Synthesis of a Next-Generation Taxoid by Rapid Methylation Amenable for 11C-Labeling.

Next-generation taxoids, such as SB-T-1214, are highly potent cytotoxic agents that exhibit remarkable efficacy against drug-resistant tumors in vivo, including those that overexpress the P-glycoprotein (Pgp) efflux pump. As SB-T-1214 is not a substrate for Pgp-mediated efflux, it may exhibit a markedly different biodistribution and tumor-accumulation profile than paclitaxel or docetaxel, which are both Pgp substrates. To investigate the biodistribution and tumor-accumulation levels of SB-T-1214 using positron emission tomography (PET), a new synthetic route has been developed to allow the incorporation of 11C, a commonly employed positron-emitting radionucleide, via methyl iodide at the last step of chemical synthesis. This synthetic route features a highly stereoselective chiral ester enolate-imine cyclocondensation, regioselective hydrostannation of the resulting ß-lactam, and the Stille coupling of the novel vinylstannyl taxoid intermediate with methyl iodide. Conditions have been established to allow the rapid methylation and HPLC purification of the target compound in a time frame amenable to 11C-labeling for applications to PET studies.

Design, synthesis and biological evaluation of a highly-potent and cancer cell selective folate-taxoid conjugate.

The folate receptor (FR) has been widely recognized as an excellent target for the tumor-selective delivery of cytotoxic agents, and four folate-drug conjugates have entered clinical evaluations for the treatment of solid tumors to date. However, most of these conjugates required structural modification of the cytotoxic warheads in order to achieve efficient drug release from the linkers. We designed and constructed a novel folate conjugate of a highly-potent next-generation taxoid, SB-T-1214, by exploiting bioorthogonal Cu-free 'click' chemistry. The synthesis was highly convergent and required no HPLC purification to obtain the final folate-taxoid conjugate 1. Conjugate 1 demonstrated highly FR-specific potency (IC50 2.1-3.5 nM) against a panel of cancer cell lines, with a >1000-fold decrease in cytotoxicity against normal human cells (IC50>5000 nM). The remarkable potency and selectivity of conjugate 1 can be attributed to highly FR-specific receptor-mediated endocytosis as well as efficient release of the unmodified cytotoxic warhead using a mechanism-based self-immolative linker.

Design, Synthesis and Application of Fluorine-Labeled Taxoids as 19F NMR Probes for the Metabolic Stability Assessment of Tumor-Targeted Drug Delivery Systems.

Novel tumor-targeting drug conjugates, BLT-F2 (1) and BLT-S-F6 (2), bearing a fluorotaxoid as the warhead, a mechanism-based self-immolative disulfide linker, and biotin as the tumor-targeting module, were designed and synthesized as 19F NMR probes. Fluorine atoms and CF3 groups were strategically incorporated into the conjugates to investigate the mechanism of linker cleavage and factors that influence their plasma and metabolic stability by real-time monitoring with 19F NMR. Time-resolved 19F NMR study on probe 1 disclosed a stepwise mechanism for release of a fluorotaxoid, which might not have been detected by other analytical methods. Probe 2 was designed to bear two CF3 groups in the taxoid moiety as "3-FAB" reporters for enhanced sensitivity and a polyethylene glycol oligomer insert to improve solubility. The clean analysis of the linker stability and reactivity of drug conjugates in blood plasma or cell culture media by HPLC and 1H NMR is troublesome, due to the overlap of key signals/peaks with background arising from highly complex ingredients in biological systems. Accordingly, the use of 19F NMR would provide a practical solution to this problem. In fact, our "3-FAB" probe 2 was proven to be highly useful to investigate the stability and reactivity of the self-immolative disulfide linker system in human blood plasma by 19F NMR. It has also been revealed that the use of polysorbate 80 as excipient for the formulation of probe 2 dramatically increases the stability of the disulfide linker system. This finding further indicates that the tumor-targeting drug conjugates with polysorbate 80/EtOH/saline formulation for in vivo studies would have high stability in blood plasma, while the drug release in cancer cells proceeds smoothly.

Fluorine-Containing Taxoid Anticancer Agents and Their Tumor-Targeted Drug Delivery.

A long-standing problem of conventional chemotherapy is the lack of tumor-specific treatments. Traditional chemotherapy relies on the premise that rapidly proliferating cancer cells are more likely to be killed by a cytotoxic agent. In reality, however, cytotoxic agents have very little or no specificity, which leads to systemic toxicity, causing undesirable severe side effects. Consequently, various "molecularly targeted cancer therapies" have been developed for use in specific cancers, including tumor-targeting drug delivery systems. In general, such a drug delivery system consists of a tumor recognition moiety and a cytotoxic "warhead" connected through a "smart" linker to form a conjugate. When a multi-functionalized nanomaterial is used as the vehicle, a "Trojan Horse" approach can be used for mass delivery of cytotoxic "warheads" to maximize the efficacy. Exploitation of the special properties of fluorine has proven successful in the development of new and effective biochemical tools as well as therapeutic agents. Fluorinated congeners can also serve as excellent probes for the investigation of biochemical mechanisms. 19F-NMR can provide unique and powerful tools for mechanistic investigations in chemical biology. This account presents our recent progress, in perspective, on the molecular approaches to the design and development of novel tumor-targeted drug delivery systems for new generation chemotherapy by exploiting the unique nature of fluorine.

Synthesis and Biological Evaluation of Novel 3'-Difluorovinyl Taxoids.

