GTP-dependent binding of the antiproliferative agent didemnin to elongation factor 1 alpha

13-Deoxytedanolide, a marine sponge-derived antitumor macrolide, binds to the 60S large ribosomal subunit

13-Deoxytedanolide is a potent antitumor macrolide isolated from the marine sponge Mycale adhaerens. In spite of its remarkable activity, the mode of action of 13-deoxytedanolide has not been elucidated. [11-3H]-(11S)-13-Deoxydihydrotedanolide derived from the macrolide was used for identifying the target molecule from the yeast cell lysate. Fractionation of the binding protein revealed that the labeled 13-deoxytedanolide derivative strongly bound to the 80S ribosome as well as to the 60S large subunit, but not to the 40S small subunit. In agreement with this observation, 13-deoxytedanolide efficiently inhibited the polypeptide elongation. Interestingly, competition studies demonstrated that 13-deoxytedanolide shared the binding site on the 60S large subunit with pederin and its marine-derived analogues. These results indicate that 13-deoxytedanolide is a potent protein synthesis inhibitor and is the first macrolide to inhibit the eukaryotic ribosome.

Marine natural products as anticancer drugs

The chemical and biological diversity of the marine environment is immeasurable and therefore is an extraordinary resource for the discovery of new anticancer drugs. Recent technological and methodologic advances in structure elucidation, organic synthesis, and biological assay have resulted in the isolation and clinical evaluation of various novel anticancer agents. These compounds range in structural class from simple linear peptides, such as dolastatin 10, to complex macrocyclic polyethers, such as halichondrin B; equally as diverse are the molecular modes of action by which these molecules impart their biological activity. This review highlights several marine natural products and their synthetic derivatives that are currently undergoing clinical evaluation as anticancer drugs.

Establishment and characterisation of a human carcinoma cell line with acquired resistance to Aplidin

Aplidin (APL) is a new antitumoral drug from marine origin currently in phase II clinical trials against a wide multiplicity of cancers. As resistance may be, as with other drugs, an important obstacle to the APL therapeutic efficacy, we have established an acquired resistance cellular model by continuous exposure of HeLa cells to the drug. The stably resistant subline generated (HeLa-APL), possessing more than 1000-fold relative resistance to APL than parental cells, did not show crossresistance to a subset of clinically relevant antitumoral agents. In addition, resistance was not related to overexpression of P-glycoprotein or differences in overall drug accumulation. Comparing to parental cells, HeLa-APL cells did not present either significant differences in the growth rate or apparent alterations in the cell cycle distribution. Aplidin induced rapid and persistent phosphorylation of both JNK and p38 MAPKs, resulting in activation of the mitochondrial apoptotic pathway in parental cells, but, notably, in HeLa-APL-resistant cells MAPKs activation only occurred in a slight and transiently manner, failing to activate the above-mentioned apoptotic machinery. These results suggest that sustained activation of JNK and p38 is essential for triggering the apoptotic programme induced by APL and that HeLa-APL cells bypass this apoptotic response by preventing the specific mechanisms that prime and sustain the long-term activation of these signalling cascades. Although far from human tumour physiology in vivo, HeLa-APL cells represent a potentially useful tool in gaining insights into the mode of action of APL, in selecting non-crossresistant APL structural analogues, as well as in investigating and developing methods to prevent resistance to this drug.

Structural basis for the binding of didemnins to human elongation factor eEF1A and rationale for the potent antitumor activity of these marine natural products

Didemnins and tamandarins are closely related marine natural products with potent inhibitory effects on protein synthesis and cell viability. On the basis of available biochemical and structural evidence and results from molecular dynamics simulations, a model is proposed that accounts for the strong and selective binding of these compounds to human elongation factor eEF1A in the presence of GTP. We suggest that the p-methoxyphenyl ring of these cyclic depsipeptides is inserted into the same pocket in eEF1A that normally lodges either the 3' terminal adenine of aminoacylated tRNA, as inferred from two prokaryotic EF-Tu.GTP.tRNA complexes, or the aromatic side chain of Phe/Tyr-163 from the nucleotide exchange factor eEF1Balpha, as observed in several X-ray crystal structures of a yeast eEF1A:eEF1Balpha complex. This pocket, which has a strong hydrophobic character, is formed by two protruding loops on the surface of eEF1A domain 2. Further stabilization of the bound depsipeptide is brought about by additional crucial interactions involving eEF1A domain 1 in such a way that the molecule fits snugly at the interface between these two domains. In the GDP-bound form of eEF1A, this binding site exists only as two separate halves, which accounts for the much greater affinity of didemnins for the GTP-bound form of this elongation factor. This binding mode is entirely different from those seen in the complexes of the homologous prokaryotic EF-Tu with kirromycin-type antibiotics or the cyclic thiazolyl peptide antibiotic GE2270A. Interestingly, the set of interactions used by didemnins to bind to eEF1A is also distinct from that used by eEF1Balpha or eEF1Bbeta, thus establishing a competition for binding to a common site that goes beyond simple molecular mimicry. The model presented here is consistent with both available biochemical evidence and known structure-activity relationships for these two classes of natural compounds and synthetic analogues and provides fertile ground for future research.

