Unmasking the Role of Uptake Transporters for Digoxin Uptake Across the Barriers of the Central Nervous System in Rat

Abstract

The role of uptake transporter (organic anion-transporting polypeptide [Oatp]) in the disposition of a P-glycoprotein (P-gp) substrate (digoxin) at the barriers of central nervous system, namely, the blood-brain barrier (BBB), blood-spinal cord barrier (BSCB), and brain-cerebrospinal fluid barrier (BCSFB), was studied using rat as a preclinical species. In vivo chemical inhibition of P-gp and Oatp was achieved using elacridar and rifampicin, respectively. Our findings show that (1) digoxin had a low brain-to-plasma concentration ratio (B/P) (0.07) in rat; (2) in the presence of elacridar, the B/P of digoxin increased by about 12-fold; (3) rifampicin administration alone did not change the digoxin B/P significantly when compared with digoxin B/P alone; (4) rifampicin administration along with elacridar resulted only in 6-fold increase in the B/P of digoxin; (5) similar fold changes and trends were seen with the spinal cord-to-plasma concentration ratio of digoxin, indicating the similarity between BBB and the BSCB; and (6) unlike BBB and BSCB, the presence of rifampicin further increased the cerebrospinal fluid-to-plasma concentration ratio (CSF/P) for digoxin, suggesting a differential orientation of the uptake transporters at the BCSFB (CSF to blood) compared with the BBB (blood to brain). The observations for digoxin uptake, at least at the BBB and the BSCB, advocate the importance of uptake transporters (Oatps). However, the activity of such uptake transporters became evident only after inhibition of the efflux transporter (P-gp).

Keywords: CNS; Digoxin; P-glycoprotein; blood-brain barrier; blood-cerebrospinal fluid barrier; blood-spinal cord barrier; organic anion transporting polypeptide.

Figure 1.

(A) Comparison of K_P,AUC,Brain of digoxin when administered alone and in combination with elacridar and/or rifampicin. The numbers above each bar indicate the fold increase in K_P,AUC,Brain compared with digoxin alone. For each group, 15 rats were used, of which 3 rats were used for each sampling time point. As the K_P,AUC,Brain was calculated using the K_P,Brain at each time point collected from different sets of rats, error bars were not included. (B) Comparison of K_P,Brain of digoxin at each time when administered alone and in combination with elacridar and/or rifampicin. Concentrations represent mean ± SD for each time point (n = 3). *P < .05 by t test by comparing the concentrations of each time point with the concentrations of the respective time point when digoxin was administered alone. D indicates digoxin; E, elacridar; R, rifampicin.

Figure 2.

Unbound concentration-time profile of (A) elacridar and (B) rifampicin in plasma. D indicates digoxin; E, elacridar; P-gp, P-glycoprotein; R, rifampicin.

Figure 3.

Comparison of K_{P,AUC,Spinal cord} of digoxin when administered alone and in combination with elacridar and/or rifampicin. The numbers above each bar indicate the fold increase in K_{P,AUC,Spinal cord} compared with digoxin alone. For each group, 15 rats were used, of which 3 rats were used for each sampling time point. As the K_{P,AUC,Spinal cord} was calculated using the K_{P,Spinal cord} at each time point collected from different sets of rats, error bars were not included. D indicates digoxin; E, elacridar; R, rifampicin.

Figure 4.

Comparison of K_P,AUC,CSF of digoxin when administered alone and in combination with elacridar and/or rifampicin. The numbers above each bar indicate the fold increase in K_P,AUC,CSF compared with digoxin alone. For each group, 15 rats were used, of which 3 rats were used for each sampling time point. As the K_P,AUC,CSF was calculated using the K_P,CSF at each time point collected from different sets of rats, error bars were not included. D indicates digoxin; E, elacridar; R, rifampicin.