Organic cation transporters in the blood-air barrier: expression and implications for pulmonary drug delivery

Insights into Inhalation Drug Disposition: The Roles of Pulmonary Drug-Metabolizing Enzymes and Transporters

The past five decades have witnessed remarkable advancements in the field of inhaled medicines targeting the lungs for respiratory disease treatment. As a non-invasive drug delivery route, inhalation therapy offers numerous benefits to respiratory patients, including rapid and targeted exposure at specific sites, quick onset of action, bypassing first-pass metabolism, and beyond. Understanding the characteristics of pulmonary drug transporters and metabolizing enzymes is crucial for comprehending efficient drug exposure and clearance processes within the lungs. These processes are intricately linked to both local and systemic pharmacokinetics and pharmacodynamics of drugs. This review aims to provide a comprehensive overview of the literature on lung transporters and metabolizing enzymes while exploring their roles in exogenous and endogenous substance disposition. Additionally, we identify and discuss the principal challenges in this area of research, providing a foundation for future investigations aimed at optimizing inhaled drug administration. Moving forward, it is imperative that future research endeavors to focus on refining and validating in vitro and ex vivo models to more accurately mimic the human respiratory system. Such advancements will enhance our understanding of drug processing in different pathological states and facilitate the discovery of novel approaches for investigating lung-specific drug transporters and metabolizing enzymes. This deeper insight will be crucial in developing more effective and targeted therapies for respiratory diseases, ultimately leading to improved patient outcomes.

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

Developing inhaled drugs for respiratory diseases: A medicinal chemistry perspective

Despite the devastating impact of many lung diseases on human health, there is still a significant unmet medical need in respiratory diseases, for which inhaled delivery represents a crucial strategy. More guidance on how to design and carry out multidisciplinary inhaled projects is needed. When designing inhaled drugs, the medicinal chemist must carefully balance the physicochemical properties of the molecule to achieve optimal target engagement in the lung. Although the medicinal chemistry strategy is unique for each project, and will change depending on multiple factors, such as the disease, target, systemic risk, delivery device, and formulation, general guidelines aiding inhaled drug design can be applied and are summarised in this review.

Lessons from Cancer Metabolism for Pulmonary Arterial Hypertension and Fibrosis

Metabolism is essential for a living organism to sustain life. It provides energy to a cell by breaking down compounds (catabolism) and supplies building blocks for the synthesis of macromolecules (anabolism). Signal transduction pathways tightly regulate mammalian cellular metabolism. Simultaneously, metabolism itself serves as a signaling pathway to control many cellular processes, such as proliferation, differentiation, cell death, gene expression, and adaptation to stress. Considerable progress in the metabolism field has come from understanding how cancer cells co-opt metabolic pathways for growth and survival. Recent data also show that several metabolic pathways may participate in the pathogenesis of lung diseases, some of which could be promising therapeutic targets. In this translational review, we will outline the basic metabolic principles learned from the cancer metabolism field as they apply to the pathogenesis of pulmonary arterial hypertension and fibrosis and will place an emphasis on therapeutic potential.

Organic Cation Transporters in the Lung-Current and Emerging (Patho)Physiological and Pharmacological Concepts

Organic cation transporters (OCT) 1, 2 and 3 and novel organic cation transporters (OCTN) 1 and 2 of the solute carrier 22 (SLC22) family are involved in the cellular transport of endogenous compounds such as neurotransmitters, l-carnitine and ergothioneine. OCT/Ns have also been implicated in the transport of xenobiotics across various biological barriers, for example biguanides and histamine receptor antagonists. In addition, several drugs used in the treatment of respiratory disorders are cations at physiological pH and potential substrates of OCT/Ns. OCT/Ns may also be associated with the development of chronic lung diseases such as allergic asthma and chronic obstructive pulmonary disease (COPD) and, thus, are possible new drug targets. As part of the Special Issue "Physiology, Biochemistry and Pharmacology of Transporters for Organic Cations", this review provides an overview of recent findings on the (patho)physiological and pharmacological functions of organic cation transporters in the lung.

