Cocrystal Systems of Pharmaceutical Interest: 2012-2014

Developments in the field of pharmaceutical cocrystals have been documented in the form of chronological review articles, and the present review encompasses articles published during 2012, 2013, and 2014. Appropriate publications were drawn from the major physical, crystallographic, and pharmaceutical journals, being sorted according to the identity of the drug substance involved. This information is contained in tabular form, where the primary sorting has been by compound name and year of publication. For each entry, the table also contains the bibliographic citation to the paper, a complete list of the authors, and the title of the paper.

Mid-Infrared Spectroscopy of Pharmaceutical Solids

It is now well established that infrared absorption spectroscopy is a powerful technique for the physical characterization of pharmaceutical solids. Besides being a preferred methodology for identification purposes, one can use trends in the energy values in the spectra as a means to study the solid-state properties of the system. FTIR spectra are often used to evaluate the type of polymorphism existing in a drug substance, can be very useful in studies of the water contained within a hydrate species, and are emerging as a technique of choice for the study of cocrystal systems. In this review, an overview of the theoretical foundations for infrared spectroscopy will be presented, which will be supported by illustrations as to how the methodology can be used.

Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy as a Forensic Method to Determine the Composition of Inks Used to Print the United States One-cent Blue Benjamin Franklin Postage Stamps of the 19th Century

Through the combined use of infrared (IR) absorption spectroscopy and attenuated total reflectance (ATR) sampling, the composition of inks used to print the many different types of one-cent Benjamin Franklin stamps of the 19th century has been established. This information permits a historical evaluation of the formulations used at various times, and also facilitates the differentiation of the various stamps from each other. In two instances, the ink composition permits the unambiguous identification of stamps whose appearance is identical, and which (until now) have only been differentiated through estimates of the degree of hardness or softness of the stamp paper, or through the presence or absence of a watermark in the paper. In these instances, the use of ATR Fourier transform infrared spectroscopy (FT-IR) spectroscopy effectively renders irrelevant two 100-year-old practices of stamp identification. Furthermore, since the use of ATR sampling makes it possible to obtain the spectrum of a stamp still attached to its cover, it is no longer necessary to identify these blue Franklin stamps using their cancellation dates.

Butyl methoxy dibenzoylmethane

A comprehensive profile on Butyl methoxy dibenzoylmethane, one of the most commonly used ultraviolet (UV) filters in topical sunscreen products, is prepared. This UV filter, often referred to as Avobenzone, has its main absorbance in the UVA I region of the spectrum and is susceptible to photodegradation. The profile contains the following sections: general information, use and mechanism of action, method of preparation, physical characteristics, methods of analysis, stability, and toxicity. The physical characteristics section includes the melting range, differential scanning calorimetry, partition coefficient, ionization constant, solubility, and UV, infrared, nuclear magnetic resonance ((1)H NMR and (13)C NMR) and mass spectrometry and X-ray powder diffractometry. The method of analysis section in addition to compendial identification and purity and assay methods includes thin-layer gas and high-performance liquid chromatography. The photostability and photostabilization of Butyl methoxy dibenzoylmethane, in addition to its toxicity, are also documented.

Pharmaceutical cocrystals: the coming wave of new drug substances

Solid crystalline phases containing two cocrystallized components offer a new development pathway whereby one can potentially improve the physical characteristics (i.e., equilibrium solubility, dissolution rate, solid-state stability, etc.) of a drug substance that exhibits a profile that is less than desirable. In this commentary, the topic of pharmaceutical cocrystals will be briefly explored, and a short exposition of the solubility and dissolution rate advantages that have been realized in various systems will be provided. The Guidance for Industry document recently proposed by United States Food and Drug Administration will be outlined, and its requirements explained. Finally, the subset of pharmaceutical cocrystals that consist of a drug substance and a salt of that substance (termed a salt cocrystal) will be examined to illustrate this additional class of pharmaceutical cocrystals that may offer significant scientific and regulatory advantages.

Fluorescence studies of the dehydration of cefadroxil monohydrate

Although cefadroxil does not exhibit the phenomenon of photoluminescence when dissolved in a fluid medium, the compound has been found to exhibit fluorescence in its solid-state monohydrate crystal form. The monohydrate was found to exhibit complicated photoluminescence, where two different sets of emission spectra could be obtained upon irradiation with an appropriate excitation wavelength. One of these photophysical systems became strongly suppressed when the monohydrate was half-dehydrated, and only one of the photophysical systems could be observed in this hemihydrate. In the fully dehydrated state, both photophysical pathways became almost totally suppressed, so that the nonsolvated cefadroxil became effectively nonfluorescent.

