A Multichannel Ca2+ Nanomodulator for Multilevel Mitochondrial Destruction-Mediated Cancer Therapy

Subcellular organelle-targeted nanoformulations for cancer theranostics are receiving increasing attention owing to their benefits of precise drug delivery, maximized therapeutic index, and reduced off-target side effects. Herein, a multichannel calcium ion (Ca2+ ) nanomodulator (CaNMCUR+CDDP ), i.e., a cisplatin (CDDP) and curcumin (CUR) co-incorporating calcium carbonate (CaCO3 ) nanoparticle, is prepared by a facile one-pot strategy in a sealed container with in situ synthesized polydopamine (PDA) as a template to enhance Ca2+ -overload-induced mitochondrial dysfunction in cancer therapy. After systemic administration, the PEGylated CaNMCUR+CDDP (PEG CaNMCUR+CDDP ) selectively accumulates in tumor tissues, enters tumor cells, and induces multilevel destruction of mitochondria by the combined effects of burst Ca2+ release, Ca2+ efflux inhibition by CUR, and chemotherapeutic CDDP, thereby observably boosting mitochondria-targeted tumor inhibition. Fluorescence imaging of CUR combined with photoacoustic imaging of PDA facilitates the visualization of the nanomodulator. The facile and practical design of this multichannel Ca2+ nanomodulator will contribute to the development of multimodal bioimaging-guided organelle-targeted cancer therapy in the future.

Structural Basis of the Modulation of the Voltage-Gated Calcium Ion Channel Cav 1.1 by Dihydropyridine Compounds*

1,4-Dihydropyridines (DHP), the most commonly used antihypertensives, function by inhibiting the L-type voltage-gated Ca2+ (Cav ) channels. DHP compounds exhibit chirality-specific antagonistic or agonistic effects. The structure of rabbit Cav 1.1 bound to an achiral drug nifedipine reveals the general binding mode for DHP drugs, but the molecular basis for chiral specificity remained elusive. Herein, we report five cryo-EM structures of nanodisc-embedded Cav 1.1 in the presence of the bestselling drug amlodipine, a DHP antagonist (R)-(+)-Bay K8644, and a titration of its agonistic enantiomer (S)-(-)-Bay K8644 at resolutions of 2.9-3.4 Å. The amlodipine-bound structure reveals the molecular basis for the high efficacy of the drug. All structures with the addition of the Bay K8644 enantiomers exhibit similar inactivated conformations, suggesting that (S)-(-)-Bay K8644, when acting as an agonist, is insufficient to lock the activated state of the channel for a prolonged duration.

Retrieving functional pathways of biomolecules from single-particle snapshots

A primary reason for the intense interest in structural biology is the fact that knowledge of structure can elucidate macromolecular functions in living organisms. Sustained effort has resulted in an impressive arsenal of tools for determining the static structures. But under physiological conditions, macromolecules undergo continuous conformational changes, a subset of which are functionally important. Techniques for capturing the continuous conformational changes underlying function are essential for further progress. Here, we present chemically-detailed conformational movies of biological function, extracted data-analytically from experimental single-particle cryo-electron microscopy (cryo-EM) snapshots of ryanodine receptor type 1 (RyR1), a calcium-activated calcium channel engaged in the binding of ligands. The functional motions differ substantially from those inferred from static structures in the nature of conformationally active structural domains, the sequence and extent of conformational motions, and the way allosteric signals are transduced within and between domains. Our approach highlights the importance of combining experiment, advanced data analysis, and molecular simulations.

Synthesis, Insecticidal Evaluation, and 3D-QASR of Novel Anthranilic Diamide Derivatives Containing N-Arylpyrrole as Potential Ryanodine Receptor Activators

To cope with the global food shortage and insect pest, there is an urgent need to discover new pesticides with novel modes of actions. Ryanodine receptor (RyR) insecticides showed great promise in integrated pest management. Herein, we report the synthesis of novel anthranilic diamide derivatives incorporating pyrrole moieties targeting at insect RyRs. The structures were confirmed by ¹H NMR, ¹³C NMR, ¹⁹F NMR, and high-resolution mass spectrometry. The preliminary bioassay results indicated that most of the title compounds showed good to excellent insecticidal activities against the oriental armyworm (Mythimna separata) and diamondback moth (Plutella xylostella). For the oriental armyworm, Ij displayed the same level of larvicidal activity as the positive control chlorantraniliprole, with an LC₅₀ value of 0.21 mg/L. For the diamondback moth, In, Io, Ip, and Iq exhibited higher insecticidal activities than chlorantraniliprole. In particular, In had 50% larvicidal activity at 0.00001 mg/L. The calcium imaging technique was applied to study the effect of Ij, In, and Ip on the intracellular calcium ion concentration ([Ca²⁺]_i) in central neurons isolated from the oriental armyworm. The results indicated that the tested compounds, such as chlorantraniliprole, could activate the insect RyRs. Furthermore, comparative molecular field analysis and density functional theory calculations were carried out to study the structure-activity relationship.

Differential Impact of Calcitriol and Its Analogs on Tumor Stroma in Young and Aged Ovariectomized Mice Bearing 4T1 Mammary Gland Cancer

(1) Background: Vitamin D compounds (VDC) are extensively studied in the field of anticancer properties, including breast cancer. Previously, we showed that calcitriol and its analogs (PRI-2191 and PRI-2205) stimulate metastasis in 4T1 murine mammary gland cancer models in young mice, whereas the reverse effect was observed in aged ovariectomized (OVX) mice; (2) Methods: We determined the phenotype of monocytes/macrophages using FACS and examined the expression of selected genes and proteins by Real-Time PCR and ELISA; (3) Results: Activities of VDC are accompanied by an increase in the percentage of Ly6Clow anti-inflammatory monocytes in the spleen of young and a decrease in aged OVX mice. Treatment of young mice with VDC resulted in an increase of CCL2 plasma and tumor concentration and Arg1 in tumor. In later stage of tumor progression the expression of genes related to metastasis in lung tissue was decreased or increased, in old OVX or young mice, respectively; (4) Conclusions: Pro- or anti-metastatic effects of calcitriol and its analogs in young or aged OVX mice, respectively, can be attributed to the differences in the effects of VDC on the tumor microenvironment, as a consequence of differences in the immunity status of young and aged mice.

Discovery of Potential Species-Specific Green Insecticides Targeting the Lepidopteran Ryanodine Receptor

Ryanodine receptors (RyRs) are homotetrameric intracellular calcium (Ca²⁺) release channels responsible for excitation-contraction coupling of muscle cells. Diamide insecticides specifically act on RyRs of Lepidoptera and Coleoptera pests and are safe for nontargeted organisms, generating big worldwide sales. Despite their popularity, several devastating agricultural pests have been reported to be resistant to them because of mutations in a small transmembrane region of their RyRs, hinting a binding pocket nearby. A potential solution to overcome resistance is to develop new insecticides targeting different binding sites in pest RyRs. Based on a high-resolution crystal structure of diamondback moth (DBM) RyR N-terminal domain (NTD) determined by our group, we carried out extensive structure-based insecticide screening targeting the intersubunit interface. We identified eight lead compounds that selectively target the open conformation of DBM RyR, which are predicted to act as channel activators similar to diamide insecticides. Binding mode analysis shows selective binding to a hydrophobic pocket of DBM NTD-A but not to the pocket of its mammalian counterpart. We tested three available compounds on the HEK293 cell lines stably expressing DBM or mammalian RyR, one of which shows good potency and selectivity against DBM RyR. The insecticidal effect of the compound was also confirmed using fruit flies. The detailed binding mode, toxicity, absorption, distribution, metabolism, and excretion, and reactivity of the compound were predicted by bioinformatic methods. Together, our study lays a foundation for developing a new class of selective RyR-targeting insecticides to control both wild-type and resistant pests.

