Development of an in vitro assay for the screening of substances capable of dissolving calcium oxalate crystals

Despite the risk of kidney damage, lithotripsy is the usual way of treating calcium oxalate (CaOx) stones, the most common type of nephrolithiasis, be cause no effective chemolytic agents are available. However, the search of new calcium chelators, less toxic than the current ones, continues, and som e of them could be tested in experimental models of nephrolithiasis, after their ability of dissolving CaOx crystals is verified. In this connection, we developed a simple assay that requires only inexpensive equipment availa ble in most laboratories for the screening of substances potentially capabl e of dissolving CaOx crystals. In particular, we decided to investigate whe ther substances previously shown to inhibit CaOx precipitation were also ca pable of dissolving this salt. Briefly, CaOx tablets of highly reproducible weight (4.55 +/- 0.07 mg) were prepared by spinning, at high speed (16,000 g), microcentrifuge tubes in which 500 mu l aliquots of 0.1 M sodium oxala te and 0.1 M calcium chloride at pH 6 were added. When these tablets were i ncubated overnight with solutions at different concentrations of EDTA, sodi um citrate, manganese chloride, sodium sulfate, sodium chloride, malic acid , succinic acid and gluconic acid, a significant dissolving activity was ob served for EDTA (similar to 25% at 0.25 M), sodium citrate (similar to 30% at 1 M) and manganese chloride (similar to 20% at 0.5 M). A good linear cor relation (r(2) = 0.84, p < 0.05) was Found between the affinity for calcium and the activity of EDTA, sodium citrate, sodium sulfate, malic acid, succ inic acid and gluconic acid, indicating that these compounds act mainly by chelating the calcium ion. Instead, manganese was supposed to act by intera cting with the oxalate ion.