Background: Monitoring bone resorption with measurements of bone density and biochemical markers is indirect. We hypothesized that bone resorption can be studied directly by serial measurements of the ratio (41)Ca/Ca in serum after in vivo labeling of calcium pools with (41)Ca. We report the preparation of an intravenous (41)Ca dose suitable for humans, an analytical method for determining (41)Ca/Ca isotope ratios in biological samples, and studies in human volunteers.
Methods: (41)Ca was formulated and aliquoted into individual vials, and to the extent possible, the (41)Ca doses were tested according to US Pharmacopeia (USP) guidelines. A 10 nCi dose of (41)Ca was administered intravenously to 4 end stage renal disease (ESRD) patients on hemodialysis and 4 healthy control individuals. Distribution kinetics were determined over 168 days. Calcium was isolated with 3 precipitation steps and a cation-exchange column, and (41)Ca/Ca ratios in serum were then measured by accelerator mass spectrometry.
Results: The dosing solution was chemically and radiologically pure, contained <0.1 endotoxin unit/mL, and passed USP sterility tests. Quantification of (41)Ca/Ca ratios was linear from 6 x 10(-14) to 9.1 x 10(-10). The run-to-run imprecision (as CV) of the method was 4% at 4.6 x 10(-11) and 6% at 9.1 x 10(-10). The area under the curve of (41)Ca in the central compartment vs time was significantly less for ESRD patients than for controls (P < 0.005).
Conclusions: Isotope ratios spanning 5 orders of magnitude can be measured by accelerator mass spectrometry with excellent precision in the range observed in samples collected from patients who have received 10 nCi of (41)Ca. The (41)Ca at this dose caused no adverse effects in 8 volunteers. This is the first report of the use of (41)Ca to monitor differences in bone turnover between healthy individuals and ESRD patients.