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

To assess the in vitro activity of the FDA-approved antihelminthic drug pyrvinium pamoate against Entamoeba histolytica and Giardia intestinalis. 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. The 50% inhibitory concentration (IC(50)) for treatment of E. histolytica with pyrvinium was 4-5 microM for both assays compared with 1-2 microM for metronidazole. For pyrvinium treatment of G. intestinalis, an IC(50) of approximately 12 microM was determined by the radiolabelled thymidine assay alone, with maximum inhibition around 60%. In contrast, the IC(50) for metronidazole treatment using this assay was approximately 2 microM. 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.