Evaluation of the DNA topoisomerase inhibitor irinotecan (CPT-11) as an alternative to irradiation for pretreatment of NK-92s prior to adoptive cell immunotherapy

Irradiation damages both proteins and DNA, which reduces the cytotoxicity of NK-92 cells as well as preventing their growth and engraftment when used for adoptive immunotherapies of cancer. We evaluated the DNA topoisomerase inhibitor irinotecan (CPT-11) as an alternative method to 10 Gy irradiation that might better preserve the perforin-granzyme cytotoxicity mediated by NK-92 cells towards tumor cells. Irinotecan is enzymatically converted into an active metabolite SN-38 which promotes its potential to become a delayed inhibitor in comparison to camptothecin which is a structurally similar topoisomerase inhibitor that is immediately active. We pretreated NK-92 cells for 4 hours with different concentrations of CPT-11, and measured NK-92 cell death and the effects on cell-mediated cytotoxicity towards Raji tumor cells (measured by 51Cr release one day after pretreatment). Previous experiments with 10 Gy irradiation were used for comparative purposes. Irinotecan had a dose response on cell death which progressed for 5 days. We found that one day after pretreatment with as little as 80 – 100 uM CPT-11, viable cell recoveries were similar to 10 Gy irradiated with over 50% compared to the counts of control cells. Activity of the viable cells after CPT-11 pretreatment, compared by lytic units, was over 30% conserved whereas irradiated cell activity was 2.9%, compared to controls. This data demonstrate that the negative effects of anti-proliferative treatment are less pronounced with irinotecan, than for irradiation. We conclude that optimization of dosage and exposure times to irinotecan may offer a means to prolong the life and function of NK-92 cells that are used for adoptive immunotherapies. Supported by the Nevada INBRE NIH GM103440, for student summer research scholarship and the UNR Foundation C.E. Hudig Fund.