The interaction of spongistatin 1 with tubulin.

A tritiated derivative of the sponge-derived natural product spongistatin 1 was prepared, and its interactions with tubulin were examined. [3H]Spongistatin 1 was found to bind rapidly to tubulin at a single site (the low specific activity of the [3H]spongistatin 1, 0.75 Ci/mmol, prevented our defining an association rate), and the inability of spongistatin 1 to cause an aberrant assembly reaction was confirmed. Spongistatin 1 bound to tubulin very tightly, and we could detect no significant dissociation reaction from tubulin. The tubulin-[3H]spongistatin 1 complex did dissociate in 8 M urea, so there was no evidence for covalent bond formation. Apparent KD values were obtained by Scatchard analysis of binding data and by Hummel-Dreyer chromatography (3.5 and 1.1 µM, respectively). The effects of a large cohort of vinca domain drugs on the binding of [3H]spongistatin 1 to tubulin were evaluated. Compounds that did not cause aberrant assembly reactions (halichondrin B, eribulin, maytansine, and rhizoxin) caused little inhibition of [3H]spongistatin 1 binding. Little inhibition also occurred with the peptides dolastatin 15, its active pentapeptide derivative, vitilevuamide, or diazonamide A, nor with the vinca alkaloid vinblastine. Strong inhibition was observed with dolastatin 10, hemiasterlin, and cryptophycin 1, all of which cause aberrant assembly reactions that might actually mask the spongistatin 1 binding site. Spongistatin 5 was found to be a competitive inhibitor of [3H]spongistatin 1 binding, with an apparent Ki of 2.2 µM. We propose that the strong picomolar cytotoxicity of spongistatin 1 probably derives from its extremely tight binding to tubulin.

Interaction of diazonamide A with tubulin.

[3H]Diazonamide A ([3H]DZA), prepared from the natural product isolated from Diazona angulata, bound to tubulin in larger aberrant assembly products (>500 kDa by sizing HPLC) but not to the αß-tubulin heterodimer. The binding reaction was rapid, but stoichiometry was low. Stoichiometry was enhanced up to 8-fold by preincubating the tubulin in the reaction mixture prior to adding the [3H]DZA. Although Mg2+ did not affect binding stoichiometry, the cation markedly increased the number of tubulin rings (diameter about 50 nm) observed by negative stain electron microscopy. Bound [3H]DZA did not dissociate from the tubulin oligomers despite extensive column chromatography but did dissociate in the presence of 8 M urea. With preincubated tubulin, a superstoichiometric amount of [3H]DZA appeared to bind to the tubulin oligomeric structures, consistent with observations that neither nonradiolabeled DZA nor DZA analogues inhibited binding of [3H]DZA to the tubulin oligomers. Only weak inhibition of binding was observed with multiple antimitotic compounds. In particular, no inhibition occurred with vinblastine, and the best inhibitors of those examined were dolastatin 10 and cryptophycin 1. We compared the aberrant assembly reaction induced by DZA to those induced by other antimitotic peptides and depsipeptides, in particular dolastatin 10, cryptophycin 1, and hemiasterlin, but the results obtained varied considerably in terms of requirements for maximal reactions, polymer morphology, and inhibitory effects observed with antimitotic compounds.

Antineoplastic Agents. 607. Emetine Auristatins.

The remarkable biological activity of the dolastatin 10 structural modifications quinstatins and isoquinstatins prompted further investigation into drug hybrids containing biologically active isoquinoline moieties. In this study, the isoquinoline alkaloid emetine was selected as one of the structural domains of a hybrid molecule. That was accomplished by covalently bonding the Dov-Val-Dil-Dap peptide sequence of dolastatin 10 peptide at the N-2' secondary amine of emetine. Three new hybrids were synthesized, 5, 9, and 10. Comparison of the biological activity of these new peptide-emetine analogues with emetine showed complete retention of activity for 5 and a 10-fold decrease for hybrids 9 and 10. The result was surprising, as the activity of emetine is usually lost or greatly reduced when substituted at the N-2' position.

Antineoplastic Agents. 605. Isoquinstatins.

