Antileishmanial Evaluation of the Leaf Latex of Aloe macrocarpa, Aloin A/B, and Its Semisynthetic Derivatives against Two Leishmania Species

The currently available antileishmanial drugs are either toxic or too expensive for routine use in developing countries where the disease is most common. Local people in the Somalia region of Ethiopia use the leaves of Aloe macrocarpa Todaro for the treatment of malaria, jaundice, and skin diseases. In our ongoing search for new, efficient, and safe antileishmanial drugs, we investigated the leaf latex of Aloe macrocarpa and its acid-hydrolyzed product aloin A/B (1), as well as the semisynthesized derivatives of aloin A/B, namely, aloe-emodin (2) and rhein (3) against promastigotes and axenically cultured amastigotes of Leishmania aethiopica and L. donovani clinical isolates. Activity study was carried out based on the fluorescence characteristic of resazurin added to drug-treated cultures. Oxidative hydrolysis of aloin A/B by ferric chloride and concentrated hydrochloric acid afforded aloe-emodin (2), which was further oxidized using sodium nitrite and concentrated sulfuric acid to furnish rhein (3). Cytotoxicity study of test substances was performed against human monocytic cell line THP-1 using Alamar Blue and cell viability was measured fluorometrically. The test compounds showed lower activity (IC(50) = 6.7 to 12.1 μM for promastigotes and IC(50) = 3.6 to 10.2 μM for axenic amastigotes) than the reference drug amphotericin B (IC(50) = 1.3 to 2.7 μM). However, amphotericin B (LC(50) = 11.1 μM) was much more toxic than the test compounds (LC(50) = 369.2 – 611.6 μM) towards human monocytic cell line (THP-1) despite its efficiency. As demonstrated in the current study, high selectivity indices (SIs) of the test compounds represent a remarkable advantage over the reference drug and highlight their potential use as templates for further development of safe leishmanicidal drugs.