Utilization of Photoaffinity Labeling to Investigate Binding of Microtubule Stabilizing Agents to P-Glycoprotein and β-Tubulin

Photoaffinity labeling approaches have historically been used in pharmacology to identify molecular targets. This methodology has played a pivotal role in identifying drug-binding domains and searching for novel compounds that may interact at these domains. In this review we focus on studies of microtubule stabilizing agents of natural product origin, specifically taxol (paclitaxel). Taxol and other microtubule interacting agents bind to both P-glycoprotein (ABCB1), a drug efflux pump that reduces intracellular drug accumulation, and the tubulin/microtubule system. Both binding relationships modulate drug efficacy and are of immense interest to basic and translational scientists, primarily because of their association with drug resistance for this class of molecules. We present this body of work and acknowledge its value as fundamental to understanding the mechanisms of taxol and elucidation of the taxol pharmacophore. Furthermore, we highlight the ability to multiplex photoaffinity approaches with other technologies to further enhance our understanding of pharmacologic interactions at an atomic level. Thus, photoaffinity approaches offer a relatively inexpensive and robust technique that will continue to play an important role in drug discovery for the foreseeable future.

Structural Refinement of the Tubulin Ligand (+)-Discodermolide to Attenuate Chemotherapy-Mediated Senescence

The natural product (+)-discodermolide (DDM) is a microtubule stabilizing agent and potent inducer of senescence. We refined the structure of DDM and evaluated the activity of novel congeners in triple negative breast and ovarian cancers, malignancies that typically succumb to taxane resistance. Previous structure-activity analyses identified the lactone and diene as moieties conferring anticancer activity, thus identifying priorities for the structural refinement studies described herein. Congeners possessing the monodiene with a simplified lactone had superior anticancer efficacy relative to taxol, particularly in resistant models. Specifically, one of these congeners, B2, demonstrated 1) improved pharmacologic properties, specifically increased maximum response achievable and area under the curve, and decreased EC₅₀; 2) a uniform dose-response profile across genetically heterogeneous cancer cell lines relative to taxol or DDM; 3) reduced propensity for senescence induction relative to DDM; 4) superior long-term activity in cancer cells versus taxol or DDM; and 5) attenuation of metastatic characteristics in treated cancer cells. To contrast the binding of B2 versus DDM in tubulin, X-ray crystallography studies revealed a shift in the position of the lactone ring associated with removal of the C2-methyl and C3-hydroxyl. Thus, B2 may be more adaptable to changes in the taxane site relative to DDM that could account for its favorable properties. In conclusion, we have identified a DDM congener with broad range anticancer efficacy that also has decreased risk of inducing chemotherapy-mediated senescence. SIGNIFICANCE STATEMENT: Here, we describe the anticancer activity of novel congeners of the tubulin-polymerizing molecule (+)-discodermolide. A lead molecule is identified that exhibits an improved dose-response profile in taxane-sensitive and taxane-resistant cancer cell models, diminished risk of chemotherapy-mediated senescence, and suppression of tumor cell invasion endpoints. X-ray crystallography studies identify subtle changes in the pose of binding to β-tubulin that could account for the improved anticancer activity. These findings support continued preclinical development of discodermolide, particularly in the chemorefractory setting.

Nanoparticle-Encapsulated Doxorubicin Demonstrates Superior Tumor Cell Kill in Triple Negative Breast Cancer Subtypes Intrinsically Resistant to Doxorubicin

The effect of size and release kinetics of doxorubicin-nanoparticles on anti-tumor efficacy was evaluated in a panel of human cancer cell lines, including triple-negative breast cancer (TNBC) cells that frequently demonstrate resistance to doxorubicin. Different nano-formulations of sol-gel-based Doxorubicin containing nanoparticles were synthesized. Increased cell kill in chemoreffactory triple-negative breast cancer cells was associated with the smallest size of nanoparticles and the slowest release of Dox. Modeling of dose-response parameters in Dox-sensitive versus Dox-resistant lines demonstrated increased EMax and area under the curve in Dox-resistant mesenchymal TNBC cells, implying potentially favorable activity in this molecular subtype of breast cancer. Mesenchymal TNBC cells demonstrated a high rate of fluorescent bead uptake suggestive of increased endocytosis, which may partially account for the enhanced efficacy of Dox-np in this subtype. Thus, manipulation of size and release kinetics of this nanoparticle platform is associated with enhanced dose-response metrics and tumor cell kill in therapeutically recalcitrant TNBC cell models. This platform is easily customizable and warrants further exploration.

Improved Dose-Response Relationship of (+)-Discodermolide-Taxol Hybrid Congeners

(+)-Discodermolide is a microtubule-stabilizing agent with potential for the treatment of taxol-refractory malignancies. (+)-Discodermolide congeners containing the C-3'-phenyl side chain of taxol (paclitaxel) were synthesized based on computational docking models predicting this moiety would fill an aromatic pocket of β-tubulin insufficiently occupied by (+)-discodermolide, thereby conferring improved ligand-target interaction. It was recently demonstrated, however, that the C-3'-phenyl side chain occupied a different space, instead extending toward the M-loop of β-tubulin, where it induced a helical conformation, hypothesized to improve lateral contacts between adjacent microtubule protofilaments. This insight led us to evaluate the biological activity of hybrid congeners using a panel of genetically diverse cancer cell lines. Hybrid molecules retained the same tubulin-polymerizing profile as (+)-discodermolide. Since (+)-discodermolide is a potent inducer of accelerated senescence, a fate that contributes to drug resistance, congeners were also screened for senescence induction. Flow cytometric and transcriptional analysis revealed that the hybrids largely retained the senescence-inducing properties of (+)-discodermolide. In taxol-sensitive cell models, the congeners had improved dose-response parameters relative to (+)-discodermolide and, in some cases, were superior to taxol. However, in cells susceptible to senescence, E_Max increased without concomitant improvements in EC₅₀ such that overall dose-response profiles resembled that of (+)-discodermolide.

