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The structures of two new didemnins ([Hysp2]didemnin B and [Hap2]didemnin B) from the Aplousobranch ascidian Trididemnum cyanophorum (Didemnidae) are described. Structures are determined by a combination of mass spectrometry and one and two-dimensional high-field NMR techniques. Complete 1H and 13C NMR spectral assignments and conformational informations are presented. The cytotoxicities of the compounds towards drug-sensitive and drug-resistant tumor cell lines are compared with those of didemnins A and B.

The activity of didemnin B, a natural product derived from the Caribbean Tunic was assessed in 16 patients with Glioblastoma multiforme. Didemnin B was administered intravenously by a short infusion at a dose of 4.3 mg/m2 and subsequently escalated to 6.3 mg/m2. No anti-tumor activity was observed. Toxicity consisted of fatigue, weakness, stomatitis, mild blood count changes, nausea and vomiting and occasional fever. Based on these results further studies with didemnin B in patients with Glioblastoma multiforme are not recommended.

A strategy is presented for the determination of amino-acid sequences of cyclodepsipeptide antibiotics. A highly specific sodium-ion interaction with the backbone lactone opens the depsipeptide ring to form a linear acylium ion or isomeric equivalent. When activated by high energy collisions on a tandem mass spectrometer or by low energy collisions on an ion trap, the acylium ion undergoes sequence-specific fragmentation to yield a simple product-ion mass spectrum. Fragmentation is charge-driven, and amino acid residues are sequentially deleted from the C-terminus of the acylium ion. Interferences from indiscriminate ring opening at backbone amide bonds are eliminated. The method is suited to the structural analyses of various cyclodepsipeptides, including those with linear peptide moieties. Results are presented for beauvericin, didemnin B, and enniatin B1, representing the three commonly encountered structural variations in cyclodepsipeptide antibiotics.

Caspases have been implicated in the induction of apoptosis in most systems studied. The importance of caspases for apoptosis was further investigated using the system of didemnin B-induced apoptosis. We found that benzyloxycarbonyl-VAD-fluoromethylketone, a general caspase inhibitor, inhibits didemnin B-induced apoptosis in HL-60 and Daudi cells. Acetyl-YVAD-chloromethylketone, a caspase-1-like activity inhibitor, inhibits didemnin B-induced apoptosis in Daudi cells, whereas the caspase-3-like activity inhibitor, acetyl-DEVD-aldehyde, has no effect. Using immunoblots to investigate cleavage of caspases-1 and -3, we found that both caspases are activated in both cell lines. We showed that the caspase substrate poly(ADP-ribose)polymerase is cleaved in these cells after didemnin B treatment. In both cell lines, poly(ADP-ribose)polymerase cleavage is inhibited by benzyloxycarbonyl-VAD- fluoromethylketone and also by acetyl-YVAD-chloromethylketone in Daudi cells. These results indicate that a caspase(s) other than caspase-3 is required for didemnin B-induced apoptosis. We show that caspases may be activated during apoptosis that are not required for the progression of apoptosis. J. Cell. Biochem. 72:269-278, 1999. r 1999 Wiley-Liss, Inc.

Based on information from X-ray, NMR, and SAR data, the N , O -diMeTyr 5 unit of didemnin B was believed to be important for biological activity. To determine the importance of aromaticity and the role of the methoxy group in this unit, two analogs were synthesized in which the N , O -diMeTyr 5 moiety was replaced with N -MeLeu and N -MePhe. Preliminary testing showed that the analogs retained antitumor activity and the ability to inhibit protein biosynthesis in vitro .

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.

Didemnin B (NSC-325319), a new depsipeptide isolated from a Caribbean tunicate, has been evaluated in a clinical phase I study. The drug was administered in a schedule of a 4 weekly intravenous injection in a six-weeks cycle. Fifty-three patients received 71 evaluable cycles in an escalated dose ranging from 0.4 mg/m2/week to 2.5 mg/m2/week. No hematological toxicity was demonstrated at any dose level. Without prophylactic antiemetics nausea and vomiting was dose limiting at 1.2 mg/m2/week. Due to the use of Cremophor EL as a solvent, hypersensitivity reactions occurred in 9 patients. These reactions occurred following prior exposure to the drug and were commonly seen at the 3rd dose. They were not dose related but became more frequent at 1.5 mg/m2/week necessitating prophylactic treatment with H1 and H2 receptor blocking agents. Non-hematological toxicities included mild diarrhea, mucositis, anorexia, headaches, and local phlebitis. The dose- limiting toxicity was generalized weakness which became severe and disabling in 3 of 6 patients treated at 2.5 mg/m2/week. No objective responses were documented in 39 patients with evaluable disease. The recommended dose for phase II studies was 2.3 mg/m2/week × 4 4 in a 6-weeks cycle given with prophylactic antiemetics and H1 and H2 receptor blocking agents.

