Maintaining Sink Conditions Research Articles

Solving a microplastic dilemma? Evaluating additive release with a dynamic leaching method for microplastic assessment (DyLeMMA)

Microplastics and plastic additives are contaminants of emerging environmental concern. Static leaching methods are commonly applied to assess the rate and extent of additive release from microplastics. However, this approach may not be representative of environmental conditions where near infinite dilution or percolation commonly occur. We evaluated three different approaches for assessing additive leaching under environmentally relevant sink conditions, culminating in the refinement and validation of DyLeMMA (Dynamic Leaching Method for Microplastic Assessment). Analysis was performed using a high-resolution liquid chromatography-mass spectrometry method enabling targeted quantification of additives and screening for non-intentionally added substances. Using four different plastics, sink conditions were maintained over the duration of the test, thereby avoiding solubility limited release and ensuring environmental relevance. Background contamination from ubiquitous additive chemicals was minimised, thereby providing good sensitivity and specificity. Resulting data, in the form of additive release curves, should prove suitable for fitting to release models and derivation of parameters describing additive leaching from microplastics.Key attributes of DyLeMMA:•Environmentally relevant dynamic leaching method for microplastics, demonstrated to maintain sink conditions over the test duration,•Simple, fast, and cost-effective approach without complication of using a solid phase sink,•Provide data suitable for understanding microplastic leaching kinetics and mechanisms.

Refining the in vitro release test method for a dapivirine-releasing vaginal ring to match in vivo performance

Previously reported in vitro release test methods for drug-releasing vaginal rings containing poorly water-soluble drugs have described use of water-alcohol systems or surfactant solutions in efforts to maintain sink conditions. Here, as part of efforts to more closely match in vitro and in vivo release for the 25 mg dapivirine matrix-type silicone elastomer vaginal ring for HIV prevention, we have investigated alternatives to the 1:1 v/v water/isopropanol medium described previously. Specifically, we evaluated dapivirine release from rings into (i) monophasic water/isopropanol mixtures of varying compositions and (ii) biphasic buffer/octanol systems using pH 4.2 and pH 7.0 buffers. The rate and mechanism of dapivirine release were dependent upon the isopropanol concentration in the release medium, in accordance with the observed trend in drug solubility. At 0 and 10% v/v isopropanol concentrations, dapivirine release followed a partition-controlled mechansim. For media containing ≥ 20% v/v isopropanol, in vitro release of dapivirine was significantly increased and obeyed permeation-controlled kinetics. Cumulative release of ~3.5 mg dapivirine over 28 days was obtained using a water isopropanol mixture containing 20% v/v isopropanol, similar to the ~4 mg dapivirine released in vivo. Dapivirine release into the biphasic buffer/octanol system (intended to mimic the fluid/tissue environment in vivo) was constrained by the limited solubility of dapivirine in the buffer component in which the ring resided, such that cumulative dapivirine release was consistently lower than that observed with the 20% v/v isopropanol in water medium. Release into the biphasic system was also pH dependent, in line with dapivirine’s pKa and with potential implications for in vivo release and absorption in women with elevated vaginal pH.Graphical abstract

Pitfalls in analyzing release from chitosan/tripolyphosphate micro- and nanoparticles

Submicron particles prepared by complexing chitosan with tripolyphosphate (TPP) attract widespread interest as potential drug, gene and vaccine delivery vehicles, and many published studies examine their release properties. Despite these sustained efforts, however, literature on the release performance of chitosan/TPP micro- and nanoparticles is filled with conflicting results, with some reporting nearly instantaneous release, while others showing the release to be sustained for up to multiple days. To resolve these opposing findings, we recently postulated that the in vitro release profiles obtained from chitosan/TPP particles by the standard “sample and separate” or “solvent replacement” method (where the solvent was periodically replaced with fresh buffer and analyzed for the released bioactive molecule content) may have been subject to strong experimental artifacts and not have reflected their true release behavior. To explore this possibility, here we examine several experimental artifacts that may arise during such in vitro experiments and show that conflicting findings on release from chitosan/TPP particles can arise from: (1) incomplete particle separation from the release media upon centrifugation; (2) irreversible particle coagulation; and (3) failure to maintain sink conditions. Moreover, we show that some of the longer-lasting release profiles may reflect the use of physiologically irrelevant (low-ionic-strength) release media. By analyzing and discussing these effects, this article provides guidelines for obtaining more reliable release profiles for chitosan/TPP micro- and nanoparticles and other/related colloidal carriers.

