Aplidin® induces JNK-dependent apoptosis in human breast cancer cells via alteration of glutathione homeostasis, Rac1 GTPase activation, and MKP-1 phosphatase downregulation

Dual-specificity phosphatases: therapeutic targets in cancer therapy resistance

PURPOSE

Therapy resistance is the principal obstacle to achieving cures in cancer patients and its successful tackling requires a deep understanding of the resistance mediators. Increasing evidence indicates that tumor phosphatases are novel and druggable targets in translational oncology and their modulation may hinder tumor growth and motility and potentiate therapeutic sensitivity in various neoplasms via regulation of various signal transduction pathways. Dual-specificity phosphatases (DUSPs) are key players of cell growth, survival and death and have essential roles in tumor initiation, malignant progression and therapy resistance through regulation of the MAPK signaling pathway. In this review, different aspects of DUSPs are discussed.

METHODS

A comprehensive literature review was performed using various websites including PubMed.

RESULTS

We provide mechanistic insights into the roles of well-known DUSPs in resistance to a wide range of cancer therapeutic approaches including chemotherapy, radiation and molecular targeted therapy in human malignancies. Moreover, we discuss the development of DUSP modulators, with a focus on DUSP1 and 6 inhibitors. Ultimately, the preclinical investigations of small molecule inhibitors of DUSP1 and 6 are outlined.

CONCLUSION

Emerging evidence indicates that the DUSP family is aberrantly expressed in human malignancies and plays critical roles in determining sensitivity to a wide range of cancer therapeutic strategies through regulation of the MAPK signaling pathways. Consequently, targeting DUSPs and their downstream molecules can pave the way for more effective cancer therapies.

Novel molecules as the emerging trends in cancer treatment: an update

As per World Health Organization cancer remains as a leading killer disease causing nearly 10 million deaths in 2020. Since the burden of cancer increases worldwide, warranting an urgent search for anti-cancer compounds from natural sources. Secondary metabolites from plants, marine organisms exhibit a novel chemical and structural diversity holding a great promise as therapeutics in cancer treatment. These natural metabolites target only the cancer cells and the normal healthy cells are left unharmed. In the emerging trends of cancer treatment, the natural bioactive compounds have long become a part of cancer chemotherapy. In this review, we have tried to compile about eight bioactive compounds from plant origin viz. combretastatin, ginsenoside, lycopene, quercetin, resveratrol, silymarin, sulforaphane and withaferin A, four marine-derived compounds viz. bryostatins, dolastatins, eribulin, plitidepsin and three microorganisms viz. Clostridium, Mycobacterium bovis and Streptococcus pyogenes with their well-established anticancer potential, mechanism of action and clinical establishments are presented.

Marine-derived pipeline anticancer natural products: a review of their pharmacotherapeutic potential and molecular mechanisms

Background

Cancer is a complex and most widespread disease and its prevalence is increasing worldwide, more in countries that are witnessing urbanization and rapid industrialization changes. Although tremendous progress has been made, the interest in targeting cancer has grown rapidly every year. This review underscores the importance of preventive and therapeutic strategies.

Main text

Natural products (NPs) from various sources including plants have always played a crucial role in cancer treatment. In this growing list, numerous unique secondary metabolites from marine sources have added and gaining attention and became potential players in drug discovery and development for various biomedical applications. Many NPs found in nature that normally contain both pharmacological and biological activity employed in pharmaceutical industry predominantly in anticancer pharmaceuticals because of their enormous range of structure entities with unique functional groups that attract and inspire for the creation of several new drug leads through synthetic chemistry. Although terrestrial medicinal plants have been the focus for the development of NPs, however, in the last three decades, marine origins that include invertebrates, plants, algae, and bacteria have unearthed numerous novel pharmaceutical compounds, generally referred as marine NPs and are evolving continuously as discipline in the molecular targeted drug discovery with the inclusion of advanced screening tools which revolutionized and became the component of antitumor modern research.

Conclusions

This comprehensive review summarizes some important and interesting pipeline marine NPs such as Salinosporamide A, Dolastatin derivatives, Aplidine/plitidepsin (Aplidin®) and Coibamide A, their anticancer properties and describes their mechanisms of action (MoA) with their efficacy and clinical potential as they have attracted interest for potential use in the treatment of various types of cancers.

Marine Power on Cancer: Drugs, Lead Compounds, and Mechanisms

Worldwide, 19.3 million new cancer cases and almost 10.0 million cancer deaths occur each year. Recently, much attention has been paid to the ocean, the largest biosphere of the earth that harbors a great many different organisms and natural products, to identify novel drugs and drug candidates to fight against malignant neoplasms. The marine compounds show potent anticancer activity in vitro and in vivo, and relatively few drugs have been approved by the U.S. Food and Drug Administration for the treatment of metastatic malignant lymphoma, breast cancer, or Hodgkin's disease. This review provides a summary of the anticancer effects and mechanisms of action of selected marine compounds, including cytarabine, eribulin, marizomib, plitidepsin, trabectedin, zalypsis, adcetris, and OKI-179. The future development of anticancer marine drugs requires innovative biochemical biology approaches and introduction of novel therapeutic targets, as well as efficient isolation and synthesis of marine-derived natural compounds and derivatives.

Marine Natural Products from Tunicates and Their Associated Microbes

Marine tunicates are identified as a potential source of marine natural products (MNPs), demonstrating a wide range of biological properties, like antimicrobial and anticancer activities. The symbiotic relationship between tunicates and specific microbial groups has revealed the acquisition of microbial compounds by tunicates for defensive purpose. For instance, yellow pigmented compounds, "tambjamines", produced by the tunicate, Sigillina signifera (Sluiter, 1909), primarily originated from their bacterial symbionts, which are involved in their chemical defense function, indicating the ecological role of symbiotic microbial association with tunicates. This review has garnered comprehensive literature on MNPs produced by tunicates and their symbiotic microbionts. Various sections covered in this review include tunicates' ecological functions, biological activities, such as antimicrobial, antitumor, and anticancer activities, metabolic origins, utilization of invasive tunicates, and research gaps. Apart from the literature content, 20 different chemical databases were explored to identify tunicates-derived MNPs. In addition, the management and exploitation of tunicate resources in the global oceans are detailed for their ecological and biotechnological implications.

Targeted Therapies for Multiple Myeloma

Multiple myeloma continues to be a challenging disorder to treat despite improved therapies and the widespread use of proteasome inhibitors and immunomodulatory drugs. Although patient outcomes have improved, the disease continues to invariably relapse, and in the majority of cases, a cure remains elusive. In the last decade, there has been an explosion of novel drugs targeting cellular proteins essential for malignant plasma cell proliferation and survival. In this review, we focus on novel druggable targets leading to the development of monoclonal antibodies and cellular therapies against surface antigens (CD38, CD47, CD138, BCMA, SLAMF7, GPRC5D, FcRH5), inhibitors of epigenetic regulators such as histone deacetylase (HDAC), and agents targeting anti-apoptotic (BCL-2), ribosomal (eEF1A2) and nuclear export (XPO1) proteins.