The syntheses of three modified peptide fragments of the cyclodepsipeptide didemnin B are reported. The HIP and isostatine (1st) units of the didemnin B macrocycle were simplified to a Z-alanine residue and the ester linkage (through threonine of the tetrapeptide) was replaced with amide linkages through the amines of glycine, D-alanine and an ethylenediamine linker. The latter permitted the attachment of a N-Me-D-Leu-Pro-Lac moiety to afford analogs 2, 3 and 4 respectively.

As part of investigations directed toward the determination of the essential/nonessential structural features for the bioactivities of didemnin B, we designed a reduced ring analog in which three moieties, namely the tyrosine side chain, the isostatine hydroxyl, and the side chain (l-lactyl-l-prolyl-N-Me-d-leucine), were in their presumed bioactive conformation. In designing the reduced ring analog, we eliminated the leucine-proline portion of the macrocycle core and replaced it with an n-butyl linker in order to elucidate its role. According to MM2 calculations (MacroModel molecular modeling), this analog was of lower energy than the natural product didemnin B, and both structures were superimposable. The synthetic strategy involved four disconnections. Macrocyclization was accomplished at the activated carboxylic acid of the α-(α-hydroxyisovaleryl)propionyl unit (HIP) and the protected amine of the n-butyl linker using a modification of Schmidt's protocol. After selective deprotection of the hydroxyl an...

Although the deprotection of amides by hydrazinolysis is a well established method in carbohydrate chemistry, this reaction is dependent upon the nearby substituents. In our facile synthesis of intermediate benzyl 2-amino-3-azido-4-O-p-methoxybenzyl-6-O-benzyl-2,3-dideoxy-α-d-glucopyranoside (3), we found that the use of trifluoroacetamides provided a more efficient protection strategy.

Three side chain analogs of didemnin B were synthesized. All three analogs of the only conventional type II β-turn present in didemnin B differed in the functionality incorporated in the threonine residue. The cytotoxic activities of these analogs were evaluated, and one of them exhibited weak activity.

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.

The structures of the two natural didemnins with a modified macrocycle, [Tyr5]didemnin B [2] and [D-Pro4]didemnin B [3], isolated from the Aplousobranch ascidian Trididemnum cyanophorum (Didemnidae) are described. Structures were determined by FABMS and NMR spectroscopy, absolute configurations by Marfey analysis of the acid hydrolisatcs. Conformational informations arc given and cytotoxic activities against human lymphoblastic leukemia cell lines evaluated.

Ten patients with previously untreated stage III/IV low grade histology non-Hodgkin's lymphoma received a 1-hour intravenous infusion of Didemnin B 2.3 mg/m2 weekly for 4 weeks repeated every 6 weeks. 40% of patients experienced significant hypersensitivity reactions, one of which was life-threatening, despite premedication with diphenhydramine and cimetidine. Other toxicities included nausea, vomiting, fatigue, diarrhea and skin rashes. No objective responses were seen. Given the serious toxicity and lack of activity in a non-pretreated group of patients, the study was closed early. Further investigation of Didemnin B at this dose and schedule is not recommended.