LPS-induced inflammation delays the transportation of ASP+ due to down-regulation of OCTN1/2 in alveolar epithelial cells

Organic cation transporters (OCTNs) can significantly affect drug disposition in alveolar epithelial cells (A549), but this process is not well understood. We investigated the expression and function of OCTN1/2 in A549 cells under different inflammatory status to examine pulmonary drug distribution. This experiment used lipopolysaccharide (LPS)-treated A549 cells to mimic inflammation in alveolar epithelial cells, and the expression of OCTN1/2, interleukin-6 (IL6), IL18, IL1β and tumour necrosis factor-alpha (TNF-α) was investigated by western blot and quantitative real-time PCR (qRT-PCR). The fluorescent compound 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP⁺) was chosen as a probe to study the activity of OCTN1/2. OCTN1/2 down-regulation induced by LPS was more pronounced than that in normal control (NC) groups. Experiments further detected the release of inflammatory factors that revealed a negative correlation between OCTN1/2 expression and inflammation secretion in human alveolar epithelial cells exposed to different concentrations of LPS. The Michaelis constant (K_m) and apparent permeability coefficient (P_app) of ASP⁺ were also decreased significantly. Our results thus show that LPS-induced inflammation could inhibit the expression and activity of OCTN1/2 in vitro and reduce the distribution of inhaled medicine in pulmonary diseases.

OCTN2-Mediated Acetyl-l-Carnitine Transport in Human Pulmonary Epithelial Cells In Vitro

The carnitine transporter OCTN2 is associated with asthma and other inflammatory diseases. The aims of this work were (i) to determine carnitine uptake into freshly isolated human alveolar type I (ATI)-like epithelial cells in primary culture, (ii) to compare the kinetics of carnitine uptake between respiratory epithelial in vitro cell models, and (iii) to establish whether any cell line was a suitable model for studies of carnitine transport at the air-blood barrier. Levels of time-dependent [³H]-acetyl-l-carnitine uptake were similar in ATI-like, NCl-H441, and Calu-3 epithelial cells, whereas uptake into A549 cells was ~5 times higher. Uptake inhibition was more pronounced by OCTN2 modulators, such as l-Carnitine and verapamil, in ATI-like primary epithelial cells compared to NCl-H441 and Calu-3 epithelial cells. Our findings suggest that OCTN2 is involved in the cellular uptake of acetyl-l-carnitine at the alveolar epithelium and that none of the tested cell lines are optimal surrogates for primary cells.

Expression of MATE1, P-gp, OCTN1 and OCTN2, in epithelial and immune cells in the lung of COPD and healthy individuals

Background

Several inhaled drugs are dependent on organic cation transporters to cross cell membranes. To further evaluate their potential to impact on inhaled drug disposition, the localization of MATE1, P-gp, OCTN1 and OCTN2 were investigated in human lung.

Methods

Transporter proteins were analysed by immunohistochemistry in lung tissue from healthy subjects and COPD patients. Transporter mRNA was analysed by qPCR in lung tissue and in bronchoalveolar lavage (BAL) cells from smokers and non-smokers.

Results

We demonstrate for the first time MATE1 protein expression in the lung with localization to the apical side of bronchial and bronchiolar epithelial cells. Interestingly, MATE1 was strongly expressed in alveolar macrophages as demonstrated both in lung tissue and in BAL cells, and in inflammatory cells including CD3 positive T cells. P-gp, OCTN1 and OCTN2 were also expressed in the alveolar epithelial cells and in inflammatory cells including alveolar macrophages. In BAL cells from smokers, MATE1 and P-gp mRNA expression was significantly lower compared to cells from non-smokers whereas no difference was observed between COPD patients and healthy subjects. THP-1 cells were evaluated as a model for alveolar macrophages but did not reflect the transporter expression observed in BAL cells.

Conclusions

We conclude that MATE1, P-gp, OCTN1 and OCTN2 are expressed in pulmonary lung epithelium, in alveolar macrophages and in other inflammatory cells. This is important to consider in the development of drugs treating pulmonary disease as the transporters may impact drug disposition in the lung and consequently affect pharmacological efficacy and toxicity.

