Hypoxia-activated
prodrugs (HAPs) are hypothesized to improve the therapeutic index of
chemotherapy drugs that are ineffective against
tumor cells in hypoxic microenvironments.
SN30000 (CEN-209) is a benzotriazine di-N-
oxide HAP that potentiates
radiotherapy in preclinical models, but its combination with
chemotherapy has not been explored. Here we apply multiple models (monolayers, multicellular spheroids and
tumor xenografts) to identify promising
SN30000/
chemotherapy combinations (with
chemotherapy drugs before, during or after
SN30000 exposure).
SN30000, unlike
doxorubicin,
cisplatin,
gemcitabine or
paclitaxel, was more active against cells in spheroids than monolayers by clonogenic assay. Combinations of
SN30000 and
chemotherapy drugs in HCT116/GFP and SiHa spheroids demonstrated
hypoxia-and schedule-dependent potentiation of
gemcitabine or
doxorubicin in growth inhibition and clonogenic assays. Co-administration with
SN30000 suppressed clearance of
gemcitabine in NIH-III mice, likely due to SN30000-induced
hypothermia which also modulated extravascular transport of
gemcitabine in
tumor tissue as assessed from its diffusion through HCT116 multicellular layer cultures. Despite these systemic effects, the same schedules that gave therapeutic synergy in spheroids (
SN30000 3 h before or during
gemcitabine, but not
gemcitabine 3 h before
SN30000) enhanced growth delay of HCT116 xenografts without increasing host toxicity. Identification of hypoxic and S-phase cells by immunohistochemistry and flow cytometry established that hypoxic cells initially spared by
gemcitabine subsequently reoxygenate and re-enter the cell cycle, and that this repopulation is prevented by
SN30000 only when administered with or before
gemcitabine. This illustrates the value of spheroids in modeling tumor microenvironment-dependent drug interactions, and the potential of HAPs for overcoming
hypoxia-mediated drug resistance.