The cytotoxic and immunosuppressive marine depsipeptide didemnin B is a potent inhibitor of protein biosynthesis in intact cells. Here, didemnin B is shown to inhibit protein synthesis in vitro during the elongation cycle, by preventing eukaryotic elongation factor 2-(eEF-2-) dependent translocation. No inhibition of aminoacyl-tRNA delivery or of peptidyltransferase activity is observed. Didemnin B stimulates eEF-1 alpha-dependent aminoacyl-tRNA binding to rabbit reticulocyte ribosomes, and eEF-1 alpha is required for inhibition of the subsequent translocation of phenylalanyl-tRNA(Phe) from the A- to the P-site. These observations suggest that didemnin B prevents translocation by stabilizing aminoacyl-tRNA bound to the ribosomal A-site, similar to the antibiotic kirromycin, and consistent with the known affinity of didemnins for elongation factor eEF-1 alpha [Crews et al. (1994) J. Biol. Chem. 269, 15411]. Unlike kirromycin, didemnin B does not prevent peptide bond formation, so inhibition is observed only at the translocation step. Inhibition of translocation by didemnin B is attenuated by increasing concentrations of eEF-2.

Didemnin B, a dipsipeptide isolated from the Caribbean tunicate Trididemnum with antitumor and antiviral activity was evaluated in a phase II trial in the treatment of metastatic, hormonally refractory adenocarcinoma of the prostate. Thirteen patients were treated with didemnin B at 3.5 mg/m2 and 20 patients were treated at 6.3 mg/m2 intravenously every 28 days. Response was assessed every 8 weeks. Of 32 evaluable patients there was one partial response for an overall response rate of 3% (95% confidence interval of 0.1-16%). The most common toxicities were nausea, vomiting, and diarrhea. Serious cardiac and pulmonary toxicities were also noted. This drug does not appear to warrant further evaluation in this disease as a single agent.

The marine natural product, didemnin B, is a 7-amino acid, cyclic depsipeptide that inhibits G1 cell cycle progression at nanomolar concentrations by undefined mechanisms. It has been reported to exhibit immunosuppressive activities in animals and is undergoing clinical trials as a potential antineoplastic drug. In addition, at higher concentrations, didemnin B has been shown to inhibit in vivo and in vitro protein synthesis. However, the mechanisms by which inhibition is achieved are unknown. To investigate didemnin's various modes of action, an affinity column was synthesized and used to purify didemnin-binding proteins. The major retained protein was the 49-kDa guanine nucleotide-binding elongation factor, EF-1 alpha, which was identified by peptide sequence analysis. Moreover, didemnin binds EF-1 alpha only in the presence of GTP but does not inhibit the GTPase activity of EF-1 alpha. Therefore, EF-1 alpha is likely to be the intracellular target responsible for didemnin B's ability to inhibit protein synthesis. Furthermore, this specificity of didemnin affinity for the GTP-bound conformation of a guanine nucleotide-binding protein with homology to the Ras superfamily suggests a possible mode of action for didemnin's antiproliferative activity.

Didemnin B is a cyclic peptide isolated from the marine tunicate Trididemnin cyanophorum. It is a known potent inhibitor of RNA, DNA and protein synthesis, with activity against the murine B16 melanoma. Fourteen patients with disseminated malignant melanoma were evaluated in a Southwest Oncology Group phase II trial of didemnin B at 4.2 mg/m2 by 30 min i.v. infusion every 28 days (SWOG-8754). Only patients with no prior chemotherapy were eligible; prior radiation therapy, surgery and at most one prior biologic regimen were allowed. Patients with brain metastasis were eligible only if the disease in the brain had been treated and controlled. All patients had to have normal renal and hepatic function and adequate granulocyte and platelet counts, a performance status of 0-2, and bidimensionally measurable disease. Fourteen patients were entered on the study; five received one and nine received two courses of didemnin B. No responses were noted among the 11 patients evaluable for response. Five patients developed unusual but reversible hypersensitivity reactions during the second course of therapy. Other toxicity in this trial was nausea and vomiting and diarrhea, none of severity greater than grade 3. Given the lack of antitumor efficacy and the unusual toxicity, further evaluation of didemnin B in this dose and schedule in malignant melanoma is not warranted.

Natural products obtained from marine invertebrates such as sponges and tunicates are attractive sources of drugs. However, a critical obstacle in the development of these compounds is the problem of supply. In most cases, neither chemical synthesis nor mariculture of invertebrates is economically feasible. Due to structural similarities, many marine natural products are suspected to be produced by associated microorganisms. A favorable strategy for the production of such compounds is to use culturable microorganisms. Here we report that didemnin B, a tunicate-derived depsipeptide, has been isolated from a culturable bacterium, Tistrella mobilis YIT 12409.

