Chk1 signaling pathways that mediated G(2)M checkpoint in relation to the cellular resistance to the novel topoisomerase I poison BNP1350

Silicon-containing analogs of camptothecin as anticancer agents

The plant pentacyclic alkaloid camptothecin and its structural analogs were extensively studied. These compounds are interesting due to the antitumor activity associated with their ability to inhibit topoisomerase I in tumor cells. During the last decades of the 20th century, a large number of the silicon-containing camptothecins (silatecans) were synthesized. 7-tert-Butyldimethylsilyl-10-hydroxy-camptothecin (DB-67 or AR-67) has enhanced lipophilicity and demonstrates a antitumor activity superior to its carbon analog. To date, certain silatecans are under clinical trials and their ultimate role in cancer therapy appears promising. In this review, we present chemical methodologies for the synthesis of silicon-containing camptothecins, their chemical properties, biological activity, and results of clinical trials.

Natural product and natural product derived drugs in clinical trials

The 25 Natural Product (NP)-derived drugs launched since 2008 and the 100 NP-derived compounds and 33 Antibody Drug Conjugates (ADCs) in clinical trials or in registration at the end of 2013 are reviewed.

Topoisomerase degradation, DSB repair, p53 and IAPs in cancer cell resistance to camptothecin-like topoisomerase I inhibitors

Topoisomerase I (TOP1) inhibitors applied in cancer therapy such as topotecan and irinotecan are derivatives of the natural alkaloid camptothecin (CPT). The mechanism of CPT poisoning of TOP1 rests on inhibition of the re-ligation function of the enzyme resulting in the stabilization of the TOP1-cleavable complex. In the presence of CPTs this enzyme-DNA complex impairs transcription and DNA replication, resulting in fork stalling and the formation of DNA double-strand breaks (DSB) in proliferating cells. As with most chemotherapeutics, intrinsic and acquired drug resistance represents a hurdle that limits the success of CPT therapy. Preclinical data indicate that resistance to CPT-based drugs might be caused by factors such as (a) poor drug accumulation in the tumor, (b) high rate of drug efflux, (c) mutations in TOP1 leading to failure in CPT docking, or (d) altered signaling triggered by the drug-TOP1-DNA complex, (e) expression of DNA repair proteins, and (f) failure to activate cell death pathways. This review will focus on the issues (d-f). We discuss degradation of TOP1 as part of the repair pathway in the processing of TOP1 associated DNA damage, give a summary of proteins involved in repair of CPT-induced replication mediated DSB, and highlight the role of p53 and inhibitors of apoptosis proteins (IAPs), particularly XIAP and survivin, in cancer cell resistance to CPT-like chemotherapeutics.

Tumor suppressor protein kinase Chk2 is a mediator of anoikis of intestinal epithelial cells

Resistance of carcinoma cells to anoikis, apoptosis that is normally induced by detachment of nonmalignant epithelial cells from the extracellular matrix, is thought to be critical for carcinoma progression. Molecular mechanisms that control anoikis of nonmalignant and cancer cells are understood poorly. In an effort to understand them we found that detachment of nonmalignant intestinal epithelial cells triggers upregulation of Chk2, a pro-apoptotic protein kinase that has never been implicated in anoikis and has been thought to kill cells mainly under the conditions compromising genome integrity. We found that enforced downregulation of Chk2 protects intestinal epithelial cells from anoikis. Chk2 can kill cells by stabilizing p53 tumor suppressor protein or via p53-independent mechanisms, and we established that Chk2-mediated anoikis of intestinal epithelial cells is p53-independent. We further found that, unlike nonmalignant intestinal epithelial cells whose anoikis is triggered by detachment-induced Chk2 upregulation, intestinal epithelial cells carrying oncogenic ras, a known inhibitor of anoikis, remain anoikis-resistant in response to enforced Chk2 upregulation. By contrast, drugs, such as topoisomerase I inhibitors, that can kill cells via Chk2-indpendent mechanisms, efficiently triggered anoikis of ras-transformed cells. Thus, oncogenic ras can prevent Chk2 from triggering anoikis even when levels of this protein kinase are elevated in cancer cells, and the use of therapeutic agents that kill cells in a Chk-2-independent, rather than Chk-2-dependent, manner could represent an efficient strategy for overcoming ras-induced anoikis resistance of these cells. We conclude that Chk-2 is an important novel component of anoikis-promoting machinery of intestinal epithelial cells.