Advances in In Vivo Predictive Dissolution Testing of Solid Oral Formulations: How Closer to In Vivo Performance?

Dissolution has become an indispensable tool to predict the in vivo performance of dosage form, especially in recent times, because of the increasing complexity of new drugs discovered. Several attempts have been made to modify dissolution, so that it mimics the in vivo behavior to maximum possible accuracy and minimizes the probability of in vivo bioequivalence failures. In this context, several advancements have been reported including drug dissolution/absorption simulating system, bionic system, dissolution/permeation model, biphasic dissolution system, Caco-2 cell monolayer in combination with compendial dissolution apparatus, artificial stomach duodenum model, dynamic gastric model, Netherlands Organization for Applied Scientific Research gastrointestinal model, and many more. The present review highlights the recent advancements in dissolution methods with a focus on in vivo predictive dissolution methods, their advantages and disadvantages, and key factors governing the results obtained. The impact of maintaining sink conditions and use of biorelevant media is also discussed briefly.

USP Apparatus 4: a Valuable In Vitro Tool to Enable Formulation Development of Long-Acting Parenteral (LAP) Nanosuspension Formulations of Poorly Water-Soluble Compounds.

Long-acting or extended release parenteral dosage forms have attracted extensive attention due to their ability to maintain therapeutic drug concentrations over long periods of time and reduce administration frequency, thus improving patient compliance. It is essential to have an in vitro release (IVR) testing method that can be used to assure product quality during routine production as well as predict and understand the in vivo performance of a formulation. The purpose of this work was to develop a discriminatory in vitro release method to guide formulation and process development of long-acting parenteral (LAP) nanosuspension formulations composed of poorly water-soluble drugs (BCS class II). Injectable nanosuspension formulations were developed to serve as test articles for method development. Several different IVR methods were evaluated for their application to the formulation screening and process development including (1) USP apparatus 2, (2) dialysis and reverse dialysis sac, and (3) continuous flow-through cell (USP apparatus 4). Preliminary data shows the promising results to support the utilization of USP 4 over more widely accepted USP 2 and dialysis methods. A combination of more representative in vivo hydrodynamics and ease of maintaining sink conditions yields the USP 4 flow-through cell method a more suitable in vitro release method for nanosuspension-based LAP formulations of poorly water-soluble compounds, such as compounds A and B.

Determining drug release rates of hydrophobic compounds from nanocarriers.

Obtaining meaningful drug release profiles for drug formulations is essential prior to in vivo testing and for ensuring consistent quality. The release kinetics of hydrophobic drugs from nanocarriers (NCs) are not well understood because the standard protocols for maintaining sink conditions and sampling are not valid owing to mass transfer and solubility limitations. In this work, a new in vitroassay protocol based on 'lipid sinks' and magnetic separation produces release conditions that mimic the concentrations of lipid membranes and lipoproteins in vivo, facilitates separation, and thus allows determination of intrinsic release rates of drugs from NCs. The assay protocol is validated by (i) determining the magnetic separation efficiency, (ii) demonstrating that sink condition requirements are met, and (iii) accounting for drug by completing a mass balance. NCs of itraconazole and cyclosporine A (CsA) were prepared and the drug release profiles were determined. This release protocol has been used to compare the drug release from a polymer stabilized NC of CsA to a solid drug NP of CsA alone. These data have led to the finding that stabilizing block copolymer layers have a retarding effect on drug release from NCs, reducing the rate of CsA release fourfold compared with the nanoparticle without a polymer coating.This article is part of the themed issue 'Soft interfacial materials: from fundamentals to formulation'.

