Role of Valsartan as an Antiplasticizer in Development of

Mar 2, 2018 - Role of Valsartan as an Antiplasticizer in Development of. Therapeutically Viable Drug−Drug Coamorphous System. Anurag Lodagekar, Rahu...
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Role of valsartan as an anti-plasticizer in development of therapeutically viable drug-drug co amorphous system Anurag Lodagekar, Rahul B Chavan, Naveen Chella, and Nalini R. Shastri Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00081 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Crystal Growth & Design

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Role of valsartan as an anti-plasticizer in development of therapeutically

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viable drug-drug co amorphous system

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Anurag Lodagekar, Rahul B Chavan, Naveen Chella, Nalini R Shastri*

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Solid State Pharmaceutical Research Group (SSPRG), Department of Pharmaceutics, National

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Institute of Pharmaceutical Education and Research (NIPER), Hyderabad, 500037, India

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*Corresponding author: Nalini R Shastri

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Tel. +91-040-23423749

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Fax. +91-040-23073751

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E-mail: [email protected],

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Address: Department of Pharmaceutics, National Institute of Pharmaceutical Education &

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Research (NIPER), Balanagar, Hyderabad, India, Pin code-500037

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Abstract

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In a rapidly growing field of co amorphous systems, selection of appropriate coformer is a

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challenging task, especially while developing drug-drug co amorphous system, as one of the

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component should possess glass forming potential, and the combination generated is required to

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be therapeutically active. The present study was hence aimed towards exploring the glass

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forming potential of valsartan for the development of therapeutically active drug-drug co

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amorphous system. Co amorphous system of valsartan-cilnidipine in 6 different weight ratios

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(4:1, 3:1, 2:1, 1:1, 1:2 and 1:3), along with the therapeutically relevant weight ratio (16:1) were

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prepared by quench cooling method and evaluated for anti-plasticization effect of valsartan using

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DSC/MDSC. Generated co amorphous systems were studied for their dissolution benefits and

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stability under accelerated conditions (40 ⁰C/ 75 % RH). DSC and FTIR outcome along with

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prediction of Tg of drug-drug mixture using Gordon-Taylor and Couchman–Karasz equation

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demonstrated that for the generation of co amorphous system, anti-plasticization activity of

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valsartan played a dominant role. Co amorphous system with higher valsartan content was found

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to provide significantly higher dissolution benefits and stability under accelerated conditions for

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1 month. This study may pave path for the development of valsartan based co amorphous system

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in the treatment of cardiovascular diseases.

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Keywords: cilnidipine, DSC, glass transition temperature, stability, dissolution

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Crystal Growth & Design

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Desired drug action or bioavailability, predominantly depends on solubility, permeability

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and potency. Except for potency, which is an intrinsic property, solubility and permeability are

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the dominant biopharmaceutical properties which play a key role in converting propitious new

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chemical entities (NCEs) into potential drug candidates.1 Numerous formulation approaches such

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as complexation,2 mesoporous drug delivery,3 nano formulations, and lipid based formulations4

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have been reported to tackle problems arising due to inadequate solubility or permeability. Solid

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state manipulation by micronization, crystal habit modification, polymorphism and

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amorphication1 are also employed to address the challenges in delivery of poorly soluble drugs.

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Apart from polymeric amorphous solid dispersion, co amorphous system, a new trending

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amorphous system has gained prominence in formulation strategy to improve the poor solubility

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of NCEs.5, 6 The term “co amorphous system” was coined by Chieng et al.,7 and was defined by

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Suresh et al., as “a multi-component single phase amorphous solid system which lacks

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periodicity in lattice and is associated by weak and discrete intermolecular interactions between

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the components”.8 One of the prime requirements while developing such multi component

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system is selection of suitable coformers which decides the fate of the formulation in terms of

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stability and performance.9 Selection process of coformers is very critical since the drug-drug

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combination generated should be pharmacological relevant and at least one drug should possess

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good glass forming ability. Because of this limitation, less than 25 drug-drug co amorphous