Anti-schistosomal action of the calcium channel agonist FPL-64176

Subversion of parasite neuromuscular function is a key strategy for anthelmintic drug development. Schistosome Ca2+ signaling has been an area of particular interest for decades, with a specific focus on L-type voltage-gated Ca2+ channels (Cavs). However, the study of these channels has been technically challenging. One barrier is the lack of pharmacological probes that are active on flatworms, since the dihydropyridine (DHP) based ligands typically used to study Cavs are relatively ineffective on schistosomes. Here, we have characterized the effect of a structurally distinct putative L-type Cav agonist, FPL-64176, on schistosomes cultured ex vivo and in an in vivo murine model of infection. Unlike DHPs, FPL-64176 evokes rapid and sustained contractile paralysis of adult Schistosoma mansoni reminiscent of the anthelmintic praziquantel. This is accompanied by tegument disruption and an arrest of mitotic activity in somatic stem cells and germ line tissues. Interestingly, this strong ex vivo phenotype was temperature dependent, with FPL-64176 treatment being less potent at 37 °C than 23 °C. However, FPL-64176 caused intra-tegument lesions at the basement membrane of worms cultured ex vivo under both conditions, as well as an in vivo hepatic shift of parasites from the mesenteric vasculature of infected mice to the liver. Gene expression profiling of worms harvested following in vivo FPL-64176 exposure reveals differences in transcripts associated with muscle and extracellular matrix function, as well as female reproduction, which is consistent with the worm phenotypes observed following ex vivo drug treatment. These data advance FPL-64176 as a useful tool to study schistosome Ca2+ signaling, and the benzoyl pyrrole core as a hit compound that may be optimized to develop new parasite-selective leads.

Calcium-Activated Cl- Channel: Insights on the Molecular Identity in Epithelial Tissues

Calcium-activated chloride secretion in epithelial tissues has been described for many years. However, the molecular identity of the channel responsible for the Ca2+-activated Cl− secretion in epithelial tissues has remained a mystery. More recently, TMEM16A has been identified as a new putative Ca2+-activated Cl− channel (CaCC). The primary goal of this article will be to review the characterization of TMEM16A, as it relates to the physical structure of the channel, as well as important residues that confer voltage and Ca2+-sensitivity of the channel. This review will also discuss the role of TMEM16A in epithelial physiology and potential associated-pathophysiology. This will include discussion of developed knockout models that have provided much needed insight on the functional localization of TMEM16A in several epithelial tissues. Finally, this review will examine the implications of the identification of TMEM16A as it pertains to potential novel therapies in several pathologies.

Novel Anthranilic Diamide Scaffolds Containing N-Substituted Phenylpyrazole as Potential Ryanodine Receptor Activators

To discover potent insecticides targeting ryanodine receptors (RyRs), a series of novel anthranilic diamides analogues (12a-12u) containing N-substituted phenylpyrazole were designed and synthesized. These compounds were characterized by (1)H NMR, (13)C NMR, and HRMS, and the structure of compound 12u was confirmed by X-ray diffraction. Their insecticidal activities indicated that these compounds displayed moderate to excellent activities. In particular, 12i showed 100 and 37% larvicidal activities against oriental armyworm (Mythimna separata) at 0.25 and 0.05 mg L(-1), equivalent to that of chlorantraniliprole (100%, 0.25 mg L(-1); and 33%, 0.05 mg L(-1)). The activity of 12i against diamondback moth (Plutella xylostella) was 95% at 0.05 mg L(-1), whereas the control was 100% at 0.05 mg L(-1). The calcium-imaging technique experiment results showed that the effects of 12i on the intracellular calcium ion concentration ([Ca(2+)]i) in neurons were concentration-dependent. After the central neurons of Helicoverpa armigera were dyed by loading with fluo-5N and treated with 12i, the free calcium released in endoplasmic reticulum indicated the target of compound 12i is RyRs or IP3Rs. The activation of RyRs by natural ryanodine completely blocked the calcium release induced by 12i, which indicated that RyRs in the central neurons of H. armigera third-instar larvae is the possible target of compound 12i.

Chiral dicarboxamide scaffolds containing a sulfiliminyl moiety as potential ryanodine receptor activators

To search for new environmentally benign insecticides with high activity, low toxicity, and low residue, novel chiral configurations introduced into dicarboxamide scaffolds containing N-cyano sulfiliminyl moieties were first studied. Four series of phthalamides with sulfur-containing side chains were designed, synthesized, and evaluated against oriental armyworm (Pseudaletia separata Walker) and diamondback moth (Plutella xylostella (L.)) for their insecticidal activities. All structures were characterized by (1)H NMR, (13)C NMR, and HRMS (or elemental analysis), and their configurations were confirmed by optical polarimetry. The biological assessment indicated that some title compounds exhibited significant insecticidal activities. For oriental armyworm, these stereoisomers exerted different impacts on biological activity following the sequence (Sc, Ss) ≥ (Sc, Rs) ≫ (Rc, Ss) > (Rc, Rs), and carbon chirality influenced the activities more strongly than sulfur. Compounds Ia and IIa reached as high an activity as commercial flubendiamide, with LC50 values of 0.0504 and 0.0699 mg L(-1), respectively, lower than that of flubendiamide (0.1230 mg L(-1)). For diamondback moth, the sequence of activity was (Sc, Ss) > (Sc, Rs), and the sulfur chirality influenced the activities more greatly than carbon. Compound IIe exhibited even higher activity than flubendiamide, whereas Ie and Ic,d reached the activity of the latter. The results indicated that the improvement of insecticidal activity probably required a coordination of both carbon and sulfur chirality. Comparative molecular field analysis calculation indicated that stereoisomers with Sc configurations containing strong electron-withdrawing groups such as as CN are important in maintaining the high activity. The chiral scaffolds containing the N-cyano sulfiliminyl moiety are also essential for high larvicidal activity. Some title compounds could be considered as potential candidates for ryanodine receptor activators.

Eudistomin D and penaresin derivatives as modulators of ryanodine receptor channels and sarcoplasmic reticulum Ca2+ ATPase in striated muscle

Eudistomin D (EuD) and penaresin (Pen) derivatives are bioactive alkaloids from marine sponges found to induce Ca(2+) release from striated muscle sarcoplasmic reticulum (SR). Although these alkaloids are believed to affect ryanodine receptor (RyR) gating in a "caffeine-like" manner, no single-channel study confirmed this assumption. Here, EuD and MBED (9-methyl-7-bromoeudistomin D) were contrasted against caffeine on their ability to modulate the SR Ca(2+) loading/leak from cardiac and skeletal muscle SR microsomes as well as the function of RyRs in planar bilayers. The effects of these alkaloids on [(3)H]ryanodine binding and SR Ca(2+) ATPase (SERCA) activity were also tested. MBED (1-5 μM) fully mimicked maximal activating effects of caffeine (20 mM) on SR Ca(2+) leak. At the single-channel level, MBED mimicked the agonistic action of caffeine on cardiac RyR gating (i.e., stabilized long openings characteristic of "high-open-probability" mode). EuD was a partial agonist at the maximal doses tested. The tested Pen derivatives displayed mild to no agonism on RyRs, SR Ca(2+) leak, or [(3)H]ryanodine binding studies. Unlike caffeine, EuD and some Pen derivatives significantly inhibited SERCA at concentrations required to modulate RyRs. Instead, MBED's affinity for RyRs (EC50 ∼ 0.5 μM) was much larger than for SERCA (IC50 > 285 μM). In conclusion, MBED is a potent RyR agonist and, potentially, a better choice than caffeine for microsomal and cell studies due to its reported lack of effects on adenosine receptors and phosphodiesterases. As a high-affinity caffeine-like probe, MBED could also help identify the caffeine-binding site in RyRs.

Selective determinants of inositol 1,4,5-trisphosphate and adenophostin A interactions with type 1 inositol 1,4,5-trisphosphate receptors

BACKGROUND AND PURPOSE

Adenophostin A (AdA) is a potent agonist of inositol 1,4,5-trisphosphate receptors (IP(3) R). AdA shares with IP(3) the essential features of all IP(3) R agonists, namely structures equivalent to the 4,5-bisphosphate and 6-hydroxyl of IP(3) , but the basis of its increased affinity is unclear. Hitherto, the 2'-phosphate of AdA has been thought to provide a supra-optimal mimic of the 1-phosphate of IP(3) .