In order to further explore quinoline-type structural modification of the powerful anticancer drug dolastatin 10, an Indian Ocean sea hare constituent and parent molecule of the very successful antibody drug conjugate (ADC) Adcetris, our recent quinstatin study has been extended by replacing the quinoline ring with an isoquinoline. The resulting isoquinstatins (4-6) were modified to N-terminal desmethylisoquinstatins (7-9) and, in turn, bonded to appropriate linker units to give linker-desmethylisoquinstatin conjugates 11-13 in preparation for eventual monoclonal antibody attachment. Comparison of the new isoquinstatins with their quinstatin counterparts against six human cancer cell lines indicated the isoquinstatins to have GI50 values that were comparable to or somewhat higher than those of the isomeric quinstatins. However, desmethylisoquinstatin 5 (7) was significantly more potent than its desmethylquinstatin 5 analogue. When evaluated against quinstatin 8, its isoquinstatin 8 (6) counterpart was somewhat less potent. In general, the isoquinstatins evaluated proved to be quite strong cancer cell growth inhibitors.

Antineoplastic Agents. 606. The Betulastatins.

The medicinal potential of the plant pentacyclic triterpene betulin has generated long-term interest focused on various SAR research avenues. The present approach was based on producing further analogues (chimeras) arising from a studied modification of betulin bonded to the Dov-Val-Dil-Dap unit of the powerful anticancer drug dolastatin 10, which provided betulastatins 1 (7b), 2 (11b), 3 (16b), and 4 (18b). Betulastatin 1, 2, and 4 exhibited modest levels of cancer cell growth inhibition against six cancer cell lines. Betulastatin 3 proved to be the most potent cancer cell growth inhibitor (GI50 0.01 µg/mL) and seems worthy of further development, as the presumed mixture of anticancer mechanisms of action may prove to be useful.

Antineoplastic Agents. 604. The Path of Quinstatin Derivatives to Antibody Drug Conjugates.

To further evaluate the exceptional cancer cell growth inhibition by the quinstatins, of which one of the series, quinstatin 8, approaches the exceptional cytotoxic activity of the parent dolastatin 10 (1), four of the quinstatins have been converted to desmethyl derivatives. Three of the four (4, 5, and 8 [7b-d]) were next bonded to the linker (8) employed in the synthesis of the very successful and structurally related anticancer drug Adcetris (3). Owing to these structural modifications, a next step could be taken by bonding to a monoclonal antibody, thereby producing an antibody drug conjugate (ADC) related to Adcetris structurally but with the possibility of a wider spectrum of activity and utility.

Antineoplastic Agents. 603. Quinstatins: Exceptional Cancer Cell Growth Inhibitors.

Discovery of the exceptionally powerful anticancer drug dolastatin 10 (1), contained in the sea hare Dolabella auricularia, opened a new frontier needed for improving human cancer treatment. Subsequently, major advances have been achieved based on results of structurally modifying this unusual natural peptide while maintaining the remarkable anticancer activity necessary for preparation of successful monoclonal antibody drug conjugates (ADC). Among the first several hundred SAR products based on dolastatin 10 our group synthesized and termed auristatins was auristatin E (2a). An anticancer activity-equivalent, desmethylaurisatin E (2b), linked to a CD30 monoclonal antibody is the very successful anticancer drug Adcetris, now approved for use in 65 countries. In the present investigation, we discovered a new subset of auristatins designated quinstatins derived from dolastatin 10 by replacing the C-terminal Doe unit with a carefully designed quinoline, which led to low or subnanomolar levels of cancer cell growth inhibition required for construction of chemically unique ADC drugs. The synthesis of quinstatins 2-8 is presented along with their cancer cell line biological data.

Antineoplastic Agents. 585. Isolation of Bridelia ferruginea Anticancer Podophyllotoxins and Synthesis of 4-Aza-podophyllotoxin Structural Modifications.

Cytotoxic constituents of the terrestrial plant Bridelia ferruginea were isolated using bioactivity-guided fractionation, which revealed the presence of the previously known deoxypodophyllotoxin (1), isopicrodeoxypodophyllotoxin (2), ß-peltatin (3), ß-peltatin-5-O-ß-D-glucopyranoside (3a), and the indole neoechinulin (4). As an extension of previous podophyllotoxin research, SAR studies were undertaken focused on 4-aza-podophyllotoxin structural modifications. A number of such derivatives were synthesized following modifications to the A and E rings. Such structural modifications with alkyl and 4-fluorobenzyl substituents at the 4-aza position provided the most potent cancer cell growth inhibitory activity (GI50 0.1 to <0.03 µg/mL) against a panel of six human cancer cell lines and one murine cancer cell line. Several compounds corresponding to 4'-demethylated modifications were also synthesized and found to be significantly less potent.

Isolation and Structure of Cancer Cell Growth Inhibitory Tetracyclic Triterpenoids from the Zimbabwean Monadenium lugardae.