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systems have been reported to date.6

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Valsartan is known to have good glass forming ability due to its high glass transition

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temperature (Tg) i.e. 76 °C.10,

11

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coformer or as an anti-plasticizing agent for the development of drug-drug co amorphous

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systems remain unexplored. Clinical data reveals that, this cardiovascular drug is used in

To the best of our knowledge, potential of valsartan as a

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combination with other cardiovascular drugs such as simvastatin and cilnidipine to improve

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therapeutic efficacy.12, 13 Preliminary screening was conducted with valsartan - simvastatin and

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valsartan - cilnidipine combinations at different weight ratios. Valsartan and simvastatin co

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amorphous system at therapeutically relevant dose of 4:1 (160 mg: 40 mg) gave single phase

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amorphous system after quench cooling. However, during stability studies, the co amorphous

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material was transformed to a rubbery mass followed by recrystallization, as observed from the

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appearance of a melting endotherm of simvastatin at 140.4 ⁰C (supplementary figure S1). As a

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result, further studies on these systems were not performed. In contrast, valsartan and cilnidipine

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co amorphous system showed promising results on stability and were selected for further studies.

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The present project was embarked with an objective to explore the glass forming

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potential of valsartan for the development of therapeutically relevant drug-drug co amorphous

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systems with cilnidipine. Barring few examples; ezetimib-lovastatin14 and nateglinide-

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metformin15, most of the reported drug-drug co amorphous systems though therapeutically

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relevant, are usually generated in 1:1 weight or molar ratio, which are generally not in a

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therapeutically administered relevant dose combination. Ergo, such combination may not be

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useful for the development of final formulation, due to the possibility of sub therapeutic or toxic

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concentration of one or both drugs in plasma after administration. Valsartan- cilnidipine in a

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therapeutically relevant weight ratio (16:1) equivalent to 160 mg: 10 mg respectively lowered the

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blood pressure significantly when compared to plain valsartan and cilnidipine, as reported in

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ClinicalTrials.gov Identifier: NCT02343250 of U.S National library of medicine. The second

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objective of this study was hence to evaluate the stabilization potential of valsartan at the

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reported valsartan- cilnidipine fixed dose combinations. The anti-plasticization effect of

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valsartan was explained by Gordon Taylor (GT) and Couchman–Karasz (CK) models. The

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Crystal Growth & Design

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difference in the solubility parameter (ΔSP) of the drugs was determined to predict the

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miscibility of the binary system. The co amorphous systems were subjected to accelerated

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stability studies to evaluate the stabilization potential of valsartan.

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For determination of the heat capacity (Cp) and glass transition temperature (Tg),

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cilnidipine (Unique Chemicals, Mumbai, India) and valsartan (Lupin Pharmaceuticals, INC.

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Pune, India) were converted into amorphous form, in situ, using a Differential Scanning

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Calorimeter (Mettler Toledo DSC system equipped with STARe software). Approximately 5-15

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mg of the drugs were sealed in pinholed aluminum pans individually, heated at a rate of 10 °C /

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min up to 130 °C and immediately cooled to 10 ⁰C at a cooling rate of -20 ⁰C/min. Quench

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cooling method was used to prepare co amorphous systems of valsartan with cilnidipine in

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various weight ratios (1:3, 1:2, 1:1, 2:1, 3:1, 4:1 and 16:1). Prior to quench cooling, all binary

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blends were gently but thoroughly mixed to avoid the inconsistency in the blends with an aid of

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mortar and pestle for 2-5 min. This crystalline blend was melted in a stainless steel spatula over a

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hot plate (130 ⁰C) and quench cooled over a crushed ice bed. Since, moisture can impact the Tg,

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appropriate precautionary measures were taken during the experiments to avoid inadvertent

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contamination with moisture, such as use of moisture proof paraffin films wrapped around the

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spatula immediately before transferring it from hot plate to crushed ice. The cooled samples were

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kept in desiccators for 12 hr for removal of any adsorbed moisture. All ratios were prepared in

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triplicates to verify the reproducibility.

