Enantiodivergent, biocatalytic routes to both taxol side chain antipodes

[Reaction: see text]. Two enantiocomplementary bakers' yeast enzymes reduced an alpha-chloro-beta-keto ester to yield precursors for both enantiomers of the N-benzoyl phenylisoserine Taxol side chain. After base-mediated ring closure of the chlorohydrin enantiomers, the epoxides were converted directly to the oxazoline form of the target molecules using a Ritter reaction with benzonitrile. These were hydrolyzed to the ethyl ester form of the Taxol side chain enantiomers under acidic conditions. This brief and atom-efficient route to both target enantiomers demonstrates both the synthetic utility of individual yeast reductases and the power of genomic strategies in making these catalysts available.

Synthetic Studies on Taxol: Highly Stereoselective Construction of the Taxol C-Ring via SN2‘ Reduction of an Allylic Phosphonium Salt

[reaction: see text] The highly stereoselective construction of the C3 stereogenic center of the taxol C-ring is described. The trans isomer at the C3-C8 position of the taxol C-ring, which is required for the total synthesis, as well as its diastereomeric cis isomer were successfully synthesized with highly diastereoselective S(N)2' reduction of the allylic phosphonium salts.

Design, synthesis, and bioactivity of simplified paclitaxel analogs based on the T-Taxol bioactive conformation

A strategy for the design and synthesis of simplified paclitaxel analogs based on the T-Taxol conformation is presented. The resulting compounds have both cytotoxic and tubulin polymerization activities, although less so than those of paclitaxel itself.

Synthesis and Biological Evaluation ofN-(Arylsulfanyl)carbonyl Analogues of Paclitaxel (Taxol)

Four new N-(arylsufanyl)carbonyl paclitaxel analogues (2a-d) were prepared from 7-(triethylsilyl)-protected baccatin III (5). Their cytotoxicities against human ovarian (A2780) and prostate cancer (PC3) cell lines, as well as their tubulin-assembly activities, were determined. In these assays, the new compounds showed rather weak activities, one two orders of magnitude below those of paclitaxel (taxol; 1). The known 3'-N-[(thiophen-2-yl)carbonyl] paclitaxel analogue 3 was also prepared. As previously reported, 3 exhibited strongly improved cytotoxicities and tubulin-assembly activities as compared to paclitaxel (1).

Highly Diastereoselective Enolate Addition of O-Protected α-Hydroxyacetate to (SR)-tert-Butanesulfinylimines: Synthesis of Taxol Side Chain

The taxol side chain (S(R),2R,3S)-N-tert-butanesulfinyl-O-Boc-3-phenylisoserine benzyl ester 4c was synthesized through a lithium enolate addition of O-Boc-alpha-hydroxyacetate benzyl ester 5c to benzylidene (S(R))-tert-butanesulfinamide 6a in excellent yield and diastereoselectivity. By similar approach, a series of enantiopure 3-substituted isoserine benzyl esters 4 useful for the semi-syntheses of taxol derivatives were also prepared in high to excellent yields and diastereoselectivities. The diastereoselective addition mechanism was discussed on the basis of the experimental observation.

Synthesis and interactions of 7-deoxy-, 10-deacetoxy, and 10-deacetoxy-7-deoxypaclitaxel with NCI/ADR-RES cancer cells and bovine brain microvessel endothelial cells

7-Deoxypaclitaxel, 10-deacetoxypaclitaxel and 10-deacetoxy-7-deoxypaclitaxel were prepared and evaluated for their ability to promote assembly of tubulin into microtubules, their cytotoxicity against NCI/ADR-RES cells and for their interactions with P-glycoprotein in bovine brain microvessel endothelial cells. The three compounds were essentially equivalent to paclitaxel in cytotoxicity against NCI/ADR-RES cells. They also appeared to interact with P-glycoprotein in the endothelial cells with the two 10-deacetoxy compounds having less interaction than paclitaxel and 7-deoxypaclitaxel.

