Rintatolimod development evolved from a 1960s synthesis by Merck & Co., a double-stranded RNA compound of inosinic and cytidylic acid residues (poly I:poly C or poly I:C). Poly I:C inhibited tumor growth by inducing IFN production, but was too toxic to use. In the mid-1970s, William A. Carter, a postdoctoral researcher at Johns Hopkins University, modified the dsRNA molecule by adding uridylic acid molecules at specific intervals along the RNA chain. The new compound, called Ampligen (for AMPLIfied GENetic activity) stimulated interferon production like poly I:C, but with much lower toxicity. It is also known as "poly I:poly C12U".