The determination of diffusion coefficients in semisolids by Fourier transform infrared (FT-IR) spectroscopy

Ocular Physiologically Based Pharmacokinetic Modeling for Ointment Formulations

Purpose

The purpose of this study is to show how the Ocular Compartmental Absorption & Transit (OCAT™) model in GastroPlus^® can be used to characterize ocular drug pharmacokinetic performance in rabbits for ointment formulations.

Methods

A newly OCAT™ model developed for fluorometholone, as well as a previously verified model for dexamethasone, were used to characterize the aqueous humor (AH) concentration following the administration of multiple ointment formulations to rabbit. The model uses the following parameters: application surface area (SA), a fitted application time, and the fitted Higuchi release constant to characterize the rate of passage of the active pharmaceutical ingredient from the ointment formulations into the tears in vivo.

Results

Parameter sensitivity analysis was performed to understand the impact of ointment formulation changes on ocular exposure. While application time was found to have a significant impact on the time of maximal concentration in AH, both the application SA and the Higuchi release constant significantly influenced both the maximum concentration and the ocular exposure.

Conclusions

This initial model for ointment ophthalmic formulations is a first step to better understand the interplay between physiological factors and ophthalmic formulation physicochemical properties and their impact on in vivo ocular drug pharmacokinetic performance in rabbits.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11095-020-02965-y.

Pharmaceutical evaluation of steroidal ointments by ATR-IR chemical imaging: distribution of active and inactive pharmaceutical ingredients

We recently used micro attenuated total reflection infrared (ATR-IR) spectroscopy to conduct imaging analysis of ointments and evaluate the distributions of the active pharmaceutical ingredient (API) and excipients. An alclometasone dipropionate (ALC) ointment was used as a model product. Almeta, a brand-name product, had a domain with absorbance at 1656 cm(-1) attributable to the carbonyl group of ALC, the API. Absorbances at 1040 and 3300 cm(-1) were also noted in this domain, indicating the presence of the solubilizer, propylene glycol. Data also suggested the presence of benzyl alcohol in this domain. More detailed analysis showed the distribution of surfactants and other excipients in the base. Similar results were obtained for Vitra, a generic version of Almeta. Imaging analysis with micro ATR-IR confirmed that both ointments are liquid droplet dispersions with ALC dissolved in propylene glycol and dispersed in a base. However, minor differences in the ingredient distributions of the two ointments were detected and reflect differences in excipient concentrations and type, or manufacturing differences. In summary, we used micro ATR-IR for imaging analysis of an original ointment, Almeta, and its generic form Vitra, and established a method for visually evaluating the distributions of the API and excipients in these ointments.

A theoretical model for transdermal drug delivery from emulsions and its dependence upon formulation

This article presents a theoretical model of transdermal drug delivery from an emulsion-type vehicle that addresses the vehicle heterogeneity and incorporates the prediction of drug transport parameters as function of the vehicle composition. The basic mass transfer model considers interfacial and diffusion resistances within the emulsion and partition/diffusion phenomena across two skin compartments in series. Drug transport parameters are predicted as follows: partition coefficients are derived from regular solutions theory, drug diffusivity in the continuous phase is computed from a free volume theory with segmental motion, and permeability of the surfactant layer around droplets is estimated based on a free surface area model. These relationships are incorporated within the basic mass transfer model, so that the overall model is able to predict temporal profiles of drug release from the vehicle and of drug concentration in plasma, as a function of vehicle composition. In this way, the proposed model provides a sound physicochemical basis to support the development of new formulations and the planning of experiments. A simulated case study regarding a nitroglycerin ointment is presented in detail, illustrating how thermodynamic and kinetic factors inherent to the emulsion vehicle can modulate drug release and subsequent systemic absorption.

