click to enlarge Figure 1: Examples of transporter mediated drug-drug interactions observed in humans (All figures from Optivia Biotechnology Inc.).

Membrane transporters have recently become an important part of the submission and approval process of new drugs because they can play a key role in safety by mediating drug-drug interactions (DDIs). In 2010, the U.S. Food and Drug Administration (FDA) communicated guidelines for assessing transporter-related DDIs for all new drugs as recommended by the FDA Advisory Committee on Pharmaceutical Science and Clinical Pharmacology.¹
 
The Advisory Committee vote on March 17, 2010 was the culmination of a process that began in 2006 when transporters were listed along with the cytochrome P450 (CYP) metabolizing enzymes as major DDI players in the FDA’s Draft Guidance on Drug Interaction Studies.² But few details were given on which transporters should be studied or what experiments needed to be done. To answer these questions, the FDA convened a Critical Path Workshop on transporter DDIs in October 2008 with follow-up working groups from the FDA, academia, and the pharmaceutical industry. The final recommendations were published in March 2010 by the International Transporter Consortium (ITC) as a journal article in Nature Reviews – Drug Discovery.³ This article has come to be commonly known as the ITC Transporter White Paper. This paper is intended to be used in conjunction with the Draft Guidance.

Specific guidelines and decision trees are given in the white paper for assessing transporter mediated drug interactions for seven transporters with demonstrated clinical relevance: P-gp, BCRP, OCT2, OAT1, OAT3, OATP1B1, and OATP1B3. Preclinical studies are used to identify potential DDIs with criteria given to trigger a clinical study for confirmation. Information from the clinical transporter DDI studies is used to help design later trials and ultimately for label language for the approved product.

The FDA published a separate commentary piece in the same issue of Nature Reviews – Drug Discovery that was written by Janet Woodcock, MD, the director of the FDA’s Center for Drug Evaluation and Research (CDER), and Shiew-Mei Huang. PhD, who has led the transporter DDI initiative from within CDER.⁴ They highlighted the importance of this new safety test and pointed out that the FDA has already been asking for data from sponsors.

“The FDA has recently asked for post-marketing studies of potential transporter mediated drug–drug interactions … more than a quarter of the clinical pharmacology PMC and PMR reports were related to drug-drug interactions. Of these, many of the more recent requests were related to evaluation of potential transporter-based drug-drug interactions”.⁴

Lei Zhang, PhD, also from CDER, presented experimental designs to the Advisory Committee to help bring together the individual decision trees from the white paper.  During the ensuing discussion, Arthur Harralson, PharmD, Associate Dean in the School of Pharmacy at Shenandoah University said, “I would be shocked if there were many drug companies that were not actually looking at this already. It’s, I think, the tip of the iceberg and we’re going to see a lot more action”.¹ The FDA Advisory Committee approved Zhang’s proposals for the routine assessment of transporter mediated drug interactions for all new drugs.

Inhibition increases exposure

click to enlarge Figure 2: Cellular localization of the seven FDA transporters implicated in clinical drug-drug interactions.

The seven transporters approved by the FDA Advisory Committee for routine drug interaction assessment were chosen because they have been well demonstrated to play a role in drug-drug interactions in humans (Figure 1). All seven transporters serve to protect the body against xenobiotics either by facilitating elimination in the liver (P-gp, BCRP, OATP1B1, and OATP1B3), facilitating elimination in the kidneys (P-gp, OCT2, OAT1, and OAT3) or by limiting absorption in the intestines (P-gp and BCRP) (Figure 2). Drugs that rely on these processes to control their exposure can rise to dangerously high levels if the action of these transporters is inhibited. For instance, when pravastatin, an OATP substrate, is co-administered with cyclosporine, a known inhibitor, the peak concentration of pravastatin is increased by 678% and an 890% increase is seen in the area under the curve (AUC) of the log plot of plasma concentration vs. time.³ This is a significant safety problem since overexposure to statins can cause rhabdomyolysis and even death.

The purpose stated in the white paper for transporter mediated DDI studies is to ensure patient safety for marketed drugs and during larger clinical trials. Two important questions must be answered:

1. Will the sponsor drug become unsafe if another drug inhibits a transporter that limits its exposure levels?

2. Will the sponsor’s drug make other drugs unsafe by inhibiting a transporter they rely on to control their disposition?

The first question is answered by assessing the sponsor drug as a substrate of the transporters and the second as an inhibitor.

Clinical studies provide the final answer on whether a given drug may cause or fall victim to a transporter DDI. Recognizing that clinical trials are costly in both time and money, the FDA allows the use of preclinical data to decide which transporters should be studied in humans (Figure 3). Guidelines are found in the detailed decision trees for the seven transporters in the white paper and the FDA Advisory Committee transcript.

Studies to identify DDI issues

click to enlarge Figure 3: Experimental strategy for transporter mediated drug interaction assessment.

A number of preclinical in vitro transporter models are available including membranes (e.g., Sf9 vesicles from insects), non-polarized cells (e.g., Xenopus laevis oocytes from frogs, CHO cells from Chinese hamster ovaries), sandwich-cultured hepatocytes, and polarized mammalian cells (e.g., MDCK-II cells from dog kidneys and Caco-2 cells from human colorectal adenocarcinomas). The white paper lists limitations of many of the models and cautions that some are more appropriate for discovery activities than development. The draft guidance notes that functional assays in polarized mammalian cells are preferred for P-gp because they have been shown to give fewer false positives and fewer false negatives than other models.2,5

The white paper lists inhibitors and probe substrates that may be used in transporter DDI clinical trials. Sponsors are encouraged to choose drugs likely to be co-administered in their patients. For example, if a cardiovascular drug is a potent in vitro inhibitor of OATP1B1, a statin would be an appropriate clinical substrate.

Conclusion
The FDA has recognized that serious adverse drug reactions, including deaths, have been caused by transporter mediated drug-drug interactions. To ensure public safety, the FDA has begun asking sponsor companies to assess their drugs as substrates and inhibitors of seven key transporters. Guidelines are found in the Draft Guidance on Drug Interaction Studies, the Transporter White Paper published in Nature Reviews – Drug Discovery in March 2010 and in the decision trees recently approved by the FDA Advisory Committee on Pharmaceutical Science and Clinical Pharmacology. Early profiling for transporter DDIs de-risks drugs and will allow companies to bring safer, more successful therapies to patients.

References
1. Meeting Transcript, FDA Advisory Committee for Pharmaceutical Science and Clinical Pharmacology, March 17, 2010, pp. 1-319.
2. FDA, Guidance for Industry: Drug Interaction Studies - Study Design, Data Analysis, and Implications for Dosing and Labeling - DRAFT GUIDANCE, September 2006, pp. 1-2.
3. Huang S-M, Tweedie D, Giacomini K, et al, Nature Reviews - Drug Discovery.2010;9(3):215-236.
4. Huang S-M, Woodcock J Nature Reviews - Drug Discovery.2010;9(3):175 – 176.
5. Polli et al.  JPET. 2001;299(2):620-628.

About the Authors
David Lustig has over 15 years of drug discovery and development experience at pharmaceutical companies. Alan Mills, PhD received his doctorate degree in Organic and Polymer Chemistry from McGill University. His experience includes research at SRI International on drugs in development.