Enhancing Oil Solubility of BCS Class II Drug Phenytoin Through Hydrophobic Ion Pairing to Enable High Drug Load in Injectable Nanoemulsion to Prevent Precipitation at Physiological pH With a Potential to Prevent Phlebitis.

This work investigates the micellar titration of phenytoin (a weakly acidic drug) with cetyltrimethylammonium hydroxide (CTAH) to form a hydrophobic ion-pair to enhance oil solubility of phenytoin, followed by an effort to formulate nanoemulsion that could potentially prevent precipitation of phenytoin at physiological pH. The ion-pair formulated in nanoemulsion was evaluated for in vitro precipitation during serial dilution at physiological pH. The formation of ion-pair during titration was explained in context of pH-solubility data. The mathematical model successfully integrated ionization and micellization equilibria to reflect on dominant mechanisms for solubilization. The micellar phenomenon during titration was confirmed using Dynamic Light Scattering (DLS). The phase changes of the excess undissolved solids during titration were evident from X-Ray Powder Diffraction (XRPD) and Fourier Transform Infrared Spectroscopy (FTIR). This analysis confirmed the conversion of phenytoin into ionized state and its subsequent ionic interaction with CTAH forming hydrophobic ion-pair complex (HIP). The complete ion pair formation was evident at pHmax (8.8 to 9.2), and its 1:1 stoichiometry was confirmed using HPLC (Phenytoin and CTAH) and H1 NMR, hence could also be called as a lipophilic salt. The ion-pair (salt) was insoluble in water and showed remarkably high partition coefficient (log P) in octanol/water. As characterized by Hot Stage Microscopy (HSM), the melting point of the ion-pair complex was lowered to 150.8°C compared to the free acid (> 300οC), this was even further lowered to 81.1 °C when evaluated in castor oil. This led to approximately eight-fold higher solubility of hydrophobic ion pair (HIP) in castor oil compared to the free acid form. The high miscibility in castor oil was suitable to formulate a high drug load injectable dispersed system. This was successfully achieved with lecithin and polysorbate as emulsifiers without leaching drug into continuous phase at pH 7.4. This nanoemulsion (<300 nm, and > +30 mV zeta potential) remain stable when evaluated over a period of one month. A serial dilution study of the nanoemulsion was performed in PBS buffer, microscopic observations suggested no birefringence despite incubation at 25°C for several hours. This result indicated that Phenytoin remained strongly partitioned within dispersed oily phase with a higher drug loading when ion-paired phenytoin was used. The higher drug load could enable a small volume slow bolus injection to meet 50 mg/min or lower delivery rate criteria for Phenytoin in the clinical set up. This provided a pathway to further explore potential injectable nano-emulsion formulations that could alleviate typical phlebitis issue associated with the injectable phenytoin solution administration at physiological pH.

Ionic liquid formation with deep eutectic forces at an atypical ratio (2:1) of naproxen to lidocaine in the solid-state, thermal characterization and FTIR investigation.

This study aimed to determine if an ionic liquid resulted from the combination of naproxen and lidocaine and the impact that varying the molar percentage of each component had on thermal behavior. Samples were prepared by mixing naproxen and lidocaine in tetrahydrofuran (THF), followed by solvent removal using a vacuum oven. Thermal events were investigated using differential scanning calorimetry (DSC) and the extent of proton transfer was probed using Fourier-Transform Infrared Spectroscopy (FTIR). The binary samples that had a higher molar percentage of naproxen formed a 2:1 naproxen to lidocaine complex, with a melting temperature in the range of 82-85 °C. This complex was determined to be a peritectic composition of naproxen and lidocaine and spectroscopic analysis confirmed the complex had ionic liquid character as well as hydrogen bonding interactions. Binary samples that contained a greater molar percentage of lidocaine formed high-order complexes of varying stoichiometry with melting temperatures below 50 °C. Spectroscopic analysis determined that a greater molar concentration of lidocaine drove more eutectic behavior, although some ionization was also present. The conclusion from this study was that naproxen and lidocaine form an ionic liquid material at a peritectic point corresponding to 2 mol of naproxen to 1 mol of lidocaine. At molar concentrations of greater lidocaine than naproxen, higher order complexes form which are largely driven by eutectic behavior.

Impact of mixed counter ion on saturation solubility of esylate salt of a weak basic drug to formulate physically stable and non-hemolytic ready to use injectable solution.

