Physical characteristics and chemical degradation of amorphous quinapril hydrochloride

Molecular complexes of drug combinations: A review of cocrystals, salts, coamorphous systems and amorphous solid dispersions

As the advancements in the medical technology and healthcare develop through the years, combinational therapy has evolved to be an important treatment modality in many disease settings, including cancer, cardiovascular disease and infectious diseases. In an effort to alleviate "pill burden" and improve patient compliance, fixed dose combinations (FDCs) have been developed to be used as effective therapeutics. Among all FDCs, the category of drug-drug molecular complexes has been proven an efficient methodology in designing and treating diseases, with many drugs being approved. Among all drug-drug molecular complexes, drug-drug cocrystals, salts, coamorphous systems and solid dispersions have been successfully developed and many have been approved by the FDA. In this review, we dwell deeply into the molecular mechanisms behind the different types of drug-drug molecular complexes, including the key functional groups involved in the intermolecular interactions, the applications of each category of molecular complexes, as well as the advantages and challenges thereof. This comprehensive review provides useful insights into the practical design and manufacture of drug-drug molecular complexes and points out the future direction for the development of new advantageous combinational therapies that benefit more patients.

Polymer-inducing chemical degradation of amorphous solid dispersions driven by drug-polymer interactions for physical stabilization

Intermolecular interactions between active pharmaceutical ingredients (APIs) and carrier polymers are important for the long-term physical stability of amorphous solid dispersions (ASDs). However, the negative impact of intermolecular interactions on chemical stability has rarely been reported. In this study, the relationship between intermolecular interactions and physical and chemical stability was investigated using two ASDs composed of API and hydroxypropyl methylcellulose acetate succinate (HPMCAS) with different stabilities: ASD1 was physically stable but chemically unstable, whereas ASD2 was physically unstable but chemically stable. Ionic-bonding between the pyridine nitrogen in the API and succinyl group in HPMCAS was found in both ASDs. The additional interaction between the succinyl group in HPMCAS and the hydroxyl group in the API was suggested only in ASD1. It was concluded that the additional interaction contributed to the physical stability of ASD1; however, it accelerated the chemical reaction between the succinyl and hydroxyl groups to generate succinyl ester owing to its close proximity. This study shows that the intermolecular interaction between the API and carrier polymer is not always beneficial for chemical stability. Understanding the molecular states of APIs and polymers in ASDs is important for their successful development.

Role of Crystal Disorder and Mechanoactivation in Solid-State Stability of Pharmaceuticals

Common energy-intensive processes applied in oral solid dosage development, such as milling, sieving, blending, compaction, etc. generate particles with surface and bulk crystal disorder. An intriguing aspect of the generated crystal disorder is its evolution and repercussion on the physical- and chemical stabilities of drugs. In this review, we firstly examine the existing literature on crystal disorder and its implications on solid-state stability of pharmaceuticals. Secondly, we discuss the key aspects related to the generation and evolution of crystal disorder, dynamics of the disordered/amorphous phase, analytical techniques to measure/quantify them, and approaches to model the disordering propensity from first principles. The main objective of this compilation is to provide special impetus to predict or model the chemical degradation(s) resulting from processing-induced manifestation in bulk solid manufacturing. Finally, a generic workflow is proposed that can be useful to investigate the relevance of crystal disorder on the degradation of pharmaceuticals during stability studies. The present review will cater to the requirements for developing physically- and chemically stable drugs, thereby enabling early and rational decision-making during candidate screening and in assessing degradation risks associated with formulations and processing.

