Current treatment centres around therapies toxic to human beings:
Leishmaniasis current therapy is mainly based on the systemic administration of toxic pentavalent antimonials or amphotericin B, drugs with several side effects, such as arrhythmia, nephro- and hepatotoxicity. Additionally, emergence of Leishmania strains resistant to antimonials has been reported. Recently, miltefosine has been approved in India for the therapy of visceral leishmaniasis, but its efficacy on the treatment of American cutaneous leishmaniasis has been shown to be variable depending on the causative species. Therefore, new alternatives for the treatment of leishmaniasis are greatly needed. (Miguel DC, Yokoyama-Yasunaka JKU, Uliana SRB (2008) Tamoxifen Is Effective in the Treatment of Leishmania amazonensis Infections in Mice. PLoS Negl Trop Dis 2(6): e249.)
Dr. Silvia Uliana has been working towards finding less-toxic treatments for Leishmaniasis. Recently, she and her colleagues found that Tamoxifen offers effective treatment of the disease at decidedly less physical cost to the patient. The paper, Tamoxifen Is Effective in the Treatment of Leishmania amazonensis Infections in Mice, was published in PLoS Neglected Tropical Diseases.
Scientist Live spoke to Dr. Uliana about her research.
How did you first come to work on this project?
This work is part of a research project that was established in my lab about 5 years ago. The aim was to investigate alternative candidates for the treatment of leishmaniasis. This particular arm of the project has been conducted by Danilo Ciccone Miguel, a PhD student in the lab, who has contributed a great deal in the development of the study.
What are the challenges of treating leishmaniasis?
To my mind, the main difficulties in the treatment of leishmaniasis are: (a) the classically available treatments (pentavalent antimonials, amphotericin B and pentamidin) have to be used by the parenteral route and are administered for a long period of time (15-30 days). Ideally, patients should receive treatment in hospitals and that is clearly unfeasible in most endemic areas. (b) These drugs exhibit several undesirable effects, some of which are potentially life-threatening. (c) Widespread resistance to the previously considered first-line drugs, i.e. pentavalent antimonials, has been identified in India. We do not know, as yet, whether resistance to antimonials is occurring in other areas as well. Considerable progress was achieved with lipid formulations of amphotericin B, with a major reduction in the doses required for treatment and consequently in the side effects. However, these formulations are unaffordable for the majority of the population affected by leishmaniasis. In recent years, miltefosine has been approved for the treatment of visceral leishmaniasis in India. The major advance here is that miltefosine is administered by the oral route. Its efficacy in other regions is still under testing.
Why have treatments involved high-toxicity drugs such as pentavalent antimonials, amphotericin B or pentamidine?
Because there were no other alternatives.
What led you to believe tamoxifen might be effective against Leishmania amazonensis?
The decision to test tamoxifen against Leishmania came from two findings reported previously in the literature. The first was that tamoxifen could induce the production of nitric oxide in tumor cells (Loo et al., 1998) and nitric oxide is the main effector against intracellular Leishmania. The second was a description of the effect of tamoxifen in reverting drug resistance to adriamycin in breast cancer cell lines (Altan et al., 1999; Chen et al., 1999). This effect was dependent upon a rapid alkalinization of acidic organelles where the drug was concentrated inside the resistant cells. With the pH shift, the drug was redistributed throughout the cell and regained function. That caught our attention because in the mammalian host, Leishmania amastigotes live inside an acidic organelle, resulting from the fusion of the parasitophorous vacuole and lysosomes. Amastigotes are adapted to survive in this acidic pH. So we presumed they could be affected by a pH shift promoted by tamoxifen. That was indeed the case. The surprise was, however, that apart from that effect, tamoxifen was also active against the parasites through other mechanisms that are not yet completely clear.
What were the key challenges to your study?
The difficulties in finding alternatives for the treatment of leishmaniasis lie mainly on the available experimental models. In vitro experiments can be used as an indication of drug activity but proof can only be derived from animal testing. Moreover, mouse infection with some species of Leishmania does not reproduce the disease characteristics in humans.
Can you please discuss your findings within the greater context of leishmaniasis research?
Finding alternatives for treating leishmaniasis is irrefutably a major goal for many groups working in tropical diseases. Research on leishmaniasis therapy has gained strength in the last years as a consequence of efforts from the World Health Organization in partnership with other groups or corporations. As a result, liposomal amphotericin B and miltefosine have been approved for this indication. But there is still room for other alternatives, due to the high cost of lipid formulations of amphotericin B and to the teratogenicity of miltefosine. In this context, the demonstration that a drug already in human use for over 30 years can work in this setting may be of great value.
What possibilities open up as a result of your lab's findings?
We are concluding the investigation on the efficacy of tamoxifen for the treatment of infections by other species of Leishmania in different rodent experimental models. If we find that tamoxifen in also active in these other models, the next step would be to proceed to trials. There is already a large clinical experience in the use of tamoxifen in women. Clinical trials will have to ascertain the safety of tamoxifen for men and children. Pending on that, clinical trials on leishmaniasis patients should be designed and carried out.
Finally, what is next for your lab in terms of research?
On one hand, we are trying to understand the mechanisms of action of tamoxifen against Leishmania. At the same time, we would like to investigate other selective estrogen receptor modulators for this application and for that we would welcome the collaboration of laboratories that have the know-how to synthesize these molecules.
Loo SA, Lesoon-Wood LA, Cooney RV. Effects of tamoxifen on nitric oxide synthesis and neoplastic transformation in C3H 10T1/2 fibroblasts. Cancer Lett. 1998;122(1-2):67-75.
Altan N, Chen Y, Schindler M, Simon SM. Tamoxifen inhibits acidification in cells independent of the estrogen receptor. Proc Natl Acad Sci U S A. 1999;96(8):4432-7.
Chen Y, Schindler M, Simon SM. A mechanism for tamoxifen-mediated inhibition of acidification. J Biol Chem. 1999;274(26):18364-73.