The solubility-permeability interplay when using cosolvents for solubilization: revising the way we use solubility-enabling formulations

Active intestinal drug absorption and the solubility-permeability interplay

The solubility-permeability interplay deals with the question: what is the concomitant effect on the drug's apparent permeability when increasing the apparent solubility with a solubility-enabling formulation? The solubility and the permeability are closely related, exhibit certain interplay between them, and ongoing research throughout the past decade shows that treating the one irrespectively of the other may be insufficient. The aim of this article is to provide an overview of the current knowledge on the solubility-permeability interplay when using solubility-enabling formulations for oral lipophilic drugs, highlighting active permeability aspects. A solubility-enabling formulation may affect the permeability in opposite directions; the passive permeability may decrease as a result of the apparent solubility increase, according to the solubility-permeability tradeoff, but at the same time, certain components of the formulation may inhibit/saturate efflux transporters (when relevant), resulting in significant apparent permeability increase. In these cases, excipients with both solubilizing and e.g. P-gp inhibitory properties may lead to concomitant increase of both the solubility and the permeability. Intelligent development of such formulation will account for the simultaneous effects of the excipients' nature/concentrations on the two arms composing the overall permeability: the passive and the active arms. Overall, thorough mechanistic understanding of the various factors involved in the solubility-permeability interplay may allow developing better solubility-enabling formulations, thereby exploiting the advantages analyzed in this article, offering oral delivery solution even for BCS class IV drugs.

Evaluating the Potential for Delivery of Irinotecan via the Buccal Route: Physicochemical Characterization and In Vitro Permeation Assessment Across Porcine Buccal Mucosa

Irinotecan (CPT-11) is used to treat advanced colorectal cancer as an intravenous therapy. Depending on pH, CPT-11 exists in either a lactone (active) or carboxylate (inactive) form, or both. In this investigation, the feasibility for systemic delivery of CPT-11 through the buccal route was evaluated. Permeation of CPT-11 across porcine buccal mucosa was studied in vitro using side-by-side flow through diffusion cells at 37°C. Experiments were performed over a pH range from 4 to 9, and the permeability of both the lactone and carboxylate forms of CPT-11 was measured. CPT-11 steady state flux was determined over a range of donor concentrations at pH 4 (0.5, 1, 5, 10, 15, 20 mg/ml) and pH 6.8 (0.5, 5, 10 mg/ml). Steady state flux increased linearly with increasing donor concentration of CPT-11 at pH 4 (r ² = 0.9935) and at pH 6.8 (r ² = 0.9886). CPT-11 permeability was independent of pH, although the distribution coefficient increased with increasing pH. Estimates of permeability for the lactone and carboxylate forms were 4.16 × 10^-5 cm/s and 2.6 × 10^-5 cm/s, respectively. These calculated permeability values were in agreement with the in vitro experimental data. Overall, CPT-11 was found to permeate through porcine buccal mucosa via passive diffusion. CPT-11 permeability was independent of pH, suggesting that the compound was transported mainly via a paracellular route. Overall, the results of this research suggest that the buccal route is a potential extravascular mode of delivery for CPT-11.

Advantageous Solubility-Permeability Interplay When Using Amorphous Solid Dispersion (ASD) Formulation for the BCS Class IV P-gp Substrate Rifaximin: Simultaneous Increase of Both the Solubility and the Permeability

Rifaximin is a BCS class IV (low-solubility, low-permeability) drug and also a P-gp substrate. The aims of this work were to assess the efficiency of different rifaximin amorphous solid dispersion (ASDs) formulations in achieving and maintaining supersaturation and to investigate the consequent solubility-permeability interplay. Spray-dried rifaximin ASDs were prepared with different hydrophilic polymers and their ability to achieve and maintain supersaturation was assessed. Then, rifaximin's apparent intestinal permeability was investigated as a function of increasing supersaturation both in vitro using the parallel artificial membrane permeability assay (PAMPA) and in vivo using the single-pass rat intestinal perfusion (SPIP) model. The efficiency of the different ASDs to achieve and maintain supersaturation of rifaximin was found to be highly polymer dependent, and the copovidone/HPC-SL formulation was found to be superior to the other two, allowing supersaturation of 200× that of the crystalline solubility for 20 h. In vitro, rifaximin flux was increased and the apparent permeability was constant as a function of increasing supersaturation level. In vivo, on the other hand, absorption rate coefficient (k _a) was first constant as a function of increasing supersaturation, but at 250×, the crystalline solubility k _a was doubled, similar to the k _a in the presence of the strong P-gp inhibitor GF120918. In conclusion, a new and favorable nature of solubility-permeability interplay was revealed in this work: delivering high supersaturation level of the BCS class IV drug rifaximin via ASD, thereby saturating the drugs' P-gp-mediated efflux transport, led to the favorable unique win-win situation, where both the solubility and the permeability increased simultaneously.

