Bone marrow and tumor cell colony-forming units and human tumor xenograft efficacy of noncamptothecin and camptothecin topoisomerase I inhibitors

Downregulation of uPAR promotes urokinase translocation into the nucleus and epithelial to mesenchymal transition in neuroblastoma

The urokinase system is involved in a variety of physiological processes, such as fibrinolysis, matrix remodeling, wound healing, and regeneration. Upon binding to its cognate receptor urokinase‐type plasminogen activator receptor (uPAR), urokinase‐type plasminogen activator (uPA) catalyzes the conversion of plasminogen to plasmin and the activation of matrix metalloproteases. Apart from this, uPA–uPAR interaction can lead to the activation of transcription factors, mitogen‐activated protein kinase signaling pathways and RTK cascades. Elevated expression of uPA and uPAR is markedly associated with cancer progression and metastasis and correlates with a poor prognosis in clinics. Targeting the urokinase system has proved to be effective in experimental models in vitro and in vivo, however, in clinics the inhibition of the uPA/uPAR system has fallen short of expectations, suggesting that the question of the functional relevance of uPA/uPAR system is far from being moot. Recently, using CRISPR/Cas9 technology, we have shown that uPAR knockout decreases the proliferation of neuroblastoma Neuro2a cells in vitro. In the present study we demonstrate that uPAR expression is essential for maintaining the epithelial phenotype in Neuro2a cells and that uPAR silencing promotes epithelial‐mesenchymal transition (EMT) and increased cell migration. Accordingly, uPAR knockout results in the downregulation of epithelial markers (E‐cadherin, occludin, and claudin‐5) and in the increase of mesenchymal markers (N‐cadherin, α‐smooth muscle actin, and interleukin‐6). In search of the molecular mechanism underlying these changes, we identified uPA as a key component. Two key insights emerged as a result of this work: in the absence of uPAR, uPA is translocated into the nucleus where it is presumably involved in the activation of transcription factors (nuclear factor κB and Snail) resulting in EMT. In uPAR‐expressing cells, uPAR functions as a uPA “trap” that binds uPA on the cell surface and promotes controlled uPA internalization and degradation in lysosomes.

Drug resistance in topoisomerase-targeting therapy

Drug resistance is a well-known phenomenon that occurs when initially responsive to chemotherapy cancer cells become tolerant and elude further effectiveness of anticancer drugs. Based on their mechanism of action, anticancer drugs can be divided into cytotoxic-based agents and target-based agents. An important role among the therapeutics of the second group is played by drugs targeting topoisomerases, nuclear enzymes critical to DNA function and cell survival. These enzymes are cellular targets of several groups of anticancer agents which generate DNA damage in rapidly proliferating cancer cells. Drugs targeting topoisomerase I are mostly analogs of camtothecin, a natural compound isolated from the bark of a tree growing in China. Drugs targeting topoisomerase II are divided into poisons, such as anthracycline antibiotics, whose action is based on intercalation between DNA bases, and catalytic inhibitors that block topoisomerase II at different stages of the catalytic cycle. Unfortunately, chemotherapy is often limited by the induction of drug resistance. Identifying mechanisms that promote drug resistance is critical for the improvement of patient prognosis. Cancer drug resistance is a complex phenomenon that may be influenced by many factors. Here we discuss various mechanisms by which cancer cells can develop resistance to topoisomerase-directed drugs, which include enhanced drug efflux, mutations in topoisomerase genes, hypophosphorylation of topoisomerase II catalytic domain, activation of NF-κB transcription factor and drug inactivation. All these events may lead to the ineffective induction of cancer cell death. Attempts at circumventing drug resistance through the inhibition of cellular efflux pumps, use of silencing RNAs or inhibition of some important mechanisms, which can allow cancer cells to survive therapy, are also presented.

