Synthesis and biological evaluation of cryptophycin analogs with substitution at C-6 (fragment C region)

Facile synthesis of 2-azaspiro[3.4]octane

Our annulation strategy utilized for the synthesis of 2-azaspiro[3.4]octane is explained. Three successful routes for the synthesis were developed. One of the approaches involved annulation of the cyclopentane ring and the remaining two approaches involved annulation of the four membered ring. All three approaches employ readily available starting materials with conventional chemical transformations and minimal chromatographic purifications to afford the title compound. The merits and limitations of the three approaches are also discussed.

Novel unit B cryptophycin analogues as payloads for targeted therapy

Cryptophycins are naturally occurring cytotoxins with great potential for chemotherapy. Since targeted therapy provides new perspectives for treatment of cancer, new potent analogues of cytotoxic agents containing functional groups for conjugation to homing devices are required. We describe the design, synthesis and biological evaluation of three new unit B cryptophycin analogues. The O-methyl group of the unit B D-tyrosine analogue was replaced by an O-(allyloxyethyl) moiety, an O-(hydroxyethyl) group, or an O-(((azidoethoxy)ethoxy)ethoyxethyl) substituent. While the former two maintain cytotoxicity in the subnanomolar range, the attachment of the triethylene glycol spacer with a terminal azide results in a complete loss of activity. Docking studies of the novel cryptophycin analogues to β-tubulin provided a rationale for the observed cytotoxicities.

Tubulin Inhibitor-Based Antibody-Drug Conjugates for Cancer Therapy

Antibody-drug conjugates (ADCs) are a class of highly potent biopharmaceutical drugs generated by conjugating cytotoxic drugs with specific monoclonal antibodies through appropriate linkers. Specific antibodies used to guide potent warheads to tumor tissues can effectively reduce undesired side effects of the cytotoxic drugs. An in-depth understanding of antibodies, linkers, conjugation strategies, cytotoxic drugs, and their molecular targets has led to the successful development of several approved ADCs. These ADCs are powerful therapeutics for cancer treatment, enabling wider therapeutic windows, improved pharmacokinetic/pharmacodynamic properties, and enhanced efficacy. Since tubulin inhibitors are one of the most successful cytotoxic drugs in the ADC armamentarium, this review focuses on the progress in tubulin inhibitor-based ADCs, as well as lessons learned from the unsuccessful ADCs containing tubulin inhibitors. This review should be helpful to facilitate future development of new generations of tubulin inhibitor-based ADCs for cancer therapy.

Cryptophycins: cytotoxic cyclodepsipeptides with potential for tumor targeting

Cryptophycins are a class of 16-membered highly cytotoxic macrocyclic depsipeptides isolated from cyanobacteria. The biological activity is based on their ability to interact with tubulin. They interfere with microtubule dynamics and prevent microtubules from forming correct mitotic spindles, which causes cell-cycle arrest and apoptosis. Their strong antiproliferative activities with 100-fold to 1000-fold potency compared with those of paclitaxel and vinblastine have been observed. Cryptophycins are highly promising drug candidates, as their biological activity is not negatively affected by P-glycoprotein, a drug efflux system commonly found in multidrug-resistant cancer cell lines and solid tumors. Cryptophycin-52 had been investigated in phase II clinical trials but failed because of its high neurotoxicity. Recently, cryptophycin conjugates with peptides and antibodies have been developed for targeted delivery in tumor therapy. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Conjugates of modified cryptophycins and RGD-peptides enter target cells by endocytosis

Tumor targeting anticancer drug conjugates that contain a tumor recognition motif (homing device) are of high current relevance. Cryptophycins, naturally occurring cytotoxic cyclo-depsipeptides, have been modified by total synthesis to provide analogues suitable for conjugation to peptide-based homing devices. An array of functionalized β(2)-amino acids was synthesized and incorporated into cryptophycins. All analogues proved to be highly active in the cytotoxicity assay using the human cervix carcinoma cell line KB-3-1 and its multidrug-resistant subclone KB-V1. Conformational analysis of cryptophycin-52 and two synthetic analogues was performed by NMR and MD methods to obtain information on the influence of the unit C configuration on the overall conformation. An azide-functionalized cryptophycin was connected by CuAAC to an alkyne-containing fluorescently labeled cyclic RGD-peptide as the homing device for internalization studies. Confocal fluorescence microscopy proved integrin-mediated internalization by endocytosis and final lysosomal localization of the cryptophycin prodrug.

Efficient, divergent synthesis of cryptophycin unit A analogues

A flexible and divergent synthesis of cryptophycin unit A analogues is described. This method relies on iridium-catalysed stereo- and enantioselective crotylation and chemoselective one-pot oxidative olefination to access common intermediate . Heck, cross metathesis, and Suzuki-Miyaura reactions are illustrated for the generation of methyl ester unit A analogues .

