Roger J. Griffin

Preclinical evaluation of a potent novel DNA-dependent protein kinase inhibitor NU7441

DNA double-strand breaks (DSB) are the most cytotoxic lesions induced by ionizing radiation and topoisomerase II poisons, such as etoposide and doxorubicin. A major pathway for the repair of DSB is nonhomologous end joining, which requires DNA-dependent protein kinase (DNA-PK) activity. We investigated the therapeutic use of a potent, specific DNA-PK inhibitor (NU7441) in models of human cancer. We measured chemosensitization by NU7441 of topoisomerase II poisons and radiosensitization in cells deficient and proficient in DNA-PK(CS) (V3 and V3-YAC) and p53 wild type (LoVo) and p53 mutant (SW620) human colon cancer cell lines by clonogenic survival assay. Effects of NU7441 on DSB repair and cell cycle arrest were measured by gammaH2AX foci and flow cytometry. Tissue distribution of NU7441 and potentiation of etoposide activity were determined in mice bearing SW620 tumors. NU7441 increased the cytotoxicity of ionizing radiation and etoposide in SW620, LoVo, and V3-YAC cells but not in V3 cells, confirming that potentiation was due to DNA-PK inhibition. NU7441 substantially retarded the repair of ionizing radiation-induced and etoposide-induced DSB. NU7441 appreciably increased G(2)-M accumulation induced by ionizing radiation, etoposide, and doxorubicin in both SW620 and LoVo cells. In mice bearing SW620 xenografts, NU7441 concentrations in the tumor necessary for chemopotentiation in vitro were maintained for at least 4 hours at nontoxic doses. NU7441 increased etoposide-induced tumor growth delay 2-fold without exacerbating etoposide toxicity to unacceptable levels. In conclusion, NU7441 shows sufficient proof of principle through in vitro and in vivo chemosensitization and radiosensitization to justify further development of DNA-PK inhibitors for clinical use.

Cyclin dependent kinase inhibitors

Cyclin-dependent kinases are involved in diverse cellular processes that include cell cycle control, apoptosis, neuronal physiology, differentiation, and transcription. Intensive screening and drug design based on CDK/inhibitor co-crystal structures and on SAR studies have led to the identification and characterization of a large variety of chemical inhibitors of CDKs. Although they all act by competing with ATP for binding at the catalytic site of the kinase, their kinase selectivity varies greatly and remains to be studied in most cases. The requirement for CDKs in many physiological processes justifies their evaluation as potential therapeutic targets against a much larger scope of diseases than initially anticipated.Cell-cycle dysregulation is one of the cardinal characteristics of neoplastic cells. For this reason, small molecule inhibitors targeting cyclin-dependent kinases (CDKs), of which flavopiridol is a prototype, have been the focus of extensive interest in cancer therapy. In addition to inhibiting cell-cycle progression, these agents exhibit a variety of other activities, including the induction of cell death. Recently, several novel mechanisms of action have been ascribed to the CDK inhibitor flavopiridol, including interference with transcription, most likely through disruption of P-TEFb (i.e. the CDK9/cyclin T complex), and induction of apoptosis, possibly a consequence of downregulation of various anti-apoptotic proteins. It has also been observed that combining CDK inhibitors with either conventional cytotoxic drugs or novel signal transduction modulators dramatically promotes neoplastic cell death in a variety of preclinical models. Efforts are underway to uncover inhibitors that selectively target specific CDKs and to develop these as a new generation of antitumour drugs. For all of these reasons, it is likely that interest in CDK inhibitors as antineoplastic agents will continue for the foreseeable future.

Potentiation of anti-cancer agent cytotoxicity by the potent poly(ADP-ribose) polymerase inhibitors NU1025 and NU1064 *