EXPERIMENTAL APPROACH

We examined the structural determinants of AdA binding to type 1 IP(3) R (IP(3) R1). Chemical synthesis and mutational analysis of IP(3) R1 were combined with (3) H-IP(3) binding to full-length IP(3) R1 and its N-terminal fragments, and Ca(2+) release assays from recombinant IP(3) R1 expressed in DT40 cells.

KEY RESULTS

Adenophostin A is at least 12-fold more potent than IP(3) in functional assays, and the IP(3) -binding core (IBC, residues 224-604 of IP(3) R1) is sufficient for this high-affinity binding of AdA. Removal of the 2'-phosphate from AdA (to give 2'-dephospho-AdA) had significantly lesser effects on its affinity for the IBC than did removal of the 1-phosphate from IP(3) (to give inositol 4,5-bisphosphate). Mutation of the only residue (R568) that interacts directly with the 1-phosphate of IP(3) decreased similarly (by ~30-fold) the affinity for IP(3) and AdA, but mutating R504, which has been proposed to form a cation-π interaction with the adenine of AdA, more profoundly reduced the affinity of IP(3) R for AdA (353-fold) than for IP(3) (13-fold).

CONCLUSIONS AND IMPLICATIONS

The 2'-phosphate of AdA is not a major determinant of its high affinity. R504 in the receptor, most likely via a cation-π interaction, contributes specifically to AdA binding.

3D-QSAR studies of insecticidal anthranilic diamides as ryanodine receptor activators using CoMFA, CoMSIA and DISCOtech

To explore the three-dimensional quantitative structure-activity relationships (3D-QSAR) and the pharmacophore model of a new class of potent activators of the anthranilic diamide ryanodine receptor (RyR), comparative molecular field analysis (CoMFA), comparative molecular similarity indices analysis (CoMSIA) and distance comparison technique (DISCOtech) were performed on 38 anthranilic diamides. Successful CoMFA and CoMSIA models yielded "leave-one-out" (LOO) cross-validated correlation coefficient (q(2)) values of 0.785 and 0.788 and non-cross-validated correlation coefficient (r(2)) values of 0.958 and 0.981, respectively. Results were graphically interpreted in terms of field contribution maps. A DISCOtech pharmacophore model containing an aromatic ring center, a hydrophobic ring center, a hydrogen bond-donor and a hydrogen bond-acceptor was constructed. This model indicated that hydrophobic interaction and hydrogen bonds have important roles in the interactions between activators and RyRs, which was consistent with CoMSIA results. The information obtained from CoMFA, CoMSIA and DISCOtech models enabled interpretation of the structure-activity relationships of anthranilic diamides. Based on the constructed models, some vital features for the interaction of anthranilic diamides with RyRs were identified, which may prove helpful in designing more potent RyR activators.

Pharmacological Regulators of Intracellular Calcium Release Channels

Intracellular Ca(2+) release channels, such as inositol 1,4,5-trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs), facilitate the release of Ca(2+) from intracellular storage organelles in response to extracellular and intracellular stimuli. Consequently, these large, tetrameric proteins play a central role in Ca(2+) signalling and Ca(2+) homeostasis in virtually all cells. Recent data suggests that intracellular Ca(2+) release channels may also have an important pathophysiological function in certain disease states, including cardiac arrhythmias and heart failure. As a result, there has been much interest in the identification and characterization of novel, selective regulators of these channels. In this article, we review the wide array of pharmacological agents that interact directly with intracellular Ca(2+) release channels and describe the mechanisms underlying their ability to modify channel function.

Effects of recombinant imperatoxin A (IpTxa) mutants on the rabbit ryanodine receptor

Imperatoxin A (IpTxa), a 3.7 kDa peptide from the African scorpion Pandinus imperator, is an agonist of the skeletal muscle ryanodine receptor (RyR1). In order to study the structure of the toxin and its effect on RyR1, IpTxa cDNA was PCR-amplified using 3 pairs of primers, and the toxin was expressed in E. coli. The toxin was further purified by chromatography, and various point mutants in which basic amino acids were substituted by alanine were prepared by site-directed mutagenesis. Studies of single channel properties by the planar lipid bilayer method showed that the recombinant IpTxa was identical to the synthetic IpTxa with respect to high-performance liquid chromatography mobility, amino acid composition and specific effects on RyR1. Mutations of certain basic amino acids (Lys19, Arg23, and Arg33) dramatically reduced the capacity of the peptide to activate RyRs. A subconductance state predominated when Lys8 was substituted with alanine. These results suggest that some basic amino acid residues in IpTxa are important for activation of RyR1, and that Lys8 plays an important role in regulating the gating mode of RyR1.

Manipulation of small-molecule inhibitory kinetics modulates MCH-R1 function

The capacity of novel benzopyridazinone-based antagonists to inhibit MCH-R1 function, relative to their affinity for the receptor, has been investigated. Three compounds that differ by the addition of either a chlorine atom, or trifluoromethyl group, have nearly identical receptor affinities; however their abilities to inhibit receptor elicited signaling events, measured as a function of time, are dramatically altered. Both the chlorinated and trifluoromethyl modified compounds have a very slow on-rate to maximal functional inhibition relative to the unmodified base compound. A similar impact on inhibitory capacity can be achieved by modifying the side-chain composition at position 2.53 of the receptor; replacement of the native phenylalanine with alanine significantly reduces the amount of time required by the chlorinated compound to attain maximal functional inhibition. The primary attribute responsible for this alteration in inhibitory capacity appears to be the overall bulk of the amino acid at this position-substitution of the similarly sized amino acids leucine and tyrosine results in phenotypes that are indistinguishable from the wild type receptor. Finally, the impact of these differential inhibitory kinetics has been examined in cultured rat neurons by measuring the ability of the compounds to reverse MCH mediated inhibition of calcium currents. As observed using the cell expression models, the chlorinated compound has a diminished capacity to interfere with receptor function. Collectively, these data suggest that differential inhibitory on rates between a small-molecule antagonist and its target receptor can impact the ability of the compound to modify the biological response(s) elicited by the receptor.

Using Dihydropyridine-Release Strategies to Enhance Load Effects in Engineered Human Bone Constructs

We report on the development of a novel biodegradable scaffold capable of enhancing mechanical signals for tissue-engineering applications. It has been shown that mechanotransduction enhances bone formation in vitro and in vivo; in tissue-engineering applications, this phenomenon is exploited through the use of mechanical bioreactors to generate bone tissue. The dihydropyridine agonist Bay K8644 (Bay) acts to increase the opening time of mechanosensitive voltage-operated calcium channels (VOCCs), specifi- cally L-type VOCCs, which are known to play a fundamental role in the early mediation of mechanotransduction. We have produced porous 3-dimensional, Bay-encapsulated biodegradable poly(L-lactide) acid scaffolds using a solvent-casting and salt-leaching technique. The effects of the released Bay on osteoid production and mineralization in human bone cell-seeded constructs following incubation in a perfusion-compression bioreactor in vitro was investigated using Western blotting techniques and a calcium assay protocol developed in our lab. Our newly developed scaffolds act by slowly releasing the calcium channel agonist Bay K8644 as observed using ultraviolet spectroscopy, maintaining the open state of mechanosensitive VOCCs responding to load, which augments the load signal at sites of strain across the scaffold. Our results demonstrate that, in the presence of physiological loading regimes in vitro, release of Bay enhances collagen I protein production and osteoid calcification more than non-Bay control constructs do. Osteopontin and alpha2delta1 VOCC subunit protein levels were also higher as a result of perfusion-compression conditioning. These results indicate that Bay-encapsulated scaffolds can be used in the presence of load to enhance the production of load-bearing engineered tissue.

Characterizing the efficacy of calcium channel agonist-release strategies for bone tissue engineering applications

We have previously reported on the use of Bay K8644-release strategies in combination with perfusion-compression bioreactor systems for up regulating bone formation in three-dimensional PLLA scaffolds. Here we report on the analysis of Bay activity following its release from our PLLA scaffolds over the culture period imposed in our tissue engineering protocol using UV spectroscopy in combination with whole cell patch clamping techniques. Bay was released continually from scaffolds within the physiological range required for agonist activity (1-10 microM). Patch clamping allowed for the effects of Bay released from scaffolds to be monitored directly with respect to osteoblast electrophysiology. A characteristic shift in the current-voltage (I-V) relationship of L-type VOCC currents was observed in rat osteoblast sarcoma (ROS) cells patched in a solution with Bay released from scaffolds following 14 and 28 days incubation, with statistically significant differences observed in peak currents compared to non-Bay controls. An increase in the magnitude of the peak inward currents was also noted. The electrophysiological response of osteoblasts in the presence of Bay released from scaffolds demonstrates that the released Bay is stable and maintains its bioactivity following culture of up to 28 days.