A folate receptor targeted didemnin B conjugate was synthesized using a hydrophilic peptide spacer linked to folate via a releasable disulfide carbonate linker. Cell cytotoxicity and TNF-α inhibition in RAW264.7 macrophage-like cells exhibited IC 50 s of 13 and 5 nM, respectively. Folate didemnin B was found to be ∼50–100 fold more potent than didemnin B itself. More importantly, activity of the prodrug was blocked by excess folic acid, demonstrating receptor-mediated cellular uptake of the conjugate.

(1S,2S,3R)-3-Hydroxy-2-methoxy-1-cyclohexanecarboxylic acid was synthesized for the construction of an amino acid to be used in a constrained ring didemnin B analog ( 2 ). The amine functionality was introduced into the cyclohexane ring by reductive amination using sodium triacetoxyborohydride or by oxime formation followed by reduction.

An X-ray crystal analysis of φ-lactone N-{1-(N-{4-{[3-hydroxy-5-methyl-1-oxo-4-(N-L-threonylamino)heptyl]oxy}-2,5-dimethyl-1,3-dioxohexyl}-L-leucyl}-L-prolyl}-N,O-dimethyl-L-tyrosine hydrobromide hydrate (1a), C 42 H 66 N 5 O 11 + .Br - .H 2 O, was obtained in order to determine the backbone folding of the macrocycle and to compare the results obtained with those reported previously for the natural product didemnin B (1b). Some differences were noted in the torsion angles of the two conformers of the hydrobromide salt, denoted (1a) and (1a'). The conformation of (1a') resembled the conformation of (1b) more closely than did that of (1a). Certain regions of both crystal backbones were more flexible than those in didemnin B ; however, the transannular hydrogen bonds in both (1a) and (1a') were somewhat stronger than in (1b).

[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).

Of the six marine-derived compounds that have reached clinical trials as antitumor agents three-didemnin B, Aplidine, and ecteinascidin 743-are derived from tunicates. Di-demnin B (DB), a cyclic depsipeptide from the compound tunicate Trididemnum solidum, was the first marine-derived compound to enter Phases I and II clinical trials. The Phase II studies, sponsored by the U. S. National Cancer Institute, indicated complete or partial remissions with non-Hodgkins lymphoma, but cardiotoxicity caused didemnin B to be dropped from further study. The closely related dehydrodidemnin B (DDB, Aplidine) was isolated in 1988 from a second colonial tunicate, Aplidium albicans, and spectroscopic studies assigned a structural formula in which a pyruvyl group in DDB replaced the lactyl group in DB and syntheses of DDB have been achieved. Aplidine is more active than DB and lacks DB's cardiotoxicity. It was introduced by PharmaMar into Phase I clinical trials in January 1999. The second family of tunicate-derived antitumor agents are the ecteinascidins (ETs), from the mangrove tunicate Ecteinascidia turbinata. The antitumor extracts of E. turbinata were first described in 1969, but the small amount of ETs in E. turbinata prevented their isolation for over a decade. The structures of ETs have been assigned mainly by spectroscopy. Phase II clinical trials with ET 743 are underway. Future supplies of ET's should be available from aquaculture or synthesis.

Sixteen evaluable patients with metastatic breast cancer were entered into a phase II trial of didemnin B. They received the drug at an initial dose of 5.6 mg/m2 every 21 to 28 days. Major toxicities noted were myalgia and nausea and vomiting while myelosuppression was mild. There were no complete responses; however, two minor responses were observed. The pharmacokinetics of didemnin B were studied in 10 patients who received the drug as 30 to 60 min i.v. infusions. A sensitive competitive inhibition enzyme immunoassay was used to quantitate didemnin B levels. Drug was observed to be rapidly cleared from plasma in a biphasic manner (t1/2 alpha = 0.12 hr, t1/2 beta = 4.8 hr). Although the assay could not identify the presence of specific metabolites, the increase of apparent didemnin B levels in plasma at later time points suggested the formation of unidentified metabolites which cross reacted with the antibody in the analytical procedure. In vitro experiments indicated that didemnin B was not bound to bovine serum albumin and only a minor portion (24%) of drug was found associated with red blood cells.

