So, how do molecules permeate across these membranes? Some molecules move across via passive diffusion, but this is not a main contributor to absorption. More significantly, molecules may be substrates for transporters protein localised in the cell membrane. “We have transporters in many biological barriers that help molecules across, and in some cases, they also recognise therapeutic molecules.” Ehrhardt gave the example of carbohydrates: sugars are hydrophilic molecules which are superficially unable to cross the cell membrane but are nonetheless are absorbed after a meal.
Transport of Inhaled Biologics: Membrane Transporters
One issue with drug uptake in the body is that different drugs may target the same transporter. “Currently, about half of newly FDA-approved molecular entities have patient information warnings about drug-drug interactions caused by transporters,” said Ehrhardt. In these interactions, two drugs will compete for the same transporter, which may impact on their pharmacokinetics, which are governed by drug absorption, distribution, metabolism and excretion.
As Ehrhardt explained, many researchers have not yet focused on transporters in the lungs, making them a relatively novel target in pulmonary drug development. He told the audience that his research had investigated ATP-binding cassette (ABC) transporters. “They have a binding site for ATP, so they can transport a substrate against the concentration gradient,” he said. These transporters efflux molecules out of the cell, which is a deleterious process in the gut as substrates are ‘kicked back out into the lumen.
“If you have this transporter sitting in the luminal side of the cell, it will recycle the drug back into the lung – that could be a potentially interesting mechanism to achieve a higher lung-residence time.”
However, in the lung the story is different. “If you have this transporter sitting in the luminal side of the cell, it will recycle the drug back into the lung,” said Ehrhardt. “That could be a potentially interesting mechanism to achieve a higher lung residence time.” The first transporter investigated for this was P-glycoprotein (P-gp/MDR1), which Ehrhardt introduced as “the best-known efflux transporter we have”.
Drug Transporter Studies
There are particular trends in inhaled drug permeability that can be monitored for, as Ehrhardt explained. “When you do a transport study – which means you grow your cells to a confluent layer on a permeable insert, you measure the bi-directional flux of the drug – if you have a passively-diffusing drug the permeability coefficients in absorptive and secretive directions should be the same.” In scenarios such as these where absorption is much slower than secretion, there is an indication that a luminal efflux transporter is present. In this instance, the transporter acts to hinder absorption.
Ehrhardt then moved on to discussing the results from a mouse model, which investigated the adsorption of P-gp substrates from isolated perfused mouse lungs. In wildtype animals P-gp substrate drugs exhibit higher dwell times. “When you inhibit the transporter genetically or pharmacologically you have a quicker absorption and clearance, which clearly shows that the transporter is involved in pharmacokinetics.”
The other transporter Ehrhardt discussed was MRP1, which is a physiologically-relevant transporter because it excretes many endogenous substrates in addition to phase 2 metabolites from the cell. MRP1 is abundantly expressed at the lung epithelial barrier, where it may influence the pulmonary deposition of inhaled drugs and contribute to variability in the therapeutic response. Observations on lung kinetics in living rats found that the wild-type animals exhibited a much faster clearance of MRP1 substrates from the lung, and when the transporter is attenuated either by knockout or pharmacological inhibition, residence time in the lung is increased.
Future Implications for Inhalation Biopharmaceutical Research
Ehrhardt wrapped up his presentation by discussing the implications for the expression of efflux transporters in human lung epithelial cells. “A number of inhaled drugs have been found to interact with membrane transporters in vitro, ex vivo, and in vivo.” P-gp increases the pulmonary retention time of inhaled drugs, while MRP1 facilitates lung drug clearance, meaning inhaled drugs stay in the lung for longer. Transporters might be novel drug targets in chronic obstructive pulmonary disease (COPD) and other smoke-related diseases, because MRP1 expression is associated with COPD severity.
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This is a pertinent area of research to focus on as similar studies may be applicable in humans to assess the effects of disease, genetic polymorphisms, or concomitant drug intake on pulmonary transporter activity. “I think we should consider these inter-individual variations, because we are not all the same and transporters are differently expressed and active in human beings,” Ehrhardt added. “And we see differences in pharmacokinetics based on that, as well as drug-drug interactions.”
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