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Quench cooled products were assessed for chemical stability by a validated HPLC

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method. The samples were analyzed on a HPLC system (e2695 Waters) consisting of a HPLC

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pump, an automated injector equipped with a Photo Diode detector (2998 PDA) and an auto

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sampler. Details of the HPLC method used are given in supporting information table S1. The

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peak purity was calculated based on the purity angle and purity threshold values of the

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chromatogram for the eluted drug; the peak was considered as pure peak when the purity angle

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was less than the purity threshold. All quench cooled samples were chemically stable and gave

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peak purity above 99.99%. Thermal characterization of the generated co amorphous systems was

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carried out using DSC. All seven valsartan: cilnidipine in weight ratio showed a single Tg values

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indicating the formation of a single phase, co amorphous system. Tg values for the ratios studied

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lie between 31 °C to 76 °C. (figure 1 and supplementary table S2). The Tg values of the

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experimentally prepared quench cooled systems were statistically similar (p> 0.05) to the Tg

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values of in situ generated co amorphous systems using DSC (supplementary figure S2 and table

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S2).

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Figure 1 DSC curves of quench cooled cilnidipine (CLD), valsartan (VAL) and VAL-CLD co amorphous systems. Arrow indicates the average of three Tg values.

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Crystal Growth & Design

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To further confirm the amorphous nature of the quench cooled samples, powder XRD

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(PXRD) patterns of the individual crystalline and amorphous forms of drugs with their physical

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mixture and binary blends were recorded on Bruker D8 Advance diffractometer (Bruker- AXS,

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Karlsruhe, Germany) with Cu-Kα X-radiation (λ= 1.5406 Å) at 2θ range 5–50° with a step time

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of 0.030 steps/0.5 s. Crystalline cilnidipine exhibited its characteristic 2θ values at 5.7, 11.65,

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12.19, 14.17, 16.35, 18.63, 19.76 and 21.65 corresponding to the reported literature values16

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(figure 2). However, crystalline valsartan did not show any Bragg’s peaks probably due to its

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smaller particle size (less than 0.1 µm) and its mesophasic nature wherein the long range-order

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of the stable crystal is limited to length of tens of nanometer scale.17 The crystallinity of

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valsartan was however confirmed by DSC (supplementary figure S3) and hot stage microscopy

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(data not shown). The quench cooled samples with halo pattern and an absence of Bragg’s peaks

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in PXRD analysis confirmed the amorphous nature of the samples, supporting the findings of

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DSC.

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Figure 2 PXRD overlay of co amorphous valsartan (VAL): cilnidipine (CLD) in different weight ratios

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FTIR spectra were used to detect the presence of intra/inter molecular interactions

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between valsartan and cilnidipine in quench cooled samples. The spectra were recorded on

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PerkinElmer FTIR C95065 spectrophotometer operating with Spectrum software in the region of

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500-4000 cm-1. FTIR analysis revealed an absence of intermolecular interactions between these

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two drugs in quench cooled samples (figure 3). Taken collectively, the DSC, PXRD and FTIR

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results suggested that quench cooled samples of cilnidipine were present in amorphous form with

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valsartan at all weight ratios.

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Figure 3 FT-IR Spectra of crystalline (a) and amorphous (b) cilnidipine, amorphous (c) and crystalline (d) valsartan, crystalline (e) and amorphous (f) physical mixture of valsartan and cilnidipine (16:1), and their co amorphous systems in different weight ratios (h) 1:3, (i) 1:2, (j) 1:1, (h) 2:1, (k) 3:1, (i) 4:1, (m) 16:1.

