Quantification of glycine crystallinity by near-infrared (NIR) spectroscopy

A Combined Near-Infrared and Mid-Infrared Spectroscopic Approach for the Detection and Quantification of Glycine in Human Serum

Serum is an important candidate in proteomics analysis as it potentially carries key markers on health status and disease progression. However, several important diagnostic markers found in the circulatory proteome and the low-molecular-weight (LMW) peptidome have become analytically challenging due to the high dynamic concentration range of the constituent protein/peptide species in serum. Herein, we propose a novel approach to improve the limit of detection (LoD) of LMW amino acids by combining mid-IR (MIR) and near-IR spectroscopic data using glycine as a model LMW analyte. This is the first example of near-IR spectroscopy applied to elucidate the detection limit of LMW components in serum; moreover, it is the first study of its kind to combine mid-infrared (25-2.5 μm) and near-infrared (2500-800 nm) to detect an analyte in serum. First, we evaluated the prediction model performance individually with MIR (ATR-FTIR) and NIR spectroscopic methods using partial least squares regression (PLS-R) analysis. The LoD was found to be 0.26 mg/mL with ATR spectroscopy and 0.22 mg/mL with NIR spectroscopy. Secondly, we examined the ability of combined spectral regions to enhance the detection limit of serum-based LMW amino acids. Supervised extended wavelength PLS-R resulted in a root mean square error of prediction (RMSEP) value of 0.303 mg/mL and R² value of 0.999 over a concentration range of 0-50 mg/mL for glycine spiked in whole serum. The LoD improved to 0.17 mg/mL from 0.26 mg/mL. Thus, the combination of NIR and mid-IR spectroscopy can improve the limit of detection for an LMW compound in a complex serum matrix.

Prediction of Solid-State Form of SLS 3D Printed Medicines Using NIR and Raman Spectroscopy

Selective laser sintering (SLS) 3D printing is capable of revolutionising pharmaceutical manufacturing, by producing amorphous solid dispersions in a one-step manufacturing process. Here, 3D-printed formulations loaded with a model BCS class II drug (20% w/w itraconazole) and three grades of hydroxypropyl cellulose (HPC) polymer (-SSL, -SL and -L) were produced using SLS 3D printing. Interestingly, the polymers with higher molecular weights (HPC-L and -SL) were found to undergo a uniform sintering process, attributed to the better powder flow characteristics, compared with the lower molecular weight grade (HPC-SSL). XRPD analyses found that the SLS 3D printing process resulted in amorphous conversion of itraconazole for all three polymers, with HPC-SSL retaining a small amount of crystallinity on the drug product surface. The use of process analytical technologies (PAT), including near infrared (NIR) and Raman spectroscopy, was evaluated, to predict the amorphous content, qualitatively and quantitatively, within itraconazole-loaded formulations. Calibration models were developed using partial least squares (PLS) regression, which successfully predicted amorphous content across the range of 0–20% w/w. The models demonstrated excellent linearity (R² = 0.998 and 0.998) and accuracy (RMSEP = 1.04% and 0.63%) for NIR and Raman spectroscopy models, respectively. Overall, this article demonstrates the feasibility of SLS 3D printing to produce solid dispersions containing a BCS II drug, and the potential for NIR and Raman spectroscopy to quantify amorphous content as a non-destructive quality control measure at the point-of-care.

Infrared Spectroscopy in Aqueous Solutions: Capabilities and Challenges

Fourier-transform infrared (FTIR) spectroscopy provides rapid, reliable, quantitative, and qualitative analysis of samples in different aggregation states, i.e., gases, thin films, solids, liquids, etc. However, when analyzing aqueous solutions, particular issues associated with the rather pronounced IR absorption characteristics of water appear to interfere with the solute determination. In this review, Fourier-transform infrared spectroscopic techniques and their analytical capabilities for analyzing aqueous solutions are reviewed, and highlight examples are discussed.