MAPK signaling pathway-targeted marine compounds in cancer therapy

PURPOSE

This paper reviews marine compounds that target the mitogen-activated protein kinase (MAPK) signaling pathway and their main sources, chemical structures, major targeted cancers and possible mechanisms to provide comprehensive and basic information for the development of marine compound-based antitumor drugs in clinical cancer therapy research.

METHODS

This paper searched the PubMed database using the keywords "cancer", "marine*" and "MAPK signaling pathway"; this search was supplemented by the literature-tracing method. The marine compounds screened for review in this paper are pure compounds with a chemical structure and have antitumor effects on more than one tumor cell line by targeting the MAPK signaling pathway. The PubChem database was used to search for the PubMed CID and draw the chemical structures of the marine compounds.

RESULTS

A total of 128 studies were searched, and 32 marine compounds with unique structures from extensive sources were collected for this review. These compounds are cytotoxic to cancer cell lines, although their targets are still unclear. This paper describes their anticancer effect mechanisms and the protein expression changes in the MAPK pathway induced by these marine compound treatments. This review is the first to highlight MAPK signaling pathway-targeted marine compounds and their use in cancer therapy.

CONCLUSION

The MAPK signaling pathway is a promising potential target for cancer therapy. Searching for marine compounds that exert anticancer effects by targeting the MAPK signaling pathway and developing them into new marine anticancer drugs will be beneficial for cancer treatment.

Converging evidence points towards a role of insulin signaling in regulating compulsive behavior

Obsessive-compulsive disorder (OCD) is a neuropsychiatric disorder with childhood onset, and is characterized by intrusive thoughts and fears (obsessions) that lead to repetitive behaviors (compulsions). Previously, we identified insulin signaling being associated with OCD and here, we aim to further investigate this link in vivo. We studied TALLYHO/JngJ (TH) mice, a model of type 2 diabetes mellitus, to (1) assess compulsive and anxious behaviors, (2) determine neuro-metabolite levels by 1 H magnetic resonance spectroscopy (MRS) and brain structural connectivity by diffusion tensor imaging (DTI), and (3) investigate plasma and brain protein levels for molecules previously associated with OCD (insulin, Igf1, Kcnq1, and Bdnf) in these subjects. TH mice showed increased compulsivity-like behavior (reduced spontaneous alternation in the Y-maze) and more anxiety (less time spent in the open arms of the elevated plus maze). In parallel, their brains differed in the white matter microstructure measures fractional anisotropy (FA) and mean diffusivity (MD) in the midline corpus callosum (increased FA and decreased MD), in myelinated fibers of the dorsomedial striatum (decreased FA and MD), and superior cerebellar peduncles (decreased FA and MD). MRS revealed increased glucose levels in the dorsomedial striatum and increased glutathione levels in the anterior cingulate cortex in the TH mice relative to their controls. Igf1 expression was reduced in the cerebellum of TH mice but increased in the plasma. In conclusion, our data indicates a role of (abnormal) insulin signaling in compulsivity-like behavior.

Plitidepsin: a potential new treatment for relapsed/refractory multiple myeloma

Plitidepsin is a marine-derived anticancer compound isolated from the Mediterranean tunicate Applidium albicans. It exerts pleiotropic effects on cancer cells, most likely by binding to the eukaryotic translation eEF1A2. This ultimately leads to cell-cycle arrest, growth inhibition and induction of apoptosis via multiple pathway alterations. Recently, a Phase III randomized trial in patients with relapsed/refractory multiple myeloma reported outcomes for plitidepsin plus dexamethasone compared with dexamethasone. Median progression-free survival was 3.8 months in the plitidepsin arm and 1.9 months in the dexamethasone arm (HR: 0.611; p = 0.0048). Here, we review preclinical data regarding plitidepsins mechanism of action, give an overview of clinical trial results across different tumor types as well as the latest results in multiple myeloma.

Ascidian Toxins with Potential for Drug Development

Ascidians (tunicates) are invertebrate chordates, and prolific producers of a wide variety of biologically active secondary metabolites from cyclic peptides to aromatic alkaloids. Several of these compounds have properties which make them candidates for potential new drugs to treat diseases such as cancer. Many of these natural products are not produced by the ascidians themselves, rather by their associated symbionts. This review will focus mainly on the mechanism of action of important classes of cytotoxic molecules isolated from ascidians. These toxins affect DNA transcription, protein translation, drug efflux pumps, signaling pathways and the cytoskeleton. Two ascidian compounds have already found applications in the treatment of cancer and others are being investigated for their potential in cancer, neurodegenerative and other diseases.

Plitidepsin: design, development, and potential place in therapy

Plitidepsin is a cyclic depsipeptide that was first isolated from a Mediterranean marine tunicate (Aplidium albicans) and, at present, is manufactured by total synthesis and commercialized as Aplidin®. Its antitumor activity, observed in preclinical in vitro and in vivo studies has prompted numerous clinical trials to be conducted over the last 17 years, alone or in combination with other anticancer agents. Single-agent plitidepsin has shown limited antitumor activity and a tolerable safety profile in several malignancies, such as noncutaneous peripheral T-cell lymphoma, melanoma, and multiple myeloma. In patients with relapsed or refractory multiple myeloma, plitidepsin activity seems to be enhanced after addition of dexamethasone while remaining well tolerated, and a Phase III trial comparing plitidepsin plus dexamethasone vs dexamethasone alone is underway. Additional studies are required to better define the role of plitidepsin in combination with other active agents in these indications. Results of plitidepsin activity in other hematological malignancies or solid tumors have been disappointing so far. Further studies analyzing its mechanisms of action and potential biomarkers will help select patients who may benefit most from this drug. In this review, we critically analyze the published studies on plitidepsin in hematological malignancies and solid tumors and discuss its current role and future perspectives in treating these malignancies. We also review its design, pharmaceutical data, and mechanism of action.

Monocrotophos Induces the Expression of Xenobiotic Metabolizing Cytochrome P450s (CYP2C8 and CYP3A4) and Neurotoxicity in Human Brain Cells

Expression of various cytochrome P450s (CYPs) in mammalian brain cells is well documented. However, such studies are hampered in neural/glial cells of human origin due to nonavailability of human brain cells. To address this issue, we investigated the expression and inducibility of CYP2C8 and CYP3A4 and their responsiveness against cyclophosphamide (CPA) and organophosphorus pesticide monocrotophos (MCP), a known developmental neurotoxicant in human neural (SH-SY5Y) and glial (U373-MG) cell lines. CPA induced significant expression of CYP2C8 and CYP3A4 in both types of cells in a time-dependent manner. Neural cell line exhibited relatively higher constitutive and inducible expression of CYPs than the glial cell line. MCP exposure alone could not induce the significant expression of CYPs, whereas the cells preexposed to CPA showed a significant response to MCP. Similar to the case of CPA induced expressions, neural cells were found to be more vulnerable than glial cells. Our data indicate differential expressions of CYPs in cultured human neural and glial cell lines. The findings were synchronized with protein ligand docking studies, which showed a significant modulatory capacity of MCP by strong interaction with CYP regulators-CAR and PXR. Similarly, the known CYP inducer CPA has also shown significant high docking scores with the two studied CYP regulators. We also observed a significant induction in reactive oxygen species (ROS), lipid peroxides (LPO), micronucleus (MN), chromosomal aberration (CA), and reduction in reduced glutathione (GSH) and catalase following the exposure of MCP. Moreover, the expressions of apoptotic markers such as caspase-3, caspase-9, Bax, and p53 were significantly upregulated, whereas the levels of antiapoptotic marker, Bcl2, was downregulated after the exposure of MCP in both cell lines. These findings confirm the involvement of ROS-mediated oxidative stress, which subsequently triggers apoptosis pathways in both human neural (SH-SY5Y) and glial (U373-MG) cell lines following the exposure of MCP.