The agents cyclosporine, tetranactin (TN), and didemnin B (DB) were compared for their ability to inhibit proliferative human T cell responses in vitro, using anti-CD3, PHA, alloantigen, or tetanus toxoid as stimuli and using monocytes or Langerhans cells as antigen-presenting cells/accessory cells (APC/AC). We found that all three agents suppressed T cell activation in a dose-dependent fashion, irrespective of the stimulus of APC/AC type used. Both T cells and APC/AC were affected by the drugs. DB appeared to be the most potent suppressive drug (IC50 = 1-4 ng/ml), whereas CsA and TN exerted approximately similar potency (IC50 = 50-60 ng/ml). Remarkably however, DB was toxic at a concentration of 10 ng/ml, which is quite close to the inhibition-inducing dose. No toxicity was observed with CsA and TN at doses up to 5000 ng/ml. The agents TN and DB could interrupt ongoing T cell responses and could block responsiveness to exogenous recombinant IL-2. Expression of IL-2 receptors was slightly inhibited by all three drugs. Expression of MHC class II molecule HLA-D and of adhesion molecules LFA-1, LFA-3, and ICAM-1 was clearly reduced by DB, giving an explanation for the observed inhibition of cluster formation between T cells and APC/AC. Except for a slight reduction of LFA-3 by TN, CsA and TN did not affect the expression of any of these cell surface markers or the formation of clusters. Differences in the effects of CsA, TN, and DB on immune responses in vitro and on the phenotype of T cells and APC/AC suggest that these immunosuppressive drugs have different inhibitory mechanisms.

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.

BACKGROUND: The process of translation occurs at a nexus point downstream of a number of signal pathways and developmental processes. Modeling activation of the PTEN/AKT/mTOR pathway in the Emu-Myc mouse is a valuable tool to study tumor genotype/chemosensitivity relationships in vivo. In this model, blocking translation initiation with silvestrol, an inhibitor of the ribosome recruitment step has been showed to modulate the sensitivity of the tumors to the effect of standard chemotherapy. However, inhibitors of translation elongation have been tested as potential anti-cancer therapeutic agents in vitro, but have not been extensively tested in genetically well-defined mouse tumor models or for potential synergy with standard of care agents. METHODOLOGY/PRINCIPAL FINDINGS: Here, we chose four structurally different chemical inhibitors of translation elongation: homoharringtonine, bruceantin, didemnin B and cycloheximide, and tested their ability to alter the chemoresistance of Emu-myc lymphomas harbouring lesions in Pten, Tsc2, Bcl-2, or eIF4E. We show that in some genetic settings, translation elongation inhibitors are able to synergize with doxorubicin by reinstating an apoptotic program in tumor cells. We attribute this effect to a reduction in levels of pro-oncogenic or pro-survival proteins having short half-lives, like Mcl-1, cyclin D1 or c-Myc. Using lymphomas cells grown ex vivo we reproduced the synergy observed in mice between chemotherapy and elongation inhibition and show that this is reversed by blocking protein degradation with a proteasome inhibitor. CONCLUSION/SIGNIFICANCE: Our results indicate that depleting short-lived pro-survival factors by inhibiting their synthesis could achieve a therapeutic response in tumors harboring PTEN/AKT/mTOR pathway mutations.

Didemnin B is a depsipeptide extracted from the marine tunicate Trididemnin cyanophorum. This agent is a potent inhibitor of L1210 growth in vitro and has activity against murine B16 melanoma, P388 leukemia, and M5076 sarcoma in vivo. The results of preclinical toxicologic tests demonstrated abnormalities in clotting parameters thought to be secondary to drug-induced liver dysfunction. Thirty-five patients with advanced cancer received didemnin B according to a 5-day bolus schedule with dose levels ranging from 0.03 to 2.00 mg/m2/d. The dose-limiting toxicity was nausea and vomiting. Sporadic elevation of the hepatic enzyme level occurred but was not dose limiting. Two patients had anaphylactic symptoms possibly related to the 5% polyoxyethylated castor oil (Cremophor EL, BASF, Ludwigshafen, Germany) vehicle during the drug infusion. Clinical bleeding was not observed and myelosuppression was not significant. No partial or complete tumor responses were seen. The recommended Phase II dose for the 5-day schedule is 1.6 mg/m2/d.

