Article pubs.acs.org/Langmuir
Green Amorphous Nanoplex as a New Supersaturating Drug Delivery System Wean Sin Cheow and Kunn Hadinoto* School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459 S Supporting Information *
ABSTRACT: The nanoscale formulation of amorphous drugs represents a highly viable supersaturating drug-delivery system for enhancing the bioavailability of poorly soluble drugs. Herein we present a new formulation of a nanoscale amorphous drug in the form of a drug−polyelectrolyte nanoparticle complex (or nanoplex), where the nanoplex is held together by the combination of a drug−polyelectrolyte electrostatic interaction and an interdrug hydrophobic interaction. The nanoplex is prepared by a truly simple, green process that involves the ambient mixing of drug and polyelectrolyte (PE) solutions in the presence of salt. Nanoplexes of poorly soluble acidic (i.e., ibuprofen and curcumin) and basic (i.e., ciprofloxacin) drugs are successfully prepared using biocompatible poly(allylamine hydrochloride) and dextran sulfate as the PE, respectively. The roles of salt, drug, and PE in nanoplex formation are examined from ternary phase diagrams of the drug−PE complex, from which the importance of the drug’s charge density and hydrophobicity, as well as the PE ionization at different pH values, is recognized. Under the optimal conditions, the three nanoplexes exhibit high drug loadings of ∼80−85% owing to the high drug complexation efficiency (∼ 90−96%), which is achieved by keeping the feed charge ratio of the drug to PE below unity (i.e., excess PE). The nanoplex sizes are ∼300−500 nm depending on the drug hydrophobicity. The nanoplex powders remain amorphous after 1 month of storage, indicating the high stability owed to the PE’s high glass-transition temperature. FT-IR analysis shows that functional groups of the drug are conserved upon complexation. The nanoplexes are capable of generating prolonged supersaturation upon dissolution with precipitation inhibitors. The supersaturation level depends on the saturation solubility of the native drugs, where the lower the saturation solubility, the higher the supersaturation level. The solubility of curcumin as the least-soluble drug is magnified 9-fold upon its transformation to the nanoplex, and the supersaturated condition is maintained for 5 h. preparation simplicity.3 However, acidic and basic drugs may not necessarily form salts, and salt formation does not necessarily guarantee enhanced saturation solubility.1 The nanoAPI strategy, however, is not limited to acidic and basic drugs because its saturation solubility enhancement relies on a particle size reduction to the nanoscale following the Ostwald−Freundlich solubility theory.4 However, the Ostwald−Freundlich equation suggests that the saturation solubility enhancement via nanoionization is significant only for size ≪100 nm.4 NanoAPI with sizes of ∼150−200 nm has been found to exhibit only a 15% higher saturation solubility than the microscale counterparts.5 Presently, established nanoAPI preparation techniques
1. INTRODUCTION The development of various methods of transforming crystalline particles of an active pharmaceutical ingredient (API) into their more soluble forms is driven by the realization that a large fraction (∼66%) of new promising drug candidates exhibit low saturation solubility in the aqueous phase.1 The low aqueous solubility of the API translates to its low oral bioavailability, which necessitates high and frequent dosing, causing high financial and pill burdens for patients. The three major formulation strategies for bioavailability enhancement include (1) transforming the API into its highly soluble salt form, (2) formulating the API into crystalline nanoparticles (or nanoAPI), and (3) delivering the API in its amorphous form.2 Because a majority of drugs are either weak organic acids or bases,2 salt formation is the first approach in increasing the saturation solubility of acidic and basic drugs because of its © 2012 American Chemical Society
Received: December 5, 2011 Revised: February 9, 2012 Published: March 22, 2012 6265
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and curcumin (CCM). Poly(allylamine hydrochloride) (PAH), a synthetic polycation commonly used in drug and gene delivery,18 is used as the PE. Ternary phase diagrams of the drug−PE complex are constructed to determine the range of compositions of salt, drug, and PE required for successful IB and CCM nanoplex formation. The findings are compared with that of the CIP nanoplex prepared under the same conditions. The phase diagrams are also used to elucidate the roles of PE, drug, and salt in nanoplex formation. The optimal formulations, in terms of the resultant production yield and complexation efficiency, are subsequently identified. The drug loading, size, zeta potential, supersaturation profile, and storage stability of the optimal nanoplex formulations are examined. The results demonstrate the amorphous nanoplex as a very capable and greener alternative to a supersaturated drug delivery system.
