How do bisphosphonates work?

My dentist explained to me that the exact way that they work isn't clearly understood--like a lot of other things, I guess.  They affect the bone metabolism by reducing resorption.  Resorption, an artifact of aging and disease, is when bone material is resorbed into the metabolic process, lessening the bone volume and density.  I think that the person who told you that dead cells attach to the bone is correctly describing what the effect is--the bone is "clad" but the armor is not new bone growth, it's more like repurposed old bone.

As I've said before, I'm not in the medical or surgical biz--just a curious patient.  I have had many many discussions with the dentist and oncologist about bisphosphonates, before they took my Actonel away from me because of all my dental problems.

Apparently, there is increasing awareness that because no one knows exactly the mechanism that biphosphonates use to affect bone metabolism, they also don't know the correct dosage for most people.  In normal patients, that's probably not a real problem--if you go over or under, you might have some se's or you might not get the job done.  But in the cases of folks who a.) get the bisphosphonate intravenously and in high dose/low frequency schedule, and b.) get too much for their particular skeletal metabolism, and c.) require and receive a dental procedure such as an extraction, the bisphosphonate is bad news along with the good.  It can result in irreversible necrosis of the jaw.  Meaning, it actually kills the jaw bone.

According to my onc and dentist, the risk/benefit highly favors use of bisphosphonates in the case of bone mets.   Not so much in early stage recurrence prevention, when someone has dental engineering like mine.  Also, no cases of ONJ have been reported in the first two years of use of the oral versions.  Also, my dentist said that when you have to take it for osteoporosis, the safest way is in smaller and more frequent doses, and orally. 

I believe there are opinions all over the map on this, I'm just giving you the info I was given and that's probably more than you want to know!  Here's an excerpt from an older web post:

 The bisphosphonates are a class of chemicals that share a basic phosphate-carbon-phosphate core and bind strongly to calcium. Over the past two decades, these drugs have assumed a significant role in the treatment of osteoporosis because of their strong avidity for the skeleton and their relative ease of use. Although the precise mechanism of action is not completely understood, these agents strongly inhibit osteoclast-mediated bone resorption.¹ As the bone remodeling cycle is slowed, formation and resorption are decreased, although resorption is inhibited more than formation. This process leads to a modest increase in bone mineral density (BMD).¹The osteogenic action of bisphosphonates and their relative lack of activity outside the skeleton have led to the increased use of these agents in treating osteoporosis."  

I worry too about cell growth factors. I'll check to see if any studies are being done, but it's hard to get answers to a question that hasn't been in a trial. There is so much that advocates don't know. For one, will the cell growth cause cancer cell growth? Heck, I'm afraid to eve drink green tea these days! lol

CBM, good info thanks, and you nailed it on dosages as well! That is a major problem with cancer treatments!