A Novel Indication for Panobinostat as a Senolytic Drug in NSCLC and HNSCC

Panobinostat (pano) is an FDA-approved histone deacetylase inhibitor. There is interest in evaluating alternate dosing schedules and novel combinations of pano for the treatment of upper aerodigestive and lung malignancies; thus we evaluated it in combination with Taxol, a chemotherapeutic with activity in both diseases. Dose-dependent synergy was observed in Non-Small Cell Lung Cancer (NSCLC) and Head and Neck Squamous Cell Carcinoma (HNSCC) cell lines and was due to senescence rather than potentiation of cell death. Senescence occurred following cisplatin- or Taxol-treatment in cell lines from both cancer types and was associated with decreased histone 3 (H3) acetylation and increased Bcl-xL expression: the latter a biomarker of senescence and target of anti-senescence therapeutics, or senolytics. Since H3 acetylation and Bcl-xL expression were altered in senescence, we subsequently evaluated pano as a senolytic in chemotherapy-treated cancer cells enriched for senescent cells. Pano caused cell death at significantly higher rates compared to repeat dosing with chemotherapy. This was associated with decreased expression of Bcl-xL and increased acetylated H3, reversing the expression patterns observed in senescence. These data support evaluating pano as a post-chemotherapy senolytic with the potential to kill persistent senescent cells that accumulate during standard chemotherapy in NSCLC and HNSCC.

Structural Basis of Microtubule Stabilization by Discodermolide

Microtubule-stabilizing agents (MSAs) are widely used in chemotherapy. Using X-ray crystallography we elucidated the detailed binding modes of two potent MSAs, (+)-discodermolide (DDM) and the DDM-paclitaxel hybrid KS-1-199-32, in the taxane pocket of β-tubulin. The two compounds bind in a very similar hairpin conformation, as previously observed in solution. However, they stabilize the M-loop of β-tubulin differently: KS-1-199-32 induces an M-loop helical conformation that is not observed for DDM. In the context of the microtubule structure, both MSAs connect the β-tubulin helices H6 and H7 and loop S9-S10 with the M-loop. This is similar to the structural effects elicited by epothilone A, but distinct from paclitaxel. Together, our data reveal differential binding mechanisms of DDM and KS-1-199-32 on tubulin.

Therapeutic Advances and New Directions for Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is a molecularly diverse grouping with poor prognosis for which chemotherapy remains the foundation of treatment. The molecular heterogeneity of the disease rationalizes its diverse biological behavior and differential response to treatment. Estimates of up to 20% of patients diagnosed have germline mutations in DNA-damage repair-pathway genes, namely BRCA1 and 2, and this can be used to select patients likely to respond to platinums and/or inhibitors of poly(ADP-ribose) polymerase (PARP). Similar strategies can be utilized in other subtypes of TNBC that have 'BRCA-like' tumor biology due to the presence of mutations in alternate DNA-damage repair genes. The diverse biological behavior of TNBC and its variable response to chemotherapy were largely decoded following genotyping studies that enabled the identification of distinct molecular subtypes, such that the biological and genetic heterogeneity of the disease could be understood. This subsequently enabled the identification of therapeutic 'vulnerabilities' for each subtype that encompass biological processes including proliferation, DNA repair, apoptosis, angiogenesis, immune modulation, and invasion and metastasis. To expedite the development of therapies for high-risk, early-stage breast cancer, we have adopted novel trial designs and re-defined endpoints as surrogates of clinical outcomes. The purpose of this review is to highlight the current standard and experimental treatment options for TNBC.

Abstract A35: Proteomic and transcriptional profiling reveal differential responses to combined MEK and PI3K-mTOR network inhibition in basal-like and mesenchymal subtypes of triple negative breast cancer

Background: Activated MAPK and PI3K/AKT/mTOR pathway signaling are associated with poor prognosis in triple negative Breast cancer (TNBC). Therefore, suppression of both arms of the MAPK/PI3K/AKT/mTOR network is a promising strategy for targeting TNBC. Here we explore the anti-tumor effects of combinations of MEK inhibitor with PI3K, AKT, or mTOR inhibitors with a focus on cell fate and biomarker development in two major subtypes of TNBC, basal-like and mesenchymal.

Methods: Combinations of a MEK inhibitor with a PI3K inhibitor, AKT inhibitor, dual mTORC 1/2 inhibitor, or rapalog were evaluated in TNBC cell lines and an orthotopically implanted patient-derived xenograft (PDX) model of TNBC. We utilized reverse-phase protein array (RPPA) and quantitative real-time PCR to interrogate the signaling architecture and transcriptional activity of the treated cell lines.

Results: All combinations synergistically suppress the growth of basal-like and mesenchymal TNBC models. RPPA confirms that all combinations suppress common network targets including pERK Th202/T204, pPRAS40 T246, pS6rp S235/236, and p4E-BP1 S65 in all cell lines. Notably, however, comparable repression of MAPK/PI3K/AKT/mTOR signaling produces distinct fates in basal-like and mesenchymal subtypes. Basal-like cell lines preferentially undergo delayed cell death with surviving cells displaying profound growth arrest. In contrast, mesenchymal cell lines respond with uniform quiescence exhibiting little or no cell death. Transcriptional analysis corroborates these phenotypic effects demonstrating differential modulation of genes regulating apoptosis and proliferation in basal-like versus mesenchymal cell lines. Drug treated mesenchymal cells exhibit transcriptional ‘reprogramming’ of epithelial-mesenchymal status as evidenced by reduced mesenchymal gene expression inferring potential effects on invasion and metastasis. In a PDX model of basal-like TNBC that is paclitaxel resistant, early and sustained suppression of tumor growth was achieved by treating once daily with the MEK inhibitor, GDC-0973, in combination with either the PI3K inhibitor, GDC-0941, or the rapalog, Temsirolimus. Concurrent dosing on this intense schedule was associated with some toxicity that could likely be diminished with dose modifications.

Conclusions: These data highlight the therapeutic potential of combined MEK and PI3K/AKT/mTOR inhibition in chemo-resistant TNBC. Importantly, they demonstrate innate differences in the response of basal-like and mesenchymal subtypes of TNBC to these combinations, supporting the concept that molecular subtype will be an important predictor of response both in this setting and for other targeted therapies. The combination of GDC-0973 and GDC-0941 performed particularly well in a taxane resistant PDX model and deserves further evaluation in clinical trials for the treatment of TNBC.

Citation Format: Sarah J. Schweber, Alicia Rodriguez-Gabin, Jinghang Zhang, Valerie Calvert, Huiping Liu, Emanuel M. Petricoin, III, Eleni Andreopoulou, Susan Band Horwitz, Hayley M. McDaid. Proteomic and transcriptional profiling reveal differential responses to combined MEK and PI3K-mTOR network inhibition in basal-like and mesenchymal subtypes of triple negative breast cancer. [abstract]. In: Proceedings of the AACR Special Conference: Targeting the PI3K-mTOR Network in Cancer; Sep 14-17, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(7 Suppl):Abstract nr A35.

