The Influence of P-glycoprotein Inhibition on Imipramine Transport across the Blood-Brain Barrier: Microdialysis Studies in the Conscious Freely Moving Rat

Fionn O’Brien1,2, Gerard Clarke2,3, Pat Fitzgerald2, Timothy Dinan2,3, Brendan Griffin1, John Cryan2,4. 1School of Pharmacy, University College Cork, Cork, Ireland, 2Alimentary Pharmabiotic Centre, University College Cork, Cork, Ireland, 3Department of Psychiatry, University College Cork, Cork, Ireland, 4Department of Anatomy, University College Cork, Cork, Ireland

Recent studies indicate that antidepressant efflux by the multidrug resistance transporter P-glycoprotein (P-gp) at the blood-brain barrier (BBB) may contribute to treatment resistant depression (TRD) by limiting intracerebral antidepressant concentrations [1]. In addition, anecdotal evidence suggests that adjunctive treatment with the P-gp inhibitor verapamil may improve the clinical outcome in TRD [2]. Therefore, the present study aimed to investigate the influence of P-gp inhibition on the transport of the tricyclic antidepressant imipramine (IMI) and its active metabolite desipramine (DMI) across the BBB.

Intracerebral microdialysis [3] was used to monitor levels of IMI and DMI in the prefrontal cortex following intravenous IMI administration in male Sprague Dawley rats (255-290 g), with or without pre-treatment with one of the P-gp inhibitors verapamil or cyclosporin A (CsA). Indwelling catheters were surgically placed into the jugular vein and carotid artery of the rats to facilitate intravenous drug administration and blood sampling, respectively. After 16-24 hours post-operative recovery, IMI (5 mg.kg-1 i.v.) was administered to all rats. Rats were separated into three groups (n = 6 per group): 1. IMI only; 2. IMI+VER: pre-treated with verapamil (20 mg.kg-1 i.p.) 90 minutes before IMI administration; 3. IMI+CsA: pre-treated with CsA (25 mg.kg-1 i.v.) 30 minutes before IMI administration. IMI and DMI concentrations in plasma and microdialysis samples (dialysates) were determined over a 4 hour period post-IMI administration by HPLC with electrochemical detection. Plasma samples were taken before IMI administration (blank) and at 5, 15, 30, 60, 120, 180 and 240 minutes post-IMI administration. Dialysate samples were collected continually at 20 minute intervals throughout the sampling period. Statistical analysis of results was carried out using one-way ANOVA, with 2-way Dunnett’s post-hoc test where appropriate. All data are presented as mean (± SEM).

Pre-treatment with either verapamil or CsA resulted in significant increases in dialysate IMI concentrations (p ≤ 0.05) relative to the IMI only group, without altering IMI levels in plasma. The dialysate IMI area under the concentration-time curve (AUC; unit: ng.ml-1.min) was increased from 1322 (± 98) in the IMI only group to 1802 (± 144) and 2108 (± 169) in the IMI+VERAP and IMI+CsA groups respectively. The mean dialysate:plasma IMI AUC ratio, which gives an indication of BBB transport, was significantly elevated by 84% in the IMI+CsA group relative to the IMI only group (0.0274 ± 0.0038 vs 0.0149 ± 0.0024; p<0.05), while the 44% increase observed in the IMI+VERAP group compared to the IMI only group did not reach statistical significance (0.0215 ± 0.0028 vs 0.0149 ± 0.0024; p>0.05). Furthermore, pre-treatment with verapamil, but not CsA, led to a significant elevation in plasma and brain levels of DMI relative to the IMI only group (p < 0.001). Plasma DMI AUC values were 46050 (± 4984) in the IMI+VERAP group, compared to 11137 (± 2393) in the IMI only group. DMI dialysate levels could only be determined in the IMI+VERAP group (158.4 ± 14), as dialysate concentrations were below the limit of quantification in the other two groups.

The present study demonstrates that P-gp inhibition can enhance intracerebral IMI concentrations. Furthermore, this study highlights a potentially important pharmacokinetic interaction between IMI and verapamil, whereby co-administration of verapamil leads to significantly elevated levels of the active IMI metabolite, DMI, both in plasma and the brain. Taken together, these findings may help to explain reports of a beneficial response to adjunctive therapy with verapamil in TRD, and highlight a potential therapeutic role for P-gp inhibitors in the clinical management of TRD.

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

[2] Clarke G, et al. (2009). Hum Psychopharmacol 24: 217-223.

[3] de Lange ECM et al. (2000). Adv Drug Deliver Rev 45: 125-148.