A facile route to paclitaxel C-10 carbamates

A general protocol for the synthesis of paclitaxel C-10 carbamates is described. The method entails MeI-mediated activation of 2'-O-TBS-7-O-TES-10-O-deacetyl-paclitaxel-10-O-carbonylimidazole prior to reaction with amines. This method is effective for the synthesis of paclitaxel C-10 derivatives, including bifunctional molecules.

A Fullerene−Paclitaxel Chemotherapeutic: Synthesis, Characterization, and Study of Biological Activity in Tissue Culture

A fullerene-paclitaxel conjugate has been synthesized as a slow-release drug for aerosol liposome delivery of paclitaxel for lung cancer therapy. The conjugate was designed to release paclitaxel via enzymatic hydrolysis and subsequently has shown a half-life of release of 80 min in bovine plasma. A liposome formulation of the conjugate has been prepared using dilauroylphosphatidylcholine (DLPC), and its IC50 is virtually identical to the IC50 for a paclitaxel-DLPC formulation in human epithelial lung carcinoma A549 cells. With both clinically relevant kinetics of hydrolysis and significant cytotoxicity in tissue culture, the conjugate holds promise for enhanced therapeutic efficacy of paclitaxel in vivo.

Cytotoxic activity of some natural and synthetic guaianolides

Several natural guaianolides and synthetic derivatives of repin (1) were tested and found to be active against tumor cell replication. Repin (1) and both mono- and di-halohydrin analogues (2, 7-9, 11, 12) showed significant antitumor potency. A more effective compound (17) was obtained by esterificating repin with the paclitaxel side chain.

Preparation and evaluation of new brominated paclitaxel analogues

Two diastereomers of 2'',3''-dibromo-7-epi-10-deacetylcephalomannine, 4 and 5, have been synthesized, purified and identified for evaluation as antitumour drugs. The cytotoxicity of the two diastereomers, assessed in cell culture against MCF-7 breast cancer, A549 lung cancer and A2780 ovarian cancer, was slightly stronger than that of paclitaxel. The cytotoxicity of 5 outweighs that of 4. In the light of the difference in cytotoxicity between the two diastereomers, we can assume that the differing configurations of C-2'' and C-3'' of the two diastereomers may result in different bioactive conformations in solution and, consequently, different biologically relevant conformations for binding to tubulin/microtubules -- a matter we are studying further.

Cellular recognition of paclitaxel-loaded polymeric nanoparticles composed of poly(gamma-benzyl L-glutamate) and poly(ethylene glycol) diblock copolymer endcapped with galactose moiety

Poly(gamma-benzyl L-glutamate) (PBLG)/poly(ethylene glycol) (PEG) diblock copolymer endcapped with galactose moiety (abbreviated as GEG) was synthesized and characterized for study of liver-specific targeting. From dynamic light scattering measurement, particle sizes of copolymeric nanoparticles were decreased with an increase of PEG in the copolymer. The morphology of GEG-3 nanoparticles observed by transmission electron micrograph was observed as almost spherical shapes and ranged about 50-300 nm. From the structural characterization using 1H nuclear magnetic resonance, both characteristic peaks of PBLG and PEG were visible in CDCl3 but the characteristic peaks of PBLG were invisible in D2O, indicating that GEG block copolymers are found to the core-shell type nanoparticles in water with PBLG innercore and PEG outershell, exposing that galactose moiety of GEG block copolymers are outerwards oriented on the nanoparticle surfaces. By galactose-specific aggregation test of particles using beta-galactose specific lectin, and flow cytometry measurement, specific interaction between asialoglycoprotein receptors (ASGPR) of HepG2, human hepatoma cell line, and galactose moieties of the GEG nanoparticles was confirmed. From cell cytotoxicity test, HepG2 cells with ASGPR are more sensitive to paclitaxel (TX)-loaded nanoparticles than free TX whereas, P388 cells, murine leukemia cell line, and SK-Hep 01, human hepatoma cell line, without ASGPR is less sensitive to TX-loaded nanoparticles than free TX, suggesting that specific interaction between HepG2 cells and galactose moiety of the nanoparticles occurred.

