Phosphorus Absorption in Healthy Adults and in Patients With Moderate Chronic Kidney Disease

• Phosphorus Absorption Test (Other): Subjects will undergo a radiophosphorus (P-33) absorption test over two days, that will include an oral administration of P-33 on the 1st day of the test, and an IV administration of P-33 on the 2nd day of the test. P-33 activity from serial serum and urine collections will be determined by scintillation counting, and fractional phosphorus absorption calculated by kinetic modeling.

Primary outcomes

• Fractional Phosphorus Absorption [Measured from serial blood draws over a 4-hour time frame]: Radiophosphorus (P-33) activity determined by liquid scintillation counting in serum and urine after oral and IV administration will be used to determine fractional phosphate absorption.

Secondary outcomes

• Serum FGF23 [Measured from baseline measures and serial blood draws over a 4-hour time frame]: serum intact and c-terminal FGF23
• Serum 1,25(OH)2D [Measured from baseline measures and serial blood draws over a 4-hour time frame]: serum 1,25-dihydroxyvitamin D
• Serum parathyroid hormone (PTH) [Measured from baseline measures and serial blood draws over a 4-hour time frame]: serum intact parathyroid hormone
• Serum phosphate [Measured from baseline measures and serial blood draws over a 4-hour time frame]: serum phosphate
• 24h Urine phosphate [measured from two, 24-hour urine collections]: 24h urine phosphate

Sex: All, ages: 30 Years to 75 Years, accepts healthy volunteers

Inclusion:

Men and women ages 30-75 years old, any race

Moderate CKD, based on the last set of labs done in the past year, defined as:

Glomerular Filtration Rate (GFR) category G3a (eGFR 45-59 mL/min) with A2 or A3 albuminuria or proteinuria by positive dipstick or urine protein/creatinine ratio (UPCR); -or- G3b (eGFR 30-44 mL/min), with or without evidence of albuminuria (any A1-A3) or proteinuria. (Figure 1)

-or- Healthy control with no diagnosis or evidence of CKD

Female subjects must be post-menopausal, surgically sterile, or confirmed not pregnant by pregnancy test and not breastfeeding.

Subjects must be on stable doses of medications (except those noted in exclusion criteria) for at least 6 weeks prior to the study

Exclusion:

Plans to initiate dialysis within 6 months

Labs based on the last set done in the past year:

Hypercalcemia defined as serum calcium > 10.5 mg/dL

Hyperkalemic > 5.5 mg/dL Prescribed a phosphate binder medication

Small bowel resection, bariatric surgery. Medically unstable or poorly controlled hypertension, diabetes, or gastrointestinal disorders in the opinion of the physicians on the study team

Calcitriol, ergocalciferol, cholecalciferol, vitamin D analogs, calcimimetics, PTH analogues, calcium supplements, multivitamins/mineral and other medications that may alter phosphorus metabolism - must be off for at least 6 weeks prior to study

• Results publication: Stremke ER, Wiese GN, Moe SM, Wastney ME, Moorthi RN, Hill Gallant KM. Intestinal Phosphorus Absorption in Moderate CKD and Healthy Adults Determined Using a Radioisotopic Tracer. J Am Soc Nephrol. 2021 Aug;32(8):2057-2069. doi: 10.1681/ASN.2020091340. Epub 2021 Jul 8. PMID 34244325