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This study demonstrates incompatibility studies of acyclovir in its physical mixtures with lactose and also in 3 different tablet brands. Initially, differential scanning calorimetery (DSC) was used to asses the compatibility of drug and lactose. The FTIR spectrum was also compared with that of pure drug and excipient. Although DSC results indicated incompatibility with lactose, FTIR spectra were mostly unmodified due to peaks overlapping. Isothermally stressed physical mixture and pure drug were stored at 95 -C for 24 hours. Residual drug was monitored using a validated HPLC method and data fitting to solid state kinetic models was performed. Drug loss followed a diffusion model. The aqueous mixture of drug and excipient was heated in order to prepare the adduct mixture. HPLC analysis revealed one extra peak that was fractionated and subsequently injected into the LC-MS/MS system. MRM chromatograms showed a similar peak with molecular mass corresponding to an acyclovir-Lactose Maillard reaction product. The presence of lactose in commercial tablets was checked using a new TLC method. Overall, incompatibility of acyclovir with lactose was successfully evaluated using the combination of thermal methods and liquid chromatography - mass spectroscopy.
Study of drug-excipient compatibility is an important process in the development of a stable solid dosage form . A new chemical entity or drug substance becomes a drug product after formulation and processing with excipients . Incompatibility between drugs and excipients can alter the stability and bioavailability of drugs, thereby affecting its safety and/or efficacy. Despite the importance of this issue, there is no universally accepted protocol for drug-excipient compatibility testing [1, 2]. In recent years thermal analysis has been used to a great extent in the development and improvement of pharmaceutical formulations [3, 4]. Thermogravimetric analysis (TGA) and differential scanning calorimerty (DSC) are the most commonly used thermal techniques in drug-excipient compatibility assessments [1, 5, 6]. Isothermal stress testing (IST) is another method that involves storing the drug-excipient blends with or without moisture at high temperature and determining the drug content [2, 7, 8]. One of the IST methods adopted by Serajuddin et al.  involved the storage of formulated samples with 20% vol/wt added water at 50 -C for 1-3 weeks. Later Sims et al. modified their method to a more rapid one by changing the storage temperature and time to 100-C and 1-3 days respectively. DSC can be used in combination with IST to evaluate compatibility of drugs with the selected excipients [1, 9].
FTIR spectroscopy is another approach used in compatibility tests based on the hypothesis that some functional groups change during drug-excipient interaction [5, 10].
In the most detailed studies degradation products can also be identified by mass spectral, NMR, and other relevant analytical techniques [2, 11-13]. The identification of degradation products in dosage formulations plays an important role in the drug development processes. During the past decade, with the commercialization of mass spectrometers using soft ionization techniques such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), the coupling of high-performance liquid chromatography (HPLC) and mass spectrometry (MS) has become one of the most powerful techniques for pharmaceutical analysis. The separation by time provided by an HPLC system combined with a mass spectrometer enables a chemist to acquire the structural information of a specific impurity or degradate without the time-consuming isolation process. LC-MS analysis is very sensitive for the detection of low-level unknowns in complex mixtures such as formulations. This great advantage of an LC-MS system is largely based on the fact that soft ionization techniques usually provide molecular weight information for the analytes. In general, protonated, ammoniated and sometimes sodiated molecules are produced in the positive ion mode, while deprotonated molecules are generated in the negative ion mode. Furthermore, these pseudomolecular ions often produce structurally informative fragment ions via collision-induced dissociation (CID) processes. Fragments of fragment ions can also be collected using tandem mass spectroscopy .
The kinetics of the reaction in the solid-state is considerably more complicated than in the case of solution phase kinetics. First, a solid system is inherently non homogeneous, making the reaction dependent on the physical configuration of the system, not only on its composition at any given time. Second, molecules in the solid state have significantly more limited molecular mobility than molecules in solution . Solid-state kinetic studies have appeared in the pharmaceutical literature over many years and can be mechanistically classified as nucleation, geometrical contraction, diffusion and reaction order models [15, 16].
