PGP and major depressive disorder: Since no predictable active efflux was observed by the P-gp for the investigated indole derivatives (5–8), they can be delivered in appropriate concentrations to the brain and used in the treatment of neurological disorders, including depression [29], whereas 3 and 4 are probable P-gp substrates due to the presence of more rotatable bonds compared to indole derivatives (5–8) [40].