One of the (many) advantages to burosumab treatment is that, unlike the old phosphorus/calcitriol treatment, it doesn’t trigger hyperparathyroidism (excessive parathyroid hormone production).
You’ve probably heard of the advice given to XLHers (and I believe also to TIO and other chronic hypophosphatemia patients) that you should NEVER take unopposed phosphorus supplements. I’ve heard a number of stories of inexpert clinicians cluelessly prescribing massive amounts of phosphorus in a desperate attempt to outrace the kidneys’ flushing of phosphorus, without also prescribing calcitriol, which opposes some of the effect of high phosphorus levels on the parathyroid glands. That combination can end up in an ever-escalating cycle of the parathyroids secreting a hormone (PTH) that tells the kidneys to dump phosphorus, only to have the phosphorus replaced with more supplements, causing the parathyroids to scramble to produce more anti-phosphorus hormones, until the glands become enlarged, and sometimes, in what’s called tertiary parathyroidism, they’re permanently triggered to secrete excessive amounts of hormone, even when the phosphorus supplements are stopped.
Even when the phosphorus and calcitriol are carefully balanced, the parathyroids may still become overactive. Plus, there’s at least anecdotal evidence that some patients have elevated PTH, presumably due to the XLH, even before they were ever given phosphorus supplements. (I’m one of those patients.)
While it appears that burosumab will not trigger the parathyroid glands the way phosphorus supplements do, some patients on burosumab may still develop hyperparathyroidism. I don’t believe anyone knows why exactly. It could be for the same (unknown) reason that some XLHers develop it even without phosphorus treatment, or it could be the result of prior phosphorus treatment. Either way, there’s now a published case report confirming that if a patient on burosumab does indeed develop hyperparathyroidism, the standard calcimimetic treatment may be safe and effective. “Combined treatment by burosumab and a calcimimetic can ameliorate hypophosphatemia … in adult XLH with tertiary hyperparathyroidism.” In that case, the patient was treated with evocalcet (similar to cinacalcet, which has been used off-label to treat hyperparathyroidism in XLH patients for many years now), which improved her hyperparathyroidism. The researchers also theorized that the untreated hyperparathyroidism was preventing the patient from getting the maximum benefit from burosumab, but were unable to say definitively whether that was true, so more research is needed.
I’m particularly interested in the issue, since my parathyroid hormone levels have risen over the last two years. The levels are reasonably well controlled, but I do have to wonder whether even a slight but chronic elevation of PTH might interfere with my getting the maximum benefit from burosumab, and what that means for patients generally when it comes to fine-tuning the burosumab dosing. It’s definitely an area that needs more research.
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Please note that the author is a well-read patient, not a doctor, and is not offering medical or legal advice.
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