For more than 100 years dithranol has been successfully used for the treatment of psoriasis . It is still not fully understood, however, how it exerts its antipsoriatic effect. From the information available to date it is clear that the actual process of decomposition is central to the therapeutic action of dithranol. Since there is an obvious parallel between the mechanism of decomposition in solution and that of decomposition/metabolism in skin stability studies were carried out in aqueous buffer (PH 5.5). The effect of various factors such as the presence of various metal ions and surfactants on the decomposition pattern (individual rates of dimer and danthron formation) of dithranol in solution were quantified. The effect of, in particular, surfactants on the skin permeation and decomposition/metabolism of dithranol were then investigated. Because of dithranol's poor water solubility it was necessary to develop an analytical technique capable of the successful quantification of low levels of dithranol and breakdown products in aqueous systems. A previously reported hplc system, which has adequate sensitivity for dithranol and danthron quantification, was used. Improved sensitivity for dimer (3 fold increase), thus allowing the accurate quantification of low levels of dimer in aqueous solutions, was achieved by a modification of the mobile phase. As it was also necessary to establish the amounts of dithranol and breakdown products on and in skin an appropriate extraction method was needed. A procedure whereby skin was first extracted using a mixture of trichloro acetic acid and methanol followed by hplc analysis of the extract was developed. Surfactant solubilisation of dithranol was used to enhance the water solubility of dithranol. Data from the solubilisation studies shows that in sodium lauryl sulphate and tween 80 (above c. m.c in aqueous buffer pH 5.5) the amount of dithranol in solution is directly proportional to the % of surfactant present. In cetrimide, solubilisatiori was observed only at low pH e.g pH 0.4. At pH 5.5, because of the interaction that takes place between dithranol and cetrimide causing dithranol to ionise, dithranol ionisation is responsible for enhanced solubility. About 3 fold more dithranol goes into such cetrimide solutions compared to equivalent concentrations of sodium lauryl sulphate and tween 80. Surfactant presence, however, had implications on dithranol's decomposition pattern. In the sodium lauryl sulphate and tween 80 solutions little change from that seen in buffer pH 5.5 was observed namely - 94 % dimer and - 4 % danthron were produced following complete decomposition. In the presence of cetrimide (PH 5.5) a marked change was seen with - 84% danthron and -14% dimer being formed on complete decomposition. The effect of the inclusion of metal ions i.e Cu2+, Zn2+ and Fe2+ on the kinetics of the decomposition of dithranol to dimer and danthron were quantified. All had a catalytic effect on the rate of dimer formation, while suppressing that of danthron. The catalytic coefficients were in the order of Cu2+ > Fe2+ > Zn2+. Concentration vs time data generated on placing surfactant solutions of dithranol in contact with animal skin (in vitro) and human skin (in vivo) allowed estimates of the amount of dithranol and breakdown products penetrating into the skin, along with the degree of skin surface decomposition taking place during the permeation process. A pronounced deviation was observed for dithranol decomposition in the formulations on the skin and when not in contact with skin. Skin surface decomposition was found to result in the formation of an ,as yet unknown, breakdown product (P4). Using the 12-0-tetradecanoylphorbol-13-acetate/hairless mouse psonasls model it was visually established that the cetrimide-dithranol formulation negated the anti inflammatory effects of dithranol . The tween 80 and sodium lauryl sulphate dithranol formulations reduced the inflammatory response in the psoriasis model to the same degree as an equivalent amount of dithranol delivered in acetone. A preliminary clinical investigation on the influence of cetrimide on the therapeutic outcome of conventional dithranol therapy was carried out using two patients with psoriasis. Cleansing the skin with a solution of cetrimide before and after treatment with dithranol resulted in both a reduction of side effects and a loss in the therapeutic effectiveness of dithranol. The data gathered allowed the discussion of the effect of dithranol's decomposition pathway on its therapeutic outcome.

