A study from the EORTC new drug development group: Open label phase II study of sabarubicin (MEN-10755) in patients with progressive hormone refractory prostate cancer

Sabarubicin

Disaccharide derivatives in the daunorubicin and in the 4-demethoxy (idarubicin) series in whichthe first sugar moiety linked to the aglycone is a non-aminated sugar, namely 2-deoxy-L-rhamnose or 2-deoxy-L-fucose andthe second moiety is daunosamine, have been obtained upon synthesis of the appropriate activated sugarintermediate and glycosylation of the corresponding aglycones. The compounds containing 2-deoxy-L-fucose exhibit superior pharmacological properties with respect to thestereoisomers containing 2-deoxy-L-rhamnose. The doxorubicinanalog 7-O-(α-L-daunosaminyl-α(1-4)-2-deoxy-L-fucosyl)-4-demethoxy-adriamycinone (sabarubicin) was prepared startingfrom 14-acetoxyidarubicinone. Solution properties and molecular interactions are compared with thoseof doxorubicin. Sabarubicin exhibits a superior antitumor efficacy, presumably related to theactivation of p53-independent apoptosis. A number of sabarubicin analogues have also been synthesized.

Anthracyclines as effective anticancer drugs

Anthracyclines have received significant attention due to their effectiveness and extensive use as anticancer agents. At present, the clinical use of these drugs is offset by drug resistance in tumours and cardiotoxicity. Therefore, a relentless search for the 'better anthracycline' has been ongoing since the inception of these drugs > 30 years ago. This review focuses on the most recent pharmacology and medicinal chemistry developments on the design and use of anthracyclines. Based on their crystal structures as well as molecular modelling, a more detailed mechanism of topoisomerase poisoning by these new anthracyclines has emerged. Chemical modifications of anthracyclines have been found to possibly change the target selectivity among various topoisomerases and, thus, vary their anticancer activity. Additionally, recent sugar modifications of anthracyclines have also been found to overcome P-glycoprotein-mediated drug resistance in cancer therapy. The continued improvement of anthracycline clinical applications so far and the clinical trials of the 'third generation' of anthracyclines (such as sabarubicin) are also discussed. To finally find the 'better' anthracycline, further areas of research still need to be explored such as: the elucidation of the topoisomerase and P-glycoprotein crystal structures, molecular modelling based on crystal structure in order to design the next generation of better anthracycline drugs, the continued modifications of the anthracycline sugar moieties, and the further improvement of anthracycline drug delivery methods.

Understanding the binding of daunorubicin and doxorubicin to NADPH-dependent cytosolic reductases by computational methods

The anthracycline anticancer agents daunorubicin (DAUN) and doxorubicin (DOX) are reduced by different NADPH-dependent cytosolic reductases into their corresponding alcohol metabolites daunorubicinol (DAUNol) and doxorubicinol (DOXol), which have been implicated in the development of chronic cardiomyopathy. To better understand the individual importance of each enzyme in the reduction and to provide deeper insight into the binding at atomic level we performed molecular docking and dynamics simulations of DAUN and DOX into the active sites of human carbonyl reductase 1 (CBR1) and human aldehyde reductase (AKR1A1). Such simulations evidenced a different behavior between the reductases with respect to DAUN and DOX suggesting major contribution of CBR1 in the reduction. The results are in agreement with available experimental data and for each enzyme and anthracycline pair provided the identification of key residues involved in the interactions. The structural models that we have derived could serve as a useful tool for structure-guided drug design studies.

An introduction to the metabolic determinants of anthracycline cardiotoxicity

Antitumor therapy with doxorubicin and other anthracyclines is limited by the possible development of cardiomyopathy upon chronic administration. Several lines of evidence suggest that a close link exists between cardiotoxicity and the amount of anthracycline that accumulates in the heart and then undergoes one- or two- electron reduction to toxic metabolites or by-products. Alternative metabolic pathways lead to an oxidative degradation of anthracyclines, possibly counteracting anthracycline accumulation and reductive bioactivation; unfortunately, however, the actual role of anthracycline oxidation is only partially characterized. Here, we briefly review the biochemical foundations of reductive versus oxidative anthracycline metabolism. We show that multiple links exist between one pathway of toxic biactivation and another, limiting the search and clinical development of "better anthracyclines" that retain antitumor activity but induce less cardiotoxicity than the available analogues.

