Pulmonary Metabolism of Substrates for Key Drug-Metabolizing Enzymes by Human Alveolar Type II Cells, Human and Rat Lung Microsomes, and the Isolated Perfused Rat Lung Model

Abstract

Significant pulmonary metabolism of inhaled drugs could have drug safety implications or influence pharmacological effectiveness. To study this in vitro, lung microsomes or S9 are often employed. Here, we have determined if rat and human lung microsomes are fit for purpose or whether it is better to use specific cells where drug-metabolizing enzymes are concentrated, such as alveolar type II (ATII) cells. Activities for major hepatic and pulmonary human drug-metabolizing enzymes are assessed and the data contextualized towards an in vivo setting using an ex vivo isolated perfused rat lung model. Very low rates of metabolism are observed in incubations with human ATII cells when compared to isolated hepatocytes and fewer of the substrates are found to be metabolized when compared to human lung microsomal incubations. Reactions selective for flavin-containing monooxygenases (FMOs), CYP1B1, CYP2C9, CYP2J2, and CYP3A4 all show significant rates in human lung microsomal incubations, but all activities are higher when rat lung microsomes are used. The work also demonstrates that a lung microsomal intrinsic clearance value towards the lower limit of detection for this parameter (3 µL/min/mg protein) results in a very low level of pulmonary metabolic clearance during the absorption period, for a drug dosed into the lung in vivo.

Drug metabolism in the lungs: opportunities for optimising inhaled medicines

“…large bronchial airways, small bronchial airways and the alveolar region) are likely to interplay with the permeability rates of the parent drug and its metabolites [34]. Lung metabolism related to regional drug deposition when administered by aerosol is particularly important as unlike hepatic metabolism where 80-90% of the organ volume consists of a single cell type responsible for metabolism, there are at least 40 different lung cell types, with varying prevalence and function dependent on lung region [16]. This approach, highlighted in Figure 5, remains a significant challenge and would necessitate combining methodology from existing software, such as those used for inhaled deposition prediction, metabolism prediction and pharmacokinetic prediction of respiratory bioavailability, to allow the prediction and elucidation of the metabolic fate of inhaled drugs.…”

“…While the functional expression of other phase I metabolism enzymes such as FMO have been confirmed in human precision-cut lung slices [15], they have received less attention than CYP enzymes. However, recently Rubin and coworkers showed higher enzyme activity for FMO than CYP enzymes with a small selection of substrates using human lung microsomes [16]. Esterases represent an important class of phase I metabolism enzymes, due to their action on several inhaled pharmaceuticals such as BDP.…”

“…Data from in vitro models may also be compared with ex vivo models, such as isolated perfused lungs, especially to relate the rate of lung metabolism to the intrinsic clearance of inhaled drugs [16]. In all the in vitro methods, enzyme kinetics studies may be performed to give estimates on the rate of metabolism for a particular inhaled drug, which may then be used independently, or in physiologically based pharmacokinetic (PBPK) modeling where the pharmacokinetics of the parent drug and metabolite(s) may be predicted.…”

Predicting systemic and pulmonary tissue barrier concentration of orally inhaled drug products

“…Other potential limitations in our current framework are a lack of device specific effects (such as single actuation content and carrier effects for DPIs and plume geometry and spray pattern for MDIs),[13, 104] a lack of other clearance mechanisms (such as drug phagocytosis by alveolar macrophages and cleared by transport to the lung- draining lymph nodes),[105, 106] and lung region-specific involvement of metabolic and transported enzymes and proteins that may modulate the lung retention and bioavailability of some drugs. [107] Hence, overall it is possible that the lung tissue concentration of inhaled drugs may be overpredicted in absence of these modules in the model framework. A goal in future versions of this model is to resolve these limitations and thereby improve the prediction process.…”

Impact of Nebulization on the Physicochemical Properties of Polymer–Lipid Hybrid Nanoparticles for Pulmonary Drug Delivery

“…The lungs possesses a large surface area exceeding 100 m 2 with an extremely thin alveolar epithelium ranging from 0.2 to 1 µm in thickness, which favors solute exchange and allows faster drug absorption and rapid drug onset following pulmonary administration [3,4]. Moreover, the relatively lower concentrations of drug-metabolizing enzymes in the lungs enhances drug bioavailability [5,6], while the non-invasiveness of the pulmonary route of delivery enhances patient compliance [7].…”