(e.g., high-pressure homogenization, wet milling) are not yet capable of consistently producing nanoAPI at sizes below 100 nm.6 Therefore, the potential of nanoAPI as a bioavailability-enhancement strategy depends on advancements in its preparation techniques. A different strategy is to exploit the metastable state of the amorphous form of the API to generate an apparent temporary increase in the solubility. The dissolution of amorphous API results in a highly supersaturated solution producing apparent solubility that is much higher than the saturation solubility of the crystalline counterparts.7 The high supersaturation level would drive drug absorption across the gastrointestinal lumen, resulting in enhanced bioavailability, provided that the supersaturation can be maintained for a time period sufficient for absorption. It is significant that the in vitro generation of high supersaturation levels by amorphous API has been shown to translate to enhanced bioavailability in vivo.8,9 Amorphous API is typically prepared in the form of microscale solid dispersions of the drug stabilized by high glass-transition-temperature polymers (e.g., hydroxypropylmethylcellulose (HPMC), poly(ethylene glycol)) by cogrinding, hot melt extrusion, or antisolvent precipitation techniques.10,11 The roles of the stabilizer, which usually makes up at least 50% (w/w) of the formulation,12 are twofold: (1) to prevent recrystallization of the amorphous API during storage by occupying its high-energy surfaces and (2) to function as precipitation inhibitors of the dissolved drug to prolong the supersaturation. The typical amorphous API formulations, however, suffer from drawbacks of (1) low drug loading due to the high stabilizer content and (2) a high propensity to recrystallize upon dissolution, attributed to slow dissolution rates of the amorphous API, particularly when slowly dissolving stabilizers (e.g., HPMC) are used,13 resulting in low apparent solubility. The in-solution recrystallization propensity of the amorphous API can be solved by having a nanoscale formulation,12 where the high specific surface area of nanoparticles ensures a high dissolution rate. A number of nanoscale amorphous APIs (or amorphous nanoAPIs) having drug loadings of up to ∼90% (w/w) have been successfully prepared by a wide range of techniques (e.g., antisolvent precipitation and sonoprecipitation).14−16 These techniques, however, exhibit several weaknesses, such as intricate processes, the heavy use of organic solvents, high-energy expense, and wide size distributions of the products. In an earlier study,17 we have developed a new method of preparing stable amorphous nanoAPIs having uniform sizes via a self-assembly drug−polyelectrolyte complexation process where the said method is simple, solvent-free, energy minimal, and fast. Using dextran sulfate (DXT) as the polyelectrolyte (PE), we have shown that a poorly soluble amphoteric API such as ciprofloxacin (CIP), after being transformed into its cations in an acidic environment below its pKa1, self-assembles upon interactions with oppositely charged DXT to form the CIP−DXT nanoparticle complex (or CIP nanoplex) in the presence of salt. The formation of the CIP nanoplex, which exhibits drug loading of up to ∼80% (w/w), is driven by CIP− DXT electrostatic interactions coupled with hydrophobic interactions of the amphiphilic CIP molecules. The stability of the dry-powder form of the amorphous CIP nanoplex after 1 month of storage has also been demonstrated. Herein we investigate the feasibility of extending the selfassembled drug−PE complexation process to prepare nanoplexes of poorly soluble weak acid APIs such as ibuprofen (IB)
2. MATERIALS AND METHODS 2.1. Materials. CIP, sodium chloride (NaCl), potassium bromide (KBr), potassium hydroxide (KOH), hydroxypropylmethylcellulose (HPMC), glacial acetic acid, and phosphate-buffered saline (PBS, pH 7.4) were purchased from Sigma-Aldrich (USA). PAH (MW 120 000− 200 000 Da), IB (an anti-inflammatory drug), and CCM (a natural agent with various therapeutic functions) were purchased from Alfa Aesar (USA). DXT (MW 5000 Da) was purchased from Wako Pure Chemical Industries (Japan). The hydrophobicities of the amphiphilic APIs, as measured by their octanol/water partition coefficient values (log P),19 are presented in Table 1. Saturation solubilities of the native API crystals and pKa19 are also presented in Table 1.
