Mathematical modeling of surface-active and non-surface-active drug transport in emulsion systems

Mathematical models were developed for the prediction of surface-active and non- surface-active drug transport in triphasic (oil, water, and micellar) emulsion systems as a function of micellar concentration. These models were evaluated by comparing experimental and simulated data. Fick's first law of diffusion with association of the surface-active or complexation nature of the drug with the surfactant was used to derive a transport model for surface-active drugs. This transport model assumes that the oil/water (O/W) partitioning process was fast compared with membrane transport and therefore drug transport was limited by the membrane. Consecutive rate equations were used to model transport of non- surface-active drugs in emulsion systems assuming that the O/W interface acts as a barrier to drug transport. Phenobarbital (PB) and barbital (B) were selected as surface-active model drugs. Phenylazoaniline (PAA) and enzocaine (BZ) were selected as non- surface-active model drugs. Transport studies at pH 7.0 were conducted using side-by-side diffusion cells and bulk equilibrium reverse dialysis bag techniques. According to the surface-active drug model, an increase in micellar concentration is expected to decrease drug-transport rates. Using the Microft EXCEL program, the non- surface-active drug model was fitted to the experimental data for the cumulative amount of the model drug that disappeared from the donor chamber. The oil/continuous phase partitioning rates (k₁) and the membrane transport rates (k₂) were estimated. The predicted data were consistent with the experimental data for both the surface-active and non- surface-active models.     

Effect of Nonionic Surfactant on Transport of Model Drugs in Emulsions

Purpose. To investigate the influence of excess surfactant on transport kinetics in emulsions, using phenylazoaniline (PAA), benzocaine, benzoic acid and phenol as model drugs. Mineral oil was chosen as the oil phase and the nonionic surfactant, polyoxyethylene oleyl ether (Brij 97) as the emulsifier. Methods. Model drug transport in emulsions was investigated using side by side diffusion cells mounted with hydrophilic dialysis or hydrophobic membranes. A novel method, involving a combination of a membrane equilibrium technique and surface tension measurement (Wilhelmy plate method), was developed to determine surfactant critical micelle concentration (CMC) in the presence of O/W emulsions. Emulsion stability was determined by droplet size analysis as a function of time, temperature and dilution using photon correlation spectroscopy and a light blockage technique. Model drug mineral oil/water partition coefficients and aqueous solubilities were determined in the presence of surfactant. Results. The emulsion CMC value was used to calculate micellar phase concentration. The transport rates of PAA and benzocaine in emulsions increased with increase in Brij 97 micellar concentration up to 1.0 % w/v and then decreased at higher surfactant concentrations. The transport rates of the more hydrophilic compounds, benzoic acid (ionized form, pH 7.0) and phenol, were not affected by the presence of micellar phase. Conclusions. Excess surfactant affected the transport rates of the model drugs in the emulsions depending on drug lipophilicity. Transport rates measured using side by side diffusion cells appeared to be governed by model drug partitioning rates from the oil to the continuous phases and by membrane type.     

Effect of Cationic Surfactant on Transport of Model Drugs in Emulsion Systems

Excess surfactant present in emulsions can influence the rates of transport of incorporated drugs by micellar solubilization, alteration of the partitioning process and by drug-surfactant complexation. Cetyltrimethylammonium bromide (CTAB), a cationic surfactant was selected to investigate these phenomena as it forms relatively stable mineral oil–water (O–W) emulsions and has the potential for ionic interaction. Phenylazoaniline, benzocaine, benzoic acid and phenol were chosen as model drugs for this study. The emulsion critical micelle concentration (CMC) for CTAB determined using a combination of a membrane equilibrium technique and surface-tension measurement was 10% w/v in 10% v/v% O-W emulsion systems. Ionic interaction between model drugs and surfactants and drug hydrophobicity affected their transport rates in the emulsion systems. The transport rates of the lipophilic drugs (benzocaine and phenylazoaniline) and the ionized hydrophillic drug (benzoic acid, pH 70) in the emulsion systems increased with increasing CTAB concentration up to 0–5% w/v micellar concentration and then decreased at higher concentrations. The rate of transport of phenol was not affected by the presence of micellar phase. Ionic interaction between surfactant and model drugs affected transport rates of model drugs in emulsion systems. The micellar phase was considered to affect the overall transport rates of model drugs.     

