Študij vpliva zgradbe in oblike kristalov na fizikalno-kemijske lastnosti zdravilnih učinkovin : doktorska disertacija

Bukovec, Polona (author), Vrečer, Franc (mentor)

Pri večini predvsem novejših zdravilnih učinkovin se v fazi razvoja farmacevtskih oblik srečujemo s problemom slabe topnosti in hitrosti raztapljanja v biorelevantnih medijih predvsem kadar razvijamo formulacije, ki vsebujejo slabo topne zdravilne učinkovine, pri katerih je izrednega pomena dobro poznavanje in razumevanje njihove fizikalno-kemijske narave. Z namenom zagotavljanja ponovljive kakovosti, torej ustreznih fizikalno-kemijskih lastnosti zdravilne učinkovine, ki jih pogojuje končna farmacevtska oblika, je potrebno zdravilno učinkovino najprej dobro okarakterizirati in kasneje ustrezno specificirati njene ključne parametre. Večina zdravilnih učinkovin je prisotnih v obliki molekulskih kristalov. Urejenost molekul v kristalu in kristalni habitus pogojuje fizikalno-kemijske lastnosti zdravilne učinkovine in tako vpliva na učinkovitost končne zdravilne oblike predvsem v smislu hitrosti raztapljanja, stabilnosti in procesibilnosti. Ustrezno in vivo obnašanje končne farmacevtske oblike lahko dosežemo na več načinov. Poleg formulacijskih pristopov, povečevanja specifične površine oz. zmanjševanja velikosti delcev ali spreminjanja notranje strukture kristalov je eden izmed uporabnih pristopov tudi spreminjanje kristalnega habitusa, ki ga lahko dosežemo prek specifično vodene kristalizacije. Obstoj različnih kristalnih habitusov nam torej lahko omogoči, da lahko samo s spremembami delcev zdravilne učinkovine reguliramo hitrost raztapljanja le-te iz končne farmacevtske oblike brez prilagajanja sestave formulacije oz. prilagajanja drugih fizikalnih lastnosti, kot sta velikost delcev ali specifična površina. To je še posebej ugodno, kadar imamo določene fizikalne lastnosti zdravilne učinkovine oz. sestavo formulacije že definirano na takem nivoju, da spremembe niso več možne oz. niso zaželene. V delu so opisani koncept in teorija kristalnega inženiringa ter razprava o potencialnih metodah, prednostih in slabostih, ki jih prinaša preučevan način povečevanja hitrosti raztapljanja. Osredotočili smo se na vpliv pogojev kristalizacije na zunanjo obliko kristalov in dokazali, da lahko oblika signifikantno vpliva na hitrost raztapljanja in posledično predvidoma tudi na peroralno absorpcijo. V raziskovalnem delu smo na molekularnem nivoju razložili, zakaj oz. kako lahko z variacijo oblike kristalov (kristalnega habitusa) ob nespremenjeni kristalni obliki, ohranjeni velikosti delcev in vrednosti specifične površine reguliramo hitrost raztapljanja modelne zdravilne učinkovine. Raziskavo smo izvedli na primeru simvastatina, katerega topnost v vodi je zelo nizka (30 μg/mL). Pri simvastatinu je namreč hitrost raztapljanja ključen omejitveni faktor za dosego ustrezne biološke uporabnosti. Glede na to, da simvastatin nad temperaturo 0 °C ne izkazuje polimorfizma, je poleg amorfizacije, mikronizacije ali formulacijskih sprememb eden izmed možnih načinov povečanja njegove hitrosti raztapljanja tudi sprememba kristalnega habitusa kristalov. Da bi potrdili to hipotezo, smo z različnimi tehnikami prekristalizacije iz neobdelanega simvastatina pripravili manjše kristale, ki so se