Cyclic depsipeptides as potential cancer therapeutics

Cyclic depsipeptides are polypeptides in which one or more amino acid is replaced by a hydroxy acid, resulting in the formation of at least one ester bond in the core ring structure. Many natural cyclic depsipeptides possessing intriguing structural and biological properties, including antitumor, antifungal, antiviral, antibacterial, anthelmintic, and anti-inflammatory activities, have been identified from fungi, plants, and marine organisms. In particular, the potent effects of cyclic depsipeptides on tumor cells have led to a number of clinical trials evaluating their potential as chemotherapeutic agents. Although many of the trials have not achieved the desired results, romidepsin (FK228), a bicyclic depsipeptide that inhibits histone deacetylase, has been shown to have clinical efficacy in patients with refractory cutaneous T-cell lymphoma and has received Food and Drug Administration approval for use in treatment. In this review, we discuss antitumor cyclic depsipeptides that have undergone clinical trials and focus on their structural features, mechanisms, potential applications in chemotherapy, and pharmacokinetic and toxicity data. The results of this study indicate that cyclic depsipeptides could be a rich source of new cancer therapeutics.

IMP2/p62 induces genomic instability and an aggressive hepatocellular carcinoma phenotype

Hepatocellular carcinoma (HCC) represents the third leading cause of cancer-related deaths and commonly develops in inflammatory environments. The IGF2 mRNA-binding protein IMP2-2/IGF2BP2-2/p62 was originally identified as an autoantigen in HCC. Aim of this study was to investigate a potential pathophysiological role of p62 in hepatocarcinogenesis. Human HCC tissue showed overexpression of IMP2, which strongly correlated with the fetal markers AFP and DLK1/Pref-1/FA-1 and was particularly elevated in tumors with stem-like features and hypervascularization. Molecular classification of IMP2-overexpressing tumors revealed an aggressive phenotype. Livers of mice overexpressing the IMP2 splice variant p62 highly expressed the stem cell marker DLK1 and secreted DLK1 into the blood. p62 was oncogenic: diethylnitrosamine (DEN)-treated p62 transgenic mice exhibited a higher tumor incidence and multiplicity than wild types. Tumors of transgenics showed a more aggressive and stem-like phenotype and displayed more oncogenic chromosomal aberrations determined with aCGH analysis. DEN-treated p62 transgenic mice exhibited distinct signs of inflammation, such as inflammatory cytokine expression and oxidative stress markers, that is, thiobarbituric acid-reactive substance (TBARS) levels. Reactive oxygen species (ROS) production was elevated in HepG2 cells, which either overexpressed p62 or were treated with DLK1. p62 induced this ROS production by a DLK1-dependent induction and activation of the small Rho-GTPase RAC1, activating NADPH oxidase and being overexpressed in human HCC. Our data indicate that p62/IMP2 promotes hepatocarcinogenesis by an amplification of inflammation.

Evaluation of plitidepsin in patients with primary myelofibrosis and post polycythemia vera/essential thrombocythemia myelofibrosis: results of preclinical studies and a phase II clinical trial

Previous data established that plitidepsin, a cyclic depsipeptide, exerted activity in a mouse model of myelofibrosis (MF). New preclinical experiments reported herein found that low nanomolar plitidepsin concentrations potently inhibited the proliferation of JAK2V617F-mutated cell lines and reduced colony formation by CD34+ cells of individuals with MF, at least in part through modulation of p27 levels. Cells of MF patients had significantly reduced p27 content, that were modestly increased upon plitidepsin exposure. On these premise, an exploratory phase II trial evaluated plitidepsin 5 mg/m(2) 3-h intravenous infusion administered on days 1 and 15 every 4 weeks (q4wk). Response rate (RR) according to the International Working Group for Myelofibrosis Research and Treatment consensus criteria was 9.1% (95% CI, 0.2-41.3%) in 11 evaluable patients during the first trial stage. The single responder achieved a red cell transfusion independence and stable disease was reported in nine additional patients (81.8%). Eight patients underwent a short-lasting improvement of splenomegaly. In conclusion, plitidepsin 5 mg/m(2) 3-h infusion q4wk was well tolerated but had a modest activity in patients with primary, post-polycythaemia vera or post-essential thrombocythaemia MF. Therefore, this trial was prematurely terminated and we concluded that further clinical trials with plitidepsin as single agent in MF are not warranted.

Renal mechanisms of salt-sensitive hypertension: contribution of two steroid receptor-associated pathways

Although salt is a major environmental factor in the development of hypertension, the degree of salt sensitivity varies widely among individuals. The mechanisms responsible for this variation remain to be elucidated. Recent studies have revealed the involvement of two important signaling pathways in renal tubules that play key roles in electrolyte balance and the maintenance of normal blood pressure: the β2-adrenergic stimulant-glucocorticoid receptor (GR)-with-no-lysine kinase (WNK)4-Na+-Cl− cotransporter pathway, which is active in distal convoluted tubule (DCT)1, and the Ras-related C3 botulinum toxin substrate (Rac)1-mineralocorticoid receptor (MR) pathway, which is active in DCT2, connecting tubules, and collecting ducts. β2-Adrenergic stimulation due to increased renal sympathetic activity in obesity- and salt-induced hypertension suppresses histone deacetylase 8 activity via cAMP/PKA signaling, increasing the accessibility of GRs to the negative GR response element in the WNK4 promoter. This results in the suppression of WNK4 transcription followed by the activation of Na+-Cl− cotransporters in the DCT and elevated Na+ retention and blood pressure upon salt loading. Rac1 activates MRs, even in the absence of ligand binding, with this activity increased in the presence of ligand. In salt-sensitive animals, Rac1 activation due to salt loading activates MRs in DCT2, connecting tubules, and collecting ducts. Thus, GRs and MRs are independently involved in two pathways responsible for renal Na+ handling and salt-sensitive hypertension. These findings suggest novel therapeutic targets and may lead to the development of diagnostic tools to determine salt sensitivity in hypertensive patients.