Table 1. Relevant Properties of the Model APIs log P pKa saturation solubility (mg/mL)
IB
CCM
CIP
5.2 4.6 1.75
2.5 8.31, 10, 10.2 0.021
1.32 6.1, 8.6 0.14
2.2. Preparation of the Amorphous Drug Nanoplex. Depending on its solubility as a function of pH, the drug was dissolved in an acidic or basic aqueous solution to form its cations or anions, respectively. CIP, being an amphoteric drug,20 forms cations when it is dissolved in aqueous acetic acid solution (AA) (pH < pKa1), whereas acidic drugs IB and CCM21,22 form anions when they are dissolved in aqueous KOH solution (pH > pKa). The ionized drug solution was subsequently added to the oppositely charged aqueous PE solution, upon which drug−PE electrostatic interaction took place to form a soluble drug−PE complex as illustrated in Figure 1. At a certain critical concentration whose value depends on the drug hydrophobicity, drug−PE complexes aggregated by means of interdrug hydrophobic interactions and finally formed the insoluble drug nanoplex in the presence of salt. The role of the salt is to provide a charge-screening effect to minimize repulsions of the like-charged PE chains, which can inhibit complex aggregation. The strong drug−PE electrostatic interaction prevented the drug molecules from assembling into ordered crystalline structures upon precipitation, resulting in the amorphous nanoplex. The detailed procedures for the CIP nanoplex preparation have been presented in Cheow and Hadinoto17 and are not repeated here for brevity. To prepare the IB and CCM nanoplexes, PAH was dissolved in 0.0, 0.1, or 0.2% (v/v) aqueous AA to form a solution of polycations, whereas the drug was dissolved in a 0.1 M KOH aqueous solution to form anionic drug molecules. Next, 1 mL of the drug solution was added to equal volume of PAH solution, with a final salt (NaCl) concentration of 0.1 M. The mixture was left for 3 h under ambient conditions to allow the complexation to equilibrate. To remove the excess drug and PE (i.e., those that do not form nanoplexes) as well as NaCl and KOH, the nanoplex suspension was washed with three cycles of centrifugation and resuspension in 6266
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Figure 2. Three model APIs used are ionized when dissolved in acid (CIP) or base (IB, CCM). calculated as the difference between the initial amount of drug added and the amount of drug remaining in the supernatant after the first centrifugation. IB, CCM, and CIP concentrations in the supernatant were measured by UV−vis spectrophotometry (UV Mini-1240, Shimadzu, Japan) at absorbance wavelengths of 263, 463, and 291 nm, respectively. The nanoplex size and zeta potential were measured by photon correlation spectroscopy (PCS) using a Brookhaven 90Plus nanoparticle size analyzer (Brookhaven Instruments Corporation, USA). The production yield is defined as the total nanoplex mass produced relative to the total mass of drug and PE initially added. The total nanoplex mass produced was determined by freeze drying an aliquot of the nanoplex suspension. Drug loading, which is defined as the percentage of drug making up the nanoplex, was determined by measuring the amount of drug released when a known amount of nanoplex was completely dissolved in PBS. The CE, yield, and drugloading characterizations were performed using at least two replicates. To verify size measurement by PCS, the freeze-dried nanoplex was sputter coated with platinum and imaged using scanning electron microscope (SEM) model JSM-6700F (JEOL, USA). The stability of the nanoplex powders was examined after storage at 25 °C and 55% relative humidity for 1 month. Powder X-ray diffraction (PXRD) patterns of the nanoplex powders were examined immediately after preparation and after one month, using a D8 Advance X-ray diffractometer equipped with Cu Kα radiation (Bruker, Germany) from 10 to 60° (2θ) with a step size of 0.02°/s. The PXRD pattern of the nanoplex powders was compared with that of the native APIs. Differential scanning calorimetry (DSC) analysis was performed using SDT Q600 (TA Instruments, USA), where 5 mg of powder was placed in an alumina pan and heated from 30 to 400 °C at 10 °C/min. Infrared (IR) spectra were recorded using a Perkin-Elmer Spectrum One FT-IR system with a spectral resolution of 4 cm−1 from 4000 to 450 cm−1. The sample pellets for FT-IR analysis were prepared by pressing a mixture of 1 mg of nanoplex powder and 100 mg of ground KBr in a die at 10 tons for 1 min. 2.4. Generation and Quantification of Supersaturation. The drug-saturation solubility presented in Table 1 was determined by adding native drugs in excess (∼100 mg) to 20 mL of PBS. After 24 h of incubation in a shaking incubator at 37 °C, a 2 mL aliquot of the incubated sample was centrifuged and filtered through a 0.1 μm filter (Puradisc, Whatman, USA), after which the dissolved drug