Topoisomerase 1B poisons: Over a half-century of drug leads, clinical candidates, and serendipitous discoveries

Topoisomerases are DNA processing enzymes that relieve supercoiling (torsional strain) in DNA, are necessary for normal cellular division, and act by nicking (and then religating) DNA strands. Type 1B topoisomerase (Top1) is overexpressed in certain tumors, and the enzyme has been extensively investigated as a target for cancer chemotherapy. Various chemical agents can act as "poisons" of the enzyme's religation step, leading to Top1-DNA lesions, DNA breakage, and eventual cellular death. In this review, agents that poison Top1 (and have thus been investigated for their anticancer properties) are surveyed, including natural products (such as camptothecins and indolocarbazoles), semisynthetic camptothecin and luotonin derivatives, and synthetic compounds (such as benzonaphthyridines, aromathecins, and indenoisoquinolines), as well as targeted therapies and conjugates. Top1 has also been investigated as a therapeutic target in certain viral and parasitic infections, as well as autoimmune, inflammatory, and neurological disorders, and a summary of literature describing alternative indications is also provided. This review should provide both a reference for the medicinal chemist and potentially offer clues to aid in the development of new Top1 poisons.

Topoisomerases as anticancer targets

Many cancer type-specific anticancer agents have been developed and significant advances have been made toward precision medicine in cancer treatment. However, traditional or nonspecific anticancer drugs are still important for the treatment of many cancer patients whose cancers either do not respond to or have developed resistance to cancer-specific anticancer agents. DNA topoisomerases, especially type IIA topoisomerases, are proved therapeutic targets of anticancer and antibacterial drugs. Clinically successful topoisomerase-targeting anticancer drugs act through topoisomerase poisoning, which leads to replication fork arrest and double-strand break formation. Unfortunately, this unique mode of action is associated with the development of secondary cancers and cardiotoxicity. Structures of topoisomerase-drug-DNA ternary complexes have revealed the exact binding sites and mechanisms of topoisomerase poisons. Recent advances in the field have suggested a possibility of designing isoform-specific human topoisomerase II poisons, which may be developed as safer anticancer drugs. It may also be possible to design catalytic inhibitors of topoisomerases by targeting certain inactive conformations of these enzymes. Furthermore, identification of various new bacterial topoisomerase inhibitors and regulatory proteins may inspire the discovery of novel human topoisomerase inhibitors. Thus, topoisomerases remain as important therapeutic targets of anticancer agents.

A novel in vivo model for predicting myelotoxicity of chemotherapeutic agents using IL-3/GM-CSF transgenic humanized mice

Evaluating myelotoxicity is essential for ensuring the safety of novel drugs before they are approved for clinical applications. Although in vivo prediction of the maximum tolerated doses (MTDs) of anticancer drugs is usually performed in rodents, the results are not always applicable to clinical treatment because drugs may have different effects in human and rodent cells. Previously, we generated a human IL-3 and GM-CSF transgenic humanized mouse (hu-IL-3/GM Tg), in which human granulocytes effectively differentiated after hematopoietic stem cell transplantation. In this study, we established a novel in vivo preclinical evaluation model for predicting human myelotoxicity of anticancer drugs using these hu-IL-3/GM Tg mice. The myelotoxicity was investigated by kinetic flow cytometry of human or murine granulocytes and by colony-forming unit granulocyte/macrophage (CFU-GM) assays. In both in vivo and in vitro analyses, topotecan was more myelotoxic to human than murine granulocytes. In contrast, oxaliplatin was more myelotoxic to murine granulocytes. The level of myelotoxicity of paclitaxel treatment was comparable between human and mouse cells. These results demonstrate that our humanized mouse model can simultaneously evaluate myelotoxicity against human and mouse cells in vivo, and provides an effective preclinical tool for predicting appropriate doses of anticancer agents for clinical treatment.

