Interestingly, PDE inhibitors were shown to inhibit several MDR transporters (e.g., ABCC4/MRP4, ABCC5/MRP5, ABCB1/P-gp, ABCG2/BCRP), thus enhancing chemotherapeutic drug accumulation in tumor cells and efficacy [50,51]. This evidence concerns the gene ABCC4 and neoplasm.