L-type calcium channels

The Ca2+ channel blockers represent a successful group of therapeutic agents directed against cardiovascular targets, including hypertension and angina. These drugs, including the first-generation verapamil, nifedipine and diltiazem are directed against a subclass of voltage-gated Ca2+ channel - the L-type channel. Other subclasses of Ca2+ channel exist and are targets for new indications. The mechanisms of actions of the L-type blockers are discussed and the origins of their cardiovascular selectivity discussed. Although new drugs of this class directed against hypertension could be developed, there are both clinical and economic reasons that argue against such development. However, there are other possible targets to investigate where antagonists and activators of the L-type channel may be useful: such targets include fertility, neuronal growth, bone formation and epilepsy. Limitations to these approaches are discussed.

Guanophostin A: Synthesis and evaluation of a high affinity agonist of the d-myo-inositol 1,4,5-trisphosphate receptor

Guanophostin A, the guanosine counterpart of the inositol 1,4,5-trisphosphate receptor agonist adenophostin A, has been synthesized and is the first synthetic adenophostin A-like analogue to be equipotent to its parent in stimulating intracellular Ca2+ release; its nucleotide moiety is proposed to interact with the receptor binding core by guanine base cation-pi stacking with Arg504 and hydrogen bonding with Glu505 and interaction of the ribosyl 2'-phosphate group with the helix-dipole of alpha6.

Investigation into the structure–activity relationship of novel concentration dependent, dual action T-type calcium channel agonists/antagonists

This paper describes the synthesis and biological evaluation of a series of straight chain analogs of a compound (1) that was previously synthesized in our research program. These compounds, which are T-type calcium channel antagonists, exhibits potent anti-proliferative activity against a variety of cancer cells. A structure-activity relationship of these analogs against a variety of cancer cells has provided insight into a logical pharmacophore for this series of compounds. Furthermore, this series of compounds has presented itself as a set of novel, concentration dependent, dual action agonists/antagonists for the T-type calcium channel.

Conformational and inframolecular studies of the protonation of adenophostin analogues lacking the adenine moiety

Four adenophostin analogues lacking the adenine moiety were subjected to 31P- and 1H-NMR titrations in order to determine the acid-base behaviour of the individual ionisable groups of the molecules and the complex interplay of intramolecular interactions resulting from the protonation process. For the two trisphosphorylated compounds, the curve pattern of the phosphorus nuclei corresponds to the superimposition of the titration curves of a monophosphorylated polyol and a polyol carrying two vicinal phosphates, suggesting that the two phosphate moieties behave independently. Also, the general shape of 1H-NMR titration curves of the studied compounds is very close to that of adenophostin A, indicating that the adenine moiety does not specifically interact with the phosphorylated sugar moieties. The curves show, however, that both trisphosphorylated compounds adopt slightly different preferential conformations which could contribute to explain the difference in their affinity for Ins(1,4,5)P3 receptor. Their macroscopic as well as the microscopic protonation constants are higher than those of adenophostin A, indicating that the adenine moiety plays a base-weakening effect on the phosphate groups. Further analysis of the microscopic protonation constants confirms that the compound whose conformation is the closest to that of adenophostin A also shows the highest biological activity. The two bisphosphorylated analogues studied behave very similarly, suggesting that the deletion of the hydroxymethyl group on the pentafuranosyl ring only weakly influences the protonation process of the phosphate groups that bear the glucopyranose moiety.

Optimising the EVA descriptor for prediction of biological activity

EVA is a multivariate molecular descriptor for use in QSAR studies. It is constructed from vibrational eigenvalues derived from either a quantum theoretical or molecular mechanical treatment of molecular structure. This paper applies the method to biological-activity data using measures of the inotropic potential of a range of Calcium channel agonists. The performance of the descriptor, as both an explanatory and a predictive tool, is analysed in relation to the way in which it is constructed using a rigorous statistical treatment. Its capabilities are examined in relation to those of previously published methodology which used a composite descriptor. It is shown to have improved performance and several procedural advantages, such as ease of calculation and operation. It is a 3-D structural descriptor which does not require prior co-alignment of structures for a QSAR study.

Topical calcitriol is degraded by ultraviolet light

Calcitriol ointment has been approved for the treatment of psoriasis in many countries around the world. It may be prescribed in conjunction with phototherapy. Our purpose was to evaluate the effect of various therapeutic ultraviolet modalities on the stability of calcitriol and, conversely, to study the effects of calcitriol ointment on transmission of different forms of ultraviolet light. Calcitriol ointment 3 microg per g was irradiated with 10 J per cm2 ultraviolet A, 100 mJ per cm2 broadband ultraviolet B, and 3.0 J per cm2 narrowband ultraviolet B, and its stability was compared with samples exposed to fluorescent light and ambient sunlight. Ultraviolet A and ultraviolet B transmission were measured through thin and thick layers of calcitriol 3 microg per g ointment and vehicle. More than 90% of calcitriol ointment is degraded upon exposure to ultraviolet A, broadband ultraviolet B, and narrowband ultraviolet B. Transmission of ultraviolet A is reduced through calcitriol ointment and its vehicle by 17%-31% and 17%-41%, respectively. Transmission of ultraviolet B is reduced by 67%-87% through vehicle and 50%-83% through calcitriol ointment. When used in conjunction with phototherapy, calcitriol ointment should be applied after ultraviolet exposure, not before. Calcipotriene, the only vitamin D analog already approved for psoriasis in the USA, has been used successfully in combination with ultraviolet B and psoralen plus ultraviolet A.

Structurally and functionally important amino acids of the agonistic conformation of the human vitamin D receptor

The crystal structures of the ligand binding domain of human vitamin D receptor (VDR) complexed with its natural ligand or the superagonists MC1288 or KH1060 have recently been reported. The crystallized ligand binding domain (LBD) of VDR, however, differs from the full-length VDR with respect to deletion of 50 amino acids between its helices 2 and 3. In this study, we investigated structurally and functionally important amino acid interactions within the ligand binding pocket of the full-length VDR in the presence of several synthetic vitamin D(3) analogs. We used site-directed mutagenesis scanning combined with limited proteolytic digestion, electrophoretic mobility shift assay, and reporter gene assay and correlated the findings with the crystal structures of truncated VDR LBD. Our results suggest that structurally different agonists have distinct ligand-receptor interactions and that the amino acid residues H229, D232, E269, F279, and Y295 are critical for the agonistic conformation of the VDR. Our biological data, which were obtained with the full-length VDR, fit well with the crystal structure of the truncated VDR LBD and suggest that removal of the insertion domain between helices 2 and 3 of the receptor does not markedly influence the functionality of the VDR.

Synthesis of glucopyranoside-based ligands for D-myo-inositol 1,4,5-trisphosphate receptors

Adenophostins A and B are naturally occurring glyconucleotides that interact potently with receptors for D-myo-inositol 1,4,5-trisphosphate, an important second messenger molecule in most cell types. Here we describe the design and synthesis of glucopyranoside-based analogues of adenophostin A lacking the adenine component. The key synthetic strategy involves glycosylation of selectively protected alcohols, derived from methyl beta-D-ribofuranoside or 1,4-anhydroerythritol, using glycosyl donors synthesised from 2,6-di-O-benzyl-D-glucopyranose derivatives. Further elaboration and deprotection of the coupled products gave two trisphosphate analogues; methyl 3-O-alpha-D-glucopyranosyl-beta-D-ribofuranoside 2,3',4'-trisphosphate ("ribophostin") and (3'S,4'R)-3'-hydroxytetrahydrofuran-4'-yl alpha-D-glucopyranoside 3,4,3'-trisphosphosphate ("furanophostin"). The route to furanophostin was further modified to give (3'S,4'R)-3'-hydroxytetrahydrofuran-4'-yl alpha-D-glucopyranoside 3'-phosphate 3,4-bisphosphorothioate, the first phosphorothioate-containing adenophostin analogue.

