The development of P-glycoprotein (P-gp) ligands remains of considerable interest, mostly for investigating the protein's structure and transport mechanism. In recent years, many different generations of ligands have been tested for their ability to modulate P-gp activity. The aim of the present work is to perform SAR studies on tetrahydroisoquinoline derivatives in order to design potent and selective P-gp ligands. For this purpose, the effect of bioisosteric replacement and the role of flexibility have been investigated, and four series of tetrahydroisoquinoline ligands have been developed: (a) 2-aryloxazole bioisosteres, (b) elongated analogues, (c) 2H-chromene, and (d) 2-biphenyl derivatives. The results showed that both 2-biphenyl derivative 20b and elongated derivative 6g behaved as strong P-gp substrates. In conclusion, important aspects for developing potent and selective P-gp ligands have been highlighted, providing a solid starting point for further optimization.