Striking the Optimal Solubility-Permeability Balance in Oral Formulation Development for Lipophilic Drugs: Maximizing Carbamazepine Blood Levels

The purpose of this research was to investigate the performance of cosolvent based solubility-enabling formulations in oral delivery of lipophilic drugs, accounting for the gastrointestinal tract (GIT) luminal solubilization processes, the solubility-permeability interplay, and the overall in vivo systemic absorption. The poorly soluble antiepileptic agent carbamazepine was formulated in three cosolvent-based formulations: 20%, 60%, and 100% PEG-400, and the apparent solubility and rat permeability of the drug in these formulations were evaluated. The performance of the formulations in the dynamic GIT environment was assessed utilizing the biorelevant pH-dilution method. Then, the overall in vivo drug exposure was investigated following oral administration to rats. The three formulations showed dramatic solubility and permeability differences; the 100% PEG-400 provided the highest solubility enhancement and the 20% the poorest, while the exact opposite was evident from the permeability point of view. The dissolution results indicated that the 20% PEG-400 formulation crashes quickly following oral administration, but both the 60% and the 100% PEG-400 formulations allowed full solubilization of the dose throughout the entire GIT-like journey. The best in vivo performing formulation was the 60% PEG-400 (F_sys > 90%), followed by the 100% PEG-400 (F_sys = 76%), and the 20% PEG-400 formulation (F_sys ≈ 60%). In conclusion, this work demonstrates the in vivo solubility-permeability trade-off in oral delivery of lipophilic drugs; when a solubility-enabling formulation is developed, minimal threshold solubility should be targeted, that is just enough to allow solubilization of the drug dose throughout the GIT, while excess solubilizer should be avoided.

Hydrotropic Solubilization of Lipophilic Drugs for Oral Delivery: The Effects of Urea and Nicotinamide on Carbamazepine Solubility-Permeability Interplay

Hydrotropy refers to increasing the water solubility of otherwise poorly soluble compound by the presence of small organic molecules. While it can certainly increase the apparent solubility of a lipophilic drug, the effect of hydrotropy on the drugs’ permeation through the intestinal membrane has not been studied. The purpose of this work was to investigate the solubility–permeability interplay when using hydrotropic drug solubilization. The concentration-dependent effects of the commonly used hydrotropes urea and nicotinamide, on the solubility and the permeability of the lipophilic antiepileptic drug carbamazepine were studied. Then, the solubility–permeability interplay was mathematically modeled, and was compared to the experimental data. Both hydrotropes allowed significant concentration-dependent carbamazepine solubility increase (up to ∼30-fold). A concomitant permeability decrease was evident both in vitro and in vivo (∼17-fold for nicotinamide and ∼9-fold for urea), revealing a solubility–permeability tradeoff when using hydrotropic drug solubilization. A relatively simplified simulation approach based on proportional opposite correlation between the solubility increase and the permeability decrease at a given hydrotrope concentration allowed excellent prediction of the overall solubility–permeability tradeoff. In conclusion, when using hydrotropic drug solubilization it is prudent to not focus solely on solubility, but to account for the permeability as well; achieving optimal solubility–permeability balance may promote the overall goal of the formulation to maximize oral drug exposure.

