Does transbilayer diffusion have a role in membrane transport of drugs?

Passive Influx and Ion Trapping Are More Relevant to the Cellular Accumulation of Highly Permeable Low-Molecular-Weight Acidic Drugs than Is Organic Anion Transporter 2

Recently published work suggests that highly permeable low-molecular-weight (LMW) acidic drugs are transported by organic anion transporter 2 (OAT2). However, an asymmetric distribution of ionizable drugs in subcellular organelles where pH gradients are significant may occur in the presence of an inhibitor relative to its absence (e.g., lysosomal trapping). In the present study, OAT2-mediated transport of highly permeable LMW anions could not be demonstrated using OAT2 transfected cells, despite robust transport of the OAT2 substrate penciclovir. Moreover, a rifamycin SV (RifSV)-dependent reduction in the accumulation of highly permeable LMW anions previously observed in hepatocytes could be qualitatively reproduced using HepG2 cells and also in Madin-Darby canine kidney (MDCK) cells, which lack expression of OAT2. Neither HepG2 nor MDCK cells demonstrated meaningful penciclovir transport, nor was the cellular accumulation of the highly permeable LMW anions sensitive to competitive inhibition by the neutral OAT2 substrate penciclovir. Both cell lines, however, demonstrated sensitivity to the mitochondrial uncoupler p-trifluoromethoxy carbonyl cyanide phenyl hydrazone (FCCP) in a manner similar to RifSV. Furthermore, the transepithelial MDCK permeability of the highly permeable LMW anions was measured in the absence and presence of RifSV and FCCP at concentrations that reduced the cellular accumulation of anions. Neither inhibitor, nor the OAT2 inhibitor ketoprofen, reduced the transepithelial flux of the anions as would be anticipated for transported substrate inhibition. The findings presented here are aligned with cellular accumulation of highly permeable LMW anions being significantly determined by ion trapping sensitive to mitochondrial uncoupling, rather than the result of OAT2-mediated transport. SIGNIFICANCE STATEMENT: The manuscript illustrates that passive influx and ion trapping are more relevant to the cellular accumulation of highly permeable low-molecular-weight acidic drugs than is the previously proposed mechanism of OAT2-mediated transport. The outcome illustrated here highlights a rare, and perhaps previously not reported, observation of anionic drug trapping in a compartment sensitive to mitochondrial uncoupling (e.g., the mitochondrial matrix) that may be confused for transporter-mediated uptake.

On the Rational Drug Design for Hypertension through NMR Spectroscopy

Antagonists of the AT1receptor (AT1R) are beneficial molecules that can prevent the peptide hormone angiotensin II from binding and activating the specific receptor causing hypertension in pathological states. This review article summarizes the multifaced applications of solid and liquid state high resolution nuclear magnetic resonance (NMR) spectroscopy in antihypertensive commercial drugs that act as AT1R antagonists. The 3D architecture of these compounds is explored through 2D NOESY spectroscopy and their interactions with micelles and lipid bilayers are described using solid state 13CP/MAS, 31P and 2H static solid state NMR spectroscopy. Due to their hydrophobic character, AT1R antagonists do not exert their optimum profile on the AT1R. Therefore, various vehicles are explored so as to effectively deliver these molecules to the site of action and to enhance their pharmaceutical efficacy. Cyclodextrins and polymers comprise successful examples of effective drug delivery vehicles, widely used for the delivery of hydrophobic drugs to the active site of the receptor. High resolution NMR spectroscopy provides valuable information on the physical-chemical forces that govern these drug:vehicle interactions, knowledge required to get a deeper understanding on the stability of the formed complexes and therefore the appropriateness and usefulness of the drug delivery system. In addition, it provides valuable information on the rational design towards the synthesis of more stable and efficient drug formulations.