Lactose (MW = 342) is one of the most commonly used pharmaceutical excipients. A survey of the Physician's Desk Reference database shows that there are many pharmaceutical formulations where amino compounds and lactose are both present [12, 17]. Recently the possible reaction of amine groups of drug entities with carbonyl groups of common tablet excipients, such as lactose, starch, and cellulose has gained a pharmaceutical interest [11-13, 18-20]
An acyclic nucleoside, acyclovir (MW = 225.2) used in the treatment of varicella infections and prophylaxis of herpes simplex infections, is an amine containing drug, making it a good candidate for Maillard reaction with a reducing agent like lactose . Tu et al., increased the liver distribution of acyclovir using acyclovir-dextran conjugate which was synthesized by formation of Schiff's base . Later Desai et al., studied the stability of low concentrations of three guanine-based antivirals (entecavir, lobucavir and acyclovir) in sucrose or maltitol solutions, and concluded the formation of isomeric adducts of the drugs and reducing sugars .
All the previous investigations have been conducted in solutions and also the possibility of the acyclovir-lactose reaction has not been investigated yet.
In this report, we focus on the determination of early-stage Maillard reaction product/s (ESMRP) between the amine-containing antiviral acyclovir (ACV) and lactose (Figure 1) in solid state mixtures and also in tablet brands. For this purpose the adduct mixture was analyzed using HPLC, FTIR and Mass spectrometry. TLC was also used to prove the presence of lactose in brand formulations. Finally acyclovir loss data with or without lactose was fitted to common solid state kinetic models.
Materials and Methods:
Acyclovir (ACV) (2-Amino-1, 9-dihydro-9-(2-hydroxyethoxymethyl)-6H-purin-6-one) and guanine (2-amino-1,7-dihydro-6H-purin-6-one) (acyclovir related compound) were obtained from Arastoo Pharmaceutical Chemicals Incorp., Tehran-Iran (Figure 1). Lactose monohydrate (Pharma grade 200 Mesh) and anhydrous lactose provided from DMV Chemical Co, Netherlands. Acetaminophen was received from Sigma Aldrich. All other chemicals were of HPLC or analytical grade obtained from Labscan analytical science, Ireland. Commercial tablets of ACV named Brand-1-3 were acquired in Iran and Australia at local pharmacies.
DSC (Differential scanning calorimetry):
A differential scanning calorimeter (DSC-60, Shimadzu, Japan) was used for thermal analysis of drug and mixtures of drug and excipient in 1:1 w/w. Individual samples (drug and excipients) as well as physical mixtures of drug and excipients were weighed to about 5 mg in the DSC aluminum pan and scanned in the temperature range of 25-300-C. A heating rate of 20 -C/min was used and thermograms obtained were observed for any interaction. Enthalpy calculations were completed using TA-60 software (version1.51).
FTIR spectroscopy (Fourier Transform Infra Red):
FTIR spectra of drug and drug-excipient blends immediately after mixing and also after heating were recorded on an FTIR spectrophotometer (Bomem, MB-100 series, Quebec, Canada) in the range of 400-4000 cm−1 using potassium bromide discs. The spectrum was a mean of ten consecutive scans on the same sample. Processing of the FTIR data was performed using GRAMS/32 version 3.04 (Galactic Industries Corporation, Salem, NH).
The HPLC system consisted of a SCL-10A XL auto injector, SCL-10A VP system controller, LC-10AT liquid chromatograph and a SPD-M10AVP, UV-VIS, photo diode array (PDA) detector and a FRC-10A fraction collector, all from Shimadzu (Kyoto, Japan). Samples were injected onto a C18 column (100 mm, 4.60 mm, 5 µm; Agilent, USA) maintained at ambient temperature. The two eluting solutions used were A (Deionized water) and B (a mixture of acetonitril: water: Formic acid (95:5:0.1)). Mobile phase was a mixture of B and A (5:95, v/v). 1 mL/min was used as the flow rate and detection performed at 250 nm. Data were analyzed with Class VP software (version: 6.14 SP1). A solution of Acetaminophen (4mg/mL in mobile phase) was used as the internal standard (Figure 1). Internal standard solution (10 µL) was added to each experimental sample (100 µL). The analytical method was validated with respect to parameters such as linearity, intermediate precision, accuracy and selectivity [24, 25].
