Drug metabolism in the lungs: opportunities for optimising inhaled medicines

Figures & data

Figure 1. Xenobiotic metabolizing enzymes expressed in human lungs. The most relevant enzymes based on expression/activity super(families) are highlighted in red with isoforms in blue. Enzymes expressed selectively in the lungs compared to other organs are asterisked, the enzymes that are most well established to be important to drug metabolism in the lung are underlined, italicized enzymes indicate that there is conflicting evidence for functional expression. CYP (Cytochromes P450), AHH (Aryl hydrocarbon hydroxylase), CES (Carboxyl esterase), FMO (Flavin-dependent monooxygenase), ADH (Alcohol dehydrogenase), ALDH (Aldehyde dehydrogenase), NQO (NAD(P)H quinone oxidoreductase), AKR (Aldo-keto reductase), EPHX (Epoxide hydrolase), GST (Glutathione-S-transferase), NAT (N-Acetyl transferase), SULT (Sulfotransferase) and UGT (UDP-glucuronosyl transferase)

Figure 2. The different cell types of the lungs. Cells with significant xenobiotic metabolizing capacity are highlighted in blue

Figure 3. (A) Metabolic pathway of beclomethasone dipropionate (BDP) to active metabolite beclomethasone 17-monopropionate (17-BMP), along with other metabolites assumed to have lower activity to the glucocorticoid receptor. Metabolism of parent compound is mediated by esterase and CYP3A enzymes within the lung epithelium. (B) Metabolic pathway of ciclesonide to active metabolite desisobutyryl-ciclesonide, along with the inactive fatty acid conjugate

Figure 4. Remdesivir and GS-44,152 metabolism to nucleoside triphosphate with antiviral properties as a potential treatment for COVID-19

Table 1. Advantages and disadvantages of different methods used to measure xenobiotic metabolism in the lungs. The models are listed in order of decreasing complexity and ability to replicate the xenobiotic exposure to biotransforming enzymes that would be produced by inhalation

Model	Advantages	Disadvantages
Isolated Perfused Lungs	Closest model to in vivo conditions. Allows drug to be administered as an aerosol, mimicking inhalation exposure. Tissue structure and metabolically competent cells are in their native structure. Potential to simultaneously obtain pharmacokinetic data.	Mostly relies on animal models (typically rat). Rat lung metabolism is typically higher than human and may overestimate metabolite formation. Many differences in rodent and human enzyme expression exist. Not amenable to high throughput assays. When used solely for the purpose of metabolite identification, metabolites may be missed especially if the parent drug is highly permeable with low lung retention.
Precision cut lung slices	Human lung tissue can be used. Native and intricate 3D architecture and cellularity is obtained with enzyme expression similar to in vivo. Rodent lung slices can be easily obtained.	Human lung slices are expensive. Tissue viability limits the time for experimentation as there are changes in enzyme expression over time. Donor-to-donor genetic variation. Rat lung metabolism is typically higher than human and may overestimate metabolite formation. Many differences in rodent and human enzyme expression exist.
Lung explants	Human lung tissue can be used. Experimentally less challenging to obtain than precision cut lung slices.	Similar disadvantages to lung slices, plus extracellular matrix are not maintained as with precision cut lung slices. Tissue viability limits the time for experimentation as there are changes in enzyme expression over time. Donor-to-donor genetic variation.
Primary cells (bronchial or alveolar)	Closer to in vivo enzyme expression than cell lines. Can include a mixture of relevant cell types, or a purified monoculture. Increased throughput in comparison to ex vivo models.	Expensive. Limited time to maintain culture and changes in enzyme expression over time. Donor-to-donor genetic variation.
Cell lines	Easier to maintain over time than primary cell models. Easier to obtain. Reduced genetic variation compared to primary models. Potential to include cocultures. Amenable to high throughput assays. The addition of cofactors are not needed.	Currently, there is a lack of cell lines that model the club cell phenotype (a cell type chiefly responsible for metabolism in the bronchial epithelium). Enzyme expression may reflect only a few cell types rather than the variety found in vivo within the lungs. Enzyme expression is often reduced compared to primary cells.
Lung cell homogenate	Amenable to high throughput assays. Very easy to obtain from both primary tissue or cell lines. Enzyme metabolism is not slowed by the need for the substrate to permeate intracellularly.	Activity may be lower than within whole cells or purified subcellular fractions. May overpredict clearance rates, due to increased availability of substrate/enzyme, compared to with whole cells in which the substrate must diffuse or be transported in. If obtained from whole tissue, activity may appear low due to dilution of metabolically active epithelial cells with other cell types within the lungs. May require the addition of cofactors (e.g. NADPH, UDPGA, PAPS).
S9 fraction	A widely used model for hepatic metabolism studies in vitro. Amenable to high throughput assays. Includes both microsomal and cytosolic enzymes (Phase I and Phase II).	May require the addition of cofactors (e.g. NADPH, UDPGA, PAPS). May overpredict clearance rates, due to increased availability of substrate/enzyme, compared to with whole cells in which the substrate must diffuse or be transported in. If obtained from whole tissue, activity may appear low due to dilution of metabolically active epithelial cells with other cell types within the lungs.
Microsomes	Amenable to high throughput assays. Allows a focus on endoplasmic reticulum bound enzymes (e.g. CYP, UGT). Metabolic rate is high compared to other models, due to concentrated enzymes. Can include recombinant enzymes with isolated CYP isoforms.	Excludes cytosolic metabolism, e.g. EST, SULT, NAT, GST. May overpredict clearance rates, due to increased availability of substrate/enzyme, compared to with whole cells in which the substrate must diffuse or be transported in. Requires NADPH cofactor.
Cell cytosol	Amenable to high throughput assays.	Excludes microsomal metabolism. May require the addition of cofactors (e.g. NADPH, UDPGA, PAPS).

Figure 5. Diagram showing potential strategy for optimizing inhaled drug development with regards to lung metabolism