LY217896 (1,3,4-thiadiazol-2-ylcyanamide) is a 2-substituted thiadiazole that is an effective inhibitor of influenza A and B viruses in vitro and in the mouse infection model. The in vitro anti-influenza activity of LY217896 is reversed by a 10-fold excess amount of guanine or guanosine. LY217896 (1 or 10μg ml−1) effected a selective 60% decrease in the levels of intracellular pools of GTP in MDCK cells. The extent of cytotoxicity of LY217896 is positively correlated with the amount of LY217896 metabolite formed intracellularly. A cell line, derived from parental MDCK cells, was selected for resistance to 50 ng of LY217896 per ml. Unlike parental MDCK cells, the resistant cells were able to undergo log phase replication in LY217896 (25 g ml−1) and were unable to metabolize the compound. Furthermore, LY217896 had no antiviral activity against influenza A/Ann Arbor (IC50 >200μg ml−1) or vaccinia virus (IC50 = 13 μg ml−1) in resistant cells. In contrast, LY217896 inhibited influenza A/Ann Arbor (IC50 = 0.5 μg ml−1) or vaccinia virus (IC50 = 0.13 μg ml−1) in the parental MDCK cells. A thiadiazole, with a guanidinyl group in the 2 position, and ribavirin were active in both the parental cells and resistant cells. Nicotinamide (up to 240-fold excess) did not reverse the anti-influenza activity of LY217896 in vitro or in the mouse infection model. A 10-fold excess of nicotinamide reversed the cytotoxicity of 2-aminothiadiazole but not that of LY217896.

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LY217896 is a substituted thiadiazole compound with anti-influenza activity in vitro and in the mouse model of infection. LY297336 is a ribosylated (N-4) derivative of LY217896. A highly polar intracellular metabolite of LY217896 was isolated by HPLC, and mass spectral analysis and treatment of the metabolite with alkaline phosphatase showed that it was a monophosphate (LY307987) derived from LY217896. The formation of LY307987 was inhibited by 43 and 63% when 10 μm of LY217896 was incubated with 100 μM of 8-aminoguanosine (8AGuo) and guanine (Gua), respectively, whereas inosine (Ino) and hypoxanthine (Hx) had no effect on the formation of LY307987. LY217896 inhibited the incorporation of [14C]-Hx into nucleic acids in cells which metabolize LY217896; however, LY217896 did not inhibit the formation of inosine 5′-monophosphate (IMP) from Hx in a cell-free HGPRT (hypoxanthine-guanine phosphoribosyltransferase)-catalysed reaction. Incubation of MDCK cells with 10 μm of LY217896 resulted in an 8-fold increase in the level of intracellular IMP. At 100 μm, neither LY217896 nor LY297336 inhibited inosine 5′-monophosphate dehydrogenase (IMPDH) and only cellular extracts which contained intracellular metabolites of LY217896 inhibited IMPDH. Quantification of the 5-phosphorylribose pyrophosphate (PRPP) levels in BS-C-1, MDCK, and MCN cells showed a positive correlation between PRPP concentration and cellular metabolism of LY217896. Combination studies of LY217896 with 2′,3′-dideoxyinosine (ddlno) or 2′,3′-dideoxyguanosine (ddGuo) showed that LY217896 enhanced the antiretroviral activities of these dideoxynucleosides, which is consistent with an inhibitory effect on IMPDH.