Figure 1. Amorphous nanoplex formation via self-assembly to yield drug−PE complexation. deionized water. The nanoplex suspension was then transformed into dry powder by freeze drying. On this note, because free CCM is prone to degradation in alkaline22 solution, the CCM solution was added to the PAH solution immediately after its preparation to form the CCM− PAH complex. The ternary phase diagram of the drug−PE complex was constructed by varying the mass compositions of the drug, PE, and salt (Mdrug/PE/salt) at different AA concentrations while keeping the total salt concentration constant at 0.1 M. The complexation products were separated into four categories: (1) a nanoplex, (2) precipitates, (3) a combination of the nanoplex and precipitates, and (4) a soluble complex. To identify nanoplex suspensions, removal of the excess salt and resuspension in DI water were necessary because the chargestabilized nanoplex was flocculated in the presence of salt. To categorize the complexation products, the samples were centrifuged for 15 min at 14 000 rpm. The sediments obtained at the bottom of the centrifuge tubes were nonredispersible pellets in the case of precipitates but were redispersible suspensions in the case of the nanoplex. Drug−PE mixtures were categorized as soluble complexes when a clear solution was observed. The drug to PE charge ratios (Rdrug/PE) were calculated from the MW and the number of charges per molecule of the drug and PE. As shown in Figure 2, IB and CIP have one charge each when fully ionized in base and acid, respectively, whereas CCM has three charges when fully ionized. For IB, the single −COO− group per IB molecule (MW 206.3 g/mol) results in a charge density of 4.8 × 10−6 mol charge/mg. The charge densities of CCM and CIP are similarly calculated, yielding 8.1 × 10−6 and 3.0 × 10−6 mol charge/mg, respectively. For the PEs, there is one −NH3+ group per PAH monomer (MW 93.5 g/mol) and 24 −OSO3− groups per DXT molecule, resulting in charge densities of 10.7 × 10−6 and 4.8 × 10−6 mol charge/mg, respectively. 2.3. Physical Characterizations of the Amorphous Drug Nanoplex. The complexation efficiency (CE) is defined as the mass percentage of drug that forms a nanoplex relative to the initial amount of drug added. The amount of drug that forms the nanoplex was 6267
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concentration was measured. Experiments with CIP and CCM were performed in opaque bottles because these compounds are prone to photolytic degradation.23 For IB and CCM, the supersaturated drug solution was generated by adding the nanoplex at a concentration equal to 25 times the saturation solubility to 50 mL of PBS at 37 °C under constant stirring. At specified time points, 2 mL of PBS was withdrawn and centrifuged, and the supernatant was filtered and diluted. To be more specific, 0.5 mL of the 2 mL filtered drug solution was immediately diluted 10-fold with PBS to prevent drug precipitation from the supersaturated solution. Afterward, the drug concentration was measured to obtain the supersaturation profile as a function of time. For comparison, the supersaturation profile from dissolution in PBS containing 1 mg/mL HPMC, an established precipitation inhibitor known to prolong and intensify the supersaturation level, was also characterized. The supersaturation profiles reported were obtained from a minimum of two replicates. For CIP, the supersaturation generation was examined at 10 times the saturation solubility because at 25 times the dissolved drug precipitates almost instantaneously, preventing the supersaturated drug solution from being properly quantified.