Chemoenzymatic Taxanes Approach Using Both Enantiomers of the Same Building Block. 2. Taxol CD Ring Unit

[reaction: see text] The enantioselective synthesis of the Taxol CD ring unit has been achieved starting from an enantiopure building block, the enantiomer of those previously utilized in the synthesis of the A ring unit. The key features of the present synthesis are astute use of both enantiomers of the same building block and complete control in the construction of five consecutive chiral centers.

Concise means for accessing an advanced precursor to 1-deoxypaclitaxel

A program directed toward a total synthesis of 1-deoxypaclitaxel is described. The most direct route consists of six steps from the previously described diketone 12 and proceeds in 18% overall yield. The transformations involved in reaching the target molecule 22 consist of stereoselective alpha-ketol generation, an EtAlCl(2)-catalyzed transannular hydride shift, regioselective monomesylation, and a Wagner-Meerwein 1,2-shift. The central issue of this synthesis is the sequential deployment of these highly controlled steps along the perimeter and across the interior gap of a nine-membered ring.

Delivery Platform for Hydrophobic Drugs: Prodrug Approach Combined with Self-Assembled Multilayers

We report the design of a platform for the delivery of hydrophobic drugs via a macromolecular prodrug approach combined with LbL-assembled polyelectrolyte multilayers. A hyaluronan ester prodrug of the chemotherapeutic drug paclitaxel has been synthesized. Conjugation of the drug to hyaluronan through a labile succinate ester did not inhibit its activity. Using quartz crystal microbalance, atomic force microscopy, and UV spectroscopy, we have shown that the presence of the hydrophobic paclitaxel moieties does not prohibit the layer-by-layer construction of the multilayers. Release of the drug from the paclitaxel-loaded multilayers upon hydrolysis of the ester linkage resulted in a drastic cell death. Application of this delivery platform to substrates such as colloids, biomedical implants, or vascular tissues may lead to new therapeutic strategies.

Synthesis and anti-tubulin activity of a 3'-(4-azidophenyl)-3'-dephenylpaclitaxel photoaffinity probe

The synthesis and biological evaluation of a novel paclitaxel photoaffinity probe is described. The synthesis involved the preparation of an azide-containing C13 side chain through a Staudinger cycloaddition followed by a lipase-mediated kinetic resolution to obtain the azetidinone in 99% ee. Coupling of the enantiopure side chain precursor to 7-TES-baccatin III and subsequent silyl ether deprotection afforded 3'-(4-azidophenyl)-3'-dephenylpaclitaxel, which was shown to be as active as paclitaxel in tubulin assembly and cytotoxicity assays.

Chemical Modification of Paclitaxel (Taxol) Reduces P-Glycoprotein Interactions and Increases Permeation across the Blood−Brain Barrier in Vitro and in Situ

The purpose of this work was to introduce a chemical modification into the paclitaxel (Taxol) structure to reduce interactions with the product of the multidrug resistant type 1 (MDR1) gene, P-glycoprotein (Pgp), resulting in improved blood-brain barrier (BBB) permeability. Specifically, a taxane analogue, Tx-67, with a succinate group added at the C10 position of Taxol, was synthesized and identified as such a candidate. In comparison studies, Tx-67 had no apparent interactions with Pgp, as demonstrated by the lack of enhanced uptake of rhodamine 123 by brain microvessel endothelial cells (BMECs) in the presence of the agent. By contrast, Taxol exposure substantially enhanced rhodamine 123 uptake by BMECs through inhibition of Pgp. The transport across BMEC monolayers was polarized for both Tx-67 and Taxol with permeation in the apical to basolateral direction greater for Tx-67 and substantially reduced for Taxol relative to basolateral to apical permeation. Taxol and cyclosporin A treatments also did not enhance Tx-67 permeation across BMEC monolayers. In an in situ rat brain perfusion study, Tx-67 was demonstrated to permeate across the BBB at a greater rate than Taxol. These results demonstrate that the Taxol analogue Tx-67 had a reduced interaction with Pgp and, as a consequence, enhanced permeation across the blood-brain barrier in vitro and in situ.

