Exploring better column selectivity choices in ultra-high performance liquid chromatography using quality by design principles

Revisiting column selectivity choices in ultra-high performance liquid chromatography-Using multidimensional analytical Design Spaces to identify column equivalency

Current guides and column selection system (CSS) platforms can provide some helpful insights with regard to the selection of alternative phases. Their practical reliability however, can also turn out to be questionable, especially considering the lack of detailed specifics, such as a clear definition of points of equivalence-appropriate running conditions under which the given analytical mixture can be satisfactorily resolved on various stationary phases. In this context, the use of multivariate modeling tools can be highly beneficial. These tools, when applied systematically, are ideal for uniquely characterizing complex LC-separation systems, a fact supported by numerous peer-reviewed papers. Revisiting our earlier work [1] and the applied systematic workflow [2], we used a Design Space modeling software (DryLab), with the main focus on building and comparing 3-dimensional separation models of amlodipine and its related impurities to identify shared method conditions under which columns are conveniently interchangeable. Our study comprised 5, C18-modified ultra-high performance liquid chromatography (UHPLC) columns in total, in some cases with surprising results. We identified several equivalences between the Design Spaces (DSs) of markedly different columns. Conversely, there were cases where, despite the predicted similarities in column data, the modeled DSs demonstrated clear differences between the selected stationary phases.

Updating the European Pharmacopoeia impurity profiling method for cetirizine and suggesting alternative column, using design space comparison

The goal of the present study was to develop a generic workflow to evaluate the chromatographic resolution in a design space and find replacement column for the new method. To attain this objective, a limited number of initial experiments have been performed, and a modeling tool was employed to study and compare design spaces obtained with different columns. By overlaying the different individual resolution maps (design spaces), it is possible to quickly identify a robust zone where the different columns meet a given resolution criterion. This new feature of the modeling tool is very useful for finding alternative columns for a given separation, rather than the usual column tests. It was also demonstrated that two different columns can be used as equivalents (replacement columns), providing sufficient resolution at the same working point and with a high degree of robustness.

The Applicability of Chromatographic Retention Modeling on Chiral Stationary Phases in Reverse-Phase Mode: A Case Study for Ezetimibe and Its Impurities

Mechanistic modeling is useful for predicting and modulating selectivity even in early chromatographic method development. This approach is also in accordance with current analytical quality using design principles and is highly welcomed by the authorities. The aim of this study was to investigate the separation behavior of two different types of chiral stationary phases (CSPs) for the separation of ezetimibe and its related substances using the mechanistic retention modeling approach offered by the Drylab software (version 4.5) package. Based on the obtained results, both CSPs presented with chemoselectivity towards the impurities of ezetimibe. The cyclodextrin-based CSP displayed a higher separation capacity and was able to separate seven related substances from the active pharmaceutical ingredient, while the cellulose-based column enabled the baseline resolution of six impurities from ezetimibe. Generally, the accuracy of predicted retention times was lower for the polysaccharide CSP, which could indicate the presence of additional secondary interactions between the analytes and the CSP. It was also demonstrated that the combination of mechanistic modeling and an experimental design approach can be applied to method development on CSPs in reverse-phase mode. The applicability of the methods was tested on spiked artificial placebo samples, while intraday and long-term (2 years) method repeatability was also challenged through comparing the obtained retention times and resolution values. The results indicated the excellent robustness of the selected setpoints. Overall, our findings indicate that the chiral columns could offer orthogonal selectivity to traditional reverse-phase columns for the separation of structurally similar compounds.