A series of 3'-difluorovinyl taxoids with C10 modifications, as well as those with C2 and C10 modifications, were strategically designed to block the metabolism by cytochrome P-450 3A4 enzyme and synthesized. These novel difluorovinyl taxoids were evaluated for their cytotoxicity against drug-sensitive human breast (MCF7), multidrug-resistant (MDR) human ovarian (NCI/ADR), human colon (HT-29) and human pancreatic (PANC-1) cancer cell lines. 3'-Difluorovinyl taxoids exhibit several to 16 times better activity against MCF7, HT-29 and PANC-1 cell lines and up to three orders of magnitude higher potency against NCI/ADR cell line as compared to paclitaxel. Structure-activity relationship study shows the critical importance of the C2 modifications on the activity against MDR cancer cell line, while the C10 modifications have a rather minor effect on the potency with some exceptions. The effect of the C2 modifications on potency against MCF7 cell line increases in the following order: H < F < Cl

Differing Mechanisms of Death Induction by Fluorinated Taxane SB-T-12854 in Breast Cancer Cells.

BACKGROUND/AIM: Classical taxanes are routinely used in cancer therapy. In this study, mechanisms involved in death induction by the novel fluorine-containing taxane SB-T-12854 were investigated. MATERIALS AND METHODS: We employed breast cancer SK-BR-3, MCF-7 and T47D cell lines to assess activation of individual caspases, changes in the expression of proteins of the Bcl-2 family, and the release of pro-apoptotic factors from mitochondria into the cytosol after SB-T-12854 treatment. RESULTS: Caspase-2, -8, and -9 were activated in SK-BR-3 and MCF-7 cells. Only caspase-8 was activated in T47D cells. Caspase-7 and -6 were activated in all tested cells while caspase-3 was activated only in SK-BR-3 cells. Pro-apoptotic Bad protein seems to be important for cell death induction in all tested cells. Anti-apoptotic Bcl-2 and pro-apoptotic Bim, Bok, Bid and Bik seem to be also associated with cell death induction in some of the tested cells. The mitochondrial apoptotic pathway was significantly activated in association with the release of cytochrome c and Smac from mitochondria, but only in SK-BR-3 cells, not in MCF-7 and T47D cells. CONCLUSION: Cell death induced by SB-T-12854, in the tested breast cancer cells, differs regarding activation of caspases, changes in levels of pro-apoptotic and anti-apoptotic proteins of the Bcl-2 family and activation of the mitochondrial apoptotic pathway.

Poly(2-oxazoline) based micelles with high capacity for 3rd generation taxoids: preparation, in vitro and in vivo evaluation.

The clinically and commercially successful taxanes, paclitaxel and docetaxel suffer from two major drawbacks, namely their very low aqueous solubility and the risk of developing resistance. Here, we present a method that overcomes both drawbacks in a very simple manner. We formulated 3rd generation taxoids, able to avoid common drug resistance mechanisms with doubly amphiphilic poly(2-oxazoline)s (POx), a safe and highly efficient polymer for the formulation of extremely hydrophobic drugs. We found excellent solubilization of different 3rd generation taxoids irrespective of the drug's chemical structures with essentially quantitative drug loading and final drug to polymer ratios around unity. The small, highly loaded micelles with a hydrodynamic diameter of less than 100nm are excellently suited for parenteral administration. Moreover, a selected formulation with the taxoid SB-T-1214 is about one to two orders of magnitude more active in vitro than paclitaxel in the multidrug resistant breast cancer cell line LCC6-MDR. In contrast, in wild-type LCC6, no difference was observed. Using a q4d×4 dosing regimen, we also found that POx/SB-T-1214 significantly inhibits the growth of LCC6-MDR orthotropic tumors, outperforming commercial paclitaxel drug Taxol and Cremophor EL formulated SB-T-1214.

Design, synthesis, and biological evaluations of tumor-targeting dual-warhead conjugates for a taxoid-camptothecin combination chemotherapy.

Novel tumor-targeting dual-warhead conjugates, 2 (DW-1) and 3 (DW-2), which consist of a next-generation taxoid, 1 (SB-T-1214), and camptothecin as two warheads, self-immolative disulfide linkers for drug release, biotin as the tumor-targeting moiety, and 1,3,5-triazine as the tripod splitter module, were designed and synthesized. The potency of 2 was evaluated against MX-1, MCF-7, ID8, L1210FR (BR+, biotin receptor overexpressed) and WI38 (BR-, normal) cell lines in the absence and presence of glutathione (GSH), which is an endogenous thiol that triggers drug release inside the cancer cells. With the GSH and resuspension protocol, 2 exhibited IC50 values of 3.22-9.80 nM against all BR+ cancer cell lines, and 705 nM against WI38. Thus, there was a two orders of magnitude higher selectivity to cancer cells. Also, a clear cooperative effect was observed for the taxoid-camptothecin combination when two drugs were delivered to the cancer cells specifically in the form of a dual-warhead conjugate.

Tumor-targeting drug delivery of new-generation taxoids.

A long-standing problem of conventional cancer chemotherapy is the lack of tumor specificity. Tumor-targeting drug-delivery systems have been explored to overcome this problem. These systems combine a powerful cytotoxic anticancer agent with a tumor-targeting molecule via a 'smart' linker to form highly efficacious drug conjugates. These drug conjugates can deliver potent cytotoxic drugs specifically to tumors and tumor cells with minimal systemic toxicity. This review describes our groups' research on the molecular approaches to the design and development of a novel drug-delivery system bearing highly potent new-generation taxoids for tumor-targeting chemotherapy in our laboratory.