Therapies for triple negative breast cancer

INTRODUCTION

Triple negative breast cancer (TNBC) is a heterogeneous disease associated with a high risk of recurrence, and therapeutic options are currently limited to cytotoxic therapy. Germ-line mutations may occur in up to 20% of unselected patients with TNBC, which may serve as a biomarker identifying which patients may have tumors that are particularly sensitive to platinums and/or inhibitors of poly(ADP-ribose)polymerase. A substantial proportion of patients with TNBCs not associated with germ-line BRCA mutations may have tumors that are ‘BRCA-like’, rendering those individuals potential candidates for similar strategies.

AREAS COVERED

The purpose of this review is to highlight the current standard and experimental treatment strategies.

EXPERT OPINION

Recent research that has illuminated the molecular heterogeneity of the disease rationalizes its diverse biological behavior and differential response to chemotherapy. Modern technology platforms provide molecular signatures that can be mined for therapeatic interventions. Target pathways that are commonly dysregulated in cancer cells control cellular processes such as apoptosis, proliferation, angiogenesis, DNA repair, cell cycle progression, immune modulation and invasion, and metastasis. Novel trial design and re-defined endpoints as surrogates to clinical outcome have been introduced to expedite the development of breakthrough therapies to treat high-risk early-stage breast cancer.

Abstract B165: The antitumor efficacy of eribulin is mediated via suppression of invasion and profound induction of cell death in triple negative breast cancer

Eribulin, a microtubule-targeting antitumor drug, is FDA-approved for the treatment of certain patients with advanced or metastatic breast cancer. Some patients treated with this drug have evidence of reduced metastatic dissemination. Triple-negative breast cancer (TNBC), which accounts for approximately 15%-20% of all breast cancers, does not respond to hormone therapies so patients are dependent on chemotherapy. A subset of these patients are intrinsically resistant to chemotherapy, or subsequently relapse and develop metastatic disease, eventually leading to mortality. We explored the mechanistic basis for the antitumor efficacy of eribulin in a panel of triple negative breast cancer cell lines that histologically are classified as basal-like or claudin-low. Specifically, we determined the ability of eribulin to induce cell death versus senescence, and secondly to modulate cancer cell invasion and epithelial to mesenchymal transition (EMT) gene expression. Senescence is a clinically relevant cell fate following chemotherapy that can propel tumor progression and EMT. Eribulin was efficacious at inducing cell death in both subtypes of TNBC, and had a remarkably low propensity to induce senescence in TNBC compared to non-small cell lung cancer and colon cancer cells. A 24-hour treatment with eribulin partially prevented tumor cell invasion in the claudin-low subtype, as assayed by Boyden chamber analysis. Drug-tolerant claudin-low cells that survived high-dose eribulin treatment were also found to have a reduction in EMT markers. In summary, in vitro analysis of cell fate and metastasis indicate that eribulin has an ability to modulate invasiveness and EMT characteristics in preclinical models of TNBC.

This research was funded by Eisai Inc.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B165.

Citation Format: Jing Hu, Susan Band Horwitz, Hayley M. McDaid. The antitumor efficacy of eribulin is mediated via suppression of invasion and profound induction of cell death in triple negative breast cancer. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B165.

Protein pathway activation mapping guided biomarker development to identify optimal combinations of MEK inhibitor with PI3K/mTOR pathway inhibitors for the treatment of triple-negative breast cancer

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Background: Activated MAPK and PI3K pathway signaling are associated with poor prognosis in triple negative breast cancer (TNBC). Although some TNBC cell models are sensitive to MEK inhibition, feedback activation of the PI3K pathway mediates resistance. Thus, suppression of both arms of the MAPK/PI3K/mTOR network is a rational approach to targeting TNBC. Here we explore the anti-tumor efficacy of combinations of MEK inhibitor with PI3K, AKT, or mTOR inhibitors with a focus on biomarker development. Methods: Combinations of the MEK inhibitor PD-0325901 with the PI3K inhibitor GDC-0941, AKT inhibitor MK-2206, dual mTORC 1/2 inhibitor Torin 1, or the rapalog temsirolimus were evaluated in TNBC cell lines. Synergy was assessed using the combination index method of Chou and Talalay. We utilized reverse-phase protein array to map the signaling architecture of the treated lines to verify target suppression and identify pharmacodynamic biomarkers. Results: All combinations demonstrated synergy that was mediated by both suppression of proliferation and cell death in a dose-dependent manner. Cell death was delayed, peaking at least 96 hours post-dosing, and was associated with sustained suppression of target proteins in both pathways, including pERKT202/Y204, pS6rpS235/236, p4EBP-1S65, and pPRAS40T246. However, suppression of pAKT (at T308 or S473) was variable and not consistently required for cell death. Pathway mapping identified a protein network ‘signature’ specific to all combination therapies that emerged at 72 hours and was associated with cell death. Thus, all combinations appear to share common downstream effectors. All combinations showed promising efficacy and will be evaluated in a human-in-mouse model of TNBC. Conclusions: These data support therapeutic strategies for TNBC that simultaneously inhibit both arms of the MAPK/PI3K/mTOR signaling network. For continued biomarker development, we stress the importance of studying the delayed effects of combination therapy. This strategy coupled with a protein network based approach uncovered a unique functional signaling ‘signature’.

Exploiting MEK inhibitor-mediated activation of ERα for therapeutic intervention in ER-positive ovarian carcinoma

While the clinical benefit of MEK inhibitor (MEKi)-based therapy is well established in Raf mutant malignancies, its utility as a suppressor of hyperactive MAPK signaling in the absence of mutated Raf or Ras, is an area of ongoing research. MAPK activation is associated with loss of ERα expression and hormonal resistance in numerous malignancies. Herein, we demonstrate that MEKi induces a feedback response that results in ERα overexpression, phosphorylation and transcriptional activation of ER-regulated genes. Mechanistically, MEKi-mediated ERα overexpression is largely independent of erbB2 and AKT feedback activation, but is ERK-dependent. We subsequently exploit this phenomenon therapeutically by combining the ER-antagonist, fulvestrant with MEKi. This results in synergistic suppression of tumor growth, in vitro and potentiation of single agent activity in vivo in nude mice bearing xenografts. Thus, we demonstrate that exploiting adaptive feedback after MEKi can be used to sensitize ERα-positive tumors to hormonal therapy, and propose that this strategy may have broader clinical utility in ERα-positive ovarian carcinoma.