Analytical quality by design-compliant retention modeling for exploring column interchangeabilities in separating ezetimibe and its related substances

There are several potential advantages of using experimental design-based retention modeling for chromatographic method development. Most importantly, through the model-delivered systematic understanding (Design Spaces), users can benefit from increased method consistency, flexibility and robustness that can efficiently be achieved at lesser amount of development time. As a result, modeling tools have always been great supplementary assets and welcomed by both the pharmaceutical industry and the regulatory authorities. Most recently published chapters of ICH however - Q2(R2) and Q14 (both currently drafts) - evidence a further paradigm shift, specifying the elements of model-based development strategies in the so-called "enhanced approach". The main aim of this study was to investigate the impact of stationary phase chemistries on chromatographic method performance in the application example of ezetimibe and its related substances. A commercial modeling software package (DryLab®) was used to outline three-dimensional experimental design frameworks and acquire model Design Spaces (DSs) of 9 tested columns. This was done by performing 12 input calibration experiments per column, systematically changing critical method parameters (CMPs) as variables such as the gradient time (tG), temperature (T) and the ternary composition (tC) of the mobile phase. The constructed models allowed studying retention behaviors of selected analytes within each separation systems. In the first part of our work, we performed single optimizations for all nine stationary phases with substantially different surface modifications based on their highest achievable critical resolution values. For these optimum points in silico robustness testing was performed, clearly showing a change of CMPs, depending on the column, and specified optimum setpoint. In the second part of our work, we simultaneously compared the three-dimensional virtual separation models to identify all method parameter combinations that could provide at least baseline separation (R_{s, crit.}>1.50). These overlapping areas between the models described a common method operational design region (MODR) where columns were considered completely interchangeable - in terms of their baseline resolving capability - regardless of their exact physicochemical properties. A final optimized, column-independent working point within the common MODR was selected for verification. Indeed, experimental chromatograms showed excellent agreement with the model; all columns in the common condition were able to yield critical resolution values higher than 2.0, only their retentivity (elution window of peaks) was found different in some cases. Our results underline that a profound understanding of the separation process is of utmost importance andthat in some cases, adequate selectivity is achievable on various stationary phases.

Development of a Fast and Robust UHPLC Method for Apixaban In-Process Control Analysis

In-process control (IPC) is an important task during chemical syntheses in pharmaceutical industry. Despite the fact that each chemical reaction is unique, the most common analytical technique used for IPC analysis is high performance liquid chromatography (HPLC). Today, the so-called “Quality by Design” (QbD) principle is often being applied rather than “Trial and Error” approach for HPLC method development. The QbD approach requires only for a very few experimental measurements to find the appropriate stationary phase and optimal chromatographic conditions such as the composition of mobile phase, gradient steepness or time (tG), temperature (T), and mobile phase pH. In this study, the applicability of a multifactorial liquid chromatographic optimization software was studied in an extended knowledge space. Using state-of-the-art ultra-high performance liquid chromatography (UHPLC), the analysis time can significantly be shortened. By using UHPLC, it is possible to analyse the composition of the reaction mixture within few minutes. In this work, a mixture of route of synthesis of apixaban was analysed on short narrow bore column (50 × 2.1 mm, packed with sub-2 µm particles) resulting in short analysis time. The aim of the study was to cover a relatively narrow range of method parameters (tG, T, pH) in order to find a robust working point (zone). The results of the virtual (modeled) robustness testing were systematically compared to experimental measurements and Design of Experiments (DoE) based predictions.

Development of an analytical method for the determination of pimavanserin and its impurities applying analytical quality by design principles as a risk-based strategy

Pimavanserin is an atypical antipsychotic indicated for the treatment of hallucinations and delusions associated with Parkinson's disease psychosis. As it is a relatively new drug on the market, limited number of pharmacokinetic information and analytical methods are available. This paper presents an ultra-high performance chromatography for the simultaneous determination of pimavanserin and its four process impurities. The method was developed applying analytical quality by design (AQbD) principles as a risk-based approach. Critical method attributes (CMAs) were selected as a resolution between the worst separated compounds (impurity B and impurity C), a duration of analysis defined by the retention time of the last eluting peak (impurity D), a capacity factor of the first eluted impurity (impurity A), a tailing factor and a theoretical plate number. Risk assessment in the early stage of method development pointed out critical method parameters (CMPs): column temperature, gradient time and pH-value of the mobile phase (water phase, eluent A). Design of experiments (DoE), using DryLab®4 software, was applied to evaluate the influence of CMPs on CMAs and to determine method operable design region (MODR). Based on the risk assessment and the results of robustness and precision tests, a control strategy with system suitability criteria was proposed. Developed method was validated according to ICH Q2 (R1) guideline with respect to the selectivity, LOD, LOQ, linearity, precision, accuracy, robustness and stability. A forced degradation study was performed to provide an evidence of the stability-indicating property of the method. Degradation products of pimavanserin were identified using ultra high-performance liquid chromatography coupled to high resolution mass spectrometry (UHPLC-qTOF). Additionally, potential degradation products were assessed in silico with the help of Zeneth® software and good agreement with experimentally identified degradation products was achieved. Main degradation products were formed during acid and base hydrolysis (m/z 223.16 [M+H]⁺ at RRT 0.37) and under oxidative stress conditions (m/z 444.26 [M+H]⁺ at RRT 0.57). The results revealed that the pimavanserin undergoes degradation through acid and base hydrolysis of urea and N-oxidation of aliphatic tertiary amine.