Accelerated Storage for Shelf-Life Prediction of Lyophiles: Temperature Dependence of Degradation of Amorphous Small Molecular Weight Drugs and Proteins

Prediction of lyophilized product shelf-life using accelerated stability data requires understanding the temperature dependence of the degradation rate. Despite the abundance of published studies on stability of freeze-dried formulations and other amorphous materials, there are no definitive conclusions on the type of pattern one can expect for the temperature dependence of degradation. This lack of consensus represents a significant gap which may impact development and regulatory acceptance of freeze-dried pharmaceuticals and biopharmaceuticals. Review of the literature demonstrates that the temperature dependence of degradation rate constants in lyophiles can be represented by the Arrhenius equation in most cases. In some instances there is a break in the Arrhenius plot around the glass transition temperature or a related characteristic temperature. The majority of the activation energies (E_a), which are reported for various degradation pathways in lyophiles, falls in the range of 8 to 25 kcal/mol. The degradation E_a values for lyophiles are compared with the E_a for relaxation processes and diffusion in glasses, as wells as solution chemical reactions. Collectively, analysis of the literature demonstrates that the Arrhenius equation represents a reasonable empirical tool for analysis, presentation, and extrapolation of stability data for lyophiles, provided that specific conditions are met.

Mechanoactivation as a Tool to Assess the Autoxidation Propensity of Amorphous Drugs

Mechanoactivation has attracted considerable attention in the pharmaceutical sciences due to its ability to generate amorphous materials and solid-state synthetic products without the use of solvent. Although some studies have reported drug degradation during milling, no studies have systematically investigated the use of mechanoactivation in predicting drug degradation in the solid state. Thus, this work explores the autoxidation of drugs in the solid state by comilling amorphous mifepristone (MFP):polyvinylpyrrolidone vinyl acetate (PVPVA) and amorphous olanzapine (OLA):PVPVA. MFP was amorphized by ball milling and OLA by quench cooling techniques. Subsequently, comilling the amorphous drugs in the presence of a 10-fold weight ratio of PVPVA (the excipient containing reactive free radicals) was performed at several milling frequencies to identify the kinetics of mechano-autoxidation over milling durations. Overall, milling led to the degradation of up to 5% drug in the solid state. The autoxidation mechanism was confirmed by performing a stress study in the solution at 50 °C for 5 h, by using a 10 mM azo-bis(isobutyronitrile) (AIBN) as a stressing agent. By deconvoluting the effect of milling frequency and the energy on the extent and kinetics of milling-induced autoxidation of amorphous drugs, it was possible to fit an extended Arrhenius model that allowed extrapolation of mechanoactivated degradation rates (K_m) to zero milling frequencies. Further, the autoxidation rates of drugs stored at high temperatures were observed to follow an Arrhenius behavior. A good degree of agreement was observed between the model predictions obtained by mechanoactivation (K_m) to the reaction rates observed under accelerated temperatures. Additionally, the impact of adding an antioxidant (e.g., butylated hydroxytoluene) to the mixture during comilling was also examined. This study can be helpful in evaluating the stability of amorphous solids stored in accelerated (non-hermetic) conditions, in screening solid-state autoxidation propensity of drugs, and for the rational selection of antioxidants.

Influence of the crystallization tendencies of pharmaceutical glasses on the applicability of the Adam-Gibbs-Vogel and Vogel-Tammann-Fulcher equations in the prediction of their long-term physical stability

Amorphization is a powerful approach for improving the aqueous solubility and bioavailability of poorly water-soluble compounds. However, it can cause chemical and physical instability, the latter of which can lead to crystallization during storage, diminishing the solubility advantage of the amorphous state. As there is no standard method for predicting the physical stability of amorphous materials, a long-term stability study is needed in drug development. This study investigated the correlation between the physical stability of amorphous compounds and molecular mobility based on the assumption that physical stability is governed by the diffusional motion of a molecule. Model compounds were evaluated for crystallization onset time, structural relaxation time, fragility, and fictive temperature. The crystallization onset time of acetaminophen glass correlated with its relaxation time calculated from the Adam-Gibbs-Vogel equation; however, that of felodipine glass correlated with the relaxation time calculated from the Vogel-Tammann-Fulcher equation. The different crystallization tendencies of these compounds can be explained by the differences in the rate limiting steps in their crystallization processes, indicating the importance of distinguishing the critical process associated with crystallization. These findings will be useful for more accurate prediction of long-term physical stability of amorphous materials.