Marine natural products and related compounds in clinical and advanced preclinical trials

The marine environment has proven to be a very rich source of extremely potent compounds that have demonstrated significant activities in antitumor, antiinflammatory, analgesia, immunomodulation, allergy, and anti-viral assays. Although the case can and has been made that the nucleosides such as Ara-A and Ara-C are derived from knowledge gained from investigations of bioactive marine nucleosides, no drug directly from marine sources (whether isolated or by total synthesis) has yet made it to the commercial sector in any disease. However, as shown in this review, there are now significant numbers of very interesting molecules that have come from marine sources, or have been synthesized as a result of knowledge gained from a prototypical compound, that are either in or approaching Phase II/III clinical trials in cancer, analgesia, allergy, and cognitive diseases. A substantial number of other potential agents are following in their wake in preclinical trials in these and in other diseases.

Antiangiogenic activity of aplidine, a new agent of marine origin

The antineoplastic compound aplidine, a new marine-derived depsipeptide, has shown preclinical activity in vitro on haematological and solid tumour cell lines. It is currently in early phase clinical trials. The exact mechanism of action of this anticancer agent still needs to be clarified. We have previously reported that aplidine blocks the secretion of the angiogenic factor vascular endothelial growth factor (VEGF) by the human leukaemia cells MOLT-4, suggesting a possible effect on tumour angiogenesis. This study was designed to investigate the antiangiogenic effect of aplidine. In vivo, in the chick embryo allantoic membrane (CAM) assay, aplidine inhibited spontaneous angiogenesis, angiogenesis elicited by exogenous VEGF and FGF-2, and induced by VEGF overexpressing 1A9 ovarian carcinoma cells. In vitro, at concentrations achievable in the plasma of patients, aplidine inhibited endothelial cell functions related to angiogenesis. It affected VEGF- and FGF-2-induced endothelial cell proliferation, inhibited cell migration and invasiveness assessed in the Boyden chamber and blocked the production of matrix metalloproteinases (MMP-2 and MMP-9) by endothelial cells. Finally, aplidine prevented the formation of capillary-like structures by endothelial cells on Matrigel. These findings indicate that aplidine has antiangiogenic activity in vivo and inhibits endothelial cell functional responses to angiogenic stimuli in vitro. This effect might contribute to the antineoplastic activity of aplidine.

Effect of Aplidin in acute lymphoblastic leukaemia cells

The cytotoxic effect of Aplidin was investigated on fresh leukaemia cells derived from children with B-cell-precursor (BCP) acute lymphoblastic leukaemia (ALL) by using stromal-layer culture system and on four cell lines, ALL-PO, Reh, ALL/MIK and TOM-1, derived from patients with ALL with different molecular genetic abnormalities. In ALL cell lines Aplidin was cytotoxic at nanomolar concentrations. In the ALL cell lines the drug-induced cell death was clearly related to the induction of apoptosis and appeared to be p53-independent. Only in ALL-PO 20 nM Aplidin treatment caused a block of vascular endothelial growth factor (VEGF) secretion and downregulation of VEGF-mRNA, but Aplidin cytotoxicity does not seem to be related to VEGF inhibition since the sensitivity of ALL-PO cells to Aplidin is comparable to that observed for the other cells used. Aplidin induced a G(1) and a G(2) M block in ALL cell lines. In patient-derived leukaemia cells, Aplidin induced a strong cytotoxicity evidenced in a stroma-supported immunocytometric assay. Cells from children with genetic abnormalities such as t(9;22) and t(4;11) translocations, associated with an inferior treatment outcome, were sensitive to Aplidin to the same extent as that observed in other BCP-ALL cases. Aplidin exerted a strong cell killing effect (>88%) against primary culture cells from five relapsed ALL cases, at concentrations much lower than those reported to be achieved in plasma of patients receiving Aplidin at recommended doses. Taken together these data suggest that Aplidin could be a new anticancer drug to be investigated in ALL patients resistant to available therapy.

In vitro cytotoxicity of aplidin and crossresistance with other cytotoxic drugs in childhood leukemic and normal bone marrow and blood samples: a rational basis for clinical development

To determine the potential of aplidin as a cytotoxic agent in pediatric leukemia, we tested bone marrow (BM) and peripheral blood (PB) samples (n=72) of children with different types of leukemia and healthy children in the methyl-thiazol-tetrazolium assay. Also, we compared these results with other cytotoxic drugs. Aplidin was cytotoxic in vitro at nanomolar concentrations, in a dose-dependent fashion. L-carnitine, that is applied in clinical studies to prevent myotoxicity caused by aplidin, had no effect on aplidin cytotoxicity in vitro. Aplidin cytotoxicity in vitro was not different when initial and relapsed acute lymphoblastic leukemia (ALL) or initial ALL and initial acute myeloid leukemia were compared. However, normal BM (n=19) and PB (n=13) cells were more resistant to aplidin than leukemic cells (median two- to seven-fold, P=0.001 and median four- to 11-fold, P&<0.0001, respectively). In leukemia samples, no significant crossresistance between aplidin and other cytotoxic drugs was found, except for a trend for correlation with 2',2'-difluorodeoxycytidine (rho=0.71, P=0.02). In normal BM samples, significant crossresistance with the epipodophyllotoxins was found, which is not readily explained by the currently known mechanisms of action of aplidin. In conclusion, we show that aplidin has selective cytotoxicity in vitro towards childhood leukemia cells and generally lacks crossresistance with other known cytotoxic drugs, which warrants clinical studies.

