Solute absorption from the airways of the isolated rat lung. IV. Mechanisms of absorption of fluorophore-labeled poly-alpha,beta-[N(2-hydroxyethyl)-DL-aspartamide]

Contemporary Formulation Development for Inhaled Pharmaceuticals

Pulmonary delivery has gained increased interests over the past few decades. For respiratory conditions, targeted drug delivery directly to the site of action can achieve a high local concentration for efficacy with reduced systemic exposure and adverse effects. For systemic conditions, the unique physiology of the lung evolutionarily designed for rapid gaseous exchange presents an entry route for systemic drug delivery. Although the development of inhaled formulations has come a long way over the last few decades, many aspects of it remain to be elucidated. In particular, a reliable and well-understood method for in vitro-in vivo correlations remains to be established. With the rapid and ongoing advancement of technology, there is much potential to better utilise computational methods including different types of modelling and simulation approaches to support inhaled formulation development. This review intends to provide an introduction on some fundamental concepts in pulmonary drug delivery and inhaled formulation development followed by discussions on some challenges and opportunities in the translation of inhaled pharmaceuticals from preclinical studies to clinical development. The review concludes with some recent advancements in modelling and simulation approaches that could play an increasingly important role in modern formulation development of inhaled pharmaceuticals.

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.

In vivo, in vitro and ex vivo models to assess pulmonary absorption and disposition of inhaled therapeutics for systemic delivery

Despite the interest in systemic delivery of therapeutic molecules including macromolecular proteins and peptides via the lung, the accurate assessment of their pulmonary biopharmaceutics is a challenging experimental task. This article reviews in vivo, in vitro and ex vivo models currently available for studying lung absorption and disposition for inhaled therapeutic molecules. The general methodologies are discussed with recent advances, current challenges and perspectives, especially in the context of their use in systemic pulmonary delivery research. In vivo approaches in small rodents continue to be the mainstay of assessment by virtue of the acquisition of direct pharmacokinetic data, more meaningful when attention is given to reproducible dosing and control of lung-regional distribution through use of more sophisticated lung-dosing methods, such as forced instillation, microspray, nebulization and aerosol puff. A variety of in vitro lung epithelial cell lines models and primary cultured alveolar epithelial (AE) cells when grown to monolayer status offer new opportunity to clarify the more detailed kinetics and mechanisms of transepithelial drug transport. While continuous cell lines, Calu-3 and 16HBE14o-, show potential, primary cultured AE cell models from rat and human origins may be of greater use, by virtue of their universally tight intercellular junctions that discriminate the transport kinetics of different therapeutic entities. Nevertheless, the relevance of using these reconstructed barriers to represent complex disposition of intact lung may still be debatable. Meanwhile, the intermediate ex vivo model of the isolated perfused lung (IPL) appears to resolve deficiencies of these in vivo and in vitro models. While controlling lung-regional distributions, the preparation alongside a novel kinetic modeling analysis enables separate determinations of kinetic descriptors for lung absorption and non-absorptive clearances, i.e., mucociliary clearance, phagocytosis and/or metabolism. This ex vivo model has been shown to be kinetically predictive of in vivo, with respect to macromolecular disposition, despite limitations concerning short viable periods of 2-3 h and likely absence of tracheobronchial circulation. Given the advantages and disadvantages of each model, scientists must make appropriate selection and timely exploitation of the best model at each stage of the research and development program, affording efficient progress toward clinical trials for future inhaled therapeutic entities for systemic delivery.

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.

The pharmacokinetics of pulmonary insulin in the in vitro isolated perfused rat lung: Implications of metabolism and regional deposition

The pharmacokinetics of several lung disposition pathways for pulmonary insulin were studied and modeled in the isolated perfused rat lung (IPRL). Insulin solution was administered by forced instillation into the airways of the IPRL as 0.1 or 0.02 ml doses of coarse spray, with or without bacitracin (BAC), N-ethylmaleimide (NEM) and atrial natriuretic peptide (ANP). Each insulin absorption profile was fitted to a kinetic model that incorporated the distribution fraction of the dose reaching the lobar region (DF) and the rate constants for absorption into perfusate (k(a)) and non-absorptive loss (k(nal)); k(nal) was shown to be due to the sum of mucociliary clearance and metabolism. Insulin absorption occurred largely by passive diffusion with values for k(a) = 0.39-0.50 h(-1). With DF = 0.91 following 0.1 ml doses, 11.9 +/- 3.4% of bioavailabilities were observed in 1h. In contrast, derived values for k(nal) = 2.34-3.45 h(-1) were significantly larger than the rate constant for mucociliary clearance determined previously in this IPRL (0.96-1.74 h(-1)) due to lung metabolism. Indeed, BAC, but neither NEM nor ANP, was found to decrease the value of k(nal), which suggested that BAC-inhibitable lung ectopeptidases, and not insulin degrading enzyme (IDE), were responsible for this pulmonary metabolism. Shallower lung distribution with DF = 0.73 following 0.02 ml doses resulted in reduced values for k(a) = 0.27 h(-1) and k(nal) = 2.79 h(-1), indicating that these kinetic processes may be lung-region dependent, even within this model and emphasizing the likely importance of reliable lung deposition in vivo.