razlikovali v kristalnem habitusu, kar smo dosegli z uporabo topil z različno hidrofilnostjo oz. polarnostjo. Iz mešanice hidrofilnih topil so kristalizirali paličasti kristali, medtem ko so iz mešanice hidrofobnih topil kristalizirali ploščati kristali. Kristali so imeli enako notranjo strukturo in se niso bistveno razlikovali v velikost oz. specifični površini, izmerjeni na materialu »in-bulk«, a so se kljub temu raztapljali z različno hitrostjo. Da bi si lahko nastale razlike razložili na molekularnem nivoju, smo v drugem delu iz enakih kombinacij topil s prilagojenimi pogoji kristalizacije izolirali tudi večje kristale, ki so izkazovali enake razlike v kristalnem habitusu kot majhni kristali. Z namenom popolne izključitve vpliva velikosti oz. specifične površine na rezultate raztapljanja smo razlike v hitrosti raztapljanja spremljali s testom intrinzične hitrosti raztapljanja na izoliranih majhnih kristalih »in-bulk« ter spremljali hitrost raztapljanja velikih monokristalov s pomočjo mikroskopa na atomsko silo. Hitrost raztapljanja paličastih kristalov, izoliranih iz hidrofilne zmesi topil, je bila bistveno višja v primerjavi s ploščatimi kristali, izoliranimi iz hidrofobne zmesi topil. Na velikih monokristalih smo indeksirali posamezne kristalne ploskve, na katerih smo nato spremljali razlike v raztapljanju s pomočjo mikroskopa na atomsko silo, ki postaja pri razvoju končnih farmacevtskih oblik vse širše uporabljena tehnika za raziskavo lastnosti trdnih delcev zdravilnih učinkovin in pomožnih snovi na nanonivoju. To nam je omogočilo, da smo lahko primerjalno za vsako posamezno kristalno ploskev določili kinetiko raztapljanja. S pomočjo mikroskopa na atomsko silo smo določili tudi razlike v mehanskih lastnostih posameznih kristalnih ploskev, ki prav tako lahko vplivajo na hitrost sproščanja zdravilne učinkovine iz končne oblike, saj razlika v elastičnosti materiala lahko zelo vpliva na razpadnost končne farmacevtske oblike in posredno na hitrost raztapljanja zdravilne učinkovine iz nje. Hitrejše raztapljanje paličastih kristalov smo tako lahko povezali z večjo pojavnostjo hidrofilnih ploskev, torej ploskev, na katerih so prisotne bolj hidrofilne funkcionalne skupine simvastatinskih molekul. Te ploskve se bolj izrazijo med kristalizacijo iz hidrofilnega topilnega sistema, posledično kristali rastejo izraziteje tudi v tretjo dimenzijo (globino). V raziskavi smo eliminirali vse ostale fizikalne parametre, ki bi potencialno lahko imeli vpliv na hitrost raztapljanja, in povezali razlike v hitrosti raztapljanja izključno z razliko v obliki kristalov oz. kristalnem habitusu kot posledica različne hitrosti rasti posameznih kristalnih ploskev zaradi uporabe zmesi topil, ki sta se razlikovali v polarnosti oz. relativni hidrofilnosti. Ugotovili smo, da je sprememba kristalnega habitusa, ki jo lahko dosežemo z izborom ustreznih kristalizacijskih oz. prekristalizacijskih topil, ena izmed metod, s katero lahko učinkovito reguliramo hitrost raztapljanja slabo topnih zdravilnih učinkovin in vplivamo na mehanske lastnosti kristalov. Potrdili smo, da nam le celosten pristop k razvoju zdravilnih oblik od razvoja zdravilne učinkovine do njenega formuliranja v končno formulacijo omogoča razvoj kakovostne, učinkovite in varne farmacevtske oblike.