3. RESULTS 3.1. Ternary Phase Diagrams and Optimal Formulations. 3.1.1. IB Nanoplex. Ternary phase diagrams of the IB−PAH complexation at three different AA concentrations (i.e., 0.0, 0.1, and 0.2% (v/v)) of the PAH solution are presented in Figure 3. The results indicate that in the absence of AA approximately two-thirds of the MIB/PAH/salt investigated produces a soluble IB−PAH complex. The percentage of soluble complex decreases to half at AA = 0.1 and 0.2% (v/v). MIB/PAH/salt that produces the soluble complex is found predominantly in the triangular regions at the bottom left of the phase diagrams as indicated by the dashed lines in Figure 3. The triangular regions are demarcated at ∼20, 25, and 30% (w/w) salt for AA = 0.0, 0.1, and 0.2% (v/v), respectively, above which only the soluble complex is formed. Outside the triangular regions, a minimum of 30% (w/w) PAH is required to produce the nanoplex below which a soluble complex or precipitates are formed. The formulation that results in the highest yield (∼45%) and CE (∼90%) is identified at 35% IB, 50% PAH, and 15% salt (w/w) for AA = 0.1% (v/v) (arrow in Figure 3B). The high CE suggests that most of the IB is recovered in the form of the nanoplex; therefore, the low yield is caused by the presence of excess PAH in the feed (i.e., RIB/PAH < 1). However, decreasing the PAH composition while simultaneously increasing the IB composition, in an attempt to improve the yield, results in precipitate formation as indicated by the solid circles below the optimal point. 3.1.2. CCM Nanoplex. Ternary phase diagrams of the CCM−PAH complexation at two different AA concentrations (i.e., 0.0 and 0.1% (v/v)) of the PAH solution are presented in Figure 4. Only a small fraction of the MCCM/PAH/salt investigated ( CCM > CIP, with all sizes 100 °C).17,24 3.4. FT-IR Spectroscopy Analysis. FT-IR spectroscopy analysis is performed to confirm the presence of API in the nanoplex formulation and to identify the API functional groups involved in the complexation. IR spectra of the IB, CCM, and PAH nanoplexes, with their corresponding native equivalents and PEs, are presented in Figure 7. The presence of IB in the nanoplex is evident because the IR spectra of both the IB nanoplex and native (Figure 7A) display peaks at 1618−1617 and 1638 cm−1 that are attributed to the aromatic CC double bond,25 an identifying functional group in the IB molecule. The presence of IB in the nanoplex is further confirmed by the peaks at 2850−2960 cm−1, which are characteristics of the stretching and deformation of methyl groups25 present in both the IB nanoplex and native but not in the PAH. With regard to the interacting IB functional group, a large peak at 1722 cm−1 present in the IB native spectrum, which is attributed to the carbonyl stretching of the carboxylic acid group (−COOH),25 is conspicuously absent from the IB nanoplex spectrum. Instead, a new peak at 1551 cm−1 due to the −COO− asymmetric stretching25 is present in the IB nanoplex spectrum. Taken together, the disappearance and emergence of the −COOH and −COO− peaks, respectively, point to the −COO− group of IB as the one interacting electrostatically with the NH3+ group of PAH to form the IB− PAH complex. For the CCM nanoplex, the phenolic C−CH groups in the CCM molecule are selected as the identifying functional group
Figure 5. SEM images of (A) IB, (B) CCM, and (C) CIP nanoplexes after freeze drying.