Inhibition of GTPase Rac1 in endothelium by 6-mercaptopurine results in immunosuppression in nonimmune cells: new target for an old drug

Azathioprine and its metabolite 6-mercaptopurine (6-MP) are well established immunosuppressive drugs. Common understanding of their immunosuppressive properties is largely limited to immune cells. However, in this study, the mechanism underlying the protective role of 6-MP in endothelial cell activation is investigated. Because 6-MP and its derivative 6-thioguanosine-5'-triphosphate (6-T-GTP) were shown to block activation of GTPase Rac1 in T lymphocytes, we focused on Rac1-mediated processes in endothelial cells. Indeed, 6-MP and 6-T-GTP decreased Rac1 activation in endothelial cells. As a result, the compounds inhibited TNF-α-induced downstream signaling via JNK and reduced activation of transcription factors c-Jun, activating transcription factor-2 and, in addition, NF κ-light-chain-enhancer of activated B cells (NF-κB), which led to decreased transcription of proinflammatory cytokines. Moreover, 6-MP and 6-T-GTP selectively decreased TNF-α-induced VCAM-1 but not ICAM-1 protein levels. Rac1-mediated generation of cell membrane protrusions, which form docking structures to capture leukocytes, also was reduced by 6-MP/6-T-GTP. Consequently, leukocyte transmigration was inhibited after 6-MP/6-T-GTP treatment. These data underscore the anti-inflammatory effect of 6-MP and 6-T-GTP on endothelial cells by blocking Rac1 activation. Our data provide mechanistic insight that supports development of novel Rac1-specific therapeutic approaches against chronic inflammatory diseases.

Trabectedin and plitidepsin: drugs from the sea that strike the tumor microenvironment

The prevailing paradigm states that cancer cells acquire multiple genetic mutations in oncogenes or tumor suppressor genes whose respective activation/up-regulation or loss of function serve to impart aberrant properties, such as hyperproliferation or inhibition of cell death. However, a tumor is now considered as an organ-like structure, a complex system composed of multiple cell types (e.g., tumor cells, inflammatory cells, endothelial cells, fibroblasts, etc.) all embedded in an inflammatory stroma. All these components influence each other in a complex and dynamic cross-talk, leading to tumor cell survival and progression. As the microenvironment has such a crucial role in tumor pathophysiology, it represents an attractive target for cancer therapy. In this review, we describe the mechanism of action of trabectedin and plitidepsin as an example of how these specific drugs of marine origin elicit their antitumor activity not only by targeting tumor cells but also the tumor microenvironment.

Nano-encapsulation of plitidepsin: in vivo pharmacokinetics, biodistribution, and efficacy in a renal xenograft tumor model

PURPOSE

Plitidepsin is an antineoplasic currently in clinical evaluation in a phase III trial in multiple myeloma (ADMYRE). Presently, the hydrophobic drug plitidepsin is formulated using Cremophor®, an adjuvant associated with unwanted hypersensitivity reactions. In search of alternatives, we developed and tested two nanoparticle-based formulations of plitidepsin, aiming to modify/improve drug biodistribution and efficacy.

METHODS

Using nanoprecipitation, plitidepsin was loaded in polymer nanoparticles made of amphiphilic block copolymers (i.e. PEG-b-PBLG or PTMC-b-PGA). The pharmacokinetics, biodistribution and therapeutic efficacy was assessed using a xenograft renal cancer mouse model (MRI-H-121 xenograft) upon administration of the different plitidepsin formulations at maximum tolerated multiple doses (0.20 and 0.25 mg/kg for Cremophor® and copolymer formulations, respectively).

RESULTS

High plitidepsin loading efficiencies were obtained for both copolymer formulations. Considering pharmacokinetics, PEG-b-PBLG formulation showed lower plasma clearance, associated with higher AUC and Cmax than Cremophor® or PTMC-b-PGA formulations. Additionally, the PEG-b-PBLG formulation presented lower liver and kidney accumulation compared with the other two formulations, associated with an equivalent tumor distribution. Regarding the anticancer activity, all formulations elicited similar efficacy profiles, as compared to the Cremophor® formulation, successfully reducing tumor growth rate.

CONCLUSIONS

Although the nanoparticle formulations present equivalent anticancer activity, compared to the Cremophor® formulation, they show improved biodistribution profiles, presenting novel tools for future plitidepsin-based therapies.

Nuclear multidrug-resistance related protein 1 contributes to multidrug-resistance of mucoepidermoid carcinoma mainly via regulating multidrug-resistance protein 1: a human mucoepidermoid carcinoma cells model and Spearman's rank correlation analysis

Background

Multidrug resistance-related protein 1 (MRP1/ABCC1) and multidrug resistance protein 1 (MDR1/P-glycoprotein/ABCB1) are both membrane-bound drug transporters. In contrast to MDR1, MRP1 also transports glutathione (GSH) and drugs conjugated to GSH. Due to its extraordinary transport properties, MRP1/ABCC1 contributes to several physiological functions and pathophysiological incidents. We previously found that nuclear translocation of MRP1 contributes to multidrug-resistance (MDR) of mucoepidermoid carcinoma (MEC). The present study investigated how MRP1 contributes to MDR in the nuclei of MEC cells.

Methods

Western blot and RT-PCR was carried out to investigate the change of multidrug-resistance protein 1 (MDR1) in MC3/5FU cells after MRP1 was downregulated through RNA interference (RNAi). Immunohistochemistry (IHC) staining of 127 cases of MEC tissues was scored with the expression index (EI). The EI of MDR1 and MRP1 (or nuclear MRP1) was analyzed with Spearman's rank correlation analysis. Using multiple tumor tissue assays, the location of MRP1 in other tissues was checked by HIC. Luciferase reporter assays of MDR1 promoter was carried out to check the connection between MRP1 and MDR1 promoter.

Results

MRP1 downregulation led to a decreased MDR1 expression in MC3/5FU cells which was caused by decreased activity of MDR1 promoter. IHC study of 127 cases of MEC tissues demonstrated a strong positive correlation between nuclear MRP1 expression and MDR1 expression. Furthermore, IHC study of multiple tumor tissue array sections showed that although nuclear MRP1 widely existed in MEC tissues, it was not found in normal tissues or other tumor tissues.

Conclusions

Our findings indicate that nuclear MRP1 contributes to MDR mainly through regulating MDR1 expression in MEC. And the unique location of MRP1 made it an available target in identifying MEC from other tumors.

c-Jun N-terminal kinase phosphorylation is a biomarker of plitidepsin activity

Plitidepsin is an antitumor drug of marine origin currently in Phase III clinical trials in multiple myeloma. In cultured cells, plitidepsin induces cell cycle arrest or an acute apoptotic process in which sustained activation of c-Jun N-terminal kinase (JNK) plays a crucial role. With a view to optimizing clinical use of plitidepsin, we have therefore evaluated the possibility of using JNK activation as an in vivo biomarker of response. In this study, we show that administration of a single plitidepsin dose to mice xenografted with human cancer cells does indeed lead to increased phosphorylation of JNK in tumors at 4 to 12 h. By contrast, no changes were found in other in vitro plitidepsin targets such as the levels of phosphorylated-ERK, -p38MAPK or the protein p27^KIP1. Interestingly, plitidepsin also increased JNK phosphorylation in spleens from xenografted mice showing similar kinetics to those seen in tumors, thereby suggesting that normal tissues might be useful for predicting drug activity. Furthermore, plitidepsin administration to rats at plasma concentrations comparable to those achievable in patients also increased JNK phosphorylation in peripheral mononuclear blood cells. These findings suggest that changes in JNK activity provide a reliable biomarker for plitidepsin activity and this could be useful for designing clinical trials and maximizing the efficacy of plitidepsin.