AZD7762, a novel checkpoint kinase inhibitor, drives checkpoint abrogation and potentiates DNA-targeted therapies

Insights from cell cycle research have led to the hypothesis that tumors may be selectively sensitized to DNA-damaging agents resulting in improved antitumor activity and a wider therapeutic margin. The theory relies on the observation that the majority of tumors are deficient in the G1-DNA damage checkpoint pathway resulting in reliance on S and G2 checkpoints for DNA repair and cell survival. The S and G2 checkpoints are regulated by checkpoint kinase 1, a serine/threonine kinase that is activated in response to DNA damage; thus, inhibition of checkpoint kinase 1 signaling impairs DNA repair and increases tumor cell death. Normal tissues, however, have a functioning G1 checkpoint signaling pathway allowing for DNA repair and cell survival. Here, we describe the preclinical profile of AZD7762, a potent ATP-competitive checkpoint kinase inhibitor in clinical trials. AZD7762 has been profiled extensively in vitro and in vivo in combination with DNA-damaging agents and has been shown to potentiate response in several different settings where inhibition of checkpoint kinase results in the abrogation of DNA damage-induced cell cycle arrest. Dose-dependent potentiation of antitumor activity, when AZD7762 is administered in combination with DNA-damaging agents, has been observed in multiple xenograft models with several DNA-damaging agents, further supporting the potential of checkpoint kinase inhibitors to enhance the efficacy of both conventional chemotherapy and radiotherapy and increase patient response rates in a variety of settings.

Natural products to drugs: natural product-derived compounds in clinical trials

Natural product and natural product-derived compounds that are being evaluated in clinical trials or are in registration (as at 31st December 2007) have been reviewed, as well as natural product-derived compounds for which clinical trials have been halted or discontinued since 2005. Also discussed are natural product-derived drugs launched since 2005, new natural product templates and late-stage development candidates.

High labeling indices of cdc25B is linked to progression of gastric cancers and associated with a poor prognosis

To clarify the significance of cdc25B, which plays an important physiologic role in regulation of the G2/M check point, in progression of gastric cancer, 125 samples of paraffin-embedded gastric cancers were investigated by immunohistochemistry. In addition, 3 human gastric cancer cell lines were studied to determine the cellular localization of cdc25B by immunohistochemistry and cell fractionation followed by Western blotting. In the cell lines cdc25B was found to be present in both nuclei and cytoplasm, but predominantly in nuclei. High labeling indices of cdc25B in invasion front of gastric cancer was observed in 31 out of 125 cases (24.8%), linked to an advanced depth of cancer invasion (P=0.02), high rates of lymphatic invasion (P=0.03), and lymph node metastasis (P<0.01). Furthermore, the Kaplan-Meier method demonstrated a poor prognosis for cdc25B high labeling indices cases (P=0.02), although multivariate analysis revealed it not to be an independent factor. In conclusion, it seems likely that cdc25B is located predominantly in nuclei when overexpressed and this has some linkage with progression of gastric cancer.

Molecular signatures of soy-derived phytochemicals in androgen-responsive prostate cancer cells: a comparison study using DNA microarray

The present study utilized microarray technology as a tool to elucidate the molecular signatures of soy-derived phytochemicals in the human androgen-responsive prostate cancer cell line LNCaP. Global gene expression pattern analysis of LNCaP cells exposed to 0, 1, 5, or 25 microM of the soy-derived phytochemicals equol and daidzein were conducted and compared. The data were further compared with previously generated data from exposure of LNCaP cells to the same doses of genistein, a soy isoflavone. Multidimensional scaling (MDS) analyses of the expression patterns suggest that these compounds exerted differential effects on gene expression in LNCaP cells. Further examination of specific gene changes revealed that these compounds differentially modulated genes in multiple cellular pathways, including the cell-cycle pathway genes. However, the three compounds also exerted similar effect on genes belonging to several other important cellular pathways. A universal effect of the three compounds on androgen-responsive genes, IGF-1 pathway gene, and MAP kinase-related pathway gene was observed. These results provide the foundation for establishing molecular signatures for equol, daidzein, and genistein. Moreover, these results also allow for the identification of candidate mechanism(s) by which soy phytochemicals and soy may act in prostate cancer cells.