Chemical defense in ascidians of the didemnidae family

Fluorescent analogues (DB1 and TA1) of the secondary metabolites didemnin B (DB) and tamandarin A (TA) were synthesized to investigate the potential chemical defense mechanisms of tunicates in the family Didemnidae. These compounds were found to alter predator-prey relations. Five species of freshwater fish and one marine fish, the damselfish Amphiprion ocellaris, were acclimated to a diet of mosquito larvae. Fish showed an immediate, negative reaction to mosquito larvae treated with >/=5 ng of DB1 or TA1, with consumption of larvae resulting in regurgitation. Both freshwater and marine fish learned to avoid tainted prey by associating species of larvae with "distaste". Distaste for a given organism also arose when depsipeptides DB1 or TA1 were transferred to the fish from the surrounding medium. Fluorescence microscopy in fish indicated that a similar processing and localization followed ingestion and absorption of DB1 or TA1. Fluorescent labeling of DB or TA provided an ideal tool to conduct short-term studies of predator-prey relationships between fish and marine invertebrate larvae.

Monoclonal antibodies to elongation factor-1alpha inhibit in vitro translation in lysates of Sf21 cells

Elongation factor-1alpha (EF-1alpha) is an enzyme that is essential for protein synthesis. Although EF-1alpha offers an excellent target for the disruption of insect metabolism, agents known to interfere with EF-1alpha activity are toxic to humans. In this article, we describe the development of monoclonal antibodies (MAbs) that can disrupt the activity of insect EF-1alpha without cross-reacting with the human enzyme. MAbs were generated to EF-1alpha from Sf21 cells derived from the fall armyworm, Spodoptera frugiperda, by immunizing mice with EF-1alpha eluted from SDS-PAGE gels. The MAbs reacted with EF-1alpha in eggs and first through fifth instars of the fall armyworm in immunoblots of SDS-PAGE gels, but did not recognize EF-1alpha in human carcinoma cells and normal tissues. MAbs with the ability to recognize EF-1alpha in its native conformation, identified through immunoprecipitation experiments, were added to Sf21 cell lysates to determine whether the antibodies could inhibit incorporation of [(35)S]methionine into newly synthesized in vitro translation products. Of the four EF-1alpha-specific MAbs tested, three significantly inhibited protein synthesis when compared to the negative control antibody (P < 0.001, one-way ANOVA; followed by Dunnett's test, P < 0.05).

Aplidin induces the mitochondrial apoptotic pathway via oxidative stress-mediated JNK and p38 activation and protein kinase C delta

Aplidin, a new antitumoural drug presently in phase II clinical trials, has shown both in vitro and in vivo activity against human cancer cells. Aplidin effectively inhibits cell viability by triggering a canonical apoptotic program resulting in alterations in cell morphology, caspase activation, and chromatin fragmentation. Pro-apoptotic concentrations of Aplidin induce early oxidative stress, which results in a rapid and persistent activation of both JNK and p38 MAPK and a biphasic activation of ERK. Inhibition of JNK and p38 MAPK blocks the apoptotic program induced by Aplidin demonstrating its central role in the integration of the cellular stress induced by the drug. JNK and p38 MAPK activation results in downstream cytochrome c release and activation of caspases -9 and -3 and PARP cleavage, demonstrating the mediation of the mitochondrial apoptotic pathway in this process. We also demonstrate that protein kinase C delta (PKC-delta) mediates the cytotoxic effect of Aplidin and that it is concomitantly processed and activated late in the apoptotic process by a caspase mediated mechanism. Remarkably, cells deficient in PKC-delta show enhanced survival upon drug treatment as compared to its wild type counterpart. PKC-delta thus appears as an important component necessary for full caspase cascade activation and execution of apoptosis, which most probably initiates a positive feedback loop further amplifying the apoptotic process.

Cell cycle phase perturbations and apoptosis in tumour cells induced by aplidine

Aplidine, dehydrodidemnin B, is a marine depsipeptide isolated from the Mediterranean tunicate Aplidium albicans currently in phase II clinical trial. In human Molt-4 leukaemia cells Aplidine was found to be cytotoxic at nanomolar concentrations and to induce both a G(1) arrest and a G(2) blockade. The drug-induced cell cycle perturbations and subsequent cell death do not appear to be related to macromolecular synthesis (protein, RNA, DNA) since the effects occur at concentrations (e.g. 10 nM) in which macromolecule synthesis was not markedly affected. Ten nM Aplidine for 1 h inhibited ornithine decarboxylase activity, with a subsequently strong decrease in putrescine levels. This finding has questionable relevance since addition of putrescine did not significantly reduce the cell cycle perturbations or the cytotoxicity of Aplidine. The cell cycle perturbations caused by Aplidine were also not due to an effect on the cyclin-dependent kinases. Although the mechanism of action of Aplidine is still unclear, the cell cycle phase perturbations and the rapid induction of apoptosis in Molt-4 cells appear to be due to a mechanism different from that of known anticancer drugs.