Plitidepsin (Aplidin) is a potent inhibitor of diffuse large cell and Burkitt lymphoma and is synergistic with rituximab

Plitidepsin (Aplidin), an antitumor agent of marine origin, presently is undergoing phase II/III clinical trials, and has shown promise for the treatment of lymphoma. Here, we describe the antitumor effects of plitidepsin alone and in combination with rituximab and investigated the effects of each drug and the combination on the cell cycle and mechanism of cell death. Several Diffuse Large Cell Lymphoma (DLCL) lines and Burkitt cell lines were tested for sensitivity to plitidepsin and rituximab. All DLCL and Burkitt lymphoma cell lines were inhibited by plitidepsin in nanomolar concentrations, while rituximab sensitivity varied among different cell lines. Ramos and the RL cell lines proved sensitive to rituximab and were used to test the effects of each of the two drugs. The two agents exhibited synergism at all tested concentrations. For in vivo studies, irradiated athymic nude mice were engrafted with the Ramos lymphoma. Treatment was initiated when the tumors were ~0.5 cm in diameter, and toxic and therapeutic effects were monitored. In the in vivo study, additive effects of the combined two drugs, was demonstrated without an increase in host toxicity. The in vitro synergy and the in vivo additive antitumor effects without an increase in host toxicity with two relatively non-marrow suppressive agents encourages further development of this combination for treatment of aggressive B-cell lymphomas.

A journey under the sea: the quest for marine anti-cancer alkaloids

The alarming increase in the global cancer death toll has fueled the quest for new effective anti-tumor drugs thorough biological screening of both terrestrial and marine organisms. Several plant-derived alkaloids are leading drugs in the treatment of different types of cancer and many are now being tested in various phases of clinical trials. Recently, marine-derived alkaloids, isolated from aquatic fungi, cyanobacteria, sponges, algae, and tunicates, have been found to also exhibit various anti-cancer activities including anti-angiogenic, anti-proliferative, inhibition of topoisomerase activities and tubulin polymerization, and induction of apoptosis and cytotoxicity. Two tunicate-derived alkaloids, aplidin and trabectedin, offer promising drug profiles, and are currently in phase II clinical trials against several solid and hematologic tumors. This review sheds light on the rich array of anti-cancer alkaloids in the marine ecosystem and introduces the most investigated compounds and their mechanisms of action.

Plitidepsin has a safe cardiac profile: a comprehensive analysis

Plitidepsin is a cyclic depsipeptide of marine origin in clinical development in cancer patients. Previously, some depsipeptides have been linked to increased cardiac toxicity. Clinical databases were searched for cardiac adverse events (CAEs) that occurred in clinical trials with the single-agent plitidepsin. Demographic, clinical and pharmacological variables were explored by univariate and multivariate logistic regression analysis. Forty-six of 578 treated patients (8.0%) had at least one CAE (11 patients (1.9%) with plitidepsin-related CAEs), none with fatal outcome as a direct consequence. The more frequent CAEs were rhythm abnormalities (n = 31; 5.4%), mostly atrial fibrillation/flutter (n = 15; 2.6%). Of note, life-threatening ventricular arrhythmias did not occur. Myocardial injury events (n = 17; 3.0%) included possible ischemic-related and non-ischemic events. Other events (miscellaneous, n = 6; 1.0%) were not related to plitidepsin. Significant associations were found with prostate or pancreas cancer primary diagnosis (p = 0.0017), known baseline cardiac risk factors (p = 0.0072), myalgia present at baseline (p = 0.0140), hemoglobin levels lower than 10 g/dL (p = 0.0208) and grade ≥2 hypokalemia (p = 0.0095). Treatment-related variables (plitidepsin dose, number of cycles, schedule and/or total cumulative dose) were not associated. Electrocardiograms performed before and after plitidepsin administration (n = 136) detected no relevant effect on QTc interval. None of the pharmacokinetic parameters analyzed had a significant impact on the probability of developing a CAE. In conclusion, the most frequent CAE type was atrial fibrillation/atrial flutter, although its frequency was not different to that reported in the age-matched healthy population, while other CAEs types were rare. No dose-cumulative pattern was observed, and no treatment-related variables were associated with CAEs. Relevant risk factors identified were related to the patient's condition and/or to disease-related characteristics rather than to drug exposure. Therefore, the current analysis supports a safe cardiac risk profile for single-agent plitidepsin in cancer patients.

CXCR4-independent rescue of the myeloproliferative defect of the Gata1low myelofibrosis mouse model by Aplidin

The discovery of JAK2 mutations in Philadelphia-negative myeloproliferative neoplasms has prompted investigators to evaluate mutation-targeted treatments to restore hematopoietic cell functions in these diseases. However, the results of the first clinical trials with JAK2 inhibitors are not as promising as expected, prompting a search for additional drugable targets to treat these disorders. In this paper, we used the hypomorphic Gata1(low) mouse model of primary myelofibrosis (PMF), the most severe of these neoplasms, to test the hypothesis that defective marrow hemopoiesis and development of extramedullary hematopoiesis in myelofibrosis is due to insufficient p27(Kip1) activity and is treatable by Aplidin, a cyclic depsipeptide that activates p27(Kip1) in several cancer cells. Aplidin restored expression of Gata1 and p27(Kip1) in Gata1(low) hematopoietic cells, proliferation of marrow progenitor cells in vitro and maturation of megakaryocytes in vivo (reducing TGF-beta/VEGF levels released in the microenvironment by immature Gata1(low) megakaryocytes). Microvessel density, fibrosis, bone growth, and marrow cellularity were normal in Aplidin-treated mice and extramedullary hematopoiesis did not develop in liver although CXCR4 expression in Gata1(low) progenitor cells remained low. These results indicate that Aplidin effectively alters the natural history of myelofibrosis in Gata1(low) mice and suggest this drug as candidate for clinical evaluation in PMF.