D-myo-inositol-1,4,5-trisphosphate and adenophostin mimics: importance of the spatial orientation of a phosphate group on the biological activity

Three different routes for the synthesis of heterocyclic analogues of the second messenger D-myo-inositol-1,4,5-trisphosphate (InsP(3)) and the natural adenophostins, starting from allyl D-xyloside are described. The two diastereoisomers at C-2 of new compounds, which we named xylophostins, were obtained. The preliminary biological studies shows that the presence of the adenine residue has a beneficial effect on the affinity for the receptor. The low potency of one of the two diastereoisomeric compounds shows that the configuration of the carbon bearing the non-vicinal phosphate group is an important requirement for a high affinity to the receptor. These results provide evidence for the existence of a binding pocket for the adenine ring nearby the InsP(3) binding site. The consequence of these stabilizing interactions should be to place the phosphate group in a suitable position to perfectly mimic InsP(3) in the more active diastereoisomer. Obviously, in the other diastereoisomer, the phosphate cannot accommodate the same orientation, thus explaining the low affinity. The existence of such a binding pocket for adenine is in line with the high potency of adenophostins.

Syntheses, calcium channel agonist-antagonist modulation activities, nitric oxide release, and voltage-clamp studies of 2-nitrooxyethyl 1,4-dihydro- 2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)pyridine-5-carboxylate enantiomers

The novel (-)-(S)-2 and (+)-(R)-3 enantiomers of 2-nitrooxyethyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)pyridine-5-carboxylate were synthesized for evaluation as calcium channel modulators. Determination of their in vitro calcium-channel-modulating activities using guinea pig left atria (GPLA) and ileum longitudinal smooth muscle (GPILSM) showed that the (-)-(S)-2 enantiomer acted as a dual cardioselective calcium channel agonist (GPLA)/smooth muscle selective calcium channel antagonist (GPILSM). In contrast, the (+)-(R)-3 enantiomer exhibited calcium channel antagonist activity on both GPLA and GPILSM. The 2-nitrooxyethyl racemate is a nitric oxide (.NO) donor that released 2.7% .NO, relative to the reference drug glyceryl trinitrate (5.3% .NO release/ONO(2) moiety), in the presence of N-acetylcysteamine. Whole-cell voltage-clamp studies using isolated guinea pig ventricular myocytes indicated that both enantiomers inhibit calcium current but that the (-)-(S)-2 enantiomer is a weaker antagonist than the (+)-(R)-3 enantiomer. These results indicate that replacement of the methyl ester substituent of (-)-(S)-methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)pyridine-5-carboxylate [(-)-(S)-1] by the 2-nitrooxyethyl ester .NO donor substituent present in (-)-(S)-2 provides a useful drug design concept to abolish the contraindicated calcium channel agonist effect of (-)-(S)-1 on vascular smooth muscle. The novel (-)-(S)-2 enantiomer is a useful lead compound for drug discovery targeted toward the treatment of congestive heart failure, and it provides a useful probe to study the structure-function relationships of calcium channels.

Absolute configuration in 4-alkyl- and 4-aryl-3,4-dihydro-2(1H)-pyrimidones: a combined theoretical and experimental investigation

Structural features (orientation of the carboxyl group, ring puckering), electronic absorption, and circular dichroism spectra of 4-alkyl- and 4-aryl-dihydropyrimidones 1-5 are calculated by semiempirical (AM1, INDO/S), ab initio (HF/6-31G, CIS/6-31G, RPA/6-31G), and density functional theory (B3LYP/6-31G) methods. These calculations allow an assignment of the absolute configuration by comparison of simulated and experimental CD spectra. Although the ab initio methods greatly overestimate electronic transition energies, the general appearance of the experimental CD spectra is quite nicely reproduced by these calculations. Thus, comparison of experimental with calculated CD spectra is a reliable tool for the assignment of the absolute configuration. For 4-methyl derivatives 1, the first enantiopure DHPM examples with no additional aromatic substituent, the stereochemistry at C4 provided by the theoretical results is confirmed by X-ray structure determination of the diastereomeric salt 6. Additional support is the consistent HPLC elution order found for all investigated DHPMs on a cellulose-derived chiral stationary phase.

3-Methyl 5-isopropyl 2-methoxyiminomethyl-6-methyl-4-(3-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate

The crystal structure of the title compound, C(20)H(23)N(3)O(7), consists of relatively isolated molecules. The substituted 1,4-dihydropyridine ring adopts a flattened boat conformation. Both ester groups, at positions 3 and 5, have cis,cis geometry. The phenyl ring is nearly planar and is approximately perpendicular to the 1,4-dihydropyridine ring (dihedral angle 87.70 degrees ).

Bicyclic analogues of D-myo-inositol 1,4,5-trisphosphate related to adenophostin A: synthesis and biological activity

The high affinity of adenophostin A for 1D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] receptors may be related to an alteration in the position of its 2'-phosphate group relative to the corresponding 1-phosphate group in Ins(1,4,5)P(3). To investigate this possibility, two bicyclic trisphosphates 9 and 10, designed to explore the effect of relocating the 1-phosphate group of Ins(1,4,5)P(3) using a novel fused-ring system, were synthesized from myo-inositol. Biological evaluation of 9 and 10 at the Ins(1,4,5)P(3) receptors of hepatocytes showed that both were recognized by hepatic Ins(1,4,5)P(3) receptors and both stimulated release of Ca(2+) from intracellular stores, but they had lower affinity than Ins(1,4,5)P(3). This finding may be explained by considering the three-dimensional structures of 9 and 10 in light of recent studies on the conformation of adenophostin A.

Structural determinants of adenophostin A activity at inositol trisphosphate receptors

Adenophostin A is the most potent known agonist of inositol 1,4,5-trisphosphate (InsP(3)) receptors. Ca(2+) release from permeabilized hepatocytes was 9.9 +/- 1.6-fold more sensitive to adenophostin A (EC(50), 14.7 +/- 2.4 nM) than to InsP(3) (145 +/- 10 nM), consistent with the greater affinity of adenophostin A for hepatic InsP(3) receptors (K(d) = 0.48 +/- 0.06 and 3.09 +/- 0.33 nM, respectively). Here, we systematically modify the structures of the glucose, ribose, and adenine moieties of adenophostin A and use Ca(2+) release and binding assays to define their contributions to high-affinity binding. Progressive trimming of the adenine of adenophostin A reduced potency, but it fell below that of InsP(3) only after complete removal of the adenine. Even after substantial modifications of the adenine (to uracil or even unrelated aromatic rings, retaining the beta-orientation), the analogs were more potent than InsP(3). The only analog with an alpha-ribosyl linkage had massively decreased potency. The 2'-phosphate on the ribose ring of adenophostin A was essential and optimally active when present on a five-membered ring in a position stereochemically equivalent to its location in adenophostin A. Xylo-adenophostin, where xylose replaces the glucose ring of adenophostin A, was only slightly less potent than adenophostin A, whereas manno-adenophostin (mannose replacing glucose) had similar potency to InsP(3). These results are consistent with the relatively minor role of the 3-hydroxyl of InsP(3) (the equivalent is absent from xylo-adenophostin) and greater role of the equatorial 6-hydroxyl (the equivalent is axial in manno-adenophostin). This is the first comprehensive analysis of all the key structural elements of adenophostin A, and it provides a working model for the design of related high-affinity ligands of InsP(3) receptors.

