One of the myths about XLH is that our kidneys are defective, so I like to remind everyone that our kidneys are not the problem. They’re just doing what they’re being told to do, and it’s the messaging TO the kidney that’s the problem.
That’s a wee bit of a simplification. It’s more accurate to say that our kidneys are not inherently defective DUE TO the XLH. Many of us end up with damaged kidneys, but it’s due to the old, terrible treatment (or some unrelated additional problem; un fortunately it’s possible to have both XLH and kidney disease).
We’ve known for a long time that phosphorus and calcitriol supplements lead to kidney calcification (and sometimes kidney stones). It’s generally not fatal, but is irreversible, and could be life-threatening if additional complications arise. I don’t believe there’s been solid, extremely detailed data collected though, on the correlation between the old treatment and the kidney damage. We’ve just had enough data to know that most kids treated with phos/calcitriol end up with some degree of kidney calcification, which is why they need to get so many kidney scans while on that treatment. Keeping the calcification from getting too bad, while still providing some phosphorus to the growing bones, is a tricky balancing act for the old, terrible treatment.
The hypothesis (not entirely proven yet, but a reasonable guess) is that burosumab treatment will not cause this kidney calcification. We don’t have enough of that data yet, but there’s work happening in Germany and Switzerland to better quantify exactly how much damage is being done by the phos/calcitriol treatment, and (eventually) how it compares to kidney health in patients on burosumab.
I only have access to the abstract for “Kidney health in children with X-linked hypophosphatemia,” but it appears to confirm, with more detailed data, what we knew anecdotally, namely that patients who have been on phos/calcitriol for any substantial period of time end up with high blood pressure, excess weight, and “significant renal comorbidity, i.e. reduced eGFR [a measurement of how well the kidneys are working], nephrocalcinosis, i…. [and] elevated urinary markers of glomerular and tubular injury and inflammation.”
Apparently they don’t have any patients in the study who have ONLY been treated by burosumab, never phos/calctriol, so they can’t yet compare burosumab-only to phos/calcitriol effects. It appears that they are planning to make that comparison when they can however, so stay tuned.
This is really important work, since part of the justification for not giving kids (or adults) the expensive burosumab is the persistent belief that phos/calcitriol works just fine, so there’s no need to spend the extra money. That’s wrong on so many levels, but particular with respect to factoring in the respective side-effects of the two treatments (serious for phos/calcitriol; minimal and largely reversible for burosumab).
Patients (and expert clinicians/researchers) have been arguing that EVEN IF phos/calcitriol mineralized bones as well as burosumab, there’s another cost that needs to be considered—the almost-universal, permanent, damage to the patient’s kidneys from phos/calcitriol. There’s sometimes resistance to that argument on the part of whoever is paying for the treatment (insurers or socialized medicine agencies), since we don’t (yet) have data to disprove the possiblity that the kidneys would have calcified anyway, even on burosumab, so we need more data on kidney health of kids (and adults) on burosumab. A natural history study would be ideal, but this research study in Germany/Switzerland, if continued, would be useful too.
P.S. For anyone wondering why there was no post last week, I’m fine. I just had a book deadline, plus plans for the long Memorial Day weekend and totally lost track of the days of the week, and all of a sudden, it was Saturday, and I hoped y’all had more fun things to do than read medical journal articles, whether you celebrate the holiday or not.
P.P.S. As long as I’m talking about scheduling, just a quick heads up that I’m planning to take the month of July off, as usual, unless there’s breaking news. Like a decision from the UK agency that was supposed to have decided months ago whether to allow adults in England access to burosumab treatment, and still hasn’t issued the decision as far as I can tell.
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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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