Absorption of ipratropium and L-carnitine into the pulmonary circulation of the ex-vivo rat lung is driven by passive processes rather than active uptake by OCT/OCTN transporters

The organic cation transporters OCT and OCTN have been reported to play a significant role in the cellular uptake of substrates within in vitro lung cells. However, no studies to date have investigated the effect of these transporters upon transepithelial absorption of substrates into the pulmonary circulation. We investigated the contribution of OCT and OCTN transporters to total pulmonary absorption of L-carnitine and the anti-muscarinic drug, ipratropium, across an intact isolated perfused rat lung (IPRL). The results obtained from the IPRL were contrasted with active transport in vitro using three human pulmonary cell lines and primary rat alveolar epithelial cells. Ex-vivo studies showed that OCT/OCTN transporters do not play a role in the overall pulmonary absorption of L-carnitine or ipratropium, as evidenced by the effect of chemical inhibition of these transporters upon pulmonary absorption. In contrast, in-vitro studies showed that OCT/OCTN transporters play a significant role in cellular accumulation of substrates with preferential uptake of ipratropium by OCTs, and of L-carnitine uptake by OCTNs. The results show that in-vitro uptake studies cannot be predictive of airway to blood absorption in-vivo. Nevertheless, localised submucosal pulmonary concentrations of inhaled drugs and their pulmonary pharmacodynamic profiles may be influenced by OCT/OCTN transport activity

Endocytic uptake, transport and macromolecular interactions of anionic PAMAM dendrimers within lung tissue

Purpose: Polyamidoamine (PAMAM) dendrimers are a promising class of nanocarrier with applications in both small and large molecule drug delivery. Here we report a comprehensive evaluation of the uptake and transport pathways that contribute to the lung disposition of dendrimers. Methods: Anionic PAMAM dendrimers and control dextran probes were applied to an isolated perfused rat lung (IPRL) model and lung epithelial monolayers. Endocytosis pathways were examined in primary alveolar epithelial cultures by confocal microscopy. Molecular interactions of dendrimers with protein and lipid lung fluid components were studied using small angle neutron scattering (SANS). Results: Dendrimers were absorbed across the intact lung via a passive, size-dependent transport pathway at rates slower than dextrans of similar molecular sizes. SANS investigations of concentration-dependent PAMAM transport in the IPRL confirmed no aggregation of PAMAMs with either albumin or dipalmitoylphosphatidylcholine lung lining fluid components. Distinct endocytic compartments were identified within primary alveolar epithelial cells and their functionality in the rapid uptake of fluorescent dendrimers and model macromolecular probes was confirmed by co-localisation studies. Conclusions: PAMAM dendrimers display favourable lung biocompatibility but modest lung to blood absorption kinetics. These data support the investigation of dendrimer-based carriers for controlled-release drug delivery to the deep lung

Spatial expression and functionality of drug transporters in the intact lung: Objectives for further research

This commentary provides a background appraising evidence in the intact lung on the spatial expression of drug transporters and, where available, evidence in the intact lung of the impact, or otherwise, that such transporters can have upon pulmonary drug absorption and disposition. Ultimately drug discovery and development scientists will wish to identify in a ‘pulmonary’ context the effect of disease upon transporter function, the potential for drug transporters to contribute to drug–drug interactions and to inter-individual variation in drug handling and response. The rate and extent of lung epithelial permeation of drugs involve an interplay between the dose and the deposition site of drug within the lung and physiological variables operational at the epithelial–luminal interface. Amongst the latter variables is the potential impact of active transporter processes which may well display regio-selective characteristics along the epithelial tract. In pulmonary tissues the spatial pattern of drug transporter expression is generally poorly defined and the functional significance of transporters within the intact lung is explored in only a limited manner. Active transporters in the lung epithelium may affect airway residence times of drug, modulate access of drug to intracellular targets and to submucosal lung tissue, and potentially influence airway to systemic drug absorption profiles. Transporters in the lung tissue may also have the capacity to mediate uptake of drug from the systemic circulation resulting in drug accumulation in the lung. Transporters have physiological roles and new drug candidates while not necessarily serving as transport substrates may modulate transporter activity and hence physiology. The commentary highlights a series of recommendations for further work in pulmonary drug transporter research

Selectivity in the impact of P-glycoprotein upon pulmonary absorption of airway-dosed substrates: A study in ex vivo lung models using chemical inhibition and genetic knockout

P-glycoprotein (P-gp) mediated efflux is recognised to alter the absorption and disposition of a diverse range of substrates. Despite evidence showing the presence of P-gp within the lung, relatively little is known about the transporter's effect upon the absorption and distribution of drugs delivered via the pulmonary route. Here, we present data from an intact isolated rat lung model, alongside two isolated mouse lung models using either chemical or genetic inhibition of P-gp. Data from all three models show inhibition of P-gp increases the extent of absorption of a subset of P-gp substrates (e.g. rhodamine 123 and loperamide) whose physico-chemical properties are distinct from those whose pulmonary absorption remained unaffected (e.g. digoxin and saquinavir). This is the first study showing direct evidence of P-gp mediated efflux within an intact lung, a finding that should warrant consideration as part of respiratory drug discovery and development as well as in the understanding of pulmonary pharmacokinetic (PK)-pharmacodynamic (PD) relationship

An <i>ex Vivo</i> Investigation into the Transurothelial Permeability and Bladder Wall Distribution of the Nonsteroidal Anti-Inflammatory Ketorolac

Transurothelial drug delivery continues to be an attractive treatment option for a range of urological conditions; however, dosing regimens remain largely empirical. Recently, intravesical delivery of the nonsteroidal anti-inflammatory ketorolac has been shown to significantly reduce ureteral stent-related pain. While this latest development provides an opportunity for advancing the management of stent-related pain, clinical translation will undoubtedly require an understanding of the rate and extent of delivery of ketorolac into the bladder wall. Using an <i>ex vivo</i> porcine model, we evaluate the urothelial permeability and bladder wall distribution of ketorolac. The subsequent application of a pharmacokinetic (PK) model enables prediction of concentrations achieved <i>in vivo</i>. Ketorolac was applied to the urothelium and a transurothelial permeability coefficient (<i>K</i>_p) calculated. Relative drug distribution into the bladder wall after 90 min was determined. Ketorolac was able to permeate the urothelium (<i>K</i>_p = 2.63 × 10^–6 cm s^–1), and after 90 min average concentrations of 400, 141 and 21 μg g^–1 were achieved in the urothelium, lamina propria and detrusor respectively. An average concentration of 87 μg g^–1 was achieved across the whole bladder wall. PK simulations (STELLA) were then carried out, using <i>ex vivo</i> values for <i>K</i>_p and muscle/saline partition coefficient (providing an estimation of vascular clearance), to predict 90 min <i>in vivo</i> ketorolac tissue concentrations. When dilution of the drug solution with urine and vascular clearance were taken into account, a reduced ketorolac concentration of 37 μg g^–1 across the whole bladder wall was predicted. These studies reveal crucial information about the urothelium’s permeability to agents such as ketorolac and the concentrations achievable in the bladder wall. It would appear that levels of ketorolac delivered to the bladder wall intravesically would be sufficient to provide an anti-inflammatory effect. The combination of such <i>ex vivo</i> data and PK modeling provides an insight into the likelihood of achieving clinically relevant concentrations of drug following intravesical administration