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

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 µg/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.