Solute Carrier Transporters in drug-drug/herb interactions and their molecular regulation in disease

Abstract

Solute Carrier Transporters (SLCs) are a group of influx transporters responsible for the cellular uptake of endogenous substances and exogenous molecules including a number of clinically important drugs. They are widely expressed in epithelium throughout the body, where they play essential roles in determining the disposition and elimination of these substances. They have been recognized as the crucial determinants to drug pharmacokinetics in humans. The dysfunction of SLC transporters contributes to the pathophysiology of several diseases, as well as largely impacts on drug pharmacokinetic performance, which in turn alters drug efficacy and toxicity. SLCs can also mediate the transport of various herbal compounds and dietary substances. Due to the wide substrate specificity of SLC transporters there is a possibility of potential drug-drug, drug-herb or drug-food interactions that might compromise therapeutic outcomes. Therefore, understanding the structure, expression, function and molecular regulation of SLCs is required to optimise therapeutic outcomes and drug development. The studies presented in this thesis were aimed to explore the physiological and pharmacological role of SLCs, understand the mechanisms through which drug-drug/herb interactions may occur and study the potential molecular regulation of the function and expression of SLCs. The inhibitory effects of five clinically relevant alkaloids were investigated using HEK293 cell line over-expressing essential SLCs. The study indicated that, tryptanthrin significantly inhibited the transport activity of OAT3, whereas, chelerythrine acted as a potent inhibitor of OATP1A2, OCT1 and OCT2. The inhibitory effects of these two natural alkaloids suggested the potential interactions of tryptanthrin and chelerythrine with OAT3, OATP1A2 and OCTs drug substrates, leading to altered pharmacokinetic profiles of these drugs. Therefore, precautions are warranted on the coadministration of these compounds with OAT3, OATP1A2 or OCT substrates due to unexpected toxicity or undesirable therapeutic outcomes. In the second study, we reported that Wnt/β-catenin pathway is involved in the molecular regulation of OATP1A2 function and expression. Transporter functional assays showed that OATP1A2 transport activity was impaired when cells were treated with Wnt-specific inhibitors FH535, 21H7; Wnt specific activator HLY78 and Wnt agonist. Treatment with FH535 reduced the transport activity of OATP1A2 by decreasing the apparent affinity of OATP1A2 for its substrate (Km), without changing the Vmax. On the other hand, 21H7, HLY78 and Wnt agonist treatments showed a reduced Vmax of E3S transport by OATP1A2, without affecting the affinity of the transporter for its substrate (Km). Immunoblotting analysis showed that 21H7, HLY78 and Wnt agonist significantly impaired the plasma membrane expression of OATP1A2. In addition, OATP1A2 overexpressing HEK293T cells co-transfected with β-catenin silencing siRNA showed significant reduction in OATP1A2 function as well as cell surface expression. Overall, Wnt modulators down-regulated the function and cell surface expression of OATP1A2, which can potentially impact the disposition of drugs that are transported by this transporter. In the third study we found that Wnt pathway is also involved in the regulation of OATs. Transporter functional assays showed that Wnt inhibitors FH535 and 21H7 reduced the transport activity of OAT1. FH535 also impaired the function of OAT4. The Wnt inhibitors did not affect the total cell and plasma membrane expression of OAT1 and OAT4. The transport function of OAT3 was impaired by Wnt-specific modulators FH535, 21H7 and HLY78. Kinetic analysis suggested that the impaired transport activity of OAT3 following FH535 treatment was due to the decreased Vmax for substrate transport, while the apparent affinity remained unchanged. FH535 treatment decreased the plasma membrane expression of OAT3. Therefore, modulation of OAT function by Wnt pathway might impact the pharmacokinetics of its drug substrates. Finally, an update on the interactions of anticancer drugs with OATPs has been discussed in chapter 5. OATPs play an important role in the disposition of anticancer drugs. Modulation of OATPs function and expression by genetic polymorphism and drug-drug/herb interactions significantly impacts the PK of anticancer drugs. Because of their abundant expression in various tumour cells and their ability to transport numerous anticancer drugs, OATPs can be considered as important therapeutic targets in anticancer drug design. Due to their physiological and pharmacological importance, SLC transporters play a significant role in human health and disease. Further research on SLC transporters is necessary to comprehensively understand their involvement in human diseases and drug development.