Improved oral bioavailability of poorly water-soluble glimepiride by utilizing microemulsion technique

The objective of this work was to prepare an oil/water glimepiride (GM) microemulsion (ME) for oral administration to improve its solubility and enhance its bioavailability. Based on a solubility study, pseudoternary phase diagrams, and Box–Behnken design, the oil/water GMME formulation was optimized and prepared. GMME was characterized by dynamic laser light scattering, zeta potential, transmission electron microscopy, and viscosity. The in vitro drug release, storage stability, pharmacodynamics, and pharmacokinetics of GMME were investigated. The optimized GMME was composed of Capryol 90 (oil), Cremophor RH40 (surfactant), and Transcutol (cosurfactant), and increased GM solubility up to 544.6±4.91 µg/mL. The GMME was spherical in shape. The particle size and its polydispersity index were 38.9±17.46 nm and 0.266±0.057, respectively. Meanwhile, the GMME was physicochemically stable at 4°C for at least 3 months. The short-term efficacy in diabetic mice provided the proof that blood glucose had a consistent and significant reduction at a dose of 375 µg/kg whether via IP injection or IG administration of GMME. Compared with the glimepiride suspensions or glimepiride-meglumine complex solution, the pharmacokinetics of GMME in Wistar rats via IG administration exhibited higher plasma drug concentration, larger area under the curve, and more enhanced oral bioavailability. There was a good correlation of GMME between the in vitro release values and the in vivo oral absorption. ME could be an effective oral drug delivery system to improve bioavailability of GM.

The solubility-permeability interplay and oral drug formulation design: Two heads are better than one

Poor aqueous solubility is a major challenge in today's biopharmaceutics. While solubility-enabling formulations can significantly increase the apparent solubility of the drug, the concomitant effect on the drug's apparent permeability has been largely overlooked. The mathematical equation to describe the membrane permeability of a drug comprises the membrane/aqueous partition coefficient, which in turn is dependent on the drug's apparent solubility in the GI milieu, suggesting that the solubility and the permeability are closely related, exhibit a certain interplay between them, and treating the one irrespectively of the other may be insufficient. In this article, an overview of this solubility-permeability interplay is provided, and the available data is analyzed in the context of the effort to maximize the overall drug exposure. Overall, depending on the type of solubility-permeability interplay, the permeability may decrease, remain unchanged, and even increase, in a way that may critically affect the formulation capability to improve the overall absorption. Therefore, an intelligent design of solubility-enabling formulation needs to consider both the solubility afforded by the formulation and the permeability in the new luminal environment resulting from the formulation.

50years of oral lipid-based formulations: Provenance, progress and future perspectives

Lipid based formulations (LBF) provide well proven opportunities to enhance the oral absorption of drugs and drug candidates that sit close to, or beyond, the boundaries of Lipinski's 'rule-of-five' chemical space. Advantages in permeability, efflux and presystemic metabolism are evident; however, the primary benefit is in increases in dissolution and apparent intestinal solubility for lipophilic, poorly water soluble drugs. This review firstly details the inherent advantages of LBF, their general properties and classification, and provides a brief retrospective assessment of the development of LBF over the past fifty years. More detailed analysis of the ability of LBF to promote intestinal solubilisation, supersaturation and absorption is then provided alongside review of the methods employed to assess formulation performance. Critical review of the ability of simple dispersion and more complex in vitro digestion methods to predict formulation performance subsequently reveals marked differences in the correlative ability of in vitro tests, depending on the properties of the drug involved. Notably, for highly permeable low melting drugs e.g. fenofibrate, LBF appear to provide significant benefit in all cases, and sustained ongoing solubilisation may not be required. In other cases, and particularly for higher melting point drugs such as danazol, where re-dissolution of crystalline precipitate drug is likely to be slow, correlations with ongoing solubilisation and supersaturation are more evident. In spite of their potential benefits, one limitation to broader use of LBF is low drug solubility in the excipients employed to generate formulations. Techniques to increase drug lipophilicity and lipid solubility are therefore explored, and in particular those methods that provide for temporary enhancement including lipophilic ionic liquid and prodrug technologies. The transient nature of these lipophilicity increases enhances lipid solubility and LBF viability, but precludes enduring effects on receptor promiscuity and off target toxicity. Finally, recent efforts to generate solid LBF are briefly described as a means to circumvent the need to encapsulate in soft or hard gelatin capsules, although the latter remain popular with consumers and a proven means of LBF delivery.