Carboxylate bioisosteres of gabapentin

A series of carboxylate bioisosteres of structures related to gabapentin 1 have been prepared. When the carboxylate was replaced by a tetrazole, this group was recognized by the alpha2-delta protein. Further characterization of alpha2-delta binding compounds 14a and 14b revealed a similar pattern of functional in vitro and in vivo activity to gabapentin 1.

Chemoenzymatic Synthesis of Cryptophycin/Arenastatin Natural Products

Microbially derived modular polyketide synthase and nonribosomal peptide synthetase biosynthetic pathways are a rich source of novel natural products. Development of these systems for the engineered biosynthesis of diverse secondary metabolites continues to progress as a robust source of chemical diversity. Recent efforts that employ individual enzymes and catalytic domains for the production or modification of small molecules have met with growing success. In this study, the thioesterase domain from the cryptophycin biosynthetic pathway was isolated and its function evaluated with a series of linear chain elongation intermediates in developing a novel chemoenzymatic synthesis of the cryptophycin/arenastatin class of antitumor agents. The results show the high efficiency of the thioesterase in generating the 16-membered depsipeptide ring of this important natural product system. Moreover, analysis of selected substrates revealed considerable tolerance for structural variation within the seco-cryptophycin unit C beta-alanine residue, but strict structural requirements at the phenyl group position of the unit A delta-hydroxy octadienoate chain elongation intermediates.

Efficient and Versatile Stereoselective Synthesis of Cryptophycins

The cryptophycins are a family of cyclic depsipeptides with four retrosynthetic units A to D which correspond to the respective amino acids and hydroxy acids. A new synthetic route to unit A allows the selective generation of all four stereogenic centres by introducing two of them in a catalytic asymmetric dihydroxylation, followed by substrate-controlled diastereoselective reactions. The diol also serves as the epoxide precursor. This approach provides selective access to stereoisomers of unit A (enantiomers, epimers) for structure-activity relationship studies. The unit A derivatives were incorporated into cryptophycin-1, cryptophycin-52 and a novel epimer of cryptophycin-52.

Biological evaluation of cryptophycin 52 fragment A analogues: Effect of the multidrug resistance ATP binding cassette transporters on antitumor activity

Cryptophycin 52 (LY355703) is a potent antiproliferative analogue of the marine natural product cryptophycin 1. It has been shown to have a broad range of antitumor activity against human tumor xenografts and murine tumors including tumors resistant to Taxol and Adriamycin. Its mechanism of action involves arresting cells in the G2-M phase of the cell cycle by binding to microtubules and suppressing their dynamics. This 16-membered depsipeptide can be divided into four major subunits or fragments (A–D). We reported previously on our synthetic efforts around fragment A and discovered that this region of the molecule was amenable to a structure-activity relationship study that resulted in highly active antiproliferative agents when evaluated in the CEM leukemia cell line. The synthetic analogues were designed to help improve the efficacy and aqueous solubility of the parent compound; therefore, many in this series contained ionizable functional groups such as an amino group, a hydroxy group, or a carboxylic acid. Although several of these analogues showed improvements in potency over cryptophycin 52 in drug-sensitive tumor xenograft models, many lost their activity against Adriamycin-resistant tumor lines. It was discovered on additional in vitro evaluation that these analogues became good substrates of the multidrug resistance transporter P-glycoprotein.

Asymmetric Syntheses of Potent Antitumor Macrolides Cryptophycin B and Arenastatin A

Efficient and highly stereoselective syntheses of cryptophycin B and arenastatin A, potent cytotoxic agents, are described. An ester-derived titanium enolate mediated syn-aldol reaction was employed to generate the stereocenters C-5 and C-6. The route is convergent and provides a convenient access to the synthesis of structural variants of cryptophycins as well as members of its family.

Synthesis of Cryptophycins via an N-Acyl-β-lactam Macrolactonization

An efficient and concise approach to the synthesis of the macrolide core of the cryptophycins has been developed. A novel macrolactonization utilizing a reactive acyl-beta-lactam intermediate incorporates the beta-amino acid moiety within the 16-membered macrolide core. This modular approach, involving a cyanide-initiated acyl-beta-lactam ring opening followed by cyclization, was successfully applied to the total synthesis of cryptophycin-24. The strategy was also used in an efficient synthesis of the 6,6-dimethyl-substituted dechlorocryptophycin-52. In this case, the cyanide-initiated ring opening of the bis-substituted 2-azetidinone followed by macrolactonization was achieved through a catalytic process.