Targeting G-Quadruplex DNA Structures by EMICORON Has a Strong Antitumor Efficacy against Advanced Models of Human Colon Cancer

We previously identified EMICORON as a novel G-quadruplex (G4) ligand showing high selectivity for G4 structures over the duplex DNA, causing telomere damage and inhibition of cell proliferation in transformed and tumor cells. Here, we evaluated the antitumoral effect of EMICORON on advanced models of human colon cancer that could adequately predict human clinical outcomes. Our results showed that EMICORON was well tolerated in mice, as no adverse effects were reported, and a low ratio of sensitivity across human and mouse bone marrow cells was observed, indicating a good potential for reaching similar blood levels in humans. Moreover, EMICORON showed a marked therapeutic efficacy, as it inhibited the growth of patient-derived xenografts (PDX) and orthotopic colon cancer and strongly reduced the dissemination of tumor cells to lymph nodes, intestine, stomach, and liver. Finally, activation of DNA damage and impairment of proliferation and angiogenesis are proved to be key determinants of EMICORON antitumoral activity. Altogether, our results, performed on advanced experimental models of human colon cancer that bridge the translational gap between preclinical and clinical studies, demonstrated that EMICORON had an unprecedented antitumor activity warranting further studies of EMICORON-based combination treatments.

Inhibition of Topoisomerase (DNA) I (TOP1): DNA Damage Repair and Anticancer Therapy

Most chemotherapy regimens contain at least one DNA-damaging agent that preferentially affects the growth of cancer cells. This strategy takes advantage of the differences in cell proliferation between normal and cancer cells. Chemotherapeutic drugs are usually designed to target rapid-dividing cells because sustained proliferation is a common feature of cancer [1,2]. Rapid DNA replication is essential for highly proliferative cells, thus blocking of DNA replication will create numerous mutations and/or chromosome rearrangements—ultimately triggering cell death [3]. Along these lines, DNA topoisomerase inhibitors are of great interest because they help to maintain strand breaks generated by topoisomerases during replication. In this article, we discuss the characteristics of topoisomerase (DNA) I (TOP1) and its inhibitors, as well as the underlying DNA repair pathways and the use of TOP1 inhibitors in cancer therapy.

Boldine, a natural aporphine alkaloid, inhibits telomerase at non-toxic concentrations

In a preliminary screening study of natural alkaloids, boldine, an aporphine alkaloid, showed an interesting dose and time dependent anti-proliferative effect in several cancer cell lines. Cytotoxicity of boldine in human fibroblasts was considerably lower than the telomerase positive embryonic kidney HEK293 and breast cancer MCF-7 and MDA-MB-231 cells. Whether boldine can inhibit telomerase was investigated here using a modified quantitative real-time telomere repeat amplification protocol (q-TRAP). This test showed that boldine inhibits telomerase in cells treated with sub-cytotoxic concentrations. Telomerase inhibition occurs via down-regulation of hTERT, the catalytic subunit of the enzyme. Boldine changed the splicing variants of hTERT towards shorter non-functional transcripts as well. A direct interaction of boldine with the enzyme may also be involved, though thermal FRET method did not detect any substantial interaction between boldine and synthetic telomere sequences. This study advocates boldine as a valuable candidate for telomerase-targeted cancer care. This study suggests that derivatives of boldine could be potent anti-cancer drugs.

Dose-dependent cytotoxic effects of boldine in HepG-2 cells-telomerase inhibition and apoptosis induction