The Zimbabwean medicinal plant Monadenium lugardae was evaluated as a potential source of new anticancer constituents. Four new tetracyclic triterpene (1-4) were isolated, accompanied by four previously known triterpenes (5-8). Against a panel of human tumor cell lines, lugardstatins 1 (1) and 2 (2) had good cancer cell growth inhibitory activity. All of the triterpene structures (1-8) were established by 1D and 2D NMR spectrometric and HR mass spectrometric analysis.

The Cephalostatins. 24. Isolation, Structure, and Cancer Cell Growth Inhibition of Cephalostatin 20.

For the purpose of advancing knowledge of the structural variations available in the natural cephalostatins contained in the marine worm Cephalodiscus gilchristi, the isolation and structure of the 20th member (1) has been accomplished (10(-7) % yield). In turn cephalostatin 20 (1) proved to be enough for an initial SAR study comprising six important human cancer cell lines. A parallel objective was aimed at the possible discovery of a natural cephalostatin with a more accessible structure for total synthesis and/or synthetic modifications, but with powerful cancer cell growth inhibition.

Antineoplastic agents. 600. From the South Pacific Ocean to the silstatins.

The recent advances in the development of antibody and other drug conjugates for targeted cancer treatment have further increased the need for powerful cancer cell growth inhibitors. Toward that objective we have extended our earlier discovery of the remarkable anticancer bacillistatins 1 and 2 from Bacillus silvestris to SAR and other structural modifications such as availability of a free hydroxy group for antibody-drug conjugate (ADC) and other prodrug linkage. That direction has resulted in seven structural modifications designated silstatins 1-8 (7a, 8a, 8b, 14a, 15a, 15b, 18a, and 18b), where the exceptional cancer cell growth inhibition of some of them are in the range GI50 10(-3)-10(-4) µM/mL. Silstatin 7 (18a) was converted to a glucuronic conjugate (28) that displayed an impressive reduction in toxicity during transport.

The Cephalostatins. 23. Conversion of Hecogenin to a Steroidal 1,6-Dioxaspiro[5.5]nonane Analogue for Cephalostatin 11.

Cephalostatin 1 (1) has proved to be a remarkably potent cancer cell growth inhibitor. Since this steroidal alkaloid constituent of the marine worm Cephalodiscus gilchristi possesses a complex structure, providing preclinical supplies by total synthesis continues to be challenging. Therefore, syntheses of less complex structural modifications of this important pyrazine have also received substantial attention. Herein are summarized the synthesis of [5.5]spiroketal 5, a simplified right-side steroidal unit of 1, in seven steps from hecogenin acetate (11) with an overall yield of 4.6%. Consistent with other SAR studies, such reduction in structural complexity compared to 1 led to loss of cancer cell growth inhibitory activity against the P388 lymphocytic leukemia cell line.

Neristatin 1 provides critical insight into bryostatin 1 structure-function relationships.

Bryostatin 1, a complex macrocyclic lactone isolated from Bugula neritina, has been the subject of multiple clinical trials for cancer. Although it functions as an activator of protein kinase C (PKC) in vitro, bryostatin 1 paradoxically antagonizes most responses to the prototypical PKC activator, the phorbol esters. The bottom half of the bryostatin 1 structure has been shown to be sufficient to confer binding to PKC. In contrast, we have previously shown that the top half of the bryostatin 1 structure is necessary for its unique biological behavior to antagonize phorbol ester responses. Neristatin 1 comprises a top half similar to that of bryostatin 1 together with a distinct bottom half that confers PKC binding. We report here that neristatin 1 is bryostatin 1-like, not phorbol ester-like, in its biological activity on U937 promyelocytic leukemia cells. We conclude that the top half of the bryostatin 1 structure is largely sufficient for bryostatin 1-like activity, provided the molecule also possesses an appropriate PKC binding domain.

Antineoplastic agents. 599. Total synthesis of dolastatin 16.

The first 23-step total synthesis of the cyclodepsipeptide dolastatin 16 (1) has been achieved. Synthesis of the dolaphenvaline and dolamethylleuine amino acid units using simplified methods improved the overall efficiency. The formation of the 25-membered macrocycle employing lactonization with 2-methyl-6-nitrobenzoic anhydride completed a key step in the synthesis. Regrettably, the synthetic dolastatin 16 (1), while otherwise identical (by X-ray crystal structure and spectral analyses) with the natural product, did not reproduce the powerful (nanomolar) cancer cell growth inhibition displayed by the natural isolate. Presumably this result can be attributed to conformation(s) of the synthetic dolastatin 16 (1) or to a chemically undetected component isolated with the natural product.

Antineoplastic agents. 595. Structural modifications of betulin and the X-ray crystal structure of an unusual betulin amine dimer.

