Histamine Receptor Antagonists, Loratadine and Azelastine, Sensitize P-gp-overexpressing Antimitotic Drug-resistant KBV20C Cells through Different Molecular Mechanisms

Background/Aim: Previously, we showed that KBV20C cancer cells highly resistant to antimitotic drugs were sensitized by co-treatment with a repositioned drug fluphenazine. Materials and Methods: Considering that fluphenazine plays a role as a histamine receptor antagonist, we investigated low doses of 21 other histamine receptor antagonists (lidocaine, cimetidine, chlorpromazine, diphenhydramine, promethazine, ranitidine, famotidine, clemastine, chlorpheniramine, desloratadine, loratadine, cyproheptadine, azelastine, brompheniramine, carbinoxamine, fexofenadine, hydroxyzine, levocetirizine, meclizine, nizatidine, and pemirolast) to identify repositioned drugs for their sensitizing effects on antimitotic drug-resistant KBV20C cells at relatively low doses. Results: Co-treatment with loratadine, and with azelastine highly sensitized KBV20C cells to vincristine treatment. Loratadine and azelastine reduced cell viability, increased G₂ arrest, and up-regulated apoptosis when co-administered with vincristine. In detailed quantitative analysis, combination of vincristine with loratadine had a higher sensitization effect than that with azelastine. Azelastine had a higher P-glycoprotein (P-gp)-inhibitory activity, similar to that of verapamil, indicating that sensitization by vincristine-azelastine involved the P-gp-inhibitory effects of azelastine. However, loratadine had a very low P-gp-inhibitory activity, suggesting that loratadine sensitization to vincristine is independent of the P-gp-inhibition. Co-treatment with eribulin and loratadine increased the sensitization of KBV20C cells, suggesting that loratadine can be combined with other antimitotic drugs to sensitize resistant cancer cells. Conclusion: These findings provide important information regarding the sensitization of drug-resistant cells and indicate that loratadine may be used in patients with potentially resistant cancer without any toxic effects from P-gp inhibition.