HDCN Article Review/Hyperlink

Delmez JA, Kelber J, Norword KY, Giles KS, Slatopolsky E

Magnesium carbonate as a phosphorus binder: A prospective, controlled, crossover study

Kidney Int (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 levels between 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 periods. 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.)