Subcellular Location of Tirapazamine Reduction Dramatically Affects Aerobic but Not Anoxic Cytotoxicity.

Hypoxia is an adverse prognostic feature of solid cancers that may be overcome with hypoxia-activated prodrugs (HAPs). Tirapazamine (TPZ) is a HAP which has undergone extensive clinical evaluation in this context and stimulated development of optimized analogues. However the subcellular localization of the oxidoreductases responsible for mediating TPZ-dependent DNA damage remains unclear. Some studies conclude only nuclear-localized oxidoreductases can give rise to radical-mediated DNA damage and thus cytotoxicity, whereas others identify a broader role for endoplasmic reticulum and cytosolic oxidoreductases, indicating the subcellular location of TPZ radical formation is not a critical requirement for DNA damage. To explore this question in intact cells we engineered MDA-231 breast cancer cells to express the TPZ reductase human NADPH: cytochrome P450 oxidoreductase (POR) harboring various subcellular localization sequences to guide this flavoenzyme to the nucleus, endoplasmic reticulum, cytosol or inner surface of the plasma membrane. We show that all POR variants are functional, with differences in rates of metabolism reflecting enzyme expression levels rather than intracellular TPZ concentration gradients. Under anoxic conditions, POR expression in all subcellular compartments increased the sensitivity of the cells to TPZ, but with a fall in cytotoxicity per unit of metabolism (termed 'metabolic efficiency') when POR is expressed further from the nucleus. However, under aerobic conditions a much larger increase in cytotoxicity was observed when POR was directed to the nucleus, indicating very high metabolic efficiency. Consequently, nuclear metabolism results in collapse of hypoxic selectivity of TPZ, which was further magnified to the point of reversing O2 dependence (oxic > hypoxic sensitivity) by employing a DNA-affinic TPZ analogue. This aerobic hypersensitivity phenotype was partially rescued by cellular copper depletion, suggesting the possible involvement of Fenton-like chemistry in generating short-range effects mediated by the hydroxyl radical. In addition, the data suggest that under aerobic conditions reoxidation strictly limits the TPZ radical diffusion range resulting in site-specific cytotoxicity. Collectively these novel findings challenge the purported role of intra-nuclear reductases in orchestrating the hypoxia selectivity of TPZ.

2-Oxo-3, 4-dihydropyrimido[4, 5-d]pyrimidinyl derivatives as new irreversible pan fibroblast growth factor receptor (FGFR) inhibitors.

A series of 2-oxo-3, 4-dihydropyrimido[4,5-d]-pyrimidinyl derivatives were designed and synthesized as new irreversible inhibitors of the FGFR family. One of the most promising compounds 2l potently inhibited FGFR1/2/3 with IC50 values of 1.06, 0.84 and 5.38 nM, respectively, whereas its potency against FGFR4 was diminished by an order of magnitude. Compound 2l strongly suppresses the proliferation of FGFR1-amplified H520 non-small cell lung cancer cells, FGFR2-amplified SUM52 breast cancer cells and FGFR3-amplified SW780 bladder cancer cells with low nanomolar IC50 values, but was significantly less potent against four FGFR-negative cancer cell lines, with low micromolar IC50 values. Biological investigation also confirmed the irreversible binding of the molecule with the FGFR1-3 target kinases. Compound 2l may serve as a promising new lead for further anticancer drug discovery.

N-(3-Ethynyl-2,4-difluorophenyl)sulfonamide Derivatives as Selective Raf Inhibitors.

A series of N-(3-ethynyl-2,4-difluorophenyl)sulfonamides were identified as new selective Raf inhibitors. The compounds potently inhibit B-Raf(V600E) with low nanomolar IC50 values and exhibit excellent target specificity in a selectivity profiling investigation against 468 kinases. They strongly suppress proliferation of a panel of human cancer cell lines and patient-derived melanoma cells with B-Raf(V600E) mutation while being significantly less potent to the cells with B-Raf(WT). The compounds also display favorable pharmacokinetic properties with a preferred example (3s) demonstrating significant in vivo antitumor efficacy in a xenograft mouse model of B-Raf(V600E) mutated Colo205 human colorectal cancer cells, supporting it as a promising lead compound for further anticancer drug discovery.