ATP-dependent adenophostin activation of inositol 1,4,5-trisphosphate receptor channel gating: kinetic implications for the durations of calcium puffs in cells

The inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) is a ligand-gated intracellular Ca(2+) release channel that plays a central role in modulating cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)). The fungal metabolite adenophostin A (AdA) is a potent agonist of the InsP(3)R that is structurally different from InsP(3) and elicits distinct calcium signals in cells. We have investigated the effects of AdA and its analogues on single-channel activities of the InsP(3)R in the outer membrane of isolated Xenopus laevis oocyte nuclei. InsP(3)R activated by either AdA or InsP(3) have identical channel conductance properties. Furthermore, AdA, like InsP(3), activates the channel by tuning Ca(2+) inhibition of gating. However, gating of the AdA-liganded InsP(3)R has a critical dependence on cytoplasmic ATP free acid concentration not observed for InsP(3)-liganded channels. Channel gating activated by AdA is indistinguishable from that elicited by InsP(3) in the presence of 0.5 mM ATP, although the functional affinity of the channel is 60-fold higher for AdA. However, in the absence of ATP, gating kinetics of AdA-liganded InsP(3)R were very different. Channel open time was reduced by 50%, resulting in substantially lower maximum open probability than channels activated by AdA in the presence of ATP, or by InsP(3) in the presence or absence of ATP. Also, the higher functional affinity of InsP(3)R for AdA than for InsP(3) is nearly abolished in the absence of ATP. Low affinity AdA analogues furanophostin and ribophostin activated InsP(3)R channels with gating properties similar to those of AdA. These results provide novel insights for interpretations of observed effects of AdA on calcium signaling, including the mechanisms that determine the durations of elementary Ca(2+) release events in cells. Comparisons of single-channel gating kinetics of the InsP(3)R activated by InsP(3), AdA, and its analogues also identify molecular elements in InsP(3)R ligands that contribute to binding and activation of channel gating.

Contribution of the adenine base to the activity of adenophostin A investigated using a base replacement strategy

Syntheses of 3'-O-alpha-D-glucopyranosyl-1-beta-D-ribofuranosidoimidazole 2',3'', 4''-trisphosphate (7) and 3'-O-alpha-D-glucopyranosyl-9-beta-D-ribofuranosidopurine 2',3'',4''- trisphosphate (8), two analogues of the superpotent 1D-myo-inositol 1,4,5-trisphosphate receptor agonist adenophostin A (2), are described. 5-O-Benzyl-1, 2-O-isopropylidene-alpha-D-ribofuranose was prepared by an improved route from 1,2-O-isopropylidene-alpha-D-xylofuranose and was coupled with 3,4-di-O-acetyl-2,6-di-O-benzyl-D-glucopyranosyl dimethyl phosphite to give 3',4'-di-O-acetyl-2',5, 6'-tri-O-benzyl-3-O-alpha-D-glucopyranosyl-1, 2-O-isopropylidene-alpha-D-ribofuranose. Removal of the isopropylidene acetal and subsequent acetylation gave the central disaccharide 1,2,3',4'-tetra-O-acetyl-2',5, 6'-tri-O-benzyl-3-O-alpha-D-glucopyranosyl-D-ribofuranose. Vorbrüggen condensation with activated imidazole or purine gave the required beta-substituted derivatives which were further elaborated to 7 and 8, respectively. Radioligand binding assays to hepatic InsP(3) receptors and functional assays of Ca(2+) release from permeabilized hepatocytes gave a rank order of potency of the ligands 2 approximately 8 > 7 approximately Ins(1,4,5)P(3) indicating that the N(6)-amino group of 2 is of little importance for activity and that a minimum of a two-fused-ring nucleobase is required for activity to exceed that of Ins(1,4,5)P(3). The role of the adenine base in the activity of the adenophostins is discussed. This general method should facilitate ready access to nucleobase-modified adenophostin analogues for SAR studies.

A new fold in the scorpion toxin family, associated with an activity on a ryanodine-sensitive calcium channel

We determined the structure in solution by (1)H two-dimensional NMR of Maurocalcine from the venom of Scorpio maurus. This toxin has been demonstrated to be a potent effector of ryanodyne-sensitive calcium channel from skeletal muscles. This is the first description of a scorpion toxin which folds following the Inhibitor Cystine Knot fold (ICK) already described for numerous toxic and inhibitory peptides, as well as for various protease inhibitors. Its three dimensional structure consists of a compact disulfide-bonded core from which emerge loops and the N-terminus. A double-stranded antiparallel beta-sheet comprises residues 20-23 and 30-33. A third extended strand (residues 9-11) is perpendicular to the beta-sheet. Maurocalcine structure mimics the activating segment of the dihydropyridine receptor II-III loop and is therefore potentially useful for dihydropyridine receptor/ryanodine receptor interaction studies. Proteins 2000;40:436-442.

Spirophostins: conformationally restricted analogues of adenophostin A

The synthesis, biological evaluation, and molecular modeling of two conformationally restricted analogues of adenophostinA (1), denominated as spirophostin (3R)-10 and (3S)-11, as novel ligands for the D-myo-inositol 1,4,5-trisphosphate receptor (IP3R), is presented. These diastereoisomeric spiroketals are synthesized by spiroketalization of D-glucose derivatives (2S)-15 and (2R)-16, separation of the protected isomers (3R)-19 and (3S)-20, followed by phosphorylation and deprotection. The spirophostins (3R)-10 and (3S)-11 display comparable biological activity, with a 3H-IP3-displacing and Ca2+-releasing potency less than IP3 and adenophostin A.

Syntheses and biological evaluation of novel 2alpha-substituted 1alpha,25-dihydroxyvitamin D3 analogues

Novel 2alpha-substituted 1alpha,25-dihydroxyvitamin D3 analogues were efficiently synthesized and their biological activities were evaluated. 2alpha-Methyl-1alpha,25-dihydroxyvitamin D3 (2), whose unique biological activities were previously reported, was modified to 2alpha-alkyl (ethyl and propyl) and 2alpha-hydroxyalkyl (hydroxymethyl, hydroxyethyl, and hydroxypropyl) analogues 3-7 by elongation of the alkyl chain and/or introduction of a terminal hydroxyl group. 2alpha-Hydroxypropyl-1alpha,25-dihydroxyvitamin D3 (7) exhibited an exceptionally potent calcium-regulating effect and a unique activity profile.

Levosimendan: effects of a calcium sensitizer on function and arrhythmias and cyclic nucleotide levels during ischemia/reperfusion in the Langendorff-perfused guinea pig heart

The majority of clinically used inotropes act by increasing cytosolic calcium levels, which may hypothetically worsen reperfusion stunning and provoke arrhythmias. We tested the hypothesis that the calcium sensitizer levosimendan (levo) given during ischemia alone or ischemia and reperfusion would improve reperfusion function without promoting arrhythmias. The Langendorff-perfused guinea pig heart, subjected to 40-min low-flow ischemia (0.4 ml/min) with or without levo (10-300 nM) given during ischemia or ischemia/reperfusion was used. Left ventricular developed pressure (LVDP) was used as an index of mechanical function. The effect of levo (300 nM) or dobutamine (0.1 microM) on the incidence of ischemia/reperfusion arrhythmias was also investigated. Control hearts (vehicle-perfused) had LVDPs of 69.4 +/- 2.1 mm Hg whereas hearts treated with levo during ischemia and reperfusion (300 nM) had LVDPs of 104.5 +/- 2.7 mm Hg (p <.05). Hearts treated with levo during ischemia alone (10 nM) had reperfusion LVDPs of 95.8 +/- 4.2 mm Hg (p <.05) after 30-min reperfusion. Hearts treated with both levo and 10 microM glibenclamide (K(ATP) channel blocker) during ischemia had reperfusion LVDPs of 73.4 +/- 4.3 mm Hg after 30-min reperfusion. Of control hearts, 25% developed reperfusion ventricular tachycardia but not ventricular fibrillation. Levo-treated hearts had no ischemia/reperfusion arrhythmias whereas 83% (p <.05 versus control) of dobutamine-treated hearts developed ventricular tachycardia and 33% (p <.05 versus levo) developed reperfusion ventricular fibrillation. Levo improved reperfusion function without promoting arrhythmias in this model. This was possibly achieved by opening the K(ATP) channels during ischemia and sensitizing myocardial contractile apparatus instead of elevating cytosolic calcium levels in reperfused hearts.

