Relapse after treatment with miltefosine for visceral leishmaniasis is associated with increased infectivity of the infecting Leishmania donovani strain

Abstract

Leishmania donovani is an intracellular protozoan parasite that causes leishmaniasis, which can range from a self-healing cutaneous disease to a fatal visceral disease depending on the infecting species. Miltefosine is currently the latest and only oral antileishmanial that came out of drug discovery pipelines in the past few decades, but recent reports indicate a significant decline in its efficacy against visceral leishmaniasis (also known as kala-azar) in the Indian subcontinent. This relapse rate of up to 20% within 12 months after treatment was shown not to be related to reinfection, drug quality, drug exposure, or drug-resistant parasites. We therefore aimed to assess other phenotypes of the parasite that may affect treatment outcome and found a significant association between the number of metacyclic parasites, parasite infectivity, and patient treatment outcome in the Indian subcontinent. Together with previous studies on resistance of L. donovani against pentavalent antimonials, these data suggest that the infectivity of the parasite, or related phenotypes, might be a more determinant factor for treatment failure in visceral leishmaniasis than drug susceptibility, warranting a reassessment of our current view on treatment failure and drug resistance in leishmaniasis and beyond.

Importance: The high miltefosine relapse rate poses a major challenge for the current Kala-Azar Elimination Program in the Indian subcontinent and other leishmaniasis control programs worldwide. This relapse rate could not be related to reinfection, drug-resistant parasites, or reduced treatment quality. Here we report that an increased infectivity of the parasite is associated with miltefosine relapse of visceral leishmaniasis (VL) patients. These results supplement those obtained with antimonial-resistant L. donovani where an increased infectivity was also observed. This challenges the current view of Leishmania drug susceptibility being the biggest parasitic factor that contributes to treatment failure in leishmaniasis. These selected more infectious parasites may pose an additional burden to leishmaniasis control programs, highlighting the importance of multifaceted control measures to achieve leishmaniasis elimination in the Indian subcontinent and other regions where leishmaniasis is endemic.

FIG 1

(A) Proportion of metacyclic parasites in stationary-phase promastigote cultures. (B) Percentage of in vitro infected macrophages 120 h postinfection. (C) Parasite susceptibility to SSG. Error bars indicate the standard errors of the means, and P values were calculated with a Mann-Whitney U test. Each symbol represents the value for an individual patient. The horizontal bars in panels A and B indicate the mean values for the groups. The broken line in panel C indicates the cutoff to determine whether a strain is SSG sensitive or SSG resistant. The # symbol indicates that the EC₅₀ of this strain sometimes exceeded the maximum concentration (60 µg/ml) to which it was exposed, and in this case, the EC₅₀ of that replicate was fixed to 60 µg/ml.