Effect of nonionic surfactant on transport of surface-active and non-surface-active model drugs and emulsion stability in triphasic systems

The effect of surfactant concentration on transport kinetics in emulsions using surface-active (phenobarbital, barbital) and non- surface-active is determined. Mineral oil was chosen as the oil phase and the nonionic surfactant polyoxyethylene-10-oleyl-ether (Brij 97) was chosen as the emulsifier. Model drug transport in the triphasic systems was investigated using side-by-side diffusion cells mounted with hydrophilic dialysis membranes (molecular weight cutoffs 1 kd and 50 kd) and a novel bulk equilibrium reverse dialysis bag technique. Emulsion stability was determined by droplet size analysis as a function of time, temperature, and the presence of model drugs, using photon correlation spectroscopy. Mineral oil/water (O/W) partition coefficients and aqueous solubilities were determined in the presence of surfactant. The transport rates of model drugs in emulsions increased with an increase in Brij 97 micellar concentrations up to 1.0% wt/vol and then decreased at higher surfactant concentrations. The transport profiles of the model drugs appeared to be governed by model drug O/W partition coefficient values and by micellar shape changes at higher surfactant concentrations. Total transport rates of phenobarbital and barbital were faster than those of phenylazoaniline and benzocaine. Excess surfactant affected the transport rates of the model drugs in the emulsions depending on drug surface activity and lipophilicity.     

Effect of Ingested Lipids on Drug Dissolution and Release with Concurrent Digestion: A Modeling Approach

Purpose

To mechanistically study and model the effect of lipids, either from food or self-emulsifying drug delivery systems (SEDDS), on drug transport in the intestinal lumen.

Methods

Simultaneous lipid digestion, dissolution/release, and drug partitioning were experimentally studied and modeled for two dosing scenarios: solid drug with a food-associated lipid (soybean oil) and drug solubilized in a model SEDDS (soybean oil and Tween 80 at 1:1 ratio). Rate constants for digestion, permeability of emulsion droplets, and partition coefficients in micellar and oil phases were measured, and used to numerically solve the developed model.

Results

Strong influence of lipid digestion on drug release from SEDDS and solid drug dissolution into food-associated lipid emulsion was observed and predicted by the developed model. Ninety minutes after introduction of SEDDS, there was 9% and 70% drug release in the absence and presence of digestion, respectively. However, overall drug dissolution in the presence of food-associated lipids occurred over a longer period than without digestion.

Conclusion

A systems-based mechanistic model incorporating simultaneous dynamic processes occurring upon dosing of drug with lipids enabled prediction of aqueous drug concentration profile. This model, once incorporated with a pharmacokinetic model considering processes of drug absorption and drug lymphatic transport in the presence of lipids, could be highly useful for quantitative prediction of impact of lipids on bioavailability of drugs.

     

Sustained release of hydrophobic and hydrophilic drugs from a floating dosage form

Floating dosage forms enable the sustained delivery of drugs in the gastro-intestinal tract. In this study, a type of multi-unit floating gel bead was synthesized with calcium alginate, sunflower oil, and a drug of interest through an emulsification/gelation process. The alginate beads with oil addition were able to continuously float over the medium for 24h under constant agitation while the non-oily beads could not. Three kinds of drugs with different hydrophilicities, ibuprofen, niacinamide and metoclopramide HCl, were tested in the study. The hydrophobic drug ibuprofen was released in a sustained manner for 24h, due to the oil partitioning. With suitable modification, the beads were able to also release the hydrophilic drugs, niacinamide and metoclopramide HCl, for a similar duration. Therefore a floating dosage form that is able to sustain release both hydrophobic and hydrophilic drugs within its extended gastric retention time has been developed.     

