Formulation and in vitro/in silico characterization of lipid-based drug delivery systems with simvastatin as model substance

nisku rastvorljivost u vodi/gastrointestinalnim (GI) tečnostima, koja je ograničavajući faktor za apsorpciju, a samim tim i biološku raspoloživost lekovite supstance. Jedan od pristupa za prevazilaženje ovog problema je izrada lipidnih formulacija, u koje spadaju i samomikroemulgujući sistemi (SMEDDS). Simvastatin je lekovita supstanca iz grupe statina, koja se koristi za snižavanje koncentracije holesterola u krvi, poseduje i vaskuloprotektivna svojstva, a najnovija istraživanja pokazuju i da ispoljava određenu efikasnost u lečenju tumora dojke, kolona i prostate. Zbog niske biološke raspoloživosti simvastatina nakon peroralne primene tableta sa trenutnim oslobađanjem, jedinog farmaceutskog oblika sa simvastatinom na tržištu, sve je veća potreba za razvojem savremenih nosača koji bi omogućili veću biološku raspoloživost ove lekovite supstance. Stoga, sveobuhvatni cilj ove doktorske disertacije je bio razvoj novih lipidnih formulacija sa aspekta potencijalne primene kao nosača nove generacije za peroralnu primenu simvastatina i njihova fizičkohemijska i biofarmaceutska karakterizacija. U prvoj fazi istraživanja formulisani su tečni SMEDDS, kao potencijalni nosači za peroralnu primenu simvastatina, koji omogućavaju povećanje rastvorljivosti i brzine rastvaranja ove lekovite supstance. Od ukupno 54 različite kombinacije ulja (oleoil makrogol-6 gliceridi, propilenglikol monokaprilat, propilenglikol monolaurat), surfaktanta (kaprilokaproil makrogol-8 gliceridi) i kosurfaktanta (polisorbat 80 ili makrogol 15 hidroksistearat), odabrani su uzorci koji su odgovarali kriterijumima za SMEDDS u pogledu brzine samoemulgovanja (< 1 min), robusnosti prema razblaživanju (transparencija ˃ 99%), veličine kapi (< 100 nm) i raspodele veličine kapi (PDI < 0,2), kao i prihvatljive stabilnosti tokom šestomesečnog skladištenja na sobnoj temperaturi (22 ± 2 °C). Simvastatin se iz odabranih tečnih SMEDDS oslobađao brzo, čime je potvrđeno da se formulisanjem tečnih SMEDDS značajno povećava brzina rastvaranja simvastatina. U drugoj fazi istraživanja, primenom kompjuterskog programa GastroPlus™, uspešno je razvijen simvastatin-specifični fiziološki zasnovan farmakokinetički (PBPK) model, koji na odgovarajući način opisuje apsorpciju, konverziju u simvastatin-kiselinu, raspodelu i eliminaciju simvastatina. Analizom literaturnih podataka, eksperimentalnih podataka i in silico predviđenih vrednosti parametara koji opisuju biofarmaceutska svojstva simvastatina i fiziološke karakteristike organizma, odabran je set ulaznih parametara koji su korišćeni za in silico modelovanje. Izgrađeni PBPK model je validiran poređenjem predviđenih sa in vivo vrednostima farmakokinetičkih parametara za simvastatin i simvastatin-kiselinu nakon peroralne primene 20 mg simvastatina u obliku tableta sa trenutnim oslobađanjem. Rezutati in vitro ispitivanja tečnih SMEDDS, punjenih u tvrde kapsule su pokazali da ove formulacije omogućavaju povećanje brzine rastvaranja simvastatina. Međutim, in silico modelovanjem je pokazano da se na ovaj način ne povećava biološka raspoloživosti simvastatina, u poređenju sa tabletama simvastatina sa trenutnim oslobađanjem. Analizom rezultata simulacija je utvrđeno da su distalni delovi GI trakta optimalno mesto za apsorpciju simvastatina, te se razvojem formulacija sa modifikovanim (ciljnim) oslobađanjem simvastatina može postići smanjenje uticaja presistemskog metabolizma i povećanje biološke raspoloživosti ove lekovite supstance. U trećoj fazi istraživanja formulisane su lipidne formulacije sa modifikovanim oslobađanjem simvastatina, punjenjem odabranih tečnih SMEDDS u acidorezistentne kapsule, kao i mešanjem odabranih tečnih SMEDDS sa kopolimerima metakrilne kiseline, koji pokazuju pH zavisnu rastvorljivost. Korišćeni su polimeri Eudragit® L100, Eudragit® S100, kao i njihova kombinacija, u odnosu 1:1, a odnos SMEDDS:polimer je bio 2:1. Rezultati in vitro ispitivanja brzine rastvaranja simvastatina pod uslovima izmene medijuma su pokazali da je punjenje tečnih SMEDDS u acidorezistentne kapsule omogućilo oslobađanje simvastatina nakon 90 min, u medijumu pH 6,4. Poređenjem rezultata in vitro ispitivanja brzine rastvaranja simvastatina iz formulacija, dobijenih mešanjem tečnih SMEDDS sa Eudragit® polimerima punjenih u tvrde kapsule, zaključeno je da postoji jasna zavisnost između brzine rastvaranja/oslobađanja simvastatina i vrste Eudragit® polimera...ABSTRACT