Pharmaceutical applications of Mid-IR and Raman spectroscopy

Mid-IR and Raman spectroscopy are versatile tools in pharmaceutics and biopharmaceutics, with a wide field of applications ranging from characterization of drug formulations to elucidation of kinetic processes in drug delivery. After an introduction to the basic principles of IR and Raman spectroscopy, new developments in applications of these methods for studying drug delivery systems, in particular topical drug delivery, will be reviewed. FTIR-ATR is a well-established standard method used to study drug release in semisolid formulations, drug penetration, and influence of penetration modifiers; it is also capable of in vivo studies. FTIR-PAS has been applied to measure drug content in semisolid and solid formulations, to determine drug penetration into artificial and biological membranes. The big advantage of this technique is the possibility of spectral depth profiling. However, FTIR-PAS is so far limited to in vitro investigations. Raman spectroscopy can be used to characterize the structure of colloidal drug carrier systems. Raman spectroscopy is readily applicable to in vivo studies, but such investigations must fulfill the relevant laser safety guideline. Recently, there has been tremendous technical improvement in vibrational microspectroscopy. FTIR imaging shows great promise in its ability to visualize the drug and excipient distribution in pharmaceutical formulations such as tablets and therapeutic transdermal systems, as well as to reveal the mechanism of drug release. Furthermore, this unique technique offers completely new possibilities to study the lateral diffusion of drugs.

ATR-FTIR characterization of transport properties of benzoic acid ion-pairs in silicone membranes

A novel technique based on Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy was used to study the transport of benzoic acid ion-pairs/salts in silicone membranes. The benzoic acid ion-pairs were prepared using various counter-ions with different degrees of lipophilicity, e.g. triethylamine (TA), diethylamine (DE), tert-butylamine (t-BA), 2-amino-2-methyl-propanol (AMP), and 2-amino-2-methyl-propanediol (AMPD). Silicone membrane, treated or untreated with propylene glycol (PG), was placed on the surface of a ZnSe crystal and the transport solution was applied to the upper surface of the membrane. A mathematical model, based on Fick's second law describing the build up of permeant concentration at the membrane/crystal interface with time was applied to determine diffusion coefficients. Absorption due to the acid (1700 cm(-1)) or benzoate anion (1555 cm(-1)) was observed at different regions without the interference from PG or silicone membrane. Benzoate anion, a charged species, was observed to permeate the membrane. The permeation of benzoate anion from sodium benzoate and polar ion-pairs of AMP and AMPD was very low in contrast to their high-saturated concentrations in PG as compared to the t-BA ion-pair. This indicated that benzoate anion preferentially permeates the membrane as an ion-pair rather than a single anion; otherwise its permeation should correspond to its concentration in PG instead of the lipophilicity of the ion-pairs. Additionally, the diffusion coefficient values of benzoic acid and benzoate anions through the treated and untreated membranes were not statistically different.

The effect of the nature of H-bonding groups on diffusion through PDMS membranes saturated with octanol and toluene

The permeation of a series of structurally related compounds across silicone membranes (PDMS) was studied. The PDMS was saturated either with toluene, to mimic a functionally inert barrier, or octanol, to mimic the polar/hydrogen bonding environment of the stratum corneum lipid barrier. Phenol, salicylic acid, benzoic acid, anisole, phenylethanol and benzyl alcohol were chosen in an attempt to relate permeation to their different H-bonding capabilities. The flux was lower through the octanol system suggesting retardation by polar/H-bonding interactions. Separation of the permeability coefficient into its thermodynamic (partition coefficient) and kinetic (diffusion coefficient) terms suggests that the effect of altering polarity within the membrane has a greater impact on the diffusion of permeant rather than its chemical potential within the membrane.

Mechanistic study into the enhanced transdermal permeation of a model beta-blocker, propranolol, by fatty acids: a melting point depression effect

Transdermal permeation of propranolol through human skin in the presence of fatty acid (lauric, capric) penetration enhancers has been investigated. Thermal analysis showed that binary mixtures of propranolol with either fatty acid were not simple mechanical mixtures of the two components. Propranolol formed 1:1 molar addition compounds with both lauric and capric acids; the addition compound produced from propranolol and lauric acid (m.p. 79 degrees C) also developed eutectic systems with both propranolol (m.p. 54 degrees C) and lauric acid (m.p. 16 degrees C). Similarly, the addition compound made from propranolol and capric acid (m.p. 97 degrees C) formed eutectic systems with propranolol (m.p. 83 degrees C) and capric acid (m.p. 15 degrees C). Infrared analyses indicated that the addition compounds were fatty acid salts of the beta-blocker. The nature of the species permeating through human epidermal membranes from binary mixtures of propranolol with the fatty acids was investigated using a novel attenuated total reflectance Fourier transform infrared method. There was no clear difference in permeation rates of the fatty acids compared with the beta-blocker, suggesting that the permeating species was the intact addition compound. The influence of melting point depression of the beta-blocker fatty acid systems on transdermal permeation was predicted from a mathematical model; predicted and experimentally determined data correlated well thus providing an alternative explanation as to the mode of action of these permeation enhancers.