Drug-surfactant interactions: effect on transport properties

The physico-chemical interactions between three model drugs and a variety of surfactants were characterized by measuring the apparent permeability coefficients of the drugs in the presence and absence of surfactants in vitro. The extent of interaction between the model drugs and the surfactants can best be described by the hydrophobic effect (primarily determined by the hydrophobic surface area of the drug molecule) and the electrostatic effect (primarily determined by the charge associated with the drug molecule as well as the surfactant molecules). For drugs that do not possess a significant hydrophobic surface area (timolol and cefoxitin), their interactions can best be described based on electrostatic effects (charge effects). This interaction being strong with oppositely charged surfactants. The interactions of L-692 585 (a model drug with appreciable hydrophobic surface area) in the presence of surfactants is dominated by the hydrophobic effect, with the electrostatic effect playing a minor secondary role. The apparent permeability coefficient of timolol as a function of the amount of surfactant in solution is modelled in light of micellar formation and entrapment and/or interaction of free drug with this micellar structure. Briefly, the extent of interaction as a function of amount of added surfactant for timolol indicates that initially as surfactant is added the activity of drug for transport declines significantly until a breaking point is reached, after which the drug activity available for transport remains relatively constant upon addition of more surfactant. A model is derived which is capable of describing this behavior and provides reasonable estimates for the critical micellar concentration of the surfactant, the affinity or binding constant for the interaction of drug with an equivalent micellar structure, and the loading capacity of the equivalent micellar structure. These observations are potentially significant for drug formulation of poorly bioavailable drugs.     

Structure‐based prediction of human intestinal membrane permeability for rapid in silico BCS classification

Human effective intestinal membrane permeability (P_eff) is one of the two important indicators for drug classification according to the Biopharmaceutical Classification System (BCS), and contributes greatly to the performance of oral drug absorption. Here, a structure‐based in silico predictive model of P_eff was developed successfully to facilitate in silico BCS classification in the early stage of drug discovery, even before the compound was synthesized. The quantitative structure–P_eff relationship for 30 drugs was constructed based on seven structural parameters. Then the model was built by the multiple linear regression method and internally validated by the residual analysis, the normal probability–probability plot and the Williams plot. For the entire data set, the R² and adjusted R² values were 0.782 and 0.712, respectively. The results indicated that the fitted model was robust, stable and satisfied all the prerequisites of the regression models. As for the 102 tested drugs, the predicted P_eff values had a good correlation with the experimental human absorbed fraction (F_a). This model was also used to perform high/low P_eff classification for 57 drugs that have been classified according to the BCS, and 72% of drugs could be classified correctly, indicating that the developed model can be used for rapid BCS classification in the early stages of drug discovery. Copyright © 2013 John Wiley & Sons, Ltd.     

Modeling of the drug delivery from a hydrophilic transdermal therapeutic system across polymer membrane.

A mathematical simulation is presented which describes the in vitro drug delivery kinetics from hydrophilic adhesive water-soluble poly-N-vinylpyrrolidone (PVP)-polyethylene glycol (PEG) matrices of transdermal therapeutic systems (TTS) across skin-imitating hydrophobic Carbosil membranes. Propranolol is employed as the test drug. The contributions of the following physicochemical determinants to drug delivery rate control have been estimated: the drug diffusion coefficients both in the matrix and the membrane; the membrane-matrix drug partition coefficient: the drug concentration in the matrix and the membrane thickness. Drug transfer from the hydrophilic matrix across the membrane is shown to be controlled by the drug partitioning from the matrix into the membrane. The best correlation between simulation data and experimental results is obtained when the effect of membrane hydration is taken into consideration during in vitro drug release.     

Dry adsorbed emulsions: an oral sustained drug delivery system

The oral sustained drug delivery system "dry adsorbed emulsion" was defined as an organized dispersion of hydrophilic and hydrophobic particles whose structure was initiated by the structure of a water-in-oil (W/O) emulsion. Sodium salicylate was dissolved in the aqueous phase of the primary W/O emulsion as an active drug. The aqueous phase of the W/O emulsion was adsorbed by a hydrophilic silica and then a hydrophobic silica was added to the preparation to obtain a stable and solid pulverulent form. The physicochemical structure of a "dry adsorbed emulsion" was described and observed by electron microscopy. The effect of different oils, castor oil and a silicone oil, on the sustained drug release was studied at two different pH values, 1.2 and 7.4, to simulate the gastric and intestinal medium, respectively. The properties of these forms were retained for more than one year at room temperature storage.     