Exploring the role of the membrane bilayer in the recognition of candesartan by its GPCR AT1 receptor

Cardiovascular diseases and hypertension in particular are major health risks worldwide and the improvement on their treatment will be beneficial for the human health. AT1R antagonists belong to the sartans family that targets the renin-angiotensin aldosterone system (RAAS) through blocking the hormone angiotensin II to exert its detrimental effects in pathological states. As a consequence, they are beneficial to treat hypertension, diabetes related kidney failure and hyperaemic episodes. Long unbiased Molecular Dynamics (MD) simulations are performed in order to explore candesartan's possible 2D and 3D diffusion mechanisms towards AT1R receptor. 3D diffusion mechanism is referred to the direct binding of the AT1 antagonist candesartan to the AT1R 3D structure (PDB ID: 4YAY). 2D diffusion mechanism involves first, the incorporation of candesartan in the bilayer core and then its localization on the AT1R binding cavity, through a diffusion mechanism. The obtained results indicate that membranes interact significantly with the neutral form of candesartan, which is indeed approaching the receptors' active site through diffusion via the lipids. On the other hand, the deprotonated form of the drug is interacting with AT1R's extracellular loop and fails to enter the membrane, pointing out the importance of the pH microenvironment around the receptor. To validate the calculated diffusion coefficients of the drug in the lipid bilayers 2D DOSY NMR experiments were recorded and they were in good agreement. Information on the impact that has the interaction of candesartan with the membrane is very important for the rationally design and development of potent ARBs. Thus, its conformational features as well as its localization in the membrane core have to be thoroughly explored.

Tuning the predictive capacity of the PAMPA-BBB model

Due to its robustness and versatility, several variations of the blood-brain barrier specific parallel artificial membrane permeability assay (PAMPA-BBB) have been reported in the central nervous system (CNS) drug discovery practice. In this study, the impact of the main assay parameters on the predictive power of PAMPA-BBB was thoroughly investigated with 27, passively BBB-transported drug molecules with in vivo logBB data. The single and combined effects of the following variables were systematically studied and optimized: incubation time and temperature (4 vs. 18h, RT vs. 37°C), type of the read-out (UV-reader vs. HPLC), solvent composition (n-dodecane/n-hexane), lipid concentration (0-10w/v % PBLE), cholesterol content (0-1.66w/v %), and thickness of the lipid membrane, and the DMSO cosolvent content (5-20v/v %), respectively. Based on our results, solvent-driven and lipid-driven mechanisms of diffusion were identified in different assay conditions. Moreover, the analysis of membrane retention (MR%; the mole fraction of solute "lost" to the membrane) data obtained at various membrane compositions (volume of solvent and concentration of phospholipids) revealed the compound-specific nature of this parameter. The optimized conditions for the PAMPA-BBB were the following: 4h incubation at 37°C, detection by HPLC-DAD, iso-pH conditions (pH=7.4) with 5v/v % DMSO content in buffer solutions, and PBLE (10w/v %; without cholesterol) as membrane dissolved in the mixture of n-hexane:n-dodecane 3:1.

Quantifying the impact of transporters on cellular drug permeability

The conventional model of drug permeability has recently been challenged. An alternative model proposes that transporter-mediated flux is the sole mechanism of cellular drug permeation, instead of existing in parallel with passive transmembrane diffusion. We examined a central assumption of this alternative hypothesis; namely, that transporters can give rise to experimental observations that would typically be explained with passive transmembrane diffusion. Using systems-biology simulations based on available transporter kinetics and proteomic expression data, we found that such observations are possible in the absence of transmembrane diffusion, but only under very specific conditions that rarely or never occur for known human drug transporters.

Interactions of anticancer drugs with biomembranes: what can we learn from model membranes?

The interactions of anticancer drugs with cell membranes are of primary importance for drug transport, accumulation and activity. However, these interactions are very difficult to investigate because of the complexity of biological membranes. Lipid model membranes have therefore been built to gain insight into the collective role of lipids in drug-membrane interactions. Membranes can act as a barrier for drug molecules, sequester them or conversely may allow them to freely diffuse, thereby modulating the accumulation of drugs into cells. Lipid membranes also affect the ability of the efflux pump Pgp to bind and efflux anticancer drugs from cells. On the other hand, anticancer drugs can alter the structure and properties of lipid membranes, which are expected to influence the functioning of embedded proteins. The relevance of lipid model membranes to assess interactions between anticancer drugs and biomembranes is evidenced.