The label for burosumab has been updated to allow for two-week dosing of adults! Not everyone needs that regimen, but it’s great to have options.
You can see the history of the burosumab label here: https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=BasicSearch.process
The previous label (summer 2025) specified the dosage for XLH adults as “1 mg/kg body weight rounded to the nearest 10 mg up to a maximum dose of 90 mg administered every four weeks.” Interestingly, the dosage for TIO started slightly lower but allowed for two-week dosing and a significantly higher maximum: “0.5 mg/kg every four weeks. Dose may be increased up to 2 mg/kg not to exceed 180 mg, administered every two weeks.” (Sorry, I don’t know why it’s higher.)
The new label specifies the same starting dosage for XLH adults as before, but adds “Dose may be increased up to 1 mg/kg rounded to the nearest 10 mg up to a maximum dose of 90 mg administered every two weeks.” The TIO dosing, which already allowed a two-week regimen, remains the same.
Why does this matter? It probably doesn’t for the majority of patients on burosumab, who respond in the “normal” pattern—a bell curve that covers a period of four weeks, never dropping (much) below normal, even at the end of the dosing period. But from what I’ve heard, some patients’ phosphorus levels do rise and fall faster, closer to a two-week cycle than a four-week cycle. If you experience that shorter cycle, you may find that you start to feel more pain/fatigue a week or more before your next (four-week) dose is due. If it’s affecting your quality of life, you should discuss with your clinician whether a change in dosing would be beneficial. If you’re not sure what your cycle looks like, it can’t hurt to check with your XLH expert clinician. Either way, if there’s a significant delay between scheduling the appointment and seeing the doctor, you might find it useful track your pain/fatigue in relation to the dose dates, so you’ll have some data. It would be simple enough to create a chart in a note app on your phone, rating your overall pain/fatigue for each day, along with the dates of your doses. If you want to get fancy, you can set up a spreadsheet with columns for pain, fatigue, and doses, or create a dedicated calendar in a calendar app where you record pain/fatigue levels on a scale of 1 to 10, plus the dose dates.
If you’re curious about my experience with doses over the course of almost ten years now, I’m basically on the standard dose, given every four weeks. For the first few years, I felt a slight increase in pain and fatigue during the last week before my next dose. It wasn’t severe, more a matter of my noticing, “Gee, I’m feeling particularly achy and tired,” and then looking at a calendar to see that my next dose was scheduled for a few days later. Eventually though, even that went away, and I don’t notice any meaningful change in my symptoms from peak phos (at mid-cycle) to the late-cycle, lower but still normal phos. I don’t know for sure whether I’ve just come to accept the increased aches/fatigue, incorporating it into what’s “normal” for me (like we become unaware of pain in general, because it’s our normal), or if my body has somehow changed its response to burosumab over time, and the response remains steady for the full four weeks. Either way, there’s no effect on my quality of life, so I don’t see any need to change to a two-week dosing. But that’s just my experience, and patients’ response to burosumab does apparently vary more significantly than I’d originally expected.
This variation in response to treatment is why having an experienced-with-XLH clinician is so important. One of the expected advantages of burosumab over the old phosphorus/calcitriol regime (besides simply being massively more effective) is that it was potentially a one-size-fits-all treatment, so more endocrinologists would be willing to treat us, even if they were more familiar with osteoporosis than the rare bone disorders. Clinicians can be reluctant to treat a rare disorder, especially when the dosing is a matter of trial and error, and there are real risks of doing more harm than good. There is far less trial-and-error with burosumab than there was with the phos/calcitriol, where the clinician had to juggle individual responses to the phosphorus, getting it high enough for long enough to be beneficial, without going too high or causing bad side effects. There are very few significant side effects with burosumab, so the clinician basically just needs to start with a low dose and increase it gradually until the blood phosphorus levels reach a safe normal range, and then making sure it stays in that range, which it generally does once the effective dose is reached.
The beenefits of having a (mostly) one-size-fits-all treatment still apply in general, but for the subset of patients whose response is a little different from the majority, getting the right dosing regimen can be a little trickier, and may require a lower dose more often, or sometimes even the regular/higher standard dose given more often. For that kind of adjustment, if the clinician does not have XLH experience, the patient may need to be more proactive than otherwise. In that case, they may want to get a second opinion, or may need a clinician who’s willing to consult with another clinician who has more XLH experience. If your clinician is willing to consult but doesn’t know where to start, you can suggest the Rare Bone Disease TeleECHO, which they can learn more about in this journal article: “The Rare Bone Disease TeleECHO Program: Leveraging Telehealth to Improve Rare Bone Disease Care.”
Bottom line: if you’re on burosumab, and you’re confident that the dosing regimen is working for you, without a severe dropoff at the end of the cycle, then this change in the label doesn’t really affect you. If, on the other hand, you notice a severe enough increase in symptoms well before your next dose is due, or a more careful monitoring of your day-to-day pain/fatigue reveals that you’ve been ignoring the increased symptoms, the change in the label gives your clinician more flexibility in the regimen, so it may be worth talking to your clinician about whether a change in the regimen would help you.
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Please note, as always, but especially on this topic, that the author is a well-read patient, not a doctor, and is not offering medical or legal advice. If you think a dosing regimen would be beneficial, you should discuss your options with a qualified medical expert.
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