The Influence of P-glycoprotein on the Blood-Brain Barrier Transport of the Antidepressant Escitalopram: Converging Evidence from In Vivo and In Vitro Studies

FE O\'Brien1,2, G Clarke2,4, TG Dinan2,4, BT Griffin1, JF Cryan2,3. 1School of Pharmacy, University College Cork, Cork, Ireland, 2Alimentary Pharmabiotic Centre, University College Cork, Cork, Ireland, 3Department of Anatomy & Neuroscience, University College Cork, Cork, Ireland, 4Department of Psychiatry, University College Cork, Cork, Ireland

Preclinical studies have shown that the drug efflux transporter P-glycoprotein (P-gp), expressed at the blood-brain barrier (BBB), restricts brain concentrations of several antidepressants, thereby potentially contributing to treatment failure (O’Brien et al., 2012). However, it is not yet known if escitalopram, one of the most commonly prescribed antidepressants, is a transported P-gp substrate. In addition, it remains unclear if findings in rodents translate to humans due to putative species differences in P-gp substrate specificity.

The impact of treatment with the P-gp inhibitor cyclosporin A (CsA) on escitalopram transport across the BBB was investigated in vivo using an integrated intracerebral microdialysis technique in conscious, freely moving male Sprague Dawley rats (250-320 g; n=5 per group). Steady state levels of escitalopram were achieved by continuous i.v. infusion (6 mg/kg bolus injection followed by 4 mg/kg/h infusion) prior to intra-arterial administration of CsA (25 mg/kg) or vehicle (1 part ethanol, 2 parts Cremophor EL®, 9 parts saline; 2 ml/kg). Data were analyzed using repeated measures one-way ANOVA with LSD post-hoc. In vitro bidirectional transport studies (n=3 per experiment) were carried out using the MDCKII-MDR1 cell line. This ABCB1-transfected canine kidney cell line expresses human P-gp in a polarized fashion when cultured as a monolayer on an appropriate transwell support. P-gp substrates exhibit a basolateral-to-apical/apical-to-basolateral transport ratio (TR) > 1.5. To account for the impact of endogenous drug transporters on escitalopram distribution, the TR in MDCKII-MDR1 cells was compared to that in wild-type MDCK cells, yielding a corrected TR (cTR). In vitro transport experiments were repeated in the presence of P-gp inhibitors (either CsA [25 µM] or verapamil [200 µM]) to confirm that P-gp efflux was the transport mechanism underlying a TR > 1.5 (Polli et al., 2001).

There was a statistically significant CsA effect on dialysate escitalopram levels (p = 0.009). Treatment with CsA resulted in a 67±14% increase from steady state in dialysate escitalopram concentrations (p < 0.001), compared to a 17±5% elevation in vehicle-treated animals (p < 0 .01). The increase in dialysate escitalopram levels observed in the vehicle-treated group mirrored the 15-20% increase in plasma escitalopram concentrations evident in both groups after CsA- or vehicle-administration. Escitalopram concentrations in brain regions harvested at termination of the experiment were also significantly higher in the prefrontal cortex (> 3-fold difference; p < 0.001) and hippocampus (> 2.5-fold difference; p < 0.001) of CsA-treated animals compared to vehicle-treated controls. In vitro: TR for escitalopram in MDCKII-MDR1 cells was 6.91, with a cTR of 3.13. Co-incubation with either one of the P-gp inhibitors verapamil or CsA reduced the TR to 1.44 and 3.19, respectively.

In vivo studies demonstrate that inhibition of P-gp leads to increased brain levels of escitalopram in rats. In vitro investigations show that escitalopram is a substrate of human P-gp. Taken together, these data indicate that P-gp plays a key role in the BBB transport of escitalopram. This finding suggests that adjunctive therapy with a P-gp inhibitor may represent a realistic approach to augment escitalopram response, or reduce peripheral side-effects while maintaining response, in the future.

References

O’Brien FE et al., (2012). Br J Pharmacol 165(2):289-312

Polli JW et al., (2001). J Pharmacol Exp Ther 299(2):620-628