observed for the IB, CCM, and CIP nanoplexes, respectively. The IB and CCM nanoplexes are spherical in shape and relatively uniform in size (Figures 5A,B), whereas the CIP nanoplex is not as spherical and is more irregular in size (Figure 5C). The PCS measurement returns nanoplex sizes that are on average ∼100− 150 nm larger than those observed in the SEM images, which can be attributed to the PE swelling in an aqueous environment contributing to larger nanoplex hydrodynamic sizes reported. 3.3. Stability of Amorphous Drug Nanoplex Powders. PXRD patterns of the native APIs and their nanoplex equivalents are presented in Figure S2 of the Supporting Information. In contrast to the sharp crystalline peaks observed in the PXRD patterns of the native APIs, the nanoplex equivalents, which are made up of ∼80−85% (w/w) drug, exhibit low, diffuse peaks denoting their highly amorphous state. Importantly, the nanoplexes remain in the amorphous state after 1 month of storage, signifying their on-shelf stability. In accordance with the PXRD results, DSC thermograms of the nanoplex similarly show characteristics of an amorphous form. The crystalline melting peaks present in the native API thermograms (Figure 6A) are replaced by small step changes indicative of glass transition in the nanoplex thermograms 6270
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1510 cm−1 peaks points to the CO group of CCM as the one responsible for complexation, where the ketone groups transform into the anionic keto−enol form and interact electrostatically with the NH3+ groups of PAH. We have shown previously that the antimicrobial efficacy of the CIP nanoplex is comparable to that of the native CIP, thus demonstrating the presence of CIP in the nanoplex.17 The interacting CIP functional group, however, has not been identified. The pyridone group of the CIP molecule is unique to CIP and thus can be used as the identifying functional group. The peak corresponding to the CO stretching of the pyridone group27 is shifted from 1618 cm−1 in the CIP native spectrum to 1629 cm−1 in the CIP nanoplex spectrum (Figure 7C). The shift is attributed to the transformation of the zwitterionic form of CIP to its cationic form having the NH2+ group. The absence of 1595 cm−1 corresponding to NH2+ bending28 in the CIP nanoplex spectrum indicates that the NH2+ group interacts electrostatically with the anionic SO3− groups of DXT. 3.5. Supersaturation Profiles. The in vitro supersaturation profiles of the three nanoplexes in PBS with or without HPMC are presented in Figure 8. The supersaturation level plotted on the y axis is defined as the ratio of the apparent solubility of the amorphous nanoplex to the saturation solubility of the native crystals. On this note, the drug is released only from the nanoplex in an aqueous salt solution such as PBS, which contains 0.14 M NaCl, where too little salt results in minimal drug release. The charge screening by the salt is believed to decomplex the drug−PE electrostatic interaction, resulting in the release of the drug.17 Figure 8A shows that transforming IB into the IB nanoplex results in only marginal supersaturation generation, where the apparent solubility is only ∼1.4 higher than the saturation solubility, with or without HPMC. Despite the low supersaturation level generated, the IB nanoplex dissolves rapidly within 5 min to reach the maximum apparent solubility attributed to its nanoscale size and amorphous state. The maximum supersaturation level is sustained for more than 2.5 h with or without HPMC. The minimal impact of HPMC on the supersaturation level indicates that the precipitation of the dissolved drug from the supersaturated solution is not the cause of the low supersaturation level. The CCM nanoplex, in the absence of HPMC, dissolves almost instantaneously as visually observed by the instantaneous color change of the dissolution medium upon nanoplex addition, which is followed within seconds by a color reversal back to a clear solution as the dissolved CCM precipitates out of the supersaturated solution. The rapid dissolution and precipitation prevent the supersaturation level from being quantified. As a result, the recorded apparent solubility is equal to the saturation solubility (Figure 8B). In contrast, rapid dissolution and precipitation of the CCM nanoplex are no longer observed in the presence of HPMC. The supersaturation level reaches the maximum point of ∼9 in 1 h before it gradually decreases to ∼2 in 5 h, providing ample time for drug absorption. The slower dissolution rate to reach the maximum supersaturation level is caused by the adsorption of HPMC onto the surface of the dissolving nanoplex, whereas the slower precipitation of the dissolved drug is attributed to the adsorption of HPMC onto the nuclei surfaces. As a side note, CCM is known to undergo significant hydrolytic degradation of up to 75% after 2.5 h of dissolution in PBS;29 therefore, various encapsulation methods have been investigated to minimize CCM degradation.30 Although we do
Figure 7. IR spectra of (A) the IB nanoplex and native, (B) the CCM nanoplex and native, and (C) the CIP nanoplex and native and their PEs.
because the same group is not present in PAH. The in-plane vibration of the phenolic C−CH groups contributes to the peaks at 1282, 1027, and 808 cm−1 in the CCM native spectrum (Figure 7B).26 The same peaks are shifted in the CCM nanoplex spectrum to 1276, 1027, and 816 cm−1, respectively, denoting the presence of CCM in the nanoplex. To identify the interacting CCM functional group, the 1627 and 1603 cm−1 peaks in the CCM native spectrum, attributed to the stretching of the CC and CO bonds,26 are contrasted with the peaks in the CCM nanoplex spectrum. The 1627 cm−1 peak is red-shifted to 1618 cm−1, but the 1603 cm−1 peak is not detected in the CCM nanoplex spectrum. In addition, a strong peak at 1510 cm−1 due to the stretching and in-plane bending of CO26 is noticeably absent in the CCM nanoplex spectrum. The disappearance of the 1603 and 6271
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at a level of ∼3 to 4 for a period of 90 min, which is considerably longer than 25 min without HPMC, hence providing a longer time window for absorption.