A methodology employing retention modeling for achieving control space in liquid chromatography method development using quality by design approach

This study reports the application of retention modeling and quality by design practices for reverse-phase liquid chromatographic method development of a new chemical entity. Prior to the retention modeling, preliminary screening experiments were performed for the selection of stationary phase, organic modifiers, and method parameters. Based on the results of preliminary method conditions, t_G-T (gradient time - temperature) 2-D modeling with 4 input runs, and t_G-T-t_c (gradient time-temperature-ternary composition) 3-D modeling with 12 input runs were designed to build a model for achieving the optimized separation. Modeling of reverse phase separations was based on the measurement of both retention times and peak areas. A design space with appropriate input variables and control strategy was established prior to optimization and robustness evaluation following the quality by design framework. DryLab^Ⓡ was used to predict the optimized gradient profile and separation temperature. The robustness evaluation was carried out using the multiple factors at a time approach and the control space was established. The interdependence of control space and the control strategy was demonstrated by evaluating method robustness using two levels of system suitability criteria. The predictive accuracy of the retention modeling was established through experimental verification of the in-silico predictions. The quality by design based method development approach demonstrated the in-silico optimization as an integral component of reverse-phase chromatographic method development to evaluate the interplay of factors such as organic modifiers, separation temperature and gradient time, which greatly integrated and enhanced method robustness during method development.

Ultra high-performance liquid chromatography method development for separation of formoterol, budesonide, and related substances using an analytical quality by design approach

The application of the Quality by Design (QbD) principles in developing a new ultra high performance liquid chromatography method for the analysis of formoterol/budesonide and related substances using Fusion QbD® software is explored. The effect of various chromatographic parameters including, column stationary phase, pH, temperature, flow rate, and gradient time on separations were systematically investigated. Results show that optimal separations of these compounds in a standard solution can be achieved using a BEH C₁₈ column (2.1 × 1.7 μm × 10 cm) applying a pH of 8.2, a temperature of 35 °C, a flow rate of 0.35 mL min^-1 and a gradient time of 25 min. Furthermore, the results show that the main parameters affecting the performance of the method were the mobile phase pH, gradient time, and the temperature. For example, the most important factor for peak tailing was the pH of the mobile phase and the critical factors affecting resolution of the analytes were the gradient time and the temperature. As an application, the method was further used to analyze budesonide and formoterol in a sample obtained from a Symbicort® metered dose inhaler and it was found to provide similar separations to those obtained with the standard solution. These findings indicate that applying the QbD principles in analytical method development can be very advantageous not only in obtaining deep understanding of the effect of input parameters but also potential regulatory flexibility.

