MicroRNAs (miRs) are post-transcriptional regulators involved in the initiation 
and progression of many tumors. Recently, naturally occurring circular RNAs 
(circRNAs) have been described in eukaryotic cells:;they comprise a new class of 
gene regulators. Naturally occurring circular miR sponges, which induce miR 
loss-of-function, can prevent endogenous onco-miRs from binding to their cognate 
mRNA targets. These findings suggest that synthetic (artificial) circular RNAs 
could be constructed as therapeutic molecular sponges to suppress harmful 
onco-miRs. Using enzymatic ligation, we designed and constructed a circular RNA 
containing both miR-21 and miR-93 binding sites. The synthetic circular sponge 
was resistant to digestion with RNase R. Luciferase assays and functional 
experiments showed that the circular multi-miR sponge was more stable than its 
linear counterpart. Moreover, endogenous miR-21 and miR-93 were inhibited by the 
circular sponge. In addition, the synthetic sponge significantly suppressed 
cellular proliferation and migration while promoting apoptosis in esophageal 
carcinoma cells. Finally, in a murine xenograft model, the circular sponge 
significantly inhibited tumor growth in vivo. Taken together, these findings 
establish that the design and construction of efficient artificial miR sponges 
represent a novel strategy to achieve miR loss-of-function in molecular cancer 
therapeutics.