Liposome transport of hydrophobic drugs: gel phase lipid bilayer permeability and partitioning of the lactone form of a hydrophobic camptothecin, DB-67

The design of liposomal delivery systems for hydrophobic drug molecules having improved encapsulation efficiency and enhanced drug retention would be highly desirable. Unfortunately, the poor aqueous solubility and high membrane binding affinity of hydrophobic drugs necessitates extensive validation of experimental methods to determine both liposome loading and permeability and thus the development of a quantitative understanding of the factors governing the encapsulation and retention/release of such compounds has been slow. This report describes an efflux transport method using dynamic dialysis to study the liposomal membrane permeability of hydrophobic compounds. A mathematical model has been developed to calculate liposomal membrane permeability coefficients of hydrophobic compounds from dynamic dialysis experiments and partitioning experiments using equilibrium dialysis. Also reported is a simple method to study the release kinetics of liposome encapsulated camptothecin lactone in plasma by comparing the hydrolysis kinetics of liposome entrapped versus free drug. DB-67, a novel hydrophobic camptothecin analogue has been used as a model permeant to validate these methods. Theoretical estimates of DB-67 permeability obtained from the bulk solubility diffusion model and the "barrier-domain" solubility diffusion model are compared to the experimentally observed value. The use of dynamic dialysis in drug release studies of liposome and other nanoparticle formulations is further discussed and experimental artifacts that can arise without adequate validation are illustrated through simulations.     

‘Dry emulsion’ — a sustained release form: Modelling of drug transfers in liquids

A galenic form made of a dry emulsion was described and tested by considering the drug release in synthetic liquids. It was obtained by mixing the two following phases: the one with water, sodium salicylate as drug and hydrophilic silica in powder form; the other being a lipidic phase with oil and hydrophobic silica. A model, based on a numerical method with finite differences, was applied to the case of spherical samples. The theoretical results for the kinetics of drug release were in good agreement with the corresponding experimental ones. Dry emulsion exhibited a significant sustained drug release, controlled by diffusion with a constant diffusivity and a coefficient of matter transfer characterizing the retardation in the transfer on the surface.     

Intestinal permeability and its relevance for absorption and elimination

Human jejunal permeability (P_eff) is determined in the intestinal region with the highest expression of carrier proteins and largest surface area. Intestinal P_eff are often based on multiple parallel transport processes. Site-specific jejunal P_eff cannot reflect the permeability along the intestinal tract, but they are useful for approximating the fraction oral dose absorbed. It seems like drugs with a jejunal P_eff > 1.5 x 10^–4 cm s^–1 will be completely absorbed no matter which transport mechanism(s) are utilized. Many drugs that are significantly effluxed in vitro have a rapid and complete intestinal absorption (i.e. >85%) mediated by passive transcellular diffusion. The determined jejunal P_eff for drugs transported mainly by absorptive carriers (such as peptide and amino acid transporters) will accurately predict the fraction of the dose absorbed as a consequence of the regional expression. The data also show that: (1) the human intestinal epithelium has a large resistance towards large and hydrophilic compounds; and (2) the paracellular route has a low contribution for compounds larger than approximately molecular weight 200. There is a need for more exploratory in vivo studies to clarify drug absorption and first-pass extraction along the intestine. One is encouraged to develop in vivo perfusion techniques for more distal parts of the gastrointestinal tract in humans. This would stimulate the development of more relevant and complex in vitro absorption models and form the basis for an accurate physiologically based pharmacokinetic modelling of oral drug absorption.     

Preparation of freeze-thaw poly(vinylalcohol) emulsion gel suppository

Poly (vinylalcohol) (PVA) emulsion gel suppositories were prepared by a given cycle of freezing and thawing. Oil phase and emulsifying agent used were Panacete 800 and a series of Pluronic L-44, respectively. The effects of polymerization degree of PVA on the gel strength and the drug release were investigated. Drug release from PVA emulsion gel suppository was compared with that from a conventional suppository. The structure of gel was observed by using a scanning electron microscope. The gel strength increased when PVA emulsion gel suppository was prepared with Panacete 800 and Pluronic L-44. The drug release of hydrophilic and hydrophobic drugs from the suppository was in agreement with a zero-order release profile. When oil phase was added into PVA gel suppository, PVA fiber became thin and the network of PVA fiber became dense.     