Characterization of the Salt and Free Base of Active Pharmaceutical Ingredients Based on NMR Relaxometry Measured by Time Domain NMR

NMR relaxometry measurement by time domain NMR (TD-NMR) is a promising technique for characterizing the properties of active pharmaceutical ingredients (APIs). This study is dedicated to identifying the salt and free base of APIs by NMR relaxometry measured by the TD-NMR technique. Procaine (PC) and tetracaine (TC) were selected as model APIs to be tested. By using conventional methods including powder X-ray diffraction and differential scanning calorimetry, this study first confirmed that the salt and free base of the tested APIs differ from each other in their crystalline form. Subsequently, measurements of T₁ and T₂ relaxation were performed on the tested APIs using TD-NMR. The results demonstrated that these NMR relaxometry measurements have sufficient capacity to distinguish the difference between the free base and salt of the tested APIs. Furthermore, quantification of the composition of the binary powder blends consisting of salt and free bases was conducted by analyzing the acquired T₁ and T₂ relaxation curves. The analysis of the T₁ relaxation curves provided a partly acceptable estimation: a good estimation of the composition was observed from PC powders, whereas for TC powders the estimation accuracy changed with the free base content in the binary blends. For the analysis on T₂ relaxation curves, a precise estimation of the composition was observed from all the samples. From these findings, the NMR relaxometry measurement by TD-NMR, in particular the T₂ relaxation measurement, is effective for evaluating the properties of APIs having different crystalline forms.

Use of Atomic Force Microscopy (AFM) to monitor surface crystallization in caffeine-oxalic acid (CAFOXA) cocrystal compacts

Our objective was to monitor the surface crystallization in disordered caffeine-oxalic acid (CAFOXA) cocrystals following exposure to elevated water vapor pressure. This was accomplished using atomic force microscopy (AFM). Disorder was induced in the cocrystal particles by the common pharmaceutical unit operations of milling and compaction. The 'activated' solid, upon exposure to elevated water vapor pressure, had a high propensity to sorb water. This led to a rise in molecular mobility and the surface underwent rapid crystallization to form needle shaped crystals of CAFOXA. Using AFM height and phase imaging, we were able to directly visualize phase transformations on the compact surface. The milled compacts exhibited higher processing induced disorder than the unmilled compacts, thereby accelerating the surface recrystallization.

Stabilizers and their interaction with formulation components in frozen and freeze-dried protein formulations

This review aims to provide an overview of the current knowledge on protein stabilization during freezing and freeze-drying in relation to stress conditions commonly encountered during these processes. The traditional as well as refined mechanisms by which excipients may stabilize proteins are presented. These stabilizers encompass a wide variety of compounds including sugars, sugar alcohols, amino acids, surfactants, buffers and polymers. The rational selection of excipients for use in frozen and freeze-dried protein formulations is presented. Lyophilized protein formulations are generally multicomponent systems, providing numerous possibilities of excipient-excipient and protein-excipient interactions. The interplay of different formulation components on the protein stability and excipient functionality in the frozen and freeze-dried systems are reviewed, with discussion of representative examples of such interactions.

Quantitative Evaluation of the Crystallinity of Indomethacin Using 1H T2 Relaxation Behaviors Measured by Time Domain NMR

We sought to demonstrate the usefulness of the T₂ relaxation behaviors measured by time domain NMR (TD-NMR) for the quantitative evaluation of the crystallinity of an active pharmaceutical ingredient (API). This study used indomethacin as a model API. After blending amorphous and crystalline indomethacin powders at a designated ratio, T₂ relaxation curves were measured by TD-NMR. Subsequently, we acquired a calibration curve to quantify crystallinity by curve fitting analysis. Validation demonstrated a good correlation between the theoretical and experimental percentage of crystallinity. Thus, this study succeeded in a precise estimation of crystallinity of indomethacin using TD-NMR. We also investigated whether the technique is practical by testing indomethacin powders with unknown proportion of crystallinity, and then compared their estimated crystallinity with that found using conventional evaluation techniques. The quantitative performance of the TD-NMR technique was comparable to that of Raman spectroscopy. Furthermore, indomethacin powder blended with excipients, which can be used to produce tablets, was tested. The TD-NMR technique was still able to quantify the crystallinity of indomethacin, even when excipients were incorporated into the sample. Therefore, the present study expands the horizon for evaluating the crystallinity of APIs in pharmaceutical sciences.