Role of polymers in the physical and chemical stability of amorphous solid dispersion: A case study of carbamazepine

Incorporating the amorphous drug in polymeric components has been demonstrated as a feasible approach to enhance the bioavailability of poorly water-soluble drugs. The objective of this study was to investigate the role of polymers in the stability of amorphous solid dispersion (ASD) by evaluating the drug-polymer interaction, microenvironmental pH, and stability of ASD. Carbamazepine (CBZ), a Biopharmaceutics Classification System Class II compound, was utilized as a model drug. Polyvinylpyrrolidone (PVP), poly(1-vinylpyrrolidone-co-vinyl acetate) (PVPVA), polyacrylic acid (PAA), and hydroxypropyl methylcellulose (HPMCAS) were selected as model polymers. CBZ ASDs were characterized by X-ray diffractometry (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and dissolution studies. Molecular modeling was conducted to understand the strength of interaction between CBZ and each polymer. FTIR spectroscopy and molecular modeling results show that the interaction between CBZ and PAA is the strongest among all the ASDs, as PAA is an acidic polymer with the potential to form strong hydrogen bonding with CBZ. Besides, hydrophobic interaction is detected between CBZ and HPMCAS. CBZ-PAA and CBZ-HPMCAS ASDs reveal better physical stability than CBZ-PVP and CBZ-PVPVA ASDs under 40 °C/75% RH for 8 weeks. However, CBZ-PAA ASD shows chemical degradation after stability testing due to its acidic microenvironmental pH. This paper shows that strong intermolecular interactions between CBZ and polymers contribute to the physical stability of the ASDs. Additionally, acidic polymers yield an acidic microenvironment pH of the ASDs that causes chemical degradation during storage. Hence, a balance between the ability of a given polymer to promote physical stability and chemical stability may need to be considered.

Structure elucidation and formation mechanistic study of a methylene-bridged pregabalin dimeric degradant in pregabalin extended-release tablets

During the pharmaceutical development of pregabalin extended-release tablets, an unknown degradant at a relative retention time (RRT) of 11.7 was observed and its nominal amount exceeded the ICH identification threshold in an accelerated stability study. The aim of this study is to identify the structure and investigate the formation mechanism of this impurity for the purpose of developing a chemically stable pharmaceutical product. By utilizing multi-stage LC-MS analysis in conjunction with mechanism-based stress study, the structure of the RRT 11.7 impurity was rapidly identified as a dimeric degradant that is caused by dimerization of two pregabalin molecules with a methylene bridging the two pregabalin moieties. The structure of the dimer was confirmed by 1D and 2D NMR measurement. The formation pathway of the dimeric degradant was also inferred from the mechanism-based stress study, which implicated that the bridging methylene could originate from formaldehyde which might be the culprit that triggers the dimerization in the first place. The subsequent API-excipients compatibility study indicated that the degradant was indeed formed in the compatibility experiments between pregabalin API and two polymeric excipients (PEO and PVPP) that are known to contain residual formaldehyde, but only in the co-presence of another excipient, colloidal silicon dioxide (SiO₂). The kinetic behavior of the degradant formation was also investigated and two kinetic models were utilized based on the Arrhenius and Eyring equations, respectively, to calculate the activation energy (E_a) as well as the enthalpy of activation (△H^‡), entropy of activation (△S^‡), and Gibbs free energy (△G^‡) of the degradation reaction. The results of this study would be useful for the understanding of similar dimeric degradant formation in finished products of drug substances containing primary or secondary amine moieties.

