Factors Differentiating the Effectiveness of Polyprotic Acids as Inhibitors of Calcium Oxalate Crystallization in Kidney Stone Disease

Abstract

Pathological crystallization of calcium oxalate monohydrate (COM) is a critical process in human kidney stone disease. Methods of curtailing COM growth involve the action of natural and synthetic polyprotic acids, which selectively bind to crystal surfaces and inhibit the anisotropic rates of growth. Here we use a combination of bulk crystallization, in situ scanning probe microscopy, density functional theory, and physiologically relevant in vitro assays to elucidate the molecular origins of COM growth inhibition, focusing on citrate (CA) and its molecular analogues hydroxycitrate (HCA) and isocitrate (ICA). These three molecules differ only in the number and/or placement of hydroxyl groups, yet each exhibit unique binding modes to COM crystal surfaces. A major breakthrough in the development of potential therapies came from the recent discovery of HCA being a potent inhibitor possessing an ability to dissolve crystal surfaces in supersaturated media. Using a combination of experiments and computational calculations, we examine the differentiating factors for isostructural analogues of citrate to prompt dissolution and introduce a vectorized displacement parameter that accounts for strain induced on a crystal lattice as a result of inhibitor adsorption. We also compare the inhibitory effects of all three polyprotic acids in a human urine assay that surprisingly revealed HCA to be a far superior inhibitor than laboratory estimates owing to an apparent synergistic cooperative effect with species in the urine milieu. Collectively, these findings identify new inhibitors of calcium oxalate crystallization and demonstrate that HCA and ICA are promising alternatives to citrate, the current therapy.