The lupane-type triterpene betulin (1) has been subjected to a series of structural modifications for the purpose of evaluating resultant cancer cell growth inhibitory activity. The reaction sequence 7→11→12 was especially noteworthy in providing a betulin-derived amine dimer. Other unexpected synthetic results included the 11 and 13/14→17 conversions, which yielded an imidazo derivative. X-ray crystal structures of dimer 12 and intermediate 25 are reported. All of the betulin modifications were examined for anticancer activity against the P388 murine and human cell lines. Significant cancer cell growth inhibition was found for 4, 8, 9, 15/16, 19, 20, 24, and 26, which further defines the utility of the betulin scaffold.

Isolation and structures of axistatins 1-3 from the Republic of Palau marine sponge Agelas axifera Hentschel .

An investigation begun in 1979 directed at the Republic of Palau marine sponge Agelas axifera Hentschel for cancer cell growth inhibitory constituents subsequently led to the isolation of three new pyrimidine diterpenes designated axistatins 1 (1), 2 (2), and 3 (3), together with the previously reported formamides 4, 5, and agelasine F (6). The structures were elucidated by analysis of 2D-NMR spectra and by HRMS. All of the isolated compounds were found to be moderate inhibitors of cancer cell growth. Axistatins 1-3 (1-3), formamide 4, and agelasine F (6) also exhibited antimicrobial activity.

Synthesis of a 2-aryl-3-aroyl indole salt (OXi8007) resembling combretastatin A-4 with application as a vascular disrupting agent.

The natural products colchicine and combretastatin A-4 are potent inhibitors of tubulin assembly, and they have inspired the design and synthesis of a large number of small-molecule, potential anticancer agents. The indole-based molecular scaffold is prominent among these SAR modifications, leading to a rapidly increasing number of agents. The water-soluble phosphate prodrug 33 (OXi8007) of 2-aryl-3-aroylindole-based phenol 8 (OXi8006) was prepared by chemical synthesis and found to be strongly cytotoxic against selected human cancer cell lines (GI50 = 36 nM against DU-145 cells, for example). The free phenol, 8 (OXi8006), was a strong inhibitor (IC50 = 1.1 µM) of tubulin assembly. The corresponding phosphate prodrug 33 (OXi8007) also demonstrated pronounced interference with tumor vasculature in a preliminary in vivo study utilizing a SCID mouse model bearing an orthotopic PC-3 (prostate) tumor as imaged by color Doppler ultrasound. The combination of these results provides evidence that the indole-based phosphate prodrug 33 (OXi8007) functions as a vascular disrupting agent that may prove useful for the treatment of cancer.

The cephalostatins. 22. synthesis of bis-steroidal pyrazine pyrones (1).

Cephalostatin 1 (1), a remarkably strong cancer cell growth inhibitory trisdecacyclic, bis-steroidal pyrazine isolated from the marine tube worm Cephalodiscus gilchristi, continues to be an important target for practical total syntheses and a model for the discovery of less complex structural modifications with promising antineoplastic activity. In the present study, the cephalostatin E and F rings were greatly simplified by replacement at C-17 with an α-pyrone (in 12), typical of the steroidal bufodienolides, and by a dihydro-γ-pyrone (in 16). The synthesis of pyrazine 12 from 5α-dihydrotestosterone (nine steps, 8% overall yield) provided the first route to a bis-bufadienolide pyrazine. Dihydro-γ-pyrone 16 was synthesized in eight steps from ketone 13. While only insignificant cancer cell growth inhibitory activity was found for pyrones 12 and 16, the results provided further support for the necessity of more closely approximating the natural D-F ring system of cephalostatin 1 in order to obtain potent antineoplastic activity.

Antineoplastic agents. 587. Isolation and structure of 3-epipancratistatin from Narcissus cv. Ice Follies.

Bioassay-guided (cancer cell line) separation of an extract prepared from Narcissus cv. Ice Follies (from The Netherlands) led to the isolation of a new Amaryllidaceae isocarbostiryl, 3-epipancratistatin (1b), as well as narciclasine (2). This Narcissus cultivar was found to be a good source of narciclasine. The structure of 1b was established by high-resolution mass and high-field 2D NMR spectroscopic analyses. Against a panel of murine and human cancer cell lines, 3-epipancratistatin (1b) led to cell growth inhibition (GI(50) 2.2-0.69 µg/mL) some 100× less than that found for pancratistatin (1a) and narciclasine (2), thereby revealing an important configurational requirement in 1a for strong cancer cell growth inhibition.