Ze-Hong Miao

Novel Indenoisoquinolines NSC 725776 and NSC 724998 Produce Persistent Topoisomerase I Cleavage Complexes and Overcome Multidrug Resistance

Camptothecin (CPT) derivatives are effective anticancer drugs, especially against solid tumors. As CPTs are chemically unstable and have clinical limitations, we have synthesized indenoisoquinolines as novel topoisomerase I (Top1) inhibitors. We presently report two indenoisoquinoline derivatives, NSC 725776 and NSC 724998, which have been selected for therapeutic development. Both are potent Top1 inhibitors and induce Top1 cleavage at unique genomic positions compared with CPT. Consistent with Top1 poisoning, protein-linked DNA breaks were detected in cells treated with NSC 725776 and NSC 724998 at nanomolar concentrations. Those drug-induced protein-linked DNA breaks persisted longer after drug removal than those produced by CPT. Studies in human cells in culture show that NSC 725776 and NSC 724998 exert antiproliferative activity at submicromolar concentrations. Furthermore, NSC 725776 and NSC 724998 show cross-resistance in cells deficient or silenced for Top1, which is consistent with their selective Top1 targeting. Similar to other known Top1 inhibitors, NSC 725776-treated and NSC 724998-treated cells show an arrest of cell cycle progression in both S and G(2)-M and a dependence on functional p53 for their cytotoxicity. Dose-dependent gamma-H2AX foci formation was readily observed in cells treated with NSC 725776 and NSC 724998. These gamma-H2AX foci were detectable at pharmacologically relevant doses for up to 24 h and thus could be used as biomarkers for clinical trials (phase 0).

Fragment-Based Drug Discovery of 2-Thiazolidinones as Inhibitors of the Histone Reader BRD4 Bromodomain.

Recognizing acetyllysine of histone is a vital process of epigenetic regulation that is mediated by a protein module called bromodomain. To contribute novel scaffolds for developing into bromodomain inhibitors, we utilize a fragment-based drug discovery approach. By successively applying docking and X-ray crystallography, we were able to identify 9 fragment hits from diffracting more than 60 crystals. In the present work, we described four of them and carried out the integrated lead optimization for fragment 8, which bears a 2-thiazolidinone core. After several rounds of structure guided modifications, we assessed the druggability of 2-thiazolidinone by modulating in vitro pharmacokinetic studies and cellular activity assay. The results showed that two potent compounds of 2-thiazolidinones have good metabolic stability. Also, the cellular assay confirmed the activities of 2-thiazolidinones. Together, we hope the identified 2-thiazolidinone chemotype and other fragment hits described herein can stimulate researchers to develop more diversified bromodomain inhibitors.

The B-RafV600E inhibitor dabrafenib selectively inhibits RIP3 and alleviates acetaminophen-induced liver injury

Receptor-interacting protein (RIP)3 is a critical regulator of necroptosis and has been demonstrated to be associated with various diseases, suggesting that its inhibitors are promising in the clinic. However, there have been few RIP3 inhibitors reported as yet. B-RafV600E inhibitors are an important anticancer drug class for metastatic melanoma therapy. In this study, we found that 6 B-Raf inhibitors could inhibit RIP3 enzymatic activity in vitro. Among them, dabrafenib showed the most potent inhibition on RIP3, which was achieved by its ATP-competitive binding to the enzyme. Dabrafenib displayed highly selective inhibition on RIP3 over RIP1, RIP2 and RIP5. Moreover, only dabrafenib rescued cells from RIP3-mediated necroptosis induced by the necroptosis-induced combinations, that is, tumor necrosis factor (TNF)α, TNF-related apoptosis-inducing ligand or Fas ligand plus Smac mimetic and the caspase inhibitor z-VAD. Dabrafenib decreased the RIP3-mediated Ser358 phosphorylation of mixed lineage kinase domain-like protein (MLKL) and disrupted the interaction between RIP3 and MLKL. Notably, RIP3 inhibition of dabrafenib appeared to be independent of its B-Raf inhibition. Dabrafenib was further revealed to prevent acetaminophen-induced necrosis in normal human hepatocytes, which is considered to be mediated by RIP3. In acetaminophen-overdosed mouse models, dabrafenib was found to apparently ease the acetaminophen-caused liver damage. The results indicate that the anticancer B-RafV600E inhibitor dabrafenib is a RIP3 inhibitor, which could serve as a sharp tool for probing the RIP3 biology and as a potential preventive or therapeutic agent for RIP3-involved necroptosis-related diseases such as acetaminophen-induced liver damage.

