The present studies were to determine whether the multi-kinase inhibitor sorafenib or its derivative regorafenib interacted with phosphodiesterase 5 (PDE5) inhibitors such as sildenafil (Viagra) to kill tumor cells. PDE5 and PDGFR (alpha and beta) were over-expressed in liver tumors compared to normal liver tissue. In multiple cell types in vitro sorafenib/regorafenib and PDE5 inhibitors interacted in a greater than additive fashion to cause tumor cell death. Knock down of PDE5 or of PDGFR (alpha and beta) recapitulated the effects of the individual drugs. The drug combination increased ROS/RNS levels that were causal in cell killing. RNS stimulated the activation of the death receptor CD95 as well as the formation of autophagosomes and autolysosomes. Inhibition of CD95 / FADD / caspase 8 signaling suppressed drug combination toxicity. Knock down of ULK-1, Beclin1 or ATG5 suppressed drug combination lethality. The drug combination inactivated ERK, AKT, p70 S6K and mTOR and activated JNK. The drug combination also reduced mTOR protein expression. Activation of ERK or AKT was modestly protective whereas re-expression of an activated mTOR protein or inhibition of JNK signaling almost abolished drug combination toxicity. Sildenafil and sorafenib / regorafenib interacted in vivo to suppress xenograft tumor growth using liver and colon cancer cells. PDE5 inhibitors and sorafenib / regorafenib are FDA approved agents, and our data is now being translated into the clinic for further determination as to whether this drug combination is a useful anti-tumor therapy for solid tumor patients.

Citation Format: Mehrad Tavallai, Laurence Booth, Jane L. Roberts, Andrew Poklepovic, Paul Dent. Regorafenib and sildenafil interact to kill tumor cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-094. doi:10.1158/1538-7445.AM2015-LB-094