In vitro activity of pyrvinium pamoate against Entamoeba histolytica and Giardia intestinalis using radiolabelled thymidine incorporation and an SYBR Green I-based fluorescence assay

Objectives: To assess the in vitro activity of the FDA-approved antihelminthic drug pyrvinium pamoate against Entamoeba histolytica and Giardia intestinalis. Methods: A head-to-head comparison of a standard radiolabelled thymidine incorporation assay and the SYBR Green I-based fluorescence assay for determination of in vitro inhibition by pyrvinium and metronidazole was performed. Results: The 50% inhibitory concentration (IC₅₀) for treatment of E. histolytica with pyrvinium was 4-5 μM for both assays compared with 1-2 μM for metronidazole. For pyrvinium treatment of G. intestinalis, an IC₅₀ of ∼12 μM was determined by the radiolabelled thymidine assay alone, with maximum inhibition around 60%. In contrast, the IC₅₀ for metronidazole treatment using this assay was ∼2 μM. Conclusions: Pyrvinium is a potential gut lumen agent for treatment of intestinal amoebiasis, but possibly not for giardiasis. SYBR Green I is an alternative screening method for E. histolytica, but not G. intestinalis.