Repair of topoisomerase I-mediated DNA damage

Topoisomerase I (Top1) is an abundant and essential enzyme. Top1 is the selective target of camptothecins, which are effective anticancer agents. Top1-DNA cleavage complexes can also be trapped by various endogenous and exogenous DNA lesions including mismatches, abasic sites and carcinogenic adducts. Tyrosyl-DNA phosphodiesterase (Tdp1) is one of the repair enzymes for Top1-DNA covalent complexes. Tdp1 forms a multiprotein complex that includes poly(ADP) ribose polymerase (PARP). PARP-deficient cells are hypersensitive to camptothecins and functionally deficient for Tdp1. We will review recent developments in several pathways involved in the repair of Top1 cleavage complexes and the role of Chk1 and Chk2 checkpoint kinases in the cellular responses to Top1 inhibitors. The genes conferring camptothecin hypersensitivity are compiled for humans, budding yeast and fission yeast.

Synergistic antitumor activity of capecitabine in combination with irinotecan

5-Fluorouracil (5-FU) and capecitabine alone and in combination with irinotecan/oxaliplatin are clinically active in the treatment of colorectal and other solid tumors. Studies of the antitumor activity and toxicity of capecitabine or irinotecan alone and in combination with each other, were compared with 5-FU and raltitrexed in human tumor xenografts of colorectal and squamous cell carcinoma of the head and neck using clinically relevant schedules. Antitumor activity and toxicity were evaluated in nude mice bearing human colon carcinomas of HCT-8 and HT-29 and in head and neck squamous cell carcinomas of A253 and FaDu xenografts using the maximum tolerable dose of single-agent capecitabine, 5-FU, or raltitrexed, or each of the drugs in combination with irinotecan. Mice were treated with capecitabine and irinotecan alone or in combination using 2 different schedules: (1) capecitabine orally once a day for 7 days and a single dose of irinotecan (50 mg/kg intravenously [I.V.]), with each drug alone or in combination, and (2) capecitabine orally 5 days a week for 3 weeks and irinotecan 50 mg/kg (I.V. injection) once a week for 3 weeks, with each drug alone or in combination. For comparative purposes, the antitumor activity of single-agent capecitabine, 5-FU, or raltitrexed, or each drug in combination with irinotecan was carried out at its maximum tolerated dose (MTD) using a 3-week schedule. Results indicated that HT-29 and A253 xenografts were de novo resistant (no cure) to capecitabine and irinotecan alone at the MTD, whereas HCT-8 and FaDu xenografts were relatively more sensitive, yielding 10%-20% cures. The combination of irinotecan/capecitabine was much more active than either drug alone against all 4 tumor models. The cure rates were increased from 0 to 20% in A253 and HT-29 xenografts and from 10%-20% to 80%-100% in HCT-8 and FaDu tumor xenografts, respectively. Irinotecan/capecitabine had clear advantage over irinotecan/5-FU and irinotecan/raltitrexed in efficacy and selectivity in that they were more active and less toxic. The extent of synergy with irinotecan/capecitabine appears to be tumor-dependent and independent of the status of p53 expression. The potential impact of the preclinical results on clinical practice for the use of these drugs in combination needs clinical validation.

Natural products to drugs: natural product derived compounds in clinical trials

Natural product and natural product-derived compounds that are being evaluated in clinical trials or in registration (current 31 December 2004) have been reviewed. Natural product derived drugs launched in the United States of America, Europe and Japan since 1998 and new natural product templates discovered since 1990 are discussed.