Investigation on SCH00013, a novel cardiotonic agent with Ca++ sensitizing action. 3rd communication: stereoselectivity of the enantiomers in cardiovascular effects

The cardiovascular effects of the enantiomers, (+)-SCH00013 and (-)-SCH00013, of a novel cardiotonic agent 4,5-dihydro-6-[1-[2-hydroxy-2-(4-cyanophenyl)ethyl]- 1,2,5,6-tetrahydropyrido-4-yl]pyridazin-3(2H)-one (SCH00013) were investigated in vitro and in vivo. The enantiomers of SCH00013 elicited an equipotent positive inotropic effect in isolated guinea-pig papillary muscles. Both of the enantiomers had a modest negative chronotropic effect in isolated guinea-pig right atria and the difference in the chronotropic effects of the enantiomers was not significant. In anesthetized dogs, both enantiomers increased LVdP/dtmax without change in heart rate and slightly decreased blood pressure. These hemodynamic effects of the enantiomers were not significantly different from each other. (+)-SCH00013 and (-)-SCH00013 increased the extent of cell shortening in association with only a small increase in the Ca++ transients in indo-1-loaded rabbit cardiomyocytes, and both the increases in cell shortening and Ca++ transients were not significantly different between the enantiomers. Both isomers equally shifted the relationships between the increases in the cell shortening and Ca++ transients to the left and upward as compared with the relationships for the elevation of extracellular Ca++ concentration and isoproterenol, which indicates that the effectiveness of the Ca++ sensitizing effects of the enantiomers are almost equivalent. The enantiomers of SCH00013 showed equipotent inhibitory effect on the phosphodiesterase (PDE) III activity. The maximal extent and the potency of prolonging effect of the two enantiomers on the effective refractory period were also the same. Thus, the efficacy and potency of the effects on the cardiovascular parameters such as myofibrillar Ca++ sensitivity, PDE III activity and the effective refractory period for the both enantiomers of SCH00013 are equivalent, indicating that the cardiovascular effects of SCH00013 may be due to equal contribution of both enantiomers.

Synthesis and voltage-clamp studies of methyl 1,4-dihydro-2, 6-dimethyl-5-nitro-4-(benzofurazanyl)pyridine-3-carboxylate racemates and enantiomers and of their benzofuroxanyl analogues

Racemic methyl 1,4-dihydro-2, 6-dimethyl-5-nitro-4-(benzofurazanyl)pyridine-3-carboxylates (+/-)-10 and (+/-)-11 and their benzofuroxanyl analogues (+/-)-12 and (+/-)-13 were prepared using a modified Hantzsch reaction that involved the condensation of nitroacetone with methyl 3-aminocrotonate and the appropriate aldehydes. The racemic mixtures were resolved into the corresponding enantiomers. Whole-cell voltage-clamp studies on L-type Ca2+ channels expressed in a rat insulinoma cell line (RINm5F) showed that all the dextrorotatory antipodes were effective agonists of L-type Ca2+ currents, while the levorotatory ones were weak Ca2+ entry blockers. The (+)-enantiomer of benzofurazan-5'-yl derivative 11 demonstrated unusual activity in that, in addition to producing a potentiation of L-type currents, it interfered with the voltage-dependent gating of L-type channels by producing a net delay of their activation at low voltages. This compound represents an interesting tool to probe L-type Ca2+ channel structure and function.

A new scorpion toxin (BmK-PL) stimulates Ca2+-release channel activity of the skeletal-muscle ryanodine receptor by an indirect mechanism

A peptide toxin isolated from the Chinese scorpion Buthus martensi Karsch (BmK-PL) stimulated Ca2+-release channel activity in both triad membranes and reconstituted ryanodine receptors partially purified from rabbit skeletal muscle. In [3H]ryanodine binding experiments, the toxin increased the affinity of ryanodine for the receptor, from a Kd of 24.3 nM to 2.9 nM, which is an enhancement similar to that seen with known receptor activators, such as ATP and high concentrations of KCl. In contrast, toxin enhancement was not observed with purified receptors, although intrinsic binding activity and stimulation by the conventional receptor activators were retained. In single channel recordings of Ca2+-release activity, the toxin increased the open channel probability (Po) from 0.019 to 0.043 (226% of control) in triad preparations. Further toxin enhancement of Po from 0.07 to 0.37 (529% of control) was observed using partially-purified receptors in the presence of ATP. When purified receptors were assayed in the presence of ATP, however, they showed a high value of Po (0.33) and no further increase was observed following application of the toxin. Results derived from two different experimental methods consistently suggest that a molecule(s) required for toxin-induced enhancement is absent from the purified receptor preparation. Western blot analysis of receptors prepared using three different protocols showed that triadin was missing from the purified receptor preparation. The scorpion toxin minimally enhanced Ca2+-release channel activity of cardiac preparations. From these results, we conclude that the toxin preferentially increases the activity of skeletal-muscle ryanodine receptors by an indirect mechanism, possibly binding to associated protein molecule(s). Triadin is a strong candidate for such a molecule.

Ionization constants and distribution coefficients of phenothiazines and calcium channel antagonists determined by a pH-metric method and correlation with calculated partition coefficients

The pH-metric technique was used to determine the ionization constants and distribution coefficients of 10 phenothiazines and five ionizable calcium channel antagonists. Because the studied compounds were poorly water soluble and quite lipophilic with partition coefficients in the range of 3.5 to 5.5, organic cosolvents had to be added for the determination of the ionization constants to avoid precipitation of the free bases. The effect of the cosolvents dioxane and methanol on the extrapolation to pure water was compared. For both cosolvents a very good agreement with accessible published ionization constants was obtained, however the slope of the regression line was much smaller for dioxane, yielding more reliable estimates according to the standard deviation of the extrapolated values. Thus, dioxane might be preferable to methanol as a cosolvent for the determination of ionization constants of sparingly water soluble bases. Also the n-octanol/water partition coefficients were determined and compared with published data and values calculated with the ClogP, ACD, and HINT programs. Although the obtained values were approximate in conformity with the published data, the calculated partition coefficients differed from the experimental ones considerably for the majority of the investigated compounds. Furthermore, the ion pair partitioning and the distribution coefficients at physiological pH 7.4 were determined. The pH-dependent distribution profiles showed the strong influence of the ionization constants and of the distribution of the ion pairs on the overall distribution. This result strongly suggests that greater use should be made of measured distribution coefficients in quantitative structure-activity relationship studies. The potentiometric method is a convenient way to determine the distribution properties of drug molecules at pH values relevant for the biological system under investigation.

Isolation of a member of the neurotoxin/cytotoxin peptide family from Xenopus laevis skin which activates dihydropyridine-sensitive Ca2+ channels in mammalian epithelial cells

We have used a sensitive bioassay of calcium-mediated volume changes in mammalian absorptive intestinal epithelial cells to screen extracts of the skin of the amphibian Xenopus laevis for the presence of factors affecting ion transport. A 66-residue peptide, purified using reversed-phase high performance liquid chromatography techniques, caused isotonic volume reduction of guinea pig jejunal villus cells in suspension. This volume reduction required extracellular Ca2+ and was prevented by the dihydropyridine-sensitive Ca2+ channel blocker niguldipine. Structural analysis demonstrated the presence of eight cysteines and a primary structure homologous to that of the neurotoxin/cytotoxin family found in the venom of certain poisonous snakes. The structure of the peptide was identical to that of xenoxin-1 purified from dorsal gland secretions of X. laevis (Kolbe, M., Huber A., Cordier, P., Rasmussen, U., Bouchon, B., Jaquinod, M., Blasak, R., Detot, E., and Kreil, G. (1993) J. Biol. Chem. 268, 16458-16464). Xenoxin-1 (10 nM) caused volume changes that required extracellular Ca2+ and were comparable in magnitude and direction to changes caused by BayK-8644 (100 nM), a dihydropyridine-sensitive Ca2+ channel agonist. The initial rate of dihydropyridine-sensitive 45Ca2+ influx was substantially increased by xenoxin-1. Staurosporine (10 nM) prevented volume changes caused by ATP (250 microM) but had no effect on volume changes caused by BayK-8644 or xenoxin-1. We conclude that xenoxin-1 directly activated dihydropyridine-sensitive Ca2+ channels in villus cells and that a mammalian homologue to xenoxin-1 may exist.