Current work investigates a typical issue in formulating a physically stable solution especially when more than one counter ions exist in the composition. The impact of different counter ions on solubilization of monohydrate esylate salt of a free base GSK-497,[BH+:C2H5SO3-:H2O] (1:1) (pKa value 8.0) was investigated to formulate ready to use small volume injectable solution. The concentration dependent aggregation was also appeared to be responsible for hemolytic nature of the drug, therefore a careful investigation was needed to select appropriate counter ion solution without compromising solubilization and leading into higher order aggregation. The esylate salt's native pH in water was closer to pHmax, thus it was risky to render the solution unbuffered. Generally, it is recommended to formulate at least two pH unit away from pHmax to prevent disproportionation related physical instability. This was achieved by buffering solution away from pHmax, using a lactate counter ion (other than esylate salt of API salt) that did not compromise solubility of the given phase and did not appear to promote higher order of aggregation. The rationale for selecting second counter ion was primarily based on the comparison of esylate salt's solubility product (Ksp), with the Ksp value generated from equilibrium solubility of the free base combined with several different counter ions (chloride, lactate, aspartate, citrate and tartrate) at equimolar molar ratio. This approach suggested that the use of a counter ion with higher Ksp (lactate and aspartate) value did not compromise the solubility of original esylate salt but a higher extent of aggregation was possible if aspartate is used to achieve higher solubility. In contrary, use of a counter ion with lower Ksp (citrate, tartrate, chloride) reduced the solubility hence did not favor higher order of aggregation. Thus, based on Ksp comparison a rationale of selecting second counter ion to buffer the salt solution is discussed in this work and optimal formulation concentration is determined based on drug aggregation threshold in solution.

Mechanism of Decarboxylation of Pyruvic Acid in the Presence of Hydrogen Peroxide.

The purpose of this work was to probe the rate and mechanism of rapid decarboxylation of pyruvic acid in the presence of hydrogen peroxide (H2O2) to acetic acid and carbon dioxide over the pH range 2-9 at 25 °C, utilizing UV spectrophotometry, high performance liquid chromatography (HPLC), and proton and carbon nuclear magnetic resonance spectrometry ((1)H, (13)C-NMR). Changes in UV absorbance at 220 nm were used to determine the kinetics as the reaction was too fast to follow by HPLC or NMR in much of the pH range. The rate constants for the reaction were determined in the presence of molar excess of H2O2 resulting in pseudo first-order kinetics. No buffer catalysis was observed. The calculated second-order rate constants for the reaction followed a sigmoidal shape with pH-independent regions below pH 3 and above pH 7 but increased between pH 4 and 6. Between pH 4 and 9, the results were in agreement with a change from rate-determining nucleophilic attack of the deprotonated peroxide species, HOO(-), on the α-carbonyl group followed by rapid decarboxylation at pH values below 6 to rate-determining decarboxylation above pH 7. The addition of H2O2 to ethyl pyruvate was also characterized.

Determination of the permeability characteristics of two sulfenamide prodrugs of linezolid across Caco-2 cells.

The purpose of this work was to study the permeability of two relatively lipophilic sulfenamide prodrugs of linezolid (clogP 0.85), N-(phenylthio)linezolid (1, clogP 2.77) and N-[(2-ethoxycarbonyl)ethylthio]linezolid (2, clogP 1.43), across Caco-2 cell monolayers. Both prodrugs were found to convert to linezolid in the donor compartment presumably from the reaction with free thiol groups on proteins on the surface of the Caco-2 cells, as no conversion was seen in the donor compartment media per se. Neither of the prodrugs could be detected in the receptor phase from either apical (AP) to basolateral (BL) or BL to AP studies. However, the appearance of linezolid in the receptor phase was biphasic with an initial rapid phase suggesting that the prodrugs were indeed more permeable, and for a short period, some prodrug was able to permeate in competition with conversion to linezolid on the donor phase surface. It appears that the prodrug was able to permeate was rapidly converted to linezolid prior to acceptor phase appearance. The second slower phase was due to the permeability of the donor-phase-formed linezolid, with the slopes similar to those from control experiments with linezolid. The limitations and possible utility of oral sulfenamide prodrugs are discussed.

An ion pairing approach to increase the loading of hydrophilic and lipophilic drugs into PEGylated PLGA nanoparticles.

