A novel nucleoside rescue metabolic pathway may be responsible for therapeutic effect of orally administered cordycepin

Abstract

Although adenosine and its analogues have been assessed in the past as potential drug candidates due to the important role of adenosine in physiology, only little is known about their absorption following oral administration. In this work, we have studied the oral absorption and disposition pathways of cordycepin, an adenosine analogue. In vitro biopharmaceutical properties and in vivo oral absorption and disposition of cordycepin were assessed in rats. Despite the fact that numerous studies showed efficacy following oral dosing of cordycepin, we found that intact cordycepin was not absorbed following oral administration to rats. However, 3'-deoxyinosine, a metabolite of cordycepin previously considered to be inactive, was absorbed into the systemic blood circulation. Further investigation was performed to study the conversion of 3'-deoxyinosine to cordycepin 5'-triphosphate in vitro using macrophage-like RAW264.7 cells. It demonstrated that cordycepin 5'-triphosphate, the active metabolite of cordycepin, can be formed not only from cordycepin, but also from 3'-deoxyinosine. The novel nucleoside rescue metabolic pathway proposed in this study could be responsible for therapeutic effects of adenosine and other analogues of adenosine following oral administration. These findings may have importance in understanding the physiology and pathophysiology associated with adenosine, as well as drug discovery and development utilising adenosine analogues.

Figure 1

In vitro biopharmaceutical assessment of cordycepin (mean ± SD). (a) Time-dependent degradation of cordycepin in rat and human plasma (n = 4). (b) Stabilisation effect of pentostatin on cordycepin tested in rat plasma at different concentrations of pentostatin (1 pM – 10 µM) (n = 4). **p < 0.01 compared to control group (without pentostatin). (c) Plasma protein binding results of cordycepin in rat and human plasma presented as fraction unbound (F_ub) in plasma (n = 4). (d) Biorelevant solubility of cordycepin tested in fasted state simulated gastric fluid (FaSSGF), fasted state simulated intestinal fluid (FaSSIF) and fed state simulated intestinal fluid (FeSSIF) (n = 4). **p < 0.01. (e) Permeability of cordycepin tested using Caco-2 cells with co-administration of pentostatin at both apical-to-basolateral (A to B) and basolateral-to-apical (B to A) directions (n = 3). (f) Permeation of 3′-deoxyinosine when Caco-2 cells are treated with cordycepin without co-administration of pentostatin. In this case, permeation of cordycepin was not detected (n = 3). (g) Cordycepin concentrations analysed before and after the permeability experiment in the donor chambers (n = 3). **p < 0.01.

Figure 2

In vivo pharmacokinetic profiles of cordycepin and its metabolite, 3′-deoxyinosine, following administration of cordycepin in rats (mean ± SD). (a) Profiles of cordycepin following intravenous administration of cordycepin at 8 or 20 mg/kg (n = 4). (b) Profiles of 3′-deoxyinosine following intravenous administration of cordycepin at 8 or 20 mg/kg (n = 4). (c) Profiles of 3′-deoxyinosine following oral administration of cordycepin at 8 or 80 mg/kg (n = 5).

Figure 3

Repression effect on inflammatory gene expression. (a) NBTI or ITu treatment prior to cordycepin treatment suppresses effect of cordycepin. RAW264.7 cells were treated with 10 µM NBTI, 100 nM ITu or DMSO (Ctrl, control) 15 min prior to cordycepin treatment. The cells were stimulated with LPS 1 h after cordycepin treatment (mean ± SD, n = 3). **p < 0.01; ***p < 0.001 compared to DMSO treatment (labelled as Ctrl). (b) 3′-deoxyinosine exerts same repression effect as cordycepin, although to a lesser degree. RAW264.7 cells were treated with 20 µM cordycepin, 20 µM 3′-deoxyinosine (3DI) or DMSO (Ctrl, control) for 1 h prior to LPS stimulation (mean ± SD, n = 3). **p < 0.01; ***p < 0.001 compared to without compound treatment (labelled as LPS).

Figure 4

Levels of cordycepin, 3′-deoxyinosine and CordyTP found following in vitro treatment with cordycepin or 3′-deoxyinosine. RAW264.7 cells maintained in 0.5 or 10% (foetal bovine serum (FBS) conditions were treated with cordycepin or 3′-deoxyinosine at 20 µM for 6 h (mean ± SD, n = 3).

Figure 5

Proposed metabolic pathways of cordycepin. The thicker arrows indicate the novel nucleoside rescue metabolic pathway proposed in this study.