The ability of the potent poly(ADP-ribose) polymerase (PARP) inhibitor, NU1025 (8-hydroxy-2-methyl-quinazolin-4-[3H]one) to potentiate the cytotoxicity of a panel of mechanistically diverse anti-cancer agents was evaluated in L1210 cells. NU1025 enhanced the cytotoxicity of the DNA-methylating agent MTIC, gamma-irradiation and bleomycin 3.5-, 1.4- and 2-fold respectively. The cytotoxicities of the thymidylate synthase inhibitor, nolatrexed, and the cytotoxic nucleoside, gemcitabine, were not increased. Potentiation of MTIC cytotoxicity by a delayed exposure to NU1025 was equally effective as by a simultaneous exposure to NU1025, indicating that the effects of NU1025 were mediated by an inhibition of the cellular recovery. The recovery from potentially lethal gamma-irradiation damage cytotoxicity in plateau-phase cells was also inhibited by NU1025. Investigation of DNA strand breakage and repair in gamma-irradiated cells by alkaline elution demonstrated that NU1025 caused a marked retardation of DNA repair. A structurally different PARP inhibitor, NU1064 (2-methylbenzimidazole-4-carboxamide), also potentiated the cytotoxicity of MTIC, to a similar extent to NU1025. NU1064 potentiated a sublethal concentration of a DNA methylating agent in a concentration-dependent manner. Collectively, these data suggest that the most suitable cytotoxic agents for use in combination with PARP inhibitors are methylating agents, bleomycin and ionizing radiation, but not anti-metabolites.

Potentiation of temozolomide-induced cytotoxicity: a comparative study of the biological effects of poly(ADP-ribose) polymerase inhibitors.

Four poly(ADP-ribose) polymerase (PADPRP) inhibitors [3-aminobenzamide, benzamide, 3,4-dihydro-5-methoxyisoquinolin-1(2H)-one (PD 128763) and 8-hydroxy-2-methylquinazolin-4(3H)-one (NU1025)] were compared with respect to their effects on a number of biological end points. The following parameters were assessed: their ability to inhibit the enzyme in permeabilised L1210 cells; their ability to potentiate the cytotoxicity of temozolomide (including the cytotoxicity of the compounds per se); their ability to increase net levels of temozolomide-induced DNA strand breaks and inhibit temozolomide-induced NAD depletion. PD 128763 and NU1025 were equipotent as PADPRP inhibitors, and 40- and 50-fold more potent than benzamide and 3-aminobenzamide respectively. All the compounds acted in a concentration-dependent manner to potentiate the cytotoxicity and increase DNA strand break levels in cells treated with temozolomide. There was an excellent correlation between the potency of the compounds as PADPRP inhibitors and their effects on cell survival and DNA repair. Temozolomide treatment caused a decrease in cellular NAD levels, and this was abolished by the PADPRP inhibitors. In conclusion, the new generation of PADPRP inhibitors are at least 50-fold more effective than 3-aminobenzamide as chemopotentiators, and can be used at micromolar rather than millimolar concentrations in intact cells.

Isoindolinone Inhibitors of the Murine Double Minute 2 (MDM2)-p53 Protein-Protein Interaction: Structure-Activity Studies Leading to Improved Potency

Inhibition of the MDM2-p53 interaction has been shown to produce an antitumor effect, especially in MDM2 amplified tumors. The isoindolinone scaffold has proved to be versatile for the discovery of MDM2-p53 antagonists. Optimization of previously reported inhibitors, for example, NU8231 (7) and NU8165 (49), was guided by MDM2 NMR titrations, which indicated key areas of the binding interaction to be explored. Variation of the 2-N-benzyl and 3-alkoxy substituents resulted in the identification of 3-(4-chlorophenyl)-3-((1-(hydroxymethyl)cyclopropyl)methoxy)-2-(4-nitrobenzyl)isoindolin-1-one (74) as a potent MDM2-p53 inhibitor (IC(50) = 0.23 ± 0.01 μM). Resolution of the enantiomers of 74 showed that potent MDM2-p53 activity primarily resided with the (+)-R-enantiomer (74a; IC(50) = 0.17 ± 0.02 μM). The cellular activity of key compounds has been examined in cell lines with defined p53 and MDM2 status. Compound 74a activates p53, MDM2, and p21 transcription in MDM2 amplified cells and shows moderate selectivity for wild-type p53 cell lines in growth inhibition assays.

