[5-(6-aminopurin-9-yl)-2-[[[[3-[2-(2-butanoylsulfanylethylcarbamoyl)ethylcarbamoyl]-3-hydroxy-2,2-dimethyl-propoxy]-hydroxy-phosphoryl]oxy-hydroxy-phosphoryl]oxymethyl]-4-hydroxy-oxolan-3-yl]oxyphosphonic acid