Updating the European Pharmacopoeia impurity profiling method for terazosin and suggesting alternative columns

This work was motivated by the demand of European Directorate for the Quality of Medicines and HealthCare (EDQM). A new liquid chromatographic (LC) method was developed for terazosin impurity profiling to replace the old European Pharmacopoeia (Ph. Eur.) method. This new method is published as part of the new Ph. Eur. monograph proposal of terazosin in Pharmeuropa issue 32.2. The aim of the method renewal was to cut the analysis time from 90 min (2 × 45 min) down to below 20 min. The Ph. Eur. monograph method is based on two different chromatographic separations to analyze the specified impurities of terazosin. The reason for the two methods is that two of the impurities are not sufficiently retained in reversed phase (RP) conditions, not even with 100% water as eluent. Therefore, next to RP, an ion-pair (IP) chromatographic method has to be applied to analyze those two impurities. With our new proposed method it was possible to appropriately increase the retention of the two critical compounds using alternative stationary phases (instead of a C18 phase which is suggested by the Ph. Eur. method). Applying a pentafluoro-phenyl (PFP) stationary phase, it was feasible to separate and adequately retain all the impurities. The detection wavelength was also changed compared to the Ph. Eur. method and is now appropriate for the detection and quantification of all impurities using perchloric acid in the mobile phase at low pH. Another goal of the present study was to develop a generic workflow and to evaluate the chromatographic resolution in a wide range of method variables and suggest some replacement columns for terazosin impurity profiling. Retention modeling was applied to study the chromatographic behavior of the compounds of interest and visualize resolution for the different columns, where a given criterion is fulfilled. A zone (set of chromatographic conditions) of a robust space could be then quickly identified by the overlay of the individual response surfaces (resolution maps). It was also demonstrated that two columns from different providers (Kinetex F5 and SpeedCore PFP) can be used as replacement columns, providing sufficient resolution at the same working point and a high degree of robustness.

Enantioselective Box Behenken Optimized HPLC-DAD Method for the Simultaneous Estimation of Alogliptin Enantiomorphs in Pharmaceutical Formulations and their Pharmacokinetic Study in Rat Plasma

Purpose: A stereoselective high performance liquid chromatographic analytical method with photodiode array detector was developed and validated as per the International Conference on Harmonization (ICH) guidelines for the determination of alogliptin (ALO) enantiomers in formulations and rat plasma.

Methods: Enantiomeric separation was performed on a Phenomenex Lux Cellulose-2 chiral column. Box-Behnken design was used to identify the optimum conditions of the three independent variables for the desired output responses.

Results: The HPLC peaks of ALO enantiomers and the internal standard pioglitazone were achieved before 8 min with a resolution of 0.77 min between R and S enantiomer and resolution of more than 2.0 between each enantiomer and pioglitazone (internal) with more than 95% recovery. The linearity range and the limit of quantification of both the enantiomers in rat plasma were 10-70 ng mL^-1 and 1.2 ng mL^-1 respectively.

Conclusion: The developed method after validation was successfully applied for estimation of ALO enantiomers in formulations. Single oral dose of 25 mg of the ALO racemate tablets were administered to a group of 6 healthy rats for a comparative pharmacokinetic study of both the enantiomers.

Combined solvent- and non-uniform temperature-programmed gradient liquid chromatography. I - A theoretical investigation

An new class of gradient liquid chromatography (GLC) is proposed and its performance is analyzed from a theoretical viewpoint. During the course of such gradients, both the solvent strength and the column temperature are simultaneously changed in time and space. The solvent and temperature gradients propagate along the chromatographic column at their own and independent linear velocity. This class of gradient is called combined solvent- and temperature-programmed gradient liquid chromatography (CST-GLC). The general expressions of the retention time, retention factor, and of the temporal peak width of the analytes at elution in CST-GLC are derived for linear solvent strength (LSS) retention models, modified van't Hoff retention behavior, linear and non-distorted solvent gradients, and for linear temperature gradients. In these conditions, the theory predicts that CST-GLC is equivalent to a unique and apparent dynamic solvent gradient. The apparent solvent gradient steepness is the sum of the solvent and temperature steepness. The apparent solvent linear velocity is the reciprocal of the steepness-averaged sum of the reciprocal of the actual solvent and temperature linear velocities. The advantage of CST-GLC over conventional GLC is demonstrated for the resolution of protein digests (peptide mapping) when applying smooth, retained, and linear acetonitrile gradients in combination with a linear temperature gradient (from 20°C to 90°C) using 300μm×150mm capillary columns packed with sub-2 μm particles. The benefit of CST-GLC is demonstrated when the temperature gradient propagates at the same velocity as the chromatographic speed. The experimental proof-of-concept for the realization of temperature ramps propagating at a finite and constant linear velocity is also briefly described.

