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

Natural-source payloads used in the conjugated drugs architecture for cancer therapy: Recent advances and future directions

Drug conjugates are obtained from tumor-located vectors connected to cytotoxic agents via linkers, which are designed to deliver hyper-toxic payloads directly to targeted cancer cells. These drug conjugates include antibody-drug conjugates (ADCs), peptide-drug conjugates (PDCs), small molecule-drug conjugates (SMDCs), nucleic acid aptamer-drug conjugates (ApDCs), and virus-like drug conjugate (VDCs), which show great therapeutic value in the clinic. Drug conjugates consist of a targeting carrier, a linker, and a payload. Payloads are key therapy components. Cytotoxic molecules and their derivatives derived from natural products are commonly used in the payload portion of conjugates. The ideal payload should have sufficient toxicity, stability, coupling sites, and the ability to be released under specific conditions to kill tumor cells. Microtubule protein inhibitors, DNA damage agents, and RNA inhibitors are common cytotoxic molecules. Among these conjugates, cytotoxic molecules of natural origin are summarized based on their mechanism of action, conformational relationships, and the discovery of new derivatives. This paper also mentions some cytotoxic molecules that have the potential to be payloads. It also summarizes the latest technologies and novel conjugates developed in recent years to overcome the shortcomings of ADCs, PDCs, SMDCs, ApDCs, and VDCs. In addition, this paper summarizes the clinical trials conducted on conjugates of these cytotoxic molecules over the last five years. It provides a reference for designing and developing safer and more efficient conjugates.

A Versatile Chemoenzymatic Synthesis for the Discovery of Potent Cryptophycin Analogs

The cryptophycins are a family of macrocyclic depsipeptide natural products that display exceptionally potent antiproliferative activity against drug-resistant cancers. Unique challenges facing the synthesis and derivatization of this complex group of molecules motivated us to investigate a chemoenzymatic synthesis designed to access new analogs for biological evaluation. The cryptophycin thioesterase (CrpTE) and the cryptophycin epoxidase (CrpE) are a versatile set of enzymes that catalyze macrocyclization and epoxidation of over 20 natural cryptophycin metabolites. Thus, we envisioned a drug development strategy involving their use as standalone biocatalysts for production of unnatural derivatives. Herein, we developed a scalable synthesis of 12 new unit A-B-C-D linear chain elongation intermediates containing heterocyclic aromatic groups as alternatives to the native unit A benzyl group. N-Acetyl cysteamine activated forms of each intermediate were assessed for conversion to macrocyclic products using wild type CrpTE, which demonstrated the exceptional flexibility of this enzyme. Semipreparative scale reactions were conducted for isolation and structural characterization of new cryptophycins. Each was then evaluated as a substrate for CrpE P450 and its ability to generate the epoxidized products from these substrates that possess altered electronics at the unit A styrenyl double bond position. Finally, biological evaluation of the new cryptophycins revealed a des-β-epoxy analog with low picomolar potency, previously limited to cryptophycins bearing epoxide functionality.

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.

The cryptophycins as potent payloads for antibody drug conjugates

The cryptophycins are a potent class of cytotoxic agents that were evaluated as antibody drug conjugate (ADC) payloads. Free cryptophycin analog 1 displayed cell activity an order of magnitude more potent than approved ADC payloads MMAE and DM1. This potency increase was also reflected in the activity of the cryptophycin ADCs, attached via a either cleavable or non-cleavable linker.

Synthetic Approach to the Psoracorylifols

Core analogs of the psoracorylifols were generated by a five-step route from 2,2-dimethyl-4-cyanobutanal.

Antibody-drug conjugates: an emerging concept in cancer therapy

Traditional cancer chemotherapy is often accompanied by systemic toxicity to the patient. Monoclonal antibodies against antigens on cancer cells offer an alternative tumor-selective treatment approach. However, most monoclonal antibodies are not sufficiently potent to be therapeutically active on their own. Antibody-drug conjugates (ADCs) use antibodies to deliver a potent cytotoxic compound selectively to tumor cells, thus improving the therapeutic index of chemotherapeutic agents. The recent approval of two ADCs, brentuximab vedotin and ado-trastuzumab emtansine, for cancer treatment has spurred tremendous research interest in this field. This Review touches upon the early efforts in the field, and describes how the lessons learned from the first-generation ADCs have led to improvements in every aspect of this technology, i.e., the antibody, the cytotoxic compound, and the linker connecting them, leading to the current successes. The design of ADCs currently in clinical development, and results from mechanistic studies and preclinical and clinical evaluation are discussed. Emerging technologies that seek to further advance this exciting area of research are also discussed.

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 .

Efficient synthesis of cryptophycin-52 and novel para-alkoxymethyl unit A analogues

Cryptophycins are a family of highly cytotoxic, cyclic depsipeptides. They display antitumour activity that is largely maintained for multi-drug-resistant tumour cells. Cryptophycins are composed of four building blocks (units A-D) that correspond to the respective amino and hydroxy acids. A new synthetic route to unit A allows the selective generation of all four stereogenic centres in a short, efficient and reliable synthesis and contributes to an easier and faster synthesis of cryptophycins. The first two stereogenic centres are introduced by a catalytic asymmetric dihydroxylation, whereas the remaining two stereogenic centres are introduced with substrate control of diastereoselectivity. The stereogenic diol function also serves as the epoxide precursor. The approach was used to synthesise the native unit A building block as well as three para-alkoxymethyl analogues from which cryptophycin-52 and three analogous cryptophycins were prepared. Macrocyclisation of the seco-depsipeptides was based on ring-closing metathesis.