Oral Delivery of a Renin Inhibitor Compound Using Emulsion Formulations

The oral delivery of O-(N-morpholino-carbonyl-3-L-phenylaspartyl-L-leucinamide of (2S,3R,4S)-2-amino-l-cyclohexyl-3,4-dihydroxy-6-methylheptane (I), a new renin inhibitor, was studied in the in vivo rat model using emulsion formulations. The components of the emulsion formulations were chosen based on their proposed effects on membrane structure, membrane fluidity, and solute transport. The percent absolute bioavailability (%AB) of I was increased from 0.3% (water suspension) to 5.1% when long-chain unsaturated fatty acid (oleic acid, linoleic acid, etc.)- and mono- and diglyceride (monolein, dilaurin, etc.)-containing emulsion formulations were used. Considering very high first-pass liver extraction of the compound (80%), it is suggested that emulsion formulations increased the intestinal transport of the compound significantly. The solubility of I in aqueous media with and without bile salt (20 mM) was found to be low (~1 µg/ml). Incubation in 0.01 N HC1 did not affect the particle size of the emulsion. The titration of oleic acid/monoolein emulsion in a pH 6.5 medium with a mixed bile salt system indicated reduction in the particle size of the emulsion. Drug precipitation was observed above 30 mM bile salt concentrations. No drug crystals could be detected in the intestinal contents of the rats when emulsion formulations were ingested. These results suggest that in the intestine of the animals, the particle size of the emulsions is reduced in the presence of bile fluid while the drug resides primarily in the oil phase. The mechanism of enhanced transport of I from the emulsion formulations is discussed along with the possibility of cotransport for the drug and oil. Emulsion formulations can be a potential delivery form for low-bioavailable lipid-soluble drugs.     

Hydrophilic polymeric matrices for enhanced transdermal drug delivery

For many drugs with various chemical structures, delivery rates from the hydrophilic polyvinylpyrrolidone (PVP)-polyethylene oxide (PEO) based pressure sensitive adhesive (PSA) matrices of transdermal therapeutic systems (TTS) are higher compared to the hydrophobic TTS matrices. Delivery of propranolol, glyceryl trinitrate (GTN) and isosorbide dinitrate (ISDN) from the hydrophilic water soluble TTS matrix across human cadaver skin epidermis or skin-imitating polydimethylsiloxane-polycarbonate block copolymer Carbosil membrane in vitro is characterized by high rate values and zero-order drug delivery kinetics up to the point of 75–85% drug release from their initial contents in matrix. Both in vitro and in vivo drug delivery rates from the TTS hydrophilic diffusion matrix are controlled by the skin or membrane permeability and may be described by Fick's law. The contributions of various physicochemical determinants to the control of transdermal drug delivery kinetics are discussed. Pharmacokinetic and pharmacodynamic properties of hydrophilic TTS matrix with propranolol, GTN and ISDN are described.     

Preparation of multi-phase microspheres of poly(D,L-lactic acid) and poly(D,L-lactic-co-glycolic acid) containing a W/O emulsion by a multiple emulsion solvent evaporation technique.

Multi-phase microspheres of poly(D,L-lactic acid) (PLA) or poly(D,L-lactic-co-glycolic acid) (PLGA) containing a water-in-oil (W/O) emulsion were prepared by a multiple emulsion solvent evaporation technique. Acetonitrile was used as the solvent for the polymers and light mineral oil as the dispersion medium for the encapsulation procedure. Process and formulation parameters to optimize the microencapsulation of a W/O emulsion containing water-soluble drugs were investigated. Drug loading efficiencies of 80-100 per cent were obtained under specific preparative conditions. The drug loading efficiency in the microspheres was dependent upon the ratio of the W/O emulsion to polymer and the concentration of surfactant in the mineral oil. Compared to conventional microspheres, in which fine drug particles are homogeneously dispersed in the polymer beads, the multi-phase microspheres permit the higher encapsulation efficiency of water-soluble drugs and eliminate partitioning into the polymer-acetonitrile phase which results in low encapsulation efficiency with conventional solvent evaporation techniques.     

