Binding of the bone-seeking agent 99mTc-1-hydroxyethylidene-1,1-diphosphonic acid to cartilage and collagen in vitro and its stimulation by Er3+ and low pH.

The bone-seeking agent 99mTc-labeled 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) unexpectedly binds to particles of human articular cartilage as well as cortical bone in vitro. Collagen also sequesters this compound, suggesting that collagen contributes to the uptake of 99mTc-HEDP by cartilage and bone. Particles of the bone mineral calcium hydroxyapatite also bind 99mTc-HEDP in vitro. Pretreatment of particles with Er3+ stimulates binding in each case. Lowering the pH of incubation to pH 2 has this effect for bone, cartilage, and collagen, but not for calcium hydroxyapatite. Mechanisms additional to the simple ionic attraction between the phosphonate groups of HEDP and metal cations such as Ca2+ are responsible for the uptake of 99mTc-HEDP by body tissues.