TLK286, a novel glutathione analog prodrug that induces irreversible G2/M cell cycle arrest and apoptosis in vitro in human lung, ovarian and colon cancer

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TLK286 is a novel anticancer prodrug that has shown significant activity in Phase 2 clinical trials in ovarian, non-small cell lung, breast and colorectal cancers. TLK286 is a glutathione analog activated by GST P1-1, an enzyme frequently elevated in ovarian, non-small cell lung, breast, colorectal, pancreatic cancer and lymphomas. Activation of TLK286 by GST P1-1 releases a phosphorodiamidate and a glutathione analog vinyl sulfone. We report here the cellular and molecular effects of TLK286 exposure in several human cancer cell lines. TLK286 treatment caused a p53-independent cell cycle arrest at G2/M in OVCAR3, DLD-1 and A549 human cancer cell lines. In the p53-deficient DLD-1 human colon cancer cells, the cell cycle block occurred in a dose and time dependent manner and the cells subsequently underwent apoptosis as measured by the activation of caspase-3 and the appearance of a sub-G0/G1 population. In addition, TLK286 treatment induced accumulation of several cell cycle regulatory proteins, including p21^waf1, p53 and cyclin B. Treatment of DLD-1 cells with the DNA alkylators melphalan and cisplatin caused apoptosis without G2/M arrest in these cells. Interestingly, the p53-positive A549 human lung cancer cells arrested irreversibly at G2/M in response to TLK286 treatment while the growth of non-transformed, primary human diploid cells was reversibly inhibited without the G2/M cell cycle arrest. These results suggest that the anticancer activity of TLK286 may be mediated by cell cycle arrest leading to subsequent cell death or growth arrest.