The LC system consisted of a SIL-10AD VP auto injector, SCL-10A VP system controller, LC-10ADVP liquid chromatograph and a DGU-12A degasser, all from Shimadzu (Kyoto, Japan).
Samples were introduced into the mass spectrometer through a C18 Gemini column (2-5-200 mm, phenomenex) eluted at a flow rate of 0.5 mL/min, at ambient temperature. Elution was performed, with 99% solvent A (0.1% formic acid in water) and 1 % solvent B (0.1% formic acid, 90% acetonitrile, 5% methanol and 5% water). Mass spectrometric detection was performed with an Applied Biosystems MDS Sciex (Ontario, Canada) API 2000 triple quadrupole mass spectrometer equipped with an electrospray ionization (ESI) interface in the positive ion mode. The tandem mass spectrometer was operated at unit resolution in the multiple reactions monitoring mode (MRM), monitoring the transition of the protonated molecular ions to the product ions. Q1 was used from 150-600 amu in a mass-resolving mode to select the parent ion. The ion source temperature was maintained at 350 °C. The ionspray voltage was set at 5500 V. The curtain gas (CUR) (nitrogen) was set at 15 and the collision gas (CAD) at 7. The collision energy (CE), declustering potential (DP), focusing potential (FP) and entrance potential (EP), were set at 25, 75, 200 and 8 V, respectively. This system was set to the multiple reaction monitoring (MRM) mode, that is, selecting precursor ions, dissociating them and finally analyzing the product ions reaching the high selectivity and sensitivity of this mode for mass analysis and detection. Two ion pairs (a=226.4|135.1, and b=550.3|194.2) were used in the MRM mode. Data acquisition and processing were accomplished using the Applied Biosystems Analyst version 1.4.1 software.
Diluent solution was a mixture of methanol and water (2:3). Developing solvent prepared as a mixture of ethyl acetate and methanol (1:3) containing 0.25% v/v glacial acetic acid. Standard solution prepared by dissolving 1 mg lactose in diluent solution. At least 20 units of each brand tablet were weighed and averages were calculated. Then assuming the whole excipient content of mean tablets weight as lactose, equivalent of 25 mg lactose was transferred to 25 ml volumetric flasks and diluted with diluent and mixed to yield Test solution. Separately 2 µL each of Standard solution and the Test solutions applied to a thin-layer chromatographic plate (20*20, Silica gel-60 F254, 0.25 mm thickness) (Merck, Germany). The spots dried, and developed in a paper-lined chromatographic chamber equilibrated with Developing solvent for about 1 hour prior to use. The chromatogram developed until the solvent front has moved about three-quarters of the length of the plate. Plate removed from the chamber, dried in a current of warm air, and the plate was sprayed evenly with a solution containing 0.5 g of thymol in a mixture of 95 mL of alcohol and 5 mL of sulfuric acid and then heated at 130 °C for 10 minutes. The principal spot obtained from the Test solution corresponds in appearance and Rf value to that obtained from Standard solution.
Preparation of ACV - lactose adduct mixture:
ACV (0.5 g) and lactose monohydrate (3.3 g) were dissolved in 50 mL of United States Pharmacopoeia (USP) borate buffer (0.1 M, pH =9.2) with the aid of stirring and ultrasound . The ionic strength of the solution was adjusted to 25 mM by sodium chloride. Triethylamine was added in an equimolar ratio with ACV to aid solubility. The clear solution was then refluxed at 60 °C in a water bath (Contherm Scientific Ltd, New Zealand) for 12 hr and dried overnight at the same temperature in an open Pyrex™ beaker using a heat oven. The dried mixture is referred to as the adduct mixture. Adducts mixture were dissolved in mobile phase to get 200 μg/mL concentration with respect to the ACV and was subjected to reversed-phase chromatography and LC-MS/MS. The presence of the brown color was also measured at 490 nm. Different samples of ACV (solid state, solutions having pH values set at 9.2), aqueous mixture of ACV and lactose (pH= 9.2) and commercial tablets were heated in order to yield degradation products. All solid and liquid samples were heated in 90°C ovens and 60°C water bath for 24 and 72 hours respectively.