The aim of this study was to enhance the loading of dalargin (enkephalin derivatives) a hydrophilic drug and loperamide HCl (non-opiate antidiarrheal agent) a lipophilic drug candidates within PEGylated nanoparticles. A novel nanoencapsulation method based on the concept of s/o/w and ion pairing followed by solvent diffusion was adopted. The copolymers with three different mPEG densities (5%, 12% and 17%) were employed separately in combination with two different grades of dextran sulphate (DS) 5000 and 500,000 MW in the preparations. Nanoparticles prepared from copolymers with increasing mPEG densities, showed an insignificant (p>0.05) increasing trend of drug loading, this was however significantly increased when DS5000 was included in the preparations. The particle size remains unchanged after dalargin loading, with no significant (p>0.05) alteration in the neutral zeta potential compared to that of the preparations without DS5000. Considering that a dalargin ion pair could also have a neutral charge, it was not advisable to conclude its incorporation, as the size remain unchanged, which would otherwise increase if an ion pair was incorporated within the core of nanoparticles. Therefore, it was expected that a dalargin ion pair might be located outside the core as a separate particulate entity or reside in the hydrophilic shell of the nanoparticles. A loperamide HCl ion pair showed significant (p<0.05) increase in size when incorporated; at the same time it provided a neutral zeta potential despite adding negatively charged DS5000 in the preparation, hence it seemed encapsulated. Inclusion of DS500,000 in the preparation further increased the drug loading of dalargin and loperamide HCl. However, a significant (p<0.05) negative zeta potential was noted in both cases which suggested that excess charge was still available on the surface of nanoparticles which could trap further amounts of drug on the surface rather than inside the core of nanoparticles. During in vitro evaluation of drug loaded nanoparticles, dalargin released as quickly as free drug, when loperamide HCl showed almost burst free sustained release profile with respect to the release of their free drug solutions, suggested that ion pairing approach was more pronounced for loperamide HCl formulation.

Comparison and validation of drug loading parameters of PEGylated nanoparticles purified by a diafiltration centrifugal device and tangential flow filtration.

This article describes drug loading validation of nanoparticles. Ultracentrifugation was avoided because of problems arising from small-sized particles. Ultrafiltration was adopted in two different modes followed by monitoring of polyvinyl alcohol (PVA), dextran sulfate (DS), and loperamide HCl contents. Diafiltration centrifugation removed all PVA at the fourth cycle and provided significantly (p = .000, .017) higher drug loading values compared with tangential flow filtration (TFF). This was due to residual PVA associated with the nanoparticles. TFF enabled satisfactory dry weight recovery (101.66 +/- 4.45%, n = 3) of nanoparticles during extended purification. Indirect drug loading (from the purification curve) was not significantly different (p = .450, .487) to the direct drug loading values. Encapsulation parameters were obtained from the purification curve once quantitative estimation of the all formulation components was established.

Purification of PEGylated nanoparticles using tangential flow filtration (TFF).

A tangential flow filtration system was evaluated to purify PEGylated nanoparticles. Two widely used surfactants, PVA and sodium cholate were efficiently removed from an empty nanoparticles suspension using the proposed system. During drug loading, surfactant (PVA) was observed to be entrapped within the core of the nanoparticle to a higher extent, hence was purified at a comparatively slower rate. The presence of dextran sulfate enhanced the drug loading but also resulted in reduced purification rate; this was described by the hypothesis of PVA inclusion within the core of the nanoparticles. Practically, it was possible to correlate the slow purification rate of PVA to its reduced filtration flow during the purification of the empty and loaded nanoparticles containing dextran sulfate. Indirectly, this system was capable of revealing the influence of an excipient and drug on the nanoparticle surface.

Comparison of diafiltration and tangential flow filtration for purification of nanoparticle suspensions.

PURPOSE: The study reports evaluation of different purification processes for removing surplus surfactant and formulating stable nanoparticle dispersions. METHODS: Nanoparticle formulations prepared from poly(D,L-lactide-co-glycolide) and polyvinyl alcohol (PVA) were purified by a diafiltration centrifugal device (DCD), using 300K and 100K molecular weight cut-off (MWCO) membranes and a tangential flow filtration (TFF) system with a 300K MWCO membrane. The effects of process parameters including MWCO, transmembrane pressure (TMP), and mode of TFF on nanoparticle purification were evaluated, and two purification techniques were compared to the commonly used ultracentrifugation technique. RESULTS: Both DCD and TFF systems (concentration mode at TMP of 10 psi) with 300K MWCO membrane removed maximal percent PVA from nanoparticle dispersions (89.0 and 90.7%, respectively). T90, the time taken to remove 90% of PVA in 200-ml sample, however, was considerably different (9.6 and 2.8 h, respectively). Purified nanoparticle dispersions were stable and free of aggregation at ambient conditions over 3 days. This is in contrast to the ultracentrifugation technique, which, although it can yield a highly purified sample, suffers from drawbacks of a level of irreversible nanoparticle aggregation and loss of fine particles in the supernatant during centrifugation. CONCLUSIONS: The TFF, in concentration mode at TMP of 10 psi, is a relatively quick, efficient, and cost-effective technique for purification and concentration of a large nanoparticle batch (>or=200 ml). The DCD technique can be an alternative purification method for nanoparticle dispersions of small volumes.