Over the past decade, transporters of medications have emerged as an important new area that is paramount to understanding drug interactions.  In fact, many drug interactions that are not easily explained by an effect on the cytochrome P450 (CYP) enzyme system can now be explained by their influence on influx and efflux cell membrane transporters.1,2,3,4  Often times these cell membrane transporters work in conjunction with drug metabolizing enzymes, such as the CYP enzyme system, for drug elimination.    

Influx cell membrane transporters are not only responsible for bringing in needed nutrients into the cell, but are also necessary for movement of medications intracellularly where they will be brought into contact with enzymes responsible for their metabolism and preparation for elimination or possibly to exert their therapeutic or biologic effect.2  As such, anything that prevents or inhibits a medication from getting inside the target cells for metabolism or to exert its therapeutic effects could result in excessive blood concentrations of the medication and cause undesired side effects.  In addition to inhibition or induction of CYP enzymes involved in drug metabolism, a number of medications can also inhibit or induce influx cell membrane transporters.  Therefore, clinicians need to become aware of the role and degree of influence that each of these systems has when investigating drug interactions.      

The most common transporters identified to date can be broken down into two main types (or super families), those that are ATP binding cassette (ABC) and those that are solute linked carriers/transporters (SLC).  ABC transporters are primary active transporters that use ATP hydrolysis to move substrates across the cell membrane; whereas, SLC transporters are facilitated or ion-coupled secondary active transporters.5,6  The genes that encode each type of transporter is partially identified by the superfamily it comes from.  For example, there are at least 49 genes for the various ABC transporters.2,5  For SLC transporters, there are known to be at least 43 families representing approximately 300 different transporters.  Therefore, each transporter will have its own gene name (see table).2  In addition, each gene may rely on a different transcription factor to turn on gene transcription for production of more transporters.  Recognizing that medications may alter gene transcription by influencing transcription factors will enable clinicians to identify medications that can inhibit or induce the activity of a transporter.  Just as medications can directly influence active CYP enzymes or influence their production to cause a change in another medication's concentration, so can medications influence transporter function.1-4  To complicate things further, some medications affect both metabolic enzymes as well as transporters.  In regard to influx transporters, most are from the SLC superfamily while efflux transporters are more commonly from the ABC superfamily.  Regardless of the type of transporter, they are both generally located in the brain, gastrointestinal tract, liver and kidney. 

Recognizing the influence of influx transporters on the movement of certain medications throughout the body is important for understanding a medication's efficacy, metabolism, elimination and potential for causing or being implicated in drug interactions.

References:

1. United States Food and Drug Administration.  Guidance for Industry.  Drug Interaction Studies - Study Design, Data Analysis, and Implications for Dosing and Labeling.  September 2006. Clinical Pharmacology. Accessed last on 5/19/2009.
2. Giacomini KM, Sugiyama Y.  Membrane transporters and drug response.  In: Goodman & Gilman's The Pharmacologic Basis of Therapeutics.  11thed. Brunton LL, Lazo JS, Parker KL eds.  McGraw-Hill Medical Publishing Division.  New York, NY. 2006;41-70.
3. Kim RB.  Transporters and xenobiotic disposition.  Toxicology 2002;181-182:291-7.
4. Dresser MJ, Leabman MK, Giamcomini KM.  Transporters involved in the elimination of drug in the kidney: organic anion transporters and organic cation transporters.  J Pharm Sci  2001;90:397-421.
5. Borst P, Elferink RO.  Mammalian ABC transporters in health and disease.  Annu Rev Biochem  2002;71:537-92.
6. Hediger MA, Coady MJ, Ikeda TS et al.  Expression cloning and cDNA sequencing of the Na+/glucose co-transporter.  Nature 1987;330:379-81.

Influx Cell Membrane Transporters, Drug Interactions, Drug Transport