Title: Assessing calcium balance in children with chronic kidney disease to optimise treatment strategies
Funder: Department of Health and Social Care

Abstract: Background: In patients with chronic kidney disease (CKD) dysregulated calcium (Ca) and phosphate (P) homeostasis is common and causally associated with reduced bone mineral density (BMD) and vascular calcification. Reduced BMD increases fracture risk and vascular calcification may lead to left ventricular hypertrophy and cardiovascular disease (CVD). The growing skeleton of children is uniquely vulnerable: 30% with early CKD and >90% on dialysis have deficient mineralization, which manifests as bone deformities, a 2-3 fold higher fracture risk compared to healthy peers and short stature in 30-50%. However, vascular calcification is present in 20%, and CVD accounts for 30% of death in children on dialysis. Medications that are universally used to prevent or treat bone disease, such as P-binders (available as Ca-based or Ca-free) and vitamin D can increase Ca intake and absorption. The absorbed Ca may get incorporated into bone or deposit in soft tissues depending on the patients' Ca balance (i.e. net balance of Ca intake and Ca excretion). In current clinical practice treatment is guided by serum Ca levels, but serum Ca reflects only 0.1% of total body Ca. Biomarkers lack specificity, bone biopsies are highly invasive, and radiological changes take months-years. There is an urgent need for Ca balance studies in order to guide treatment aimed at two key patient level outcomes fracture risk and cardiovascular health. Two Cochrane reviews, NICE and an international guideline development group, KDIGO, have identified a gap in knowledge and called for Ca balance studies in CKD patients. Ca balance studies traditionally involved radioactive isotopes or laborious mass balance experiments requiring a metabolic ward' setting. To date, only two Ca balance studies have been performed in (adult) CKD patients. We propose a novel, non-invasive method of assessing Ca balance by natural Ca isotope fractionation. Natural Ca isotopes (44Ca and 40Ca) are not radioactive, stable over millions of years and can be measured in very small volumes of multiple bodily fluids ( Hypothesis: Natural Ca isotope fractionation is a sensitive and specific method of monitoring Ca balance in children with CKD and on dialysis in order to provide safe and effective treatment that prevents Ca deficiency or overload. Objectives: Validate the natural Ca isotope fractionation technique in CKD and healthy controls Determine Ca balance in healthy children of different ages and pubertal stage Measure Ca balance in children with CKD and on dialysis Perform a randomised cross-over trial comparing the effect of Ca-based and Ca-free P-binders on changes in Ca balance Methods: Ca isotope fractionation (expressed as 44/40Ca) will be measured by thermal ionisation mass-spectrometry in diet, blood, urine and faeces, and described as net Ca gain or loss using a compartment model based on Ca kinetics. The percentage bone gain (or loss) can be determined, and annualized change in BMD calculated. 44/40Ca values will be correlated with BMD (measured by peripheral quantitative CT and dual-energy x-ray absorptiometry) and bone-specific biomarkers. Effects of medications and dialysis prescription will be assessed, adjusting for age, gender and pubertal stage. Outcomes: The Ca isotope fractionation technique will provide a powerful tool to guide therapy in CKD patients so as to prevent Ca deficiency and fractures or Ca overloading and cardiovascular disease. We will validate it for routine clinical use as a cost-effective, easily reproducible and sensitive marker of bone mineralisation. Ca isotope fractionation can be widely applied in different patient cohorts to monitor effect of medications like anti-resorptive agents and glucocorticoids on bone mineralisation.

Lay abstract: Background: Patients with chronic kidney disease (CKD) can have weak bones that fracture easily, and also a high risk of dying from heart failure. Bone disease and heart disease are linked, and happen because of changes in the balance between two important minerals in the body - calcium (Ca) and phosphate (P). Ca is removed from the bones, making bones soft, causing bone pain, bone deformities and a high risk of fractures. The growing bones of children are particularly vulnerable. Nearly 90% of children on dialysis (a treatment for severe CKD) have soft bones. Boys and girls with even early stages of CKD are at a 2 3 times greater risk of fractures compared to their healthy peers. However, it is also possible that too much Ca can get deposited in other parts of the body, such as the arteries. This can make the arteries thick and hard, which puts extra strain on the heart. This is a problem for approximately 20% of children on dialysis, and causes heart failure, the commonest cause of death in children on dialysis. Why we need this research: To prevent and treat bone disease, CKD patients are regularly given medicines called phosphate binders (P-binders) and vitamin D. Some P-binders and vitamin D increase Ca intake into the body. So, if a patient has less Ca in their body, this increased Ca goes into their bones and makes them stronger. But, if a patient has too much Ca already, the extra Ca can get dumped in the arteries. This means that the very medicines used to treat children may be causing them serious long-term harm. To prescribe medicines correctly, it is important that we know how much Ca is in the body this is called Ca balance. Unfortunately, there is no easy way to measure Ca balance. There are only 2 small studies looking at this in CKD, but they were both performed in adults. International experts have identified this as a gap in knowledge. Our group of young people with kidney disease has urged us to perform this study as it focuses on outcomes that matter most to them: long-term bone and heart health. A new test for Ca balance: We will use a new method to measure Ca balance that looks at the proportion of different natural forms of Ca (called isotopes) in the body. Natural Ca isotopes are found in food and taken up in the bones or passed out in urine and stool in fixed proportions. By measuring the proportion of different Ca isotopes in food, blood, urine and stool we can calculate if there is too much or too little Ca in the body. Research goals: In order to develop this new way of measuring Ca balance which could be used in day-to-day care, we will: 1. Test our method against the currently used Ca balance technique that we are already researching in our hospital. 2. Measure Ca balance in healthy boys and girls of different ages 3. Measure Ca balance in children with CKD and on dialysis 4. Perform a study to compare the effect of two different types of P- binders (one with Ca and the other without Ca) on the Ca balance. Both types of P-binders are regularly used in children, but we do not know which type is the correct treatment for an individual patient. Ca balance measurements will be compared with two different x-ray measurements of bone density and special blood tests for bones. Benefits of our research: By measuring the Ca balance of children with CKD using a carefully developed technique, we can determine if a child has too little Ca for their bones (and therefore needs medications that provide more Ca), just the right amount, or too much (in which case Ca containing medicines can be harmful). Knowing a person's Ca balance is the only way we can give them the correct medicines for their bone and heart health. Importantly, bone and cardiovascular complications of CKD are preventable, so early diagnosis that can allow appropriate treatment is crucial. This can potentially reduce bone pain, deformities and fractures, reduce hospitalisation and improve quality of life. Thus, data from our study will benefit our research participants too. Also, our unique technique of measuring Ca balance can be applied to patients with many other types of bone diseases (like osteoporosis) to manage their medicines accurately.

Research activity codes: 4.2 EVALUATION OF MARKERS AND TECHNOLOGIES
Health categories: RENAL AND UROGENITAL