A novel drug–drug coamorphous system without molecular interactions: improve the physicochemical properties of tadalafil and repaglinide

Tadalafil and repaglinide, categorized as BCS class II drugs, have low oral bioavailabilities due to their poorly aqueous solubilities and dissolutions. The aim of this study was to enhance the dissolution of tadalafil and repaglinide by co-amorphization technology and evaluate the storage and compression stability of such coamorphous system. Based on Flory–Huggins interaction parameter (χ ≤ 0) and Hansen solubility parameter (δ_t ≤ 7 MPa^0.5) calculations, tadalafil and repaglinide was predicted to be well miscible with each other. Coamorphous tadalafil–repaglinide (molar ratio, 1 : 1) was prepared by solvent-evaporation method and characterized with respect to its thermal properties, possible molecular interactions. A single T_g (73.1 °C) observed in DSC and disappearance of crystallinity in PXRD indicated the formation of coamorphous system. Principal component analysis of FTIR in combination with Raman spectroscopy and Ss ¹³C NMR suggested the absence of intermolecular interactions in coamorphous tadalafil–repaglinide. In comparison to pure crystalline forms and their physical mixtures, both drugs in coamorphous system exhibited significant increases in intrinsic dissolution rate (1.5–3-fold) and could maintain supersaturated level for at least 4 hours in non-sink dissolution. In addition, the coamorphous tadalafil–repaglinide showed improved stability compared to the pure amorphous forms under long-term stability and accelerated storage conditions as well as under high compressing pressure. In conclusion, this study showed that co-amorphization technique is a promising approach for improving the dissolution rate of poorly water-soluble drugs and for stabilizing amorphous drugs.

The coamorphous tadalafil–repaglinide (molar ratio, 1 : 1) prepared by solvent-evaporation method significantly improve the physicochemical properties of tadalafil and repaglinide.

Effects of the Preparation Process on the Properties of Amorphous Solid Dispersions

The use of amorphous solid dispersions to improve the bioavailability of active ingredients from the BCS II and IV classifications continues to gain interest in the pharmaceutical industry. Over the last decade, methods for generating amorphous solid dispersions have been well established in commercially available products and in the literature. However, the amorphous solid dispersions manufactured by different technologies differ in many aspects, primarily chemical stability, physical stability, and performance, both in vitro and in vivo. This review analyzes the impact of manufacturing methods on those properties of amorphous solid dispersions. For example, the chemical stability of drugs and polymers can be influenced by differences in the level of thermal exposure during fusion-based and solvent-based processes. The physical stability of amorphous content varies according to the thermal history, particle morphology, and nucleation process of amorphous solid dispersions produced by different methods. The in vitro and in vivo performance of amorphous formulations are also affected by differences in particle morphology and in the molecular interactions caused by the manufacturing method. Additionally, we describe the mechanism of manufacturing methods and the thermodynamic theories that relate to amorphous formulations.

Effect of milling on the plastic and the elastic stiffness of lactose particles

The purpose of this study was to investigate the effect of degree of disorder of a series of α-lactose monohydrate powders, prepared by milling for different time periods, on the plastic and the elastic stiffness of the particles. As references, a series of physical mixtures consisting of original crystalline particles and amorphous particles obtained by spray-drying was used. In addition, the effect of powder pre-storage humidity on the mechanical properties was investigated. For milled particles of a low degree of disorder, a decreased particle size increased the particle plastic stiffness. For milled particles of constant particle size, the plastic stiffness decreased with an increased degree of disorder while the elastic stiffness seemed nearly independent of the degree of disorder. The presence of moisture caused a recrystallisation of milled particles with low degree of disorder which increased their plastic stiffness. For the physical mixtures of crystalline and amorphous particles, similar relationships between plastic stiffness and amorphous content as for the milled powders were obtained. A reasonable explanation is that the nature of the milled particles is represented by a two-state system with crystalline and amorphous domains.

Process Development and Robust Control of Physical Attributes of an Amorphous Drug Substance

Control of physical attributes of amorphous active pharmaceutical ingredients (APIs) can be challenging due to processability issues, their wide variation during processing and the requirement to control them to specific ranges. In this article, we report our efforts to develop a robust isolation process for boceprevir, which delivers specific surface areas between 3.0 and 9.4 m²/g. We developed mechanistic process understanding by utilizing a new method to measure glass transition temperature of API suspensions. Boceprevir processability and surface area are determined by the interplay between the suspension operating conditions and glass transition temperature. Processing time and thermal history also influence API surface area evolution, rendering a dynamic nature to it. A control strategy was developed consisting of 2 elements: a continuous tee mixer precipitation process, which delivers API in the 40-60 m²/g range and 1 of 2 annealing step variants. In the first, surface area was controlled in 4 batches to 6.7-7.5 m²/g by equilibrating the API suspension above its glass transition temperature and a subsequent vacuum distillation. The second annealing variant controlled surface area to 4.5-6.5 m²/g for over 70 commercial batches through a dynamic vacuum distillation step with prescribed temperature and % batch volume distilled profiles versus time.

