STEREOSELECTIVE PERCUTANEOUS ABSORPTION AND TRANSDERMAL DELIVERY OF CHIRAL DRUGS

The selection of the enantiomeric or racemate form of a drug is a critical issue in the development of transdermal dosage forms. Indeed, the stratum corneum, the rate-limiting barrier to percutaneous absorption, is mainly composed by keratin and ceramides, which could potentially provide a chiral environment. Therefore, the different binding of enantiomers to keratin or the interactions with ceramides may give rise to differences in the permeation profiles of the enantiomers of chiral molecules. Whereas the implication of different physicochemical properties between enantiomers and racemate in the in vitro skin permeation profiles have been studied, other aspects such as the effect of different counter-ions of ionisable chiral drug and different enantiomer of chiral permeation enhancers on skin permeability have not yet been extensively investigated. This work was therefore focused on these two topics to deep actual understanding and to obtain information useful to rationalize the development of transdermal patches. Among the chiral drugs, propranolol (PR) and ibuprofen (IB) were selected. PR is a good candidate as its biological activity is largely associated to a single enantiomer. In humans, S-PR is about 100 times more potent than the R-enantiomer as \u3b2-blocker and the information reported in literature about the in vitro skin permeability suggest that it can be a suitable candidate to transdermal delivery. In the case of IB, largely used in the treatment of musculoskeletal injuries, only the S-enantiomer (S-IB) is therapeutically active, and extensive unidirectional chiral enantiomeric inversion of R to S occurs in vivo. Studies indicates that there aren\u2019t any evidences for the metabolic inversion after topical administration. Till now, although the biological activity of IB resides with the S-enantiomer, the topical preparations available on the market contain the racemate. This work aimed to determinate the skin permeability profiles of the selected drugs and in some case of the their salts, in order to evaluate the enantio-selectivity in the diffusion process. The salts were obtained salifing the drug, either as racemate and pure enantiomer, with different counter-ions, selected on the bases of their physico-chemical characteristics. The experiments were performed by the modified Franz cell method, using human skin epidermis as a membrane and selecting the donor phase on the bases of the drug solubility. Suitable analytical methods for the quantitative determination of the drugs were developed. As a first step the skin permeability test was conducted by using saturated or diluted solutions. On the bases of the preliminary permeability data, the formulation study of monolayer transdermal patches containing the drug in its most appropriate form , and, if necessary, the suitable promotion enhancer, was carried out

DEVELOPMENT OF ALTERNATIVE METHODS TO IN VIVO AND IN VITRO ASSAYS FOR THE PREDICTION OF SKIN PERMEABILITY TO CHEMICAL COMPOUNDS

