In vitro phosphate-binding ability of calcium-based agents is augmented by co-administration of activated charcoal

Calcium carbonate is widely used as a phosphate binder in patients on maintenance hemodialysis. An unwanted side effect of calcium carbonate is hypercalcemia and vascular calcification. Oral activated charcoal (AC) is a non-selective and highly effective adsorbent. We hypothesized that AC augments the phosphate binding capacity of calcium-based agents. We performed an in vitro study to test this hypothesis.

Simulated gastric fluid and intestinal fluid were prepared with a phosphate concentration of 10 mmol/l. Different dosages of calcium chloride (0.083 g, 0.167 g, and 0.250 g), AC (0.15 g, 0.30 g, and 0.45 g) or a combination of both were added to either gastric or intestinal fluid for phosphate binding. After a reaction time of 2 hours, phosphate concentrations in the supernatant were measured, and absolute reduction and percent reduction of phosphate were calculated. The phosphate-binding abilities of calcium chloride, AC, and a combination of both were compared.

In simulated intestinal fluid there was no significant difference in the percent reduction of phosphate concentrations among the different calcium chloride concentration groups (28.90 ± 2.04 vs. 33.33 ± 3.90 vs. 31.86 ± 5.23) and there was still no significant difference in phosphate concentrations among the different AC groups (3.33 ± 0.08 vs. 3.26 ± 0.01 vs. 3.36 ± 0.11). In simulated gastric fluid phosphate concentrations at each of the time points (before the reaction, 1 hour after calcium chloride was added, and 2 hours after AC was added) were not significantly different. In simulated intestinal fluid the percent decrease in phosphate concentration in the calcium chloride + AC group was significantly higher than that in the calcium chloride group (48.23 ± 5.55 vs. 30.72 ± 6.11).

AC alone had no phosphate-binding ability in either gastric or intestinal fluid. The phosphate-binding ability of calcium chloride was improved by AC in intestinal fluid.