Natural products as probes of cell biology: 20 years of didemnin research

The discovery of the didemnin family of marine depsipeptides launched an exciting and intriguing chapter in natural product chemistry. The unusual structure of the didemnin congeners has led to several total syntheses by research groups from around the world. The impressive in vitro and in vivo biological activities of the didemnins resulted in the first human clinical trials in the U.S. of a marine natural product against cancer, and additional clinical trials of a second-generation didemnin, dehydrodidemnin B (aplidine), are underway. As we mark the 20-year anniversary of the discovery of the didemnins, this class of natural products continues to stimulate active research in fields ranging from synthetic and medicinal chemistry to clinical oncology and cell biology. While some progress was made in dissecting the molecular mechanism of action and in establishing structure-activity relationships, there are still more questions than answers. This review covers the recent didemnin literature, highlighting the work directed towards understanding how this group of natural products interact with fundamental processes such as cell proliferation, protein biosynthesis, and apoptosis. The didemnin field illustrates how natural product chemistry may be used as a critical tool for the study of cell biology.

In vitro hematotoxicity of Aplidine on human bone marrow and cord blood progenitor cells

Aplidine is a cyclic depsipeptide that was isolated from a Mediterranean marine tunicate, Aplidium albicans. In experimental animals, Aplidine mediated an in vivo inhibitory effect in a number of tumor cell types. In humans, Aplidine is currently used in phase I clinical trials. Aiming to predict the hematotoxicity of Aplidine in humans, samples from human bone marrow (BM) and cord blood (CB) were exposed in vitro to increasing concentrations of the drug and then assayed for the clonogenic ability of myeloid (CFU-GM), erythroid (BFU-E), megakaryocitic (CFU-Meg) and pluripotent (CFU-Mix) hematopoietic progenitors. We investigated whether predictions of the hematotoxicity of Aplidine based on bone marrow (BM) cultures were reproduced when a more readily available source of human hematopoietic cells, cord blood cells, was used in experiments involving 24-h exposures. Although hematopoietic progenitors derived from bone marrow were generally more sensitive than those derived from cord blood, differences on the IC50, IC70 and IC90 varied within a relatively small range of 1.6-6.2-fold. Moreover, data obtained from cord blood cultures confirmed the observation made in bone marrow assays indicating that the myeloid (CFU-GM) and the erythroid (BFU-E) progenitors were the least sensitive to Aplidine. Regardless of the origin of the hematopoietic progenitors (bone marrow or cord blood) the toxicity of Aplidine in human hematopoietic progenitors (IC50: 150-2250 nM) was lower than that observed in previous studies with tumoral cell lines.

Synthesis and biological evaluation of didemnin photoaffinity analogues

The synthesis of four benzophenone-containing analogues of the antiproliferative natural product didemnin B is presented. In vitro protein biosynthesis inhibition potency and antitumor activity were evaluated. The results indicate that all four analogues are biologically active and could serve as photoaffinity reagents for the study of receptor-binding interactions of didemnins. These analogues could also be useful in studying antitumor effects of didemnins.

Mitochondrial cytochrome c release is caspase-dependent and does not involve mitochondrial permeability transition in didemnin B-induced apoptosis

Permeability transition, and a subsequent drop in mitochondrial membrane potential (DeltaPsi(m)), have been suggested to be mechanisms by which cytochrome c is released from the mitochondria into the cytosol during apoptosis. Furthermore, a drop in DeltaPsi(m) has been suggested to be an obligate early step in the apoptotic pathway. Didemnin B, a branched cyclic peptolide described to have immunosuppressive, anti-tumour, and anti-viral properties, induces rapid apoptosis in a range of mammalian cell lines. Induction of apoptosis by didemnin B in cultured human pro-myeloid HL-60 cells is the fastest and most complete ever described with all cells being apoptotic after 3 h of treatment. By utilizing the system of didemnin B-induced apoptosis in HL-60 cells, and the potent inhibitors of mitochondrial permeability transition, cyclosporin A and bongkrekic acid, we show that permeability transition as determined by changes in DeltaPsi(m) and mitochondrial Ca2+ fluxing, is not a requirement for apoptosis or cytochrome c release. In this system, changes in mitochondrial membrane potential and cytochrome c release are shown to be dependent on caspase activation, and to occur concurrently with the release of caspase-9 from mitochondria, genomic DNA fragmentation and apoptotic body formation.

Total syntheses and biological investigations of tamandarins A and B and tamandarin A analogs

Tamandarins A (1) and B (2), two natural products similar in structure to didemnin B (3), were recently isolated from a Brazilian marine ascidian of the family Didemnidae. The cytotoxicity of 1 was reported to be somewhat more potent in vitro than that of 3 against various human cancer cell lines. The present account describes the first total syntheses of 1 and 2, and the syntheses of tamandarin A side chain analogues. The cytotoxicity data for these compounds show that the side chain modifications exhibit a parallel effect for both didemnins and tamandarins. This observation supports tamandarins' role as didemnins' mimic.

