molecules Article Assembling the Puzzle of Taxifolin Polymorphism Roman P. Terekhov 1,* , Irina A. Selivanova 1, Nonna A. Tyukavkina 1, Igor R. Ilyasov 1, Anastasiya K. Zhevlakova 1, Alexander V. Dzuban 2, Anatoliy G. Bogdanov 3, Georgiy N. Davidovich 3, Gennadii V. Shylov 4, Andrey N. Utenishev 1,4, Dmitriy Yu. Kovalev 5 , Anatoliy A. Fenin 6 and Tatyana G. Kabluchko 7 1 Department of Chemistry, Sechenov First Moscow State Medical University, Trubetskaya st. 8-2, 119991 Moscow, Russia; [email protected] (I.A.S.); [email protected] (N.A.T.); [email protected] (I.R.I.); [email protected] (A.K.Z.); [email protected] (A.N.U.) 2 Department of Chemistry, Lomonosov Moscow State University, Leninskiye Gory 1-3, 119991 Moscow, Russia; [email protected] 3 Faculty of Biology, Lomonosov Moscow State University, Leninskiye Gory 1-32, 119991 Moscow, Russia; [email protected] (A.G.B.); [email protected] (G.N.D.) 4 Laboratory of Structural Chemistry, Institute of Problems of Chemical Physics, Russian Academy of Sciences, Acad. Semenov av. 1, 143432 Chernogolovka, Russia; [email protected] 5 Laboratory of X-ray Investigation, Merzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Acad. Osipyan str. 8, 142432 Chernogolovka, Russia; [email protected] 6 Institute of Materials for Modern Power Engineering and Nanotechnology, Mendeleev University of Chemical Technology of Russia, Miusskaya sq. 9, 125947 Moscow, Russia; [email protected] 7 Department of Technology, Ametis JSC, Naberezhnaya st. 68, 675000 Blagoveshchensk, Russia; [email protected] * Correspondence: terekhov_r_p@staff.sechenov.ru; Tel.: +7-965-232-5122 Academic Editor: Carlos Eduardo Sabino Bernardes Received: 15 October 2020; Accepted: 16 November 2020; Published: 20 November 2020 Abstract: A large amount of the current literature dedicated to solid states of active pharmaceutical ingredients (APIs) pays special attention to polymorphism of flavonoids. Taxifolin (also known as dihydroquercetin) is an example of a typical flavonoid. Some new forms of taxifolin have been reported previously, however it is still unclear whether they represent polymorphic modifications. In this paper, we tried to answer the question about the taxifolin polymorphism. Taxifolin microtubes and taxifolin microspheres were synthesized from raw taxifolin API using several methods of crystal engineering. All forms were described with the help of spectral methods, scanning electron microscopy (SEM), X-ray powder diffraction (XRPD), and thermal analysis (TA). SEM reveals that the morphology of the solid phase is very specific for each sample. Although XRPD patterns of raw taxifolin and microtubes look similar, their TA profiles differ significantly. At the same time, raw taxifolin and microspheres have nearly identical thermograms, while XRPD shows that the former is a crystalline and the latter is an amorphous substance. Only the use of complex analyses allowed us to put the puzzle together and to confirm the polymorphism of taxifolin. This article demonstrates that taxifolin microtubes are a pseudopolymorphic modification of raw taxifolin. Keywords: taxifolin; flavonoids; active pharmaceutical ingredient; polymorphism; scanning electron microscopy; X-ray diffraction; variable-temperature powder X-ray diffraction; thermal analysis 1. Introduction Development of drugs is a long and expensive process. That is why pharmaceutical companies are poised to invest more money in research [1,2]. In such circumstances, crystal engineering provides Molecules 2020, 25, 5437; doi:10.3390/molecules25225437 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 16 1. Introduction MoleculesDevelopment2020, 25, 5437 of drugs is a long and expensive process. That is why pharmaceutical companies2 of 15 are poised to invest more money in research [1,2]. In such circumstances, crystal engineering providesan opportunity an opportunity to reduce theto reduce cost of the drug cost design of drug and design to modify and biomedical to modify propertiesbiomedical of properties well-known of wellcompounds-known [compounds3,4]. [3,4]. One ofof the the primary primary goals goals of crystal of crystal engineering engineering in drug in design drug consists design in consists obtaining in the obtaining polymorphic the polymorphiclandscape of thelandscape active pharmaceutical of the active ingredientspharmaceutical (APIs). ingredients Vernadsky (API defineds). Vernadsky polymorphism defined as a polymorphismgeneral property as of a materialsgeneral property [5]. This of term materials has various [5]. This definitions term has depending various definitions on the scientific depending field [on6]. theAccording scientific to Europeanfield [6]. According Pharmacopeia, to European polymorphism Pharmacopeia is