Pharmaceutical solvates, hydrates and amorphous forms: A special emphasis on cocrystals

Active pharmaceutical ingredients (APIs) may exist in various solid forms, which can lead to differences in the intermolecular interactions, affecting the internal energy and enthalpy, and the degree of disorder, affecting the entropy. Differences in solid forms often lead to differences in thermodynamic parameters and physicochemical properties for example solubility, dissolution rate, stability and mechanical properties of APIs and excipients. Hence, solid forms of APIs play a vital role in drug discovery and development in the context of optimization of bioavailability, filing intellectual property rights and developing suitable manufacturing methods. In this review, the fundamental characteristics and trends observed for pharmaceutical hydrates, solvates and amorphous forms are presented, with special emphasis, due to their relative abundance, on pharmaceutical hydrates with single and two-component (i.e. cocrystal) host molecules.

Processing thermally labile drugs by hot-melt extrusion: The lesson with gliclazide

The formation of molecularly dispersed amorphous solid dispersions by the hot-melt extrusion technique relies on the thermal and mechanical energy inputs, which can cause chemical degradation of drugs and polymeric carriers. Additionally, drug degradation may be exacerbated as drugs convert from a more stable crystalline form to a higher energy amorphous form. Therefore, it is imperative to study how drug degrades and evaluate methods to minimize drug degradation during the extrusion process. In this work, gliclazide was used as a model thermally labile drug for the degradation kinetics and process optimization studies. Preformulation studies were conducted using thermal analyses, and liquid chromatography-mass spectroscopy to identify drug degradation pathways and to determine initial extrusion conditions. Formulations containing 10% drug and 90% AFFINISOL™ HPMC HME 100LV were then extruded using a twin screw extruder, and the extrudates were characterized using X-ray powder diffraction, modulated dynamic scanning calorimetry, and potency testing to evaluate physicochemical properties. The energies of activation for both amorphous gliclazide, crystalline gliclazide, and gliclazide solution were calculated using the Arrhenius equation to further guide the extrusion optimization process. Preformulation studies identify two hydrolysis degradation pathways of gliclazide at elevated temperatures. The activation energy study indicates a significantly higher degradation rate for the amorphous gliclazide compared to the crystalline form. After optimization of the hot-melt extrusion process, including improved screw designs, machine setup, and processing conditions, gliclazide amorphous solid dispersion with ∼95% drug recovery was achieved. The ability to process thermally labile drugs and polymers using hot-melt extrusion will significantly expand the possible applications of this manufacturing process.

Unintended and in situ amorphisation of pharmaceuticals

Amorphisation of poorly water-soluble drugs is one approach that can be applied to improve their solubility and thus their bioavailability. Amorphisation is a process that usually requires deliberate external energy input. However, amorphisation can happen both unintentionally, as in process-induced amorphisation during manufacturing, or in situ during dissolution, vaporisation, or lipolysis. The systems in which unintended and in situ amorphisation has been observed normally contain a drug and a carrier. Common carriers include polymers and mesoporous silica particles. However, the precise mechanisms by which in situ amorphisation occurs are often not fully understood. In situ amorphisation can be exploited and performed before administration of the drug or possibly even within the gastrointestinal tract, as can be inferred from in situ amorphisation observed during in vitro lipolysis. The use of in situ amorphisation can thus confer the advantages of the amorphous form, such as higher apparent solubility and faster dissolution rate, without the disadvantage of its physical instability.