Impaired dephosphorylation renders G6PD-knockdown HepG2 cells more susceptible to H(2)O(2)-induced apoptosis

Glucose-6-phosphate dehydrogenase (G6PD) plays a key role in the regeneration of NADPH and maintenance of cellular redox balance. In the present study, we investigate the effect of G6PD deficiency on H(2)O(2)-elicited signaling in HepG2 cells. H(2)O(2) was found to inhibit cellular protein tyrosine phosphatase (PTP) activity, resulting in activation of MAPKs. MKP-1 expression increased in the late phase of H(2)O(2) signaling. Using RNAi technology, we found that G6PD knockdown enhanced the inhibitory effect of H(2)O(2) on PTPs and led to sustained MAPK activation. This was accompanied by delayed expression and inhibition of MKP-1. Using a pharmacological inhibitor and siRNA, we demonstrate that MKP-1 acts as a regulator of MAPK activation in H(2)O(2) signaling. The prolonged MAPK activation in G6PD-knockdown cells was associated with an increased susceptibility to H(2)O(2)-induced apoptosis and growth retardation. Treatment with p38 and JNK inhibitors or N-acetylcysteine ameliorated such cellular effect, while triptolide and MKP-1-siRNA did the opposite. Glucose oxidase treatment had similar effects as addition of H(2)O(2). Taken together, these findings suggest that G6PD knockdown enhances the magnitude and duration of H(2)O(2)-induced MAPK signaling through inhibition of cellular PTPs, and the resultant anomalous signaling may lead to cell demise.

Phase I study of weekly plitidepsin as 1-hour infusion combined with carboplatin in patients with advanced solid tumors or lymphomas

This dose-escalating phase I clinical trial was designed to determine the recommended dose (RD) and to assess the safety and feasibility of weekly plitidepsin (1-hour i.v. infusion, Days 1, 8 and 15) combined with carboplatin (1-hour i.v. infusion, Day 1, after plitidepsin) in 4-week (q4wk) cycles given to patients with advanced solid tumors or lymphomas. Twenty patients were enrolled and evaluable for both safety and efficacy. The starting dose was plitidepsin 1.8 mg/m(2) and carboplatin area under the curve (AUC) = 5 min*mg/ml; dose escalation proceeded based on worst toxicity in the previous cohort. The maximum tolerated dose (MTD) was plitidepsin 3.0 mg/m(2) and carboplatin AUC = 5 min*mg/ml, with grade 3 transaminase increases as the most common dose-limiting toxicities (DLTs). The RD for phase II studies was plitidepsin 2.4 mg/m(2) and carboplatin AUC = 5 min*mg/ml, with fatigue, myalgia and nausea as the most common drug-related adverse events (AEs). No unexpected toxicity was seen. Twelve patients (60%), ten of whom were heavily pretreated (≥2 previous chemotherapy lines) showed stable disease (SD), with a median time to progression (TTP) of 4.4 months. In conclusion, plitidepsin 2.4 mg/m(2) and carboplatin AUC = 5 min*mg/ml is a safe dose for future phase II studies evaluating the use of this combination in cancer patients potentially sensitive to platinum-based therapy.

Phase II study of plitidepsin 3-hour infusion every 2 weeks in patients with unresectable advanced medullary thyroid carcinoma

OBJECTIVES

To evaluate the antitumor response, time-to-event efficacy endpoints and toxicity of plitidepsin (Aplidin) 5 mg/m as a 3-hour intravenous (i.v.) infusion every 2 weeks in patients with unresectable advanced medullary thyroid carcinoma (MTC).

METHODS

Sixteen patients with MTC and disease progression or large tumor burden received plitidepsin. Tumor response and time-related parameters were evaluated according to Response Evaluation Criteria in Solid Tumors. Secondary efficacy endpoints were marker response (calcitonin and carcinoembryonic antigen), clinical benefit and quality of life. Safety was assessed using the National Cancer Institute Common Toxicity Criteria.

RESULTS

A total of 141 cycles (median, 9 per patient; range, 1-24) were administered. No complete responses or partial responses (PR) were found, and 12 patients had stable disease for >8 weeks. Median follow-up was 15.0 months. Median time to progression was 5.3 months. Median overall survival could not be calculated, but 86.7% and 66.0% of patients were alive at 6 and 12 months. Marker response included 1 unconfirmed PR and 2 stabilizations for calcitonin, and 1 unconfirmed PR and 4 stabilizations for calcitonin and carcinoembryonic antigen. One patient showed clinical benefit. Quality of life scores generally decreased during the study. Most treatment-related adverse events were mild or moderate. Grade 3 lymphopenia was the only severe hematological toxicity found (2 patients). Severe nonhematological toxicities were grade 3 creatine phosphokinase increase (2 patients, with no myalgia or muscular weakness) and transient grade 3 alanine aminotransferase increase (5 patients).

CONCLUSIONS

Single-agent plitidepsin given as 3-hour i.v. infusions every 2 weeks was generally well tolerated but showed limited clinical activity in patients with unresectable advanced MTC.

Heparin and suramin alter plitidepsin uptake via inhibition of GPCR coupled signaling

Plitidepsin (Aplidin) is a novel antitumor agent, derived from the mediterranean tunicate Aplidium albicans, and is currently in phase ii clinical trials with evidence of activity in heavily pretreated multiple myeloma, renal cell carcinoma, melanoma and neuroblastoma patients. As compared to its parental compound didemnin B, plitidepsin has shown a better therapeutic index with less bone marrow toxicity, cardiotoxicity and neurotoxicity in patients and a more potent cytotoxic effect in several tumor cell lines. As sensitivity to the drug varies between cell lines and fresh leukemia samples, we performed studies on transport of plitidepsin in leukemia and lymphoma cell lines to determine the mechanism of uptake. The drug is taken up by an active transport process, i.e. the process is temperature and energy dependent, and has a high-affinity binding site with Kt =212 nM and Vmax = 15 pmoles/min. Importantly, once inside the cell, efflux of plitidepsin is minimum, suggesting that the drug is bound to intracellular macromolecules. Further work showed that plitidepsin binds to G-Protein Coupled Receptors (GPCRs), since GPCR and GRK (GPCR kinases) inhibitors suramin and heparin respectively, markedly reduce the drug uptake and its cytotoxic activity. Signaling via Jak/Stat pathway is inhibited by pharmacological concentrations of plitidepsin, further confirming the relationship between plitidepsin and GPCRs.

Aplidin as a potential adjunct to radiation therapy: in vitro studies

PURPOSE

Aplidin (plitidespin) is a novel cyclic depsipeptide, currently in Phase II clinical trials for solid and hematologic malignancies. We examined the effects of oxygen on the cytotoxicity of Aplidin and the interactions between Aplidin and radiation. These factors will be important if Aplidin is used clinically in combination with radiotherapy.

MATERIALS

Exponentially-growing EMT6 mouse mammary tumour cells in monolayer cultures were treated with Aplidin and 250 kV X-rays.

RESULTS

The cytotoxicity of Aplidin was not altered either by incubation in moderate hypoxia before and during a 24 h drug treatment or by incubation in severe hypoxia before and during a 2 h drug treatment. Treatment with Aplidin plus radiation produced cytotoxicities compatible with additive or supraadditive cytotoxicities. Cells treated with 1 microM Aplidin for 24 h then killed by 100 Gy of radiation were toxic to untreated cells co-cultured with them.

CONCLUSIONS

The cytotoxicity of Aplidin is independent of the oxygenation during treatment. Aplidin, or an active metabolite of Aplidin, is retained in the cells and later released as the radiation-sterilised cells die, producing a Bystander effect that kills neighbouring cells. This Bystander effect could affect the outcome of therapeutic regimens combining Aplidin and radiation.