A convergent approach to cryptophycin 52 analogues: synthesis and biological evaluation of a novel series of fragment a epoxides and chlorohydrins

Cryptophycin 52 is a synthetic derivative of Cryptophycin 1, a potent antimicrotubule agent isolated from cyanobacteria. In an effort to increase the potency and water solubility of the molecule, a structure-activity relationship study (SAR) was initiated around the phenyl ring of fragment A. These Cryptophycin 52 analogues were accessed using a Wittig olefination reaction between various triphenylphosphonium salts and a key intermediate aldehyde prepared from Cryptophycin 53. Substitution on the phenyl ring of fragment A was well tolerated, and several of these analogues were equally or more potent than Cryptophycin 52 when evaluated in vitro in the CCRF-CEM leukemia cell line and in vivo against a murine pancreatic adenocarcinoma.

Cryptophycin affinity labels: synthesis and biological activity of a benzophenone analogue of cryptophycin-24

An efficient synthesis of a C16 side chain benzophenone analogue of cryptophycin-24 using a crotylboration reaction and Heck coupling as key steps is described. In an in vitro tubulin assembly assay, the benzophenone analogue of the beta isomer (IC(50)=7.4 microM) is twice as active as cryptophycin-24 (IC(50)=15 microM).

The cryptophycins: their synthesis and anticancer activity

The cryptophycins are a unique family of 16-membered macrolide antimitotic agents isolated from the cyanobacteria Nostoc sp. Their molecular target is tubulin protein wherein they are the most potent known stabilizers of microtubule dynamics and depolymerize microtubules at higher concentrations. They also deactivate the Bcl2 protein and produce apoptotic response much more quickly and at considerably lower concentrations than clinically utilized compounds. The presence of several amide and ester linkages within the cryptophycin core provides access to very convergent total synthetic approaches. Likewise, the modularity of the structure renders their synthesis amenable to structure-activity studies in several regions of the molecule. The in vivo hydrolytic instability of the C5 ester was a key obstacle to the successful identification of a clinical candidate. This problem was ameliorated by increased substitution at C6 as in the presence of gem-dimethyl substitution in the clinical candidate, cryptophycin-52.

Rapid entry into the cryptophycin core via an acyl-beta-lactam macrolactonization: total synthesis of cryptophycin-24

[see structure]. An efficient, concise approach to the macrolide core of the cryptophycins, potent antimitotic agents, has been achieved. The reaction sequence features a novel macrolactonization utilizing a reactive acyl-beta-lactam intermediate that incorporates the beta-amino acid moiety within the 16-membered macrolide core. This highly modular approach, which allows for multiple alterations throughout the structure, was successfully applied to the total synthesis of cryptophycin-24.

Total synthesis of cryptophycin-24 (Arenastatin A) amenable to structural modifications in the C16 side chain

Two efficient protocols for the synthesis of tert-butyl (5S,6R,2E, 7E)-5-[(tert-butyldimethylsilyl)oxy]-6-methyl-8-phenyl-2, 7-octadienoate, a major component of the cryptophycins, are reported. The first utilized the Noyori reduction and Frater alkylation of methyl 5-benzyloxy-3-oxopentanoate to set two stereogenic centers, which became the C16 hydroxyl and C1' methyl of the cryptophycins. The second approach started from 3-p-methoxybenzyloxypropanal and a crotyl borane reagent derived from (-)-alpha-pinene to set both stereocenters in a single step and provided the dephenyl analogue, tert-butyl (5S,6R,2E)-5-[(tert-butyldimethylsilyl)oxy]-6-methyl-2, 7-octadienoate, in five steps. This compound was readily converted to the 8-phenyl compound via Heck coupling. The silanyloxy esters were efficiently deprotected and coupled to the C2-C10 amino acid fragment to provide desepoxyarenastatin A and its dephenyl analogue. The terminal olefin of the latter was further elaborated via Heck coupling. Epoxidation provided cryptophycin-24 (arenastatin A).

Synthesis and biological property of carba and 20-deoxo analogues of arenastatin A

The carba analogue, in which a methylene group is substituted for the oxygen atom linked to C-15, and 20-deoxo analogue of arenastatin A, a potent cytotoxic spongean depsipeptide, were synthesized. Both analogues lacking the 15,20-ester function, which was easily metabolized in serum, showed good stability in serum as well as moderate cytotoxic activity against KB cells and better solubility.

Synthesis of cryptophycin 52 using the sharpless asymmetric dihydroxylation: diol to epoxide transformation optimized for a base-sensitive substrate

A synthesis of cryptophycin 52 (2) is reported using a Sharpless asymmetric dihydroxylation (AD) strategy to install the epoxide moiety. The high stereoselectivity of the AD reaction that allows for an efficient means of preparing the epoxide is in contrast to the standard direct epoxidation of cryptophycin substrates, which proceeds with poor diastereoselectivity. Methodology for conversion of the diol AD product to the requisite epoxide is disclosed. The transformation has been optimized to proceed in high yield in the presence of base sensitive functionality.