Mini Reviews in Medicinal Chemistry - Volume 8, Issue 6, 2008

The tetracyclic indolo[3,2-b]quinoline ring system constitutes an important structural moiety in natural products exhibiting numerous biological activities. In particular, indolo [3, 2-b]quinoline, commonly known as linear quindoline is of particular interest, because of its rigid structure and scope of derivatization. Although the core linear quindoline skeleton shows little or no activity in several biological systems, introduction of a methyl group on the N-5 atom leading to cryptolepine induces remarkable activity against a broad spectrum of biological targets. A number of analogs of quindoline and cryptolepine have been synthesized, incorporating various functional groups on the core quindoline skeleton leading to improved biological activities. In this review, we describe various synthetic methodologies leading to the quindoline scaffold, the biological activities and the structure activity relationships (SAR) of quindoline derivatives toward different disease states to give a better picture of the importance of this moiety in medicinal chemistry.

The plasma membrane redox (PMR) system is important for cell metabolism and survival; it is also crucial for blood coagulation and thrombosis. This review will give an update on the PMR system, with a particular regard to platelets, and on the role of antioxidant vitamins belonging to this system.

Tamoxifen, a selective estrogen receptor modulator (SERM), has been used for many decades as the “gold standard” adjuvant treatment for patients with hormone-receptor-positive early breast cancer. This drug, when administered for 5 years, reduces the risk for recurrence, contralateral breast cancer (BC) and death. The optimal duration of tamoxifen in the adjuvant setting has not been established yet, but it has been demonstrated that 5 years are better than shorter treatment while it is still unclear if a prolongation of the treatment for more than 5 years is worthwhile. In the last few years, third generation aromatase inhibitors (AIs), either steroidal (exemestane) or non-steroidal (anastrozole, letrozole), have shown to be an effective alternative to tamoxifen in postmenopausal patients with BC regardless of its stage. These agents act by blocking the aromatase enzyme which converts androgens into estrogens. The goal of this article was to review the results of recent randomized trials comparing AIs to tamoxifen in postmenopausal women in the adjuvant setting. Two strategies have been utilized: a direct upfront comparison in which both tamoxifen and AIs were given for 5 years or an early switch in which AIs were administered after 2-3 years of tamoxifen for 3-2 years or vice versa. Both strategies have shown a superiority of AIs over tamoxifen and a different safety profile but, the optimal treatment modality has yet to be defined. Moreover, in an attempt to further reduce patients' risk of recurrence after the administration of tamoxifen for 5 years, three trials have evaluated the role of prolonging the adjuvant treatment with AIs for 5 more years in comparison to placebo (late switch). A significant improvement of disease-free survival and of overall survival in the subgroup of nodepositive patients, at least in one trial, has been observed with AIs. Despite these important results several unanswered questions remain and the results of ongoing trials will hopefully clarify some of them.

Saponins are plant glycosides with favorable anti-tumorigenic properties. Several saponins inhibit tumor cell growth by cell cycle arrest and apoptosis with IC50 values of up to 0.2 μM. We discuss diverse groups of saponins (dioscins, saikosaponins, julibrosides, soy saponins, ginseng saponins and avicins) investigated in relation to tumor therapy and focus on cellular and systemic mechanisms of tumor cell growth inhibition both in vitro and in vivo. The review also describes saponins in combination with conventional tumor treatment strategies, which result in improved therapeutic success. Some combinations of saponins and anti-tumorigenic drugs induce synergistic effects with potentiated growth inhibition.

An intermolecular disulfide bond serves as a thioredoxin-dependent redox-sensing switch for the regulation of the enzymatic activity of 3-mercaptopyruvate sulfurtransferase (MST, EC.2.8.1.2). A cysteine residue on the surface of each subunit was oxidized to form an intersubunit disulfide bond so as to decrease MST activity, and thioredoxin-specific conversion of a dimer to a monomer increased MST activity. Further, a low redox potential sulfenate was reversibly formed at a catalytic site cysteine so as to inhibit MST, and thioredoxin-dependent reduction of the sulfenate restored the MST activity. Concludingly, MST partly contributes to the maintenance of cellular redox homeostasis via exerting control over cysteine catabolism.

Toll-like receptor 9 has been the focus of considerable research attention for the ability to modulate its activity, and subsequent innate immune responses, through DNA-based immunotherapeutics. Nucleic acids are attractive as therapeutics for their low cost, chemical stability and ease of production. While the ability for TLR9 to be differentially regulated by nucleic acids of varying sequences and structures offers flexibility for immunotherapeutic design, it also necessitates a more comprehensive characterization of these agonists in terms of how these structural parameters correlate with the activation of unique cellular responses. Despite the utilization of TLR9 agonists in human trials these issues have not been adequately addressed. While a wealth of cell stimulation experiments demonstrate the preferential ability for nucleic acids which contain unmethylated cytosine-phosphate-guanine (CpG) motifs to initiate innate immune responses this has not been supported by binding investigations from which largely contradictory information has emerged with respect to the ability of TLR9 to bind nucleic acids in a sequence-dependent fashion. Recent models help to reconcile this apparent contradiction by suggesting that while TLR9 activation is specific for CpG-containing nucleic acids, the receptor binds, and is functionally influenced by, nucleic acids in a sequence-independent fashion. We have proposed a model in which the absolute concentration of nucleic acids modulates the sensitivity of the receptor in a sequence-dependent fashion while activation is specifically achieved by CpG-containing ligands. In this review we reconsider the literature from the perspective of this new appreciation of the functional complexity of TLR9 ligand binding and higher-order regulation with discussion of the implications for immunotherapeutic targeting of TLR9.

The mode of action of retinoids in relation to their activity in the adult central nervous system and the potential of synthetic retinoid analogues is reviewed. Investigation into the activity of such molecules will further our understanding of the retinoid pathway during nervous system development and in various neurological disease states.

Rho GTPases, which control processes such as cell proliferation and cytoskeleton remodeling, are often hyperexpressed in tumors. Several members, such as RhoA/B/C, must be isoprenylated to interact with their effectors. Statins, by inhibiting the synthesis of prenyl groups, may affect RhoA/B/C activity and represent a promising tool in anticancer therapy.

Endothelial Lipase (EL) is a newly identified member of the triacylglycerol lipase family. Recent studies suggested that EL may be an important determinant of HDL-metabolism and inflammation acting at the level of the vessel wall. The aim of this review is to summarize important facts derived from experimental approaches and from epidemiologic human studies to provide a comprehensive view on the role of EL in inflammation and atherogenesis as well as target for potential pharmaceutical interventions.

The Src family of non-transmembrane protein kinases is comprised of eleven homologous members in mammals. Together, these kinases regulate a wide variety of cellular processes including cell survival, proliferation, differentiation, and motility. One member of this family, Lck, plays a pivotal role in T-cell signaling. Inhibition of Lck with small molecules has significant potential for therapeutic immunosuppression and treatment of diseases such as rheumatoid arthritis and asthma. Critical for successful clinical use of any Lck inhibitor is high specificity for Lck as inhibition of other members of the Src kinase family may result in unwanted side effects. In this review we provide an overview of the various synthetic compounds currently under investigation as Lck-specific inhibitors. In addition we provide an analysis of the properties of these compounds that account for the specificity required for the inhibition of one of eleven highly similar kinases.