Hydroxycitrate: A Novel Therapy for Calcium Phosphate Urinary Stones

项目摘要/摘要

PROJECT SUMMARY / ABSTRACT The incidence of calcium phosphate (CaP) stone disease has markedly increased over the last four decades. CaP stone formers experience high rates of stone recurrence and frequently need repeat stone surgery, indicating that current prophylactic medical regimen are suboptimal. The most common metabolic abnormalities identified in CaP stone formers are hypercalciuria, hypocitraturia, and elevated urine pH, all factors that increase CaP saturation. Alkali therapy (potassium citrate) is well-established to prevent calcium oxalate stones, in part by raising urine citrate. However, when tested in CaP stone formers at doses targeted at normalizing citrate excretion, alkali therapy may worsen CaP stones due to a concomitant rise in urine pH. Hydroxycitrate is a molecule closely related to citrate that has been shown to prevent CaP nucleation in vitro. However, its efficacy against CaP stone formation in vivo has not been previously studied. Our preliminary studies suggest that: 1) Hydroxycitrate can exert a powerful inhibitory effect on CaP crystal growth at low concentrations; 2) Oral hydroxycitrate ingestion is well-tolerated and raises urine OHCit excretion to concentrations that reduce CaP crystallization in urine; 3) Oral hydroxycitrate administration increases urine citrate without significantly increasing urine pH. In this proposal, we will therefore test the overall hypothesis that hydroxycitrate can serve as a treatment to prevent recurrence of CaP stone disease. To test this hypotheses, we will combine three complementary approaches: 1) In human metabolic studies conducted in CaP stone formers, we will examine the short-term impact of a. escalating doses of hydroxycitrate and b. comparative effectiveness of hydroxycitrate vs alkali therapy (potassium citrate) on urinary biochemical and physicochemical parameters, and CaP crystal growth and dissolution using atomic force microscopy and bulk crystallization studies conducted on participants’ urine specimen. 2) In an animal model of CaP precipitation, we will assess the long-term impact of the same interventions to be tested in humans with measurement of urine chemistry, crystalluria, and development of calcification in kidneys. 3) In vitro, we will manipulate hydroxycitrate, calcium, citrate, and pH, and assess CaP crystal growth and solubility by a unique combination of techniques spanning multiple length scales. Notably, we will examine microscopic surface growth by atomic force microscopy, macroscopic crystal nucleation and growth in a microfluidic device, and the kinetics of bulk crystallization over a range of clinically relevant hydroxycitrate concentrations. We will test the mechanisms by which hydroxycitrate exerts its impact on CaP crystal growth and dissolution, and assess the cooperative effects of hydroxycitrate and citrate concentration using a range of clinically relevant pH and ionic strengths. Overall, based on our strong preliminary data, our innovative but rigorous techniques, and our cross-disciplinary complimentary studies, we hope that this proposal will help identify optimal therapeutics for the increasingly encountered and difficult to treat CaP stones.