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Prediction of Tg of values of valsartan: cilnidipine mixture using Gordon- Taylor (GT)

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equation and Couchman–Karasz (CK) equation were performed to obtain insights on the

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mechanism by which valsartan stabilizes the co amorphous systems. This study helps in

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Crystal Growth & Design

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assessing whether intermolecular interactions or anti plasticization effect of valsartan play an

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important role in stabilization. The GT equation is based on the additivity of free volumes of the

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individual components characteristic of ideal mixing. The equation helps in estimating the

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theoretical Tg of a co amorphous system as a function of composition. The GT equation reads as: ܶ௚௠௜௫ =

‫ݓ‬ଵ ∗ ܶ௚ଵ + ‫ݓ ∗ ்ீܭ‬ଶ ∗ ܶ௚ଶ ‫ݓ‬ଵ + ‫ݓ ∗ ்ீܭ‬ଶ

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Where w1 and w2 are the weight fraction of each component, Tg1 and Tg2 are the Tg of each

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component (wherein Tg1 ≤ Tg2). KGT is the ratio of the free volumes of individual components

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and is calculated by ‫= ்ீܭ‬

ߩଵ ∗ ∆ߙଵ ߩଵ ∗ ܶ௚ଵ ≈ ߩଶ ∗ ∆ߙଶ ߩଶ ∗ ܶ௚ଶ

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Where ρ, Tg and ∆α are, respectively, the density, the Tg, and the change in the thermal

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expansivities of each component at the respective Tgs. The Tg values of in situ quench cooled

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cilnidipine and valsartan generated using MDSC were found to be 23.36 ⁰C and 76.25 ⁰C,

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respectively, while, the true density values for cilnidipine and valsartan were found to be 1.24

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and 1.2 g/ml, respectively.

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CK equation results from the expression of total molar entropy of the blend and reads as, ln ܶ௚ =

‫ݔ‬ଵ ∗ ln ܶ௚ଵ + ‫ܭ‬஼௄ ∗ ሺ1 − ‫ݔ‬ଵ ሻ ∗ ܶ௚ଶ ‫ݔ‬ଵ + ‫ܭ‬஼௄ ∗ ሺ1 − ‫ݔ‬ଵ ሻ ‫ܭ‬஼௄ =

∆‫ܥ‬௣ଶ ∆‫ܥ‬௣ଵ

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Where, ∆Cp1 and ∆Cp2 are the changes in specific heat capacity of each component at their

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respective Tgs. The Cp1 (cilnidipine) and Cp2 (valsartan) generated from MDSC experiments were

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found to be 0.359 and 0.48 J/g, respectively.

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For a co amorphous system, Tg can be accurately predicted by GT equation, which

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suggest whether two components mix ideally and are fully miscible at molecular level. Deviation

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of experimental Tg values from the theoretical values indicate strong inter molecular interactions

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and non ideal mixing.18 At lower ratios, no significant deviation was observed between the

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experimental Tg and the Tg obtained from the GT and CK model fit as determined from the slope

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and coefficient comparisons. However, significant deviation was observed at higher valsartan:

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cilnidipine weight ratio i.e. 4:1 and 16:1, which indicated an absence of intermolecular

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interactions between valsartan and cilnidipine in glass formation (figure 4). This deviation might

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be due to the contribution of the steric hindrance along with anti-plasticization effect.19

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Figure 4 Compositional variation of the Tg of co amorphous systems (a) Gordon Taylor (GT) and (b) Couchman–Karasz (CK) equations

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The findings from GT and CK curve fitting, the absence of molecular interaction in the

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quench cooled samples, coupled with high Tg values of valsartan suggested anti-plasticization

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effect of valsartan to be a dominating factor in generation and stabilization of co amorphous

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systems.20 With higher molar ratio of valsartan, the co amorphous system gave Tg values near to

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Tg value of valsartan demonstrating a direct correlation between the Tg values of the co

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Crystal Growth & Design

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amorphous systems generated and the amount of valsartan present in the systems. Additionally,

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the solubility parameter values for valsartan and cilnidipine were predicted using Materials

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Studio 7.0 (Accelrys Inc., USA) software. Solubility parameters for valsartan and cilnidipine

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were found to be 21.903 and 21.096, respectively. Minimal difference between the solubility

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parameter indicates that both components are miscible with each other.21, 22 Miscibility of the

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components in a blended system is of prime importance for the stabilization of an amorphous

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system, since it is generally believed that miscibility at molecular level is necessary to achieve

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maximum physical stabilization.

