In vitro activity: CX-5461 is found to selectively inhibit rRNA synthesis (Pol I IC50=142 nM; Pol II IC50> 25 μM; selectivity ~200-fold) in the HCT-116 cells. Selective inhibition of rRNA synthesis by CX-5461 is confirmed in two other human solid tumor cell lines; melanoma A375 (Pol I IC50 = 113 nM; Pol II IC50> 25 μM) and pancreatic carcinoma MIA PaCa-2 (Pol I IC50=54 nM; Pol II IC50 ~25 mM). CX-5461 possesses 250- to 300-fold selectivity for inhibition of rRNA transcription versus DNA replication and protein translation. CX-5461 exhibits broad antiproliferative potency in a panel of cancer cell lines in human cancer cell lines, but has minimal effect on viability of nontransformed human cells. The median EC50 across all tested cell lines is 147 nM, yet all normal cell lines have EC50 values of approximately 5, 000 nM. Evaluation of the antiproliferative dose response for HCT-116, A375, and MIA PaCa-2 cell lines yield EC50 values of 167, 58, and 74 nM. CX-5461 induces autophagy and senescence in solid tumor cancer cells, rather than apoptosis, through a p53-independent process.

Kinase Assay: Two short-lived RNA transcripts (half-lives ~20-30 minutes), one produced by Pol I and another by Pol II, are quantitated by qRT-PCR as a measure of CX-5461-related effects on transcription. The 45S pre-rRNA served as the Pol I transcript and the mRNA for the protooncogene c-myc served as the comparator Pol II transcript. Both Pol I and Pol II transcription are known to be affected by general cellular stress. To minimize the potential effects of such stress, cells are exposed to test agents for only a short period of time (2 hours). This is sufficient time for these transcripts to be reduced by greater than 90% if CX-5461 affects their synthesis.

Cell Assay: Cells are plated on 96-well plates and treated the next day with dose response of CX-5461 for 96 hours. Cell viability is determined using Alamar Blue and CyQUANT assays.

Cancer Res. 2011 Feb 15;71(4):1418-30.