Introduction: Verapamil is an L-type calcium channel blocker and has been used in the treatment
of hypertension, angina pectoris, cardiac arrhythmia, and most recently, cluster headaches.
Increasing evidence shows that Verapamil is able to enhance the cytotoxic effect of
certain chemotherapies and reverse multidrug resistance by competitively inhibiting
drug transport through P-glycoprotein. The side population (SP) is a small subtype
of tumor cells with stem-like properties which could substantially be blocked by verapamil.
In this study, we focus on the therapeutic potential of verapamil on stem-like SP
cells and its positive effect as a chemotherapy promoter in pancreatic cancer.
Material and methods: From the highly metastatic pancreatic cancer cell line L3.6pl a gemcitabine resistant
variant (L3.6plGem) was developed and further validated by IC-50 determination. Hoechst33342 staining
was used to detect the SP proportion in both sensitive and resistant L3.6pl cells. Proliferation assays, clonogenic assays, migration assays and apoptosis assays were
performed in the presence of verapamil alone and combined with gemcitabine treatment.
SP cells sorted from L3.6plGem were implanted orthotopically in nude mice and treated with verapamil by daily i.p
injections.
Results: After continuous treatment with gemcitabine, the IC50 of L3.6plGem significantly increased from 6.11ng/ml±0.93 to 119.77ng/ml±5.12 (p<1E-9). In parallel,
the percentage of SP cells rose from 0.9%±0.22 to 5.38%±0.99 (p<5E-17). Verapamil
treatment inhibited cell proliferation in both sensitive and resistant cells and increased
chemosensitivity to gemcitabine. After 4 weeks daily therapy with verapamil, the in
vivo data indicated a significant inhibition of primary tumor growth initiated by
chemoresistant SP cells as compared to control groups.
Conclusion: Overall, our findings demonstrate that verapamil could reverse the sensitivity of
gemcitabine in resistant pancreatic cancer cells and might inhibit tumorigenesis by
targeting side population providing evidence for a new clinical feature of this 'old'
reagent.