David J.W. Grant

Crystal structures of drugs: advances in determination, prediction and engineering

Most marketed pharmaceuticals consist of molecular crystals. The arrangement of the molecules in a crystal determines its physical properties and, in certain cases, its chemical properties, and so greatly influences the processing and formulation of solid pharmaceuticals, as well as key drug properties such as dissolution rate and stability. A thorough understanding of the relationships between physical structures and the properties of pharmaceutical solids is therefore important in selecting the most suitable form of an active pharmaceutical ingredient for development into a drug product. In this article, we review the different crystal forms of pharmaceuticals, the challenges that they present and recent advances in crystal structure determination. We then discuss computational approaches for predicting crystal properties. Finally, we review the analysis of crystal structures in furthering crystal engineering to design novel pharmaceutical compounds with desired physical and mechanical properties.

Influence of crystal structure on the tableting properties of sulfamerazine polymorphs

AbstractPurpose. To understand the influence of polymorphic structure on the tableting properties of sulfamerazine. Methods. Bulk powders of sulfamerazine polymorph I and of two batches, II(A) and II(B) of different particle size, of polymorph II were crystallized. The powders were compressed to form tablets whose porosity and tensile strength were measured. The relationships between tensile strength, porosity and compaction pressure were analyzed by the method developed by Joiris, E., et al. Pharm. Res.15:1122-1130 (1998). Results. The sensitivity of tensile strength to compaction pressure, known as the tabletability, follows the order, I >> II(A) > II(B) and the porosity at the same compaction pressure, which measures the compressibility, follows the order, I << II(A) < II(B). Therefore, the superior tabletability of I over II(A) or II(B) is attributed to its greater compressibility. Molecular simulation reveals slip planes in crystals of I but not in II. Slip planes provide I crystals greater plasticity and therefore greater compressibility and tabletability. Larger crystal size of II(B) than of II(A) leads to fewer contact points between crystals in the tablets and results in a slightly lower tabletability. Conclusions. Slip planes confer greater plasticity to crystals of I than II and therefore greater tabletability.

Solid-state characterization of nifedipine solid dispersions

The purpose of this study is to characterize the nature and solid-state properties of a solid dispersion system of nifedipine (33.3% w/w) in a polymer matrix consisting of Pluronic F68 (33.3% w/w) and Gelucire 50/13 (33.3% w/w). The nature of nifedipine dispersed in the matrix was studied by powder X-ray diffractometry (PXRD), differential scanning calorimetry (DSC) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The rate and extent of water uptake of the solid dispersion were determined by weight gain. The dissolution rate of nifedipine solid dispersion was determined using Apparatus 2 of USP XXIII (1995). Quantitative PXRD showed that the saturation solubility of nifedipine in the polymer matrix is 2.1-3.0% w/w and indicated an excess of crystalline nifedipine in the solid dispersion. The maximum water uptake by the solid dispersion exposed to 75% RH at 45 degrees C was 3.3 times higher than for the dispersion exposed to 65% RH at 25 degrees C. Over 8 weeks, PXRD and DRIFTS of the nifedipine matrix stored at 25 or 4 degrees C were unchanged, showing constancy of crystallinity and intermolecular interactions. For a given mass of nifedipine (20 mg) and for a given particle size of nifedipine (<850 microm), the initial release rate of nifedipine from the solid dispersion was faster (46.2% of the nifedipine dissolved in 20 min) than that of the pure drug (1.2% of the nifedipine dissolved in 20 min). The results indicate that the nifedipine solid dispersion is physically stable over 8 weeks. Nifedipine is released faster from the solid dispersion than from the pure crystalline drug of the same particle size.

