Biomimetic artificial membrane permeability assay over Franz cell apparatus using BCS classified drugs

: A major parameter controlling the extent and rate of oral drug absorption is permeability through the lipid bilayer of intestinal epithelial cells. Here, a biomimetic artificial membrane permeability assay (Franz-Bampa) was validated using Franz cells apparatus. Both high and low permeability drugs (metoprolol and mannitol, respectively) were used as external standards. Biomimetic properties of Franz-Bampa were also characterized by electron paramagnetic resonance spectroscopy (EPR). Moreover, the permeation profile for the 14 BCS class I-IV drugs cited in the FDA guidance (including other drugs as acyclovir, cimetidine, diclofenac, ibuprofen, piroxicam, and trimethoprim) were measured across Franz-Bampa. Apparent permeability (Papp) was compared to literature fraction dose absorbed in humans (Fa%). Papp in Caco-2 cells and Corti artificial membrane were likewise compared to Fa% to assess Franz-Bampa performance. Mannitol and metoprolol Papp values across Franz-Bampa were lower (3.20 x 10 -7 and 1.61 x 10 -5 cm/s, respectively) than those obtained across non-impregnated membrane (2.27 x 10 -5 and 2.55 x 10 -5 cm/s, respectively), confirming lipidic barrier resistivity. Performance of the Franz cell permeation apparatus using an artificial membrane showed similar log linear correlation (R2 = 0.664) with Fa%, as seen for Papp in Caco-2 cells (R2 = 0.805). Data support the validation of the Franz-Bampa method for use during drug discovery process.


Introduction
Favorable absorption, distribution, metabolism, and excretion (ADME) of orally administrated drugs are essential for therapeutic activity in vivo. Poor oral bioavailability contributes to a very high failure rate during pre-clinical drug development [1,2]. In this regard, the Biopharmaceutic Classification System (BCS) proposed by Amidon and co-workers [3] have been widely used as an important tool to support early drug development [4][5][6]. For orally administered drugs, gastrointestinal physiology is a key factor impacting on the rate and extent of drug absorption [7]. Transcellular passive diffusion across membranes is the major route and is governed by several molecular properties such as partition and distribution coefficient, as well as molecular weight [8,9] Currently, important tools based on physicochemical properties and in vitro assays are used to predict in vivo gastrointestinal absorption [10]. In vitro methodologies include animal [11,12] or human tissues [13], cultured cells [14,15] and artificial membranes [16][17][18]. Caco-2 cell monolayers in vitro model is thoroughly studied and generally mimics major transport pathways in the gastrointestinal tract [19]. However, this method is limited by long cell growth and differentiation cycles, risks of microbial contamination, and high implementation costs [19][20][21] Cell-free permeation systems using artificial membranes are gaining progressively more interest as an alternative model to cell-based systems that can be simpler, less time consuming, and costeffective [22,23]. Depending on the composition of the barrier, it can be classified as biomimetic barrier which is constructed from (phospho)lipids or, alternatively, from non-biomimetic barrier containing dialysis membrane [24].
Here, a biomimetic artificial membrane permeability assay systems (Franz-Bampa) comprises a microfilter impregnated by a phospholipid solution attempting to simulate gastrointestinal permeation and provide rapid information about passively transported drugs [25,26]. It was prepared according to Corti and co-workers [5], but mounted on a Franz-cell diffusion apparatus with stirring [20]. Therefore, the aim of this study was to validate this Franz-Bampa system by evaluating the correlation between Papp (apparent permeability) from 14 drugs to their fraction of drug absorbed in humans (Fa%).