R16, a novel amonafide analogue, induces apoptosis and G2-M arrest via poisoning topoisomerase II.

Amonafide, a naphthalimide derivative, although selected for exploratory clinical trials for its potent anticancer activity, has long been challenged by its unpredictable side effects. In the present study, a novel amonafide analogue, 2-(2-dimethylamino)-6-thia-2-aza-benzo-[def]-chrysene-1,3-diones (R16) was synthesized by substituting 5′-NH2 of the naphthyl with a heterocyclic group to amonafide, with additional introduction of a thiol group. In a panel of various human tumor cell lines, R16 was more cytotoxic than its parent compound amonafide. It was also effective against multidrug-resistant cells. Importantly, the i.p. administration of R16 inhibited tumor growth in mice implanted with S-180 sarcoma and H22 hepatoma. The molecular and cellular machinery studies showed that the R16 functions as a topoisomerase II (topo II) poison via binding to the ATPase domain of human topo IIα. The superior cytotoxicity of R16 to amonafide was ascribed to its potent effects on trapping topo II–DNA cleavage complexes. Moreover, using a topo II catalytic inhibitor aclarubicin, ataxia-telangiectasia-mutated (ATM)/ATM- and Rad3-related (ATR) kinase inhibitor caffeine and topo II–deficient HL-60/MX2 cells, we further showed that R16-triggered DNA double-strand breaks, tumor cell cycle arrest, and apoptosis were in a topo II–dependent manner. Taken together, R16 stood out by its improved anticancer activity, appreciable anti–multidrug resistance activities, and well-defined topo II poisoning mechanisms, as comparable with the parent compound amonafide. All these collectively promise the potential value of R16 as an anticancer drug candidate, which deserves further development. [Mol Cancer Ther 2007;6(2):484–95]

Pseudolaric Acid B Inhibits Angiogenesis and Reduces Hypoxia-Inducible Factor 1α by Promoting Proteasome-Mediated Degradation

Purpose: Pseudolaric acid B (PAB), the naturally occurring diterpenoid isolated from the root bark of Pseudolarix kaempferi Gordon tree (Pinaceae), possesses potent antifungal and pregnancy-terminating effects that may be tightly associated with angiogenesis. This study was to examine its angiogenic inhibition, impact on vascular endothelial growth factor (VEGF) secretion from tumor cells and the possible mechanism of action. Experimental Design: Angiogenesis inhibition was assessed by the human umbilical vascular endothelial cell proliferation, migration, and tube-formation assays, as well as the chorioallantoic membrane assay. ELISA, reverse transcription-PCR, and Western blotting analyses were performed to examine VEGF protein secretion, mRNA expression, and the possible mechanism in hypoxic MDA-MB-468 cells. Results: PAB displayed potent in vitro antiangiogenic activity shown by inhibiting VEGF-stimulated proliferation and migration and fetal bovine serum-stimulated tube formation of human umbilical vascular endothelial cells in a concentration-dependent manner. Moreover, PAB (10 nmol per egg) significantly suppressed in vivo angiogenesis in the chorioallantoic membrane assay. On the other hand, PAB abrogated hypoxia-induced VEGF secretion from MDA-MB-468 cells via reducing HIF-1α protein. Additional analyses using LY294002 and U0126 indicated that the increase in hypoxia-inducible factor 1 (HIF-1)α protein level was highly dependent on phosphatidylinositol 3′-kinase and p42/p44 mitogen-activated protein kinase activities in hypoxic MDA-MB-468 cells. However, PAB treatment did not affect the active (phosphorylated) forms of Akt and Erk. Interestingly, the selective proteasome inhibitor MG-132 completely reversed the reduction of HIF-1α protein in the PAB-treated MDA-MB-468 cells. Conclusions: PAB displays the dual antiangiogenic activities of directly inhibiting endothelial cells and abrogating paracrine stimulation of VEGF from tumor cells due to reducing HIF-1α protein by promoting its proteasome-mediated degradation in MDA-MB-468 cells, which has potential clinical relevance.