Chemometrics-assisted solid-state characterization of pharmaceutically relevant materials. Polymorphic substances

Current regulations command to properly characterize pharmaceutically relevant solid systems. Chemometrics comprise a range of valuable tools, suitable to process large amounts of data and extract valuable information hidden in their structure. This review aims to detail the results of the fruitful association between analytical techniques and chemometrics methods, focusing on those which help to gain insight into the characteristics of drug polymorphism as an important aspect of the solid state of bulk drugs and drug products. Hence, the combination of Raman, terahertz, mid- and near- infrared spectroscopies, as well as instrumental signals resulting from X-ray powder diffraction, ¹³C solid state nuclear magnetic resonance spectroscopy and thermal methods with quali-and quantitative chemometrics methodologies are examined. The main issues reviewed, concerning pharmaceutical drug polymorphism, include the use of chemometrics-based approaches to perform polymorph classification and assignment of polymorphic identity, as well as the determination of given polymorphs in simple mixtures and complex systems. Aspects such as the solvation/desolvation of solids, phase transformation, crystallinity and the recrystallization from the amorphous state are also discussed. A brief perspective of the field for the next future is provided, based on the developments of the last decade and the current state of the art of analytical instrumentation and chemometrics methodologies.

Characterisation of crystalline-amorphous blends of sucrose with terahertz-pulsed spectroscopy: the development of a prediction technique for estimating the degree of crystallinity with partial least squares regression

The control of the amorphous and crystalline states of drugs and excipients is important in many instances of product formulation, manufacture, and packaging, such as the formulation of certain (freeze-dried) fast melt tablets. This study examines the use of terahertz-pulsed spectroscopy (TPS) coupled with two different data analytical methods as an off-line tool (in the first instance) for assessing the degree of crystallinity in a binary mixture of amorphous and polycrystalline sucrose. The terahertz spectrum of sucrose was recorded in the wave number range between 3 and 100 cm(-1) for both the pure crystalline form and for a mixture of the crystalline and amorphous (freeze-dried) form. The THz spectra of crystalline sucrose showed distinct absorption bands at ∼48, ∼55, and ∼60 cm(-1) while all these features were absent in the amorphous sucrose. Calibration models were constructed based on (1) peak area analysis and (2) partial least square regression analysis, with the latter giving the best LOD and LOQ of 0.76% and 2.3%, respectively. The potential for using THz spectroscopy, as a quantitative in-line tool for percent crystallinity in a range of complex systems such as conventional tablets and freeze-dried formulations, is suggested in this study.

Use of infrared spectroscopy to monitor protein structure and stability

One of the most versatile methods for monitoring the structure of proteins, either in solution or in the solid state, is Fourier transform infrared spectroscopy. Also known as mid-range infrared, which covers the frequency range from 4000 to 400 cm(-1), this wavelength region includes bands that arise from three conformationally sensitive vibrations within the peptide backbone (amide I, II and III). Of these vibrations, amide I is the most widely used and can provide information on secondary structure composition and structural stability. One of the advantages of infrared spectroscopy is that it can be used with proteins that are either in solution or in the solid state. The use of infrared to monitor protein structure and stability is summarized herein. In addition, specialized infrared methods are presented, such as techniques for the study of membrane proteins and oriented samples. In addition, there is a growing body of literature on the use of infrared to follow reaction kinetics and ligand binding in proteins, as well as a number of infrared studies on protein dynamics. Finally, the potential for using near-infrared spectroscopy to study protein structure is introduced.

