Mitoxantrone permeability in MDCKII cells is influenced by active influx transport

Case Study 8: Status of the Structural Mass Action Kinetic Model of P-gp-Mediated Transport Through Confluent Cell Monolayers

In the first edition of this book, we presented the basics of explicitly incorporating the lipid biochemistry into a confluent cell monolayer transport model and the novel findings of this model up to 2013, including the use of global optimization to fit the elementary rate constants and the efflux active P-glycoprotein (P-gp) membrane concentrations for the transport of four P-gp substrates across MDCKII-hMDR1-NKI confluent cell monolayers. This chapter is an update on that model, which has been focused primarily on discovering how microvilli morphology regulates the efflux active P-gp and the existence of, as yet, unidentified uptake transporters of P-gp substrates in all of the commonly used P-gp expressing cell lines used in the pharmaceutical industry, thereby adding new players to DDI predictions and IVIVE. The structural mass action kinetic model uses the general mass action reactions for P-gp binding and efflux, with the membrane structural parameters for the confluent cell monolayer to predict drug transport over time. Binding of drug to P-gp occurs within the cytosolic monolayer of the apical membrane, according to (a) the molar partition coefficient of the drug to the cytosolic monolayer and (b) the association rate constant, k₁ (M^-1 s^-1), of the drug from the basolateral or apical outer monolayers into the P-gp binding site. Release of substrate from P-gp back into the cytosolic monolayer occurs with a dissociation rate constant k_r (s^-1) or, much less frequently, into the apical aqueous chamber with an efflux rate constant k₂ (s^-1). The model fits the efflux active P-gp concentration, T(0), i.e., the P-gp whose effluxed drug actually reaches the apical aqueous chamber, as opposed to the majority of P-gp whose effluxed drug is reabsorbed back into the same or neighboring microvilli prior to reaching the apical aqueous chamber. Efflux active P-gp largely resides near the tips of the microvilli. We have shown using kinetics and structured illumination microscopy that: (a) efflux active P-gp is controlled by microvilli morphology; (b) there are apical (AT) and basolateral (BT) uptake transporters for P-gp substrates in most, if not all, P-gp expressing cell lines used in the pharmaceutical industry, which exist, but which remain unidentified; (c) the lab-to-lab variability in P-gp IC₅₀ values observed in the P-gp IC₅₀ initiative was due to the conflated inhibition of P-gp and the basolateral digoxin uptake transporters by all 15 P-gp substrates tested in that study; (d) even the IC₅₀ values for P-gp inhibition alone do not obey the Cheng-Prusoff relationship; (e) the fitted elementary rate constants and the molecular dissociation constant Ki for this kinetic model are system independent; and (f) the time dependence of product formation for these confluent cell monolayers is correlated with the P-gp V_max/K_m, when defined by its fitted elementary rate constants and uptake transporter clearances, without any steady-state assumptions.

Investigation of Enantioselective Membrane Permeability of α-Lipoic Acid in Caco-2 and MDCKII Cell

α-Lipoic acid (LA) contains a chiral carbon and exists as two enantiomers (R-α-lipoic acid (RLA) and S-α-lipoic acid (SLA)). We previously demonstrated that oral bioavailability of RLA is better than that of SLA. This difference arose from the fraction absorbed multiplied by gastrointestinal availability (F_a × F_g) and hepatic availability (F_h) in the absorption phase. However, it remains unclear whether F_a and/or F_g are involved in enantioselectivity. In this study, Caco-2 cells and Madin–Darby canine kidney strain II cells were used to assess the enantioselectivity of membrane permeability. LA was actively transported from the apical side to basal side, regardless of the differences in its steric structure. Permeability rates were proportionally increased in the range of 10–250 µg LA/mL, and the permeability coefficient did not differ significantly between enantiomers. Hence, we conclude that enantioselective pharmacokinetics arose from the metabolism (F_h or F_g × F_h), and definitely not from the membrane permeation (F_a) in the absorption phase.

Altered Expression of Transporters, its Potential Mechanisms and Influences in the Liver of Rodent Models Associated with Diabetes Mellitus and Obesity

Diabetes mellitus is becoming an increasingly prevalent disease that concerns patients and healthcare professionals worldwide. Among many anti-diabetic agents in clinical uses, numerous reports are available on their altered pharmacokinetics because of changes in the expression of drug transporters and metabolic enzymes under diabetic states. These changes may affect the safety and efficacy of therapeutic agents and/or drug-drug interaction with co-administered agents. Therefore, the changes in transporter expression should be identified, and the underlying mechanisms should be clarified. This review summarizes the progress of recent studies on the alterations in important uptake and efflux transporters in liver of diabetic animals and their regulatory pathways.

