The inhibition of calcium oxalate crystal growth by multidentate organic phosphonates

The effect of a number of structurally related multidentate organic phosphonates on the rate of crystal growth of calcium oxalate was studied as a function of pH. Rate constants were obtained at various concentrations for the phosphonates ethane-1-hydroxy-1,1-diphosphonate (EHDP), nitrilotri(methylenephosphonic acid) (NTMP), N,N,N',N'-ethylene-diaminetetra(methylenephosphonic acid) (ENTNP), and N,N,N',N'-hexamethylenediaminetetra(methylenephosphonic acid (HMTMP), at pH 5.00, 6.00, and 7.00. The effect of pH on the inhibitory activity of each of the phosphonates was considerable with effective concentrations of inhibitor decreasing two orders of magnitude, in some cases, as the pH was increased. At a given pH the potentially hexadentate ligands, ENTMP and HMTMP, were generally the most effective inhibitors. The results suggested that EHDP, at currently administered doses, provides only a moderate increase in the capacity of human urine to inhibit calcium oxalate crystal growth.