Screening test and commercial tablets:
For screening tests an IST derived from Serajuddin et al.  and Sims et al.  with minor modifications was employed to monitor the probable solid state interaction of drug with excipient in an accelerated manner. Briefly drug and excipients (Table 1) were weighed directly in 4ml glass vials (n = 2) and mixed by 1:1 molar ratio on a vortex mixer for 2 min. all samples were kept in silica gel containing desiccators for 3 days at room temperature. After drying whenever needed 1% w/w magnesium stearate and or 20 % v/w water were added to the mixture. The total weight of drug: excipient blend in a vial was kept at 200 mg. Each vial was tightly capped and stored at 95-C hot air oven. Controls were also done (Table 1). These samples were examined after 24 hours of storage at the above conditions using HPLC. Solid samples were dissolved in the mobile phase to yield appropriate concentrations and centrifuged. The supernatant filtered through 0.45μm nylon membrane filters and then injected to HPLC system.
The presence of lactose in commercial tablets was initially examined according to the British Pharmacopoeia (BP), that is heating of a mixture of lactose (equivalent to 0.25 mg) with added ammonia (5 mL) and water (5 mL). Development of red color confirms the presence of lactose in the formulations . As some tablets were colored, TLC method was used to confirm the color test results. Twenty commercial tablets of three different brands were finely powdered and assayed according to the United States Pharmacopoeia (USP) and then were kept at 95 °C for 24 hr with or without water (Table 1).
Solid-State Kinetic study:
Lactose and ACV were mixed (1:1 molar ratio) thoroughly with a mortar and pestle and 200mg of the mixture was added to at least 10 glass vials (4mL). The vials were dried for 3 days in silicagel chambers and then capped and placed in equilibrated oven at 95 °C. Samples of the solids were removed at 2, 6, 24, 48, 72 and 240 h and assayed. Pure ACV was also heated in the same conditions as control.
Results and Discussion:
Selected DSC scans of drug, excipient and drug-excipient mixtures are shown in Figure 2. The thermal behavior of pure drug, respective excipient and the combination of drug and excipient is compared in the DSC thermograms. Peak temperature and heat of fusion or enthalpy values for drug, excipient and dug -excipient mixture have been summarized in Table 2.
The ACV presented its melting point at (255.27°C) and heat of fusion of (46.75 Jg-1). The endothermic peak of anhydrous lactose was at 241.72°C in the pure sample. Melting endothermic peak of ACV was missing in Acv-anhydrous lactose mixture, which suggested incompatibility (Fig 2-A). A new endothermic peak was also appeared at 227 -C (starting from 205.53 -C and ending at 259.34 -C which may be due to drug and excipient incompatibility.
The monohydrate lactose showed two peaks, first one due to dehydration at (152.7°C) and the second one related to the melting point at (218.38°C) (Fig 2-B). The lactose melting peak (218°C) was characteristic of a monohydrate α-lactose form . In the DSC thermogram of ACV in the presence of lactose monohydrate the ACV melting peak was missing which can be related to the drug and excipient interaction. It should be noted that, the peak at 277.04 °C in ACV and anhydrous lactose mixture, was not appeared in this sample. The second peak in the acv-lactose monohydrate thermogram (Fig 2-B) at 218.58 starts nearly in the same time as the second peak of pure lactose monohydrate (218.38°C), but ends almost differently at the later time (253.57°C Vs 229.05°C). This finding indicates that, the broad peak appeared at 218.58 °C in acv-lactose monohydrate mixture is not the same as the peak in the pure lactose and may contain another peak covered (Fig 2-C). According to Table 2, this can be proved as the âˆ†H value for this peak in the pure sample is 137.68 J/g, but in the mixture increases to 314.62 J/g. Thus it can be concluded that there is an incompatibility between ACV and monohydrate lactose as well.
The absorption pattern of ACV, lactose, ACV-lactose blends immediately after mixing and adducts mixture of drug and excipient is shown in Figure 3-a, 3-b and 3-c respectively. The possible interaction between ACV and lactose can be related to a Maillard type reaction leading to an imine band formation known as Schiff's base. The C=N stretching band appears at 1630-1650 cm-1 in the infrared spectra of imine containing compounds [25, 27-29].