Characterization of a human βV-tubulin antibody and expression of this isotype in normal and malignant human tissue

There are seven distinct β-tubulin isotypes and eight α-tubulin isotypes in mammals that are hypothesized to have tissue- and cell-specific functions. There is an interest in the use of tubulin isotypes as prognostic markers of malignancy. βV-tubulin, like βIII-tubulin, has been implicated in malignant transformation and drug resistance, however little is known about its localization and function. Thus, we generated for the first time, a rabbit polyclonal antibody specific for human βV-tubulin. The antibody did not cross-react with mouse βV-tubulin or other human β-tubulin isotypes and specifically labeled βV-tubulin by immunoblotting, immunofluorescence and immunohistochemistry. Immunohistochemistry of various human normal tissues revealed that βV-tubulin was expressed in endothelial cells, myocytes and cells with muscle differentiation, structures with transport and/or secretory function such as renal tubules, pancreatic ducts and bile ducts, and epithelium with secretory function such as prostate. βV-tubulin was also specifically expressed in pancreatic islets and intratubular germ cell neoplasia, where it may have diagnostic utility. Initial studies in breast, lung and ovarian cancers indicated aberrant expression of βV-tubulin, suggesting that this isoform may be associated with tumorigenesis. Thus, βV-tubulin expression is a potentially promising prognostic marker of malignancy.

Abstract 4230: Senescence reversion contributes to acquired drug resistance

Cells undergo a diverse range of responses to cytotoxic drugs such as Taxol, including apoptosis, mitotic catastrophe, necrosis and accelerated senescence. Senescent cells are resistant to growth factor-stimulated proliferation and often have a secretory phenotype (the Senescence-Associated Sectetory Phenotype, or SASP) that mediates extracellular matrix remodeling and immune clearance. Although poorly defined, in some situations senescent cells do not get cleared and persist. A growing body of evidence suggests that cancer cells made senescent from cytotoxic therapy have a pro-inflammatory SASP that promotes malignant transformation and EMT of surrounding cells. Furthermore, we have demonstrated that senescent cells undergo reversion (PNAS 2010; 108, p391-396.), although the frequency of this phenomena in human malignancies is unknown. Specifically, we generated a human lung cancer cell line that overcame senescence caused by the microtubule-stabilizing drug discodermolide. These cells are resistant to discodermolide and other cytotoxic drugs; therefore we hypothesized that senescent reverted cells retain some characteristics of their senescent precursors, and that this contributes to drug resistance. To explore this possibility, we profiled global gene expression and promoter methylation of drug-resistant cell lines derived from the A549 lung cancer cells and their senescent counterparts. These were made resistant to different tubulin-polymerizing drugs that induce accelerated senescence. Bioinformatic and subsequent ingenuity pathway analysis identified common pathways that were statistically significantly altered in senescent cells and their resistant counterparts. Candidate genes were selected for validation of gene expression and methylation changes by qRT-PCR and MassArray, respectively. Drug-induced senescent cells had increased levels of the SASP genes IL6 (16.8-fold), IL8 (4.15-fold) and CXCL1 (4.35-fold); as well as other genes not typically associated with SASP phenotypes. Interestingly, all three resistant cells retained high expression of specific genes in pro-inflammatory signaling pathways, including IL8 and CXCL1. The increased expression of CXCL1 in senescent cells was via transcriptional activation, whereas the even higher expression observed in senescent reverted cells (resistant cells) was via promoter hypomethylation. Ongoing experiments to modulate gene expression using shRNA-mediated knockdown will help elucidate the role of CXCL1 in senescence reversion and how this contributes to drug resistance. Additional pathways including Wnt and G-protein coupled receptor signaling were also strongly implicated in senescence reversion and will be presented. This is the first study to document retention of SASP signaling in senescent reverted cells and to explore whether these contribute to acquired drug resistance.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4230. doi:1538-7445.AM2012-4230

Insights into 4E-BP1 and p53 mediated regulation of accelerated cell senescence

Senescence is a valid tumor suppressive mechanism in cancer. Accelerated cell senescence describes the growth arrested state of cells that have been treated with anti-tumor drugs, such as doxorubicin that induce a DNA damage response. Discodermolide, a microtubule-stabilizing agent, is a potent inducer of accelerated cell senescence. Resistance to discodermolide is mediated via resistance to accelerated cell senescence, and is associated with reduced expression of the mTORC1 substrate, 4E-BP1 and increased expression of p53 [1]. Although the association of p53 with senescence induction is well-characterized, senescence reversion in the presence of high expression of p53 has not been well-documented. Furthermore, studies addressing the role of mTOR signaling in regulating senescence have been limited and recent data implicate a novel, senescence-associated role for 4E-BP1 in crosstalk with the transcription factor p53. This research perspective will address these somewhat contradictory findings and summarize recent research regarding senescence and mTORC1 signaling.

Abstract 1228: Resistance to discodermolide, a microtubule stabilizing agent and senescence inducer, is 4E-BP1 dependent

Discodermolide is a microtubule stabilizing agent that induces accelerated cell senescence. A discodermolide-resistant cell line, AD32, was generated from the human lung cancer cell line A549. We hypothesize that the major resistance mechanism in these cells is escape from accelerated senescence. AD32 cells have decreased levels of 4E-BP1 mRNA and protein, relative to the parental discodermolide-sensitive A549 cells. Lentiviral-mediated re-expression of wild type 4E-BP1 in AD32 cells increased the proliferation rate and reverted resistance to discodermolide via restoration of discodermolide-induced accelerated senescence. Consistent with this, cell growth and response to discodermolide was confirmed in vivo using tumor xenograft models. Furthermore, re-introduction of a non-phosphorylatable mutant (Thr 37/46 Ala) of 4E-BP1 was able to partially restore sensitivity and enhance proliferation in AD32 cells, suggesting that these effects are independent of phosphorylation by mTORC1. Microarray profiling of AD32 resistant cells versus sensitive A549 cells, and subsequent unbiased gene ontology analysis, identified molecular pathways and functional groupings of differentially expressed mRNAs implicated in overcoming discodermolide-induced senescence. The most-statistically significant classes of differentially expressed genes included p53 signaling, G2/M checkpoint regulation and genes involved in the role of BRCA1 in the DNA damage response. Consistent with this, p53 protein expression was up-regulated and had increased nuclear localization in AD32 cells relative to parental A549 cells. Furthermore, the stability of p53 was enhanced in AD32 cells. Our studies propose a role for 4E-BP1 as a regulator of discodermolide-induced accelerated senescence.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1228. doi:10.1158/1538-7445.AM2011-1228