Chemosensitization of Cancer Cells by KU-0060648, A Dual Inhibitor of DNA-PK and PI-3K

DNA double-strand breaks (DSB) are the most cytotoxic lesions induced by topoisomerase II poisons. Nonhomologous end joining (NHEJ) is a major pathway for DSB repair and requires DNA-dependent protein kinase (DNA-PK) activity. DNA-PK catalytic subunit (DNA-PKcs) is structurally similar to PI-3K, which promotes cell survival and proliferation and is upregulated in many cancers. KU-0060648 is a dual inhibitor of DNA-PK and PI-3K in vitro. KU-0060648 was investigated in a panel of human breast and colon cancer cells. The compound inhibited cellular DNA-PK autophosphorylation with IC50 values of 0.019 μmol/L (MCF7 cells) and 0.17 μmol/L (SW620 cells), and PI-3K–mediated AKT phosphorylation with IC50 values of 0.039 μmol/L (MCF7 cells) and more than 10 μmol/L (SW620 cells). Five-day exposure to 1 μmol/L KU-0060648 inhibited cell proliferation by more than 95% in MCF7 cells but only by 55% in SW620 cells. In clonogenic survival assays, KU-0060648 increased the cytotoxicity of etoposide and doxorubicin across the panel of DNA-PKcs–proficient cells, but not in DNA-PKcs–deficient cells, thus confirming that enhanced cytotoxicity was due to DNA-PK inhibition. In mice bearing SW620 and MCF7 xenografts, concentrations of KU-0060648 that were sufficient for in vitro growth inhibition and chemosensitization were maintained within the tumor for at least 4 hours at nontoxic doses. KU-0060648 alone delayed the growth of MCF7 xenografts and increased etoposide-induced tumor growth delay in both in SW620 and MCF7 xenografts by up to 4.5-fold, without exacerbating etoposide toxicity to unacceptable levels. The proof-of-principle in vitro and in vivo chemosensitization with KU-0060648 justifies further evaluation of dual DNA-PK and PI-3K inhibitors. Mol Cancer Ther; 11(8); 1789–98. ©2012 AACR.

DNA-Dependent Protein Kinase Is a Therapeutic Target and an Indicator of Poor Prognosis in B-Cell Chronic Lymphocytic Leukemia

Purpose: del(17p), del(11q), and associated p53 dysfunction predict for short survival and chemoresistance in B-cell chronic lymphocytic leukemia (CLL). DNA-dependent protein kinase (DNA-PK) is activated by DNA damage and mediates DNA double-strand break repair. We hypothesized that inhibiting DNA-PK would sensitize CLL cells to drug-induced DNA damage and that this approach could increase the therapeutic index of agents used to treat CLL. Experimental Design: Fifty-four CLL cases were characterized for poor prognosis markers [del(17p), del(11q), CD38, and ZAP-70]. In selected cases, DNA-PK catalytic subunit (DNA-PKcs) expression and activity and p53 function were also measured. Ex vivo viability assays established sensitivity to fludarabine and chlorambucil and also tested the ability of a novel DNA-PK inhibitor (NU7441) to sensitize CLL cells to these drugs. The effects of NU7441 on fludarabine-induced DNA damage repair were also assessed (Comet assays and detection of γH2AX). Results: DNA-PKcs levels correlated with DNA-PK activity and varied 50-fold between cases but were consistently higher in del(17p) (P = 0.01) and del(11q) cases. NU7441 sensitized CLL cells to chlorambucil and fludarabine, including cases with del(17p), del(11q), p53 dysfunction, or high levels of DNA-PKcs. NU7441 increased fludarabine-induced double-strand breaks and abrogated drug-induced autophosphorylation of DNA-PKcs at Ser2056. High DNA-PK levels predicted for reduced treatment-free interval. Conclusions: These data validate the concept of targeting DNA-PKcs in poor risk CLL, and demonstrate a mechanistic rationale for use of a DNA-PK inhibitor. The novel observation that DNA-PKcs is overexpressed in del(17p) and del(11q) cases indicates that DNA-PK may contribute to disease progression in CLL.