Analysis of the cryptophycin P450 epoxidase reveals substrate tolerance and cooperativity

Cryptophycins are potent anticancer agents isolated from Nostoc sp. ATCC 53789 and Nostoc sp. GSV 224. The most potent natural cryptophycin analogues retain a beta-epoxide at the C2'-C3' position of the molecule. A P450 epoxidase encoded by c rpE recently identified from the cryptophycin gene cluster was shown to install this key functional group into cryptophycin-4 (Cr-4) to produce cryptophycin-2 (Cr-2) in a regio- and stereospecific manner. Here we report a detailed characterization of the CrpE epoxidase using an engineered maltose binding protein (MBP)-CrpE fusion. The substrate tolerance of the CrpE polypeptide was investigated with a series of structurally related cryptophycin analogues generated by chemoenzymatic synthesis. The enzyme specifically installed a beta-epoxide between C2' and C3' of cyclic cryptophycin analogues. The kcat/Km values of the enzyme were determined to provide further insights into the P450 epoxidase catalytic efficiency affected by substrate structural variation. Finally, binding analysis revealed cooperativity of MBP-CrpE toward natural and unnatural desepoxy cryptophycin substrates.

In vitro metabolism of the epoxide substructure of cryptophycins by cytosolic glutathione S-transferase: species differences and stereoselectivity

The enzyme kinetics of the glutathione (GSH) conjugation of cryptophycin 52 (C52, R-stereoisomer) and cryptophycin 53 (C53, S-stereoisomer) by cytosolic glutathione S-transferases (cGSTs) from human, rat, mouse, dog and monkey liver were studied. Vmax, Km, and CLint values for glutathione conjugation of C52 (R-stereoisomer) were 0.10 +/- 0.01 nmol min-1 mg-1, 3.24 +/- 0.23 microM, and (3.15 +/- 0.09) x 10(-2) ml min-1 mg-1, respectively, in human cytosol. Due to limited solubility relative to the Km, only CLint values were determined in rat ((7.76 +/- 0.10) x 10-2 ml min-1 mg-1) and mouse ((7.61 +/- 0.50) x 10(-2) ml min-1 mg-1) cytosol. Enzyme kinetic parameters could not be determined for C53 (S-stereoisomer). Microsomal GSH conjugation in human, rat, and mouse was attributed to cytosolic contamination. No GSH conjugation was seen in any biological matrix from dog or monkey. There was little GSH conjugation of C53 by cytosol or microsomes from any species. The metabolism of C52 and C53 by epoxide hydrolase was also investigated. No diol product was observed in any biological matrix from any species. Thus, cGSTs are primarily responsible for C52 metabolism.

Enantiomeric separation and determination of absolute stereochemistry of asymmetric molecules in drug discovery—Building chiral technology toolboxes

The application of Chiral Technology, or the (extensive) use of techniques or tools for the determination of absolute stereochemistry and the enantiomeric or chiral separation of racemic small molecule potential lead compounds, has been critical to successfully discovering and developing chiral drugs in the pharmaceutical industry. This has been due to the rapid increase over the past 10-15 years in potential drug candidates containing one or more asymmetric centers. Based on the experiences of one pharmaceutical company, a summary of the establishment of a Chiral Technology toolbox, including the implementation of known tools as well as the design, development, and implementation of new Chiral Technology tools, is provided.

Biosynthetic characterization and chemoenzymatic assembly of the cryptophycins. Potent anticancer agents from cyanobionts

The lichen cyanobacterial symbiont Nostoc sp. ATCC 53789 and its close relative Nostoc sp. GSV 224 are prolific producers of natural products, generating >25 derivatives of the cryptophycin class of secondary metabolites. Cryptophycin 1, the prototypic member of the class, is a potent tubulin-depolymerizing agent, and several semisynthetic derivatives are being developed as anticancer therapeutics. Here we provide a detailed characterization of the cryptophycin metabolic pathway by stable-isotope labeling experiments and through cloning, sequencing, and annotating the cryptophycin biosynthetic gene cluster. A comparative secondary metabolomic analysis based on polyketide (PK)/non-ribosomal peptide gene clusters from the phylogenetically related, non-cryptophycin producing cycad symbiont, Nostoc punctiforme ATCC 29133, was used to identify the cryptophycin biosynthetic genes that encompass approximately 40 kb within the lichen symbiont Nostoc sp. ATCC 53789 genome. The pathway encodes a collinear set of enzymes, including three modular PK synthases, two non-ribosomal peptide synthetase modules, and an integrated adenylation/ketoreductase didomain for elaboration of the leucic acid subunit. In addition, genes encoding key tailoring steps, including a FAD-dependent halogenase and CYP450 epoxidase, were identified. The inherent flexibility of the cryptophycin biosynthetic enzymes was harnessed to generate a suite of new analogues by altering the pool of PK starter units and selected amino acid extender groups. Characterization of the cryptophycin CYP450 enabled development of the first stereospecific synthesis of cryptophycin 2, through a tandem chemoenzymatic synthesis from the natural seco-cryptophycin 4 chain elongation intermediate.

Synthesis of 15,20-triamide analogue with polar substituent on the phenyl ring of arenastatin A, an extremely potent cytotoxic spongean depsipeptide

In order to increase metabolic stability and water solubility of arenastatin A, an extremely potent cytotoxic depsipeptide from the Okinawan marine sponge of Dysidea arenaria, several 15,20-triamide analogues with a polar substituent on the phenyl ring were synthesized. The 15,20-triamide analogues with a polar substituent (24, 30, and 31) showed increased solubility to MeOH and stronger cytotoxicity against KB cells in comparison with the parental 15,20-triamide analogue (2). Furthermore, the diethylamine analogue (30) exhibited in vivo anti-tumor activity against subcutaneously implanted murine sarcoma.

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.