Abstract 5472: Loss of TSC2 function predicts optimal therapeutic response to combinations of MEK and mTOR inhibitors

The MAPK-RAS-PI3K signaling network is oncogenic in many human malignancies. The plasticity of cancer cell ‘circuitry’ is an important consideration in the rational design of drug combinations, particularly those targeting this signaling network. Therapies directed at MEK, a downstream RAS effector, have been in various stages of clinical development and shown to exhibit single-agent activity in RAF-mutant tumors. However, there is significant redundancy in the MAPK-PI3K pathway that mediates acute resistance to drugs that target either axis of this network, therefore the optimal use for MEK inhibitors will be in combination.

We have previously demonstrated that the combination of MEK and mTOR inhibitors is synergistic in a broad range of tumor cell lines and xenofrafts and mechanistically this is mediated via potent suppression of cap-dependent translation. To better define which cohort of patients would best benefit from this combination modality, we have evaluated the efficacy of MEK and rapalog combinations in TSC2 defective cells. The TSC1/2 complex integrates AKT/mTOR and MAPK circuitry and normally inhibits mTOR via Rheb. However disruption of the TSC complex occurs by mutational inactivation, LOH and inactivating phosphorylation via various kinases, including ERK. Loss of TSC2 function hyperactivates mTOR signaling and confers increased sensitivity to mTOR-directed therapies, including rapalogs. In spite of the known participation of MAPK signaling in TSC function, pharmacological inhibitors of MAPK or other components have not been evaluated in TSC2 null tumors.

Cells defective in TSC2 had increased sensitivity to both single agent MEK or rapalog therapies, relative to TSC2 WT cells, and had a much more synergistic interaction that was associated with potent dephosphorylation of the translational repressor, 4E-BP1. Synergy was associated with autophagy induction in TSC2 null cells followed by profound growth arrest that did not respond to growth factor stimulation. Cells with intact TSC2 function did not undergo autophagy after combined MEK and rapalog-inhibitor treatment and did not exhibit the same degree of growth arrest. Bioenergetic profiling indicated that TSC2 null cells had increased levels of oxygen consumption relative to WT cells, indicating changes in metabolic output associated with mTOR activation. In conclusion, these data support the use of combined MEK and rapalog-inhibitor therapy in tumors with defective TSC2 signaling.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5472. doi:10.1158/1538-7445.AM2011-5472

Abstract IA2-2: Targeting redundancy in RAS-PI3K signaling in NSCLC

Cytotoxic chemotherapy has formed the backbone of cancer treatment for decades. The clinical use of such agents was established using dosing strategies that favored efficacy (optimal tumor regression) coupled with minimal toxicity. Conceptually, little regard was given to mechanisms governing survival circuitry, or the genetic alterations in a given tumor, which we now appreciate, are important prognosticators of clinical response for both cytotoxic drugs and therapies that target aberrant signaling pathways involved in cancer pathogenesis. Pharmacogenomic profiling is focused on characterizing genetic changes in tumor cells that drive malignant growth and render cells ‘addicted’ to certain pathways for survival and growth. By identifying these genes and the pathways in which they participate, one can devise therapeutic strategies to disable them.

An underappreciated fact in cancer therapeutics is that cancer cells respond to and counteract the effects of any drugs that threaten their survival. Signaling pathways have evolved with innate adaptive abilities to form a regulatory circuitry via positive and negative feedback. These adaptive properties present in normal cells are exploited further in cancer genomes due to their genomic instability and the resulting plasticity.

The MAPK-RAS-PI3K signaling network is oncogenic in many human malignancies, including NSCLC, though somatic mutations in PI3K are rare in this disease. Efforts to therapeutically target MAPK dysfunction have been focused on RAF and RAS, since their high frequency of mutations in human tumors renders these druggable targets (activating point mutations in K-RAS occur in approximately 30% of lung adenocarcinomas). Therapeutic strategies to inhibit oncogenic RAS alone have not been successful since these have focused on disrupting effectors downstream of the canonical RAS pathway, without perturbing other RAS effectors. One such RAS- binding protein is type I PI3K, which is required for RAS-driven tumorigenesis: therefore, the PI3K / mTOR component is an important target in RAS-mutant tumors.

Therapies directed at MEK, a downstream RAS effector, have been in therapeutic development and shown to exhibit optimal singleagent activity in RAF-mutant tumors. However, the enthusiasm for their clinical application in lung cancer is moderated by the low prevalence of RAF mutations/amplifications in the disease and the appreciation that there is significant redundancy in the MAPK-PI3K pathway that mediates acute resistance to these agents. Therefore, like most targeted therapies, the optimal use for this drug will be in combination. The plasticity of cancer cell circuitry is an important consideration in the rational design of drug combinations, particularly those targeting this signaling network. By characterizing the mechanisms that RAS and RAF mutant cancer cells use to escape MEK-inhibitor therapy, one can use combinations of drugs that target addicted pathways and also prevent cancer cells activating secondary pathways to evade death.

We have focused on strategies to counteract these mechanisms of resistance and exploit them therapeutically. An example of this is using MEK inhibitors with rapamycin, which is synergistic in a diverse range of NSCLC cell lines and xenograft models. We have identified proteins in the RAS-PI3K network that moderate the therapeutic response to the MEK-rapamycin combination. In addition, other combinations of MEK inhibitors with drugs that target the estrogen receptor and erbB receptor tyrosine kinases, respectively, will be discussed.

Citation Information: Clin Cancer Res 2010;16(14 Suppl):IA2-2.

