In this study, a multiscale, cell-based compartmental model was developed to analyze the transport of cell-permeant, weakly basic or acidic, monoprotic small molecules in the lung. To predict drug absorption, the transcellular diffusion of small molecules across the phospholipid bilayer was modeled with Fick’s and Plank-Nernst’s equations. The ionization state and lipid partitioning of neutral and ionized forms of the molecules was used to compute the free aqueous fraction of concentration for both neutral and ionized form in each subcellular compartment. Based on the concentration gradient of neutral and ionized, free aqueous species, the concentration and amount at different compartments within the lung could be calculated through time, in accordance with the histological and morphological architecture of the airways and alveolar region.
For development of locally-targeted small molecule drugs, a cell-based biophysical model is able to capture how the behavior of small molecules in airways and alveolar region are different. Our results illustrate how such a model can provide quantitative insights about the relationship between the physicochemical properties and absorption and tissue retention in the upper airways vs. alveolar region. The simulations suggest that a molecule is absorbed at a slower rate in the upper airway, and has more tissue retention than in alveolar region. Organelle sequestration also has a far more significant effect on upper airway pharmacokinetics. Therefore, the upper airway appears as the preferred drug targeting site for local (inhaled) drug therapy with monoprotic weakly basic small molecules. Conversely, the alveolar region is a far more challenging site to target with locally-active inhaled, cell-permeant, monoprotic weakly basic molecules, while it would be the preferred site for facilitating their absorption into the systemic circulation.
The predicted absorption kinetics in lung are very rapid compared with that of the GI tract, and were all consistent in time scale with respect to experimentally-measured absorption kinetics in rat lungs. For model validation, the correlation between predicted and reported values for small lipophilic, monobasic or monoacid molecules within the size range of most drugs (i.e 4< Petitjean radius<9) was found to be significant (R2 = 0. 86). For molecules of larger or smaller size, the molecular radius is far more important than logP and pKa in determining absorption kinetics. Only by accounting for radius with a semi-empirical formula, the correlation between predicted and reported values for compounds that included molecules with extreme size range was significant (R2 = 0.87). As a caveat, the correlation (R2) was 0.60 (P = 0.06 (one tail)) if losartan was removed, which points to the need of acquiring more experimental data for furthering model refinement and validation.
Compared to the other organ systems, the effect of active transporters on drug absorption in the lung have not been extensively studied, although there are suggestions that bioavailability of inhaled medications is minimally affected by such transporters (24
). We used talinolol as a model P-gp substrate in simulations to quantify the potential role of transporters on the absT50
using the in vitro
kinetic parameters (i.e. Vmax
). P-gp expression in lung cell lines and in vivo
is lower than in Caco-2 cells with measured Vmax
only around 2 × 10−12
). Therefore, the predicted much larger Vmax
) suggests that efflux by P-gp at the apical side of lung epithelial cells cannot account for talinolol’s slower-than-expected absorption. As an alternative explanation, the absorption of talinolol could be mostly limited by its larger size and low solubility (24
By running simulations with and without mitochondria and lysosomes, the effect of organelle sequestration on small molecule retention and absorption in airways vs. alveoli was also analyzed. The results indicate that organelle sequestration slows down of absorption of monobasic small molecules in the upper airways when pKa > 10.5. This effect is largely due to the membrane potential-dependent uptake of positively-charged, protonated species in the mitochondria. The organelle sequestration effect in alveoli is minimal, due to the larger apical and basolateral membrane surface areas in relation to the mitochondrial surface area of alveolar epithelial and endothelial cells, as well as the absence of interstitial cells. For molecules with pKa < 10.5 the passive diffusion of the neutral species of the molecule does not allow for prolongued retention in mitochondria or lysosomes, with minimal effect on absT50 in both airways and alveoli.
While helping formulate quantitative hypothesis, these results illustrate how a cell-based computational model can help us interpret experimental data on the absorption and retention of small compounds in the lung in the context of the branching structure of the airways, and the cellular organization of the walls of the airways. Presently, the scope of the model is constrained to basic or acidic, monoprotic compounds which are cell permeant, of a limited size range for which the transcellular route is the primary absorption pathway. Admittedly, the uncertainty and inter-individual variability in the estimated or measured parameters for cell types are factors that can affect the accuracy of the model (31
). Nevertheless, the parameter sensitivity (error propagation) analysis indicates that the predictions are robust and error-tolerant within the aforementioned constraints and the range of uncertainty of the estimated model parameters.
While this cell-based, mechanistic model can be further elaborated and improved, one of its important applications may reside in its ability to help design inhaled compounds with optimal physiochemical properties at early stage of drug design, thereby improving drug targeting and delivery in the lung. It can also help make predictions about the bioaccumulation and biodistribution properties of inhaled chemical agents, for toxicity risk assessment. More importantly, because the model incorporates quantitative species-specific information about the anatomy, physiology and histology of the lung, it can be scaled to predict human lung absorption. Its potential to bridge the gap between animal species and humans may be particularly valuable when clinical lung absorption data is scarce.
To summarize, a cell-based biophysical model of drug absorption in the lungs is a computational tool that can provide mechanistic insights about a relatively unexplored site of drug targeting and delivery. As additional small molecule absorption experiments are performed, the model can be further validated, refined and elaborated, to increase its accuracy and extend its domain of applicability. Planned experiments and future development effort will aim at exploring the size dependency of transport behavior, modeling paracellular transport routes of more hydrophilic compounds and macromolecules, probing active transport effects, as well as extending the model to zwitterionic and multivalent molecules.