Lecithin microemulsions in dimethyl ether and propane for the generation of pharmaceutical aerosols containing polar solutes

Water soluble compounds have been incorporated into solution phase metered dose inhalers (MDIs) utilizing lecithin inverse microemulsions in dimethyl ether (DME) and propane. DME and propane acted as both solvent and propellant. Experiments utilizing model propellants (dimethylethyleneglycol (DMEG) and hexane) were used to investigate microemulsion physicochemical phenomena, and the results were used to design and interpret the technically more challenging MDI experiments. NMR and viscosity experiments with model propellants were consistent with a "sphere-to-string" micellar shape change as the solvent was varied from pure DMEG to pure hexane. Water soluble solutes, including selected peptides and fluorescently labeled poly-alpha, beta-[N-(2-hydroxyethyl) D,L-aspartamide] (fPHEAs), dissolved in DME/propane dependent on lecithin and water content. MDIs containing microemulsions generated aerosols with mass median aerodynamic values ranging from 2.7 to 3.1 microns, within the range of commercially available formulations. Fine particle fraction values (50-70%) exceeded those of commercial formulations. fPHEA up to 18 kDa did not adversely affect the aerosol characteristics. Deposition of the aerosol onto a water surface resulted in the formation of liposomes with partially entrapped solute.

Biochemical evidence for transcytotic absorption of polyaspartamide from the rat lung: effects of temperature and metabolic inhibitors

Airway-to-perfusate polyhydroxyethylaspartamide (PHEA) absorption was studied in the isolated perfused rat lung at a reduced temperature and by the use of metabolic inhibitors, to kinetically clarify the mechanisms and cellular pathways of its active absorption. Fluorophore-labeled PHEA (F-PHEA; 7.4 kDa) was administered into the airways, and its absorption followed with time at 25 degrees C and in the presence of 2,4-dinitrophenol (DNP), ouabain (OUA), monensin (MON), and nocodazole (NOC). Across-dose absorption profiles were analyzed using a kinetic model incorporating active (V(max,P) and K(m,P)) and passive (k(a,P)) absorption from the pulmonary lung region alongside the competing, pulmonary-to-bronchial mucociliary escalator (k(E)). The model was validated at 25 degrees C and a lack of perturbation on the k(a,P) and k(E) values for passively absorbed solutes confirmed by studying the disposition of sodium fluorescein and 4.4 kDa fluorescein isothiocyanate-labeled dextran. F-PHEA absorption was significantly suppressed at 25 degrees C, compared with 37 degrees C, because of a significant decrease in the value of the maximum rate of active absorption, V(max,P) (4.37 --> 0.67 microg/min; p < 0.05), whereas the carrier-affinity term, K(m,P), was statistically unchanged. F-PHEA's active absorption was also significantly inhibited by DNP (> or =0.5 mM), OUA (> or =50 microM), MON (> or =10 microM), and NOC (> or =1 microM), whereas these inhibitors had no significant effect on the values for k(a,P) and k(E). Thus, F-PHEA's pulmonary active absorption in the rat lung was temperature- and adenosine 5'-triphosphate-derived intracellular energy-dependent (DNP and OUA inhibition) and apparently mediated via transcytosis through cytoplasmic endosomes and microtubules (MON and NOC inhibition).

Solute disposition in the rat lung in vivo and in vitro: determining regional absorption kinetics in the presence of mucociliary escalator

Solute absorption from the airways was compared and modeled in vivo and in vitro isolated perfused rat lung (IPRL), and its regional kinetic descriptors in the presence of competing mucociliary escalator were estimated. 7.4 kDa fluorophore-labeled polyhydroxyethylaspartamide (F-PHEA), FITC-labeled dextran 40 (FD-4) and sodium fluorescein (F-Na) were used as model solutes. They were reproducibly administered into the airways in a range of doses in vivo and in vitro IPRL, and their initial deposition and subsequent absorption profiles compared. Each of the absorption data was fitted across doses to a kinetic model in which rate constants for Michaelis-Menten-type active (V(max,P) and K(m,P)) and/or first-order passive (k(a,P)) absorption and mucociliary escalator (k(E)) were estimated simultaneously. Statistically indistinguishable initial solute distribution was ensured in vivo and in vitro. The absorption profiles for F-PHEA were kinetically identical in vivo and in vitro, and their modeling analysis revealed the presence of competing, solute-independent pulmonary-to-bronchial mucociliary escalator with a half-life of 28.9 min. F-PHEA's active absorption was found to be 77 times faster than its passive absorption, yet this was present only in the pulmonary region. Passive solute absorption was inversely related to solute molecular weight [F-PHEA < FD-4 < F-Na]. Bronchial absorption was shown for F-Na in vivo and its rate indistinguishable from that from the pulmonary region. Thus, a single kinetic model was developed, enabling regional absorption kinetic analysis both in vivo and in vitro, in the presence of competing, solute-independent mucociliary escalator.

Methods used to assess pulmonary deposition and absorption of drugs

The assessment of pulmonary drug absorption and deposition is becoming increasingly important in drug development. Absorption information can be used to maximize pulmonary selectivity, to screen drug candidates and to help evaluate the bioequivalence of generic inhalation products. Several methods are available to investigate pulmonary drug absorption and deposition, ranging from in vitro experiments to in vivo pharmacokinetic and pharmacodynamic analyses. In combination, these methods can indicate the fate of an inhaled drug.