Percutaneous absorption is an interdisciplinary topic which is relevant to a number of divergent fields. Indeed, the knowledge of the diffusion of a given compound after skin contact plays a crucial role for the risk assessment of toxic substances, the safety of cosmetic ingredients and the design and optimization of pharmaceutical dosage forms. The skin\u2019s barrier capacity is related to the molecular organization of the lipids in the extracellular space of the stratum corneum (SC). The well-known equation by Potts and Guy, correlating the SC permeability coefficient Kp to the permeant's partition coefficient and molecular volume or weight, can already explain almost 70% of the variability in the data contained in Flynn's set. Since experimental Kp values in the data set may have up to 30% variability, improving the Potts and Guy equation is difficult. Though room for a quantitative improvement of existing predictive models seems to be limited, there is still a quest for a better understanding of the skin permeation process at a molecular level. In this respect, Molecular Dynamics is a promising tool, since it naturally accounts for the non homogeneity and anisotropy of the SC. In this work, after building a stable and equilibrated model for the stratum corneum (SC) lipid bilayer, SMD simulations were performed to study the penetration of 80 permeants of known permeability coefficient through the SC. MD has been utilized before by other Authors for studying permeation of chemicals through plasma membranes, notably the work by Marrink and Berendsen, whose approach was later applied to the SC by Das, Noro and Olmsted. A screening of a large number of molecules with MD simulations, though has never been attempted to our knowledge. The goal of the screening was to assess the possibility of using MD to assist in the development of predictive equations of skin permeability, more than gaining new insight in the mechanics of SC permeation. The approach developed (for plasma membranes) by Marrink and Berendsen for the prediction of permeability couldn't be applied, though, because of the unfeasibility of Free Energy calculations from SMD simulations in the screening of 80 permeants, due to the large amount of trajectories required. Instead, SMD has been used in an indirect way as a mean to explore the conformational and property space of the permeants in the different microenvironments of the stratum corneum. SMD was also used in a direct way to calculate averaged diffusion coefficients in different regions of the stratum corneum. What proved to be the best option, was a mixed approach where the averaged diffusion coefficients in the plane parallel to the bilayer surface were calculated in different regions of the SC, and then used together with the physicochemical properties in the correlation equation. This approach has allowed us to correlate the permeability coefficient to averaged physicochemical properties, improving on existing semi-empirical methods (employing the same quantities determined experimentally). The obtained equation compares well with the Potts and Guy equation. Further improvement of the correlation coefficient seems difficult, considering the heterogeneity of the experimental data sets and the experimental error involved in the measurement of the permeability coefficient. Further study of SMD data may still allow us to gain a better understanding of the physical process. Performing multiple SMD simulations for a small number of permeants, or designing a new protocol using only equilibrium simulations in specific regions of the lipid matrix model, will allow us to make free energy calculations. So, the work done here is just a first step in the direction of actually employing Molecular Dynamics in developing viable predictive equations of skin permeability. As a final point, our work has shown that, as stated above, the uncertainty of experimental values of the permeability coefficient is too high to expect significant improvements in the predictive power of equations. The Fully Validated set represents a great advance in reducing the bias of Flynn's set and making the variability of experimental conditions explicit. We feel, though, that this is not enough. As an effect of Regulation (EC) No 1223/2009 on cosmetic products, testing of finished cosmetic products and ingredients on animals is prohibited in the European Union, as well as the marketing of cosmetic products which have been tested on animals. If governmental Authorities feel that the development of alternative methods to in vivo and in vitro assays for the determination of skin permeability to chemical compounds is worth consideration and funding, then developing a research project for measuring skin permeability coefficients of a large set of molecules in controlled and reproducible conditions would be an important step in the right direction

TRANSDERMAL AND TRANSMUCOSAL PHARMACEUTICAL DOSAGE FORMS FOR PALLIATIVE CARE IN CANCER THERAPY

Pain is recognized as one of the most distressing cancer-related syndromes and treatment side effects and is linked to decreased quality of life among patients. Despite the improvements of pain management guidelines proposed in the last decades, therapeutic issues are still unsolved, above all in the treatment of loco-regional painful symptoms. For example, a proper pharmacological therapy to treat cisplatin-induced ototoxicity is not currently available. Pain associated to cutaneous wounds is treated by an off-label use of systemic analgesics with high incidence of related side effects. Conventional dosage forms applied in the buccal cavity are unable to achieve suitable efficacy in the case of oral mucositis. Hence, there is a need to design novel drug delivery systems, which can be easily used in the clinical practice for an effective treatment of loco-regional painful syndromes. This doctoral thesis aimed to investigate the critical aspects of drug delivery correlated to three loco-regional syndromes and propose technological solutions to rationalize drug delivery. In particular, the experimental work focused on: (1) the development of a mucoadhesive microparticle suspension intended for treating oral mucositis and designed to combine the peculiarities of prolonged release mucoadhesive systems with those of an immediate release oromucosal solution; (2) the optimization of a biodegradable nanoparticle system intended to deliver resveratrol to cochlea in the therapy of cisplatin-induced ototoxicity; (3) the rationalization of the use of morphine derivatives, according to their chemical structure, in the management of cutaneous painful syndromes