Impregnation of membrane support
Membranes were impregnated by immersion for 60 min (22±1 o C) with a lipid solution (mixture of phospholipids), as previously reported [5]. Briefly, the lipid phase solution for impregnation was a mixture of 1.7% phospholipids (Lipoid® E 80, Ludwigshafen, Germany), 2.1% cholesterol (Sigma-Aldrich Chemical Co., Milan, Italy) and 96.2% n-octanol (Synth, Diadema -Brazil). Excess lipid was absorbed with cellulose filter paper over 30 min. Next, all impregnated membranes (N=20) were weighed in a microanalytical scale (Mettler Toledo, mod. XPE56DR) and evaluated to check for its accuracy (211.2 mg ± 6.0%). Prior to use, impregnated membranes were protected from moisture atmosphere and, refrigerated (-8 o C, 24 h). Worth mentioning that all membranes were stabilized, previously to use. Stabilization was confirmed by EPR spectra which did not show any signals of physicochemical degradation: none of membranes showed any difference on 14N-hyperfine coupling constant value (14.8 G) demonstrating its stability [27]. EPR signals were compared just after 24h refrigeration and post-run permeability studies as well as after a month of refrigerated storage time (data not shown).

Electronic Paramagnetic Resonance (EPR)
The biomimetic membranes were impregnated, as described above. Spin labeling technique was employed to examine the conformational structure of the membrane using 5-DSA or 16-DSA. EPR was performed using a Bruker ESP 300 spectrometer (Bruker, Rheinstetten, Germany) equipped with an ER 4102 ST resonator. The instrument settings were microwave power of 2 mW; modulation frequency of 100 KHz; modulation amplitude of 1.0 G; magnetic field scan of 100 G; sweep time of 168 s; and a detector time constant of 41 ms. EPR spectral simulations were performed using the NLLS program for an isotropic model. The biomimetic membrane was introduced into flat, quartz EPR cell to perform the EPR measurements at room temperature (~25 o C).

Permeation studies
Permeation studies were performed using a Franz vertical diffusion cell (MicroettePlus, Hanson Research, California, USA). Impregnated artificial membranes (Franz-Bampa) were positioned between upper and lower part of diffusion cells and, the donor (1 mL) and receptor (7 mL) compartments holding phosphate-buffered solution (PBS), pH 7.4 (USP 32). In order to minimize the unstirred water layer (UWL), receptor compartment media was stirred (500 rpm). Temperature was kept constant (37.0 ± 0.5 ºC). Each drug (n=3) was added in the donor compartment at a fixed concentration ( ⁄ =10 mg/mL). Thus, the adjusted dose number (D0*) follow the equation: * = ⁄ where Mo was a fixed dose, Vo is the uptake volume (1 mL) and Cs is the drug solubility and, If D0 is higher than unit, the administered dose can be dissolved in the water volume taken and so, such drugs can be classified as highly soluble [20]. In this work, the dosage strength and volume used were adapted to experimental conditions i.e. 10 mg and 10 mL, respectively. Likewise, Kasim et al (2004) [28] used metoprolol as reference drug to set permeability.
Samples from permeation studies were collected during 12 h (0.25; 0.5; 1.0; 2.0; 3.0; 4.0; 5.0; 6.0; 10.0 and 12.0 h) and analyzed by HPLC (Shimadzu Class VP or Agilent 1220), according to official compendiums (USP 32 or Brazilian Pharmacopeia 4th edition). Sampling volume was immediately replaced with the same volume of fresh PBS prewarmed solution at 37O ± 0.5 ºC. Calibration curves were performed at least at three concentrations levels for each drug tested, in a GLP accredited laboratory (Institute of Pharmaceutical Sciences, Goiânia, Goiás, Brazil). The validated chromatographic conditions used for drug permeability assay are given in Table 1.