Total synthesis of a conformationally constrained didemnin B analog

The total synthesis of a didemnin B analogue containing a conformationally constrained replacement for the isostatine moiety is reported. Synthetic highlights include an improved preparation of 2-hydroxy-3-cyclohexenecarboxylic acid and a new strategy for accessing the macrocycle.

Using model-system genetics for drug-based target discovery

The combination of medicinal chemistry and model-organism genetics is emerging as a powerful tool for the discovery and validation of drug targets. Model systems can be used to identify the cognate target for compounds that demonstrate in vivo efficacy but have unknown mechanisms of action. Alternatively, drugs with known cognate targets can be used to probe biochemical pathways in model organisms, revealing new targets and mechanisms within these pathways. In both cases, the availability of human genomic sequence data is opening up new opportunities for accelerating target discovery.

[Lys3]Didemnins as potential affinity ligands

The synthesis and biological activity of N(epsilon)-Z-[Lys3]didemnin B are reported. This novel analogue retains antiproliferative, cytotoxic, and protein biosynthesis inhibition activities, but at reduced levels. This result suggests the use of [Lys3]didemnin derivatives as potential affinity probes for studying the molecular target(s) of the didemnin class of natural products.

Radicicol binds and inhibits mammalian ATP citrate lyase

Six different biotinylated radicicol derivatives were synthesized as affinity probes for identification of cellular radicicol-binding proteins. Derivatives biotinylated at the C-17 (BR-1) and C-11 (BR-6) positions retained the activity of morphological reversion in v-src-transformed 3Y1 fibroblasts. Two radicicol-binding proteins, 120 and 90-kDa in size, were detected in HeLa cell extracts by employing BR-1 and BR-6, respectively. The 90-kDa protein bound to BR-6 was identified to be Hsp90 by immunoblotting. The 120-kDa protein bound to BR-1 was purified from rabbit reticulocyte lysate, and its internal amino acid sequence was identical to that of human and rat ATP citrate lyase. The identity of the 120-kDa protein as ATP citrate lyase was confirmed by immunoblotting. Interaction between BR-1 and ATP citrate lyase was blocked by radicicol but not by herbimycin A that interacts with Hsp90. These results suggest that radicicol binds the two proteins through different molecular portions of its structure. BR-1-bound ATP citrate lyase isolated from rabbit reticulocyte lysate showed no enzymatic activity. The activity of rat liver ATP citrate lyase was inhibited by radicicol and BR-1 but not by BR-6. Kinetic analysis demonstrated that radicicol was a non-competitive inhibitor of ATP citrate lyase with K(i) values for citrate and ATP of 13 and 7 microm, respectively.

Inhibition of protein synthesis by didemnins: cell potency and SAR

Synthetic and naturally occurring didemnins are potent and specific inhibitors of protein synthesis in vitro. Structure-activity analysis indicates a requirement for the intact macrocycle; however, the smaller ring size represented by the didemnin analogue, tamandarin A, is equipotent to didemnin B. Replacement of the N,O-dimethyltyrosine by a N-methylphenylalanine or N-methylleucine residue is also well-tolerated. The rank order for inhibition of protein synthesis in vitro appears to be retained in MCF-7 cells, albeit at much higher potency. This increase in potency is explained for the first time by data indicating that MCF-7 cells can accumulate didemnin B up to 2-3 orders of magnitude compared to the growth medium.

Pharmaceutical development of anticancer agents derived from marine sources

The marine ecosystem is more and more acknowledged as a source of potential anticancer agents. After the identification of a potential substance several hurdles have to be overcome before a marine candidate can enter the clinic. Amongst these are the establishment of a method which ensures sufficient supply and, which is the focus of this review, the development of a clinically useful pharmaceutical formulation. General issues with respect to the pharmaceutical development of marine anticancer agents will be discussed, which will be illustrated by highlighting aspects of the pharmaceutical development and clinical use of some representative compounds.

Phase II trail of didemnin B in previously treated non-Hodgkin's lymphoma: an Eastern Cooperative Oncology Group (ECOG) Study

Patients with non-Hodgkin's lymphoma (NHL) who fail initial therapy have a poor prognosis. We conducted a phase II study to determine the efficacy and toxicity of didemnin B, a non-myelosuppressive marine compound, in patients with NHL who relapsed or progressed after receiving one or two previous chemotherapy regimens. Fifty-one eligible patients were registered on this phase II study. Twenty-nine patients had intermediate or high grade (IG/HG) disease and 22 patients had low grade (LG) disease. Twenty-five patients received didemnin B at a dose of 6.3 mg/m2 and the remainder received 5.6 mg/m2, administered intravenously every 28 days. The patients had an Eastern Cooperative Oncology Group (ECOG) performance status of 0-2 and biopsy-proven relapsed disease. Objective responses were observed in two (7%) patients (one complete remission [CR] and one partial remission [PR]) with IG/HG disease and five (23%) patients (one CR and four PR) with LG disease. Patients with IG/HG disease had a median time to treatment failure (TTF) of 1.6 months and a median survival of 8.0 months. In contrast, the group with LG disease had a median TTF of 4.6 months and a median survival of 2.7 years. There were five grade V, 12 grade IV, and 57 grade III toxicities. Didemnin B appears to have modest activity in low grade NHL. However, the drug has considerable toxicity in this population of patients.