Effect of Absorption Behavior of Solubilizers on Drug Dissolution in the Gastrointestinal Tract: Evaluation Based on In Vivo Luminal Concentration-Time Profile of Cilostazol, a Poorly Soluble Drug, and Solubilizers

The purpose of this study was to evaluate the effect of absorption behavior of solubilizers on drug dissolution in the gastrointestinal tract. After oral administration of FITC-dextran (FD-10), a nonabsorbable marker, and cilostazol (CZ), a low-solubility drug, with or without solubilizers (dimethyl sulfoxide [DMSO], and d-α-tocopherol polyethylene glycol 1000 succinate [TPGS]), the in vivo rat luminal concentrations of these compounds were determined by direct sampling of residual water in each segment of the gastrointestinal tract. DMSO was rapidly absorbed and not detected in the middle small intestine. Conversely, the TPGS concentration increased by 1.5- and 2-fold relative to the initial dose concentration in the middle and lower small intestine, respectively, owing to condensation. Then, normalized area under the luminal concentration-time curve of solid CZ was calculated from the luminal concentration-time profiles of FD-10 and solid CZ to evaluate in vivo dissolution behavior of CZ. The dissolution of CZ was marked when administered with TPGS compared with that when administered with DMSO, especially in the lower small intestine. This clearly indicates that absorbability of solubilizers is one of the important factors in determining the solubilizing effect. These findings may be beneficial to development of oral lipophilic drugs.

Challenges and Strategies in Thermal Processing of Amorphous Solid Dispersions: A Review

Thermal processing of amorphous solid dispersions continues to gain interest in the pharmaceutical industry, as evident by several recently approved commercial products. Still, a number of pharmaceutical polymer carriers exhibit thermal or viscoelastic limitations in thermal processing, especially at smaller scales. Additionally, active pharmaceutical ingredients with high melting points and/or that are thermally labile present their own specific challenges. This review will outline a number of formulation and process-driven strategies to enable thermal processing of challenging compositions. These include the use of traditional plasticizers and surfactants, temporary plasticizers utilizing sub- or supercritical carbon dioxide, designer polymers tailored for hot-melt extrusion processing, and KinetiSol® Dispersing technology. Recent case studies of each strategy will be described along with potential benefits and limitations.

A simple and effective method to improve bioavailability of glimepiride by utilizing hydrotropy technique

The purpose of this study was to improve the solubility and bioavailability of glimepiride (GLMP) by utilizing hydrotropy technique. Meglumine (MU) as a hydrotrope could form the stable complex with glimepiride. The optimal glimepiride and meglumine (GLMP-MU) complex powder was obtained by using lyophilization. GLMP-MU powder was characterized by Fourier transform infrared spectroscopy (FT IR), X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC). The formation of hydrogen bond between glimepiride and meglumine was confirmed by FT IR. The XRD studies indicated the amorphous state of glimepiride was appeared in the GLMP-MU. The DSC results were further confirmed GLMP-MU complex was prepared successfully. Moreover, the in vitro drug release rate of GLMP-MU powder was dramatically faster than that of glimepiride. Meanwhile, the AUC of GLMP-MU solution at an i.g./or i.v. dose of 5mg/kg in rat was significantly higher than that of the glimepiride suspensions. Together our results showed that hydrotropy technique was a simple and effective method to increase the solubility of glimepiride.

The use of captisol (SBE7-β-CD) in oral solubility-enabling formulations: Comparison to HPβCD and the solubility-permeability interplay

The aim of this research was to study the interaction of sulfobutyl ether7 β-cyclodextrin (captisol) and 2-hydroxypropyl-β-cyclodextrin (HPβCD) with the poorly soluble antiarrhythmic drug amiodarone, and to investigate the consequent solubility-permeability interplay. Phase-solubility studies of amiodarone with the two cyclodextrins, followed by PAMPA and rat intestinal permeability experiments, were carried out, and the solubility-permeability interplay was then illustrated as a function of increasing cyclodextrin content. Equimolar levels of captisol allowed ∼10-fold higher amiodarone solubility than HPβCD, as well as binding constant. With both captisol and HPβCD, decreased in vitro and in vivo amiodarone apparent permeability was evident with increasing CD levels and increased apparent solubility. A theoretical model assuming direct proportionality between the apparent solubility increase allowed by the CD and permeability decrease was able to accurately predict the solubility-permeability tradeoff as a function of CD levels. In conclusion, the addition of ionic interactions (e.g. amiodarone-captisol) to hydrophobic interactions of the inclusion complex formation may result in synergic effect on solubilization; however, it is not merely the solubility that should be examined when formulating an oral poorly soluble compound, but the solubility-permeability balance, in order to maximize the overall drug exposure.

