Functional CRISPR and shRNA Screens Identify Involvement of Mitochondrial Electron Transport in the Activation of Evofosfamide

Evofosfamide (TH-302) is a hypoxia-activated DNA-crosslinking prodrug currently in clinical development for cancer therapy. Oxygen-sensitive activation of evofosfamide depends on one-electron reduction, yet the reductases that catalyze this process in tumors are unknown. We used RNA sequencing, whole-genome CRISPR knockout, and reductase-focused short hairpin RNA screens to interrogate modifiers of evofosfamide activation in cancer cell lines. Involvement of mitochondrial electron transport in the activation of evofosfamide and the related nitroaromatic compounds EF5 and FSL-61 was investigated using 143B ρ ⁰ (ρ zero) cells devoid of mitochondrial DNA and biochemical assays in UT-SCC-74B cells. The potency of evofosfamide in 30 genetically diverse cancer cell lines correlated with the expression of genes involved in mitochondrial electron transfer. A whole-genome CRISPR screen in KBM-7 cells identified the DNA damage-response factors SLX4IP, C10orf90 (FATS), and SLFN11, in addition to the key regulator of mitochondrial function, YME1L1, and several complex I constituents as modifiers of evofosfamide sensitivity. A reductase-focused shRNA screen in UT-SCC-74B cells similarly identified mitochondrial respiratory chain factors. Surprisingly, 143B ρ ⁰ cells showed enhanced evofosfamide activation and sensitivity but had global transcriptional changes, including increased expression of nonmitochondrial flavoreductases. In UT-SCC-74B cells, evofosfamide oxidized cytochromes a, b, and c and inhibited respiration at complexes I, II, and IV without quenching reactive oxygen species production. Our results suggest that the mitochondrial electron transport chain contributes to evofosfamide activation and that predicting evofosfamide sensitivity in patients by measuring the expression of canonical bioreductive enzymes such as cytochrome P450 oxidoreductase is likely to be futile.

Cellular pharmacology of evofosfamide (TH-302): A critical re-evaluation of its bystander effects

Evofosfamide (TH-302) is a clinical-stage hypoxia-activated prodrug with proven efficacy against hypoxic cells in preclinical tumour models. TH-302 is designed to release the DNA crosslinking agent bromo-isophosphoramide mustard (Br-IPM) when reduced in hypoxic tissue. Br-IPM is considered to diffuse locally from hypoxic regions, eliciting additional tumour cell killing, but the latter 'bystander effect' has not been demonstrated directly. Previous studies with multicellular co-cultures that included cells expressing the E. coli nitroreductase NfsA as a model TH-302 reductase have provided clear evidence of a bystander effect (which we confirm in the present study). However, NfsA is an oxygen-insensitive two-electron reductase that is not expected to generate the nitro radical intermediate that has been demonstrated to fragment to release Br-IPM. Here, we use mass spectrometry methods to characterise TH-302 metabolites generated by one-electron reduction (steady-state radiolysis by ionising radiation and cellular metabolism under hypoxia, including HCT116 cells that overexpress P450 oxidoreductase, POR) or by NfsA expressed in HCT116 cells under oxic conditions, and investigate the stability and cytotoxicity of these products. Br-IPM is shown to have very low cytotoxic potency when added to extracellular culture medium and to be rapidly converted to other hydrophilic products including dichloro-isophosphoramide mustard (IPM). Only traces of Br-IPM or IPM were detected in the extracellular medium when generated by cellular metabolism of TH-302. We identify, in NfsA-expressing cells, the hydroxylamine metabolite of TH-302, and downstream products resulting from rearrangement or hydration of the imidazole ring, and demonstrate that formation of these candidate bystander effect mediators is suppressed by hypoxia. This characterisation of the cellular pharmacology of TH-302 implies that bystander effects from hypoxic activation of TH-302 are unlikely to contribute to its anticancer activity.

Next-Generation Hypoxic Cell Radiosensitizers: Nitroimidazole Alkylsulfonamides

Innovations in the field of radiotherapy such as stereotactic body radiotherapy, along with the advent of radio-immuno-oncology, herald new opportunities for classical oxygen-mimetic radiosensitizers. The role of hypoxic tumor cells in resistance to radiotherapy and in suppression of immune response continues to endorse tumor hypoxia as a bona fide, yet largely untapped, drug target. Only nimorazole is used clinically as a radiosensitizer, and there is a dearth of new radiosensitizers in development. Here we present a survey of novel nitroimidazole alkylsulfonamides and document their cytotoxicity and ability to radiosensitize anoxic tumor cells in vitro. We use a phosphate prodrug approach to increase aqueous solubility and to improve tumor drug delivery. A 2-nitroimidazole and a 5-nitroimidazole analogue demonstrated marked tumor radiosensitization in either ex vivo assays of surviving clonogens or tumor regrowth delay.