Protection of DNA against low-energy electrons by amino acids: a first-principles molecular dynamics study

The physical shielding and chemical stabilizing of the low energy electron by glycine around the thymine through proton transfer.

Methods of amorphization and investigation of the amorphous state

The amorphous form of pharmaceutical materials represents the most energetic solid state of a material. It provides advantages in terms of dissolution rate and bioavailability. This review presents the methods of solid- -state amorphization described in literature (supercooling of liquids, milling, lyophilization, spray drying, dehydration of crystalline hydrates), with the emphasis on milling. Furthermore, we describe how amorphous state of pharmaceuticals differ depending on the method of preparation and how these differences can be screened by a variety of spectroscopic (X-ray powder diffraction, solid state nuclear magnetic resonance, atomic pairwise distribution, infrared spectroscopy, terahertz spectroscopy) and calorimetry methods.

Application of partial least-squares (PLS) modeling in quantifying drug crystallinity in amorphous solid dispersions

Among the different experimental methods that can be used to quantify the evolution of drug crystallinity in polymer-containing amorphous solid dispersions, powder X-ray diffractometry (PXRD) is commonly considered as a frontline method. In order to achieve accurate quantification of the percent drug crystallinity in the system, calibration curves have to be constructed using appropriate calibration samples and calculation methods. This can be non-trivial in the case of partially crystalline solid dispersions where the calibration samples must capture the multiphase nature of the systems and the mathematical model must be robust enough to accommodate subtle and not so subtle changes in the diffractograms. The purpose of this study was to compare two different calculation and model-building methods to quantify the proportion of crystalline drug in amorphous solid dispersions containing different ratios of drug and amorphous polymer. The first method involves predicting the % drug crystallinity from the ratio of the area underneath the Bragg peaks to total area of the diffractogram. The second method is multivariate analysis using a Partial Least-Squares (PLS) multivariate regression method. It was found that PLS analysis provided far better accuracy and prediction of % drug crystallinity in the sample. Through the application of PLS, root-mean-squared error of estimation (RMSEE) values of 2.2%, 1.9%, and 4.7% drug crystallinity was achieved for samples containing 25%, 50%, and 75% polymer, respectively, compared to values of 11.2%, 17.0%, and 23.6% for the area model. In addition, construction of a PLS model enables further analysis of the data, including identification of outliers and non-linearity in the data, as well as insight into which factors are most important to correlate PXRD diffractograms with % crystallinity of the drug through analysis of the loadings.

A review of near infrared spectroscopy and chemometrics in pharmaceutical technologies

Near-infrared spectroscopy (NIRS) is a fast and non-destructive analytical method. Associated with chemometrics, it becomes a powerful tool for the pharmaceutical industry. Indeed, NIRS is suitable for analysis of solid, liquid and biotechnological pharmaceutical forms. Moreover, NIRS can be implemented during pharmaceutical development, in production for process monitoring or in quality control laboratories. This review focuses on chemometric techniques and pharmaceutical NIRS applications. The following topics are covered: qualitative analyses, quantitative methods and on-line applications. Theoretical and practical aspects are described with pharmaceutical examples of NIRS applications.

Analytical techniques for quantification of amorphous/crystalline phases in pharmaceutical solids

The existence of different solid-state forms such as polymorphs, solvates, hydrates, and amorphous form in pharmaceutical drug substances and excipients, along with their downstream consequences in drug products and biological systems, is well documented. Out of these solid states, amorphous systems have attracted considerable attention of formulation scientists for their specific advantages, and their presence, either by accident or design is known to incorporate distinct properties in the drug product. Identification of different solid-state forms is crucial to anticipate changes in the performance of the material upon storage and/or handling. Quantitative analysis of physical state is imperative from the viewpoint of both the manufacturing and the regulatory control aimed at assuring safety and efficacy of drug products. Numerous analytical techniques have been reported for the quantification of amorphous/crystalline phase, and implicit in all quantitative options are issues of accuracy, precision, and suitability. These quantitative techniques mainly vary in the properties evaluated, thus yielding divergent values of crystallinity for a given sample. The present review provides a compilation of the theoretical and practical aspects of existing techniques, thereby facilitating the selection of an appropriate technique to accomplish various objectives of quantification of amorphous systems.