Permeability calculations
The diffusion area (A) was calculated from the radius of the Franz cell and was 1.77 cm2. Flux through membrane to receptor compartment (J; μg/cm2/sec) was calculated dividing the amount of drug accumulated in the receptor compartment by A. The Fick´s first law was derived to calculate flux (J) at steady state (Eq. 1): where dQ is the amount of drug across the membrane (in moles), dt the permeation time (in seconds) and A the diffusion area (in cm 2 ). Note that J was obtained from the slope of the curve at steady state from typical mean cumulative concentration-time plots (minimum of triplicates), as further shown (Figs 2-5). Coefficient of variation (CV) of flux for each drug was also measured.
The apparent permeability (Papp) was calculated normalizing the flux (J) over the drug concentration in the donor compartment C0, as described by the following equation (2): This approximation was used in all cases, even when sink conditions do not hold and donor concentrations changes with time, as already described for some experiments [29]. In addition, the following equation was used to account for the fact that in most cases sink conditions were not maintained [30] where Creceiver,t is the drug concentration in the receiver chamber at time t, Qtotal is the total amount of drug in both chambers, Vreceiver and Vdonor are the volumes of each chamber, Creceiver,t−1 is the drug concentration in receiver chamber at previous time, f is the sample replacement dilution factor, S is the surface area of the membrane, Δt is the time interval and Peff is the permeability coefficient. This equation considers a continuous change of the donor and receiver concentrations, and it is valid in either sink or non-sink conditions. The curve-fitting is performed by non-linear regression, by minimization of the Sum of Squared Residuals (SSR), where: Cr,i,obs is the observed receiver concentration at the end of interval i, and Cr,I (tend,i) is the corresponding concentration at the same time calculated according to Eq. 3 [29].
Classification as high permeability was stablished if the calculated permeability (under sink or non-sink conditions was higher than 0.8* Metoprolol Permeability [31] The in vitro permeability (Papp) of each drug studied was compared to in vivo absorption in humans (Fa%), Papp in Corti artificial membrane [16], and Papp in Caco-2 cells.

EPR analysis and membrane stability
The Franz-Bampa was characterized by EPR spectroscopy of lipid spin labels of doxyl class. The spectra showed a movement consistent with lipid bilayer (Fig. 1). Two analogs of stearic acid, 5-DSA and 16-DSA, and two analogs of phosphatidylcholine, 5-PC and 16-PC, having the nitroxide radical positioned at the 5th and 16th carbon atom of the acyl chain, respectively, were used to examine the molecular dynamic at two regions into the bilayer. The EPR spectra of these four spin labels are shown in Fig. 1.
The EPR parameter -isotropic 14 N-hyperfine coupling constant, a0 -increased with increasing dielectric constant (i.e. solvent polarity) in which the nitroxide radical is dissolved. The measured value of 14.8 G is consistent with a spin label in a membrane [32]. The spin labels 5-DSA and 5-PC with the nitroxide moiety in the region near the polar head group of the bilayer showed more restricted rotational motion relative to their positional isomers 16-DSA and 16-PC, in which the nitroxide radical is more deeply inserted in the hydrophobic core. These results indicate the existence of a gradient of flexibility along the acyl chain, with more restricted motion in the polar region. This pattern is consistent with the properties of lipid bilayers from eukaryotic cells. The rotational motion at the polar interface of the membrane was more restricted for the spin label analog of phosphatidylcholine (5-PC) with C of 14.2 x 10 -10 s than for the stearic acid one (5-DSA) whose C was of 8.4 x 10 -10 s (Fig. 1). Membrane barriers from similar models such as PAMPA and PVPA have been proven to be stable in a pH range from 2 to 8 [26]. Here, EPR spectra were also recorded before and after permeation studies to check for the integrity of biomimetic membranes. No leaching of barrierconstituents such as phosphatidylcholine and lipids into the donor compartment could be evidenced as none of membranes showed any difference on 14 N-hyperfine coupling constant value (14.8 G) demonstrating its stability [33]. Likewise, using the same chemical composition as   [5], acidic and basic drugs also showed pH-dependent permeability according to the pH partition theory [27,34]. Accordingly, close Person´s correlation coefficient was seen (r=0,7355) to our data from Franz-Bampa X PAMPA pH 7.4 ( Table 2).
In this regard, pHs of drug solutions were all measured to assure buffer capacity and drug stability. Some authors correlated membrane flux with the fraction absorbed in human, showing that the flux through the egg lecithin / dodecane membrane correlated better than octanol/water logD values with the fraction absorbed in humans [17]. Later, an in-depth investigation of pH impact on drugs will be necessary to evaluate more accurately biomimetic and absorption predictive power of the Franz-Bampa method. Although, studying all those influences were not beyond the scope of this work at the time.