Phase II study of biweekly plitidepsin as second-line therapy for advanced or metastatic transitional cell carcinoma of the urothelium

The objective of this exploratory, open-label, single-arm, phase II clinical trial was to evaluate plitidepsin (5 mg/m(2)) administered as a 3-hour continuous intravenous infusion every two weeks to patients with locally advanced/metastatic transitional cell carcinoma of the urothelium who relapsed/progressed after first-line chemotherapy. Treatment cycles were repeated for up to 12 cycles or until disease progression, unacceptable toxicity, patient refusal or treatment delay for >2 weeks. The primary efficacy endpoint was objective response rate according to RECIST. Secondary endpoints were the rate of SD lasting > or = 6 months and time-to-event variables. Toxicity was assessed using NCI-CTC v. 3.0. Twenty-one patients received 57 treatment cycles. No objective tumor responses occurred. SD lasting <6 months was observed in two of 18 evaluable patients. With a median follow-up of 4.6 months, the median PFR and the median OS were 1.4 months and 2.3 months, respectively. The most common AEs were mild to moderate nausea, fatigue, myalgia and anorexia. Anemia, lymphopenia, and increases in transaminases, alkaline phosphatase and creatinine were the most frequent laboratory abnormalities. No severe neutropenia occurred. Treatment was feasible and generally well tolerated in this patient population; however the lack of antitumor activity precludes further studies of plitidepsin in this setting.

Initial testing of aplidin by the pediatric pre-clinical testing program

Aplidin was tested in vitro at concentrations ranging from from 0.1 nM to 1.0 microM and in vivo at a dose of 0.6 mg/kg administered intraperitoneally on an every 4 days x 3-schedule that was repeated at day 21. In vitro, Aplidin was most active against acute lymphoblastic leukemia (ALL) cell lines. In vivo, Aplidin induced significant differences in EFS distribution in 12 of 28 (43%) solid tumor models and 2 of 6 evaluable ALL models. Aplidin showed potent in vitro activity and induced significant in vivo tumor growth inhibition in some xenografts, but did not induce tumor regressions.

Phase II study of biweekly plitidepsin as second-line therapy in patients with advanced malignant melanoma

The objective of this study was to evaluate the antitumor activity and safety profile of 5 mg/m2 plitidepsin administered as a 3-h continuous intravenous infusion every 2 weeks to patients with advanced malignant melanoma who relapsed or progressed after one line of systemic therapy. Objective response rate (primary efficacy endpoint) was evaluated according to Response Evaluation Criteria In Solid Tumors and toxicity was assessed using National Cancer Institute -Common Toxicity Criteria Version 2.0. Of 39 enrolled patients (median age: 53 years), 37 patients were treated who received a total of 167 treatment cycles (median: 3 cycles per patient; range: 1-32). All patients had received prior systemic therapy with a median of one line per patient (range: 1-6 lines). Of the 35 evaluable patients, two dacarbazine-resistant patients (5.7%) with metastatic cutaneous melanoma achieved partial responses. Five other patients (14.3%) reported stable disease (median stable disease duration: 3.5 months; range: 2.2-15.8 months). Therefore, the rate of tumor growth control was 20.0%. With a median follow-up of 11.0 months, the median progression-free survival was 1.3 months and the median overall survival was 3.5 months. Six patients (16.2%) had the following treatment-related grade 3/4 adverse events: myalgia (n = 3), injection-site reaction (n = 2), hypersensitivity, hypotension, and fatigue (n = 1 each). One patient was withdrawn from the trial because of grade 4 hypersensitivity reaction and hypotension. No severe neutropenia was reported. Plitidepsin showed a minor degree of antitumor activity in patients with refractory advanced malignant melanoma. Further evaluation of plitidepsin in combination schedules may be warranted.

Marine Natural Products as Inhibitors of Hypoxic Signaling in Tumors

Marine natural products have become a major source of new chemical entities in the discovery of potential anticancer agents that potently suppress various antitumor molecular targets. As a consequence of insufficient vascularization, hypoxic regions form within rapidly growing solid tumor masses. Specific alterations of gene expression in these hypoxic tumor cells help facilitate the survival and metastatic spread of solid tumors. The transcriptional response to cellular hypoxia is primarily mediated by the transcription factor hypoxia-inducible factor-1 (HIF-1) that regulates the expression of more than 100 genes involved in cellular adaptation and survival under hypoxic stress. Clinical studies in cancer patients indicate that HIF-1 activation is directly correlated with advanced disease stages and treatment resistance. HIF-1 has emerged as an important tumor-selective molecular target for anticancer drug discovery. As a result, natural product-based inhibitors of HIF-1 activation have been identified from plants and microorganisms. Recently, structurally unique natural products from marine sponges, crinoids, and algae have been identified as HIF-1 activation inhibitors. The US National Cancer Institute's Open Repository of marine invertebrate and algae extracts has proven to be a valuable source of natural product HIF-1 inhibitors. Among the active compounds identified, certain marine natural products have also been shown to suppress the hypoxic induction of HIF-1 target genes such as vascular endothelial growth factor (VEGF). Some of these marine HIF-1 inhibitors act by interfering with the generation of mitochondrial signaling molecules in hypoxic cells. However, the precise mechanisms of action for many newly identified marine natural product HIF-1 inhibitors remain unresolved.

Population pharmacokinetics meta-analysis of plitidepsin (Aplidin) in cancer subjects

OBJECTIVE

To characterize the population pharmacokinetics of plitidepsin (Aplidin) in cancer patients.

METHODS

A total of 283 patients (552 cycles) receiving intravenous plitidepsin as monotherapy at doses ranging from 0.13 to 8.0 mg/m(2) and given as 1- or 24-h infusions every week; 3- or 24-h infusion biweekly; or 1-h infusion daily for 5 consecutive days every 21 days were included in the analysis. An open three-compartment pharmacokinetic model and a nonlinear binding to red blood cells model were used to describe the plitidepsin pharmacokinetics in plasma and blood, respectively, using NONMEM V software. The effect of selected covariates on plitidepsin pharmacokinetics was investigated. Model evaluation was performed using goodness-of-fit plots, posterior predictive check and bootstrap.

RESULTS

Plasma clearance and its between subject variability (%) was 13.6 l/h (71). Volume of distribution at steady-state was calculated to be 4791 l (59). The parameters B (max) and C (50) of the non-linear blood distribution were 471 microg/l (56) and 41.6 microg/l, respectively. Within the range of covariates studied, age, sex, body size variables, aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), total bilirubin, creatinine clearance, albumin, total protein, performance status, co-administration of inhibitors or inducers of CYP3A4 and presence of liver metastases were not statistically related to plitidepsin pharmacokinetic parameters. Bootstrap and posterior predictive check evidenced the model was deemed appropriate to describe the time course of plitidepsin blood and plasma concentrations in cancer patients.

CONCLUSIONS

The integration of phase I/II pharmacokinetic data demonstrated plitidepsin linear elimination from plasma, dose-proportionality up to 8.0 mg/m(2), and time-independent pharmacokinetics. The distribution to red blood cells can be considered linear at doses lower than 5 mg/m(2) administered as 3-h or longer infusion. No clinically relevant covariates were identified as predictors of plitidepsin pharmacokinetics.