Development and Validation of a Discriminating Dissolution Method for Atorvastatin Delayed-Release Nanoparticles Using a Flow-Through Cell: A Comparative Study Using USP Apparatus 4 and 1

Atorvastatin, an HMG CoA reductase inhibitor, is widely used for the treatment of dyslipidemia and prevention of cardiovascular disease. It belongs to Class 2 of the Biopharmaceutics Classification System owing to its low solubility and high permeability. In vitro dissolution testing is an essential tool for the design of a dosage form. Appropriate selection of dissolution test conditions is essential since it will help establish an appropriate in vitro-in vivo correlation (IVIVC), which is imperative for a waiver of costly bioequivalence studies. Compounds belonging to BCS Class 2 are suitable for establishing a significant IVIVC. USP Apparatus 4 (flow-through cell) is an attractive alternative to conventional dissolution methods like Apparatus 1 and 2. The ability of the apparatus to maintain sink conditions over a longer period of time has proved useful in the development of dissolution methods for poorly soluble drugs, making the development of IVIVCs easier for such drugs. Nanoparticles of atorvastatin were formulated by an emulsion-solvent evaporation method to overcome its bioavailability problems. Development of a discriminatory dissolution method for the same was attempted using a flow-through cell (Apparatus 4) because dissolution performed using Apparatus 1 did not provide complete release of the drug within 60 min. Phosphate buffer pH 6.8 with 1.5% SLS was selected as the dissolution medium based on the results of the solubility studies. Various parameters such as flow rate and sample-loading method were optimized. The dissolution method was validated with respect to various parameters as per ICH guidelines.

Comparison of Dissolution Profiles for Sustained Release Resinates of BCS Class I Drugs Using USP Apparatus 2 and 4: A Technical Note

In vitro dissolution specifications are established to ensure batch-to-batch consistency while pointing out any potential problems with in vivo bioavailability among different drug products (1). At early stages of formulation development, in vitro dissolution testing provides guidance on optimizing drug release from formulations. While at later stages, it may be employed as an indicator of the in vivo performance of drug products to potentially reduce the number of bioavailability/bioequivalence studies. An established IVIVC can be further used to set meaningful dissolution specifications that take clinical consequences into account in the hope of facilitating regulatory approvals of post approval changes. Among the different in vitro systems available for testing, USP Dissolution apparatus 4 offers a viable option for carrying out dissolution of novel dosage forms (2,3). USP Dissolution apparatus 4 is helpful in: testing robustness of the formulation with respect to the variations in the intralumenal environment, maintaining sink conditions for the poorly soluble drugs (4,5) and non tedious method to change pH of the dissolution medium (6). USP Dissolution apparatus 4 has been extensively studied for the prediction of in vivo performance of orally administered dosage forms like amoxicillin capsules (7), paracetamol (8), carbamazepine (9), danazole capsules (10), spironolactone (11). It is the method of choice for extended release and poorly soluble drugs. Point to point correlation of in vitro release data using flow through cell apparatus is established for monolithic extended release formulations of highly permeable isosorbide-5-mononitrate (12). Similarly, good in vitro in vivo correlation was reported for vincamine extended release formulations (13). The development of flow through cell came about as an attempt to provide controlled hydrodynamic environment and renewable stream to the sample and has proven to be a useful tool in determining the in vitro release of the extended release formulations. Extended release formulation can be formulated using polymer (14), lipids/waxes (15), ion exchange resins (16) etc. Ion exchange resins have recently been gaining importance in pharmaceutical industries in the manufacturing of extended release formulations. IER are water insoluble, cross-linked polymers containing salt forming groups in repeating positions on the polymer chains. IER form a complex with the drug through weak ionic bonding. Drug release from resinate depends on pH and electrolyte concentration within the gastrointestinal tract. With basic drugs, strong cationic exchange resins (sulfonic acid) yield strong drug resin bonding resulting in slower elution of drug from resinates and this principle has exhibited the potential of resins as sustained release agents (17,18). The presence of released drug and ion exchange resin with free active sites has a significant effect on the further release of drug from resinate. This effect will be more pronounced while dealing with BCS class I drugs. Butler et al. (6) has mentioned the usage of USP Dissolution apparatus 4 particularly for drugs with low solubility as it can maintain the sink condition, independent of drug solubility. Hitherto there are no reports in the literature that cite the necessity of maintaining sink conditions while performing in vitro release of sustained release resinates of highly soluble drug. Release data based on a well thought out test is of tremendous value in selection of right formulation. Therefore, the present work was focused on studying the effect of USP Dissolution apparatus 2 and 4 on the in vitro release of drug from resinates of BCS class I drugs.