Abstract 4152: Therapeutic targeting of the estrogen receptor in ovarian carcinoma by MEK inhibition

The regulation and function of the estrogen receptor alpha (ERα) by the ligand estrogen (ES) in ovarian tumorigenesis is not well established. Expression of ERα in ovarian carcinoma, like breast carcinoma, is regulated epigenetically and also via changes in growth-factor receptor signaling and intracellular signaling pathways, such as the Mitogen-activated Protein Kinase (MAPK). Numerous studies have documented an inverse relationship between MAPK activity and ERα expression. Oncogenic MAPK signaling can occur via mutation of K-RAS and B-RAF, which are not prevalent in ovarian carcinomas, and overexpression of receptor tyrosine kinases such as HER2. The aim of these studies were to elucidate (i) the relationship between MAPK signaling and ERα expression, in a panel of ovarian carcinoma cell lines and (ii) the utility of PD0325901, a selective allosteric inhibitor of MEK1/2, in sensitizing ERα expressing ovarian cancer to antiestrogen therapy.

We compared ERα expression in a panel of 5 ovarian carcinoma cell lines (A2780, Hey, SKOV3, OVCAR8, IGROV) under physiologic and ES deprived conditions and also after treatment with PD0325901 by immunoblot analysis. The proliferation rate of all cell lines was reduced in E2-free conditions and the PI3K mutant SKOV3 cell line was most sensitive to ES deprivation. A2780 and SKOV3 (PTEN null and PIK3CA mutant, respectively) expressed ERα. The PTEN and BRCA1-null IGROV cells were MEK-dependent and hypersensitive to PD0325901, whereas SKOV3 cells were highly resistant, due to increased phosphorylation and cross-activation of HER2 by PD0325901. Unlike our previous studies in lung cancer, constitutive activation of PIK3CA /AKT did not predict resistance to MEK-inhibitors. Treatment of SKOV3 cells with PD0325901 was associated with increased expression of ERα and altered mobility by immunoblot analysis. These effects were not observed in A2780 cells that express low levels of ERα. Finally, the interaction of the estrogen receptor antagonist fulvestrant and PD0325901 was also evaluated in ERα expressing SKOV3 and A2780 cells. A greater than additive cytotoxic effect was observed in SKOV3, while an antagonistic / minimally additive interaction was noted for A2780 cells that do not show changes in ERα expression after treatment with PD0325901.

Our data support the utility of MEK-inhibitors as modulators of ERα expression and function in ovarian cancer cell lines. Ongoing studies will focus on delineating the mechanism of PD0325901-mediated changes in ERα in various histological subtypes of ovarian carcinoma (such as the clear cell carcinoma cell line SKOV3). This may identify subsets of ovarian cancers that may benefit from therapeutic intervention by estrogen antagonists in conjunction with MEK inhibitors.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4152.

Targeting protein translation in human non small cell lung cancer via combined MEK and mammalian target of rapamycin suppression

Lung cancer is a genetically heterogeneous disease characterized by the acquisition of somatic mutations in numerous protein kinases, including components of the rat sarcoma viral oncogene homolog (RAS) and AKT signaling cascades. These pathways intersect at various points, rendering this network highly redundant and suggesting that combined mitogen-activated protein/extracellular signal-regulated kinase (MEK) and mammalian target of rapamycin (mTOR) inhibition may be a promising drug combination that can overcome its intrinsic plasticity. The MEK inhibitors, CI-1040 or PD0325901, in combination with the mTOR inhibitor, rapamycin, or its analogue AP23573, exhibited dose-dependent synergism in human lung cancer cell lines that was associated with suppression of proliferation rather than enhancement of cell death. Concurrent suppression of MEK and mTOR inhibited ribosomal biogenesis by 40% within 24 h and was associated with a decreased polysome/monosome ratio that is indicative of reduced protein translation efficiency. Furthermore, the combination of PD0325901 and rapamycin was significantly superior to either drug alone or PD0325901 at the maximum tolerated dose in nude mice bearing human lung tumor xenografts or heterotransplants. Except for a PTEN mutant, all tumor models had sustained tumor regressions and minimal toxicity. These data (a) provide evidence that both pathways converge on factors that regulate translation initiation and (b) support therapeutic strategies in lung cancer that simultaneously suppress the RAS and AKT signaling network.

Peripheral blood mononuclear and tumor cell pharmacodynamics of the novel epothilone B analogue, ixabepilone

BACKGROUND

We previously demonstrated that peak microtubule bundle formation (MBF) in peripheral blood mononuclear cells (PBMCs) occurs at the end of drug infusion and correlates with drug pharmacokinetics (PK). In the current study, a new expanded evaluation of drug target effect was undertaken.

PATIENTS AND METHODS

Patients with advanced solid malignancies were treated with ixabepilone 40 mg/m2 administered as a 1-h i.v. infusion every 3 weeks. Blood, plasma, and tumor tissue sampling was carried out to characterize pharmacodynamics and PK.

RESULTS

Forty-seven patients were treated with 141 cycles of ixabepilone. In both PBMCs (n=27) and tumor cells (n=9), peak MBF occurred at the end of infusion; however, at 24-72 h after drug infusion, the number of cells with MBF was significantly greater in tumor cells, relative to PBMCs. A Hill model (EC50=109.65 ng/ml; r2=0.94) was fitted, which demonstrated a relationship between percentage of PBMCs with MBF and plasma ixabepilone concentration. The percentage of PBMCs with MBF at the end of infusion also correlated with severity of neutropenia (P=0.050).

CONCLUSIONS

Plasma ixabepilone concentration and severity of neutropenia correlate with the level of MBF in PBMCs. Therefore, this technically straightforward assay should be considered as a complement to the clinical development of novel microtubule-binding agents.

Potentiation of taxol efficacy and by discodermolide in ovarian carcinoma xenograft-bearing mice

PURPOSE

To evaluate the drug combination of discodermolide and Taxol in human ovarian cancer cells and in an in vivo model of ovarian carcinoma.

EXPERIMENTAL DESIGN

The combination index method was used to evaluate the interaction of Taxol and discodermolide in human ovarian SKOV-3 carcinoma cells. Data were correlated with alterations in cell cycle distribution and caspase activation. In addition, SKOV-3 xenograft-bearing mice were treated with either Taxol, discodermolide, or a combination of both drugs given concurrently to evaluate the antitumor efficacy and toxicity of this combination. The Matrigel plug assay and CD31 immunohistochemistry were done to assess antiangiogenic effects.