Influence of bioluminescence imaging dynamics by D-luciferin uptake and efflux mechanisms

Bioluminescence imaging (BLI) detects light generated by luciferase-mediated oxidation of substrate and is used widely for evaluating transgene expression in cell-based assays and in vivo in relevant preclinical models. The most commonly used luciferase for in vivo applications is firefly luciferase (fLuc), for which D-luciferin serves as the substrate. We demonstrated previously that the expression of the ABCG2 efflux transporter can significantly reduce BLI signal output and that HhAntag-691 can inhibit the efflux of D-luciferin, thereby enhancing BLI signal. Here we show that an HhAntag-691-sensitive uptake mechanism facilitates the intracellular concentration of D-luciferin and that the BLI dynamics of different cell lines are coregulated by this uptake mechanism in conjunction with ABCG2-mediated efflux. After administration of D-luciferin, the HhAntag-691-sensitive uptake mechanism generates a rapid increase in BLI signal that decreases over time, whereas ABCG2-mediated efflux stably reduces signal output. We implicate SLC22A4 (OCTN1), a member of the organic cation/zwitterion uptake transporter family, as one potential mediator of the HhAntag-691-sensitive D-luciferin uptake. These findings provide insight into mechanisms that contribute to the cellular uptake kinetics and in vivo biodistribution of D-luciferin.

Contribution of tumoral and host solute carriers to clinical drug response

Members of the solute carrier family of transporters are responsible for the cellular uptake of a broad range of endogenous compounds and xenobiotics in multiple tissues. Several of these solute carriers are known to be expressed in cancer cells or cancer cell lines, and decreased cellular uptake of drugs potentially contributes to the development of resistance. As result, the expression levels of these proteins in humans have important consequences for an individual's susceptibility to certain drug-induced side effects, interactions, and treatment efficacy. In this review article, we provide an update of this rapidly emerging field, with specific emphasis on the direct contribution of solute carriers to anticancer drug uptake in tumors, the role of these carriers in regulation of anticancer drug disposition, and recent advances in attempts to evaluate these proteins as therapeutic targets.

Organic cation uptake is enhanced in bcrp1-transfected MDCKII cells

Stably transfected cell models are routinely used to examine drug-transporter interactions. In one such model of bcrp1-transfected MDCKII cells, we observed a significant enhancement of organic cation intracellular accumulation. Therefore, our goal was to further explore the expression and functional consequences of this cation transport system. Transport assays were carried out in wild-type and bcrp1-transfected MDCKII cells to examine uptake of [3H]-prazosin (bcrp1 positive control), [3H]-agmatine, [3H]-TEA, and [14C]-choline. RT-PCR was employed to determine the mRNA levels of bcrp1 and OCT2/OCT3. Western blots were used to evaluate corresponding protein levels. Accumulation studies determined a significant increase in the uptake of the organic cations agmatine, TEA, and choline in bcrp1-transfected cells when compared to wild-type cells. Directional transport of [3H]-agmatine showed a significantly greater apical (A) to basolateral (B) than B-to-A flux in both cell types. In spite of this, the A-to-B flux was significantly lower in bcrp1-transfected cells. RT-PCR revealed 10-fold higher OCT2 mRNA levels in bcrp1-transfected cells, with no changes in OCT3. OCT2 protein expression was approximately 3.5-fold higher in bcrp1-transfected cells. The upregulation of OCT2 in bcrp1-transfected MDCKII cells contributed to a significant enhancement in the uptake of several organic cations. These results are consistent with the endogenous expression of OCT2 in the kidney tubule, and may be related to the expression and function of bcrp1. Our findings illustrate the importance of understanding how endogenous transporters, which may compete for common substrates, may be influenced by the overexpression and enhanced function of recombinant transport systems.

Effects of single and multiple flavonoids on BCRP-mediated accumulation, cytotoxicity and transport of mitoxantrone in vitro

PURPOSE

The objective of our study was to investigate the effect of single and multiple flavonoids on the accumulation and cytotoxicity of mitoxantrone in BCRP-overexpressing breast cancer cells and on the transport of mitoxantrone in BCRP-expressing normal cells.

