Pharmacologic modulation of hand pain in osteoarthritis: a double-blind placebo-controlled functional magnetic resonance imaging study using naproxen

OBJECTIVE

In an attempt to shed light on management of chronic pain conditions, there has long been a desire to complement behavioral measures of pain perception with measures of underlying brain mechanisms. Using functional magnetic resonance imaging (fMRI), we undertook this study to investigate changes in brain activity following the administration of naproxen or placebo in patients with pain related to osteoarthritis (OA) of the carpometacarpal (CMC) joint.

METHODS

A placebo-controlled, double-blind, 2-period crossover study was performed in 19 individuals with painful OA of the CMC joint of the right hand. Following placebo or naproxen treatment periods, a functionally relevant task was performed, and behavioral measures of the pain experience were collected in identical fMRI examinations. Voxelwise and a priori region of interest analyses were performed to detect between-period differences in brain activity.

RESULTS

Significant reductions in brain activity following treatment with naproxen, compared to placebo, were observed in brain regions commonly associated with pain perception, including the bilateral primary somatosensory cortex, thalamus, and amygdala. Significant relationships between changes in perceived pain intensity and changes in brain activity were also observed in brain regions previously associated with pain intensity.

CONCLUSION

This study demonstrates the sensitivity of fMRI to detect the mechanisms underlying treatments of known efficacy. The data illustrate the enticing potential of fMRI as an adjunct to self-report for detecting early signals of efficacy of novel therapies, both pharmacologic and nonpharmacologic, in small numbers of individuals with persistent pain.

Multivariate decoding of cerebral blood flow measures in a clinical model of on-going postsurgical pain

Recent reports of multivariate machine learning (ML) techniques have highlighted their potential use to detect prognostic and diagnostic markers of pain. However, applications to date have focussed on acute experimental nociceptive stimuli rather than clinically relevant pain states. These reports have coincided with others describing the application of arterial spin labeling (ASL) to detect changes in regional cerebral blood flow (rCBF) in patients with on-going clinical pain. We combined these acquisition and analysis methodologies in a well-characterized postsurgical pain model. The principal aims were (1) to assess the classification accuracy of rCBF indices acquired prior to and following surgical intervention and (2) to optimise the amount of data required to maintain accurate classification. Twenty male volunteers, requiring bilateral, lower jaw third molar extraction (TME), underwent ASL examination prior to and following individual left and right TME, representing presurgical and postsurgical states, respectively. Six ASL time points were acquired at each exam. Each ASL image was preceded by visual analogue scale assessments of alertness and subjective pain experiences. Using all data from all sessions, an independent Gaussian Process binary classifier successfully discriminated postsurgical from presurgical states with 94.73% accuracy; over 80% accuracy could be achieved using half of the data (equivalent to 15 min scan time). This work demonstrates the concept and feasibility of time-efficient, probabilistic prediction of clinically relevant pain at the individual level. We discuss the potential of ML techniques to impact on the search for novel approaches to diagnosis, management, and treatment to complement conventional patient self-reporting.

Safety and tolerability of LBR-101, a humanized monoclonal antibody that blocks the binding of CGRP to its receptor: Results of the Phase 1 program

BACKGROUND

LBR-101 is a fully humanized monoclonal antibody that binds to calcitonin gene-related peptide.

OBJECTIVE

The objective of this article is to characterize the safety and tolerability of LBR-101 when administered intravenously to healthy volunteers, by presenting the pooled results of the Phase 1 program.

METHODS

LBR-101 was administered to 94 subjects, while 45 received placebo. Doses ranged from 0.2 mg to 2000 mg given once (Day 1), as a single IV infusion, or up to 300 mg given twice (Day 1 and Day 14).

RESULTS

Subjects receiving placebo reported an average of 1.3 treatment-emerging adverse events vs 1.4 per subject among those receiving any dose of LBR-101, and 1.6 in those receiving 1000 mg or higher. Treatment-related adverse events occurred in 21.2% of subjects receiving LBR-101, compared to 17.7% in those receiving placebo. LBR-101 was not associated with any clinically relevant patterns of change in vital signs, ECG parameters, or laboratory findings. The only serious adverse event consisted of "thoracic aortic aneurysm" in a participant later found to have an unreported history of Ehlers-Danlos syndrome.

CONCLUSION

Single IV doses of LBR-101 ranging from 0.2 mg up to 2000 mg and multiple IV doses up to 300 mg were well tolerated. Overt safety concerns have not emerged. A maximally tolerated dose has not been identified.