Membrane Transport in Hepatic Clearance of Drugs I: Extended Hepatic Clearance Models Incorporating Concentration-Dependent Transport and Elimination Processes

Purpose. The objective of the present study was to develop hepatic clearance models which incorporate a unidirectional carrier-mediated uptake and bidirectional diffusional transport processes for drug transport in the sinusoidal membrane of hepatocytes as well as nonlinear intrinsic elimination. Methods. Two models were derived which view the liver as two separate compartments, i.e., sinusoid and hepatocyte. Model I assumes the instantaneous complete mixing of drugs within each compartment (similar to that of the 'well-stirred' model), while model II assumes that the drug concentrations in both compartments decrease progressively in the direction of the hepatic blood flow path (similar to that of the 'parallel-tube' model). Computer simulations were performed using a range of steady-state infusion rates for a substrate, while varying theV _max (capacity) and K _m (Michaelis-Menten constant) for the carrier-mediated uptake process, the diffusional clearance, the V _max and K _m for the intrinsic elimination process, blood flow and protein binding. Results. Simulations in which V _max and K _m for the sinusoidal membrane transporter and the diffusional clearance were varied, demonstrated that these membrane transport processes could affect the clearance of drugs to a significant extent in both models. The estimates for clearance of substrates with the same pharmacokinetic parameters are always lower in model I than in model II, although the quantitative differences in parameter estimates between models varied, depending on the steady state infusion rates. Conclusions. These more general hepatic clearance models will be most useful for describing the hepatic clearance of hydrophilic compounds, such as organic anions or cations, which exhibit facilitated uptake and limited membrane diffusion in hepatocytes.     

The Effect of β-Turn Structure on the Passive Diffusion of Peptides Across Caco-2 Cell Monolayers

Purpose. To investigate the relationships between the -turn structure of a peptide and its passive diffusion across Caco-2 cell monolayers, an in vitro model of the intestinal mucosa. Methods. Linear hydrophilic peptides (Ac-TyrProXaaZaaVal-NH₂; Xaa = Gly, Ile and Zaa = Asp, Asn) and hydrophobic (Ac-YaaPro-XaaIleVal-NH₂; Yaa = Tyr, Phe and Xaa = Gly, Ile: and Ac-PhePro-XaaIle-NH₂; Xaa = Gly, He) peptides were synthesized and their effective permeability coefficients (P_eff) were determined across Caco-2 cell monolayers. The lipophilicities of the peptides were estimated by measuring their partition coefficients (P_o/w) between 1-octanol and HBSS. Two-dimensional NMR (2D-NMR) spectroscopy and circular dichroism (CD) spectroscopy was used to determine the solution structures of these model peptides. Results. Using 2D-NMR spectroscopy and CD spectroscopy, the hydrophilic Gly-containing peptides (Ac-TyrProGlyZaaVal-NH₂; Zaa = Asp, Asn) were shown to exhibit a higher degree of -turn structure in solution than the Ile-containing peptides (Ac-TyrProIleZaaVal-NH₂; Zaa = Asp, Asn). CD spectroscopy was used to show that the Gly-containing hydrophobic peptides (Ac-YaaProGlyIleVal-NH₂; Yaa = Tyr, Phe: and Ac-PheProGlyIle-NH₂) exhibited a higher degree of -turn structure in solution than the Ile-containing hydrophobic peptides. The P_eff values of all four hydrophilic peptides across unperturbed Caco-2 cell monolayers were very low and no statistically significant differences were observed between the Gly- and Ile-containing penta-peptides within either the Asp or Asn series. The P_eff values for the hydrophobic Gly-containing peptides were significantly greater than the P_eff values determined for their Ile-containing counterparts. The Gly-containing penta- and tetrapeptides in the Phe series, which exhibited high permeation, were shown to be metabolically unstable. In contrast, the Gly- and Ile-containing pentapeptides in the Tyr series and the Ile-containing penta- and tetrapeptides in the Phe series, which exhibited low permeation, were metabolically stable. Conclusions. Hydrophobic peptides that exhibit significant -turn structure in solution are more lipophilic as measured by log P_o/w and more readily permeate Caco-2 cell monolayers via the transcellular route than hydrophobic peptides that lack this type of solution structure. The ability of these peptides to permeate Caco-2 cell monolayers via the transcellular route also exposed them to metabolism, presumably by cytosolic endopeptidases. Similar secondary structural features in hydrophilic peptides do not appear to sufficiently alter the physicochemical properties fo the peptides so as to alter their paracellular flux through unperturbed Caco-2 cell monolayers.