RESULTS

Taxol and discodermolide interact synergistically over a range of concentrations and molar ratios that cause drug-induced aneuploidy in ovarian carcinoma cells. In SKOV-3 xenograft-bearing mice, the combination is significantly superior to either single agent, and induces tumor regressions without notable toxicities. Immunohistochemical analysis of CD31 and Matrigel plug analysis show decreased vessel formation in mice treated with the combination relative to either drug alone.

CONCLUSIONS

The synergistic activity of Taxol and discodermolide in cells is most potent at drug concentrations that result in drug-induced aneuploidy rather than mitotic arrest. Moreover, in an animal model of ovarian carcinoma, this is a well-tolerated combination that induces tumor regressions and suppresses angiogenesis. These data confirm the potency of this combination and support the use of concurrent low doses of Taxol and discodermolide for potential use in cancer therapeutics.

Enhancement of the therapeutic efficacy of taxol by the mitogen-activated protein kinase kinase inhibitor CI-1040 in nude mice bearing human heterotransplants

Taxol may contribute to intrinsic chemoresistance by activating the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) cytoprotective pathway in human cancer cell lines and tumors. We have previously shown additivity between Taxol and the MEK inhibitor, U0126 in human cancer cell lines. Here, the combination of Taxol with an orally bioavailable MEK inhibitor, CI-1040, was evaluated in human lung tumors heterotransplanted into nude mice. Unlike xenograft models that are derived from cells with multiple genetic alterations due to prolonged passage, heterotransplanted tumor models are more clinically relevant. Combined treatment with both drugs resulted in inhibition of tumor growth in all models and tumor regressions in three of four models tested, supporting our previous observation that Taxol's efficacy is potentiated by MEK inhibition. Concurrent administration was superior to intermittent dosing. Pharmacodynamic assessments of tumors indicated that suppression of MEK was associated with induction of S473 phosphorylated Akt and reduced proliferation in the combination groups relative to single agents, in addition to suppression of fibroblast growth factor-mediated angiogenesis and reduced expression of vascular endothelial growth factor. These findings are significant and indicate that this combination may have broad therapeutic applications in a diverse range of lung tumors with different intrinsic chemosensitivities.

Validation of the pharmacodynamics of BMS-247550, an analogue of epothilone B, during a phase I clinical study

The primary aims of this study were to evaluate the timecourse and dose response of microtubule bundle formationin peripheral blood mononuclear cells (PBMCs) and to correlate these data with BMS-247550 pharmacokinetics. The data presented here were obtained from 17 patients enrolled in a Phase I trial who received five dose levels of BMS-247550 (7.4-59.2 mg/m(2)), given as a 1-h infusion once every 3 weeks. Plasma drug exposure or area under the curve (AUC), and tubulin bundle formation in PBMCs were assessed in cycles 1 and 2. Similar analyses were also performed on tumor biopsies from one eligible patient. PBMCs exhibited dramatic microtubule bundle formation 1 h after infusion that declined by 24 h, showing a positive correlation with AUC((0-24)) for cycles 1 and 2. A similar pattern of tubulin bundle formation also was observed in a smaller proportion of breast tumor cells from one patient who exhibited a partial response to BMS-247550. This patient's tumor expressed multidrug resistance (MDR1) and MDR-associated protein (MRP1), and in addition poly(ADPribose) polymerase cleavage, a marker of cell death, was observed within 23 h after drug infusion. This patient was also heterozygous for a novel polymorphism at the extreme COOH terminus of beta-tubulin (Gly 437 Gly/Ser), although the relevance of the polymorphism to the response is unknown. In summary, microtubule bundle formation in PBMCs occurs within 1 h of treatment with BMS-247550 and is related to plasma AUC. Similar bundle formation was seen in one tumor sample, despite expression of MDR1 and MRP1. Cell death occurred 23 h after peak microtubule bundle formation in these tumor cells. These findings validate in vitro pharmacodynamic observations.

Selective potentiation of paclitaxel (taxol)-induced cell death by mitogen-activated protein kinase kinase inhibition in human cancer cell lines

Activation of the mitogen-activated protein kinase (MAPK) pathway in HeLa and Chinese hamster ovary cells after treatment with paclitaxel (Taxol) and other microtubule interacting agents has been investigated. Using a trans-reporting system, the phosphorylation of the nuclear transcription factors Elk-1 and c-jun was measured. Concentration- and time-dependent activation of the Elk-1 pathway, mediated primarily by the extracellular signal-regulated kinase (ERK) component of the MAPK family, was observed. Inactive drug analogs and other cytotoxic compounds that do not target microtubules failed to induce similar levels of activation, thereby indicating that an interaction between these drugs and the microtubule is essential for the activation of MAPKs. Evaluation of the endogenous levels of MAPK expression revealed cell-dependent expression of the ERK, c-jun N-terminal kinase, and p38 pathways. In the case of HeLa cells, time-dependent activation of ERK coincided with increased poly(ADP-ribose) polymerase (PARP) cleavage, phosphatidylserine externalization, and increased accumulation of cells in G2/M. In both cell lines, inhibition of ERK activity potentiated paclitaxel-induced PARP cleavage and phosphatidylserine externalization, suggesting that ERK activity coincided with, but did not mediate, the cytotoxic effects of paclitaxel. We evaluated the nature of the interaction between paclitaxel and the MAPK kinase inhibitor U0126 in three cell lines, on the basis of a potential chemotherapeutic advantage of paclitaxel plus ERK inhibition. Our data confirmed additivity in those cells lines that undergo paclitaxel-induced ERK activation, and antagonism in cells with low ERK activity, suggesting that in tumors with high ERK activity, there may be an application for this strategy in therapy.

Taxol and discodermolide represent a synergistic drug combination in human carcinoma cell lines

Recently, three natural products have been identified, the epothilones, eleutherobin, and discodermolide, whose mechanism of action is similar to that of Taxol in that they stabilize microtubules and block cells in the mitotic phase of the cell cycle. In this report, we have compared and contrasted the effects of these new agents in Taxol-sensitive and -resistant cell lines. We also have taken advantage of a human lung carcinoma cell line, A549-T12, that was isolated as a Taxol-resistant cell line and found to require low concentrations of Taxol (2-6 nM) for normal cell division. This study then examined the ability of these new compounds to substitute for Taxol in sustaining the growth of A549-T12 cells. Immunofluorescence and flow cytometry have both indicated that the epothilones and eleutherobin, but not discodermolide, can substitute for Taxol in this Taxol-dependent cell line. In A549-T12 cells, the presence of Taxol significantly amplified the cytotoxicity of discodermolide, and this phenomenon was not observed in combinations of Taxol with either the epothilones or eleutherobin. Median effect analysis using the combination index method revealed a schedule-independent synergistic interaction between Taxol and discodermolide in four human carcinoma cell lines, an effect that was not observed between Taxol and epothilone B. Flow cytometry revealed that concurrent exposure of A549 cells to Taxol and discodermolide at doses that do not induce mitotic arrest caused an increase in the hypodiploid population, thereby indicating that a possible mechanism for the observed synergy is the potentiation of apoptosis. Our results suggest that Taxol and discodermolide may constitute a promising chemotherapeutic combination.

