Miglioramento delle caratteristiche farmacocinetiche e della tollerabilità digestiva della diacereina attraverso modificazione dello stato solido

Abstract

Lo scopo di questa tesi è stato principalmente quello di studiare l’interazione tra la reina e il sucralfato. Questo obiettivo è stato perseguito attraverso diversi studi di caratterizzazione al fine di valutare la fattibilità di un approccio formulativo che preveda il sequestro della reina da parte del sucralfato a livello del colon. Il sucralfato è stato scelto come farmaco modello per il miglioramento della tollerabilità della diacereina. Le sue caratteristiche lo rendono molto promettente per contrastare l’effetto indesiderato della diacereina, sia per quanto riguarda la sua capacità di proteggere la mucosa gastrointestinale che per la sua tendenza a formare composti di interazione con diversi tipi di molecole. Nella prima parte del lavoro sono state eseguite analisi sulle sostanze pure ed è stato allestito un metodo per studiare l’interazione tra reina e sucralfato. Gli studi in soluzione hanno prodotto campioni i quali sono stati esaminati mediante spettrofotometria UV-vis. In seguito sono state condotte analisi al fine di ottenere informazioni sulle attrazioni intermolecolari del complesso. Questo tipo di studi hanno riguardato dapprima una parte sperimentale in cui le soluzioni di interazione sono state analizzate mediante spettrofotometria UV, HPLC ed NMR in soluzione. Inoltre, sono stati condotti esperimenti d’interazione tra reina e alluminio idrossido per valutare il ruolo di quest’ultimo nell’interazione con la reina. Infine, l’ultima parte del lavoro ha previsto la caratterizzazione dello stato solido del composto d’interazione tra le due sostanze attraverso microscopia ottica, calorimetria differenziale a scansione (DSC), diffrazione di raggi X su polvere (PXRD).

The improvement of biopharmaceutical characteristics of drugs is one of the leading purpose of pharmaceutical research. Some active principles, though provided with significant therapeutical efficiency cannot be employed for their negative chemical, physical and biological features. Frequently, the most troublesome elements are the molecule solubility, permeability and dissolution rate but also its potential adverse effects. According to present technologies, it is possible to improve drugs solubility with methods more efficient than simple mechanical or chemical processes. Solid state chemistry (co-crystal production, polymorph research) and lipid or polymer–based formulations are the most promising approaches exploited to obtain an increase in solubility but also in dissolution rate of active pharmaceutical ingredients. Solubility and permeability improvement of a drug means the opportunity to reduce its daily administration dose. This feature grants a proportional decrement of adverse effects that can moreover be removed with particular technological operations. Some specific compounds can therefore protect from adverse effects when administered in association with another drug (for example non-steroideal anti-inflammatory drugs with gastro-protective molecules) or when administered by an alternative route that by-passes the area subjected to the side effects (injection of drugs or in-situ release). Side effects can also be avoided in a more specific way, by avoiding contact with the area where it performs its negative activity. This purpose is easily attained by taking advantage of the sequestrating ability of some compounds that bind active principles avoiding their undesired activity. Diacerein is classified as a slow-acting anti-arthritic drug. It is administered by oral route and it is metabolised in rhein by enterocytes and hepatocytes before reaching the systemic circulation. Diacerein is a safe drug with a different mechanism of action with respect to non-steroideal anti-inflammatory drugs but it is not frequently used in therapy for two principal reasons: its low solubility in water and the laxative effect carried out by its metabolite, rhein, at colon level. The improvement of biopharmaceutical characteristics and digestive tolerability of diacerein are fundamental for its safe use in therapy. A promising technique to avoid a drug adverse effects could be carried out mimicking the procedures used in detoxifying practice, namely by identifying a molecule-specific method that could remove rhein side effect at colon level. The in situ formation of an interaction product between rhein and a binding agent would create an insoluble and/or not absorbable compound that could prevent the active ingredient side effect. Sucralfate is a drug mainly used in therapy to treat and prevent gastric ulcer and also to protect gastro-intestinal membrane. Sucralfate has often demonstrated a good tendency to sequestrate drugs with formation of complexes. For these reasons sucralfate may be selected as candidate compound for the diacerein tolerability improvement. In this thesis the interaction between rhein and sucralfate has been studied. This purpose has been pursued through characterization studies in order to evaluate the opportunity to produce a pharmaceutical formulation that allows the sequestration of rhein by sucralfate at colon level. Sucralfate characteristics make it a promising drug to counteract diacerein side effect at colon level where it would also exploits its mucosa-protecting action. Analyses on raw materials have been carried out at first, then a process for rhein and sucralfate interaction study has been set up. Molecular interactions between the two drugs have been studied in solution by means of Uv-vis spectroscopy, HPLC and 1H-NMR. Experiments between rhein and aluminium hydroxide have also been conducted to evaluate the role of this moiety in the interaction process between sucralfate and rhein. Solid-state characterization has moreover been carried out to investigate the solid compound obtained from rhein and sucralfate interaction. Optical microscopy, differential scanning calorimetry and powder X-ray diffraction contributed to define the profile of the interaction product. The results collected in this thesis describe a complex pattern for rhein and sucralfate interaction. Data indicate that the two drugs interact in solution giving rise to a complex. In particular, Uv-vis spectra and solution 1H-NMR analysis provides clear evidences of the formation of said complex. The solubility of the interaction compound remains an issue that the present experimentation has not completely addressed. As a matter of fact, most studies underline the formation of an insoluble complex. Nevertheless there are some experimental evidences that support the opposed hypothesis, namely the formation of a soluble compound that subtract rhein from the detection of the analytical instrumentation. In this case the product obtained reveals a stoichiometry of one mole of rhein for two moles of sucralfate in concentrated solutions. Sucralfate content increase for more diluted solutions. Another hypothesis can be considered, that is the formation of a solid solution between sucralfate and the interaction compound. The solid phase isolated from the two ingredients interaction reveals an amorphous and glassy structure. The initial hypothesis to exploit in therapy the interaction between rhein and sucralfate, remains in any case valid to reduce the heavy side effects of rhein. The preparation of an oral pharmaceutical formulation with appropriate delivery kinetics would permit a safe use of diacerein in therapy with the complete removal of its major and more problematic side effect. This approach has however to be confirmed with an in vivo study.