insoluble in water but very soluble in alcohol and chloroform (Reynolds and Prasad, 1982).
It is very common nowadays to deal with poorly soluble drugs in drug design and formulation since approximately 40% of drugs currently on the market are estimated to exhibit poor solubility behavior in water hence, poor bioavailability (Williams et al., 2013). Generally, formulating poorly soluble drugs into desired therapeutic dosage forms has always been a challenge for many years. Consequently, various strategies have been extensively studied in the past and recent years. Currently, a strategy based on combining two or more solid compounds with active pharmaceutical ingredient (API) by non-covalent interactions via the cocrystallization process has attracted the interest of many researchers. Thus, the strategy has been employed for the production of multicomponent crystalline solids including among others, cocrystals, salts, and eutectic systems (Cherukuvada and Guru Row, 2014).
In pharmaceutics, multicomponent crystalline solids have found many applications in drug delivery given their addictive effect, enhanced physicochemical properties, and bioavailability. Usually, pharmaceutical formulations infer applying multicomponent systems based on API-API and API-excipient mixtures. Lidocaine for instance has been reported to have formed eutectic mixtures with other APIs and excipients including lipids by which the crystallinity of LID was altered. Monocarboxylic fatty acids including hexanoic acid, capric acid, myristic acid, stearic acid (Bica et al., 2011) and lauric acid (Lazerges et al., 2015) are among the lipids reported to have formed eutectic systems with LID.
Lipids are classified as amphipathic organic molecules since they are made of two parts with different properties: a hydrophobic tail and a hydrophilic head. Lipids are a broad family and encompass many organic compounds including fatty acids, fatty alcohols, and fatty esters. Because of their variability, lipids have found many applications in various fields. They are often used as emulsifiers, solubilizers, z stabilizers, lubricators, and permeation enhancers in food, cosmetics, and pharmaceutics (Rosiaux et al., 2015). Their wide application in drug delivery is very much also related to its low cost, good safety profile, and for being in the list of excipients approved by the FDA.
In pharmaceutics, most lipid-containing multicomponent systems involve fatty acid species (Inoue, Hisatsugu, Suzuki, et al., 2004). To obtain a good understanding of the physical properties of lipids and their related binary mixtures with LID, it was decided to take this study involving other lipid species including dicarboxylic FAs and fatty alcohols, since it would give useful information regarding the influence of alkyl chain length and polar head moieties in physical properties and phase behavior of different lipids and their mixtures.
Normally, should two solid compounds (where one is at least an API), form a eutectic mixture, triggers changes in the physicochemical properties of API including, solubility, melting point, and bioavailability. It is well known that these changes are influenced by the molar ratio of components and the type of interactions involved (Agafonova, Moshchenskii, and Tkachenko, 2013; Stoler and Warner, 2015). The molecular structure does not easily identify the intricate noncovalent interactions that govern many areas of biology and chemistry, including the design of new materials and drugs. We develop an approach to detect noncovalent interactions in real space, based on the electron density and its derivatives. (Johnson, 2010)
With the application of crystal engineering, various researchers have dedicated their time and resources to generate multi-component crystals with improved physical properties such as solubility and chemical stability (Umeda et al., 2009).
- Crystalline Multicomponent
A multicomponent system consists of two or more constituents associated with intermolecular interactions where a component is an atom, ion, or molecule. In Pharmaceutics, multicomponent crystals systems have been known for more than 50 years and their adoption in favor of single component crystals offers better drug performance with improved physicochemical properties including crystallinity, solubility, hygroscopicity, particle size, and stability (Childs, Stahly, and Park, 2007; Umeda et al., 2009; Cruz-Cabeza, 2012).
Multicomponent solid forms incorporate all noncovalent derivatives classified in different classes including salts, cocrystals, hydrates, polymorphs, and eutectics. They are therefore viewed as unique chemical entities with new order structures different from their parent compounds (Foxman et al., 1998). Because they are closely related, often they are misdescribed solid forms, hence, resulting in an ambiguous classification. Thermal analysis and spectroscopic analysis have found common applications for the characterization of these multicomponent systems and they are used as combine techniques for proper interpretation of complex phenomena (Giron et al., 1997). Below is how some multicomponent of relevance is defined:
- Eutectic – is a mixture of two or more crystal compounds that at a given composition ratio melts and freezes at the same temperature, which is lower than each component.
