In a patient who fractures after several years of denosumab therapy, would you wait 6 months after the last denosumab injection to start an anabolic agent or could start an anabolic agent sooner?

In a patient on any antiresorptive drug who fractures, we should consider an anabolic drug. A fracture does not always mean the drug has failed but we should consider changing to an anabolic since we have good evidence that anabolics have superior fracture risk reduction. 

Denosumab is of course more complicated because of the potential rapid bone loss associated with cessation of denosumab. I personally do not wait. If I think an anabolic is appropriate I start as soon as I can. Even an anabolic may not completely protect from the bone loss of stopping denosumab. Maybe getting it started before the 6th month could be helpful but we have no good data. I do not believe in combination data because the biology of the PTH anabolics is remodeling based new bone formation. Inhibition of osteoclast resorption does not make great sense to me when I give a PTH drug. 

BMD testing is very complex and a change in BMD with an antiresorptive is mostly more mineral. An increase in BMD with a PTH anabolic is a young new bone that may not be mature with minerals for a while. The PTH drugs increase cortical porosity as part of their biology. Therefore, cortical BMD (like femoral neck or 1/3 Radius) may go down even if the PTH is working. The bottom line is BMD is not the best to determine if you are giving the patient benefit in the short term. 

I make my choice of anabolic. I like to see a rise in the PINP (by at least 10) and I ask about new fx. If no new fractures I treat with the usual 2 years and then back to an antiresorptive like denosumab or bisphosphonate.