The absorption and metabolism of xenobiotics in the lung

Harnessing inhaled nanoparticles to overcome the pulmonary barrier for respiratory disease therapy

The lack of effective treatments for pulmonary diseases presents a significant global health burden, primarily due to the challenges posed by the pulmonary barrier that hinders drug delivery to the lungs. Inhaled nanomedicines, with their capacity for localized and precise drug delivery to specific pulmonary pathologies through the respiratory route, hold tremendous promise as a solution to these challenges. Nevertheless, the realization of efficient and safe pulmonary drug delivery remains fraught with multifaceted challenges. This review summarizes the delivery barriers associated with major pulmonary diseases, the physicochemical properties and drug formulations affecting these barriers, and emphasizes the design advantages and functional integration of nanomedicine in overcoming pulmonary barriers for efficient and safe local drug delivery. The review also deliberates on established nanocarriers and explores drug formulation strategies rooted in these nanocarriers, thereby furnishing essential guidance for the rational design and implementation of pulmonary nanotherapeutics. Finally, this review cast a forward-looking perspective, contemplating the clinical prospects and challenges inherent in the application of inhaled nanomedicines for respiratory diseases.

Factors Affecting Drug Exposure after Inhalation

Administration of drugs by inhalation is mainly used to treat lung diseases and is being investigated as a possible route for systemic drug delivery. It offers several benefits, but it is also fraught with many difficulties. The lung is a complex organ with complicated physiology and specific pharmacokinetic processes. Therefore, the exposure and subsequently efficacy of a drug after inhalation is affected by a number of factors. In this review, we summarize the main variables that may affect drug fate after inhalation delivery, such as physicochemical properties of the drug, pulmonary clearance and metabolism, pathophysiological factors and inhalation device. Factors that have impact on pharmacokinetic processes need to be considered during development as their correct setting can lead to new effective inhaled drugs.

Using Polar Ion-Pairs to Control Drug Delivery to the Airways of the Lungs

The rapid absorptive clearance of drugs delivered to the airways of the lungs means that many inhaled medicines have a short duration of action. The aim of this study was to investigate whether forming polar ion-pairs can modify drug absorption to slow down clearance from the airways. Salbutamol was used as a model drug and was formulated as ion-pairs in an aqueous solution with three negatively charged hydrophilic counterions: sulfate (molecular weight (MW) 142), gluconate (MW 218), and phytate (MW 736) (association constants of 1.57, 2.27, and 4.15, respectively) and one negatively charged hydrophobic counterion, octanoate (MW 166) (association constant, 2.56). All of the counterions were well tolerated by Calu-3 human bronchial epithelial cells when screened for toxicity in vitro using conditions that in silico simulations suggested maintain >80% drug-counterion association. The transport of salbutamol ion-pairs with higher polar surface area (PSA), i.e., the sulfate (PSA 52%), gluconate (PSA 50%), and phytate (PSA 79%) ion-pairs, was significantly lower compared to that of the drug alone (PSA 30%, p < 0.05). In contrast, the octanoate ion-pair (PSA 23%) did not significantly alter the salbutamol transport. The transport data for the gluconate ion-pair suggested that the pulmonary absorption half-life of the ion-paired drug would be double that of salbutamol base, and this illustrates the promise of increasing drug polarity using noncovalent complexation as an approach to control drug delivery to the airways of the lungs.

Drug-Induced Lung Toxicity

The number of blood-borne chemotherapeutic agents implicated in drug-induced lung toxicity continues to increase, although problems in detection remain. The initiation of drug-induced lung injury can have an immunologic or nonimmunologic basis. If endothelial cells are injured, interstitial pulmonary edema may result. Regardless of the source of injury, the progression of drug-induced lung toxicity is often quite similar, involving (1) parenchymal damage, (2) recruitment of inflammatory cells, and (3) progression of the inflammatory process. If the inflammatory reponse is sufficiently severe and disperse, increased collagen can be deposited in interstitial and intra-alveolar areas. The resulting attenuation of gas exchange can induce dyspnea and possibly death. Recent research suggests mediation of the fibrogenic process via cytokines such as transforming growth factor-β and tumor necrosis factor. Preliminary results demonstrating amelioration of cytokine mediated lung-induced fibrosis in animal models with appropriate antibodies suggest a possible future modality of therapy. Certain amphiphilic drugs are capable of eliciting a more specific form of lung toxicity. This class of drugs can interfere with phospholipid metabolism in pulmonary macrophages. In these cases, phospholipidosis results from phospholipid accumulation. The physiologic sequelae in human phospholipidosis is still uncertain.