An investigation into the crystallization tendency/kinetics of amorphous active pharmaceutical ingredients: A case study with dipyridamole and cinnarizine

Amorphous drug formulations have great potential to enhance solubility and thus bioavailability of BCS class II drugs. However, the higher free energy and molecular mobility of the amorphous form drive them towards the crystalline state which makes them unstable. Accurate determination of the crystallization tendency/kinetics is the key to the successful design and development of such systems. In this study, dipyridamole (DPM) and cinnarizine (CNZ) have been selected as model compounds. Thermodynamic fragility (mT) was measured from the heat capacity change at the glass transition temperature (Tg) whereas dynamic fragility (mD) was evaluated using methods based on extrapolation of configurational entropy to zero [Formula: see text] , and heating rate dependence of Tg [Formula: see text] . The mean relaxation time of amorphous drugs was calculated from the Vogel-Tammann-Fulcher (VTF) equation. Furthermore, the correlation between fragility and glass forming ability (GFA) of the model drugs has been established and the relevance of these parameters to crystallization of amorphous drugs is also assessed. Moreover, the crystallization kinetics of model drugs under isothermal conditions has been studied using Johnson-Mehl-Avrami (JMA) approach to determine the Avrami constant 'n' which provides an insight into the mechanism of crystallization. To further probe into the crystallization mechanism, the non-isothermal crystallization kinetics of model systems were also analysed by statistically fitting the crystallization data to 15 different kinetic models and the relevance of model-free kinetic approach has been established. The crystallization mechanism for DPM and CNZ at each extent of transformation has been predicted. The calculated fragility, glass forming ability (GFA) and crystallization kinetics are found to be in good correlation with the stability prediction of amorphous solid dispersions. Thus, this research work involves a multidisciplinary approach to establish fragility, GFA and crystallization kinetics as stability predictors for amorphous drug formulations.

Stabilization of proteins in solid form

Immunogenicity of aggregated or otherwise degraded protein delivered from depots or other biopharmaceutical products is an increasing concern, and the ability to deliver stable, active protein is of central importance. We review characterization approaches for solid protein dosage forms with respect to metrics that are intended to be predictive of protein stability against aggregation and other degradation processes. Each of these approaches is ultimately motivated by hypothetical connections between protein stability and the material property being measured. We critically evaluate correlations between these properties and stability outcomes, and use these evaluations to revise the currently standing hypotheses. Based on this we provide simple physical principles that are necessary (and possibly sufficient) for generating solid delivery vehicles with stable protein loads. Essentially, proteins should be strongly coupled (typically through H-bonds) to the bulk regions of a phase-homogeneous matrix with suppressed β relaxation. We also provide a framework for reliable characterization of solid protein forms with respect to stability.

Kinetics and mechanism of solid state imidapril hydrochloride degradation and its degradation impurities identification

A detailed stability testing of solid state imidapril hydrochloride (IMD) was performed and its degradation products were identified. The analysis was conducted according to ICH guidelines Q1A(R2). Pure IMD samples were exposed to stress conditions of elevated temperature and relative humidity (T = 363 K, RH = 76.4%) in order to accelerate degradation. The regular loss of IMD content with time, and the formation of two degradation impurities were observed. The appropriate reaction rate constants k (for IMD degradation and for the formation of product I and II) were calculated using Prout-Tompkins equation. The obtained degradation products were separated and identified by means of LC-MS technique. Based on the obtained m/z values, the masses and the structures of the formed degradation impurities were established. Also IMD degradation scheme was constructed. It was demonstrated that under the applied analytical conditions, IMD degradation follows an autocatalytic reaction model with the rate constant k = (4.764 ± 0.34)×10 −6 s −1 and with the parallel formation of two degradation products: imidaprilat and the diketopiperazine derivative. The obtained experimental results are in agreement with IMD degradation pathways proposed theoretically.