Quantifying the test-retest reliability of cerebral blood flow measurements in a clinical model of on-going post-surgical pain: A study using pseudo-continuous arterial spin labelling

Arterial spin labelling (ASL) is increasingly being applied to study the cerebral response to pain in both experimental human models and patients with persistent pain. Despite its advantages, scanning time and reliability remain important issues in the clinical applicability of ASL. Here we present the test–retest analysis of concurrent pseudo-continuous ASL (pCASL) and visual analogue scale (VAS), in a clinical model of on-going pain following third molar extraction (TME). Using ICC performance measures, we were able to quantify the reliability of the post-surgical pain state and ΔCBF (change in CBF), both at the group and individual case level. Within-subject, the inter- and intra-session reliability of the post-surgical pain state was ranked good-to-excellent (ICC > 0.6) across both pCASL and VAS modalities. The parameter ΔCBF (change in CBF between pre- and post-surgical states) performed reliably (ICC > 0.4), provided that a single baseline condition (or the mean of more than one baseline) was used for subtraction. Between-subjects, the pCASL measurements in the post-surgical pain state and ΔCBF were both characterised as reliable (ICC > 0.4). However, the subjective VAS pain ratings demonstrated a significant contribution of pain state variability, which suggests diminished utility for interindividual comparisons. These analyses indicate that the pCASL imaging technique has considerable potential for the comparison of within- and between-subjects differences associated with pain-induced state changes and baseline differences in regional CBF. They also suggest that differences in baseline perfusion and functional lateralisation characteristics may play an important role in the overall reliability of the estimated changes in CBF. Repeated measures designs have the important advantage that they provide good reliability for comparing condition effects because all sources of variability between subjects are excluded from the experimental error. The ability to elicit reliable neural correlates of on-going pain using quantitative perfusion imaging may help support the conclusions derived from subjective self-report.

•

Test-retest reliability of pCASL is considered in a post-surgical pain model.

•

Pain-state and ∆CBF measurements were reliable at the group and individual level.

•

Single or average baseline measurements improve reliability of ∆CBF.

•

pCASL is a reliable technique for detecting cerebral responses to on-going pain.

Assessment of the pharmacology and tolerability of PF-04457845, an irreversible inhibitor of fatty acid amide hydrolase-1, in healthy subjects

AIMS To evaluate the pharmacology and tolerability of PF-04457845, an orally available fatty acid amide hydrolase-1 (FAAH1) inhibitor, in healthy subjects.

METHODS

Double-blind, randomized, placebo-controlled single and multiple rising dose studies and an open-label, randomized, food effect study were conducted. Plasma and urine PF-04457845 concentrations, plasma fatty acid amide concentrations and FAAH1 activity in human leucocytes were measured. Tolerability, including effects on cognitive function, were assessed.

RESULTS

PF-04457845 was rapidly absorbed (median t(max) 0.5-1.2 h). Exposure increased supraproportionally to dose from 0.1 to 10 mg and proportionally between 10 and 40 mg single doses. The pharmacokinetics appeared dose proportional following 14 days once daily dosing between 0.5 and 8 mg. Steady-state was achieved by day 7. Less than 0.1% of the dose was excreted in urine. Food had no effect on PF-04457845 pharmacokinetics. FAAH1 activity was almost completely inhibited (>97%) following doses of at least 0.3 mg (single dose) and 0.5 mg once daily (multiple dose) PF-04457845. Mean fatty acid amide concentrations increased (3.5- to 10-fold) to a plateau and then were maintained following PF-04457845. FAAH1 activity and fatty acid amide concentrations returned to baseline within 2 weeks following cessation of dosing at doses up to 4 mg. There was no evidence of effects of PF-04457845 on cognitive function. PF-04457845, at doses up to 40 mg single dose and 8 mg once daily for 14 days, was well tolerated.

CONCLUSIONS

PF-04457845 was well tolerated at doses exceeding those required for maximal inhibition of FAAH1 activity and elevation of fatty acid amides.

Effects of gabapentin on experimental somatic pain and temporal summation

BACKGROUND AND OBJECTIVES

Gabapentin is used for treatment of neuropathic pain, but its effect on different somatic pain modalities and integrative mechanisms are not completely understood. The aim of this double-blind, placebo-controlled experimental pain study, conducted on 20 healthy volunteers, was to examine the effect of a single dose of 1200 mg gabapentin on multi-modal experimental cutaneous and muscle pain models.