Plant metabolites are valuable sources of novel therapeutic compounds. In an anti-telomerase screening study of plant secondary metabolites, the aporphine alkaloid boldine (1,10-dimethoxy-2,9-dihydroxyaporphine) exhibited a dose and time dependent cytotoxicity against hepatocarcinoma HepG-2 cells. Here we focus on the modes and mechanisms of the growth-limiting effects of this compound. Telomerase activity and expression level of some related genes were estimated by real-time PCR. Modes of cell death also were examined by microscopic inspection, staining methods and by evaluating the expression level of some critically relevant genes. The growth inhibition was correlated with down-regulation of the catalytic subunit of telomerase (hTERT) gene (p < 0.01) and the corresponding reduction of telomerase activity in sub-cytotoxic concentrations of boldine (p < 0.002). However, various modes of cell death were stimulated, depending on the concentration of boldine. Very low concentrations of boldine over a few passages resulted in an accumulation of senescent cells so that HepG-2 cells lost their immortality. Moreover, boldine induced apoptosis concomitantly with increasing the expression of bax/bcl2 (p < 0.02) and p21 (p < 0.01) genes. Boldine might thus be an interesting candidate as a potential natural compound that suppresses telomerase activity in non-toxic concentrations.

Microscopic Modes and Free Energies for Topoisomerase I-DNA Covalent Complex Binding with Non-campothecin Inhibitors by Molecular Docking and Dynamics Simulations

Topoisomerase I (Topo1) has been identified as an attractive target for anticancer drug development due to its central role in facilitating the nuclear process of the DNA. It is essential for rational design of novel Topo1 inhibitors to reliably predict the binding structures of the Topo1 inhibitors interacting with the Topo1-DNA complex. The detailed binding structures and binding free energies for the Topo1-DNA complex interacting with typical non-camptothecin (CPT) Topo1 inhibitors have been examined by performing molecular docking, molecular dynamic (MD) simulations, and binding free energy calculations. The computational results provide valuable insights into the binding modes of the inhibitors binding with the Topo1-DNA complex and the key factors affecting the binding affinity. It has been demonstrated that the - stacking interaction with the DNA base pairs and the hydrogen bonding with Topo1 have the pivotal contributions to the binding structures and binding free energies, although the van der Waals and electrostatic interactions also significantly contribute to the stabilization of the binding structures. The calculated binding free energies are in good agreement with the available experiment activity data. The detailed binding modes and the crucial factors affecting the binding free energies obtained from the present computational studies may provide valuable insights for future rational design of novel, more potent Topo1 inhibitors.

Analyses of the combination of 6-MP and dasatinib in cell culture

A major tenet of cancer therapeutics is that combinations of anticancer agents with different mechanisms of action and different toxicities may be effective treatment regimens. Evaluation of additivity/synergy in cell culture may be used to identify drug combination opportunities and to assess risk of additive/synergistic toxicity. The combination of 6-mercaptopurine and dasatinib was assessed for additivity/synergy using the combination index (CI) method and a response surface method in six human tumor cell lines including MCF-7 and MDA-MB‑468 breast cancer, NCI-H23 and NCI-H460 non‑small cell lung cancer, and A498 and 786-O renal cell cancer, based on two experimental end‑points: ATP content and colony formation. Clonal colony formation by human bone marrow CFU-GM was used to assess risk of enhanced toxicity. The concentration ranges tested for each drug were selected to encompass the clinical Cmax concentrations. The combination regimens were found to be additive to sub‑additive by both methods of data analysis, but synergy was not detected. The non-small cell lung cancer cell lines were the most responsive among the tumor lines tested and the renal cell carcinoma lines were the least responsive. The bone marrows CFU-GM were more sensitive to the combination regimens than were the tumor cell lines. Based upon these data, it appears that the possibility of enhanced efficacy from combining 6-mercaptopurine (6-MP) and dasatinib would be associated with increased risk of severe bone marrow toxicity, so the combination is unlikely to provide a therapeutic advantage for treating solid tumor patients where adequate bone marrow function must be preserved.

Development and validation of an immunoassay for quantification of topoisomerase I in solid tumor tissues

BACKGROUND

Topoisomerase I (Top1) is a proven target for cancer therapeutics. Recent data from the Fluorouracil, Oxaliplatin, CPT-11: Use and Sequencing (FOCUS) trial demonstrated that nuclear staining of Top1 correlates with chemotherapeutic efficacy. Such a correlation may help identify patients likely to respond to Top1 inhibitors and illuminate their mechanism of action. Cellular response to Top1 inhibitors is complex, but Top1 target engagement is a necessary first step in this process. This paper reports the development and validation of a quantitative immunoassay for Top1 in tumors.