Inhibition of protein synthesis by didemnin B: how EF-1alpha mediates inhibition of translocation

The antineoplastic cyclic depsipeptide didemnin B (DB) inhibits protein synthesis in cells and in vitro. The stage at which DB inhibits protein synthesis in cells is not known, although dehydrodidemnin B arrests translation at the stage of polypeptide elongation. Inhibition of protein synthesis by DB in vitro also occurs at the elongation stage, and it was shown previously that DB prevents EF-2-dependent translocation in partial reaction models of protein synthesis. This inhibition of translocation displays an absolute requirement for EF-1alpha; however, the dependence upon EF-1alpha was previously unexplained. It is shown here that DB binds only weakly to EF-1alpha/GTP in solution, but binds to ribosome. EF-1alpha complexes with a dissociation constant K(d) = 4 microM. Thus, the inhibition of protein synthesis by DB appears to involve an interaction with both EF-1alpha and ribosomes in which all three components are required. Using diphtheria toxin-mediated ADP-ribosylation to assay for EF-2, it is demonstrated that DB blocks EF-2 binding to pre-translocative ribosome.EF-1alpha complexes, thus preventing ribosomal translocation. Based on this model for protein synthesis inhibition by DB, and the proposed mechanism of action of fusidic acid, evidence is presented in support of the Grasmuk model for EF-1alpha function in which this elongation factor does not fully depart the ribosome during polypeptide elongation.

Tamandarins A and B: new cytotoxic depsipeptides from a Brazilian ascidian of the family Didemnidae

The structures of two new, naturally occurring cytotoxic depsipeptides, tamandarins A and B (1 and 2), are presented. The tamandarins were isolated from an unidentified Brazilian marine ascidian of the family Didemnidae. The structures of the new cytotoxins were assigned by interpretation of FABMS data and by extensive 2D NMR analyses. The absolute configurations of the tamandarins were assigned by acid and alkaline hydrolysis to yield their corresponding amino acids, which were then analyzed as their Marfey derivatives. The cytotoxicity of tamandarin A (1) was evaluated against various human cancer cell lines and shown to be slightly more potent than didemnin B. A qualitative discussion of the conformation of tamandarin A (1) in solution, obtained from NMR J-value data, variable temperature experiments, and NOESY/ROESY data, is included.

The first total synthesis of (-)-tamandarin A

[formula: see text] Tamandarin A (1), a newly isolated natural product similar in structure to didemnin B (2), was shown to be somewhat more active in vitro than 2 against pancreatic carcinoma with an ED50 value 1.5 to 2 ng/mL. We report here the first total synthesis of 1. The key steps include a practical stereoselective synthesis of the Hiv-isostatine unit, high-yielding linear precursor formation, a successful macrocyclization, and coupling of the macrocycle with the side chain to afford tamandarin A (1).

Inhibition of protein synthesis by didemnin B is not sufficient to induce apoptosis in human mammary carcinoma (MCF7) cells

Didemnin B (DB) is one member of a class of natural cyclic depsipeptides that display potent cytotoxicity in vitro. The detailed mechanism of action of DB is unknown, although it appears to involve the inhibition of protein biosynthesis. Additional activities of DB have established DB as a rapid and potent inducer of apoptosis in HL-60 cells. Our aim was to determine if the induction of apoptosis by DB is mediated through inhibition of protein synthesis in MCF-7 human breast carcinoma cells. Apoptosis was observed only at > or = 100 nM DB, even though inhibition of protein synthesis occurred at much lower DB concentrations (IC50 = 12 nM). DB-induced apoptosis was mediated by caspase activation, since cleavage of the caspase substrate poly(ADPribose) polymerase was observed as early as 6 hr after DB exposure. Two additional protein synthesis inhibitors, cycloheximide (CHX) and emetine (ET), failed to induce apoptosis at concentrations that completely inhibited protein synthesis. Moreover, DB-induced apoptosis was enhanced only slightly by pre- and co-treatment with CHX and ET. Thus, inhibition of protein synthesis alone was not sufficient to induce apoptosis in these cells. As a measure of antiproliferative potential, DB (1-5 nM) inhibited the colony forming ability of MCF7 cells regardless of pretreatment with CHX. In conclusion, additional effects of DB, independent of protein synthesis inhibition, are proposed to account for its ability to induce apoptosis and prevent cell proliferation.