In Vitro Assessment of Uptake and Lysosomal Sequestration of Respiratory Drugs in Alveolar Macrophage Cell Line NR8383

Purpose

To assess accumulation and lysosomal sequestration of 9 drugs used in respiratory indications (plus imipramine as positive control) in the alveolar macrophage (AM) cell line NR8383.

Methods

For all drugs, uptake at 5 μM was investigated at 37 and 4°C to delineate active uptake and passive diffusion processes. Accumulation of basic clarithromycin, formoterol and imipramine was also assessed over 0.1–100 μM concentration range. Lysosomal sequestration was investigated using ammonium chloride (NH₄Cl), monensin and nigericin. Impact of lysosomal sequestration on clarithromycin accumulation kinetics was investigated.

Results

Both cell-to-medium concentration ratio (K_p) and uptake clearance (CL_uptake) ranged > 400-fold for the drugs investigated. The greatest K_p was observed for imipramine (391) and clarithromycin (82), in contrast to no accumulation seen for terbutaline. A concentration-dependent accumulation was evident for the basic drugs investigated. Imipramine and clarithromycin K_p and CL_uptake were reduced by 59–85% in the presence of NH₄Cl and monensin/nigericin, indicating lysosomal accumulation, whereas lysosomal sequestration was not pronounced for the other 8 respiratory drugs. Clarithromycin uptake rate was altered by NH₄Cl, highlighting the impact of subcellular distribution on accumulation kinetics.

Conclusions

This study provides novel evidence of the utility of NR8383 for investigating accumulation and lysosomal sequestration of respiratory drugs in AMs.

Electronic supplementary material

The online version of this article (doi:10.1007/s11095-015-1753-8) contains supplementary material, which is available to authorized users.

A lung-on-a-chip array with an integrated bio-inspired respiration mechanism

We report a lung-on-a-chip array that mimics the pulmonary parenchymal environment, including the thin alveolar barrier and the three-dimensional cyclic strain induced by breathing movements. The micro-diaphragm used to stretch the alveolar barrier is inspired by the in vivo diaphragm, the main muscle responsible for inspiration. The design of this device aims not only at best reproducing the in vivo conditions found in the lung parenchyma but also at making the device robust and its handling easy. An innovative concept, based on the reversible bonding of the device, is presented that enables accurate control of the concentration of cells cultured on the membrane by easily accessing both sides of the membranes. The functionality of the alveolar barrier could be restored by co-culturing epithelial and endothelial cells that form tight monolayers on each side of a thin, porous and stretchable membrane. We showed that cyclic stretch significantly affects the permeability properties of epithelial cell layers. Furthermore, we also demonstrated that the strain influences the metabolic activity and the cytokine secretion of primary human pulmonary alveolar epithelial cells obtained from patients. These results demonstrate the potential of this device and confirm the importance of the mechanical strain induced by breathing in pulmonary research.

Challenges in inhaled product development and opportunities for open innovation

Dosimetry, safety and the efficacy of drugs in the lungs are critical factors in the development of inhaled medicines. This article considers the challenges in each of these areas with reference to current industry practices for developing inhaled products, and suggests collaborative scientific approaches to address these challenges. The portfolio of molecules requiring delivery by inhalation has expanded rapidly to include novel drugs for lung disease, combination therapies, biopharmaceuticals and candidates for systemic delivery via the lung. For these drugs to be developed as inhaled medicines, a better understanding of their fate in the lungs and how this might be modified is required. Harmonized approaches based on 'best practice' are advocated for dosimetry and safety studies; this would provide coherent data to help product developers and regulatory agencies differentiate new inhaled drug products. To date, there are limited reports describing full temporal relationships between pharmacokinetic (PK) and pharmacodynamic (PD) measurements. A better understanding of pulmonary PK and PK/PD relationships would help mitigate the risk of not engaging successfully or persistently with the drug target as well as identifying the potential for drug accumulation in the lung or excessive systemic exposure. Recommendations are made for (i) better industry-academia-regulatory co-operation, (ii) sharing of pre-competitive data, and (iii) open innovation through collaborative research in key topics such as lung deposition, drug solubility and dissolution in lung fluid, adaptive responses in safety studies, biomarker development and validation, the role of transporters in pulmonary drug disposition, target localisation within the lung and the determinants of local efficacy following inhaled drug administration.

