Recebido em 24/5/04; aceito em 14/9/04; publicado na web em 17/2/05 LIPOSOMES: PHYSICOCHEMICAL AND PHARMACOLOGICAL PROPERTIES, APPLICATIONS IN ANTIMONY-BASED CHEMOTHERAPY. The use of organoantimonial complexes in the therapeutic of leishmaniasis and schistosomiasis has been limited mainly by the need for daily parenteral administration, their adverse side-effects and the appearance of drug resistance. Liposome encapsulation has been so far the most effective means to improve the efficacy of pentavalent antimonials against visceral leishmaniasis. Pharmacologically-and pharmaceutically-acceptable liposomal compositions are still being investigated through manipulation of preparation method, lipid composition and vesicle size. Recently, the encapsulation of a trivalent antimonial within "stealth" liposomes was found to reduce its acute toxicity and effectively deliver this compound to the parasite in experimental schistosomiasis.Keywords: liposomes; antimony; chemotherapy. INTRODUÇÃONo início do século passado, Gaspar Vianna, pesquisador pioneiro em doença de Chagas e leishmaniose, relatou a eficácia do complexo de antimônio trivalente (Sb(III)), tártaro emético, no tratamento da leishmaniose muco-cutânea 1 . Da mesma forma, o tártaro emético foi o primeiro medicamento empregado com êxito no tratamento da esquistossomose 2,3 . Entretanto, o uso clínico deste composto foi interrompido, por causa de seus severos efeitos colaterais e da descoberta de novos fármacos menos tóxicos.A partir da década de 1940, complexos de antimônio pentavalente (Sb(V)) começaram a ser utilizados na terapêutica das leishmanioses 4 . Os principais antimoniais atualmente em uso são complexos de Sb(V) com o N-metil-glucamina (antimoniato de meglumina) e com o gluconato de sódio (estibogluconato de sódio). Até hoje, nem a estrutura desses compostos, nem seu mecanismo de ação foram completamente elucidados. Foi sugerido que o Sb(V) seria uma pró-droga, sendo reduzido no organismo hospedeiro a Sb(III) que seria a forma ativa e tóxica 5 . Recentemente, foi mostrado que os tióis podem estar envolvidos nesse processo de redução 6 . Embora os antimoniais pentavalentes continuem sendo os medicamentos de primeira escolha no tratamento de todas as formas de leishmanioses, o seu uso clínico apresenta várias limitações. Esses compostos devem ser administrados por via parenteral (injeção intravenosa ou intramuscular), diariamente, num período de 20-40 dias. Nesse contexto, efeitos colaterais são freqüentes Essencialmente duas estratégias diferentes estão atualmente disponíveis para o desenvolvimento de novos medicamentos. Uma estratégia envolve o planejamento/síntese de novas substâncias ativas ou de fármacos já conhecidos com modificações químicas; a outra envolve a associação reversível de fármacos já em uso a um sistema transportador, visando direcionar o fármaco para a célula alvo e evitar os locais indesejáveis onde o fármaco exerce toxicidade. Esta última estratégia, além de prolongar a validade de proteção por patente no uso do fármaco, oferece um...

The toxicity and antileishmanial effectiveness of a novel liposome formulation of meglumine antimoniate in mongrel dogs with visceral leishmaniasis (VL) obtained from a region where VL is endemic in Brazil have been investigated. Groups of 12 animals received by the intravenous route four doses (with 4-day intervals) of either liposomal meglumine antimoniate (group I [GI], 6.5 mg Sb/kg of body weight/dose), empty liposomes (GII), or isotonic saline (GIII). Evaluation of markers of hematopoietic, hepatic, and renal functions before and just after treatment showed no significant change. On the other hand, transitory adverse reactions, including prostration, defecation, tachypnea, and sialorrhea, were observed during the first 15 min after injections in GI and GII. Parasitological evaluation of sternal bone marrow 4 days after the last dose showed a significant reduction of parasite burden in GI, compared to the other groups. Immunocytochemical evaluations of the skin, bone marrow, cervical lymph nodes, livers, and spleens of dogs for parasites, 150 days after treatment, indicated significant parasite suppression (higher than 95.7%) in the lymph nodes, livers, and spleens of GI, compared to control groups. Feeding of Lutzomyia longipalpis phlebotomines on dogs from GI, 150 days after treatment, resulted in a significant reduction of sand fly infection efficiency, compared to feeding on animals from GII and GIII. This is the first report of both long-term parasite suppression and reduction of infectivity to sand flies in naturally infected dogs following treatment with a liposome-encapsulated drug. Importantly, this was achieved using a 20-fold-lower cumulative dose of Sb than is used for conventional antimonial treatment.

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The aim of the present study was to evaluate the impact of a multiple dose regimen of a liposomal formulation of meglumine antimoniate (LMA) on the pharmacokinetics of antimony in the bone marrow of dogs with visceral leishmaniasis and on the ability of LMA to eliminate parasites from this tissue. Dogs naturally infected with Leishmania chagasi received 4 intravenous doses of either LMA (6.5 mg antimony/kg body weight, N = 9), or empty liposomes (at the same lipid dose as LMA, N = 9) at 4-day intervals. A third group of animals was untreated (N = 8). Before each administration and at different times after treatment, bone marrow was obtained and analyzed for antimony level (LMA group) by electrothermal atomic absorption spectrometry, and for the presence of Leishmania parasites (all groups). There was a significant increase of antimony concentration from 0.76 µg/kg wet organ (4 days after the first dose) to 2.07 µg/kg (4 days after the fourth dose) and a half-life of 4 days for antimony elimination from the bone marrow. Treatment with LMA significantly reduced the number of dogs positive for parasites (with at least one amastigote per 1000 host cells) compared to controls (positive dogs 30 days after treatment: 0 of 9 in the LMA group, 3 of 9 in the group treated with empty liposomes and 3 of 8 in the untreated group). However, complete elimination of parasites was not achieved. In conclusion, the present study showed that multiple dose treatment with LMA was effective in improving antimony levels in the bone marrow of dogs with visceral leishmaniasis and in reducing the number of positive animals, even though it was not sufficient to achieve complete elimination of parasites.

The achievement of complete cure in dogs with visceral leishmaniasis is currently a great challenge, since dogs are the main reservoir for the transmission of visceral leishmaniasis to humans and they respond poorly to conventional treatment with pentavalent antimonials. In order to improve the efficacy of treatment, we developed a novel formulation for meglumine antimoniate based on the encapsulation of this drug in freeze-dried liposomes (LMA). The aim of the present study was to evaluate the biodistribution of antimony (Sb) in dogs following a single intravenous bolus injection of LMA. Four healthy male mongrel dogs received LMA at 3.8 mg Sb/kg body weight and were sacrificed 3, 48 and 96 h and 7 days later.