Dissolution test development for complex veterinary dosage forms: oral boluses.

Fundamental aspects of electrolyte chemistry were used to design an appropriate dissolution medium with the capacity to maintain sink conditions throughout the test. Dissolution of various bolus dosage forms was studied using USP Apparatus II at various stirring speeds. Complete dissolution of each drug in the designed medium was achieved, and there is evidence that such a dissolution test could be discriminating. This review details the development of potentially discriminating in vitro dissolution tests for veterinary boluses using USP Apparatus II and examines the potential role of such testing during product quality assessments, in the evaluation of postapproval manufacturing changes and for the establishment of the generic equivalence of veterinary products.

Evaluation of a novel diffusion cell for in vitro transdermal permeation: effects of injection height, volume and temperature

The objective of this study was to evaluate the performance of a new, compact, dynamic diffusion cell for in vitro transdermal permeation. These so-called Kelder-cells were developed as an automated alternative to the static Franz diffusion cells. The new cells were used in combination with the ASPEC-system (automatic sample preparation with extraction columns) which was initially designed for the automation of solid-phase extractions. Three variables were tested to optimize the performance of the new cell system: injection height into the inlet compartment, volume flowing through the receptor compartment and temperature. Experiments were performed using the tritium labelled anticholinergic [ 3H]dexetimide permeating through an artificial membrane (Silastic®). The injection height of the needle into the inlet compartment of the cell should be programmed at −34 mm to ensure complete air tightness, thus forcing the buffer to flow through the cell. The volume of buffer flow through the receptor compartment is important in maintaining sink conditions: a volume of 117 μl was chosen to replace the total content of the cell (84 μl) every 2 min. The temperature was precisely controlled in a thermostatic cabinet to minimize variations in experimental conditions. For [ 3H]dexetimide, an increase in temperature of 20°C reduced the lag time by a factor of approximatley two, however the influence on the flux was negligible. The data for the Kelder-cells were comparable with static Franz diffusion cells at a pseudo-steady state, however Kelder-cells have the advantage of automatic sampling, continuous replacement of the receptor solution, and unattended operation over at least 24 h.

Method for evaluating drug release from liposomes in sink conditions

An in vitro method was developed for the evaluation of the drug release from disperse systems, such as liposomes, under sink conditions. Determination of in vitro release of lipophilic drugs from liposomes requires a dissolution medium that maintains sink conditions without damaging the lipid membrane and a method to separate the released drug from the liposomal drug. We propose a new in vitro technique for the evaluation of drug release from liposomes using a hydrophilic β-cyclodextrin derivative in the dissolution medium to maintain sink conditions. A liposomal drug dispersion was placed in a magnetically stirred dialysis bag (Mw cut-off 300 000) containing cyclodextrins to provide sink conditions. The released drug was sampled from the outside of the bag. Release of hydrocortisone and budesonide from different multilamellar liposomes was measured. True release rate was evaluated from the release data of liposomal and free drug from the dialysis bags, respectively. Release of steroids from the liposomes was relatively fast, but they were retained longer in gel-phase, distearoyl- l- α-phosphatidylcholine (DSPC) liposomes than in liquid-phase, l- α-phosphatidylcholine (EPC) liposomes. Leakage of encapsulated calcein from the liposomes was not affected by cyclodextrins suggesting that they do not disturb the structure of the lipid bilayers. This method is capable of distinguishing different true release rates of drugs from colloidal carriers and it is easy to perform.