Non-invasive systemic drug delivery: developability considerations for alternate routes of administration

Over the past few decades alternate routes of administration have gained significant momentum and attention, to complement approved drug products, or enable those that cannot be delivered by the oral or parenteral route. Intranasal, buccal/sublingual, pulmonary, and transdermal routes being the most promising non-invasive systemic delivery options. Considering alternate routes of administration early in the development process may be useful to enable new molecular entities (NME) that have deficiencies (extensive first-pass metabolism, unfavorable physicochemical properties, gastro-intestinal adverse effects) or suboptimal pharmacokinetic profiles that are identified in preclinical studies. This review article describes the various delivery considerations and extraneous factors in developing a strategy to pursue an alternate route of administration for systemic delivery. The various delivery route options are outlined with their pros and cons; key criteria and physicochemical attributes that would make a drug a suitable candidate are discussed; approaches to assess delivery feasibility, toxicity at the site of delivery, and overall developability potential are described; and lastly, product trends and their disease implications are highlighted to underscore treatment precedence that help to build scientific rationale for the pursuit of an alternate route of administration.

Improvement of pulmonary absorption of insulin and other water-soluble compounds by polyamines in rats

The absorption enhancing effects of polyamines, spermine (SPM), spermidine (SPD) and putrescine (PUT) on the pulmonary absorption of poorly absorbable drugs were studied in rats. Insulin, 5(6)-carboxyfluorescein (CF), and fluorescein isothiocyanate-labeled dextrans (FDs) were chosen as models of poorly absorbable drugs. The absorption of insulin from the lung was enhanced in the presence of SPM and SPD, while PUT had almost no absorption enhancing effect for improving the pulmonary absorption of insulin in rats. SPM also improved the pulmonary absorption of FDs with various molecular weights, although we found almost no significant difference in the pulmonary absorption of CF with or without SPM. As for the pulmonary membrane toxicity of SPM, there was no significant difference in the release of protein and lactate dehydrogenase (LDH) with or without SPM in bronchoalveolar lavage fluid (BALF), indicating that SPM did not cause any membrane damage to the lung tissues. Furthermore, SPM did not affect the stability of insulin in BALF, suggesting that SPM might increase the permeability of insulin across the alveolar epithelium. In conclusion, polyamines, especially SPM can effectively improve the pulmonary absorption of insulin and other macromolecules without any membrane damage to the lung tissues.

Systemic Delivery of Drugs to Humans via Inhalation

Unwanted systemic absorption of drugs delivered for the local treatment of respiratory disease is well documented. Methods to minimize this now exist, especially for reduction of oropharyngeal deposition. While small molecules appear to be absorbed also from the airways, it is the alveolated regions that provide a large absorptive surface. Lung has been used as a portal for systemic delivery of substances such as anesthetics, nicotine and a number of illicit drugs. Much research has lead to the solutions of the fundamental technical hurdles of practicable delivery of systemic therapeutic drugs in milligram quantities to the lung efficiently and reproducibly. Commercial manufacturing processes exist for production of delivery systems suitable for this purpose. Generally, the deposition of small molecules in the "deep lung" leads to high absorption rates, making the inhalation delivery attractive for drugs with intended rapid onset of action. Many therapeutics, especially peptides and proteins, that cannot be delivered systemically non-invasively, are absorbed with various degrees of systemic bioavailability via inhalation. The critical factor for efficient and reproducible systemic delivery is lung deposition which depends on the properties of drug particles (size, shape, density, hygroscopicity, velocity, charge) and the state of the respiratory system (including the individual's anatomy, age, sex, disease, lung volume). While concerns exist about the potential adverse reactions of the immune system to therapeutic proteins and peptides delivered to and through the lung, there is not much data on the immune response or its link to any safety issues with inhaled biologics. Desirable systemic immune effects have been demonstrated by cytokine delivery to the lung.