[Antiradical activity and membranotropic action of cardio- and vasotropic compounds of varying chemical nature]

Antiradical activity and ability to interaction with phosphatidylcholine bilayer of the physiological active compounds (PhAC), concerning the classes of phenylalkilamines (dobutamine, verapamil), dihydro pyridines (BAY-K-4688, nifedipine), analogues of crown ether (carbicyle, diol) was studied. By means of the method of microcalorimetry and spectrophotometry it was shown the complexing ability PhAC with the phospholipid bilayer of the model membrane. It was stated the simbasity in the changes of the thermal effect of the compounds with the negative inotropic activity during its reaction with phosphatidylcholine bilayer and antiradical activity. That show on the presence of antiradical component in the mechanism that compounds' action.

Synthesis, rotamer orientation, and calcium channel modulation activities of alkyl and 2-phenethyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(3- or 6-substituted-2-pyridyl)-5-pyridinecarboxylates

A group of racemic alkyl and 2-phenethyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(3- or 6-substituted-2-pyridyl)-5-pyridinecarboxylates (13a-q) was prepared using a modified Hantzsch reaction that involved the condensation of a 3- or 6-substituted-2-pyridinecarboxaldehyde (7a-j) with an alkyl or 2-phenethyl 3-aminocrotonate (11a-d) and nitroacetone (12). Nuclear Overhauser (NOE) studies indicated there is a significant rotamer fraction in solution where the pyridyl nitrogen is oriented above the 1,4-dihydropyridine ring, irrespective of whether a substituent is located at the 3- or 6-position. A potential H-bonding interaction between the pyridyl nitrogen free electron pair and the suitably positioned 1,4-dihydropyridine NH moiety may stablize this rotamer orientation. In vitro calcium channel antagonist and agonist activities were determined using guinea pig ileum longitudinal smooth muscle (GPILSM) and guinea pig left atrium (GPLA) assays, respectively. Compounds having an i-Pr ester substituent acted as dual cardioselective calcium channel agonists (GPLA)/smooth muscle-selective calcium channel antagonists (GPILSM), except for the C-4 3-nitro-2-pyridyl compound which exhibited an antagonist effect on both GPLA and GPILSM. In contrast, the compounds with a phenethyl ester group, which exhibited antagonist activity (IC50 = 10(-5)-10(-7) M range) on GPILSM, were devoid of cardiac agonist activity on GPLA. Structure-activity relationships showing the effect of a substituent (Me, CF3, Cl, NO2, Ph) at the 3- or 6-position of a C-4 2-pyridyl moiety and a variety of ester substituents (Me, Et, i-Pr, PhCH2CH2-) upon calcium channel modulation are described. Compounds possessing a 3- or 6-substituted-2-pyridyl moiety, in conjuction with an i-Pr ester substituent, are novel 1,4-dihydropyridine calcium channel modulators that offer a new drug design approach directed to the treatment of congestive heart failure and may also be useful as probes to study the structure-function relationships of calcium channels.

Synthesis and calcium channel-modulating effects of alkyl (or cycloalkyl) 1,4-dihydro-2,6-dimethyl-3-nitro-4-pyridyl-5-pyridinecarboxylate racemates and enantiomers

A group of racemic alkyl (or cycloalkyl) 1,4-dihydro-2,6- dimethyl-3-nitro-4-(2-, 3-, or 4-pyridyl)-5-pyridinecarboxylate isomers (6-14) were prepared using a modified Hantzsch reaction that involved the condensation of nitroacetone with an alkyl (or cycloalkyl) 3-aminocrotonate and 2-, 3-, or 4-pyridinecarboxaldehyde. Determination of their in vitro calcium channel-modulating activities using guinea pig ileum longitudinal smooth muscle (GPILSM) and guinea pig left atrium (GPLA) assays showed that the 2-pyridyl isomers acted as dual cardioselective calcium channel agonists (GPLA)/smooth muscle selective calcium channel antagonists (GPILSM). In contrast, the 3-pyridyl and 4-pyridyl isomers acted as calcium channel agonists on both GPLA and GPILSM. In the C-4 2-pyridyl group of compounds, the size of the C-5 alkyl (or cycloalkyl) ester substituent was a determinant of GPILSM antagonist activity where the relative activity profile was cyclopentyl and cyclohexyl > t-Bu, i-Bu, and Et > MeOCH2CH2 > Me. The point of attachment of the C-4 pyridyl substituent was a determinant of GPLA agonist activity where the potency order was generally 4- and 3-pyridyl > 2-pyridyl. (+)-Cyclohexyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-pyridyl)-5- pyridinecarboxylate [(+)-14a] was a less potent calcium antagonist (IC50 = 5.27 x 10(-6) M) than the (-)-enantiomer (IC50 = 7.48 x 10(-8) M) on GPILSM. In the GPLA assay, (+)-14a exhibited a much more potent agonist effect (EC50 = 8.45 x 10(-6) M) relative to the marginal agonist effect produced by (-)-14a. The C-4 2-pyridyl compounds (enantiomers) constitute a novel type of 1,4-dihydropyridine calcium channel modulator that could provide a new drug design concept directed toward the treatment of congestive heart failure, and for use as probes to study the structure-function relationships of calcium channels.

Modes of action of anthelmintic drugs

Modes of action of anthelmintic drugs are described. Some anthelmintic drugs act rapidly and selectively on neuromuscular transmission of nematodes. Levamisole, pyrantel and morantel are agonists at nicotinic acetylcholine receptors of nematode muscle and cause spastic paralysis. Dichlorvos and haloxon are organophosphorus cholinesterase antagonists. Piperazine is a GABA (gamma-amino-butyric acid) agonist at receptors on nematode muscles and causes flaccid paralysis. The avermectins increase the opening of glutamate-gated chloride (GluCl) channels and produce paralysis of pharyngeal pumping. Praziquantel has a selective effect on the tegument of trematodes and increases permeability of calcium. Other anthelmintics have a biochemical mode of action. The benzimidazole drugs bind selectively to beta-tubulin of nematodes, cestodes and fluke, and inhibit microtubule formation. The salicylanilides: rafoxanide, oxyclozanide, brotianide and closantel and the substituted phenol, nitroxynil, are proton ionophores. Clorsulon is a selective antagonist of fluke phosphoglycerate kinase and mutase. Diethylcarbamazine blocks host, and possibly parasite, enzymes involved in arachidonic acid metabolism, and enhances the innate, nonspecific immune system.

Bastadin 20 and bastadin O-sulfate esters from Ianthella basta: novel modulators of the Ry1R FKBP12 receptor complex

New compounds bastadin 20 (9), 15,34-O-disulfatobastadin 7 (10), and 10-O-sulfatobastadin 3 (11) were isolated from Ianthella basta collected in Exmouth Gulf, Western Australia. Compounds 10 and 11 exhibited moderate differential activity as SR Ca2+ channel agonists (EC50 13.6 and 100 microM, respectively) of the Ry1R FKBP12 complex, while the potency of 9 was almost half that of 10 (EC50 20.6 microM). The problem of dereplication of bastadins was addressed using 1H-NMR "fingerprinting" of MeO signals in the corresponding permethyl bastadin derivatives.

A new type of Ca-channel modulation by a novel class of 1,4-dihydropyridines

Both Ca-antagonistic as well as Ca-agonistic 1,4-dihydropyridines (DHPs) have become extremely important tools to investigate the role of Ca-channels under various physiological and pathophysiological conditions. While Ca-antagonists stabilize the inactivated state of the Ca-channel without influencing the voltage dependent open and closed times, Ca-agonists prolong the mean open time of the channel. We here report for the first time the effects of a novel DHP, BAY Y 5959, which modulates Ca-channel gating in a unique manner: It increases both the mean open time and the mean closed time of the Ca-channel by binding to the DHP receptor. This results in a reduced rate of Ca-current activation, an increased peak current, and a strongly prolonged tail current decay. All these effects are strongly voltage dependent. Therefore it depends on resting membrane potential and shape of the action potential whether and how the Ca-influx into the cell is influenced. This novel mode of action of BAY Y 5959 results in an interesting pharmacological profile: It has a strong positive inotropic effect in the heart without influencing vessel tone. Therefore the term Ca-promoter is suggested; it could become a new approach for the drug treatment of congestive heart failure.