Cytotoxicity, apoptosis induction and downregulation of MDR-1 expression by the anti-topoisomerase II agent, salvicine, in multidrug-resistant tumor cells.

Salvicine, a novel topoisomerase II inhibitor and a diterpenoid quinone compound, exerts potent in vitro and in vivo antitumor effects. In our study, we show that salvicine effectively kills multidrug‐resistant (MDR) sublines, such as K562/A02, KB/VCR and MCF‐7/ADR, and parental K562, KB and MCF‐7 cell lines to an equivalent degree. These cytotoxic activities of salvicine were much more potent than those of several classical anticancer drugs (average resistance factor: 1.42 for salvicine vs. 344.35, 233.19 and 71.22 for vincristine, doxorubicin and etoposide, respectively). Flow cytometry and DNA agarose gel electrophoresis demonstrated that salvicine induced similar levels of apoptosis in MDR K562/A02 and parental cells. The compound activated caspase‐1 and ‐3 (but not caspase‐8) and increased the ratio of bax to bcl‐2 mRNA via reduction of bcl‐2 mRNA expression in the same cells. Furthermore, salvicine induced the downregulation of mdr‐1 gene and P‐gp expression but had no effect on MRP and LRP gene expression in MDR K562/A02 cells. These results suggest that the reduction of mdr‐1 and bcl‐2 expression by salvicine possibly contributes to its cytotoxicity and apoptotic induction in this system. The effectiveness, broad‐spectrum activity and possibly novel mechanism of killing MDR tumor cells in vitro of salvicine signify promising in vivo and clinical activity. The novel chemical structure of this compound further implies a role for salvicine in future MDR tumor therapy.

Chimmitecan, a Novel 9-Substituted Camptothecin, with Improved Anticancer Pharmacologic Profiles In vitro and In vivo

Purpose: This study aimed to evaluate antitumor activities and pharmacologic profiles of chimmitecan, a novel 9-small-alkyl–substituted lipophilic camptothecin, in comparison with irinotecan (CPT-11) and topotecan. Experimental Design: The in vitro cytotoxities of chimmitecan in human tumor cell lines and multidrug resistance (MDR) cells were evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and sulforhodamin B assays. DNA relaxation, cleavage assays, and cellular band depletion assay were combined to delineate its effects on topoisomerase I. DNA damage, cell cycle arrest, and apoptosis were assessed using comet assay, flow cytometry, and DNA ladder analysis, respectively. The in vivo antitumor activities were measured in nude mice bearing human tumor xenografts. Results: Chimmitecan displayed more potent cytotoxicity than SN38 and topotecan. Neither a cross-resistance to chimmitecan in MDR cells nor an influence of human serum albumin in its cytotoxity was observed. Chimmitecan exhibited comparable effects on topoisomerase I compared with the reference drugs, including inhibiting topoisomerase I catalytic activity and trapping and stabilizing covalent topoisomerase I-DNA complexes. Furthermore, nanomolar levels of chimmitecan caused impressive DNA damage, G2-M phase arrest, and apoptosis in human leukemia HL60 cells. I.v. administration of chimmitecan inhibited the growth of HCT-116, MDA-MB-435, BEL-7402, and A549 human carcinoma xenografts in nude mice, with greater potency than CPT-11 against the latter two tumors models. Chimmitecan presented potent efficacy in A549 tumor model when given orally. Conclusions: Chimmitecan is a potent inhibitor of topoisomerase I and displays outstanding activity in vitro and in vivo. The substitution at the 9-position benefits chimmitecan a salient anti-MDR activity, stability in human serum albumin, improved solubility, and oral availability, which might favorably promise its therapeutic potential in clinical settings.