The biopharmaceutics of successful controlled release drug product: Segmental-dependent permeability of glipizide vs. metoprolol throughout the intestinal tract

The purpose of this work was to study the challenges and prospects of regional-dependent absorption in a controlled-release scenario, through the oral biopharmaceutics of the sulfonylurea antidiabetic drug glipizide. The BCS solubility class of glipizide was determined, and its physicochemical properties and intestinal permeability were thoroughly investigated, both in-vitro (PAMPA and Caco-2) and in-vivo in rats. Metoprolol was used as the low/high permeability class boundary marker. Glipizide was found to be a low-solubility compound. All intestinal permeability experimental methods revealed similar trend; a mirror image small intestinal permeability with opposite regional/pH-dependency was obtained, a downward trend for glipizide, and an upward trend for metoprolol. Yet the lowest permeability of glipizide (terminal Ileum) was comparable to the lowest permeability of metoprolol (proximal jejunum). At the colon, similar permeability was evident for glipizide and metoprolol, that was higher than metoprolol's jejunal permeability. We present an analysis that identifies metoprolol's jejunal permeability as the low/high permeability class benchmark anywhere throughout the intestinal tract; we show that the permeability of both glipizide and metoprolol matches/exceeds this threshold throughout the entire intestinal tract, accounting for their success as controlled-release dosage form. This represents a key biopharmaceutical characteristic for a successful controlled-release dosage form.

Provisional in-silico biopharmaceutics classification (BCS) to guide oral drug product development

The main objective of this work was to investigate in-silico predictions of physicochemical properties, in order to guide oral drug development by provisional biopharmaceutics classification system (BCS). Four in-silico methods were used to estimate LogP: group contribution (CLogP) using two different software programs, atom contribution (ALogP), and element contribution (KLogP). The correlations (r(2)) of CLogP, ALogP and KLogP versus measured LogP data were 0.97, 0.82, and 0.71, respectively. The classification of drugs with reported intestinal permeability in humans was correct for 64.3%-72.4% of the 29 drugs on the dataset, and for 81.82%-90.91% of the 22 drugs that are passively absorbed using the different in-silico algorithms. Similar permeability classification was obtained with the various in-silico methods. The in-silico calculations, along with experimental melting points, were then incorporated into a thermodynamic equation for solubility estimations that largely matched the reference solubility values. It was revealed that the effect of melting point on the solubility is minor compared to the partition coefficient, and an average melting point (162.7 °C) could replace the experimental values, with similar results. The in-silico methods classified 20.76% (± 3.07%) as Class 1, 41.51% (± 3.32%) as Class 2, 30.49% (± 4.47%) as Class 3, and 6.27% (± 4.39%) as Class 4. In conclusion, in-silico methods can be used for BCS classification of drugs in early development, from merely their molecular formula and without foreknowledge of their chemical structure, which will allow for the improved selection, engineering, and developability of candidates. These in-silico methods could enhance success rates, reduce costs, and accelerate oral drug products development.

Analytical technologies for real-time drug dissolution and precipitation testing on a small scale

Objectives

This review focuses on real-time analytics of drug dissolution and precipitation testing on a comparatively small scale.

Key findings

Miniaturisation of test equipment is an important trend in pharmaceutics, and several small-scale experiments have been reported for drug dissolution and precipitation testing. Such tests typically employ analytics in real-time. Fibre optic ultraviolet (UV) analytics has become a well-established method in this field. Novel imaging techniques are emerging that use visible or UV light; also promising is Fourier transform infrared imaging based on attenuated total reflection. More information than just a rate constant is obtained from these methods. The early phase of a dissolution process can be assessed and drug precipitation may eventually be observed. Some real-time techniques are particularly well suited to studying drug precipitation during formulation dispersion; for example, turbidity, focused beam reflectance measurement and Raman spectroscopy.

Summary

Small-scale dissolution tests equipped with real-time analytics have become important to screen drug candidates as well as to study prototype formulations in early development. Future approaches are likely to combine different analytical techniques including imaging. Miniaturisation started with mini-vessels or small vials and future assays of dissolution research will probably more often reach the level of parallel well plates and microfluidic channels.