defined, polymorphism as follows: “the is ability defined of a as compound follows: “thein the ability solid stateof a compound to exist in di inff erentthe solid crystalline state to forms exist havingin different the same crystalline chemical forms compound”. having the The same U.S. chemicalFDA classifies compound amorphous,”. The solvate, U.S. andFDA hydrate classifies forms amorphous, as polymorphic. solvate, Polymorphism and hydrate of APIsforms is anas polymorphic.important subject Polymorphism in pharmaceutical of APIs scienceis an important because thesubject molecular in pharmaceutical packing may science have abecause significant the molecularimpact on physicochemicalpacking may propertieshave a significant [7,8], biopharmaceutical impact on parametersphysicochemical [9], and properties pharmacological [7,8], biopharmaceuticalactivity [10]. parameters [9], and pharmacological activity [10]. A significant significant part of the current literature on polymorphism pays particular attention to flavonoids.flavonoids. From From the the point point of of view of chemical sciences, this group of compounds is a derivative of 1,3-diphenylpropane.1,3-diphenylpropane. They They are are secondary metabolites of of plants [11] [11].. Flavonoids Flavonoids are are well well know knownn as bioactive compounds [12 [12–15]. Implementation Implementation of of these these materials materials in in medical medical practice practice is is restricted restricted by by the limited limited water water solubility solubility and and low low bioavailability bioavailability of of flavonoids flavonoids [16]. [16 ].Researchers Researchers have have attempt attempteded to modifyto modify the the properties properties of ofthese these compounds compounds via via dev developingeloping new new polymorphic polymorphic forms forms using using crystal engineering [17]. [17]. Taxifolin, also also known known as as dihydroquercetin, dihydroquercetin, is isa commercially a commercially available available flavonoid flavonoid (Figure (Figure 1). 1Its). mainIts main natural natural source source is the is thebutt butt-log-log portion portion of Larix of Larix spp.spp. wood. wood. Taxifolin Taxifolin has reducing has reducing properties properties and isand well is wellknown known as a food as a foodingredient ingredient thanks thanks to its toantioxidant its antioxidant activity activity [18–20 [].18 It–20 is]. placed It is placedin the Europeanin the European Union Unionmarket market as a asfood a foodsupplement. supplement. This Thiscompound compound is also is alsocharacteri characterizedzed by capillaryby capillary-protective-protective [21,22], [21,22 neuroprotective], neuroprotective [23,24], [23,24 ],hepatoprotecti hepatoprotectiveve [25,26], [25,26], regenerative regenerative [27], [27], antitumor [[28],28], anti-inflammatoryanti-inflammatory [ 29[29],], antidiabetic antidiabetic [30 [30],], and and antiviral antiviral activities activit [ies31, 32[31,32].]. Due Due to a wideto a widerange range of pharmacological of pharmacological effects, effects, this substance this substance was registered was registered as an as API an in API Russia in Russia and it and is being it is beingindustrially industrially produced produ byced Ametis by Ametis JSC. JSC. Figure 1. Chemical structure of taxifolin. Figure 1. Chemical structure of taxifolin. In order to obtain a standard sample, a complex investigation of taxifolin was performed. ThereIn exist order several to obtain articles a standard that focus sample, on the a complex X-ray analysis investigation of such of samples, taxifolin produced was performed. by a diff Thereerent existmanufacturer several articles [33,34]. that Physicochemical focus on the propertiesX-ray analysis of taxifolin of such have samples, previously produced been adjustedby a different using manufacturerchemical and physical[33,34]. Physicochemical modification [35 properties,36], lyophilization of taxifolin [37 ],have and previously nanodispersion been adjusted formation using [38]. chemicalThere is still and uncertainty, physical modification however, regarding [35,36], lyophilization whether different [37], polymorphic and nanodispersion forms of formation taxifolin exist. [38]. ThereThis is still paper uncertainty, relays the however, analysis ofregarding different whether forms of different taxifolin withpolymorphic the objective forms to of try taxifolin to categorize exist. themThis as polymorphic paper relays modifications. the analysis of different forms of taxifolin with the objective to try to categorize them as polymorphic modifications. Molecules 2020, 25, x; doi: FOR PEER REVIEW www.mdpi.com/journal/molecules Molecules 2020, 25, 5437 3 of 15 Molecules 2020, 25, x FOR PEER REVIEW 3 of 16 2. Results