Delmez JA, Kelber J, Norword KY, Giles KS, Slatopolsky E
Magnesium carbonate as a phosphorus binder: A
prospective, controlled, crossover study
(Jan) 49:163-166 1996
Delmez et al describe the use of magnesium carbonate as a phosphorus
binder in 15 highly compliant, well dialyzed patients. In a rather
complicated protocol they first demonstrated compliance with and
effectiveness of calcium carbonate. After patients had serum calcium
9.5 and 10.5 mg% for four weeks, they were randomly assigned to either
continue calcium carbonate (average dose 2.9 g/day) or to take a
combination of magnesium carbonate (750 mg/day) and calcium carbonate
(1.2 g/day). After 4 weeks on this therapy calcitriol was added to both
patient groups, given intravenously at a dose of 2.0 mcg three times a
week. The dose of calcitriol was increased every 2 weeks until it
reached 4 mcg three times a week. If hypercalcemia occurred the dose was
reduced. The maximum achievable calcitriol dose was sustained for 10
weeks. The patients were then restarted on calcium carbonate as at the
beginning and assigned to the opposite limb of the protocol (with regards
to magnesium or not) to which they had previously been assigned.
Dialysate calciums were 5.0 mg% and magnesiums 1.8 mg%, except when
patients were receiving oral magnesium it was reduced to 0.6 mg%.
In this study the patients' calcium, phosphate and magnesium
levels did not differ when they were treated with magnesium versus
calcium alone. There were significantly fewer episodes of hypercalcemia
during the magnesium treatment (1.8/patient vs 2.7). Calcitriol dose
averaged nearly twice as high (1.5 mcg/treatment vs 0.8). PTH levels
(using the CH9 antibody), however, were similar during the two treatment
The authors suggest that magnesium may be a partial alternative to
calcium as a phosphorus binder, particularly in patients who develop
hypercalcemia on calcitriol therapy.
Comment: This careful study is somewhat disappointing in that
magnesium doesn't seem to be
much better than calcium as a phosphorus binder. On the other hand,
it is gratifying to find an
alternative and it is useful to know that magnesium can be given safely.
Whether the difficulty of customizing the dialysate can be easily
resolved remains to be seen. It is curious that while the investigators
were able to
give nearly twice as much calcitriol when the patients were on the
magnesium, the PTH levels did not differ between the groups. One would
have expected greater PTH suppression with the higher calcitriol dose.
This result might be an artefact of the PTH assay they used. It would be
of interest to see the effects on the more commonly used intact PTH
assay. It is also possible that it may take longer for the PTH
suppression to be seen with the assay they used. In any case these are
useful data. The search for a better binder must continue, unfortunately.
(Donald J. Sherrard, M.D.)