Electronic supplementary material

The online version of this article (10.1186/s12931-018-0760-9) contains supplementary material, which is available to authorized users.

Enhanced expression of Organic Cation Transporters in bronchial epithelial cell layers following insults associated with asthma - Impact on salbutamol transport

Increasing evidence suggests Organic Cation Transporters (OCT) might facilitate the absorption of inhaled bronchodilators, including salbutamol, across the lung epithelium. This is essentially scarred and inflamed in asthma. Accordingly, the impact of epithelial insults relevant to asthma on OCT expression and salbutamol transport was evaluated in air-liquid interfaced layers of the human broncho-epithelial cell line Calu-3. These were physically injured and allowed to recover for 48h or exposed to the pro-inflammatory stimulant lipopolysaccharide (LPS) for 48h and the aeroallergen house dust mite (HDM) for 8h twice over 48h. Increases in transporter expression were measured following each treatment, with the protein levels of the OCTN2 subtype consistently raised by at least 50%. Interestingly, OCT upregulation upon LPS and HDM challenges were dependent on an inflammatory event occurring in the cell layers. Salbutamol permeability was higher in LPS exposed layers than in their untreated counterparts and in both cases, was sensitive to the OCT inhibitor tetraethylammonium. This study is the first to show epithelial injury, inflammation and allergen abuse upregulate OCT in bronchial epithelial cells, which might have an impact on the absorption of their substrates in diseased lungs.

Organic cation rhodamines for screening organic cation transporters in early stages of drug development

The aim of this study was to investigate the suitability of rhodamine-123, rhodamine-6G and rhodamine B as non-radioactive probes for characterizing organic cation transporters in respiratory cells. Fluorescent characteristics of the compounds were validated under standard in vitro drug transport conditions (buffers, pH, and light). Uptake/transport kinetics and intracellular accumulation of the compounds were investigated. Uptake/transport mechanisms were investigated by comparing the effect of pH, temperature, concentration, polarity, OCTs/OCTNs inhibitors/substrates, and metabolic inhibitors on the cationic dyes uptake in Calu-3 cells. Fluorescence stability and intensity of the compounds were altered by buffer composition, light, and pH. Uptake of the dyes was concentration-, temperature- and pH-dependent. OCTs/OCTNs inhibitors significantly reduced intracellular accumulation of the compounds. Whereas rhodamine-B uptake was sodium-dependent, pH had no effect on rhodamine-123 and rhodamine-6G uptake. Transport of the dyes across the cells was polarized: (AP→BL>BL→AP transport) and saturable: {V_max=14.08±2.074, K_m=1821±380.4 (rhodamine-B); V_max=6.555±0.4106, K_m=1353±130.4 (rhodamine-123) and V_max=0.3056±0.01402, K_m=702.9±60.97 (rhodamine-6G)}. The dyes were co-localized with MitoTracker®, the mitochondrial marker. Cationic rhodamines, especially rhodamine-B and rhodamine- 6G can be used as organic cation transporter substrates in respiratory cells. During such studies, buffer selection, pH and light exposure should be taken into consideration.

Betulinic Acid: Recent Advances in Chemical Modifications, Effective Delivery, and Molecular Mechanisms of a Promising Anticancer Therapy

An important method of drug discovery is examination of diverse life forms, including medicinal plants and natural products or bioactive compounds isolated from these sources. In cancer research, lead structures of compounds from natural sources can be used to design novel chemotherapies with enhanced biological properties. Betulinic acid (3β-hydroxy-lup-20(29)-en-28-oic acid or BetA) is a naturally occurring pentacyclic triterpene with a wide variety of biological activities, including potent antitumor properties. Non-malignant cells and normal tissues are not affected by BetA. Because BetA exerts its effects directly on the mitochondrion and triggers death of cancerous cells, it is an important alternative when certain chemotherapy drugs fail. Mitochondrion-targeted agents such as BetA hold great promise to circumvent drug resistance in human cancers. BetA is being developed by a large network of clinical trial groups with the support of the U.S. National Cancer Institute. This article discusses recent advances in research into anticancer activity of BetA, relevant modes of delivery, and the agent's therapeutic efficacy, mechanism of action, and future perspective as a pipeline anticancer drug. BetA is a potentially important agent in cancer therapeutics.