The low/high BCS permeability class boundary: physicochemical comparison of metoprolol and labetalol

Although recognized as overly conservative, metoprolol is currently the common low/high BCS permeability class boundary reference compound, while labetalol was suggested as a potential alternative. The purpose of this study was to identify the various characteristics that the optimal marker should exhibit, and to investigate the suitability of labetalol as the permeability class reference drug. Labetalol's BCS solubility class was determined, and its physicochemical properties and intestinal permeability were thoroughly investigated, both in vitro and in vivo in rats, considering the complexity of the whole of the small intestine. Labetalol was found to be unequivocally a high-solubility compound. In the pH range throughout the small intestine (6.5-7.5), labetalol exhibited pH-dependent permeability, with higher permeability at higher pH values. While in vitro octanol-buffer partitioning (Log D) values of labetalol were significantly higher than those of metoprolol, the opposite was evident in the in vitro PAMPA permeability assay. The results of the in vivo perfusion studies in rats lay between the two contradictory in vitro studies; metoprolol was shown to have moderately higher rat intestinal permeability than labetalol. Theoretical distribution of the ionic species of the drugs was in corroboration with the experimental in vitro and the in vivo data. We propose three characteristics that the optimal permeability class reference drug should exhibit: (1) fraction dose absorbed in the range of 90%; (2) the optimal marker drug should be absorbed largely via passive transcellular permeability, with no/negligible carrier-mediated active intestinal transport (influx or efflux); and (3) the optimal marker drug should preferably be nonionizable. The data presented in this paper demonstrate that neither metoprolol nor labetalol can be regarded as optimal low/high-permeability class boundary standard. While metoprolol is too conservative due to its complete absorption, labetalol has been shown to be a substrate for P-gp-mediated efflux transport, and both drugs exhibit significant segmental-dependent permeability along the gastrointestinal tract. Nevertheless, the use of metoprolol as the marker compound does not carry a risk of bioinequivalence: Peff value similar to or higher than metoprolol safely indicates high-permeability classification. On the other hand, a more careful data analysis is needed if labetalol is used as the reference compound.

The complexity of intestinal permeability: Assigning the correct BCS classification through careful data interpretation

While the solubility parameter is fairly straightforward when assigning BCS classification, the intestinal permeability (Peff) is more complex than generally recognized. In this paper we emphasize this complexity through the analysis of codeine, a commonly used antitussive/analgesic drug. Codeine was previously classified as a low-permeability compound, based on its lower LogP compared to metoprolol, a marker for the low-high permeability class boundary. In contrast, high fraction of dose absorbed (Fabs) was reported for codeine, which challenges the generally recognized Peff-Fabs correlation. The purpose of this study was to clarify this ambiguity through elucidation of codeine's BCS solubility/permeability class membership. Codeine's BCS solubility class was determined, and its intestinal permeability throughout the small intestine was investigated, both in vitro and in vivo in rats. Codeine was found to be unequivocally a high-solubility compound. All in vitro studies indicated that codeine's permeability is higher than metoprolol's. In vivo studies in rats showed similar permeability for both drugs throughout the entire small-intestine. In conclusion, codeine was found to be a BCS Class I compound. No Peff-Fabs discrepancy is involved in its absorption; rather, it reflects the risk of assigning BCS classification based on merely limited physicochemical characteristics. A thorough investigation using multiple experimental methods is prudent before assigning a BCS classification, to avoid misjudgment in various settings, e.g., drug discovery, formulation design, drug development and regulation.

Oral delivery of lipophilic drugs: the tradeoff between solubility increase and permeability decrease when using cyclodextrin-based formulations

The purpose of this study was to investigate the impact of oral cyclodextrin-based formulation on both the apparent solubility and intestinal permeability of lipophilic drugs. The apparent solubility of the lipophilic drug dexamethasone was measured in the presence of various HPβCD levels. The drug’s permeability was measured in the absence vs. presence of HPβCD in the rat intestinal perfusion model, and across Caco-2 cell monolayers. The role of the unstirred water layer (UWL) in dexamethasone’s absorption was studied, and a simplified mass-transport analysis was developed to describe the solubility-permeability interplay. The PAMPA permeability of dexamethasone was measured in the presence of various HPβCD levels, and the correlation with the theoretical predictions was evaluated. While the solubility of dexamethasone was greatly enhanced by the presence of HPβCD (K_1∶1 = 2311 M⁻¹), all experimental models showed that the drug’s permeability was significantly reduced following the cyclodextrin complexation. The UWL was found to have no impact on the absorption of dexamethasone. A mass transport analysis was employed to describe the solubility-permeability interplay. The model enabled excellent quantitative prediction of dexamethasone’s permeability as a function of the HPβCD level. This work demonstrates that when using cyclodextrins in solubility-enabling formulations, a tradeoff exists between solubility increase and permeability decrease that must not be overlooked. This tradeoff was found to be independent of the unstirred water layer. The transport model presented here can aid in striking the appropriate solubility-permeability balance in order to achieve optimal overall absorption.