Targeting senescence pathways to reverse drug resistance in cancer

Irreversible proliferation arrest (also called senescence) has emerged recently as a drug-responsive program able to influence the outcome of cancer chemotherapy. Since the drug amounts required for induction of proliferation arrest are much lower than those necessitated for induction of cell death, forcing cancer cells to undergo senescence may represent a less aggressive approach to control tumor progression. However, to achieve a long-standing control of proliferation, the ability of cancer cells to escape senescence and become drug resistant must be inhibited. Therefore, a clear understanding of the mechanisms that govern drug-induced senescence is critical and can lead to discovery of novel approaches to suppress drug resistance. The present review discusses the relevance of senescence in response to chemotherapy and the onset of drug resistance development. Particular emphasis is directed toward the utilization of findings from the field of research on aging, that can be applied to induction of senescence in cancer cells and reversal of their drug resistance phenotype. Proof of principle for this relationship is represented by the identification of inhibitors of aging associated proteases such as the proteasome and cathepsin L as novel and potent cancer drug resistance reversing agents.

Enhanced 7-Ethyl-10-hydroxycamptothecin (SN-38) Lethality by Methylselenocysteine Is Associated with Chk2 Phosphorylation at Threonine-68 and Down-Regulation of Cdc6 Expression

Methylselenocysteine (MSC) is an organic selenium compound in preventative clinical trials involving prostate, lung, and colon carcinoma. We found that methioninase-activated MSC potentiates 7-ethyl-10-hydroxycamptothecin (SN-38)-induced cell lethality in vitro in the p53-defective human head and neck carcinoma A253 cells. Activated MSC increases chk2 phosphorylation at threonine-68 induced by SN-38, with no significant effect on chk1 phosphorylation. Cell cycle arrest induced by SN-38, however, was not abrogated or potentiated by MSC. These results suggest that the enhanced cellular lethality of SN-38 by MSC was not associated with cell cycle regulation pathways. Because chk2, in addition to its role in cell cycle arrest, can induce apoptosis by phosphorylation/activation, we examined whether increased chk2 phosphorylation could induce preapoptotic DNA fragmentation. DNA damage analysis showed that megabase DNA fragmentation is decreased, accompanied by the increased 30 to 300 kilobase pairs of DNA fragmentation after exposure to SN-38 with MSC, compared with SN-38 alone. No significant changes in the amount of DNA fragments were observed in cells treated with SN-38 or MSC alone. Moreover, proteolytic destruction of DNA replication-associated proteins cdc6, MCM2, and cdc25A may induce a DNA damage checkpoint response. The observed down-regulation of DNA replication proteins cdc6, MCM2, and cdc25A after exposure to SN-38 with MSC further indicates a relationship between drug response and DNA damage. Exposure to SN-38 with MSC resulted in a significant increase of poly(ADP-ribose) polymerasecleavage and caspase 3 activation. All together, the data support the hypothesis that enhanced lethality of this combination is associated with increased chk2 phosphorylation at Thr68 and down-regulation of specific DNA replication-associated proteins, which result in poly(ADP-ribose) polymerase cleavage, caspase 3 activation, and the induction of 30 to 300 kilobase pairs of DNA fragmentation.

Mechanisms of apoptosis induction by nucleoside analogs

Nucleoside analogs are structurally, metabolically, and pharmacodynamically related agents that nevertheless have diverse biological actions and therapeutic consequences. This class of agents affects the structural integrity of DNA, generally after incorporation during replication or DNA excision repair synthesis, leading to stalled replication forks and chain termination. The DNA damage sensors ATM, ATR and DNA-PK recognize these events. These and other protein kinases activate checkpoint pathways that arrest cell cycle progression, and also signal for DNA repair. In addition, if these survival mechanisms are overwhelmed by the damage caused, a third function of these sensors is to activate signaling pathways that initiate apoptotic processes. A review of the spectrum of responses that are activated by clinically relevant nucleoside analogs begins to provide a mechanistic basis for diverse outcomes in cell viability. Such information, when coupled with an understanding of the intrinsic apoptotic potential of a tumor cell type may provide a rational basis for the design of therapeutic strategies.