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Co amorphous systems are known to elicit and prolong the supersaturated solution state,

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hence in vitro dissolution study was conducted for all seven weight ratios of co amorphous

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systems in USP type II apparatus under non sink condition and compared with crystalline

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cilnidipine, amorphous cilnidipine and crystalline physical mixture of valsartan and cilnidipine in

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16:1 weight ratio. All samples were passed through sieve number ASTM #44 prior to the sample

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addition into the discriminating dissolution medium (6.8 pH buffer with 0.4% SLS) to maintain

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uniform particle size for all samples. Dissolution samples were analyzed using the above

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mentioned HPLC method. Peak purity was determined to ascertain the purity of each eluted

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component. DE15 (dissolution efficiency at 15 min) and f2 (similarity factor) values were

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calculated using DDsolver23 to compare the dissolution profiles.

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Plain crystalline cilnidipine showed only 27.53 % release in 120 min (figure 5). The

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cilnidipine release from physical mixture was found to be similar to crystalline cilnidipine (f2=

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60.71) as only 24.78 % of cilnidipine was released in 120 min (supplementary figure S4).

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However, cilnidipine release from the co amorphous systems was enhanced compared to its

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crystalline counterpart. Similarly, cilnidipine release showed an increment as the concentration

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Crystal Growth & Design

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of valsartan was increased in the co amorphous systems. Within 30 min, 100 % of cilnidipine

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was released from valsartan: cilnidipine co amorphous system in 16:1 ratio. The release patterns

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of other valsartan: cilnidipine co amorphous systems (4:1, 3:1, 2:1 and 1:1) were between the

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release pattern of 16:1 ratio and crystalline cilnidipine. The cilnidipine release from the 16:1

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valsartan: cilnidipine co amorphous system was 21 times more that the drug release from

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crystalline cilnidipine as seen from the DE15 value (Supplementary table S3). This significant

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enhancement in the drug release was attributed to the existence of cilnidipine as a co amorphous

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system with valsartan. No drug release was recorded from amorphous cilnidipine as it became

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rubbery immediately after preparation due to its low Tg value. Similarly, valsartan: cilnidipine at

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1:2 and 1:3 weight ratios also developed a rubbery state and failed to release the drug during the

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dissolution study. 120 100 % drug release

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CLD

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VAL:CLD 3:1

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60 Time (min) VAL:CLD 1:1

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VAL:CLD 4:1

100

120

VAL:CLD 2:1 VAL:CLD 16:1

Figure 5 Release profiles of cilnidipine from crystalline cilnidipine (CLD) and its co amorphous systems with valsartan (VAL) in different weight ratios

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Drug release of valsartan from its crystalline and amorphous systems (figure 6) was

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found to be similar (f2 = 90.34). This can be attributed to the microcrystalline nature of valsartan

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in its crystalline counterpart. The release from lower ratios was not significantly different to

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crystalline and amorphous valsartan, as reflected by the similarity factor f2 >50 (supplementary

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table S4). Valsartan release from co amorphous systems at 16:1 weight ratios was more when

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compared to crystalline and amorphous valsartan. The amount of valsartan released from 16:1

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formulation nearly doubled at the end of 90 min when compared to crystalline valsartan.

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However, the enhancement in the release was not as significant as that observed with cilnidipine

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release, indicating that cilnidipine did not have a major impact on the dissolution of valsartan. 50

40

% Drug release

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Crystal Growth & Design

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20

10

0 0

239 240 241

20

40

60 Time (min)

80

C VAL

A VAL

VAL:CLD 1:1

VAL:CLD 3:1

VAL:CLD 4:1

VAL:CLD 16:1

100

120

VAL:CLD 2:1

Figure 6 Release profiles of valsartan from crystalline valsartan (CVAL), amorphous VAL (AVAL) and its co amorphous systems with cilnidipine (CLD) in different weight ratios