METHODS

The effect of flavonoids on mitoxantrone accumulation and cytotoxicity was studied in the human breast cancer MCF-7 MX100 cell line. Mitoxantrone transport in the presence of flavonoids was studied in human and murine BCRP-transfected MDCK cell lines, and mitoxantrone concentrations were determined by HPLC.

RESULTS

Our results demonstrated that multiple flavonoid combinations act additively and exhibit strong BCRP inhibition for increasing mitoxantrone accumulation in breast cancer cells. Kaempferide, biochanin A, 5,7-dimethoxyflavone, and 8-methylflavone greatly increased the cytotoxicity of mitoxantrone in BCRP-overexpressing breast cancer cells. Additionally, the basolateral-to-apical membrane-directed transport of mitoxantrone in murine Bcrp1- and human BCRP-expressing MDCK cells, in the presence of 2.5 microM of these flavonoids, was also significantly decreased.

CONCLUSION

The results indicate that flavonoids are potent BCRP inhibitors and that they exert additive effects when used in combination. Flavonoids demonstrate MDR-reversing effects, but also may influence the disposition of mitoxantrone and cause pharmacokinetic interactions.

Identification and functional characterization of breast cancer resistance protein in human bronchial epithelial cells (Calu-3)

Breast cancer resistance protein (BCRP), a 72 kDa protein belongs to the subfamily G of the human ATP-binding cassette transporter superfamily. Overexpression of BCRP was found to play a major role in the development of resistance against various chemotherapeutic agents. BCRP plays an important role in absorption, distribution and elimination of several therapeutic agents. BCRP expression and functional activity across human bronchial epithelium and its impact on pulmonary drug accumulation has not been established. The objective of this study was to identify and characterize the BCRP efflux transporter across human bronchial epithelium. Calu-3, a human bronchial epithelial cell line was employed as a model for this study. Reverse transcription-polymerase chain reaction (RT-PCR), Western blot and immunocytochemical studies were performed to identify and characterize the expression of BCRP. RT-PCR studies detected ABCG2 mRNA levels in Calu-3 cells. A strong band for BCRP with a molecular weight of approximately 72 kDa was observed in Western blot analysis. Immunocytochemical studies confirmed the presence of BCRP on the apical membrane of human bronchial epithelium. Functional activity of BCRP was determined by performing uptake of radioactive substrate [3H]-mitoxantrone in the presence and absence of BCRP inhibitors. Uptake of [3H]-mitoxantrone was elevated significantly in the presence of GF120918 and fumitremorgin C. An increase in the accumulation of Hoechst 33342, a fluorescent dye was also detected in the presence of BCRP inhibitors when compared to control. In summary, this study provides evidence for the presence of an ATP dependent, membrane bound efflux transporter BCRP across human bronchial epithelial cell line, Calu-3.

Kinetic identification of membrane transporters that assist P-glycoprotein-mediated transport of digoxin and loperamide through a confluent monolayer of MDCKII-hMDR1 cells

A robust screen for compound interaction with P-glycoprotein (P-gp) has some obvious requirements, such as a cell line expressing P-gp and a probe substrate that is transported solely by P-gp and passive permeability. It is actually difficult to prove that a particular probe substrate interacts only with P-gp in the chosen cell line. Using a confluent monolayer of MDCKII-hMDR1 cells, we have determined the elementary rate constants for the P-gp efflux of amprenavir, digoxin, loperamide, and quinidine. For amprenavir and quinidine, transport was fitted with just P-gp and passive permeability. For digoxin and loperamide, fitting required a basolateral transporter (p < 0.01), which was inhibited by the P-gp inhibitor N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918). This means that when digoxin is used as a probe substrate and a compound is shown to inhibit digoxin flux, it could be that the inhibition occurs at the basolateral transporter rather than at P-gp. Digoxin basolateral>apical efflux also required an apical importer (p < 0.05). We propose that amprenavir and quinidine are robust probe substrates for assessing P-gp interactions using the MDCKII-hMDR1 confluent cell monolayer. Usage of another cell line, e.g., LLC-hMDR1 or Caco-2, would require the same kinetic validation to ensure that the probe substrate interacts only with P-gp. Attempts to identify the additional digoxin and loperamide transporters using a wide range of substrates/inhibitors of known epithelial transporters (organic cation transporters, organic anion transporters, organic ion-transporting polypeptide, uric acid transporter, or multidrug resistance-associated protein) failed to inhibit the digoxin or loperamide transport through their basolateral transporter.