In the next several months, if you have rheumatoid arthritis or osteoporosis, you will be hearing more about a substance called RANK ligand.

Let me explain. Bone is a living tissue. New bone is constantly replacing old bone. This is done through a series of processes called: activation, resorption, reversal, and formation.

In activation, osteoclasts, cells that chew up old bone are recruited to the site of old bone.

Resorption is the phase where old bone is removed.

Reversal is the next step. Osteoclasts, having completed their task of removing old bone, now begin to die and are replaced with osteoblasts.

Formation is when osteoblasts begin to lay down new bone.

It turns out that osteoclasts have receptors on their surface which are activated by a substance called RANK ligand. Once osteoclasts come into contact with RANK ligand, they multiply and become activated.

When osteoclasts are activated, they lead to bone loss. Many different factors such as parathyroid hormone, tumor necrosis factor, and interleukin-1 cause bone loss. They do this by activating RANK ligand.

What controls this process and prevents total bone destruction is that the body has a naturally protective protein, osteoprotegrin, which neutralizes the effects of RANK ligand. Osteoprotegrin acts as a decoy receptor by binding with RANK ligand and preventing the activation of osteoclasts.

So the balance between RANK ligand and osteoprotegrin determines whether there is net bone loss or bone gain.

Conditions where there is a deleterious imbalance so that RANK ligand overwhelms osteoprotegrin occurs with disease states such as estrogen deficiency, glucocorticoid use (cortisone), rheumatoid arthritis, and bony metastases (spread of cancer to bone).

What's very interesting- and how all this relates to rheumatoid arthritis- is that osteoprotegrin is a member of the tumor necrosis factor receptor family.

So what does this have to do with you?

If you have either rheumatoid arthritis or osteoporosis, a new drug called denosumab, will possibly be approved by the FDA within the next year. This drug is an antibody that blocks the effects of RANK ligand. What that does is reduce the effects of osteoclasts so that less bone loss occurs.

Denosumab is given by subcutaneous injection, so that the patient can give themselves the shot or- if they're needle-phobic- they can elect to get the shot in the doctor's office. The frequency of injection is once every six months.

Where denosumab will fit in treatment is difficult to say right now.

For people with osteoporosis, it may be used after a patient has either failed or has become intolerant to drugs like Fosamax, Actonel, or Boniva. But since it really doesn't build new bone, it will not replace parathyroid hormone (Forteo).

Patients with rheumatoid arthritis likely might benefit from the bone protective effects since rheumatoid arthritis causes erosions in bones. However, many of the anti-TNF drugs used already in rheumatoid arthritis do the same thing so that denosumab use in rheumatoid arthritis is less predictable.