-fed infants and dairy consumers. Some of these ingredients include physiologically relevant compounds such as vitamins, peptides, neuroactive compounds and hormones. Conversely, milk may contain substances-drugs, pesticides, carcinogens, environmental pollutants-which have undesirable effects on health. The transfer of these compounds into milk is unavoidably linked to the function of transport proteins. Expression of transporters belonging to the ATP-binding cassette (ABC-) and Solute Carrier (SLC-) superfamilies varies with the lactation stages of the mammary gland. In particular, Organic Anion Transporting Polypeptides 1A2 (OATP1A2) and 2B1 (OATP2B1), Organic Cation Transporter 1 (OCT1), Novel Organic Cation Transporter 1 (OCTN1), Concentrative Nucleoside Transporters 1, 2 and 3 (CNT1, CNT2 and CNT3), Peptide Transporter 2 (PEPT2), Sodium-dependent Vitamin C Transporter 2 (SVCT2), Multidrug Resistance-associated Protein 5 (ABCC5) and Breast Cancer Resistance Protein (ABCG2) are highly induced during lactation. This review will focus on these transporters overexpressed during lactation and their role in the transfer of products into the milk, including both beneficial and harmful compounds. Furthermore, additional factors, such as regulation, polymorphisms or drug-drug interactions will be described.

Enantioselective Drug Recognition by Drug Transporters

Drug transporters mediate the absorption, tissue distribution, and excretion of drugs. The cDNAs of P-glycoprotein, multidrug resistance proteins (MRPs/ABCC), breast cancer resistance protein (BCRP/ABCG2), peptide transporters (PEPTs/SLC15), proton-coupled folate transporters (PCFT/SLC46A1), organic anion transporting polypeptides (OATPs/SLCO), organic anion transporters (OATs/SLC22), organic cation transporters (OCTs/SLC22), and multidrug and toxin extrusions (MATEs/SLC47) have been isolated, and their functions have been elucidated. Enantioselectivity has been demonstrated in the pharmacokinetics and efficacy of drugs, and is important for elucidating the relationship with recognition of drugs by drug transporters from a chiral aspect. Enantioselectivity in the transport of drugs by drug transporters and the inhibitory effects of drugs on drug transporters has been summarized in this review.

Stereoselectivity of chiral drug transport: a focus on enantiomer-transporter interaction

Drug transporters and drug metabolism enzymes govern drug absorption, distribution, metabolism and elimination. Many literature works presenting important aspects related to stereochemistry of drug metabolism are available. However, there is very little literature on stereoselectivity of chiral drug transport and enantiomer-transporter interaction. In recent years, the experimental research within this field showed good momentum. Herein, an up-to-date review on this topic was presented. Breast Cancer Resistance Protein (BCRP), Multidrug Resistance Proteins (MRP), P-glycoprotein (P-gp), Organic Anion Transporters (OATs), Organic Anion Transporting Polypeptides (OATPs), Organic Cation Transporters (OCTs), Peptide Transport Proteins (PepTs), Human Proton-Coupled Folate Transporter (PCFT) and Multidrug and Toxic Extrusion Proteins (MATEs), have been reported to exhibit either positive or negative enantio-selective substrate recognition. The approaches utilized to study chirality in enantiomer-transporter interaction include inhibition experiments of specific transporters in cell models (e.g. Caco-2 cells), transport study using drug resistance cell lines or transgenic cell lines expressing transporters in wild type or variant, the use of transporter knockout mice, pharmacokinetics association of single nucleotide polymorphism in transporters, pharmacokinetic interaction study of racemate in the presence of specific transporter inhibitor or inducer, molecule cellular membrane affinity chromatography and pharmacophore modeling. Enantiomer-enantiomer interactions exist in chiral transport. The strength and/or enantiomeric preference of stereoselectivity may be species or tissue-specific, concentration-dependent and transporter family member-dependent. Modulation of specific drug transporter by pure enantiomers might exhibit opposite stereoselectivity. Further studies with integrated approaches will open up new horizons in stereochemistry of pharmacokinetics.

Stereoselective interaction of pantoprazole with ABCG2. II. In vitro flux analysis

(-)Pantoprazole [(-)PAN] accumulated in rat milk stereoselectively, and this accumulation was attributed to rat Abcg2 (rAbcg2). In contrast, flux experiments at 25 μM showed that (+)pantoprazole [(+)PAN] was preferentially transported by rAbcg2. The purpose of the current study was to comprehensively evaluate the transport of PAN isomers in empty-Madin-Darby canine kidney II (MDCKII) and MDCKII cells expressing the human/rat (ABCG2/rAbcg2) isoforms at concentrations ranging from 3 to 200 μM. The apical-to-basolateral and basolateral-to-apical directional flux and the asymmetry efflux ratios were virtually identical for both isomers in empty (mock transfected)-MDCKII monolayers but were concentration dependent for both isomers in ABCG2 (human/rat)-MDCKII. Kinetic analysis using predicted cellular concentrations showed that (-)PAN had an 8-fold lower K(M) compared with (+)PAN for both rAbcg2 (0.25 versus 1.85 μM) and ABCG2 (0.6 versus 5.32 μM). (+)PAN had a 3-fold higher T(Max) compared with the (-)PAN for both rAbcg2 (7.86 versus 2.49 nmol/h · cm(2)) and ABCG2 (10.2 versus 3.29 nmol/h · cm(2)). Effective ABCG2 surface-area permeability of (-)PAN was 9920 and 5480 (μl/h)/cm(2) for rAbcg2 and ABCG2, respectively, compared with the (+)PAN isomer (4250 and 1920 μl/h · cm(2), respectively). These results indicate a stereoselective interaction of PAN with similar kinetic parameters for both human and rat ABCG2. (-)PAN is a better substrate than (+)PAN for ABCG2/rAbcg2 and provide a rationale for the preferential accumulation of (-)PAN into rat milk.