Natural product triptolide mediates cancer cell death by triggering CDK7-dependent degradation of RNA polymerase II

Triptolide is a bioactive ingredient in traditional Chinese medicine that exhibits diverse biologic properties, including anticancer properties. Among its many putative targets, this compound has been reported to bind to XPB, the largest subunit of general transcription factor TFIIH, and to cause degradation of the largest subunit Rpb1 of RNA polymerase II (RNAPII). In this study, we clarify multiple important questions concerning the significance and basis for triptolide action at this core target. Triptolide decreased Rpb1 levels in cancer cells in a manner that was correlated tightly with its cytotoxic activity. Compound exposure blocked RNAPII at promoters and decreased chromatin-bound RNAPII, both upstream and within all genes that were examined, also leading to Ser-5 hyperphosphorylation and increased ubiqutination within the Rbp1 carboxy-terminal domain. Notably, cotreatment with inhibitors of the proteasome or the cyclin-dependent kinase CDK7 inhibitors abolished the ability of triptolide to ablate Rpb1. Together, our results show that triptolide triggers a CDK7-mediated degradation of RNAPII that may offer an explanation to many of its therapeutic properties, including its robust and promising anticancer properties.

Chk1 and Chk2 are differentially involved in homologous recombination repair and cell cycle arrest in response to DNA double-strand breaks induced by camptothecins

Camptothecins (CPT) activate S or G2-M arrest and the homologous recombination (HR) repair pathway in tumor cells. In this process, both checkpoint kinases 1 and 2 (Chk1 and Chk2, respectively) are activated, but their differential roles, especially in the coordination of checkpoint and repair control, and potential clinic relevance remain to be fully elucidated. In this study, the repairable double-strand breaks were induced in human colon cancer HCT116 cells by 1-h exposure to 25 or 100 nmol/L CPT and its novel derivative chimmitecan. The cellular disposal of double-strand breaks was reflected as the progressive dispersal of γ-H2AX foci, reduction of “comet” tails, dynamic activation of RAD51-mediated HR repair, and reversible G2-M arrest. In this model, the differential kinetics of Chk1 and Chk2 activation was characterized by the progressively increased phosphorylation of Chk2 until 72 h, the degradation of Chk1, and the disappearance of phosphorylated Chk1 48 h after drug removal. Using RNA interference, we further showed that Chk2 was essential to G2-M arrest, whereas Chk1 was mainly required for HR repair in CPT-treated HCT116 cells. Moreover, Chk2, rather than Chk1, predominated over the control of cell survival in this model. The differential roles of Chk1 and Chk2 in regulating HR repair and G2-M phase arrest were also confirmed in HT-29 colon cancer cells. Together, these findings systematically dissect the differential roles of Chk1 and Chk2 in a favorable model pursuing CPT-driven DNA damage responses, providing critical evidence to further explore checkpoint modulation, especially Chk2 inhibition as a therapeutic strategy in combination with CPT. [Mol Cancer Ther 2008;7(6):1440–9]

Reactive oxygen species contribute to cell killing and P-glycoprotein downregulation by salvicine in multidrug resistant K562/A02 cells

Salvicine, a novel diterpenoid quinone compound, displays potent antitumor activities in vitro and in vivo, which is under Phase II clinical trials for cancer therapy. Our previous studies have shown that salvicine effectively kills multidrug-resistant (MDR) cells and down-regulates mdr-1 and P-glycoprotein (P-gp) levels by activation of transcription factor c-Jun in MDR K562/A02 cells. Recent studies have further demonstrated that salvicine-formed reactive oxygen species (ROS) contribute to its induction of cytotoxicity, DNA double strand breaks and apoptosis. In this study, we showed that salvicine induced equal ROS generation and glutathione depletion in both sensitive K562 and MDR K562/A02 cells. Pre-incubation with thiol antioxidants glutathione or N-acetyl-cysteine (NAC, precursor of intracellular glutathione) almost abolished the cytotoxicity of salvicine, which also could be attenuated by the H2O2-specific scavenger catalase. Moreover, NAC abrogated salvicine-induced DNA double strand breaks and apoptosis. Notably, both H2O2 and vitamin C potentiated the cytotoxicity and apoptotic induction of salvicine in parental K562 and MDR K562/A02 cells, and catalase could remove such potentiation. Furthermore, pretreatment of K562/A02 cells with NAC eliminated P-gp downregulation, JNK phosphorylation and c-Jun activation induced by salvicine. Our data collectively indicate that salvicine-generated ROS contribute to both cell killing and P-gp downregulation in MDR K562/A02 cells, thus extending our prior related studies. This study also opens the possibility of the combination therapy of salvicine and vitamin C in the future.