Polymorphism and solvatomorphism 2005

Papers and patents that deal with polymorphism (crystal systems for which a substance can exist in structures characterized by different unit cells, but where each of the forms consists of exactly the same elemental composition) and solvatomorphism (systems where the crystal structures of the substance are defined by different unit cells, but where these unit cells differ in their elemental composition through the inclusion of one or molecules of solvent) have been summarized in an annual review. The works cited in this review were published during 2005, and were drawn primarily from the major physical, crystallographic, and pharmaceutical journals. The review is divided into sections that cover articles of general interest, computational and theoretical studies, preparative and isolation methods, structural characterization and properties of polymorphic and solvatomorphic systems, studies of phase transformations, effects associated with secondary processing, and United States patents issued during 2005.

Solid-state fluorescence of the trihydrate phases of ampicillin and amoxicillin

The purpose of this work was to study the effects of crystal structure on the solid-state photoluminescence of the trihydrate phases of ampicillin and amoxicillin, and to contrast these spectra with analogous spectra obtained on the molecules dissolved in a solution phase. The polymorphic identity of the analytes was established using x-ray powder diffraction and Fourier transform infrared absorption spectroscopy, and the solid-state luminescence spectra obtained under ambient conditions. It was found that the solid-state excitation and emission spectra of ampicillin trihydrate and amoxicillin trihydrate were dominated by energy transfer and exciton effects, which were manifested as decreases in the energy of the excitation and emission bands of the solid-state systems relative to those of the free molecule in solution. The photoluminescence data revealed that in spite of the known structural similarity of ampicillin trihydrate and amoxicillin trihydrate, the magnitude of the Davydov splitting, and the degree of band energy shifting differed between the 2 systems. This finding indicates that the small differences in crystal structure existing between the 2 compounds leads to measurable differences in the patterns of energy transfer.

Solid-state fluorescence studies of some polymorphs of diflunisal*

PURPOSE

The solid-state luminescence spectroscopy of organic molecules is strongly affected by the effects of excited state energy transfer, with the fluorescence of solids often differing significantly from the fluorescence of the molecule dissolved in a solution phase. Because the magnitude of these solid-state effects is determined by the crystallography of the system, solid-state fluorescence studies can be used to gain insight into the polymorphism of the system. To this end, the spectroscopic properties of four polymorphs of diflunisal have been obtained, and compared to the properties of the molecule in the solution phase.

METHODS

Fluorescence excitation and emission spectra were obtained on four polymorphic forms of diflunisal, and on the compound dissolved in water.

RESULTS

It was found that exciton effects dominate the excitation spectra of diflunisal in the four studied polymorphic forms. These phenomena lead to a decrease in the energy of the excitation bands relative to that observed for the free molecule in fluid solution, and in a splitting of the excitation peak into two Davydov components.

CONCLUSIONS

The trends in the excitation and emission spectra led to the grouping of diflunisal Forms I, II, and III into one category, and diflunisal Form IV into a separate category. Because other work has established that Form IV is characterized by the highest crystal density and consequent degree of intermolecular interaction, the magnitude of the exciton coupling can be used to estimate the degree of face-to-face overlap of the salicylate-type fluorophores.

Foundations of chemical microscopy. 3. derivatives of some chiral phenylalkylamines and phenylalkylamino acids with 5-nitrobarbituric acid

At one time, 5-nitrobarbituric acid (also known as dilituric acid) was extensively used as a chemical reagent for the qualitative identification of a variety of basic substances by light microscopy. This methodology was based on evaluation of observed crystal morphologies, because skilled observers could associate a unique crystal habit with the products formed with analytes when those were crystallized using standardized methods. As part of a study to understand the scientific foundations that permitted chemical microscopy to function as a useful analytical technique during its heyday, the products formed by dilituric acid with resolved and racemic phenylalkylamines and phenylalkylamino acids were characterized using a variety of physical analytical techniques. It was found that the different crystal morphologies associated with each of the crystalline adducts were derived from the ability of the systems to form differing structural types and/or hydrate crystal forms upon crystallization.

Fluorescence studies of the transformation of carbamazepine anhydrate form III to its dihydrate phase

It has been found that both the anhydrous Form III and dihydrate phases of carbamazepine exhibit fluorescence in the solid state. The fluorescence intensity associated with the dihydrate phase was determined to be significantly more intense than that associated with the anhydrate phase, and this difference was exploited to develop a method for study of the kinetics of the aqueous solution-mediated phase transformation between these forms. Studies were conducted at temperatures over the range 18-40 degrees C, and it was found that the phase transformation was adequately characterized by first-order reaction kinetics. The temperature dependence in the calculated rate constants was used to calculate an activation energy of 11.2 kcal/mol (47.4 cal/g) for the anhydrate-to-dihydrate phase conversion.

Effects of mechanical processing on phase composition

One factor that must be considered during drug development process is that various types of pharmaceutical manufacturing can alter the physical characteristics of the drug entity. These effects become particularly important during scale-up of processing operations, because new and unanticipated results can become manifest in systems of insufficient characterization. Any transformed drug substance or altered dosage form could exhibit an altered solubility or dissolution rate that might produce an undesirable bioavailability profile. Some of the more interesting mechanical manipulations that have the potential to yield problems include particle size reduction and compression, and such investigations are the focus of this minireview.