Membrane validation and performance
Studies here deals with a modified PAMPA method over Franz cell apparatus. The biomimetic membrane (Franz-Bampa) has been previously described by Corti and coworkers [5] as a modified version from Kansy et al (1998) [35]. Mannitol and metoprolol were used as a marker for the cutoff point between low and high permeability drugs.
Membrane performance was assessed using 14 representative drugs from all four BCS classes, cited in the FDA BCS guidance [36], except for acyclovir, cimetidine, diclofenac, ibuprofen, piroxicam, and trimethoprim. Class I compounds were caffeine, metoprolol, propranolol and verapamil. Class II compounds were diclofenac, ibuprofen, naproxen and, piroxicam. Class III compounds were atenolol, cimetidine, ranitidine, and trimethoprim. Class IV compounds were acyclovir, furosemide and hydrochlorothiazide.
Cumulative drug transport through Franz-Bampa was plotted over 12 h and the Papp was calculated from the slopes obtained from linear regressions (Fig. 3, Table 2). Of the 21 compounds studied by Corti and coworkers and of the 14 compounds studied here, there were 11 common compounds tested in both studies: acyclovir, atenolol, caffeine, cimetidine, furosemide, hydrochlorothiazide, metoprolol tartrate, naproxen, propranolol, ranitidine, and trimethoprim. For these drugs Caco-2 Papp values were also surveyed from literature and compared here ( Table 2).
For low permeability drugs (BCS III and IV, Table 2), the Papp coefficient found were consistently much lower than high permeability drugs. Franz-Bampa, Caco-2, and Corti membrane provided value ranges of 0.2 -24.6 x10 -6 cm/s, 0.1 -83.0 x10 -6 cm/s, and 3.2 -45.5 x10 -6 cm/s, respectively. Permeability of most drugs tested here showed Papp 1.0 x10 -5 cm/s (Fig 2). Typically, PAMPA methods are affected by high variability and so, data can be somehow noisy for poorly permeable drugs. Variability is also an issue that impacts on permeability for Caco-2 [5] and other in situ [19] and in vivo [24] models. For low permeability drugs (Fa80%), Avdeef and coworkers (2003) [6] measured variability for more than 200 different drugs accounting for more than 600 measurements. Papp values close to 10 x10 -6 cm/s showed variability of around 10%. Such error can increase slightly for higher Papp values, but is larger for Papp 0.1 x10 -6 (60%), with 0.01 x10 -6 values exhibiting variability of 100% or more. Papp BAMPA x D 0    (17), 2  Likewise, permeability of small hydrophilic compounds is frequently underestimated in PAMPA since the membrane has hydrophobic nature besides being a cell-free system [38]. For the FDA-listed drugs, PAMPA Papp displayed values ranging from 0.00 to 2.35×10 −5 cms −1 , indicating it was not sensitive enough to discriminate and rank poorly permeable compounds. Unlike, Franz-Bampa showed values in a wider Papp range of 0.4 -68.1 x10 -6 cm/s. This could be tentatively explained due to hydrophilic nature of membrane support and pH-dependent characteristics of the drugs [22,24,31]. Also, Franz cell stirring clearly reduces the unstirred water layer resistance in the system.
Additionally, variability of Papp values was also addressed by the calculation methods. A more sophisticated analysis by using per Arthursson equation [15] for sink and non-sin conditions as for checking the impact of extracting a permeability coefficient from data that are not at true steady state and thus, possible impacted by dose depletion. Note that for both sink and non-sink equation, Papp values showed a particularly good correlation between them (0.8984). Similarly, Papp values obtained by us showed to be very alike to values calculated according to Arthurson´s non sink equation (Table 2, Figure 4). The reason is that we used the same systematic procedure i.e. the best fit method through the linear portion, to calculate all the slopes characterizing an accurate permeability flow. So that, the impact from dose depletion is considered not above average. As a result, all drugs got the same BCS classification in both methods. In this context, Franz-Bampa profile is mimicking biological permeation in a graphical pattern related to permeation through Caco-2 cells (R 2 = 0.826). Obtained Papp values versus fraction of dose absorbed in humans (Fa%) showed log linear correlation (Fig. 5), as also described by Zhu et al [37] when analyzing permeability performance of 93 commercial drugs as for artificial membranes. As expected, Franz-Bampa also showed a significantly improved log linear correlation (R 2 = 0.6982) when actively transported compounds ranitidine, trimethoprim and verapamil were not incorporated in the regression analysis. Contrasting, Fa% vs Corti membrane correlation was linear (R 2 = 0.904). This difference from Franz-Bampa and Caco-2 reflects the greater passive permeability of tested drugs through Corti membrane, especially for low and moderate permeability drugs, as discussed elsewhere [39].
Currently, a promising biomimetic barrier also adapted to Franz diffusion cells Permeapad™ - [22] was reported for six drugs concurrent to our model (acyclovir, atenolol, caffeine, ibuprofen, and metoprolol). Even if a satisfactorily comparative analysis was not straightforward, BCS classification of most drugs (4 out of 5) showed to be identical with similar Papp rank order (Table 2).  . Actively transported drugs were removed for R 2 calculation. Correlation coefficients from Franz-Bampa and Caco-2 cell membranes were higher (R 2 = 0.698 and R 2 = 0.805, respectively) without actively transported drugs (verapamil, ketoprofen, ranitidine, and trimethoprim). Meanwhile, Corti membrane Papp () correlation to %Fa (R 2 = 0.890) was essentially unchanged.