Influence of water activity in organic solvent + water mixtures on the nature of the crystallizing drug phase. 1. Theophylline

The hydration state of a hydrate depends on the water activity, aw in the crystallization medium. Selection of an appropriate ratio of water to cosolvent in the crystallization medium of a hydrate is critical and is often semi-empirical. This study attempts to elucidate this selection process by studying the conditions of physical stability of the solid phases of theophylline, which comprise an anhydrate and a monohydrate. A mixture of the anhydrate and the monohydrate may sometimes be obtained, if the system is not in equilibrium. The excess solid phase was characterized by powder X-ray diffractometry and the water content was measured by Karl-Fischer titrimetry. In contact with methanol + water or 2-propanol (isopropyl alcohol, IPA)+ water mixtures, at aw 0.25 in either solvent mixture, the monohydrate was obtained as the most stable form at equilibrium. These results suggest (a) that water activity is the major factor determining the nature of the solid phase of theophylline which crystallizes from methanol + water or IPA + water mixtures and (b) that the system, theophylline anhydrate ⇌ theophylline monohydrate, is in equilibrium at aw = 0.25 and at 25°C. The solubilities of the two solid forms in each of the mixed solvent systems were also measured and are discussed. The concepts presented, tested and discussed may, in principle, be applied to any pharmaceutical system consisting of an anhydrate and a hydrate, or a lower hydrate and a higher hydrate.

Identifying the Stable Polymorph Early in the Drug Discovery–Development Process

The thermodynamically most stable polymorph under ambient conditions is almost without exception the most desirable crystalline form for development by a pharmaceutical company. It is, therefore, beneficial to discover and to characterize this polymorph at the earliest possible stage of development. A screen for discovering the stable polymorph of a pharmaceutical compound early in the drug discovery–development process is developed and described. In this screen, a small amount of compound is suspended in a diverse group of solvents for two weeks in an effort to crystallize the most stable polymorph. The solubility of the compound in each solvent utilized in the stable polymorph screen is also simultaneously determined using a simple gravimetric method. Ritonavir and an early development candidate (Pfizer compound A) are used as model compounds to demonstrate the utility of the screen for finding the stable polymorph early in the drug discovery–development process.

In Situ Dehydration of Carbamazepine Dihydrate: A Novel Technique to Prepare Amorphous Anhydrous Carbamazepine

The purposes of this project were to prepare amorphous carbamazepine by dehydration of crystalline carbamazepine dihydrate, and to study the kinetics of crystallization of the prepared amorphous phase. Amorphous carbamazepine was formed and characterized in situ in the sample chamber of a differential scanning calorimeter (DSC), a thermogravimetric analyzer (TGA), and a variable temperature x-ray powder diffractometer (VTXRD). It has a glass transition temperature of 56°C and it is a relatively strong glass with a strength parameter of 37. The kinetics of its crystallization were followed by isothermal XRD, under a controlled water vapor pressure of 23 Torr. The crystallization kinetics are best described by the three-dimensional nuclear growth model with rate constants of 0.014, 0.021, and 0.032 min1 at 45, 50, and 55°C, respectively. When the Arrhenius equation was used, the activation energy of crystallization was calculated to be 74 kJ/mol in the presence of water vapor (23 Torr). On the basis of the Kissinger plot, the activation energy of crystallization in the absence of water vapor (0 Torr water vapor pressure) was determined to be 157 kJ/mol. Dehydration of the dihydrate is a novel method to prepare amorphous carbamazepine; in comparison with other methods, it is a relatively gentle and effective technique.

Improved tableting properties of p-hydroxybenzoic acid by water of crystallization: a molecular insight.

AbstractPurpose. To understand the influence of water in the crystal structure on the compaction properties of otherwise structurally similar crystals, p-hydroxybenzoic acid anhydrate (HA) and the monohydrate (HM) were used as model compounds. Methods. Bulk powder of HM was prepared by exposing HA powder to 97% relative humidity at 23°C. Each powder, HA or HM, was uniaxially compressed and triaxially decompressed under various pressures to form square-faced tablets. The tensile strength and porosity of the tablets were measured. Results. Incorporation of water into the crystal lattice results in greater tablet strength and larger reduction in volume for HM crystals than for HA crystals. Both HA and HM crystals contain hydrogen-bonded, zigzag-shaped layers that lie parallel to the (401) plane. When HA crystals are compressed, the zigzag-shaped layers mechanically interlock, inhibiting slip and reducing plasticity. However, water molecules in the HM crystals assume a space-filling role, which increases the separation of the layers. This effect allows easier slip between layers and provides greater plasticity of HM crystals, which increases the interparticulate bonding area under the same compaction pressure. However, the water molecules in the HM crystals increase their lattice energy by forming a three-dimensional hydrogen-bonding network. The greater bonding strength that results is reflected in greater tensile strength of HM compacts at zero porosity. Conclusions. The presence of water molecules in the crystal structure of p-hydroxybenzoic acid facilitates plastic deformation of HM crystals, thereby enhancing their bonding strength and giving much stronger tablets than of HA crystals.