Tris(hydroxymethyl) aminomethane salt of ramipril: synthesis, structural characterization from X-ray powder diffraction and stability studies

Tris(hydroxymethyl) aminomethane (tris) salt of API ramipril was synthesized, and characterized by FTIR, TG-DSC and ab initio X-ray powder structure analysis. The compound, ramipril-tris (II), crystallizes in the monoclinic space group P2(1) with a=24.3341(15), b=6.4645(5), c=9.5357(7) Å, β=96.917(3)° and V=1489.1(3) Å(3). The crystal structure has been determined from laboratory X-ray powder diffraction data using direct space global optimization strategy (simulated annealing) followed by the Rietveld refinement. A network of intermolecular OH…O, CH…N and CH…O hydrogen bonds between the ramipril-ramipril, tris-tris and ramipril-tris components in the compound generates a two-dimensional molecular assembly in (110) plane. A comparative study of solid-state stabilities of ramipril-tris (II) with that of ramipril (I) and ramipril-erbumine (III) indicates that ramipril-tris (II) is the most stable one among the three, and the conversion to impurity D after 72 h at 80 °C is only 1.5%. The solution phase analysis at different pH values also reveals a greater stability of ramipril-tris (II) over ramipril (I).

Advances in simultaneous DSC-FTIR microspectroscopy for rapid solid-state chemical stability studies: some dipeptide drugs as examples

The solid-state chemistry of drugs has seen growing importance in the pharmaceutical industry for the development of useful API (active pharmaceutical ingredients) of drugs and stable dosage forms. The stability of drugs in various solid dosage forms is an important issue because solid dosage forms are the most common pharmaceutical formulation in clinical use. In solid-state stability studies of drugs, an ideal accelerated method must not only be selected by different complicated methods, but must also detect the formation of degraded product. In this review article, an analytical technique combining differential scanning calorimetry and Fourier-transform infrared (DSC-FTIR) microspectroscopy simulates the accelerated stability test, and simultaneously detects the decomposed products in real time. The pharmaceutical dipeptides aspartame hemihydrate, lisinopril dihydrate, and enalapril maleate either with or without Eudragit E were used as testing examples. This one-step simultaneous DSC-FTIR technique for real-time detection of diketopiperazine (DKP) directly evidenced the dehydration process and DKP formation as an impurity common in pharmaceutical dipeptides. DKP formation in various dipeptides determined by different analytical methods had been collected and compiled. Although many analytical methods have been applied, the combined DSC-FTIR technique is an easy and fast analytical method which not only can simulate the accelerated drug stability testing but also at the same time enable to explore phase transformation as well as degradation due to thermal-related reactions. This technique offers quick and proper interpretations.

Stabilization of quinapril by incorporating hydrogen bonding interactions

In the present study stability of various known solvates of quinapril hydrochloride has been compared with nitromethane solvate. Nitromethane solvate was found to be more stable compared to other known solvates. Single crystal X-ray diffraction analysis of quinapril nitromethane solvate shows intermolecular hydrogen bonding between quinapril molecule and nitromethane. Stabilization of quinapril by forming strong hydrogen bonding network as in case of co-crystals was further studied by forming co-crystal with tris(hydroxymethyl)amino methane. Quinapril free base forms a stable salt with tris(hydroxymethyl)amino methane not reported earlier. Quinapril tris(hydroxymethyl)amino methane salt found to be stable even at 80° for 72 h i.e. hardly any formation of diketopiperazine and diacid impurity. As expected single crystal X-ray diffraction analysis reveals tris(hydroxymethyl)amino methane salt of quinapril shows complex hydrogen bonding network between the two entities along with ionic bond. The properties of this stable salt - stable in solid as well as solution phase, might lead to an alternate highly stable formulation.

Effect of sugars on the molecular motion of freeze-dried protein formulations reflected by NMR relaxation times

PURPOSE

To relate NMR relaxation times to instability-related molecular motions of freeze-dried protein formulations and to examine the effect of sugars on these motions.

METHODS

Rotating-frame spin-lattice relaxation time (T(1ρ)) was determined for both protein and sugar carbons in freeze-dried lysozyme-sugar (trehalose, sucrose and isomaltose) formulations using solid-state (13)C NMR.