Bioactive agents from natural sources: trends in discovery and application

About 30% of the worldwide sales of drugs are based on natural products. Though recombinant proteins and peptides account for increasing sales rates, the superiority of low-molecular mass compounds in human diseases therapy remains undisputed mainly due to more favorable compliance and bioavailability properties. In the past, new therapeutic approaches often derived from natural products. Numerous examples from medicine impressively demonstrate the innovative potential of natural compounds and their impact on progress in drug discovery and development. However, natural products are currently undergoing a phase of reduced attention in drug discovery because of the enormous effort which is necessary to isolate the active principles and to elucidate their structures. To meet the demand of several hundred thousands of test samples that have to be submitted to high-throughput screening (HTS) new strategies in natural product chemistry are necessary in order to compete successfully with combinatorial chemistry. Today, pharmaceutical companies have to spend approximately US $350 million to develop a new drug. Currently, approaches to improve and accelerate the joint drug discovery and development process are expected to arise mainly from innovation in drug target elucidation and lead finding. Breakthroughs in molecular biology, cell biology, and genetic engineering in the 1980 s gave access to understanding diseases on the molecular or on the gene level. Subsequently, constructing novel target directed screening assay systems of promising therapeutic significance, automation, and miniaturization resulted in HTS approaches changing the industrial drug discovery process drastically. Furthermore, elucidation of the human genome will provide access to a dramatically increased number of new potential drug targets that have to be evaluated for drug discovery. HTS enables the testing of an increasing number of samples. Therefore, new concepts to generate large compound collections with improved structural diversity are desirable.

Synthesis and biological activity of [Tic5] didemnin B

A didemnin B analog containing a Tic (1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) as a conformationally restrained replacement for tyrosine has been synthesized and shown to have comparable potency as a protein biosynthesis inhibitor. Synthetic highlights include an oxidation of an alcohol to an acid in the presence of the sensitive Tic heterocycle and a modified Schmidt-type one-pot macrocyclization.

The antiproliferative agent didemnin B uncompetitively inhibits palmitoyl protein thioesterase

Dynamic protein palmitoylation has been proposed to regulate GTP-binding proteins by controlling their membrane association and thus their access to key signaling proteins. While the palmitoyl protein thioesterase(s) responsible for depalmitoylation of plasma membrane-associated signaling proteins has (have) not been identified, the lysosomal palmitoyl protein thioesterase 1 (PPT1) has proven useful in in vitro studies of membrane localization requirements of GTP-binding proteins. We have previously reported the binding of the antiproliferative cyclic depsipeptide didemnin B to PPT1. To investigate the nature of this binding and its possible effects on PPT1 enzymatic activity, human PPT1 was expressed in an insect cell baculoviral system, and inhibition assays were performed using both [3H]palmitoyl Ha-Ras and myristoyl-CoA as PPT1 substrates. Didemnin B was shown to inhibit recombinant human PPT1 with a Ki of 92 nM. Kinetic analysis of this inhibition revealed that didemnin B inhibits PPT1 uncompetitively. Providing biochemical support for an uncompetitive mode of inhibition, in vitro binding studies of PPT1 and didemnin indicate that the natural product binds preferentially to the enzyme-substrate complex PPT1-palmitoyl-CoA. As the first described inhibitor of PPT1, didemnin B may prove to be a useful tool in the investigation of protein palmitoylation regulation.

An antibody diagnostic for hymenopteran parasitism is specific for a homologue of elongation factor-1 alpha

An enzyme-linked immunosorbent assay (ELISA) useful for identifying noctuid pests parasitized by hymenopteran endoparasitoids was recently described. The ELISA employed a monoclonal antibody (MAb 9A5) that appeared highly polyspecific for parasitoid antigens, yielding banding patterns more typical of a polyclonal antiserum than of a monoclonal antibody in immunoblots of parasitoid homogenates subjected to SDS-PAGE. Although MAb 9A5 appeared capable of binding to dozens of parasitoid antigens, no cross-reactivity for noctuid antigens was evident by either immunoblotting or ELISA. In the study described here, immunoprecipitation, SDS-PAGE, and N-terminus amino acid sequencing were used to identify the protein recognized by MAb 9A5 as a homologue of elongation factor-1 alpha (EF-1 alpha). The propensity for EF-1 alpha to bind to cytoskeletal components, the additional subunits of EF-1, and other proteins may account for the apparent polyspecificity of MAb 9A5 in immunoblots of whole-body parasitoid homogenates. The presence of a unique hymenopteran epitope suggests that EF-1 alpha molecules from other insect groups could similarly express novel determinants. These determinants may prove useful not only for insect detection, but also as targets for selective insecticides that act by inhibiting protein synthesis.

Site-directed mutants of post-translationally modified sites of yeast eEF1A using a shuttle vector containing a chromogenic switch

Eukaryotic elongation factor 1A (eEF1A, formerly eEF-1 alpha) carries aminoacyl-tRNAs into the A-site of the ribosome in a GTP-dependent manner. In order to probe the structure/function relationships of eEF1A, we have generated site-directed mutants using a modification of a highly versatile yeast shuttle vector, which consists of the insertion of a 66 base long synthetic DNA fragment in the vector's polylinker. Via oligonucleotide-directed mutagenesis, the modification permits the identification of mutant clones based on a chromogenic screen of beta-galactosidase activity. Mutagenesis reactions are performed with two or more oligonucleotides, one introducing the chromogenic shift, and the other(s) introducing the mutation(s) of interest in eEF1A. Several rounds of chromogenic shifts and additional mutations can be performed in succession on the same vector. To address the possible function of the methylated lysines in yeast eEF1A, we have changed the post-translationally modified lysines (residue 30, 79, 316 and 390) to arginines using the above methodology. Yeast with eEF1A mutants that substitute arginine in all four sites do not show any phenotypic change. There is also an apparent equivalency of wild-type and mutant yeast eEF1A in in vitro assays. It is concluded that the post-translational modifications of eEF1A are not of major importance for eEF1A's role in translation.