Influence of Elastic Deformation of Particles on Heckel Analysis

The Heckel equation is one of the most useful equations for describing the compaction properties of pharmaceutical powders. Important material properties (e.g., yield strength) of powders can be derived using Heckel analysis. Two types of Heckel analysis are in common use. One is the “out-of-die,” or “zero-pressure” method, the other is the “in-die” or “at-pressure” method. Because particles undergo elastic deformation under pressure, which tends to lower the porosity of the powder bed, the “out-of-die” method describes powder consolidation and compaction more accurately than the “in-die” method. However, “in-die” Heckel analysis has been widely used because of the speed and ease of data collection. Using L-lysine monohydrochloride dihydrate as a model compound, this work analyzes quantitatively the effects of elastic deformation on the calculation of porosity of a tablet, and therefore on the Heckel analysis. The effects of a small change in porosity, ε, on Heckel analysis are presented mathematically. It is found that a decrease in porosity of 0.001, when the porosity is lower than 0.05, causes a significant increase in the value of −ln ε. Therefore, data at ε < 0.05 should be interpreted with caution when using Heckel analysis. Elastic deformation causes positive deviations in the Heckel plot, and therefore leads to a yield strength that is lower than the true value. The lower the elastic modulus of the powder, the greater is the deviation from the true value. Therefore, the “in-die” method gives values of yield strength that are significantly lower than the true values for most pharmaceutical powders.

Influence of Crystal Shape on the Tableting Performance of L-Lysine Monohydrochloride Dihydrate

The purpose of this study is to understand the influence of crystal shape on the tableting performance of L-lysine monohydrochloride (LMH) dihydrate, using the method of data analysis developed by Joiris E et al. 1998. Pharm Res 15:1122-1130. Phase-pure crystals of LMH dihydrate, prism-shaped (S) and plate-shaped (T), were prepared by adjusting the composition of the crystallization solvent. At the same compaction pressure, T always gives stronger tablets than S, (i.e.; the tabletability of T is greater). The porosity of tablets from T crystals is always greater than that of S crystals when compressed at the same pressure, (i.e.; the compressibility of T is lower). The tensile strength of T tablets, at the same porosity, is greater than that of S tablets, (i.e.; the compactibility of T is greater). Therefore, the greater tabletability of T is a result of its better compactibility that overcomes the negative effects by its lower compressibility. The greater compactibility of T is related to favorable orientation of the slip planes in the tablet, corresponding to greater plasticity under load. The yield strengths of T and S crystals are essentially the same (20 MPa). Therefore, the crystal shape influences the tableting performance but does not, in principle, affect the yield strength of LMH dihydrate.

Stabilization of a metastable polymorph of sulfamerazine by structurally related additives

Abstract The influence of structurally related additives, namely N4-acetylsulfamerazine (NSMZ), sulfadiazine (SD) or sulfamethazine (SM), on the rate of the solvent-mediated polymorphic transformation (I→II) of sulfamerazine in acetonitrile (ACN) at 24°C was studied. The transformation rate is controlled by the crystallization rate of the more stable Polymorph II. All three impurities exhibit inhibitory effects on the crystallization of Polymorph II and hence stabilize the metastable Polymorph I in ACN suspension. The rank order of the inhibitory effect (NSMZ⪢SD>SM) is the same as the rank order of the binding energy of the impurity molecule to the surface of the host crystal. The relationship between the concentration of the impurity and the inhibitory effect was fitted to various models and was found to be best described by a model based on the Langmuir adsorption isotherm.