Conclusions
The Franz-Bampa method provided close permeability pattern to those from Caco-2. Methodologically, the advantages of Franz-Bampa over Caco-2 are the lower costs and simplicity of membrane preparation (e.g. reagents and artificial membrane are commercially available). Furthermore, the method is very versatile as a high-throughput in vitro method to detect and classify compounds absorbed by passive diffusion.
Using metoprolol as a high permeability marker (Papp=1.61 x10 -5 cm/s; Fig. 1), seven drugs were classified as highly permeable (best fit method): atenolol, caffeine, cimetidine, diclofenac, ibuprofen, naproxen, and propranolol (Table 2). Only atenolol and cimetidine were misclassified as highly permeable drugs, relative to their prior literature classification as BCS 3 drugs.
Additionally, ten out of seventeen drugs were classified as low permeability drugs in Franz-Bampa. Nevertheless, only naproxen, piroxicam and verapamil (3 out of 10) had their permeability underestimated according to BCS, as they performed as low permeability drugs instead of BCS2 drugs. This could be tentatively explained by their solubility [1] profile once they are weak acidic/basic drugs and therefore, not very well soluble at the experimental pH 7.4 used here at donor and acceptor chambers. Moreover, verapamil (AT) is a actively transported drug and is classified as BDDCS class 1 drug [1,4].We are aware that the Papp in free-cells membranes can be pH-dependent, especially for weak acids and bases drugs showing pka close to buffer pH of incubation solution [39], such as most of the previously misclassified drugs. Also, permeation of acids is regularly underestimated when measuring the permeation in vitro at pH 7.4 only.
Summing up, a potential limitation of our study is that the Papp have been calculated with an equation in which the underlying assumptions are constant donor concentration and sink conditions. In order to account for that, we also did the calculations to estimate permeability values under nonsink conditions. The obtained values are about the same compared with the true values (i.e assuming donor concentration change and non-sink conditions). Although, the relative estimation error do change across high versus low permeability compounds [29], the practical implications for predicting oral fraction absorbed would only be a "shifted" to the left in the x scale. In the case of a direct correlation with Caco-2 values it would be reflected in a different slope, but it would not change the significance of the regression line. In the case of the use of the apparent permeabilities for classification of compounds, the reference value of metoprolol is also underestimated, so the classification outcome would not be changed [29].
As a final comment, the ability of Franz-Bampa to classify drugs was good and can be potentially challenged at different pH conditions to predict intestinal permeability of drugs showing passive transport. Eventually, the Franz-Bampa cell diffusion can be modulated in lipid composition and may be a suitable alternative for studying other biological barriers such as blood-brain barrier, skin, and mucosal barriers as buccal or nasal. The current dataset adds valuable information for future analysis of drug-molecular interactions at the lipid layer and in silico model development. Additionally, all apparatus and supplies experimentally used on Franz-Bampa are commercially available and affordable to facilitate drug discovery method application.