Cell culture models of the respiratory tract relevant to pulmonary drug delivery

The respiratory tract holds promise as an alternative site of drug delivery due to fast absorption and rapid onset of drug action, with avoidance of hepatic and intestinal first-pass metabolism as an additional benefit compared to oral drug delivery. At present, the pharmaceutical industry increasingly relies on appropriate in vitro models for the faster evaluation of drug absorption and metabolism as an alternative to animal testing. This article reviews the various existing cell culture systems that may be applied as in vitro models of the human air-blood barrier, for instance, in order to enable the screening of large numbers of new drug candidates at low cost with high reliability and within a short time span. Apart from such screening, cell culture-based in vitro systems may also contribute to improve our understanding of the mechanisms of drug transport across such epithelial tissues, and the mechanisms of action how advanced drug carriers, such as nanoparticles or liposomes, can help to overcome these barriers. After all, the increasing use and acceptance of such in vitro models may lead to a significant acceleration of the drug development process by facilitating the progress into clinical studies and product registration.

Drug absorption from the isolated perfused rat lung--correlations with drug physicochemical properties and epithelial permeability

The pulmonary absorption of nine low-molecular-weight (225-430 Da) drugs (atenolol, budesonide, enalaprilat, enalapril, formoterol, losartan, metoprolol, propranolol and terbutaline) and one high-molecular-weight membrane permeability marker compound (FITC-dextran 10000 Da) was investigated using the isolated, perfused and ventilated rat lung (IPL). The relationships between pulmonary transport characteristics, epithelial permeability of Caco-2 cell monolayers and drug physicochemical properties were evaluated using multivariate data analysis. Finally, an in vitro-in vivo correlation was made using in vivo rat lung absorption data. The absorption half-life of the investigated drugs ranged from 2 to 59 min, and the extent of absorption from 21 to 94% in 2 h in the isolated perfused rat lung model. The apparent first-order absorption rate constant in IPL (ka(lung)) was found to correlate to the apparent permeability (P(app)) of Caco-2 cell monolayers (r = 0.87), cLog D(7.4) (r = 0.70), cLog P, and to the molecular polar surface area (%PSA) (r = -0.79) of the drugs. A Partial Least Squares (PLS)-model for prediction of the absorption rate (log ka(lung)) from the descriptors log P(app), %PSA and cLogD(7.4) was found (Q2 = 0.74, R2 = 0.78). Furthermore, a strong in vitro-in vivo correlation (r = 0.98) was found for the in vitro (IPL) drug absorption half-life and the pulmonary absorption half-life obtained in rats in vivo, based on a sub-set of five compounds.

Pharmacokinetics of a benzodithiin (RD3-0028) following aerosol treatment in rat

1. RD3-0028, a benzodithiin compound, has potent antiviral activity against respiratory syncytial virus (RSV) in cell culture. The compound also inhibits growth of RSV and improves pathologic changes of interstitial pneumonia in the immunosuppressed mouse when delivered by small-particle aerosol. 2. In the present study, the absorption, distribution and excretion of 14C-RD3-0028 were compared in rat following either a single aerosol treatment or oral administration. 3. The plasma concentration was maintained at the same level from 5 min to 1 h, and decreased with a half-life of 2.2 +/- 0.1 h for 1-8 h. 4. The excretion of radioactivity in the urine and faeces at 24 h after aerosol treatment was 89.3 and 4.5%, respectively, indicating that almost all the radioactivity was rapidly excreted in the urine. The excretion of total radioactivity was 98.9% within 168 h. 5. The concentrations of radioactivity in the lung and trachea following aerosol treatment were higher than those in other tissues, and were detected even at 72 h. 6. These results suggest that the aerosol treatment might be useful for delivering RD3-0028 to the respiratory tract of RSV-infected patients.

Microparticulate uptake mechanisms of in-vitro cell culture models of the respiratory epithelium