- Cocrystal – is a crystalline complex of two or more neutral molecular constituents bound together in the crystal lattice through noncovalent interactions, often including hydrogen bonding (Jones, Motherwell, and Trask, 2006).
- Salt – is an ionic complex formed by an acid-base reaction involving a proton transfer or a neutralization reaction (Berge, Bighley and Monkhouse, 1977).
- Solid solution – is a monophasic system in which two compounds crystallize on a single lattice, wherein a compound is incorporated substitutionally in the lattice of another component and sustained by cohesive interactions (Cherukuvada and Nangia, 2013; Maximo et al., 2014; Cherukuvada, Kaur and Guru Row, 2016).
- Ion liquids – are generally salts with melting points below 100 oC (many liquids at ambient temperature), and whose melts are composed of discrete cations and anions (Hough et al., 2007).
- Salts vs Cocrystals
Salts and cocrystals, for example, are multicomponent crystals that are generally formed by a mixture of an acid and a base. However, salts are considered ionic compounds and are distinguished from cocrystals for the latter being neutral (Umeda et al., 2009). Moreover, predicting salts and cocrystals, based on our values is widely accepted. But, although they serve as good indicators to predict salts when ∆pKa > 3, or cocrystals when ∆pKa < 0, there is a little confidence of crystal derivative predictability when ∆pKa is between the range 0 to 3 (Childs, Stahly and Park, 2007). The salt crystal lattice is distinct from that of parent compounds and differs from other salts of the same compound. Using thermal analysis and other means of characterization can be essential to identify salts from other solid forms.
- Cocrystals vs Eutectics
Cocrystals and eutectics are intimately related crystal multicomponent adducts but differ on binary compositions properties and phase diagrams. Thus, DSC analysis data for eutectic binary mixtures exhibit a “V” shaped phase diagram with a single eutectic point while, cocrystals display the “W” shaped phase diagram, presenting two eutectic points and a cocrystal region between the two eutectic points. On PXRD spectroscopy, eutectic system PXRD patterns are a sum of parent PXRD patterns; this because of the involvement of weaker adhesive interactions created between the heteromorphous materials giving the eutectics a similar lattice to the parent components; in contrast, cocrystal exhibits new or distinct peaks from the parent components result from strong adhesive interactions between isomorphous materials resulting from crystal lattice different from parent components (Stoler and Warner, 2015; Cherukuvada, 2016). The use of these microscopic techniques, namely hot stage microscope has been essential for differentiating cocrystal form eutectic in addition to other techniques.
It happens in given situations that the multi-component solid forms mentioned above fail to form even when using components with complementary functional groups. This is the case where components coexist unreacted as physical mixtures. The DSC and PXRD data can distinguish them from other multi-component forms since the latter would exhibit melting points and spectra corresponding to the corresponding pure components respectively (Widjaja et al., 2008).
This study proposes the formation of multicomponent adducts between LID and i) monocarboxylic (hexanoic acid, capric acid, lauric acid, and myristic acid), ii) dicarboxylic (adipic acid, sebacic acid, dodecanedioic acid and tetradecanedioic acid) saturated fatty acids, and iii) fatty alcohols (3-methyl-3-pentanol, 1-decanol, 1-dodecanol, and 1-tetradecanol) (see table 1).
Bica and colleagues (2011) proposed the formation of multicomponent adducts between LID and some aliphatic (saturated and unsaturated) fatty acids with different chain lengths with assorted phase diagrams. Later, a study by Lazerges and Co (2015), proposed a simple eutectic phase diagram for LID and lauric acid binary mixture, which correlated to some systems reported by Bica and group. Driven by these studies, we propose to investigate the impact of saturated chain lengths on the formation of LID-lipid adducts and establish a correlation of these systems with the type of phase diagrams.