Profiling biopharmaceutical deciding properties of absorption of lansoprazole enteric-coated tablets using gastrointestinal simulation technology

The aim of the present study was to correlate in vitro properties of drug formulation to its in vivo performance, and to elucidate the deciding properties of oral absorption. Gastrointestinal simulation technology (GST) was used to simulate the in vivo plasma concentration-time curve and was implemented by GastroPlus™ software. Lansoprazole, a typical BCS class II drug, was chosen as a model drug. Firstly, physicochemical and pharmacokinetic parameters of lansoprazole were determined or collected from literature to construct the model. Validation of the developed model was performed by comparison of the predicted and the experimental plasma concentration data. We found that the predicted curve was in a good agreement with the experimental data. Then, parameter sensitivity analysis (PSA) was performed to find the key parameters of oral absorption. The absorption was particularly sensitive to dose, solubility and particle size for lansoprazole enteric-coated tablets. With a single dose of 30 mg and the solubility of 0.04 mg/ml, the absorption was complete. A good absorption could be achieved with lansoprazole particle radius down to about 25 μm. In summary, GST is a useful tool for profiling biopharmaceutical deciding properties of absorption of lansoprazole enteric-coated tablets and guiding the formulation optimization.

The interaction of nifedipine with selected cyclodextrins and the subsequent solubility-permeability trade-off

The purpose of this study was to investigate the interaction of 2-hydroxypropyl-β-cyclodextrin (HPβCD) and 2,6-dimethyl-β-cyclodextrin (DMβCD) with the lipophilic drug nifedipine and to investigate the subsequent solubility-permeability interplay. Solubility curves of nifedipine with HPβCD and DMβCD in MES buffer were evaluated using phase solubility methods. Then, the apparent permeability of nifedipine was investigated as a function of increasing HPβCD/DMβCD concentration in the hexadecane-based PAMPA model. The interaction with nifedipine was CD dependent; significantly higher stability constant was obtained for DMβCD in comparison with HPβCD. Moreover, nifedipine displays different type of interaction with these CDs; a 1:1 stoichiometric inclusion complex was apparent with HPβCD, while 1:2 stoichiometry was apparent for DMβCD. In all cases, decreased apparent intestinal permeability of nifedipine as a function of increasing CD level and nifedipine apparent solubility was obtained. A quasi-equilibrium mass transport analysis was developed to explain this solubility-permeability interplay; the model enabled excellent quantitative prediction of nifedipine's permeability as a function of CD concentrations. This work demonstrates that when using CDs in solubility-enabling formulations, a trade-off exists between solubility increase and permeability decrease that must not be overlooked. This trade-off was found to be independent of the type of CD-drug interaction. The transport model presented here can aid in striking the appropriate solubility-permeability balance in order to achieve optimal overall absorption.

Regional-dependent intestinal permeability and BCS classification: elucidation of pH-related complexity in rats using pseudoephedrine

Based on its lower Log P value relative to metoprolol, a marker for the low/high-permeability (P(eff)) class boundary, pseudoephedrine was provisionally classified as BCS low-permeability compound. On the other hand, following oral administration, pseudoephedrine fraction dose absorbed (F(abs)) and systemic bioavailability approaches 100%. This represents a challenge to the generally recognized P(eff)-F(abs) correlation. The purpose of this study was to elucidate the underlying mechanisms behind the confusion in pseudoephedrine's BCS classification. Pseudoephedrine's BCS solubility class was determined, and its physicochemical properties and intestinal permeability were thoroughly investigated, both in vitro and in vivo in rats, considering the complexity of the whole of the small intestine. Pseudoephedrine was found to be unequivocally a high-solubility compound. All of the permeability studies revealed similar phenomenon; at any given intestinal segment/pH, the permeability of metoprolol was higher than that of pseudoephedrine, however, as the intestinal region becomes progressively distal, and the pH gradually increases, pseudoephedrine's permeability rises above that of metoprolol in the former segment. This unique permeability pattern likely explains pseudoephedrine's complete absorption. In conclusion, pseudoephedrine is a BCS Class I compound; no discrepancy between P(eff) and F(abs) is involved in its absorption. Rather, it reflects the complexity behind P(eff) when considering the whole of the intestine. We propose to allow high-permeability classification to drugs with P(eff) that matches/exceeds the low/high class benchmark anywhere throughout the intestinal tract and not restricted necessarily to the jejunum.

