Regulation of Drug Transport Proteins-From Mechanisms to Clinical Impact: A White Paper on Behalf of the International Transporter Consortium

Abstract

Membrane transport proteins are involved in the absorption, disposition, efficacy, and/or toxicity of many drugs. Numerous mechanisms (e.g., nuclear receptors, epigenetic gene regulation, microRNAs, alternative splicing, post-translational modifications, and trafficking) regulate transport protein levels, localization, and function. Various factors associated with disease, medications, and dietary constituents, for example, may alter the regulation and activity of transport proteins in the intestine, liver, kidneys, brain, lungs, placenta, and other important sites, such as tumor tissue. This white paper reviews key mechanisms and regulatory factors that alter the function of clinically relevant transport proteins involved in drug disposition. Current considerations with in vitro and in vivo models that are used to investigate transporter regulation are discussed, including strengths, limitations, and the inherent challenges in predicting the impact of changes due to regulation of one transporter on compensatory pathways and overall drug disposition. In addition, translation and scaling of in vitro observations to in vivo outcomes are considered. The importance of incorporating altered transporter regulation in modeling and simulation approaches to predict the clinical impact on drug disposition is also discussed. Regulation of transporters is highly complex and, therefore, identification of knowledge gaps will aid in directing future research to expand our understanding of clinically relevant molecular mechanisms of transporter regulation. This information is critical to the development of tools and approaches to improve therapeutic outcomes by predicting more accurately the impact of regulation-mediated changes in transporter function on drug disposition and response.

Figure 1

Mechanisms of transporter regulation. (a) Nuclear receptors (NRs) bind ligands and attach to specific DNA sequences, often dimerized with another NR, to initiate transcription. (b) DNA methylation can decrease gene expression by disturbing the binding of transcription factors or co‐activators. Histone acetylation can unfold chromatin leading to a decrease in the binding affinity between histones and DNA, thereby resulting in an increase in gene expression. (c) microRNAs (miRNAs) are small non‐coding RNAs that can suppress (or potentially activate) translation by binding to 3′‐UTR regions of mRNA or initiate mRNA degradation through perfect complementarity with the mRNA. (d) With alternative splicing, multiple mRNAs can be produced from one gene, which can then result in different proteins. In humans, exon skipping is the most common form of alternative splicing. ORF, open reading frame; UTR, untranslated region.

Figure 2

The turnover model as applied to drug‐drug interactions. The dynamics of transporter regulation via induction/suppression is represented by a turnover model in which the amount of transporter at equilibrium reflects the balance between its rate of synthesis and its rate of degradation, defined by a first‐order rate constant (k_deg). The required perpetrator data are either: (1) maximum fold induction/suppression over vehicle control (Ind_max; Ind_max < 1 indicates suppression, Ind_max > 1 indicates induction); (2) The slope of the fold induction/suppression versus transporter inhibitor concentration ([I]_t) plot when induction/suppression is linear within the range of perpetrator concentrations (Ind_slope in µM⁻¹); or (3) perpetrator concentration that supports half‐maximal induction/suppression (IndC₅₀ in µM) together with Ind_slope. This option will link the perpetrator concentration over a range of concentrations directly to induction/suppression and hence will best translate to changes in protein levels. For the IndC₅₀ determination, the fraction of unbound drug in the in vitro incubation (fu _inc) also should be considered. Required system data (population information) are: baseline protein level (T₀) and k_deg. T_t is the state variable describing the relative change in the transporter level with respect to baseline because of induction/suppression. Without induction/suppression T_t = 1.