The purpose of this study was to investigate two promising immunosuppressive agents, didemnin B (DB) and FK506 (FK), during pregnancy to assess potential adverse maternal or fetal effects. Pregnant C3H mice were randomized into control and high- and low-dose treatment groups for each drug. Animals received daily injections from day 1 to day 16, and on day 17 of gestation the maternal condition, litter size, fetal resorption rates, and fetal/placental unit weights were determined. Immunoglobulin (IgG) levels were obtained for DB treatment groups. Delayed type hypersensitivity was assessed in virgin females. Both DB and FK had dose-dependent immunosuppressive activity in the DTH assay, and DB caused elevated IgG concentrations. High doses of DB caused diarrhea and maternal wasting with no fetal survival; with low-dose DB, maternal weight gain was depressed, but pregnancy outcome was not different from control animals. High-dose FK had no obvious detrimental effects on maternal health but caused resorption of all fetuses; administration of low-dose FK resulted in a higher number of resorptions, but fetuses that survived did not appear different from controls. We conclude that these immunosuppressive drugs can have adverse effects on pregnancy, but the maternal and fetal toxicity are dose-dependent.

Nordidemnin (2), a natural analogue of the marine cyclodepsipeptide didemnin B (1b), showed cytotoxic activity against L1210 leukemia and antineoplastic activity against P388 leukemia as well as B16 melanoma; nordidemnin (2) was as active as didemnin B (1b). The influence of synthetic modifications in the linear peptidic chain on in vitro and in vivo activity was also studied. Replacement of the terminal lactyl residue by mandelyl and 3-(p-hydroxyphenyl)propionyl residues in compounds 3 and 4, respectively, did not affect the cytotoxic activity against L1210 leukemia (ID50 of 1.1 nM and 1.2 nM, respectively) or the in vivo activity against P388 leukemia. Unlike these aromatic substituants, the lipophilic palmityl residue induced a dramatic loss in cytotoxic activity. The inverted chirality of the MeLeu joining residue in compound 6 caused a marked reduction in the in vitro activity.

Didemnin B (NSC 325,319), a cyclic depsipeptide isolated from a Caribbean tunicate, exhibits potent preclinical antitumor activity. In previous phase I studies, 3.47 mg/m2 was the maximally tolerated dose, with nausea and vomiting being the dose-limiting toxicity. The drug was given in a single bolus infusion over 30 min every 28 days. In the current study, 30 patients presenting with previously treated non-small-cell lung cancer (NSCLC) received 46 courses of the drug at doses ranging from 3.47 to 9.1 mg/m2. Neuromuscular toxicity was dose-limiting. Nausea and vomiting appeared to be correlated with dose levels and were ameliorated by a combination of antiemetics including dexamethasone. Other side effects included a mild rise in hepatic enzymes and an allergic reaction that was preventable by the addition of corticosteroids to the premedication regimen. In all, 2 minor responses were seen among 24 evaluable patients. Because neuromuscular toxicity is dose-limiting, we recommend that routine measurements of creatine kinase and aldolase, a careful neurologic evaluation, and electromyography and muscle biopsy (if indicated) be incorporated into phase II trials. The recommended dose for phase II studies using a single bolus schedule is 6.3 mg/m2, following the premedication of patients with antiemetics.

Didemnins A, B, and C were efficiently prepared in a stereocontrolled manner, producing the common macrocycle and, in a separate step, introducing the substituents on the amino group of L-threonine as optically pure units. We envisaged that disconnections between L-leucine and the HIP group (2S,4S) and between L-threonine and isostatine (3S,4R,5S) would afford two units: a HIP-isostatine unit (I) and a tetrapeptide unit (II)

Several homo- and heteronuclear two-dimensional NMR techniques were used to assign all H- and C-resonances of the two conformers A and B of [7-(N-methyl-L-leucine)]didemnin B. Didemnine is a biologically highly active cytostaticum and immunosuppressivum. The assignment of the aliphatic C-atoms were done by the inverse H,C-COSY with TOCSY transfer which connects complete proton spin systems and represents them on C-atoms. The structure of both conformers (A and B) in (D6)DMSO solution was derived from homo- and heteronuclear couplings (J), temperature dependencies of NH protons, and NOE effects. Distances determined from the latter were used for refinements by restrained MD calculations using the GROMOS program. The solution structure of [Me-L-Leu7]didemnin B (A and B) was compared to that of didemnin B. The backbone structure of the macrocyclic ring and of the linear side-chain moiety are very similar in conformer A and didemnin B, though the Ist1-Hip2 region of the ring is slightly ex tended in conformer A. This may be caused by the influence of the Me-L-Leu7 residue in A and may be responsible for its reduced biological activity in comparison to didemnin B. The more weakly populated conformer B exhibits a βVI turn in the linear side-chain moiety.

