13126 Background: CKD601, a newly developed telomerase inhibitor, shows an anti-cancer effect through its inhibitory effect on telomerase, by intercalation of the drug into the structures of the G-quadruplex. No study has been conducted to assess the anti-cancer effects of CKD601 in regards to gastric cancer. We attempted to confirm the anti-cancer effect of CKD601 in the gastric cancer cell line, and to investigate the mechanisms of the anti-cancer effect and resistance in some cell lines. Methods: After long-term drug exposure, we performed Southern analysis, TRAP, and β-Gal staining about the extracted DNA, RNA, and protein from the gastric cancer cell lines and the U2OS cell line to confirm the anti-cancer effect of CKD601. We attempted to investigate the change in the hTERT expression of cancer cells as a result of exposure to CKD601 by RT-PCR and real-time PCR, and to confirm the presence of the ALT (alternate lengthening of telomere) mechanism by metaphase telomere FISH and IF. Results: The anticancer effect of CKD601, including the shortening of telomere, inhibition of telomerase activity, cellular aging, and decreased growth rates, was observed in some gastric cancer cell lines (SNU-1 and SNU-601). SNU-484 and SNU-668 cell lines showed no anti-cancer effect of CKD601. The resistance mechanism of SNU-484 was the significant overexpression of hTERT following exposure to CKD601. ALT, another mechanism that functions in the maintenance of telomere length, was detected in SNU-668 following exposure to CKD601, and it is the resistance mechanism against CKD601. Conclusions: CKD601 is active in gastric cancer by the inhibition of telomerase activity. The resistance mechanisms of gastric cancer cell lines against CKD601 are the induction of the overexpression of hTERT and the ALT mechanism. [Table: see text] No significant financial relationships to disclose.