Organic cation transporter function in different in vitro models of human lung epithelium

Organic cation transporters (OCT) encoded by members of the solute carrier (SLC) 22 family of genes are involved in the disposition of physiological substrates and xenobiotics, including drugs used in the treatment of chronic obstructive lung diseases and asthma. The aim of this work was to identify continuously growing epithelial cell lines that closely mimic the organic cation transport of freshly isolated human alveolar type I-like epithelial cells (ATI) in primary culture, and which consequently, can be utilised as in vitro models for the study of organic cation transport at the air-blood barrier. OCT activity was investigated by measuring [(14)C]-tetraethylammonium (TEA) uptake into monolayers of Calu-3, NCI-H441 and A549 lung epithelial cell lines in comparison to ATI-like cell monolayers in primary culture. Levels of time-dependent TEA uptake were highest in A549 and ATI-like cells. In A549 cells, TEA uptake had a saturable and a non-saturable component with Km=528.5±373.1μM, Vmax=0.3±0.1nmol/min/mg protein and Kd=0.02μl/min/mg protein. TEA uptake into Calu-3 and NCI-H441 cells did not reach saturation within the concentration range studied. RNAi experiments in A549 cells confirmed that TEA uptake was mainly facilitated by OCT1 and OCT2. Co-incubation studies using pharmacological OCT modulators suggested that organic cation uptake pathways share several similarities between ATI-like primary cells and the NCI-H441 cell line, whereas more pronounced differences exist between primary cells and the A549 and Calu-3 cell lines.

Functional characterization of the organic cation transporters (OCTs) in human airway pulmonary epithelial cells

Organic cation transporters (OCT1-3) mediate the transport of organic cations including inhaled drugs across the cell membrane, although their role in lung epithelium hasn't been well understood yet. We address here the expression and functional activity of OCT1-3 in human airway epithelial cells A549, Calu-3 and NCl-H441. Kinetic and inhibition analyses, employing [(3)H]1-methyl-4-phenylpyridinium (MPP+) as substrate, and the compounds quinidine, prostaglandine E2 (PGE2) and corticosterone as preferential inhibitors of OCT1, OCT2, and OCT3, respectively, have been performed. A549 cells present a robust MPP+ uptake mediated by one high-affinity component (Km~50μM) which is identifiable with OCT3. Corticosterone, indeed, completely inhibits MPP+ transport, while quinidine and PGE2 are inactive and SLC22A3/OCT3 silencing with siRNA markedly lowers MPP+ uptake. Conversely, Calu-3 exhibits both a high (Km<20μM) and a low affinity (Km>0.6mM) transport components, referable to OCT3 and OCT1, respectively, as demonstrated by the inhibition analysis performed at proper substrate concentrations and confirmed by the use of specific siRNA. These transporters are active also when cells are grown under air-liquid interface (ALI) conditions. Only a very modest saturable MPP+ uptake is measurable in NCl-H441 cells and the inhibitory effect of quinidine points to OCT1 as the subtype functionally involved in this model. Finally, the characterization of MPP+ transport in human bronchial BEAS-2B cells suggests that OCT1 and OCT3 are operative. These findings could help to identify in vitro models to be employed for studies concerning the specific involvement of each transporter in drug transportation.

Beta-2 Adrenergic Agonists Are Substrates and Inhibitors of Human Organic Cation Transporter 1