Understanding and controlling the cell cycle with natural products

Small molecule natural products have aided in the discovery and characterization of many proteins critical to the progression and maintenance of the cell cycle. Identification of the direct target of a natural product gives scientists a tool to control a specific aspect of the cell cycle, thus facilitating the study of the cell-cycle machinery.

Structure--activity relationships of the didemnins

Bioactivities of 42 didemnin congeners, either isolated from the marine tunicates Trididemnun solidum and Aplidium albicans or prepared synthetically and semisynthetically, have been compared. The growth inhibition of various murine and human tumor cells and plaque reduction of HSV-1 and VSV grown on cultured mammalian cells were used to assess cytotoxicity and antiviral activity. Biochemical assays for macromolecular synthesis (protein, DNA, and RNA) and enzyme inhibition (dihydrofolate reductase, thymidylate synthase, DNA polymerase, RNA polymerase, and topoisomerases I and II) were also performed to specify the mechanisms of action of each analogue. Immunosuppressive activity of the didemnins was determined using a mixed lymphocyte reaction (MLR) assay. These assays revealed that the native cyclic depsipeptide core is an essential structural requirement for most of the bioactivites of the didemnins, especially for cytotoxicities and antiviral activities. The linear side-chain portion of the peptide can be altered with a gain, in some cases, of bioactivities. In particular, dehydrodidemnin B, tested against several types of tumor cells and in in vivo studies in mice, as well as didemnin M, tested for the mixed lymphocyte reaction and graft vs host reaction in murine systems, showed remarkable gains in their in vitro and in vivo activities compared to didemnin B.

Antiproliferative effect of dehydrodidemnin B (DDB), a depsipeptide isolated from Mediterranean tunicates

The biological effects of dehydrodidemnin B(DDB), a novel depsipeptide isolated from Aplidium albicans, were studied on Ehrlich carcinoma growing in vivo and in primary cultures, and compared with those reported for Didemnin B (DB). Daily administration of DB or DDB (2.5 micrograms/mouse) almost duplicated the animal life-span and total number of tumour cells decreased by 70-90%. Results suggest a major effect of DDB when administered in the lag phase of growth. DDB behaved as a very potent inhibitor of protein synthesis; consequently, ornithine decarboxylase activity (ODC, EC 4.1.1.17) is drastically reduced by DDB-treatment.

Crystal and molecular structure of didemnin A, an antiviral depsipeptide

The molecular structure of didemnin A, the parent compound of a series of antiviral cytotoxic depsipeptides extracted from a marine tunicate Trididemnum solidum of the family of Didemnidae, has been determined by single-crystal X-ray diffraction. In the crystal, didemnin A molecules form pseudo-symmetric dimeric pair. The two molecules in the dimer are held together by strong N--H center dot center dot center dot O and N--H center dot center dot center dot N hydrogen bonds. A chloride ion, placed almost symmetrically between the dimeric pair, forms N--H center dot center dot center dot Cl hydrogen bonds (3.19 and 3.23 Angstrom) with both the molecules. The two independent molecules in the structure have closely similar geometry. For each molecule, the 23-membered depsipeptide ring assumes a folded conformation in the shape of a 'bent figure-of-eight' similar to that observed in the didemnin B crystal structure. The major conformational differences in the macrocycle of didemnin A and didemnin B are around the Hip residue. The root mean-square (RMS) difference of 20 of the 23 endocyclic torsion angles for the two structures is less than 10 degrees, while the three bond torsions in the Hip residue vary by about 50 degrees. The macrocycle conformation is stabilized by a transannular N--H center dot center dot center dot O hydrogen bond linking the isostatine amide group with the leucine carbonyl group. The truncated linear chain is folded back toward the macrocyclic ring and is held by a N--H center dot center dot center dot O hydrogen bond between the leucine amide group and Me-Leu carbonyl group. The transannular hydrogen bond in the didemnin A structure (N4--H center dot center dot center dot O3 = 2.83 Angstrom in both molecule a and molecule b) is noticeably stronger than that observed in the didemnin B structure (3.02 Angstrom). The X-ray structure of didemnin A is generally consistent with that obtained by NMR studies. Within the crystal, the molecules are packed in zig-zag chains formed by intermolecular O--H center dot center dot center dot O hydrogen bonds. The crystal structure and packing of didemnin A are quite different from that of the didemnin B structure.

Fishing for drugs from the sea: status and strategies

Marine organisms represent an enormous, essentially unexploited, resource of natural products. Globally, the race to develop marine-derived drugs is well under way with many pharmaceutical companies positioning themselves to reap large profits by the exploitation of the ocean's rich chemical diversity. Targeted strategies, often in combination with high-throughput screening, are being employed in this hunt for novel pharmacotherapeutic agents. David de Vries and Phil Beart examine the potential, problems and technologies of an international pharmaceutical search that has important ethical considerations.