An active metabolite, ddATP, of didanosine that is an analogue of purine-nucleoside (a component of nucleic acid) was known to inhibit the activity of DNA polymerase for E. coli. In 1985, Dr. Michiya et al. of NCI reported that didanosine and ddA inhibited replication of the human immunodeficiency virus (HIV). This discovery led to the clinical application of both the compounds. Didanosine, after being uptaken into a cell, becomes an active metabolite, ddATP, to inhibit a reverse transcriptase of HIV. Compared with zidovudine, didanosine has weak cytotoxicity both in vitro and in vivo. Didanosine, which is recommended as a first-line therapy drug in the Japanese Guideline on an anti-HIV Infection Therapy, was approved as twice-daily Videx Tablet and Dry Syrup formulations for launch in June 1992. In March 2001, a once-daily Videx EC Capsule formulation was approved and launched, having expected adherence improvements in HIV/AIDS patients.