Beta-2-adrenergic agonists are first line therapeutics in the treatment of asthma and chronic obstructive pulmonary disease (COPD). Upon inhalation, bronchodilation is achieved after binding to β2-receptors, which are primarily localized on airway smooth muscle cells. Given that β2-adrenergic agonists chemically are bases, they carry net positive charge at physiologic pH value in the lungs (i.e., pH 7.4). Here, we studied whether β2-agonists interact with organic cation transporters (OCT) and whether this interaction exerted an influence on their passage across the respiratory epithelium to their target receptors. [14C]-TEA uptake into proximal (i.e., Calu-3) and distal (i.e., A549 and NCI-H441) lung epithelial cells was significantly reduced in the presence of salbutamol sulfate, formoterol fumarate, and salmeterol xinafoate in vitro. Expression of all five members of the OCT/N family has been confirmed in human pulmonary epithelial cells in situ and in vitro, which makes the identification of the transporter(s) responsible for the β2-agonist interaction challenging. Thus, additional experiments were carried out in HEK-293 cells transfected with hOCT1-3. The most pronounced inhibition of organic cation uptake by β2-agonists was observed in hOCT1 overexpressing HEK-293 cells. hOCT3 transfected HEK-293 cells were affected to a lesser extent, and in hOCT2 transfectants only marginal inhibition of organic cation uptake by β2-agonists was observed. Bidirectional transport studies across confluent NCI-H441 cell monolayers revealed a net absorptive transport of [3H]-salbutamol, which was sensitive to inhibition by the OCT1 modulator, verapamil. Accordingly, salbutamol uptake into hOCT1 overexpressing HEK-293 cells was time- and concentration-dependent and could be completely blocked by decynium-22. Taken together, our data suggest that β2-agonists are specific substrates and inhibitors of OCT1 in human respiratory epithelial cells and that this transporter might play a role in the pulmonary disposition of drugs of this class.

Current status and future prospects of drug delivery systems

This is an overview of the current drug delivery systems (DDS) starting with various routes of drug administration. Various drug formulations are then described as well as devices used for drug delivery and targeted drug delivery. There has been a considerable increase in the number of new biotechnology-based therapeutics. Most of these are proteins and peptides, and their delivery presents special challenges. Cell and gene therapies are sophisticated methods of delivery of therapeutics. Nanoparticles are considered to be important in refining drug delivery; they can be pharmaceuticals as well as diagnostics. Refinements in drug delivery will facilitate the development of personalized medicine in which targeted drug delivery will play an important role. There is discussion about the ideal DDS, commercial aspects, challenges, and future prospects.

Biopharmaceutical in vitro characterization of CPZEN-45, a drug candidate for inhalation therapy of tuberculosis

BACKGROUND

The caprazamycin derivative, CPZEN-45 has previously demonstrated antitubercular activity against Mycobacterium tuberculosis H37Rv. Here, the authors report a basic biopharmaceutical characterization of the compound focusing on in vitro permeability and cytotoxicity, with respect to the suitability of CPZEN-45 hydrochloride for inhalation treatment of tuberculosis.

RESULTS

MTT assays confirmed that CPZEN-45 HCl had no acute cytotoxic effects up to 3 mg/ml. In transport studies, apparent permeability coefficients of CPZEN-45 HCl across Calu-3 monolayers in absorptive and secretive directions were 0.43 ± 0.20 × 10(-6) cm/s and 0.38 ± 0.12 × 10(-6) cm/s, respectively. Across ATI-like monolayers, apparent permeability values were 12.10 ± 4.31 × 10(-6) cm/s and 8.50 ± 1.83 × 10(-6) cm/s. CPZEN-45 HCl formed colloidal complexes at concentrations above 0.38 mg/ml; however, these complexes were not micelles, as assessed by Orange OT encapsulation assay.

CONCLUSION

CPZEN-45 is an interesting new drug candidate with potential to be used in aerosol therapy of tuberculosis.

Mechanisms of absorption and elimination of drugs administered by inhalation

Pulmonary drug delivery is an effective route for local or systemic drug administration. However, compared with other routes of administration, there is a scarcity of information on how drugs are absorbed from the lung. The different cell composition lining the airways and alveoli makes this task extremely complicated. Lung cell lines and primary culture cells are useful in studying the absorption mechanisms. However, it is imperative that these cell cultures express essential features required to study these mechanisms such as intact tight junctions and transporters. In vivo, the drug has to face defensive physical and immunological barriers such as mucociliary clearance and alveolar macrophages. Knowledge of the physicochemical properties of the drug and aerosol formulation is required. All of these factors interact together leading to either successful drug deposition followed by absorption or drug elimination. These aspects concerning drug transport in the lung are addressed in this review.