Analysis of Chemical Shift Changes Reveals the Binding Modes of Isoindolinone Inhibitors of the MDM2-p53 Interaction

In this study we present a method for defining the binding modes of a set of structurally related isoindolinone inhibitors of the MDM2-p53 interaction. This approach derives the location and orientation of isoindolinone binding, based on an analysis of the patterns of magnitude and direction of chemical shift perturbations for a series of inhibitors of the MDM2-p53 interaction. The MDM2-p53 complex is an attractive target for therapeutic intervention in cancer cells with intact tumor suppressor p53, as it offers the possibility of releasing p53 by blocking the MDM2-p53 binding site with a small molecule antagonist to promote apoptosis. Isoindolinones are a novel class of MDM2-antagonists of moderate affinity, which still require the development of more potent candidates for clinical applications. As the applicability of conventional structural methods to this system is limited by a number of fundamental factors, the exploitation of the information contained in chemical shift perturbations has offered a useful route to obtaining structural information to guide the development of more potent compounds. For a set of 12 structurally related isoindolinones, the data suggests 4 different orientations of binding, caused by subtle changes in the chemical structure of the inhibitors.

Potentiation of paclitaxel-induced apoptosis by the novel cyclin-dependent kinase inhibitor NU6140: a possible role for survivin down-regulation

Cyclin-dependent kinases (CDK) play a crucial role in the control of the cell cycle. Aberrations in the control of cell cycle progression occur in the majority of human malignancies; hence, CDKs are promising targets for anticancer therapy. Here, we define the cellular effects of the novel CDK inhibitor NU6140, alone or in association with paclitaxel, with respect to inhibition of cell proliferation and cell cycle progression and induction of apoptosis in HeLa cervical carcinoma cells and in comparison with purvalanol A. Both CDK inhibitors induced a concentration-dependent cell cycle arrest at the G2-M phase and an increase in the apoptotic rate, with a concomitant down-regulation of the antiapoptotic protein survivin, a member of the inhibitors of apoptosis protein family. Notably, the addition of NU6140 to paclitaxel-treated cells resulted in markedly increased cytotoxic effect and apoptotic response in comparison with the paclitaxel-purvalanol A combination (86 ± 11% and 37 ± 8%, respectively). Similarly, the extent of caspase-9 and caspase-3 activation in paclitaxel-NU6140–treated cells was ∼4-fold higher than after the paclitaxel-purvalanol A combination. Moreover, an almost complete abrogation of the expression of the active, Thr34-phosphorylated form of survivin was observed in cells exposed to the paclitaxel-NU6140 combination. A synergistic effect of the paclitaxel-NU6140 combination, as a consequence of survivin inhibition and increased activation of caspase-9 and caspase-3, was also observed in OAW42/e ovarian cancer line but not in the derived OAW42/Surv subline ectopically expressing survivin. Results from this study indicate that NU6140 significantly potentiates the apoptotic effect of paclitaxel, with inhibition of survivin expression/phosphorylation as the potential mechanism.

Characterisation of a Tip60 Specific Inhibitor, NU9056, in Prostate Cancer

Tip60 (KAT5) is a histone acetyltransferase (HAT enzyme) involved in multiple cellular processes including transcriptional regulation, DNA damage repair and cell signalling. In prostate cancer, aggressive cases over-express Tip60 which functions as an androgen receptor co-activator via direct acetylation of lysine residues within the KLKK motif of the receptor hinge region. The purpose of this study was to identify and characterise a Tip60 acetylase inhibitor. High-throughput screening revealed an isothiazole that inhibited both Tip60 and p300 HAT activity. This substance (initially identified as 4-methyl-5-bromoisothiazole) and other isothiazoles were synthesised and assayed against Tip60. Although an authentic sample of 4-methyl-5-bromoisothiazole was inactive against Tip60, in an in vitro HAT assay, 1,2-bis(isothiazol-5-yl)disulfane (NU9056) was identified as a relatively potent inhibitor (IC50 2 µM). Cellular activity was confirmed by analysis of acetylation of histone and non-histone proteins in a prostate cancer cell line model. NU9056 treatment inhibited cellular proliferation in a panel of prostate cancer cell lines (50% growth inhibition, 8–27 µM) and induced apoptosis via activation of caspase 3 and caspase 9 in a concentration- and time-dependent manner. Also, decreased androgen receptor, prostate specific antigen, p53 and p21 protein levels were demonstrated in response to treatment with NU9056. Furthermore, pre-treatment with NU9056 inhibited both ATM phosphorylation and Tip60 stabilization in response to ionising radiation. Based on the activity of NU9056 and the specificity of the compound towards Tip60 relative to other HAT enzymes, these chemical biology studies have identified Tip60 as a potential therapeutic target for the treatment of prostate cancer.