Structure-activity profiles of eleutherobin analogs and their cross-resistance in Taxol-resistant cell lines

PURPOSE

Eleutherobin, a natural product, is an antimitotic agent that promotes the polymerization of stable microtubules. Although its mechanism of action is similar to that of Taxol, its structure is distinct. A structure-activity profile of synthetic eleutherobin derivatives that have modifications at C3, C8 and C15 was undertaken to define the structural requirements for microtubule stabilization and cross-resistance in Taxol-resistant cell lines.

METHODS

The biological activity of five eleutherobin analogs was assessed using three techniques: (1) cytotoxicity and drug-resistance in three paired Taxol-sensitive and -resistant cell lines; (2) polymerization of microtubule protein in vitro in the absence of GTP and (3) induction of microtubule bundle formation in NIH3T3 cells.

RESULTS

Eleutherobin had an IC50 value comparable to that of Taxol, whereas neoeleutherobin, which has a carbohydrate domain that is enantiomeric with that of the parent compound, was less cytotoxic and had 69% of the maximum microtubule polymerization ability of eleutherobin. Both of these compounds exhibited cross-resistance in MDRI-expressing cell lines. Removal or replacement of the C15 sugar moiety resulted in reduced microtubule polymerization and cytotoxicity compared to eleutherobin and loss of cross-resistance in the cell lines SKVLB and J7-T3-1.6, both of which express high levels of P-glycoprotein. By contrast, removal of the urocanic acid group at C8 resulted in virtually complete abrogation of biological activity. The compound lost its ability to polymerize microtubules, and its cytotoxicity was reduced by a minimum of 2000-fold in lung carcinoma A549 cells.

CONCLUSIONS

Removal or modification of the sugar moiety alters the cytotoxic potency of eleutherobin and its pattern of cross-resistance in Taxol-resistant cells, although such compounds retain a small percentage of the microtubule-stabilizing activity of eleutherobin. The N(1)-methylurocanic acid moiety of eleutherobin, or perhaps some other substituent at the C8 position, is essential for Taxol-like activity. These findings will be important for the future design and the synthesis of new and more potent eleutherobin derivatives.

Increased expression of the RIalpha subunit of the cAMP-dependent protein kinase A is associated with advanced stage ovarian cancer

The primary element in the cAMP signal transduction pathway is the cAMP-dependent protein kinase (PKA). Expression of the RIalpha subunit of type I PKA is elevated in a variety of human tumours and cancer cell lines. The purpose of this study was to assess the prognostic importance of RIalpha expression in patients with ovarian cancer. We have evaluated the expression of RIalpha in a panel of human ovarian tumours (n = 40) and five human ovarian cancer cell lines using quantitative reverse transcription polymerase chain reaction (RT-PCR) and Western blot analysis. The human ovarian cell lines OAW42 and OTN14 express high endogenous levels of RIalpha mRNA and protein (at significantly higher mRNA levels than high tissue expressors, P < 0.05). The ovarian cell line A2780 expresses low endogenous levels of RIalpha mRNA and protein (also at higher mRNA levels than low tissue expressors, P < 0.05). Quantitative RT-PCR revealed no significant difference in RIalpha mRNA expression between different ovarian histological subtypes in this study. No associations were found between RIalpha mRNA expression and differentiation state. RIalpha mRNA expression was significantly associated with tumour stage (P = 0.0036), and this remained significant in univariate analysis (P = 0.0002). A trend emerged between RIalpha mRNA expression levels and overall survival in univariate analysis (P = 0.051), however, by multivariate analysis, stage remained the major determinant of overall survival (P = 0.0001). This study indicates that in ovarian epithelial tumours high RIalpha mRNA expression is associated with advanced stage disease. RIalpha expression may be of predictive value in ovarian cancer and may be associated with dysfunctional signalling pathways in this cancer type.

Synergistic interaction between paclitaxel and 8-chloro-adenosine 3',5'-monophosphate in human ovarian carcinoma cell lines

Taxol is a unique anticancer agent that is used in treatment of advanced ovarian cancer. Taxol exposure results in the polymerization and stabilization of the microtubule skeleton of eukaryotic cells, hence blocking replication and intracellular motility. 8-Chloro-adenosine 3',5'-monophosphate (8-Cl-cAMP) is a cAMP analogue, currently in Phase II clinical trials, that displays growth inhibition at micromolar concentrations. The aim of this study was to assess the nature of the interaction between 8-Cl-cAMP and paclitaxel using the combination index (CI) method of Chou and Talalay, which uses the median-effect analysis. Two ovarian cancer cell lines, A2780 and OAW42, which differ in sensitivity to both drugs, were tested using the fixed-ratio design using various scheduling regimens. Concurrent exposure of both drugs resulted in highly synergistic interactions in both cell lines. CIs (mean +/- SE) with this schedule were 0.182 +/- 0.016, 0.315 +/- 0.32, and 0.618 +/- 0.637 at 20, 50, and 80% cell kill, respectively, in A2780 cells and 0.001 +/- 0.0009, 0.016 +/- 0.0075, and 0.184 +/- 0.168 at 20, 50, and 80% cell kill, respectively, in OAW42 cells. In both cell lines, synergy was effective over a 4-fold log range of concentration for either drug. Sequencing with paclitaxel for 24 h prior to 8-Cl-cAMP was the most effective regimen; it resulted in consistently low CIs of up to the 90% cell kill level for both cell lines. Exposure to 8-Cl-cAMP prior to paclitaxel was the least effective regimen. In conclusion, the combination of paclitaxel and 8-Cl-cAMP is highly synergistic in ovarian carcinoma cell lines, suggesting that 8-Cl-cAMP may stimulate the antitumor effect of the taxanes.