Prodrogues glucuronylées du paclitaxel (Taxol®) activables au niveau des tumeurs

Three glucuronyl prodrugs of paclitaxel have been synthesized in order to be activated by the B-glucuronidase present within the necrotic areas of tumors. As three compartments containing prodrugs, they include a specifier, a self immolative spacer and the drug. In vitro testing clearly indicates that the two former prodrugs are stable and are more or less detoxified. As expected, in the presence of E. coli B-glucuronidase, the glycosidic linkage is hydrolyzed with a rate depending on the nature of the spacer but, once this hydrolysis has occurred, the self immolative spacer is soon eliminated leading to the liberation of the paclitaxel.

Preparation and Characterization of Water-Soluble Prodrug, Liposomes and Micelles of Paclitaxel

Alternative formulations of paclitaxel were developed in order to improve its aqueous solubility, and characterized in vitro. A methacrylic acid based nanoconjugate of paclitaxel was synthesized by a simple esterification reaction with molecular weight of 1657 Da. The in vitro hydrolysis study on the prodrug of paclitaxel in presence of rat plasma has shown that the ester bond was quite stable (less than 1% of paclitaxel was liberated from prodrug in 24 h). This water-soluble prodrug was encapsulated into polyethylene glycol coated liposomes optimized with saturated lipids, to overcome the physical instability associated with paclitaxel. Under in vitro testing, prodrug liposomes seem very impressive with release of only 45% of payload in 180 h. Further, chemical as well as physical stability studies have shown that liposomes were stable without any signs of crystallization of paclitaxel. In addition, paclitaxel was covalently coupled to poloxamer via methacrylic acid linker to obtain a micelle forming conjugate. Evidence for self-assembly of this conjugate into micelles was provided by fluorescence spectroscopy, light scattering and differential scanning calorimetry techniques. Micellization of the conjugate was thermodynamically favored and the core of resulting micelles exhibited higher microviscosities (than poloxamer micelles). Release studies using dialysis technique along with high performance liquid chromatography revealed that paclitaxel is liberated from micelle in the form of methacrylic acid oligomer based prodrug in a gradual manner. These preliminary studies provided indication on the performance and feasibility of testing these carrier systems as a safer alternative to the Cremophor EL based paclitaxel formulation.

Synthesis and biological evaluation of a paclitaxel immunoconjugate

Targeted cancer therapy is a promising strategy for the treatment of this disease. In this approach, a cytotoxic agent (CA), such as a drug or a radionuclide, is attached, usually covalently, to a "targeting" vehicle (TV), which in turn is capable of recognizing specific receptor motifs on the surface of the tumor cells. Once administered systemically, the construct would localize on the tumor through the TV moiety and would release the CA cargo, resulting in the destruction of the malignant tissue. Small-molecule peptides as well as monoclonal antibodies have been used as TVs. The synthesis, antigen binding, and cytotoxicity of a covalent conjugate of the anticancer drug paclitaxel (taxol) to the anti-epidermal growth factor receptor (EGFR) monoclonal antibody C225 (IMC-C225; Erbitux, ImClone Systems, Somerville, NJ) are described in this chapter to illustrate the methods used for the construction and in vitro evaluation of these conjugates.

