P-glycoprotein reduces the ability of amitriptyline metabolites to cross the blood brain barrier in mice after a 10-day administration of amitriptyline

Abstract

P-glycoprotein (P-gp) is a 170-kDa membrane protein and the gene product of the multiple drug resistance (MDR1 or ABCB1) gene. It constitutes an important part of the blood-brain barrier and actively exports a number of molecules across the blood-brain barrier back into the vascular space, subsequently reducing central nervous system (CNS) bioavailability of these substances. The aim of the present study was to investigate the pharmacokinetics of amitriptyline and its metabolites in P-gp (also called mdr1ab or abcb1ab) knockout mice and controls after a long-term adminstration for 10 days. Knockout mice and controls received s.c. injections of amitriptyline (10 microg/g bodyweight) twice daily for 10 days. After 10 days, the animals were sacrificed and the concentrations of amitriptyline and nortriptyline and both their E-10-OH and Z-10-OH metabolites were measured with high-performance liquid chromatography in the cerebrum, plasma, spleen, kidney, testes, lung, liver, muscle and fat. Except for amitriptyline, the brain concentrations of all other examined substances were significantly higher in the P-gp knockout mice. Compared to controls, concentrations of nortriptyline were 2.6-fold higher, E-10-OH-nortriptyline 10-fold higher, Z-10-OH-nortriptyline seven-fold higher, E-10-OH-amitriptyline two-fold higher and Z-10-OH-amitriptyline five-fold higher. The present study confirms that P-gp plays an important role in the interaction between CNS drugs and the blood-brain barrier. Without P-gp at the blood-brain barrier, the brain concentrations of the substances were up to 10-fold higher, showing that P-gp plays an active role in exporting CNS drugs out of the brain. Recent clinical studies showing different side-effects in patients with P-gp polymorphisms confirm the clinical importance of these findings.

The Antidepressant Desipramine Requires the ABCB1 (Mdr1)-Type p-Glycoprotein to Upregulate the Glucocorticoid Receptor in Mice

“…However, it is important to note that chronic treatment is different from a single injection; indeed, these same authors have found that after 10 days of administration (20 mg/kg/d in two i.p. injections, as in our study) only nortriptyline brain levels were elevated in abcb1ab (À/À) mice (approximately threefold), whereas amitriptyline brain levels were identical to those of FVB/N controls (Grauer and Uhr, 2004). Nevertheless, even if brain levels of desipramine were increased in abcb1ab (À/À) mice, there is no evidence that higher doses of tricyclic could lead to hippocampal GR downregulation rather than upregulation.…”

“…These would have been interesting data, as previous studies (Uhr et al, 2000 have shown, also using abcb1ab (À/À) mice, that MDR PGP limits the access to the brain of the tricyclic antidepressants, doxepine, amitriptyline, nortriptyline and trimipramine, after a single injection (although no data are available on desipramine). However, our study protocol was aimed at detecting persisting changes in the HPA axis, rather than the steady-state brain concentrations of antidepressants, and, therefore, the mice were killed 16 h after the last injection, whereas in previous studies measuring brain antidepressants levels, the animals were killed 15-60 min after single injections (Glotzbach and Preskorn, 1982;Uhr et al, 2000 or 4 h after the last injections in chronic-treatment studies (Grauer and Uhr, 2004); this could explain our negative findings. Theoretically, it is possible that abcb1ab (À/À) mice in our study have higher brain concentrations of desipramine, and that this might explain the differences in the GR expression.…”

Expression of multi-drug resistance 1 mRNA in human and rodent tissues: reduced levels in Parkinson patients

“…The presence of Mdr1 in the meninges and the blood-brain barrier is critical for controlling the entry of drugs into the nervous system. Mice lacking one or both of the Mdr1a and Mdr1b genes show decreased drug elimination and increased drug levels in many tissues, in particular the brain (Schinkel et al 1994;Liu et al 2002;Grauer and Uhr 2004). In the present study, we have observed significantly reduced MDR1 mRNA levels in striatum of Parkinson patients compared with controls, indicating reduced MDR1 gene activity or decreased mRNA stability.…”

ABCB1 (P-Glycoprotein/MDR1) Gene G2677T/A Sequence Variation (Polymorphism): Lack of Association with Side Effects and Therapeutic Response in Depressed Inpatients Treated with Amitriptyline

“…Recent studies showed that penetration of amitriptyline and its metabolites (including nortriptyline) into the brain is enhanced in mice lacking P-gp (4,5 ). Therefore, dysfunctional sequence variants of the ABCB1 gene might lead to diminished P-gp activity and higher bioavailability of P-gp substrates in the central nervous system.…”