The objective of this study was to examine the uptake mechanisms of fluorescent polystyrene microspheres of various diameters and surface chemistry by two human cell lines derived from the respiratory epithelium, A549 and Calu-3. Briefly, A549 and Calu-3 cells were grown to confluence in 12-well cluster plates and the uptake of fluorescent microspheres by the cells was determined at various time points. The amount of microspheres internalized by the cells was determined by correcting for non-specific binding to the cell surface. The data showed that A549 cells appeared to have more phagocytic activity than Calu-3 cells. Albumin-coated microspheres as large as 3 microm diameter can be internalized by A549 cells. The amount of internalization by A549 cells observed for 0.5-microm diameter albumin-coated microspheres was approximately 10-times greater than that observed for 1-microm diameter spheres and approximately 100-times greater than values observed for 2- and 3-microm diameter beads. Transmission electron micrographs confirmed that the microspheres were internalized by the cells. Uptake experiments conducted with Calu-3 cells indicated that albumin-coated microspheres were neither bound nor internalized by the cells. The effect of microsphere surface chemistry on the uptake mechanism indicated that amidine microspheres were internalized more rapidly and to a greater extent by both A549 and Calu-3 cells than carboxylate microspheres and non-coated microspheres. This phenomenon is thought to be attributed to masking of the negative polystyrene core by the positive amidine functional group; this effect was less marked for the carboxylate microspheres. These results suggest that A549 and Calu-3 cells can internalize microspheres and that size and effective charge played an important role in the uptake process.

Human airway epithelial cell lines for in vitro drug transport and metabolism studies

The pharmaceutical industry relies on appropriate in vitro models for the evaluation of drug absorption and metabolism. Despite increasing interest in drug delivery via the lung, there is currently no widely accepted cell culture model of the airway epithelium. This review considers the airway epithelium, the culture of airway epithelial cells and the need for cell lines which can model the airway epithelium. Three of the most promising human bronchial cell lines, 16HBE14o-, Calu-3 and BEAS-2B, are reviewed, with emphasis on their recent application for the study of drug transport, drug metabolism and gene delivery. Current limitations and future directions for the development of these cell lines as models of the airway epithelium are discussed.

Identification of GS 4104 as an orally bioavailable prodrug of the influenza virus neuraminidase inhibitor GS 4071

GS 4071 is a potent carbocyclic transition-state analog inhibitor of influenza virus neuraminidase with activity against both influenza A and B viruses in vitro. GS 4116, the guanidino analog of GS 4071, is a 10-fold more potent inhibitor of influenza virus replication in tissue culture than GS 4071. In this study we determined the oral bioavailabilities of GS 4071, GS 4116, and their respective ethyl ester prodrugs in rats. Both parent compounds and the prodrug of the guanidino analog exhibited poor oral bioavailability (2 to 4%) and low peak concentrations in plasma (Cmaxs; Cmax <0.06 microg/ml). In contrast, GS 4104, the ethyl ester prodrug of GS 4071, exhibited good oral bioavailability (35%) as GS 4071 and high Cmaxs of GS 4071 (Cmax = 0.47 microg/ml) which are 150 times the concentration necessary to inhibit influenza virus neuraminidase activity by 90%. The bioavailability of GS 4104 as GS 4071 was also determined in mice (30%), ferrets (11%), and dogs (73%). The plasma of all four species exhibited high, sustained concentrations of GS 4071 such that at 12 h postdosing the concentrations of GS 4071 in plasma exceeded those necessary to inhibit influenza virus neuraminidase activity by 90%. These results demonstrate that GS 4104 is an orally bioavailable prodrug of GS 4071 in animals and that it has the potential to be an oral agent for the prevention and treatment of influenza A and B virus infections in humans.

Intratracheal delivery of peptide and protein agents: absorption from solution and dry powder by rat lung

Proteins of high molecular weight and low lipophilicity must be administered parenterally to achieve the desired therapeutic blood levels. We investigated the absorption of peptide and protein agents by rat lung following their intratracheal administration, expressing it as percent bioavailability. An aqueous solution and/or a dry powder of calcitonin, insulin, thyrotropin stimulating hormone (TSH), follicle stimulating hormone (FSH), and human chorionic gonadotropin (HCG) was delivered into the exposed trachea of anesthetized rats, and blood was sampled from the jugular vein at specified intervals. The bioavailabilities of TSH, FSH, and HCG delivered in a solution of neutral pH were 2.5, 2.3, and 0.2 %, respectively. Transpulmonary absorption of a solution of these agents, administered with a surfactant or under acidic conditions, was 2-30 times greater than the values obtained in controls. The bioavailabilities of calcitonin, insulin, TSH, FSH, and HCG, given intratracheally as a dry powder, were 11.5, 6.5, 1.6, 0.6, and 0.1%, respectively. Following intratracheal administration, we noted a negative association between molecular weight and bioavailability. The intratracheal route may thus be useful for delivering peptide and protein agents.