New design of a flow-through permeation cell for studying in vitro permeation studies across biological membranes

A newly designed diffusion cell has been developed for in vitro permeation studies across biological or artificial membranes. The cell has several unique properties to make it suitable for transdermal drug delivery studies: (i) it is two-chambered, allowing application of a donor with a finite or infinite dose; (ii) it allows continuous flow-through permeation of the acceptor phase, maintaining sink conditions and allowing automatic sampling to obtain an accurate permeation profile; (iii) it features a spiral acceptor fluid flow pattern, ensuring equal transport along — and moisturization of — the membrane and minimizing the influence of volume and flow rate on mass transfer by eliminating any stagnant domains in the acceptor compartment.

Controlled-release coevaporates of dipyridamole prepared with acrylic polymers

The objective of this study was to obtain Controlled-release dosage forms of dipyridamole chosen as a model drug practically insoluble above pH 5. The coevaporates were prepared by the solvent method using enteric and insoluble acrylic polymers as well as their mixtures in different proportions. The in vitro dissolution test results were determined by the USP XXII paddle method at different pH values during a period of 8 h. These results showed that the progressive pH-dependent release of dipyridamole could only be achieved by blending enteric and insoluble polymers. To overcome the difficulty in maintaining sink conditions with the paddle method, the dissolution profiles of some of the formulations were investigated using the open flow-through cell method.

ADS-12: An Automated Programmable 12-tablet Dissolution Testing System

A novel computerized automated system is described which conforms to the basic requirements set forth in the compendia for testing the dissolution characteristics of solid dosage forms. This modular system can test 12 tablets simultaneously. It can maintain sink conditions at all times and can handle any chemistry amenable to an automatic analyzer. Dissolution, sampling, chemistry, and readout are all accomplished simultaneously, reducing the complete test time for 12 tablets to the dissolution time itself.

Computerized Automated System for Determining Dissolution Rate Profiles for Solid Dosage Forms

A unique automated system was developed for the measurement of dissolution rates of tablets and capsules. This system, which adheres to principles set forth in the compendia, can test six dosage forms simultaneously. It is modular in form and capable of maintaining sink conditions, and it can handle any type of chemistry amenable to an automatic analyzer. In addition to the usual dissolution apparatus, the system includes a 12‐channel combination sampling and solvent addition pump, a sequence control module, pertinent automated analyzer components, a spectrophotometer or fluorometer, an optional recorder, a specially designed digitizing system, and a teletype equipped with a paper tape punch reader and acoustic coupler. Each dissolution flask is automatically sampled every 6min. Standards may be run either before or after the samples. Since dissolution, sampling, chemistry, and readout are all accomplished simultaneously, the complete test time is essentially reduced to the dissolution time itself. At the completion of sample dissolution, raw dissolution profile data are on the punched paper tape ready for computer processing via a time‐sharing system. Developed software provides for the printout of a complete test report in less than 15min.

Design and Evaluation of a Rotating Filter–Stationary Basket in vitro Dissolution Test Apparatus II: Continuous Fluid Flow System

The rotating filter-stationary basket apparatus reported previously for the determination of dissolution rates in fixed fluid volumes was modified for use as a continuous fluid flow dissolution test system. The modified apparatus, consisting of a smaller size fluid container, provides a convenient means of maintaining sink conditions during the dissolution process by a continuous exchange of a portion of the dissolution medium with fresh solvent. Equations are developed to calculate the total amount of drug dissolved as a function of time from continuous-flow dissolution rate profiles. Dissolution rate results for several tablet samples of an antidiabetic drug evaluated under continuous fluid flow conditions are comparable with the results obtained under fixed fluid volume conditions, and they also correlate with in vivo blood sugar-lowering activity of the tablets.

Establishment of sink conditions in dissolution rate determinations. Theoretical considerations and application to nondisintegrating dosage forms

The importance of approximating perfect sink conditions for the determination of dissolution rates which may be correlated with in vivo results is pointed out. A theoretical treatment of zero and first-order dissolution under perfect sink and nonsink conditions is presented. It is shown that unless sink conditions are maintained, in vitro results will bear little relationship to in vivo observations. The use of an organic solvent phase which can function as a reservoir for dissolved drug and thereby maintain sink conditions in the aqueous dissolution fluid is demonstrated for a model nondisintegrating tablet exhibiting zero-order drug release. The use of an organic solvent reservoir for the determination of first-order dissolution rates is also discussed.