Multidrug resistance (MDR) caused by overexpressed permeability–glycoprotein (P-gp) in cancer cells is the main barrier for the cure of cancers. P-gp can pump many chemotherapeutic drugs, which is a viable target to overcome P-gp-mediated MDR by efficient inhibitors of P-gp. However, limited understanding of the efflux mechanism by human P-gp hinders the development of efficient inhibitors. Herein, the transport of a P-gp inhibitor, verapamil, by human P-gp has been investigated using targeted molecular dynamics simulations and energetics analysis based on our previous research on the transport of a drug (doxorubicin). The energetics analysis identifies that the driving forces for the transport of verapamil are electrostatic repulsions contributed by the positively charged residues in the initial stage and then hydrophobic interactions contributed by the important residues in the later stage. This scenario is generally consistent with that in the transport of doxorubicin. However, the positively charged residues and the important residues for the transport of verapamil are incompletely consistent with the relative residues for the transport of doxorubicin. Moreover, the binding free energy contributions of the positively charged residues for the transport of verapamil are generally higher than them for the transport of doxorubicin, while the important residues constitute significantly different binding free energy compositions in the transports of the two substrates. Consequently, the pathway for the transport of verapamil is identified, which shares only two residues (F336 and M986) with the pathway of doxorubicin. This may imply the weak competitiveness of verapamil with doxorubicin in the substrate efflux. Taken together, this work provided new insights into the efflux mechanisms by human P-gp and would be beneficial in the design of potent P-gp inhibitors.

癌细胞中过度表达的通透性糖蛋白（P-gp）引起的多药耐药性（MDR）是治愈癌症的主要障碍。P-gp可以泵送许多化疗药物，这是通过有效的P-gp抑制剂克服P-gp介导的MDR的可行目标。然而，对人P-gp对外排机制的了解有限，阻碍了有效抑制剂的开发。在此，基于我们先前对药物（阿霉素）的运输研究，使用靶向分子动力学模拟和能量学分析，研究了人P-gp对P-gp抑制剂维拉帕米的运输。能量学分析表明，维拉帕米转运的驱动力是在最初阶段是由带正电荷的残基促成的静电排斥，然后是在重要阶段是由重要残基促成的疏水相互作用。这种情况通常与阿霉素的运输情况一致。然而，带正电的残基和维拉帕米转运的重要残基与阿霉素的相对残基不完全一致。此外，带正电的残留物在维拉帕米运输中的结合自由能贡献通常要比在阿霉素的运输中高，而重要的残基在两种底物的转运中构成了明显不同的结合自由能组成。因此，确定了维拉帕米的运输途径，该途径仅与阿霉素途径共有两个残基（F336和M986）。这可能暗示维拉帕米与阿霉素在底物外排中的竞争力较弱。两者合计，这项工作为人类P-gp的外排机制提供了新的见识，并将对有效P-gp抑制剂的设计有益。