Selectivity in the impact of P-glycoprotein upon pulmonary absorption of airway-dosed substrates: a study in ex vivo lung models using chemical inhibition and genetic knockout

P-glycoprotein (P-gp) mediated efflux is recognised to alter the absorption and disposition of a diverse range of substrates. Despite evidence showing the presence of P-gp within the lung, relatively little is known about the transporter's effect upon the absorption and distribution of drugs delivered via the pulmonary route. Here, we present data from an intact isolated rat lung model, alongside two isolated mouse lung models using either chemical or genetic inhibition of P-gp. Data from all three models show inhibition of P-gp increases the extent of absorption of a subset of P-gp substrates (e.g. rhodamine 123 and loperamide) whose physico-chemical properties are distinct from those whose pulmonary absorption remained unaffected (e.g. digoxin and saquinavir). This is the first study showing direct evidence of P-gp mediated efflux within an intact lung, a finding that should warrant consideration as part of respiratory drug discovery and development as well as in the understanding of pulmonary pharmacokinetic (PK)-pharmacodynamic (PD) relationships.

Organic cation transporters in human nasal primary culture: expression and functional activity

BACKGROUND

The majority of drugs cross epithelial cells by either passive diffusion or via carrier-mediated drug transporters. The aim of this study was to investigate the transport characteristics, protein expression and localization of organic cation transporters in human nasal epithelium.

METHODS & RESULTS

The expression, localization and transport characteristics of the transporters were investigated using permeation, PCR and immunohistochemistry. The uptake of 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide followed Michaelis-Menten kinetics. Its intracellular accumulation of the compound was inhibited by organic cation transporters (OCTs) and carnitine/organic cation transporter (OCTNs) inhibitors. Detected OCT1-3, OCTN1 and OCTN2 gene transcripts correlated with immunohistological staining for OCT1-3, OCTN1 and OCTN2 antibodies. Except for OCTN1, the antibodies were generally localized on the apical side of the epithelial cells.

CONCLUSION

Based on the immunohistochemical and uptake/transport studies, we conclude that the human nasal epithelium expresses OCT1-3, OCTN1 and OCTN2 transporters mainly on the apical side of the nasal cells.

The verapamil transporter expressed in human alveolar epithelial cells (A549) does not interact with β2-receptor agonists

  Affinity of different organs for verapamil is highly variable and organ-specific. For example, the drug exhibits high levels of accumulation in lung tissues. A transporter recognising verapamil as a substrate has previously been identified in human retinal pigment epithelial (RPE) and in rat retinal capillary endothelial (TR-iBRB2) cells. This transporter is distinct from any of the cloned organic cation transporters. Therefore, we hypothesised that the verapamil transporter is also functionally expressed in the human respiratory mucosa. Moreover, we tested the hypothesis that this transporter interacts with pulmonary administered cationic drugs such as β2-agonists. The uptake of [(3)H]verapamil was studied in A549 human alveolar epithelial cell monolayers at different times and concentrations. The influence of extracellular proton concentration and various organic cations on verapamil uptake was determined. Verapamil uptake into A549 cells was time- and concentration-dependent, sensitive to pH and had a Km value of 39.8 ± 8.2 µM. Verapamil uptake was also sensitive to inhibition by amantadine, quinidine and pyrilamine, but insensitive to other typical modulators of organic cation and choline transporters. Whilst we demonstrated functional activity of the elusive verapamil transporter at the lung epithelium, our data suggest that this transporter does not interact with β2-agonists at therapeutic concentrations.

Finding novel pharmaceuticals in the systems biology era using multiple effective drug targets, phenotypic screening and knowledge of transporters: where drug discovery went wrong and how to fix it