Assessment of column selection systems using Partial Least Squares

Column selection systems based on calculation of a scalar measure based on Euclidean distance between chromatographic columns, suffer from the same issue. For diverse values of their parameters, identical or near-identical values can be calculated. Proper use of chemometric methods can not only provide a remedy, but also reveal underlying correlation between them. In this work, parameters of a well-established column selection system (CSS) developed at Katholieke Universiteit Leuven (KUL CSS) have been directly correlated to parameters of selectivity (retention time, resolution, and peak/valley ratio) toward pharmaceuticals, by employing Partial Least Squares (PLS). Two case studies were evaluated, separation of alfuzosin, lamotrigine, and their impurities, respectively. Within them, comprehensive correlation structure was revealed, which was thoroughly interpreted, confirming a causal relationship between KUL parameters and parameters of column performance. Furthermore, it was shown that the developed methodology can be applied to any distance-based column selection system.

Quality by design in the chiral separation strategy for the determination of enantiomeric impurities: development of a capillary electrophoresis method based on dual cyclodextrin systems for the analysis of levosulpiride

Quality by design (QbD) concepts, in accordance with International Conference on Harmonisation Pharmaceutical Development guideline Q8(R2), represent an innovative strategy for the development of analytical methods. In this paper QbD principles have been comprehensively applied in the set-up of a capillary electrophoresis method aimed to quantify enantiomeric impurities. The test compound was the chiral drug substance levosulpiride (S-SUL) and the developed method was intended to be used for routine analysis of the pharmaceutical product. The target of analytical QbD approach is to establish a design space (DS) of critical process parameters (CPPs) where the critical quality attributes (CQAs) of the method have been assured to fulfil the desired requirements with a selected probability. QbD can improve the understanding of the enantioseparation process, including both the electrophoretic behavior of enantiomers and their separation, therefore enabling its control. The CQAs were represented by enantioresolution and analysis time. The scouting phase made it possible to select a separation system made by sulfated-β-cyclodextrin and a neutral cyclodextrin, operating in reverse polarity mode. The type of neutral cyclodextrin was included among other CPPs, both instrumental and related to background electrolyte composition, which were evaluated in a screening phase by an asymmetric screening matrix. Response surface methodology was carried out by a Doehlert design and allowed the contour plots to be drawn, highlighting significant interactions between some of the CPPs. DS was defined by applying Monte-Carlo simulations, and corresponded to the following intervals: sulfated-β-cyclodextrin concentration, 9-12 mM; methyl-β-cyclodextrin concentration, 29-38 mM; Britton-Robinson buffer pH, 3.24-3.50; voltage, 12-14 kV. Robustness of the method was examined by a Plackett-Burman matrix and the obtained results, together with system repeatability data, led to define a method control strategy. The method was validated and was finally applied to determine the enantiomeric purity of S-SUL in pharmaceutical dosage forms.

Design of Experiment (DOE) Utilization to Develop a Simple and Robust Reversed-Phase HPLC Technique for Related Substances' Estimation of Omeprazole Formulations

A simple, fast, and sensitive reversed-phase HPLC method with UV detection was developed for the quantitation of omeprazole and its eleven related compounds (impurities) in pharmaceutical formulation using the Thermo Accucore C–18 (50 mm × 4.6 mm, 2.6 μm) column. The separation among all the compounds was achieved with a flow rate of 0.8 mL min⁻¹ employing a gradient program of mobile phase A [0.08 M glycine buffer pH 9.0: acetonitrile; 95:05 (v/v)] and mobile phase B [acetonitrile: methanol; 65:35 (v/v)]. The chromatographic detection was carried out at a wavelength of 305 nm. The method was validated for specificity, linearity, and recovery. The huskiness of the method was determined prior to validation using the Design of Experiments (DOE). The ANOVA analysis of DOE with a 95% confidence interval (CI) confirmed the buffer pH of mobile phase A (p <0.0001) and column temperature (p<0.0001) as significant Critical Method Parameters (CMPs).