Carbon Isotopes Profiles of Human Whole Blood, Plasma, Red Blood Cells, Urine and Feces for Biological/Biomedical ¹⁴C-Accelerator Mass Spectrometry Applications

Abstract

Radiocarbon (14C) is an ideal tracer for in vivo human ADME (absorption, distribution, metabolism, elimination) and PBPK (physiological-based pharmacokinetic) studies. Living plants preferentially incorporate atmospheric 14CO2, vs 13CO2, vs 12CO2, which result in unique signature. Furthermore, plants and the food chains they support also have unique carbon isotope signatures. Humans, at the top of the food chain, consequently acquire isotopic concentrations in the tissues and body fluids depending on their dietary habits. In preparation of ADME and PBPK studies, 12 healthy subjects were recruited. The human baseline (specific to each individual and their diet) total carbon (TC) and carbon isotope 13C (δ13C) and 14C (Fm) were quantified in whole blood (WB), plasma, washed red blood cell (RBC), urine, and feces. TC (mg of C/100μL) in WB, plasma, RBC, urine, and feces were 11.0, 4.37, 7.57, 0.53, and 1.90, respectively. TC in WB, RBC, and feces was higher in men over women, P < 0.05. Mean δ13C were ranked low to high as follows, feces < WB = plasma = RBC = urine, P < 0.0001. δ13C was not affected by gender. Our analytic method shifted δ13C by only ± 1.0 ‰ ensuring our Fm measurements were accurate and precise. Mean Fm were ranked low to high as follows, plasma = urine < WB = RBC = feces, P < 0.05. Fm in feces was higher for men over women, P < 0.05. Only in WB, 14C levels (Fm) and TC were correlated with one another (r = 0.746, P < 0.01). Considering the lag time to incorporate atmospheric 14C into plant foods (vegetarian) and or then into animal foods (non-vegetarian), the measured Fm of WB in our population (recruited April 2009) was 1.0468 ± 0.0022 (mean±SD), the Fm of WB matched the (extrapolated) atmospheric Fm of 1.0477 in 2008. This study is important in presenting a procedure to determine a baseline for a study group for human ADME and PBPK studies using 14C as a tracer.

Automated Combustion Accelerator Mass Spectrometry for the Analysis of Biomedical Samples in the Low Attomole Range

“…To convert a 14 C/ 12 C ratio to mBq/mL, eq is used, whereby 1.3614 × 10 –12 corresponds to the certified ratio for OXII and 0.3077 to the amount of Bq/g for OXII. In literature, the average plasma carbon concentration is reported as 44 mg carbon/mL, and the natural abundance of 14 C is 1.17 × 10 –12 . Consequently, the theoretical background level of activity in plasma can be calculated using eq and equals 11.73 mBq/mL.…”

A phase I study to assess the mass balance, excretion, and pharmacokinetics of [14C]-ixazomib, an oral proteasome inhibitor, in patients with advanced solid tumors

“…A typical batch contained unknown samples, certified standards to normalize all measurements, machine blanks ( 14 C–free graphite of natural origin) to assess the sensitivity of the spectrometer, and chemical blanks (samples prepared with a 14 C–free substance) to characterize the extraneous carbon introduced during sample preparation. For total carbon measurement, a reference value of 43.7 mg C/mL and 109.8 mg C/mL for plasma and whole blood, respectively, was used [23]. The AMS isotope ratio was converted to ng-eq/mL of whole blood, plasma, urine, or fecal homogenate.…”

Biological and biomedical 14C-accelerator mass spectrometry and graphitization of carbonaceous samples

“…57 Sample type, size, graphitization yield, and other considerations for bio-14 C-AMS Table 3 shows the common biological samples used in bio-14 C-AMS and their carbon contents. 7,59 Blood, plasma, red blood cell (RBC), urine, and/or fecal matter are mainly used for the bio-14 C-AMS studies in vivo in human. These samples contained 4-17% of carbon, so a small volume or mass of these samples can easily provide 1 mgC required for the accurate, precise, and reproducible bio-14 C-AMS measurement.…”

“…However, carbon Table 3 Carbon contents in common biological samples for bio-14 C-AMS applications. 7,59 Some information in this table were adopted and edited from the prior publication 7 55 So, the S or N has to be eliminated or separated from the urine sample during the graphitization process to obtain an optimal graphitization yield.…”