The twofold advantage of the amorphous form as an oral drug delivery practice for lipophilic compounds: increased apparent solubility and drug flux through the intestinal membrane

The purposes of this study were to assess the efficiency of different nifedipine amorphous solid dispersions (ASDs) in achieving and maintaining supersaturation and to investigate the solubility-permeability interplay when increasing the apparent solubility via ASD formulations. Spray-dried ASDs of nifedipine in three different hydrophilic polymers, hydroxypropyl methylcellulose acetate succinate (HPMC-AS), copovidone, and polyvinylpyrrolidone (PVP), were prepared and characterized by powder X-ray diffraction and differential scanning calorimetry. The ability of these formulations to achieve and maintain supersaturation over 24 h was assessed. Then, nifedipine's apparent intestinal permeability was investigated as a function of increasing supersaturation in the parallel artificial membrane permeability assay model and in the single-pass rat intestinal perfusion model. The efficiency of the different ASDs to achieve and maintain supersaturation of nifedipine was found to be highly polymer dependent; while a dispersion in HPMC-AS enabled supersaturation 20× that of the crystalline aqueous solubility, a dispersion in copovidone enabled 10×, and PVP allowed supersaturation of only 5× that of the crystalline solubility. Nifedipine flux across the intestine from supersaturated solutions was increased, and the apparent intestinal permeability was constant, irrespective of the degree of supersaturation or the polymer being used. In conclusion, while with other solubility-enabling approaches (e.g., surfactants, cyclodextrins, cosolvents), it is not enough to increase the apparent solubility, but to strike the optimal solubility-permeability balance, which limits the chances for successful drug delivery, the amorphous form emerges as a more advantageous strategy, in which higher apparent solubility (i.e., supersaturation) will be readily translated into higher drug flux and overall absorption.

A win-win solution in oral delivery of lipophilic drugs: supersaturation via amorphous solid dispersions increases apparent solubility without sacrifice of intestinal membrane permeability

Recently, we have revealed a trade-off between solubility increase and permeability decrease when solubility-enabling oral formulations are employed. We have shown this trade-off phenomenon to be ubiquitous, and to exist whenever the aqueous solubility is increased via solubilizing excipients, regardless if the mechanism involves decreased free fraction (cyclodextrins complexation, surfactant micellization) or simple cosolvent solubilization. Discovering a way to increase drug solubility without concomitant decreased permeability represents a major advancement in oral delivery of lipophilic drugs and is the goal of this work. For this purpose, we sought to elucidate the solubility-permeability interplay when increased apparent solubility is obtained via supersaturation from an amorphous solid dispersion (ASD) formulation. A spray-dried ASD of the lipophilic drug progesterone was prepared in the hydrophilic polymer hydroxypropyl methylcellulose acetate succinate (HPMC-AS), which enabled supersaturation up to 4× the crystalline drug's aqueous solubility (8 μg/mL). The apparent permeability of progesterone from the ASD in HPMC-AS was then measured as a function of increasing apparent solubility (supersaturation) in the PAMPA and rat intestinal perfusion models. In contrast to previous cases in which apparent solubility increases via cyclodextrins, surfactants, and cosolvents resulted in decreased apparent permeability, supersaturation via ASD resulted in no decrease in apparent permeability with increasing apparent solubility. As a result, overall flux increased markedly with increasing apparent solubility via ASD as compared to the other formulation approaches. This work demonstrates that supersaturation via ASDs has a subtle yet powerful advantage over other solubility-enabling formulation approaches. That is, increased apparent solubility may be achieved without the expense of apparent intestinal membrane permeability. Thus, supersaturation via ASDs presents a markedly increased opportunity to maximize overall oral drug absorption.