- Aims and Objectives
The main aim of this study is to investigate possible eutectic mixtures between lidocaine and lipids and to obtain a good understanding of resulting multicomponent crystal structures by a comprehensive solid-state characterization. The main objectives of this study are:
- To determine the crystal lattice of novel organic multicomponent systems formed by the binary combination of lidocaine and a series of lipids
- To construct eutectic phase diagrams of the binary systems and correlate them with alkyl chain length and/or polar head moiety effect
- To assess the usefulness of the thermal microscopic technique in the characterization of eutectic systems
Table 1. Molecular structures and other indicators of all compounds used in the binary mixtures
|Compound name||Structure||Molar mass (g/mol)||pKa|
|Lidocaine C14H22N2O (a,b)||234.34 kg/mol||7.86||2.44|
|Hexanoic acid C6H12O2 (a)||
|Adipic acid C6H10O4 (a)||
||146.142 kg/mol||4.44 & 5.44||0.49|
|3-Methyl-3-pentanol C6H14O (a)||
|Capric acid C10H20O2 (a)||
|Sebacic acid C10H18O4 (a)||
||202.25 kg/mol||4.72 & 5.45||2.27|
|1-Decanol C10H22O (a)||
|Lauric acid C12H24O2 (a)||
|Dodecanedioic acid C12H22O4 (a)||
||230.304 kg/mol||4.45 & 5.05||3.16|
|1-Dodecanol C12H26O (a)||
|Myristic acid C14H28O2 (a)||
|Tetradecanedioic acid C14H26O4 (a)||
||258.354 kg/mol||4.75 & 5.24||4.05|
|1-Tetradecanol C14H30O (a)||
References (a) (PubChem, 2020)
(b) (Benet, Broccatelli and Oprea, 2011)
- Materials and Apparatus
Lidocaine also is known as 2-(diethylamino) -N-(2,6-dimethyl phenyl)-acetamide, was purchased from Swati Spentose Private Ltd (Mumbai, India);
Whereas, lipids, – capric acid (CA), adipic acid (AA), hexanoic acid (Hex), sebacic acid (SeB), lauric acid (LA), 1-decanol (1DC), 1-tetradecanol (1TDC), 1-dodecanol (1DOD), tetradecanedioic acid (TTDD), 3-methyl-3-pentanol (3MetPen), dodecanedioic acid (DDDA) and myristic acid (MA) were all purchased from Sigma-Aldrich (Dorset, UK).
All chemicals were used as received without further purification.
- Differential Scanning Calorimetry (DSC) Q100, TA Instruments (New Castle, USA).
- ATR/FT-IR spectrometer (Spectrum TWO, PerkinElmer, Inc., Waltham, Massachusetts, USA).
- Olympus polarised light microscope, KeyMed Ltd (Essex, UK).
- Rigaku Miniflex II desktop X-Ray diffractometer (Kent, UK).
- Thermal Gravimetric Analysis (TGA) Q500, TA Instruments (New Castle, USA).
it is essential, at the early stages of drug design and formulation, to ascertain the utmost temperature material can endure before it undergoes chemical or physical decomposition process. Figure 1 and Table 3 present the thermogravimetric thermograms and 5% weight loss of raw materials used.
2.4.1. Differential Scanning Calorimetry (DSC)
This calorimetric technique measures heat flow as a function of temperature. It provides material thermal behaviors such as crystallization, melting, and glass transitions (Agroui, Collins, and Farenc, 2012). All analyses were carried out under a stream of liquid nitrogen as a purge gas at a rate of 50 ml min-1 by using Q100 (TA Instrument). The equipment calibration process was followed according to the manufacturer’s protocols by using Indium and Zinc standard values of melting point and heat of fusion as references. Samples of pure compounds were accurately weighed (3 – 8 mg) directly into aluminum DSC pans using an analytical balance. Whereas molar compositions of binary mixtures were physically mixed using mortar and pestle and the mixture was subsequently weighed as above before being carefully transferred into aluminum DSC pans. All loaded DSC pans (Perkin-Elmer Corporation) were crimped with their correspondent aluminum pan lids using a Perkin-Elmer press. All samples were subjected to heat-cool-heat cycles at a constant rate of 10 o C min-1 (Teixeira et al., 2010). The scanning temperature ranges were adjustable according to firstly, to the melting points of the components forming a binary mixture with lidocaine (see table 2) and secondly, by the outcomes of the preliminary screening experiments carried out.