Near-infrared analysis of protein secondary structure in aqueous solutions and freeze-dried solids

Near-infrared spectroscopy (NIR) of various proteins (bovine serum albumin, lysozyme, ovalbumin, gamma-globulin, beta-lactoglobulin, myoglobin, cytochrome-c) was investigated as a possible analytical method of the protein secondary structure in various physical states. The spectra of proteins in aqueous solutions (transmission mode, solvent-compensated) and those in freeze-dried solids (nondestructive diffuse reflection mode) showed several bands at similar frequencies in the combination (4000-5000 cm(-1)) and first overtone (5600-6600 cm(-1)) spectral regions. The normalized second-derivative near-infrared spectra of proteins in aqueous solutions suggested that some bands indicated alpha-helix (4090, 4365-4370, 4615, and 5755 cm(-1)) and beta-sheet (4060, 4405, 4525-4540, 4865, and 5915-5925 cm(-1)) structures. The proteins mostly maintained spectra characteristic of their native structure after freeze-drying, although some reductions in alpha-helical structure and increase in unordered or beta-sheet structures were observed. The near-infrared analysis also showed beta-sheet formation of heat-treated BSA in aqueous solutions and in subsequently freeze-dried solids. The present results thus indicated that the nondestructive near-infrared analysis can be used for the investigation of dehydration-induced changes in protein secondary structures.

Quantitative determination of amorphous cyclosporine in crystalline cyclosporine samples by Fourier transform infrared spectroscopy

The purpose of this study is the development of a quantification method to detect the amount of amorphous cyclosporine using Fourier transform infrared (FTIR) spectroscopy. The mixing of different percentages of crystalline cyclosporine with amorphous cyclosporine was used to obtain a set of standards, composed of cyclosporine samples characterized by different percentages of amorphous cyclosporine. Using a wavelength range of 450-4,000 cm(-1), FTIR spectra were obtained from samples in potassium bromide pellets and then a partial least squares (PLS) model was exploited to correlate the features of the FTIR spectra with the percentage of amorphous cyclosporine in the samples. This model gave a standard error of estimate (SEE) of 0.3562, with an r value of 0.9971 and a standard error of prediction (SEP) of 0.4168, which derives from the cross validation function used to check the precision of the model. Statistical values reveal the applicability of the method to the quantitative determination of amorphous cyclosporine in crystalline cyclosporine samples.

Noninvasive Determination of Protein Conformation in the Solid State Using Near Infrared (NIR) Spectroscopy

Fourier transform infrared (FTIR) spectroscopy is a powerful tool for monitoring structural changes in lyophilized protein formulations. However, direct measurement of IR spectra requires significant handling time and effort. The possibility of using near infrared (NIR) spectroscopy as a rapid and noninvasive alternative to FTIR is explored in this study. NIR and conventional FTIR spectra were collected for two model proteins, alpha-chymotrypsinogen A and cytochrome c, under conditions of varying stability and structural perturbation. NIR was then compared to FTIR and whereby calibration model was generated by partial least square (PLS) regression to correlate NIR data with FTIR spectra. There is a strong correlation of certain NIR bands with the amide I region of FTIR spectra. It appears that NIR can distinguish damage caused by elevated temperatures and freeze-drying stresses. The ability of sucrose to stabilize the structure of these two proteins can be detected by both methods. It appears that NIR spectroscopy has the potential to provide detailed information on the secondary structure of proteins in the solid state. However, many more examples will be needed to demonstrate fully the ability of NIR to replace FTIR as the standard tool for characterizing lyophilized protein formulations.