Phase I clinical and pharmacokinetic study of plitidepsin as a 1-hour weekly intravenous infusion in patients with advanced solid tumors

PURPOSE

Plitidepsin, given as a 1-hour weekly i.v. infusion for 3 consecutive weeks during a 4-week treatment cycle, was investigated in patients with solid tumors to determine the maximum tolerated dose and the recommended dose (RD) using this administration schedule.

EXPERIMENTAL DESIGN

Consecutive cohorts of patients with metastatic solid tumors or non-Hodgkin's lymphomas were to be treated at escalating doses of plitidepsin in a conventional phase I study including pharmacokinetic analyses of plitidepsin in plasma, whole blood, and blood cell pellets.

RESULTS

Forty-nine patients with solid tumors were enrolled, and 48 were treated with plitidepsin (doses from 0.133 to 3.6 mg/m2/week). Dose-limiting toxicities (defining 3.6 mg/m2/week as the maximum tolerated dose) included myalgia, increased creatine phosphokinase levels, and sustained grade 3/4 increases of hepatic enzyme levels. The RD was established at 3.2 mg/m2/week. The most common toxicities were fatigue, vomiting/nausea, anorexia, injection site reaction, and pain, mostly of mild or moderate severity. Muscular toxicity manifested by mild-moderate myalgia, weakness, and/or creatine phosphokinase elevations occurred in approximately 25% of patients and seemed to be dose related. Transient transaminase elevations were frequent but achieved grade 3 or 4 in only approximately 10% of patients. Plitidepsin lacked significant hematologic toxicity. No complete or partial tumor responses were observed; however, five patients had disease stabilization (including one patient with medullary thyroid carcinoma with an unconfirmed partial response and one patient with renal carcinoma with major tumor shrinkage in lung metastases). Pharmacokinetic results for the RD indicated a long plasma half-life give value (16.8 +/- 7.7 hour) and a high volume of distribution value (525.2 +/- 219.3 L).

CONCLUSIONS

The recommended dose for plitidepsin given as a weekly 1-hour schedule was 3.2 mg/m2/week. Muscular and liver toxicity were dose limiting at 3.6 mg/m2/week. Additional evaluation of this dose dense schedule is warranted.

Marine pharmacology in 2005-2006: antitumour and cytotoxic compounds

During 2005 and 2006, marine pharmacology research directed towards the discovery and development of novel antitumour agents was reported in 171 peer-reviewed articles. The purpose of this article is to present a structured review of the antitumour and cytotoxic properties of 136 marine natural products, many of which are novel compounds that belong to diverse structural classes, including polyketides, terpenes, steroids and peptides. The organisms yielding these bioactive marine compounds included invertebrate animals, algae, fungi and bacteria. Antitumour pharmacological studies were conducted with 42 structurally defined marine natural products in a number of experimental and clinical models which further defined their mechanisms of action. Particularly potent in vitro cytotoxicity data generated with murine and human tumour cell lines were reported for 94 novel marine chemicals with as yet undetermined mechanisms of action. Noteworthy is the fact that marine anticancer research was sustained by a global collaborative effort, involving researchers from Australia, Belgium, Benin, Brazil, Canada, China, Egypt, France, Germany, India, Indonesia, Italy, Japan, Mexico, the Netherlands, New Zealand, Panama, the Philippines, Slovenia, South Korea, Spain, Sweden, Taiwan, Thailand, United Kingdom (UK) and the United States of America (USA). Finally, this 2005-2006 overview of the marine pharmacology literature highlights the fact that the discovery of novel marine antitumour agents continued at the same active pace as during 1998-2004.

Phase II study of weekly plitidepsin as second-line therapy for small cell lung cancer

OBJECTIVE

To evaluate the antitumor activity and safety profile of plitidepsin administered as a 1h weekly intravenous (i.v.) infusion of 3.2mg/m(2) to patients with small cell lung cancer (SCLC) who relapsed or progressed after one line of chemotherapy.

PATIENTS AND METHODS

This was a multicenter, open-label, single-arm, exploratory, phase II clinical trial. Treatment lasted until disease progression, unacceptable toxicity, patient refusal or treatment delay for >2 weeks. Objective response rate (primary efficacy endpoint) was evaluated according to response evaluation criteria in solid tumors (RECIST). The rate of stable disease (SD) lasting for at least 6 months and time-to-event variables were secondary endpoints of efficacy. Toxicity was assessed using National Cancer Institute Common Toxicity Criteria (NCI-CTC) version 2.0.

RESULTS

Twenty pretreated SCLC patients (median age, 60 years) with extensive (n = 13) or limited-stage disease (n = 7) received a total of 24 treatment cycles (median, one cycle per patient; range, 1-2). Objective tumor responses were not observed and only one of the 17 evaluable patients had SD. With a median follow-up of 11.8 months, the progression-free survival and the median overall survival were 1.3 months and 4.8 months, respectively. The most troubling or common toxicities were fatigue, muscle weakness, lymphopenia, anemia (no patients showed neutropenia), and asymptomatic, non-cumulative increase of transaminases levels and alkaline phosphatase.

CONCLUSION

This clinical trial shows that a cycle of 1h weekly i.v. infusion of plitidepsin (3.2mg/m(2)) was generally well tolerated other than fatigue and muscle weakness in patients with pretreated SCLC. One patient died due to multi-organ failure. The absence of antitumor activity found here precludes further studies of this plitidepsin schedule as second-line single-agent treatment of SCLC.

Antitumour and antiangiogenic effects of Aplidin in the 5TMM syngeneic models of multiple myeloma

Aplidin® is an antitumour drug, currently undergoing phase II evaluation in different haematological and solid tumours. In this study, we analysed the antimyeloma effects of Aplidin in the syngeneic 5T33MM model, which is representable for the human disease. In vitro, Aplidin inhibited 5T33MMvv DNA synthesis with an IC₅₀ of 3.87 nM. On cell-cycle progression, the drug induced an arrest in transition from G0/G1 to S phase, while Western blot showed a decreased cyclin D1 and CDK4 expression. Furthermore, Aplidin induced apoptosis by lowering the mitochondrial membrane potential, by inducing cytochrome c release and by activating caspase-9 and caspase-3. For the in vivo experiment, 5T33MM-injected C57Bl/KaLwRij mice were intraperitoneally treated with vehicle or Aplidin (90 μg kg⁻¹ daily). Chronic treatment with Aplidin was well tolerated and reduced serum paraprotein concentration by 42% (P<0.001), while BM invasion with myeloma cells was decreased by 35% (P<0.001). Aplidin also reduced the myeloma-associated angiogenesis to basal values. This antiangiogenic effect was confirmed in vitro and explained by inhibition of endothelial cell proliferation and vessel formation. These data indicate that Aplidin is well tolerated in vivo and its antitumour and antiangiogenic effects support the use of the drug in multiple myeloma.