The effect of hydrogen bonding on diffusion across model membranes: consideration of the number of H-bonding groups

The diffusion of a series of phenols across simple silicone membranes impregnated with either octanol or toluene was studied. These solvents are taken up and saturate the membrane. The presence of the solvents in a solid membrane allows them to interact with any permeant that cross the membrane. This membrane was used to simulate a bio-membrane, e.g. the skin, capable of hydrogen bonding with the permeant. As the number of H-bonding groups was increased the flux across both the octanol and toluene impregnated membranes decreased. However, deconvolution of the data showed that for the octanol impregnated membrane the diffusion coefficient (Dm) decreased significantly with the number of H-bonding groups. This was not the case for the toluene impregnated membrane. Furthermore the spatial configuration of the -OH groups around the aromatic ring had a significant effect on the decrease in Dm. These findings have considerable implications in understanding the absorption of permeants across bio-membranes capable of H-bonding.

Penetration of compounds through human stratum corneum as studied by Fourier transform infrared photoacoustic spectroscopy

The penetration of the lipophilic model permeant, 1-cyanodecane, into isolated human stratum corneum (SC) was followed nondestructively by step-scan Fourier transform infrared (FTIR) photoacoustic spectroscopy (PAS) with phase modulation technique. The uptake of the compound in the SC was quantified by monitoring the alterations in the spectra in the course of the penetration using multivariate analysis. Step-scan technique in conjunction with phase modulation offers the possibility for controllable depth profiling (sampling depth up to 30 microm) during the penetration process. Based on Fick's second law and assuming a virtually layered structure of the membrane, depth-dependent diffusion coefficients were derived by numerical fitting of the spectroscopic data. For 1-cyanodecane, the diffusion coefficient in the inner region of the SC is 1.6-fold that measured in the outer region.

Investigation of drug release from suspension using FTIR-ATR technique: part I. Determination of effective diffusion coefficient of drugs

Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy was used to study directly the release of drug particles (ketoconazole) in a liquid medium (paraffinum liquidum). In the case of the release experiment, the formulation is placed on the ATR crystal and the acceptor membrane on the top of the ointment. The decrease of the drug content in the sediment near the interface ATR crystal-formulation in the course of the release process was quantified by monitoring the changes of the IR spectrum in relevant spectral ranges using multivariate analysis. A mathematical model based on Fick's second law with appropriate initial and boundary conditions was applied in order to determine the diffusion coefficient of the drug in the liquid medium. Knowing this value, it is possible to calculate the effective diffusion coefficient of the drug in heterogeneous semisolid formulation (Vaseline) as a function of the volume fraction of the solid phase.

Drug and excipient diffusion and solubility in acrylate adhesives measured by infrared-attenuated total reflectance (IR-ATR) spectroscopy

Infrared-attenuated total reflectance (IR-ATR) was used to measure drug and excipient diffusion in acrylate pressure-sensitive adhesives. The first part describes diffusion of drug and excipient from one adhesive layer to another. The IR-ATR spectrometer is used to continuously monitor the rate at which drug and excipient diffuse into the 'receptor' adhesive layer. In this way, the ability of the drug and/or excipient to leave an adhesive can be determined without any influence of receptor fluids or skin membranes. Data is reported here for terpineol and testosterone diffusion in isooctyl acrylate (IOA) and IOA-acrylic acid (AA) adhesives. It is shown that the diffusion rate is much higher in IOA adhesive than in IOA-AA adhesive. The second part describes the use of IR-ATR to measure the solubility of liquids in adhesives. In this method, a liquid excipient is placed in direct contact with an adhesive layer containing no excipient. The IR-ATR spectrometer is used to continuously monitor the rate at which excipient diffuses into the 'receptor' adhesive layer. At equilibrium, the IR spectrum can be compared to both the pure adhesive spectrum and the pure excipient spectrum to determine the solubility of the excipient in the adhesive. Data are reported here for terpineol in an IOA adhesive and for several liquids in an IOA-vinyl acetate adhesive.