Histone deacetylase inhibitors sensitize prostate cancer cells to agents that produce DNA double-strand breaks by targeting Ku70 acetylation

This study reports a histone deacetylation-independent mechanism whereby histone deacetylase (HDAC) inhibitors sensitize prostate cancer cells to DNA-damaging agents by targeting Ku70 acetylation. Ku70 represents a crucial component of the nonhomologous end joining repair machinery for DNA double-strand breaks (DSB). Our data indicate that pretreatment of prostate cancer cells with HDAC inhibitors (trichostatin A, suberoylanilide hydroxamic acid, MS-275, and OSU-HDAC42) led to increased Ku70 acetylation accompanied by reduced DNA-binding affinity without disrupting the Ku70/Ku80 heterodimer formation. As evidenced by increased Ser(139)-phosphorylated histone H2AX (gammaH2AX), impaired Ku70 function diminished cellular capability to repair DNA DSBs induced by bleomycin, doxorubicin, and etoposide, thereby enhancing their cell-killing effect. This sensitizing effect was most prominent when cells were treated with HDAC inhibitors and DNA-damaging agents sequentially. Mimicking acetylation was done by replacing K282, K317, K331, K338, K539, or K542 with glutamine via site-directed mutagenesis, which combined with computer docking analysis was used to analyze the role of these lysine residues in the interactions of Ku70 with DNA broken ends. Mutagenesis of K282, K338, K539, or K542 suppressed the activity of Ku70 to bind DNA, whereas mutagenesis of K317 or K331 with glutamine had no significant effect. Moreover, overexpression of K282Q or K338Q rendered DU-145 cells more susceptible to the effect of DNA-damaging agents on gammaH2AX formation and cell killing. Overall, the ability of HDAC inhibitors to regulate cellular ability to repair DNA damage by targeting Ku70 acetylation underlies the viability of their combination with DNA-damaging agents as a therapeutic strategy for prostate cancer.

Ongoing phase I and II studies of novel anthracyclines

Many anthracyclines are currently in clinical development with the common aim of improving selectivity. This could be achieved by improving tumor drug delivery through the identification and development of molecules with new structure, prodrugs with low molecular weight for selective release and activation, prodrugs with high molecular weight conjugated to antibody with active targeting or macromolecules with enhanced permeability and retention. There are still interfering factors to be defined, in particular chemical, with degradation steps in tumor tissues, biological, related to tumor proteases, pharmacological, with inter-individual tumor differences in the extent of accumulation. Another way to improve selectivity is to activate the drug at the tumor site, a good example of which is provided by Nemorubicin (2\"-(S)-methoxymorpholinodoxorubicin hydrochloride) in hepatocellular carcinoma. The favorable characteristics of Nemorubicin in terms of broad spectrum of significant antitumor activity in liver malignancies models, lower cardiotoxicity than Doxorubicin, make Nemorubicin a promising third-generation anthracycline, suitable for intrahepatic administration.

In vitro modeling of the structure–activity determinants of anthracycline cardiotoxicity

Doxorubicin and other anthracyclines rank among the most effective anticancer drugs ever developed. Unfortunately, the clinical use of anthracyclines is limited by a dose-related life-threatening cardiotoxicity. Understanding how anthracyclines induce cardiotoxicity is essential to improve their therapeutic index or to identify analogues that retain activity while also inducing less severe cardiac damage. Here, we briefly review the prevailing hypotheses on anthracycline-induced cardiotoxicity. We also attempt to establish cause-and-effect relations between the structure of a given anthracycline and its cardiotoxicity when administered as a single agent or during the course of multiagent chemotherapies. Finally, we discuss how the hypotheses generated by preclinical models eventually translate into phase I-II clinical trials.