Sixteen evaluable patients with advanced or recurrent epithelial ovarian carcinoma following progression on combination chemotherapy were treated with 4.2 mg/m2 of Didemnin B (NSC #325319) intravenously every 28 days until progression of disease. All patients had prior cisplatin-containing chemotherapy. There were no responders. Seven patients had stable disease (43.7%) and nine (53.3%) had increasing disease. The toxicities were significant, consisting of nausea and vomiting in seven patients (44%), one grade 3 hepatic toxicity, and three instances of grade 4 toxicities (1 leukopenia, 1 GI, and 1 GU). When used with this schedule Didemnin B is ineffective in patients with previously treated epithelial ovarian cancer.

Aplidin is a cyclic depsipeptide of the family of didemnins, originally isolated from the colonial tunicate Aplidium albicans. Aplidin is being evaluated in a phase III clinical trial in patients with relapse or refractory multiple myeloma. As part of its antitumor activity, Aplidin induces rapid oxidative stress, activation of Rac1 and phosphorylation of p38 and JNK1 stress kinases, which together trigger the apoptotic death of tumor cells. Didemnin B (DB), a molecule closely related to Aplidin, has been previously shown to interact with the GTP-bound conformation of the eukaryotic elongation factor eEF1A, an interaction that was related to the didemnin's B ability to inhibit protein synthesis (J. Biol. Chem. 1994, 269:15411-14). A structural model for this interaction has been proposed (J. Med. Chem. 2004, 47:4439-52). We wanted to investigate whether eEF1A had any role in the mechanism of action of Aplidin. Using the DARTS technique, we observed that Aplidin treatment of tumor cells and subsequent digestion of the cellular extracts with different proteases, resulted in a significant increase in the stabililty of eEF1A against protease digestion, suggesting a direct effect of Aplidin on this protein. We previously generated, by continuous exposure to increasing concentrations of the drug, a HeLa derivative cell line (HeLa-APL-R) that showed specific resistance to Aplidin as well as to other related didemnins and tamandarins (Br. J. Cancer 2004, 91:1405-13). We investigated whether there was any difference in the expression levels of eEF1A between HeLa-wt and HeLa-APL-R cell lines. Since two isoforms of the elongation factor are expressed in tumor cells, eEF1A1 and eEF1A2, we checked the relative amount of both proteins at the mRNA and protein levels using DNA microarrays and iTRAQ, respectively. Remarkably, we observed that the mRNA and protein levels of eEF1A2 isoform were lower in HeLa-APL-R resistant cells as compared to their parental cell line. No significant changes were seen in the levels of eEF1A1. The reduced levels of eEF1A2 protein in HeLa-APL-R cells were further confirmed by western blotting using isoform-specific antibodies. To explore the effect of the restoration of the eEF1A2 levels in the HeLa resistant clone, we generated two cell lines stably overexpressing eEF1A1 or eEF1A2 and checked their sensitivity to Aplidin in dose-response cytotoxicity experiments. Both cell lines partially recovered the sensitivity to Aplidin, with the eEF1A2-overexpressing cell line having an even slightly higher sensitivity to the compound. In eEF1A overexpressing cells, Aplidin induced a robust cytostatic effect. At the molecular level, Aplidin induced the phosphorylation of p38 as well as ERK MAPKs, but not JNK phosphorylation or PARP cleavage, two key events in the cytotoxic signaling of the drug. These results could indicate a role of eEF1A in the biological activity of Aplidin in tumor cells. Citation Format: Alejandro Losada, Juan F. Martínez-Leal, Federico Gago, Carmen Cuevas, Luis F. Garcia-Fernández, Carlos M. Galmarini. Role of the eukaryotic elongation factor eEF1A in the mechanism of action of Aplidin. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5467. doi:10.1158/1538-7445.AM2014-5467