RESULTS

The temperature dependence of T(1ρ) for the lysozyme carbonyl carbons in lysozyme with and without sugars was describable with a model that includes two different types of molecular motion with different correlation times (τ(c)) for the carbon with each τ(c) showing Arrhenius temperature dependence. Both relaxation modes have much smaller relaxation time constant (τ(c)) and temperature coefficient (Ea) than structural relaxation and may be classified as β-relaxation and γ-relaxation. The τ(c) and Ea for γ-relaxation were not affected by sugars, but those for β-relaxation were increased by sucrose, changed little by trehalose, and decreased by isomaltose, suggesting that the β-mobility of the lysozyme carbonyl carbons is decreased by sucrose and increased by isomaltose.

CONCLUSION

T(1ρ) determined for the lysozyme carbonyl carbons can reflect the effect of sugars on molecular mobility in lysozyme. However, interpretation of relaxation time data is complex and may demand data over an extended temperature range.

Manufacture and performance evaluation of a stable amorphous complex of an acidic drug molecule and Neusilin

In this paper, we explore the use of Neusilin, an inorganic magnesium aluminometasilicate, to stabilize the amorphous form of an acidic drug Sulindac. Both cryomilling and ball milling of the drug with Neusilin were found to produce the amorphous phase. However, the ball-milled (BM) material exhibited superior physical stability when compared with the cryomilled material at 40°C/75% relative humidity. (13) C solid-state nuclear magnetic resonance investigation of the BM material revealed an acid-base reaction between Sulindac and Neusilin. Optimal milling conditions and the kinetics of salt formation were also established. As benchtop milling is a laboratory-scale process, a scalable process was developed to make Sulindac-Neusilin amorphous drug complex using hot-melt extrusion (HME). The dissolution properties of the resulting HME material was found to have been improved over the material made by benchtop milling while maintaining similar physical stability. The HME material was used to make tablets using a direct compression method. The HME tablets were found to have better dissolution properties than tablets made from crystalline Sulindac. For the broad class of acidic drugs containing the carboxyl moiety, inorganic silicates such as Neusilin would offer a better choice than organic polymers to stabilize the amorphous phase.

Fast surface crystallization of amorphous griseofulvin below T g

PURPOSE

To study crystal growth rates of amorphous griseofulvin (GSF) below its glass transition temperature (T (g)) and the effect of surface crystallization on the overall crystallization kinetics of amorphous GSF.

METHODS

Amorphous GSF was generated by melt quenching. Surface and bulk crystal growth rates were determined using polarized light microscope. X-ray powder diffraction (XRPD) and Raman microscopy were used to identify the polymorph of the crystals. Crystallization kinetics of amorphous GSF powder stored at 40 degrees C (T (g)-48 degrees C) and room temperature (T (g)-66 degrees C) was monitored using XRPD.

RESULTS

Crystal growth at the surface of amorphous GSF is 10- to 100-fold faster than that in the bulk. The surface crystal growth can be suppressed by an ultrathin gold coating. Below T (g), the crystallization of amorphous GSF powder was biphasic with a rapid initial crystallization stage dominated by the surface crystallization and a slow or suspended late stage controlled by the bulk crystallization.

CONCLUSIONS

GSF exhibits the fastest surface crystallization kinetics among the known amorphous pharmaceutical solids. Well below T (g), surface crystallization dominated the overall crystallization kinetics of amorphous GSF powder. Thus, surface crystallization should be distinguished from bulk crystallization in studying, modeling and controlling the crystallization of amorphous solids.

Deliquescence-Induced Caking in Binary Powder Blends

In this study, moisture-induced caking of deliquescent crystalline powder blends was investigated. Physical mixtures of sugars and citric acid anhydrous showed significant cake formation when cycled above and below the mixture critical relative humidity. It was found that combinations of glucose and citric acid underwent efflorescence to form crystalline solid bridges while fructose and citric acid cakes contained amorphous material. In conclusion, the reduced deliquescence point in deliquescent solid mixtures was found to cause caking.