• Language: Slovenian
• Keywords: farmacevtska tehnologija, farmacevtske oblike, zdravilne učinkovine, topnost, stabilnost, fizikalno-kemijske lastnosti, kristali, oblika, kristalni habitus, kristalizacija, simvastatin, disertacije
• Work type: Dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FFA - Faculty of Pharmacy
• Place of publishing: Ljubljana
• Publisher: [P. Bukovec]
• Year: 2015
• Number of pages: 125 str.
• PID: 20.500.12556/RUL-143751
• UDC: 661.12.065.5(043.3)
• COBISS.SI-ID: 282809600
• Publication date in RUL: 11.01.2023

Secondary language

• Language: English
• Title: Study of the influence of crystal structure and shape on physico-chemical properties of active substances

Abstract

When developing a new drug product we are frequently facing problems connected with poor solubility and low dissolution rate of the active pharmaceutical ingredient in bio-relevant media. Poorly water soluble active pharmaceutical ingredients in general require an in depth knowledge of their physico-chemical properties. To ensure consistent quality of the active pharmaceutical ingredient in terms of proper physico-chemical properties, which are dependent of the final dosage form, the active pharmaceutical ingredient has to be thoroughly characterised and the critical parameters should be specified accordingly. Most active pharmaceutical ingredients are present in the form of molecular crystals. The arrangement of the molecules in the crystal and the crystal habit determine the physico-chemical properties of the active pharmaceutical ingredient and consequently determine the effectiveness of the final dosage forms, especially in terms of dissolution rate, stability and processibility Proper in-vivo performance of the final dosage form can be achieved by several different approaches. Besides the commonly known formulation approaches, the enlargement of the specific surface area or the minimization of active ingredient’s particle size or the modifications of its internal structure, also changes of the crystal habit itself is one of the possible ways by which we can regulate the dissolution rate. This can be achieved throughout specifically controlled crystallization procedure. The existence of different crystal habits thus makes it possible to regulate the dissolution rate by only changing the active pharmaceutical ingredient itself without the alteration of other physico-chemical parameters, such as particle size or surface area. This is especially favourable when those properties are already defined on such a level which does not allow additional changes and/or when the composition of the final dosage form is fixed. In the present work we have presented a concept and theory of crystal engineering, the discussion about the required analytical methods and described the advantages and disadvantages which the studied way of dissolution rate enhancement could bring. We have focused the research on understanding the influence of the crystallization parameters on the crystal habit and demonstrated that the crystal shape can significantly influence the dissolution rate and consequently also the oral absorption of the active pharmaceutical ingredients in the gastro intestinal tract. In the continuation of our research we have explained on the molecular level how it is possible to regulate the dissolution rate of the active pharmaceutical ingredient throughout the variation of only crystal shape (crystal habit) without changing the internal structure, particle size and specific surface area. For the present study, simvastatin was selected as a model poorly soluble API with aqueous solubility of 30 μg/mL. For simvastatin, solubility is a crucial rate limiting factor to achieve its desired level in systemic circulation for pharmacological response. Since simvastatin does not exhibit polymorphic phenomena above 0°C, one of the possible ways to enhance the dissolution rate next to amorphisation, micronization or changes in the composition is the transformation of its crystal habit. In order to confirm this hypothesis we have prepared small crystals which differed in their crystal habit from the untreated simvastatin. We have accomplished this by using solvents with different polarity or hydrophilic values. Crystals isolated from hydrophilic solvent mixture crystalized in form of rod-like crystals, whereas crystals isolated from more hydrophobic solvent mixture crystallized in the form of plates. Crystals exhibited the same internal structure and did not significantly differ in their size or specific surface area measured in-bulk, although the dissolution behaviour was significantly different. In order to explain these differences on the molecular level, in the second part of the research, we have isolated large simvastatin crystals that were expressing the same difference in their crystal habit using the same solvent mixtures by only altering some of the crystallization conditions. In order to completely exclude the influence of particle size or the specific surface area on the dissolution results we have determined the differences in the dissolution behaviour by the analysis of the intrinsic dissolution rate on the smaller crystals and measured the differences of the dissolution of larger crystals by using the atomic force microscope. Dissolution rate of the rod-like crystals isolated from the hydrophylic solvent mixture was significantly higher in comparison to the plate like crystals isolated form more hydrophobic solvent mixture. On large monocrystals we have indexed individual crystal planes on which we have than determined differences in the dissolution behaviour by using the atomic force microscope which is becoming a valuable method for the characterisation of the solid particles during the research and development of the new final dosage forms. This enabled us to compare the dissolution kinetics for each crystal face. We have determined the differences in the mechanical properties of each individual crystal face, which can also influence the dissolution rate of the active pharmaceutical ingredient from the final dosage form. Namely the differences in the elasticity of the material can significantly influence the disintegration of the final dosage form and consequently the dissolution rate of the active pharmaceutical ingredient. We have correlated faster dissolution rate of the rod-like crystals with higher incidence of the hydrophilic faces, e.g. faces where more hydrophilic functional groups of the simvastatin molecule were present on the surface. Such faces are more likely to grow during crystallization from the hydrophilic solvent mixtures; consequently such crystals have a more pronounced third dimension (thickness). During our research we have eliminated all other physical parameters which could potentially influence the dissolution rate and correlated the differences in the dissolution rate only with the differences in the crystal habit as a consequence of the different growth rate of the individual crystal faces because of different solvent mixtures with different polarity were used for their crystallization. We have shown that the change in the crystal habit which can be achieved by careful selection of the crystallization or recrystallization solvents is one of the possible methods by which one can regulate the dissolution rate of the poorly water soluble active pharmaceutical ingredient and change its mechanical properties. Only a comprehensive approach to the development of the final dosage forms from developing the active pharmaceutical ingredient to the incorporation of such ingredient into the final dosage form enables us to develop a high-quality, efficient and safe product.