Determination of diffusion coefficients of sodium p-aminosalicylate in sheep nasal mucosae and dialysis membranes by Fourier transform infrared horizontal attenuated total reflectance spectroscopy

Fourier transform infrared horizontal attentuated total reflectance (FT-IR-H-ATR) spectroscopy was employed to determine the diffusion coefficients of sodium p-aminosalicylate (PAS) in sheep nasal mucosae and dialysis membranes. The system configuration, which comprises a closed system with an aqueous layer and a membrane layer, represents diffusion from a solution of limited volume. Data analysis involved fitting a truncated (seven term) Fourier series to the total mass transport into the membrane as a function of time. Comparison of diffusion coefficients of PAS in dialysis membranes obtained by this technique to those obtained by a standard steady-state permeation method showed excellent agreement. Apparent diffusion coefficients were approximately 4.33 (+/- 0.38) x 10(-7) and approximately 9.62 (+/- 5.30) x 10(-7) cm2/s for dialysis membranes and sheep nasal mucosae, respectively. These values are substantially smaller than the diffusion coefficient of PAS in aqueous solution, indicating that the rate-limiting step was diffusion in the membrane. The effect of purified gastric mucin solution (concentration up to approximately 6% w/v) on the apparent diffusion coefficient of PAS in the membranes was also investigated. The results showed no statistically significant change in the apparent diffusion coefficient in the presence of mucin for either sheep nasal mucosae or dialysis membranes. Although it was reported that mucin in solution retards the diffusion of PAS as compared to buffer alone, the mass transport within the membrane was the rate-limiting step for this hydrophilic compound.

The relative effect of Azone and Transcutol on permeant diffusivity and solubility in human stratum corneum

PURPOSE

The purpose of this work was to analyse the mechanism of the enhancement of percutaneous penetration demonstrated by the known enhancers Azone and Transcutol.

METHODS

Enhancer induced changes in the diffusivity and solubility of a model permeant (4-cyanophenol) in human stratum corneum were monitored (in-vitro) using Attenuated total reflectance Fourier transform infra-red (ATR-FTIR) spectroscopy and compared to the gross effects of the enhancers on flux as measured using simple Franz-type diffusion cells.

RESULTS

It has been shown by both the well-established Franz diffusion cell technique and the use of ATR-FTIR spectroscopy that the enhancers studied both increase the flux of cyanophenol across human skin in-vitro by a factor of approximately two. Furthermore, it has been demonstrated by ATR-FTIR that these enhancers are likely to exert their effects by different mechanisms. It is probable that Azone reduces the diffusional resistance of the stratum corneum and that Transcutol increases the solubility of the penetrant in this barrier.

CONCLUSIONS

There is increasing interest in the apparently synergistic nature in which certain enhancers appear to work. The exact nature of these multiplicative and/or additive effects is not known although there are numerous suggestions in the current literature. The application of ATR-FTIR spectroscopy to such enhancing systems will allow the mechanisms of the observed enhancements to be probed in greater depth.

Use of ATR-FTIR spectroscopy to study the diffusion of ethanol through glycerogelatin films

The use of ATR-FTIR spectroscopy to study the permeability of a glycerogelatin film is described. Measurement of the diffusion coefficient of ethyl alcohol-d in the film showed excellent reproducibility. Comparison of results from this technique with those previously obtained using an air-flow receptor phase diffusion cell show good agreement in terms of lag time assessed diffusion coefficients. ATR-FTIR spectroscopy revealed time-dependent changes in the composition of the glycerogelatin film during the diffusional process. It was also demonstrated that the concurrent assessment of both diffusant penetration and film composition is feasible.