The absorption band at about 1723 cm−1 in Figure 3-a and 3-c is consistent with ACV carboxylic group vibration (Figure 6-b). The only difference seen between the adduct mixture and pure drug is a visible shift of this band to 1688 cm−1 which can be related to intermolecular hydrogen bonding. Shepherd et al. have reported the similar shift in IR spectra of ACV . According to FTIR results no interaction between ACV and lactose can be detected. This finding can be explained by common phenomena of peak overlaying in IR spectroscopy as the absorption region of the expected imine and the carboxylic group overlaps.
Although different HPLC methods have been used in ACV identification, but only a few number have been performed using internal standard. Guanine, vanillin and salicylic acid [31-33] that have been previously used as internal standards were not suitable with the method used in this study. Finally acetaminophen was tried as a new internal standard, and acceptable results were produced. ACV and internal standard chromatogram is presented in Figure 4-A.
HPLC Method Validation
The standard solutions for linearity test were prepared five times at different concentration levels. Peak area ratios of ACV to internal standard were calculated and plotted versus respective concentrations and linear regression analysis was performed. The constructed calibration curve was linear over the concentration range of 0.98-250 µg/mL. Correlation coefficient was found to be more than 0.999 with relative standard deviation (RSD) values ranging from 0.21-2.08% within the concentration ranges studied. Repeatability of measurements of peak area was carried out using seven replicates of the same concentration (200 µg/mL). The RSD was found to be 0.234 %.
The intra- and inter-day precision of the method was carried out at four different concentrations (31.25, 62.5, 125 and 250 µg/ml). The low RSD values of within-day and day-to-day variations revealed that the proposed method is precise (Table 3).
Limit of detection (LOD) and limit of quantification (LOQ) were determined based on signal-to-noise ratios using an analytical response of three and ten times the background noise, respectively . The LOD and LOQ were found to be 1.3 and 3.9 µg/mL. Selectivity of the method was tested using heated samples of ACV with or without lactose. Chromatograms are presented in Figure 4. Some useful standard chromatographic parameters have been calculated and reported in Table 4.
HPLC Analysis of the adduct mixtures
The adduct mixture was dissolved in mobile phase to produce a solution with a nominal ACV concentration of 200 µg/mL. Control was done using heated ACV without lactose (Fig 4-C).
Heated ACV showed a peak (labeled as c) which was related to guanine. The chromatogram of Spiked sample with guanine is shown in Figure 4-B. In comparison to control, HPLC analysis of the ACV-lactose adduct mixture revealed one extra peak (labeled as d) and named as Unknown-1 (Figure 4-D and 4-C), eluted before ACV. Lactose samples either anhydrous or hydrous that were heated alone showed no extra peaks in the same chromatographic conditions compared to that obtained with the mixture.
Adduct mixture solution was prepared and injected to LC-MS/MS system. A mass spectrometer compatible (salt-free) method was developed to give similar separation to the HPLC method. Mass spectra (MS2) are presented in figures 5 (A-D). Manually collected HPLC fractions of Unknown-1 were injected to tandem mass system. The full-scan positive ion electrospray product ion mass spectra showed that the precursor ions of ACV and Unknown-1 were the protonated molecule, [M + H]+, of m/z 226.3 and 550.3 respectively [Fig. 5-A, 5-C]. ACV was also identified as a trimmer shown in Fig. 5-B. After collision-induced dissociation, the characteristic ions in the product ion mass spectrum were at m/z 152.2 and 194.2 respectively. Similar molecular   and daughter ions  have been previously introduced for ACV LC-MS/MS analysis. Proposed structures for Unknown-1 have been presented in figure 6. The nominal molecular mass of Unknown-1 is consistent with the ACV-lactose condensation product formed by the elimination of a single molecule of water from the parent compounds (Figure 5-C). This reaction has been known as the Maillard reaction . The MRM mode was set for ACV (226.4/152.2) and Unknown-1 (550.3/194.2). MRM chromatogram shown in Figure 5D indicates the presence of condensation product or the shiff's base before ACV in LC-MS/MS system which is in accordance to HPLC results (Fig 4-D).