Syntheses and bioactivities of macrocyclic paclitaxel bis-lactones

Five macrocyclic paclitaxel bis-lactones and their corresponding open chain taxoids were synthesized as models of the tubulin-binding conformation of paclitaxel. Macrocyclic lactones with a 19-21-membered ring underwent isomerization to form smaller rings. The lactones were evaluated for cytotoxicity and tubulin-polymerization ability. All five macrocyclic paclitaxel lactones were active, but less so than paclitaxel, while the rearranged macrocyclic lactones and the corresponding open-chain taxoids were much less active or inactive.

Tumor specific novel taxoid-monoclonal antibody conjugates

The basic principle of the targeted delivery approach is that the conjugation of a drug to a tumor-specific molecule renders the drug inactive until it reaches the target site. Monoclonal antibodies (mAbs), which have shown high binding specificity for tumor-specific antigens, could be used as targeting agents. Paclitaxel has brought significant impact on the current cancer chemotherapy, but seriously suffers from the lack of tumor specificity. A series of paclitaxel-monoclonal antibody conjugates via C-2' ester linkage were reported. Taking into account the fact that the cytotoxicity of paclitaxel is not good enough and thus not applicable to this target delivery prodrug approach, new taxoids bearing methyldisulfanyl(alkanoyl) groups were designed, synthesized, and their activities evaluated. A highly cytotoxic C-10 methyldisulfanyl-propanoyl taxoid, was conjugated to monoclonal antibodies recognizing the epidermal growth factor receptor (EGFR). These conjugates were shown to possess remarkable target-specific antitumor activity in vivo against EGFR-expressing A431 tumor xenografts in SCID mice, resulting in complete inhibition of tumor growth in all the treated mice without any noticeable toxicity to the animals.

Synthesis, modeling, and anti-tubulin activity of a D-seco paclitaxel analogue

[reaction: see text] We have previously described a model of paclitaxel-microtubule binding that led to the prediction that analogues of paclitaxel lacking any D ring could stabilize microtubules as well as paclitaxel if the substituent present at C4 did not have unfavorable steric interactions with the binding pocket. We report the synthesis of a 4-methyl paclitaxel analogue, compound 1, which bears this prediction out. Compound 1 is as potent as paclitaxel at microtubule stabilization in vitro; however, it has only about one-four-hundredth the cytotoxicity of paclitaxel.

Synthesis, physico-chemical and biological characterization of a paclitaxel macromolecular prodrug

Paclitaxel was attached to poly(hydroxyethylaspartamide) via a succinic spacer arm by a two-step protocol: (1) synthesis of 2'-O-succinyl-paclitaxel; (2) synthesis of PHEA-2'-O-succinyl-paclitaxel. The 2'-O-succinyl-paclitaxel derivative and the macromolecular conjugate were characterized by UV, IR, NMR and mass spectrometry analysis. The reaction yields were over 95% and the purity of products over 98%. Paclitaxel release and degradation from 2'-O-succinyl-paclitaxel occurred at a faster rate at pH 5.5 than 7.4. After 30 h of incubation at pH 5.5 and 7.4 the released free paclitaxel was about 40 and 20%, respectively. In plasma both drug release and degradation were found to occur at a higher rate than in buffer at pH 7.4 suggesting that an enzymatic mechanism could be involved. The paclitaxel release and degradation from PHEA-2'-O-succinyl-paclitaxel were negligible at pH 5.5 and 7.4 and very slow in plasma. Investigation carried out using murine myeloid cell line showed that the polymeric prodrug maintains partial pharmacological activity of paclitaxel. The DL50 of the conjugate (over 40 ng/ml) as compared to free paclitaxel (about 1 ng/ml) was correlated to the slow drug release. Finally a pharmacokinetic study carried out by intravenous inoculation of the macromolecular prodrug to mice demonstrated that the polymer conjugation modify dramatically the in vivo fate of the drug. The conjugate disappeared from the bloodstream much more quickly as compared to both free drug and naked polymer. Massive accumulation of bioconjugate in the liver (80% of the dose) was found to persist throughout 1 week.