METHODOLOGY/PRINCIPAL FINDINGS

We have developed and validated a two-site enzyme chemiluminescent immunoassay for quantifying Top1 levels in tumor biopsies. Analytical validation of the assay established the inter-day coefficient of variation at 9.3%±3.4% and a 96.5%±7.3% assay accuracy. Preclinical fit-for-purpose modeling of topotecan time- and dose-effects was performed using topotecan-responsive and -nonresponsive xenografts in athymic nude mice. Higher baseline levels of Top1 were observed in topotecan-responsive than -nonresponsive tumors. Top1 levels reached a maximal decrease 4 to 7 hours following treatment of engrafted mice with topotecan and the indenoisoquinoline NSC 724998.

CONCLUSIONS/SIGNIFICANCE

Our analysis of Top1 levels in control and treated tumors supports the previously proposed mechanism of action for Top1 inhibitor efficacy, wherein higher baseline Top1 levels lead to formation of more covalent-complex-dependent double-strand break damage and, ultimately, cell death. In contrast, xenografts with lower baseline Top1 levels accumulate fewer double-stand breaks, and may be more resistant to Top1 inhibitors. Our results support further investigation into the use of Top1 levels in tumors as a potential predictive biomarker. The Top1 immunoassay described in this paper has been incorporated into a Phase I clinical trial at the National Cancer Institute to assess pharmacodynamic response in tumor biopsies and determine whether baseline Top1 levels are predictive of response to indenoisoquinoline Top1 inhibitors.

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.

Testing of the topoisomerase 1 inhibitor Genz-644282 by the pediatric preclinical testing program

BACKGROUND

Genz-644282 is a novel non-camptothecin topoisomerase I poison that is in clinical development.

PROCEDURES

Genz-644282 was tested against the PPTP in vitro panel (0.1 nM to 1 µM), and in vivo using three times per week × 2 schedule repeated at day 21 at its maximum tolerated dose (MTD) of 4 mg/kg. Subsequently Genz-644282 was tested at 4, 3, 2, and 1 mg/kg in 3 models to assess the dose-response relationship. mRNA gene signatures predictive for Genz-644282 response in vitro were applied to select 15 tumor models that were evaluated prospectively.

RESULTS

In vitro, Genz-644282 demonstrated potent cytotoxic activity with a median IC(50) of 1.2 nM (range 0.2-21.9 nM). In vivo, Genz-644282 at its MTD (4 mg/kg) induced maintained complete responses (MCR) in 6/6 evaluable solid tumor models. At 2 mg/kg Genz-644282 induced CR or MCR in 3/3 tumor models relatively insensitive to topotecan, but there were no objective responses at 1 mg/kg. Further testing at 2 mg/kg showed that Genz-644282 induced objective regressions in 7 of 17 (41%) models. There was a significant correlation between predictive response scores based on Affymetrix U133Plus2 baseline tumor expression profiles and the observed in vivo responses to Genz-644282.

CONCLUSIONS

Genz-644282 was highly active within a narrow dose range (2-4 mg/kg), typical of other topoisomerase I poisons. As with other topoisomerase I poisons, how accurately these data will translate to clinical activity will depend upon the drug exposures that can be achieved in children treated with this agent.

