Pharmacokinetics, tissue distribution and mass balance of radiolabeled dihydroartemisinin in male rats

Abstract

Background: Dihydroartemisinin (DHA), a powerful anti-malarial drug, has been used as monotherapy and artemisinin-based combination therapy (ACT) for more than decades. So far, however, the tissue distribution and metabolic profile of DHA data are not available from animal and humans.

Methods: Pharmacokinetics, tissue distribution, mass balance, and elimination of [14C] DHA have been studieded in rats following a single intravenous administration. Protein binding was performed with rat and human plasma. Drug concentrations were obtained up to 192 hr from measurements of total radioactivity and drug concentration to determine the contribution by the parent and metabolites to the total dose of drug injected from whole blood, plasma, urine and faecal samples.

Results: Drug was widely distributed after 1 hr and rapidly declined at 24 hr in all tissues except spleen until 96 hrs. Only 0.81% of the total radioactivity was detected in rat brain tissue. DHA revealed a high binding capacity with both rat and human plasma proteins (76-82%). The concentration of total radioactivity in the plasma fraction was less than 25% of that in blood total. Metabolism of DHA was observed with high excretion via bile into intestines and approximately 89-95% dose of all conjugations were accounted for in blood, urine and faeces. However, the majority of elimination of [14C] DHA was through urinary excretion (52% dose). The mean terminal half-lives of plasma and blood radioactivity (75.57-122.13 h) were significantly prolonged compared with that of unchanged DHA (1.03 h).

Conclusion: In rat brain, the total concentration of [14C] was 2-fold higher than that in plasma, indicating the radioactivity could easily penetrate the brain-blood barrier. Total radioactivity distributed in RBC was about three- to four-fold higher than that in plasma, suggesting that the powerful anti-malarial potency of DHA in the treatment of blood stage malaria may relate to the high RBC binding. Biliary excretion and multiple concentration peaks of DHA have been demonstrated with high urinary excretion due to a most likely drug re-absorption in the intestines (enterohepatic circulation). The long lasting metabolites of DHA (> 192 hr) in the rats may be also related to the enterohepatic circulation.

Figure 1

Multiple peaks of mean concentration were shown in whole plasma (dash line with triangular markers) and in blood (solid line with circular markers) with the radioactivity (ng equivalents per ml), as well as PK profile of unchanged DHA in plasma (solid line) following a single 3 mg/20 μCi/kg intravenous injection of [¹⁴C]-dihydroartemisinin (14C-DHA) in rats (n = 6).

Figure 2

Mean distribution profile of total radioactivity (ng equivalents per gram tissue) of dihydroartemisinin (DHA) in various tissues (column) at 1, 6, 24, 48, 72, 96, and 192 hr following a single 3 mg/20 μCi/kg intravenous injection of [¹⁴C] DHA in rats (n = 5).

Figure 3

Individual elimination profile of total radioactivity (μg equivalents per rat) of dihydroartemisinin (DHA, circular markers) in urine (top) and faeces (bottom) and computer fitted curves (solid line) by pharmacokinetic parameters following a single 3 mg/20 μCi/kg intravenous injection of [¹⁴C] DHA in rats during 8 days collection (n = 5).

Figure 4

Individual protein binding of ¹⁴C-DHA in male human (top) and male rat (bottom) plasma (triangle markers) and mean value in the human plasma (solid line) after incubation in variable concentrations of ¹⁴C-DHA (0.15, 0.74, 3.7, 18.5, 92.5, 462, 2312, 11560, and 57800 ng/ml) at 37°C for 5 hours (n = 3).