A large part of the work presented in this thesis describes the development and use of a novel electrochemical detector designed to allow the electrochemical characterisation of compounds in flowing solution by means of cyclic voltammetry. The detector was microprocessor controlled, which provides digital generation of the potential waveform and collection of data for subsequent analysis. Microdisk working electrodes are employed to permit both thermodynamic and kinetically controlled processes to be studied under steady-state conditions in flowing solutions without the distortion or hysteresis normally encountered with larger sized electrodes. The effect of electrode size, potential scan rate, and solution flow rate are studied extensively with the oxidation of ferrocene used as an example of a thermodynamically controlled process and a series of catecholamines as examples of a kinetically controlled process. The performance of the detector was best demonstrated when used as a HPLC post-column detector. The 3-dimensional chromatovoltammograms obtained allow on-line characterisation of each fraction as it elutes from the column. The rest of the work presented in this thesis involves the study of the oxidative degradation pathway of dithranol. The oxidative pathway was shown to involve a complex free radical mechanism, dependent on the presence of both oxygen and, in particular light. The pathway is further complicated by the fact that dithranol may exist in either a keto or enol form, the enol being most susceptible to oxidation. A likely mechanism is proposed from studies performed with cyclic voltammetry and controlled potential electrolysis, then defined by subsequent kinetic studies.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Dithranol is very effective drug for the topical treatment of psoriasis. However, it has some adverse effects, such as irritation and stain in the skin that difficult its application and patient adherence to treatment. Its instability to light, high pH values, metals and the presence of oxygen, configure as a limiting step for use. So, the inclusion of drug in nanocarriers was the main objective of this work. Lipid core nanocapsules and nanoemulsions containing 0.5 mg/mL of dithranol and 0.05% of EDTA or 0.02% of ascorbic acid were prepared by interfacial deposition of preformed polymer and spontaneous emulsification methods, respectively, and evaluated in relation to its physicochemical characteristics (drug content, encapsulation efficiency, pH, mean size, polydispersity index and zeta potential). The nanocapsules, after preparation, showed satisfactory characteristics: drug content near to the theoretical concentration, encapsulation efficiency about 100%, nanometric mean size (220- 250 nm), polydispersity index below 0.25, negative zeta potential, and pH values from 5.6 to 4.4. Instead, low drug content was verified for the nanoemulsions (approximately 80%) after preparation. In photodegradation study against UVA light it was observed a higher stability of the dithranol-loaded nanocapsules comparing to solution containing the free drug (t1/2 = 4 and 1 h for nanocapsule and free drug solution containing EDTA, respectively; t1/2 = 17 and 7,5 h for nanocapsule and free drug solution containing ascorbic acid, respectively). Irritation test by HET-CAM method was conducted to evaluate the safety of the formulations. From the results it was found that nanoencapsulation of the drug decreased its toxicity compared to the effects observed for free drug. Subsequently, hydrogels containing nanocapsules were prepared employing Carbopol® 940 and Aristoflex® AVC as gel-forming polymers. The semisolid formulations showed suitable properties for topical application and higher stability when compared to nanocapsules suspensions and the hydrogel containing the free drug. Furthermore, a higher stability of dithranol was verified for hydrogels prepared with Aristoflex® AVC. / O ditranol é um fármaco muito eficaz no tratamento tópico da psoríase. Entretanto, apresenta alguns efeitos adversos, como irritação e manchas na pele que dificultam sua utilização e adesão dos pacientes ao tratamento. Sua instabilidade frente à luz, altos valores de pH, metais e a presença de oxigênio, configuram, também, um passo limitante para o seu uso. Desta forma, a inclusão do fármaco em nanocarreadores constituiu o principal objetivo deste trabalho. Nanocápsulas de núcleo lipídico e nanoemulsões contendo 0,5 mg/mL de ditranol e 0,05% de EDTA ou 0,02% de ácido ascórbico foram preparadas pelos métodos de deposição interfacial do polímero pré-formado e emulsificação espontânea, respectivamente, e avaliadas em relação as suas características físico-químicas (teor de fármaco, eficiência de encapsulamento, pH, diâmetro médio de partícula, polidispersão e potencial zeta). As nanocápsulas, após preparação, apresentaram características satisfatórias: teor de fármaco próximo ao teórico, eficiência de encapsulamento de, aproximadamente, 100%, diâmetro de partícula na faixa nanométrica (220-250 nm), índice de polidispersão abaixo de 0,25, potencial zeta negativo e valores de pH de 5,6 a 4,4. Ao contrário, um baixo teor de fármaco foi verificado para as nanoemulsões (aproximadamente, 80%) após preparação. No estudo de fotodegradação frente à luz UVA se observou uma maior estabilidade do fármaco nas nanocápsulas em comparação à solução do fármaco livre (t1/2 = 4 e 1 hora para a nanocápsula e solução do fármaco livre contendo EDTA, respectivamente; t1/2 = 17 e 7,5 horas para a nanocápsula e solução do fármaco livre contendo ácido ascórbico, respectivamente). O ensaio de irritação pelo método de HET-CAM foi realizado para a avaliação da segurança das formulações. A partir dos resultados verificou-se que a encapsulação do fármaco diminuiu sua toxicidade em relação aos efeitos observados para o fármaco livre. Posteriormente, hidrogéis contendo as nanocápsulas foram preparados empregando-se Carbopol® 940 e Aristoflex® AVC como polímeros formadores de gel. As formulações semissólidas desenvolvidas apresentaram propriedades adequadas para a aplicação tópica e maior estabilidade quando comparadas às suspensões de nanocápsulas e ao hidrogel contendo o fármaco livre. Além disso, uma maior estabilidade do ditranol foi verificada para os hidrogéis preparados com Aristoflex® AVC.