Table 2. Summary of temperature range used on DSC heat-cool-heat cycles for each system
|1st heating cycle
- Powder X-ray Diffraction (PXRD)
Crystalline patterns on pure solid samples and of selected solid mixtures were analyzed at room temperature using a MiniFlex II desktop powder X-ray diffractometer (Rigaku Corporation, Kent, England) equipped with Cu radiation, at a voltage of 30 kV and a current of 15 mA. A powder x-ray diffraction process consists of an x-ray source (usually an x-ray tube), a sampling stage, a detector, and a means to vary the angle θ. The X-rays are focused on the sample at a certain angle θ, while the detector opposite the source reads the intensity of the X-rays it receives 2θ from the path of the source. Powder samples were loaded into the X-ray machine via sample holder and scanned within the angular range of 3−60° 2 θ in continuous mode with a sampling width of 0.02° and a scan speed of 2.0°/min.
- Thermal Gravimetric Analysis (TGA)
The TGA studies are fundamental at pre-formulation stages since they help to establish the temperature limit of which pure compounds and mixtures can withstand before the chemical decomposition occurs. The decomposition temperature for each raw compound and the respective mixtures were determined using a Thermal Advantage model Q500 TGA (TA Instruments, Leatherhead, U.K.) and the results were collected with Universal Analysis 2000 (TA instruments, Leatherhead, UK). The test was run in ramp mode and the temperature under investigation ranged from 0 oC to 300 oC. Approximately 15 mg of sample was put into a tared open aluminum pan and heated at a rate of 10 o C min-1 and nitrogen was used as a purge gas at 60 ml min-1. The temperatures at which onset decomposition and 5% weight loss occurred were recorded.
- Fourier Transform Infrared (FTIR) Spectroscopy
Fourier-Transform Infrared spectroscopy was used to understand possible intermolecular interactions between lidocaine and all lipids. IR spectra of tested samples were obtained using an IR spectrometer (Spectrum TWO, PerkinElmer, Inc., Waltham, Massachusetts, USA) equipped with an in-built designed ATR device with zinc selenide crystal. Each spectrum was obtained from an average of 10 scans in the range of 4000-400 cm-1 with a resolution of 4 cm-1 after placing the sample (solid or liquid) in contact with the zinc selenide crystal. The data was analyzed using the KnowItAll (R) Informatic System, Wiley Software Solution Version 2018.
- Hot-Stage Microscopy (HSM)
Thermal microscopic analysis was carried out with Linkham polarised light microscope (PLM). With LINKAM hot stage provided, the heating or cooling cycles (from -90 to 165 oC) were possible using the temperature controller LINKAM TMS and allowed visual observation of the events reported during DSC analysis(Umeda et al., 2007). The analysis was conducted according to the Koffler mixed fusion method (Berry et al., 2008; Lekšić, Pavlović, and Meštrović, 2012). Generally, upon recrystallization of both components, a mixing zone comparable to DSC binary phase diagram is formed. In the zone, one pure compound will be at one side of the microscope slide and the other at another side with a concentration gradient across the zone where the number of phases present in the system is viewed (Berry et al., 2008). The LINKAM Dewar filled with liquid nitrogen was applied for cooling cycles below the ambient temperatures. The Dewar was attached to control unit pumps to allow cooling the system temperature to as lower as -90 oC. The physical and morphological changes of samples were observed with Pixelink Megapixel Firewire camera and Pixelink software (Scorpion Vision Ltd., Lymington, U.K.) at a reduced heating rate of 1 oC min-1 at temperatures close to the transitions.
The eutectic systems are discontinuous solid solutions Whereas, continuous solid solutions occur when the molecules of the drug are dispersed (be interstitial or substitutional) among the molecules of excipients in the solid-state (Grant and Abougela, 1982).
- Results and Discussion
2.5.1. Thermal Analysis
126.96.36.199. Thermogravimetric Analysis (TGA)
The TGA technique was used to determine the percentage mass change of materials as a function of temperature increase.
Figure 1. Thermogravimetric analysis presenting the thermal endurance of the raw material.
The thermogravimetric thermograms revealed that the lipids in liquid state form namely 3MetPen, Hex, and 1DC were the least thermostable pure compounds (Table 3). The 5% weight loss for 3MetPen was rapid and occurred at 42.3 oC, followed by Hex and 1DC with onset 5% weight loss temperatures at 100.7 and 104.6 oC respectively. Whereas, the dicarboxylic FAs exhibited high thermal stability. Overall, 3MetPen was the only pure compound that exhibited a 5% weight loss within the temperature range used for DSC analysis.