Current Medicinal Chemistry - Volume 10, Issue 8, 2003

Small conductance calcium-activated potassium (SK) channels are found in many types of neurons as well as in some other cell types. These channels are selective for K+ and open when intracellular Ca2+ rises to ∼500 nM. In neurons, this occurs during and after an action potential. Activation of SK channels hyperpolarizes the membrane, thus reducing cell excitability for several tens or hundreds of milliseconds. This phenomenon is called a afterhyperpolarization (AHP). Three subtypes of SK channels (SK1, SK2, SK3) have been cloned and exhibit a differential localization in the brain.SK channels may play a role in physiological and pathological conditions. They may be involved in the control of memory and cognition. Moreover, they are heavily expressed in the basal ganglia (in particular in the substantia nigra, pars compacta) and in the limbic system, suggesting that they may modulate motricity and emotional behaviour.Based on these facts, SK channel subtypes may be a suitable target for developing novel therapeutic agents, but more work is needed to validate these targets. Hence, there is a great need for selective ligands. Moreover, although the risk of peripheral side-effects for SK channel modulators appears to be low, some questions remain to be investigated.Currently, different molecules are known as SK channel modulators. Apamin is a very potent peptidic agent, it produces a strong blockade of these targets which is only very slowly reversible and it has limited selectivity. Dequalinium was found to be an effective blocker. Different chemical modulations on the dequalinium structure led to the discovery of highly potent bis-quinolinium derivatives such as UCL 1684. Other bis-(2- amino-benzimidazole) derivatives are in development. On the other hand, quaternary salts of bicuculline were reported to be effective in inhibiting AHPs. More recent developments on structurally-related molecules revealed that methyl-laudanosine is a new interesting tool for exploring SK channel pharmacology. Finally, a family of compounds has been shown to facilitate SK channel opening. Such compounds may be useful in treating disorders involving neuronal hyperexcitability.

Large-conductance Ca2+-activated K+ (BK-Ca) channels differ from most of other K+ channels in that their activation is under dual control, i.e., activated by either increase in intracellular Ca2+ or membrane depolarization. These channels, which are widely distributed in a variety of cells, can control Ca2+ influx as well as a number of Ca2+-dependent physiological processes. In neurons or neuroendocrine cells, BK-Ca channels are believed to play an important role in controlling hormonal secretion by altering the duration and frequency of action potentials. The activity of BK-Ca channels functionally expressed in vascular endothelial cells can control K+ efflux and affect intracellular Ca2+ concentration. Experimental observations have revealed that a variety of compounds can directly modulate BK-Ca channel activity. Epoxyeicosatrienoic acids, a metabolite of arachidonic acid, and the increase in intracellular cyclic GMP with vinpocetine or YC-1 can stimulate BK-Ca channel activity. The increased activity of BK-Ca channels thus serves as a negative feedback mechanism to limit Ca2+ influx in excitable cells. Clotrimazole, an imidazole P-450 inhibitor used for the management of sickle cell anemia, can directly suppress BK-Ca channel activity. Riluzole, a drug used for the treatment of amyotrophic lateral sclerosis, can directly enhance channel activity in neuroendocrine cells. This effect may explain its inhibitory action on excitatory neurotransmission. 2-Methoxyestradil, an endogenous metabolite of 17β-estradiol, suppresses BK-Ca channel activity, whereas resveratrol, a natural phytoalexin present in grapes and wine, directly stimulates BK-Ca channel activity in vascular endothelial cells. These effects may be responsible for their actions on functional activities of endothelial cells. The fenamates, a family of nonsteroidal anti-inflammatory drugs, are also openers of BK-Ca channels. Therefore, the modulation of BK-Ca channel activity in excitable and non-excitable cells can be important for therapeutic interventions.

Microwaves are used in medical applications, so their eventual toxicity effects must be carefully evaluated. An integral toxicity test, based on the monitoring of the respiratory activity of yeast cells, is proposed to evaluate the damage from microwave exposure. Different exposure times and microwave powers were considered. On supposing that the damages occur at enzymatic levels, the inhibiting effects of microwave exposure on two enzymes (glucose oxidase (GOD) and superoxide dismutase (SOD)), assumed like models as present in the human organism, was evaluated.

While photoaffinity ligands have been widely used to probe the structures of many receptors and nucleic acid binding proteins, their effective use in the study of cytochrome P450 structure is less established. Nevertheless, significant advances in this field have been made since the technique was first applied to P450cam in 1979. In several cases, especially studies involving P450s of the 1A and 2B families, peptides covalently modified with photoaffinity ligands have been isolated and characterized. Some of these peptides were predicted by molecular modeling to line substrate binding regions of the enzymes. Other data obtained from such studies were more difficult to reconcile with theory. This review addresses the status of photoaffinity labeling as a tool for studying cytochrome P450 structure. In addition, potential future directions in this field are discussed, including the development of heme-directed agents and validation of their effectiveness as photoaffinity ligands using sperm whale myoglobin as a test protein. The potential for hydroxyaromatic compounds to serve as photoactivated probes of active site nucleophiles is also discussed. This class of compounds and its derivatives has long been known in the fields of photochemistry and photophysics to be precursors of reactive radicals and quinone methides that are likely to serve as effective active site probes of the P450s.

The more profound understanding of the genetics and molecular pathways driving human tumorigensis is paralleled by an ongoing interest to translate this knowledge into development of cancer biomarkers, termed molecular tumor markers. The molecular changes observed in tumors frequently constitute early events which are detectable as signatures of malignancy in body fluids and their occurrence may preceede clinical cancer diagnosis. Thus, beyond applicability on tissue samples, molecular markers for tumor signatures should allow noninvasive or minimally invasive diagnosis in blood and / or other body fluid samples. However, to qualify as a clinically useful molecular tumor marker for initial diagnosis and detection of recurrent disease, a molecular tumor marker must have better test characteristics (sensitivity, specifity) than currently applied tumor markers. A molecular tumor marker should also be suitable for screening purposes by defining the subset of individuals for which definite cancer diagnosis by more invasive and/or expensive additional investigations is indicated. In addition, there is a demand for molecular tumor markers to be used as reliable surrogate endpoints in cancer prevention trials. Recommondation for the use of individual molecular tumor markers within evidence-based medicine criteria should ideally be derived from their overall efficacy to reduce tumor-specific mortality. This review focuses on DNA based, RNA based and proteomics based molecular methods of noninvasive cancer detection in bodily fluids and asseses the value of these methods for the current and future clinical management of cancer patients.

The dramatically increasing number of compounds that become available for biological evaluation presents a significant challenge for database design, management, and mining. Computational approaches for screening, profiling, or filtering of large compound collections are by now widely used in pharmaceutical research. Among popular compound classification and database mining techniques, partitioning methods are computationally very efficient and particularly suitable for the analysis of increasingly large molecular databases, as they do not depend on pair-wise comparisons of compounds to assess molecular similarity or diversity. Promising applications of partitioning algorithms include diversity selection, searching for compounds with desired biological activity, or the derivation of predictive models from screening datasets. Compound partitioning is introduced here in the context of virtual screening and different partitioning methods are discussed that operate in low-dimensional or other chemical descriptor spaces, including a number of practical drug-discovery-related applications.