More than 50% of the marketed drugs and approximately 90% of new drug entities are poorly soluble in water/gastrointestinal (GI) fluids, which is a limiting factor for their absorption and bioavailability. One of the approaches to overcome this issue is based on the design of lipid-based drug delivery systems, including self-microemulsifying drug delivery systems (SMEDDS). Simvastatin belongs to statins, the class of drugs used to lower cholesterol levels in the blood; it also has vasculoprotective properties, and recent research shows that simvastatin exhibits certain activity in the treatment of breast, colon and prostate cancer. Due to low bioavailability of this drug following oral administration of immediate-release tablets, the only dosage form of simvastatin available on the market, there is a growing need for the development of modern carriers that would improve its oral bioavailability. Therefore, the overall aim of this doctoral dissertation was to develop novel lipid-based drug delivery systems as carriers for oral administration of simvastatin, along with their physicochemical and biopharmaceutical characterization. In the first phase of the study, liquid SMEDDS were formulated, as potential carriers for oral administration of simvastatin, which improved the drug solubility and dissolution rate. Out of 54 different combinations of oils (oleoyl macrogol-6 glycerides, propylene glycol monocaprylate, propylene glycol monolaurate), surfactant (caprylocaproyl macrogol-8 glycerides) and cosurfactant (polysorbate 80 or macrogol 15 hydroxystearate), only formulations with acceptable selfemulsification rate (< 1 min), robustness to dilution (transparency ˃ 99%), droplet size (< 100 nm) and polydispersity index (PDI < 0.2), which proved to be stable during six-month storage at room temperature (22 ± 2 °C) were selected. Simvastatin was rapidly released from the selected liquid SMEDDS, confirming that liquid SMEDDS significantly improved the dissolution rate of simvastatin. In the second phase of the study, using the GastroPlus™ software, a drug-specific physiologically-based pharmacokinetic (PBPK) model was successfully developed, which appropriately describes the absorption, conversion to simvastatin-acid, distribution and elimination of simvastatin. After analyzing literature and experimental data, together with in silico predicted values to describe simvastatin biopharmaceutical properties and physiological characteristics of the human body, a set of input parameters used for in silico modeling was selected. The constructed PBPK model was validated by comparing the estimated values with the in vivo data on pharmacokinetic parameters for simvastatin and simvastatin-acid following oral administration of 20 mg simvastatin immediate-release tablets. In vitro drug dissolution profiles from liquid SMEDDS, filled into hard gelatin capsules, showed that these formulations improved the dissolution rate of simvastatin. However, in silico results demonstrated that this strategy would not lead to the enhancement of drug oral bioavailability compared to immediate-release tablets. In addition, simulation results showed that distal parts of the GI tract are the optimal place for simvastatin absorption, and that formulation of modified (targeted)-release oral dosage forms of simvastatin could reduce the effects of presystemic metabolism and improve oral bioavailability of this drug. In the third phase of the study, modified-release lipid-based drug delivery systems were formulated by filling the selected liquid SMEDDS into acid-resistant capsules, as well as by mixing the selected liquid SMEDDS with pH-dependent methacrylic acid copolymers. Eudragit® L100 and Eudragit® S100 polymers, as well as their combination (in the ratio 1:1) were used, whereas the ratio SMEDDS:polymer was 2:1. In vitro dissolution results under medium-change conditions showed that liquid SMEDDS filled into acid-resistant capsules enabled complete drug release after 90 min in medium pH 6.4. Comparison of the in vitro dissolution test results for formulations obtained by mixing liquid SMEDDS with Eudragit® polymers filled into hard gelatin capsules, pointed out that there was a clear relationship between simvastatin release rate and the type of Eudragit® polymer. Simvastatin was completely released from all formulations with Eudragit® L100 after 180 min in medium pH 6.9. The presence of Eudragit® S100 in the formulations enabled complete simvastatin release after 300 min in medium pH 7.4. .