Potentiation of motexafin gadolinium cytotoxicity by celecoxib but not other cyclooxygenase-2 inhibitors

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Motexafin gadolinium (MGd, Xcytrin®) is a tumor-selective redox mediator that catalytically oxidizes intracellular reducing metabolites and produces reactive oxygen species (ROS). In some hematopoietic tumor-derived cell lines such as HF-1, MGd treatment results in apoptosis and growth inhibition, whereas in others such as Ramos and DHL-4, MGd treatment results in growth inhibition. Celecoxib, a cyclooxygenase-2 (COX-2) inhibitor, has been shown to induce apoptosis and growth inhibition by itself and potentiate the effects of chemotherapeutic agents. These effects of celecoxib may occur through COX-2 inhibition and/or a COX-2-independent pathway since celecoxib can also disrupt a survival pathway mediated by Akt kinase. To determine if celecoxib could potentiate the effects of MGd, we treated lymphoma cell lines HF-1, Ramos and DHL-4 with MGd (50 μM), celecoxib (10-25 μM) or the combination MGd/celecoxib. In all three cell lines, MGd/celecoxib treatment resulted in enhanced growth inhibition compared to MGd or celecoxib treatment alone. In HF-1 and Ramos, MGd/celecoxib resulted in increased apoptosis as assessed by annexin V binding and caspase activity assays compared to either agent alone. Further experiments in Ramos cells showed that only MGd/celecoxib treatment resulted in decreased levels of phosphorylated Akt kinase (to about 30% of normal), and activation of the caspase cascade as demonstrated by cleavage of caspases-9, -8, -3 and cleavage of substrates Bid and Parp. The specific COX-2 inhibitors, rofecoxib and valdecoxib (which are not reported to interfere with Akt phosphorylation), did not potentiate the effects of MGd in Ramos cells. These results suggest that celecoxib potentiation of MGd may be COX-2 independent. The combination of MGd/celecoxib was also active in an animal model since treatment with MGd (46 mg/kg IV x 5 days)/celecoxib (25 mg/kg oral gavage x 5 days) suppressed the growth of Ramos xenografts in nude mice at doses where neither MGd nor celecoxib were effective as single agents. Our pre-clinical data suggest that the addition of celecoxib to MGd-containing clinical regimens to enhance cytotoxicity or growth suppression of tumors should be explored.