Process for producing 4-amino-1-hydroxybutylidene-1,1-bisphosphonic acid and its trihydrated monosodium salt

This invention relates to an improved process for the preparation of 4-amino-1-hydroxybutylidene-1,1-bisphosphonic acid and of the trihydrated monosodium salt thereof, respectively known by the International Nonproprietary Names (INN) thereof as alendronic acid and sodium alendronate, and which are useful for the treatment and prevention of diseases involving bone resorption, such as malign hypercalcemia, Paget's disease, osteoporosis, etc. The invention relates to a process for the preparation of 4-amino-1-hydroxybutylidene-1,1-bisphosphonic acid and of the trihydrated monosodium salt thereof, consisting of reacting 4-aminobutyric acid with a phosphonation mixture formed by phosphorous acid and methanesulfonic anhydride and thereafter hydrolyzing the product of said reaction and isolating the products by adjustment of the pH.