Molecular and cellular pharmacology of the novel noncamptothecin topoisomerase I inhibitor Genz-644282

Camptothecin derivatives are powerful anticancer drugs because of their ability to trap topoisomerase I (Top1)-DNA cleavage complexes. However, they exhibit clinical limitations due to the instability of their α-hydroxylactone six-membered E-ring structure. In addition, they exhibit bone marrow and intestinal toxicity, especially in adults, and are drug efflux substrates. Here, we report a novel Top1 inhibitor, Genz-644282. We show that Genz-644282 and its metabolites induce Top1 cleavage at similar, as well as unique genomic positions, compared with camptothecin. The compound also induces protein-linked DNA breaks and Top1-DNA cleavage complexes that persist longer after compound removal than camptothecin. Concentration-dependent and persistent γH2AX formation was readily observed in cells treated with Genz-644282, and was present in greater than 50% of the cell population following 24 hours compound exposure. The compound shows partial cross-resistance in cell lines resistant to camptothecin. These cell lines include the human prostate DU145RC0.1 and the leukemic CEM/C2 cells. Limited cross-resistance to Genz-644282 was also found in the Top1 knockdown colon cancer (HCT116) and breast cancer (MCF7) cell lines and in human adenocarcinoma cells (KB31/KBV1) that overexpress (P-glycoprotein, ABCB1), a member of the ATP-binding cassette family of cell surface transport proteins known to confer MDR. Together, our results provide the first molecular and cellular characterization of Genz-644282 and its clinically relevant metabolites.

14-Aminocamptothecins: their synthesis, preclinical activity, and potential use for cancer treatment

14-Aminocamptothecins were synthesized in good yields by treating camptothecin (1a) and 7-ethylcamptothecin (1b) with 90% fuming nitric acid either neat or in acetic anhydride and then followed by reduction of the resulting 14-nitrocamptothecins (2). 14-Aminocamptothecin (3a) and 7-ethyl-14-aminocamptothecin (3b) demonstrated excellent cytotoxic potency against human tumor cell lines in vitro, and they are not substrates for any of the major clinically relevant efflux pumps (MDR1, MRP1, and BCRP). 3a and 3b showed similar cytotoxicity against human and mouse bone marrow progenitor cells. This is in contrast to many camptothecin analogues, which are substrates for efflux pumps and are dramatically more toxic to human marrow cells relative to murine. 3a and 3b demonstrated significant brain penetration when dosed orally in mice. 3b showed significantly better efficacy relative to topotecan when dosed orally in the three ectopic xenograft models, H460, HT29, and PC-3. On the basis of its favorable in vitro and in vivo profile, 3b warrants future development.

Bone marrow CFU-GM and human tumor xenograft efficacy of three tubulin binding agents

PURPOSE

The dynamic instability of microtubules in cells is one of the key targets of anticancer therapeutics. Microtubule-disrupting agents such as vinca alkaloids and microtubule-stabilizing agents such as taxanes are important antitumor agents. The bone marrow toxicity and human tumor xenograft activity of three tubulin-binding compounds, vincristine, paclitaxel, and tasidotin were compared.

METHODS

Mouse and human bone marrow were subjected to colony-forming (CFU-GM) assays over a 5-log concentration range in culture. In vivo, a range of tasidotin doses was compared with vincristine, paclitaxel, and docetaxel for efficacy in several human tumor xenografts.

RESULTS

The IC(90) concentrations for vincristine and paclitaxel for mouse CFU-GM were 30 and 27 nM, and for human CFU-GM were 3 and 9 nM, giving mouse to human differentials of ten- and threefold. Tasidotin produced IC(90)s of >300 nM in mouse and 65 nM in human CFU-GM, thus a >4.6-fold differential between species. In vivo, tasidotin resulted in a dose-dependent increase in tumor growth delay in the RL lymphoma, the RPMI 8226 multiple myeloma, and MX-1 breast carcinoma models. Vincristine and tasidotin were also very effective against these tumors. The PC-3 prostate carcinoma was very responsive to full-dose paclitaxel and docetaxel while tasidotin generated a dose dependent effect.

CONCLUSIONS

Bringing together bone marrow toxicity data, pharmacokinetic parameters, and human tumor xenograft efficacy provides valuable information for the translation of preclinical findings to the clinic.