A member of the class of phosphonic acids that is methylphosphonic acid in which one of the methyl hydrogens is replaced by a [(2R)-1-(6-amino-9H-purin-9-yl)propan-2-yl]oxy group. An inhibitor of HIV-1 reverse transcriptase, the bis(isopropyloxycarbonyloxymethyl) ester (disoproxil ester) prodrug is used as the fumaric acid salt in combination therapy for the treatment of HIV infection.