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The stability of the generated co amorphous samples were evaluated by placing the

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samples in open and closed containers in a stability chamber (Osworld OPSH-G-16-GMP series,

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Osworld Scientific Equipments Pvt. Ltd., India) at 40 ⁰C ± 2 ⁰C and 75 ± 5% relative humidity

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(RH). Samples were withdrawn at specified period (0, 1, 2, 3 and 4 week) and analyzed by DSC

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for monitoring the crystallinity. Additionally, at the end of the stability period, the stable

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formulations and the therapeutically relevant valsartan: cilnidipine ratio 16:1 was subjected to

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dissolution studies. As mentioned earlier, amorphous cilnidipine became rubbery immediately

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after preparation and crystallized within first week on stability studies. Similarly, co amorphous

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systems with higher cilnidipine content (1:2 and 1:3) also crystallized under stability conditions.

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Melting endotherms in DSC analysis of the stability samples of these two weight ratios confirm

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the instability (supplementary figure S5). Whereas, co amorphous valsartan: cilnidipine with

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higher valsartan content (1:1, 2:1, 3:1, 4:1 and 16:1) were found to be stable for 1 month of

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storage, as the respective samples did not show any melting endotherm of either drug in DSC

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(figure S6). Evidence of amorphous nature was also confirmed by cross polarized microscopy by

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absence of birefringence (data not shown). Valsartan: cilnidipine 16:1 co amorphous system on

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storage at the end of four week period showed similar dissolution profile (supplementary figure

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S7) as that of the freshly prepared co amorphous system (similarity factor value f2=62.14).

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Valsartan is an old drug and is extensively studied for its thermal behavior. However, its

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potential in formation of co amorphous system has remained unexplored. Use of valsartan for the

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development of therapeutically active valsartan: cilnidipine co amorphous system was

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demonstrated successfully in this present study. Outcome of the DSC and PXRD confirmed the

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formation of co amorphous system by quench cooling method. Theoretical Tg prediction of

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valsartan- cilnidipine mixture using GT and CK equation and FTIR demonstrated that anti

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plasticization effect of valsartan was prevalent in formation of co amorphous system. Evaluation

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of the generated co amorphous systems with higher valsartan content was stable for 1 month

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under accelerated stability conditions. Similarly, compared to crystalline cilnidipine, nearly 5

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fold enhancement in the dissolution at end of 2 h, was observed with valsartan: cilnidipine co

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amorphous system at 16:1 weight ratio (which is the therapeutically active dose ratio). This

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preliminary study on potential of valsartan as a coformer in the development of co amorphous

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system may pave path for more therapeutically active drug-drug co amorphous system. Clinical

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data has demonstrated that administration of valsartan cilnidipine combination in 16:1 ratio

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provides maximum benefits in lowering the blood pressure. Development of co amorphous

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system at the same ratio may thus help in achieving the same therapeutic benefits.

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Supporting information

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DSC thermograms of stability samples, in situ quench cooled systems, crystalline drugs.

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Dissolution profile of stability samples, Chromatographic parameters for HPLC analysis, Glass

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transition temperatures, Comparison of drug release.

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Acknowledgement

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The authors acknowledge the financial support from the Department of Pharmaceuticals (DoP),

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Ministry of Chemicals and Fertilizers, Govt. of India.

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Conflict of interest

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The authors declare no competing financial interest.

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References

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For Table of Contents Use Only

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Role of valsartan as an anti-plasticizer in development of therapeutically viable drug-drug co amorphous system Anurag Lodagekar, Rahul B Chavan, Naveen Chella, Nalini R Shastri

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Synopsis

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Anti-plasticization potential of valsartan was explored for the first time in development of

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therapeutically active drug-drug co amorphous system with cilnidipine. The therapeutically

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administered relevant dose combination of valsartan: cilnidipine (16:1) was stable for 1 month

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under accelerated stability conditions (40 ⁰C/ 75 % RH) and showed 5 fold improvement in

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release of cilnidipine.

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