Liquid chromatography/mass spectrometry (LC/MS)-based techniques provide unique capabilities for pharmaceutical analysis. LC/MS methods are applicable to a wide range of compounds of pharmaceutical interest, and they feature powerful analytical figures of merit (sensitivity, selectivity, speed of analysis and cost effectiveness) . Harmon et al., have been detected lactose-hydrochlorothiazide condensation product (m/z=622 and 620) using mass spectrometry. Similar approach has been used by Qiu et al., and Wirth et al., in lactose - metoclopramide and lactose- flouxetin condensation products respectively [11, 12, 20].
Screening tests and Commercial Tablets
For ease of comparing, screening test results have been shown in table 1. As it was expected [13, 26], the presence of water promoted the degradation reactions including the Maillard reaction (even samples except 20). The interesting finding was that, Unknown-1 was only detected in wet physical mixtures of drug and excipient under test conditions used here (samples 2, 4, 6 and 8). In addition no coloring was seen even in samples with detectable amounts of Unknown-1 as a marker for the occurrence of Maillard reaction. These results may be explained by the fact that the reaction has not been reached to the end point and no melanoidines which are responsible for brown color have been produced yet . This may be due to the storage conditions used in this study. According to table 1 the reaction has been appeared in wet physical mixtures of drug and both excipients (monohydrate and anhydrous lactose) even in the presence of magnesium stearate.
Although positive red color test was the evidence for the presence of lactose in all commercial tablets,TLC was performed to confirm this finding as some tablets were initially colored which was interfering with the red color visualization. The method used for TLC is novel and have used tailing reducer agent (Glacial acetic acid) . TLC results have been shown in Figure 7. It is clear that all the tested brands did have lactose in their formulations. The amount of remaining ACV in heated brands with or without water was analyzed using HPLC. Unknown-1 or ACV-lactose condensation produc 9Shiff`s base) was checked with HPLC and LC-MS/MS. It is interesting to report that, Brand-1 and brand 2, containing lactose as excipient did not show any evidence of Maillard reaction even in wet conditions. The only detectable reaction product was seen in brand 3 only in wet condition. These findings indicate that real formulations having different ingredients along with lactose or the same but in different ratios may not undergo the Maillard reaction even in accelerated test conditions used in this study.
Attempts have been done to develop a kinetic model for the loss of the amine in the solid-state Maillard reaction from a pharmaceutical aspect. ACV loss was correlated to different kinetic models and the best fit was accomplished using diffusion model (D3). Data have been presented in Table 5. Recently Khawam and Flanagan have reviewed the Basics and applications of solid-state kinetics and Solid-State Kinetic Models in two separate articles [15, 16]. Solid state reactions are not usually controlled by mass transfer except for a few reversible reactions or when a large evolution or consumption of heat occurs. Diffusion usually plays a role in the rates of reaction between two reacting solids when reactants are in separate crystal lattices.
In diffusion controlled reactions, the rate-limiting step is the diffusion of reactants into reaction sites or products away from reaction sites and the rate of product formation decreases proportionally with the thickness of the product barrier layer. The model proposed by Qiu et al. describing the metoclopramide loss kinetics, can not be used here because the molar ratio of ACV lactose in our study was 1, resulting in Ln 1, to be zero and turning all equation to zero. Full kinetic analysis as a function of the amine, carbohydrate, and reaction conditions is left for future studies.
Compatibility studies were done using different analytical methods. Although DSC, HPLC and tandem mass spectroscopy provided useful information on ACV-lactose compatibility, but FTIR was not helpful due to peaks overlapping phenomena.
The ability to predict reactions in dosage forms depends on the similarity of the prepared mixture to the formulation. The novel TLC method introduced in this study is a valuable and fast test method to follow lactose in commercial solid dosage forms. Low levels of ACV-lactose reaction condensation product was confirmed by liquid chromatography- tandem mass spectrometry (LC-MS/MS) indicating a covalent linkage between ACV and lactose with the loss of only a single water molecule. Although all tested brands contained lactose in their formulations but except one brand no Schiff's base adduct was observed even in wet conditions. It can be concluded that the Maillard reaction of acyclovir and lactose in solid state formulation is less possible than in aqueous phase. Thus there may be some other important factors such as restricted mobility, affecting the Maillard reaction in real solid dosage forms. It is also advisable to avoid wet conditions in formulation process and or storage of acyclovir solid state dosage forms containing lactose.