Despite the sequencing of the human genome, the rate of innovative and successful drug discovery in the pharmaceutical industry has continued to decrease. Leaving aside regulatory matters, the fundamental and interlinked intellectual issues proposed to be largely responsible for this are: (a) the move from 'function-first' to 'target-first' methods of screening and drug discovery; (b) the belief that successful drugs should and do interact solely with single, individual targets, despite natural evolution's selection for biochemical networks that are robust to individual parameter changes; (c) an over-reliance on the rule-of-5 to constrain biophysical and chemical properties of drug libraries; (d) the general abandoning of natural products that do not obey the rule-of-5; (e) an incorrect belief that drugs diffuse passively into (and presumably out of) cells across the bilayers portions of membranes, according to their lipophilicity; (f) a widespread failure to recognize the overwhelmingly important role of proteinaceous transporters, as well as their expression profiles, in determining drug distribution in and between different tissues and individual patients; and (g) the general failure to use engineering principles to model biology in parallel with performing 'wet' experiments, such that 'what if?' experiments can be performed in silico to assess the likely success of any strategy. These facts/ideas are illustrated with a reasonably extensive literature review. Success in turning round drug discovery consequently requires: (a) decent systems biology models of human biochemical networks; (b) the use of these (iteratively with experiments) to model how drugs need to interact with multiple targets to have substantive effects on the phenotype; (c) the adoption of polypharmacology and/or cocktails of drugs as a desirable goal in itself; (d) the incorporation of drug transporters into systems biology models, en route to full and multiscale systems biology models that incorporate drug absorption, distribution, metabolism and excretion; (e) a return to 'function-first' or phenotypic screening; and (f) novel methods for inferring modes of action by measuring the properties on system variables at all levels of the 'omes. Such a strategy offers the opportunity of achieving a state where we can hope to predict biological processes and the effect of pharmaceutical agents upon them. Consequently, this should both lower attrition rates and raise the rates of discovery of effective drugs substantially.

The promiscuous binding of pharmaceutical drugs and their transporter-mediated uptake into cells: what we (need to) know and how we can do so

A recent paper in this journal sought to counter evidence for the role of transport proteins in effecting drug uptake into cells, and questions that transporters can recognize drug molecules in addition to their endogenous substrates. However, there is abundant evidence that both drugs and proteins are highly promiscuous. Most proteins bind to many drugs and most drugs bind to multiple proteins (on average more than six), including transporters (mutations in these can determine resistance); most drugs are known to recognise at least one transporter. In this response, we alert readers to the relevant evidence that exists or is required. This needs to be acquired in cells that contain the relevant proteins, and we highlight an experimental system for simultaneous genome-wide assessment of carrier-mediated uptake in a eukaryotic cell (yeast).

Characterization of Calu-3 cell monolayers as a model of bronchial epithelial transport: organic cation interaction studies

BACKGROUND

To fully exploit organic cation transporters for targeted drug delivery in the lung, the use of a readily available and well-characterized tissue culture model and cheap easily detectable substrates is indispensable.

OBJECTIVES

To investigate the suitability of Calu-3 as tissue model for characterizing organic cation permeation across the bronchial cells using a fluorescent dye, 4-(4-(Dimethylamino)styryl)-N-methylpyridinium iodide (4-DI-1-ASP).

METHODS

Substrate uptake, inhibition, and transport were performed to establish active transport mechanism. Organic cation transporter expression was determined with quantitative polymerase chain reaction (qPCR), immune-histochemistry, and fluorescent microscopy.

RESULTS

4-Di-1-ASP uptake in Calu-3 cells was concentration (K(m) = 2.7 ± 0.3 mM, V(max) = 4.6 ± 2.6 nmol/µg protein/30 min), temperature (uptake at 37°C>>4°C), and pH dependent (higher uptake at pH ≥ 7). L-carnitine, verapamil, and corticosterone significantly inhibited its uptake with IC(50) of 28.2, 0.81, and 0.12 mM, respectively. Transport of the dye across the cells was polarized (AP→BL transport was 2.5-fold > BL→AP), saturable (Km = 43.9 ± 3.2) (µM; Vmax =0.0228 ± nmol/cm(2)/sec) and reduced 3-fold by metabolic inhibition. The expression pattern of the organic cation transporters (OCT) and carnitine/organic cation transporter (OCTN) isoforms was: OCT1<<OCT3 <OCTN1<OCTN2; OCT2 was not detected.

CONCLUSIONS

Based on qPCR, immunohistochemistry, uptake and transport data, the Calu-3 cells can be used as a model for not only studying strategies for optimizing the effect of inhaled organic cations, but also for cross-validating newly-developed respiratory cell lines.