4. DISCUSSION 4.1. Elucidating the Roles of Salt, PE, and Drugs in Nanoplex Formation. The use of drug−PE complexation for nanoscale drug delivery is not completely new. A number of recent studies have taken advantage of the drug−PE electrostatic interaction for controlled drug release purposes, where the release is prolonged by encapsulating the API into nanoparticles made of PE as the matrix.31,32 The drug loading for such a nanoplex, however, is low ( CIP > IB, coinciding with the saturation solubility order of the native APIs that is CCM < CIP < IB. In other words, the lower the saturation solubility of the API, the higher the solubility enhancement gained from its transformation into the nanoplex. Nevertheless, the higher apparent solubility generated by the nanoplex is obtained at the expense of a slower dissolution rate. Therefore, amorphous nanoplex formulation is most suitable for poorly soluble APIs, whose therapeutic effectiveness requires them to be maintained at high concentrations for a prolonged period. The supersaturation behavior of the nanoplex in an acidic environment mimicking that of the stomach will be investigated in future studies.
respectively). The increased percentage of precipitates formed at higher AA concentrations is attributed to the increased hydrophobicity and lower solubility of the drugs at lower pH. In contrast to the soluble complex and precipitates formed by IB and CCM complexed with PAH, CIP complexation with DXT results only in nanoplex suspensions. Owing to its strong polyelectrolyte nature, DXT molecules are fully charged under all conditions investigated. The full ionization of DXT under the present operating condition is in contrast to that of PAH, where the addition of the basic drug solution results in the partial ionization of PAH. Despite the successful nanoplex formation at all MCIP/DXT/salt values investigated, this by no means implies that the CIP nanoplex can be produced under any conditions. More importantly, it does not mean that all of the CIP nanoplexes produced have the same characteristics (e.g., CE and size). 4.2. Significance of Rdrug/PE on CE and Yield. Although successful nanoplex formation is governed by the pHdependent ionization of the PE and Mdrug/PE/salt, the CE and yield are more or less governed by Rdrug/PE. In this regard, Rdrug/PE > 1 denotes the presence of more drug than PE available for complexation, resulting in low CE. However, Rdrug/PE < 1 denotes excess PE in the feed, which ensures high CE but with the trade-off of having a low yield. Therefore, the optimal formulation in terms of both the CE and yield is generally found at Rdrug/PE values equal to or slightly lower than unity, which is demonstrated in the present results. Figure 9 presents the CE and yield of all of the IB, CCM, and CIP nanoplexes (precipitates are excluded) plotted against their
5. CONCLUSIONS We have demonstrated the feasibility of employing an amorphous drug nanoplex as a supersaturating drug delivery system to improve the bioavailability of poorly soluble drugs. Both anionic (i.e., IB and CCM) and cationic (i.e., CIP) drug molecules can be transformed into the nanoplex using a simple, green process, which involves only mixing of the drug and the PE solution in the presence of salt under ambient conditions. The roles of the salt, PE, and drug in nanoplex formation have been elucidated by a close examination of the drug−PE complex phase diagrams. Specifically, the importance of (1) the drug’s hydrophobicity and charge density and (2) the PE’s degree of ionization has been discussed. The optimal nanoplex formulations exhibit high drug loading in the range of ∼80−85% that is attributed to the high CE achieved at Rdrug/PE ≤ 1. The nanoplex powders remain amorphous after 1 month of storage, denoting its stability attributed to the high Tg of the PE. The supersaturation level generated by the nanoplex in the presence of HPMC depends on the saturation solubility of the native API, where the lower the saturation solubility, the higher the supersaturation level. For CCM as the least-soluble API, high supersaturation levels of between ∼3 and 9 times the saturation solubility are generated by the CCM nanoplex over a 5 h period. The future direction is to extend the amorphous nanoplexation to poorly soluble APIs (