METHODS

The following pain models were applied: (1) pain thresholds to single and repeated cutaneous and intramuscular electrical stimulation (temporal summation to 5 stimuli delivered at 2 Hz); (2) stimulus-response function relating pain intensity scores (visual analog scale, VAS) to increasing current intensities for electrical skin and muscle stimuli (single and repeated, determined at baseline); and (3) the pain intensity (VAS) and pain areas after intramuscular injection of hypertonic saline. Pain assessments were performed prior to, and at 4, 6, and 8 hours after medication.

RESULTS

When responses were averaged across the post-dose times, gabapentin: (1) significantly increased the temporal summation pain threshold in skin compared with placebo (P = .03); (2) significantly reduced the area under the pain intensity curve to hypertonic saline injections in the muscle (P = .02); and (3) significantly reduced the area of pain evoked by hypertonic saline (P = .03).

CONCLUSIONS

Gabapentin reduces temporal summation of skin stimuli at pain threshold intensities; this may have potential as a biomarker for drugs with efficacy on neurogenic pain. The data also suggest that tonic muscle pain is responsive to gabapentin treatment and suggest further clinical studies.

Cognitive testing in early-phase clinical trials: Development of a rapid computerized test battery and application in a simulated Phase I study

BACKGROUND

Inclusion of cognitive assessment in Phase I trials of novel pharmaceutical agents may help identify subtle yet meaningful CNS effects early in clinical development, and lead to a greater understanding of the pharmacokinetic/pharmacodynamic relationship prior to entering pivotal late-phase trials.

AIMS

To examine issues surrounding the inclusion of a computerised cognitive test battery in Phase I clinical trials.

METHODS

A 12-minute battery of five computerized cognitive tasks was administered to 28 healthy males in a double-blind, single ascending dose study using three doses of midazolam (0.6 mg, 1.75 mg and 5.25 mg) with placebo insertion. Subjects were enrolled and assessed at two Phase I units. Statistical analyses sought to determine the sensitivity of the test battery to sedation-related cognitive dysfunction, any between-site differences in outcome, and also the effects of repeated test administration (i.e., practice or learning effects).

RESULTS

There were no significant differences in data collected between sites. All standard safety measurements were completed. No substantial technical issues were noted. No learning effects were observed on four of the five cognitive tasks. ANOVA comparing baseline to post-baseline results revealed significant cognitive deterioration on all five cognitive tasks 1 h following administration of 5.25 mg midazolam. The magnitude of these changes were very large according to conventional statistical criteria. Smaller but significant changes were observed on a subset of memory and learning tasks at 1 h post-dosing in 1.75 mg condition, and at 2 h post-dosing in the 5.25 mg condition.

CONCLUSIONS

The cognitive test battery was well tolerated by subjects and research unit staff. The tests demonstrated minimal learning effects, were unaffected by language and cultural differences between sites, and were sensitive to the sedative effects of midazolam. Inclusion of this cognitive test battery in future studies may allow identification of cognitive impairment or enhancement early in the clinical development cycle.

Selective Urokinase-Type Plasminogen Activator Inhibitors. 4. 1-(7-Sulfonamidoisoquinolinyl)guanidines†

1-isoquinolinylguanidines were previously disclosed as potent and selective inhibitors of urokinase-type plasminogen activator (uPA). Further investigation of this template has revealed that incorporation of a 7-sulfonamide group furnishes a new series of potent and highly selective uPA inhibitors. Potency and selectivity can be achieved with sulfonamides derived from a variety of amines and is further enhanced by the incorporation of sulfonamides derived from amino acids. The binding mode of these 1-isoquinolinylguanidines has been investigated by X-ray cocrystallization studies. uPA inhibitor 26 was selected for further evaluation based on its excellent enzyme potency (Ki 10 nM) and selectivity profile (4000-fold versus tPA and 2700-fold versus plasmin). In vitro, compound 26 is able to inhibit exogenous uPA in human chronic wound fluid (IC50=0.89 microM). In vivo, in a porcine acute excisional wound model, following topical delivery, compound 26 is able to penetrate into pig wounds and inhibit exogenous uPA activity with no adverse effect on wound healing parameters. On the basis of this profile, compound 26 (UK-371,804) was selected as a candidate for further preclinical evaluation for the treatment of chronic dermal ulcers.

Cognitive testing in early phase clinical trials: outcome according to adverse event profile in a Phase I study

BACKGROUND

It has been proposed that objective cognitive testing provides additional information to that collected via adverse event (AE) recordings. However, in clinical trials of compounds with potentially negative effects on cognition, the results of cognitive testing may overlap with AE recordings.

AIMS

To examine cognitive function in subjects who do and do not report sedation-related AEs in a Phase I clinical trial.

METHODS

Five computerized cognitive tasks were administered to 28 healthy male volunteers enrolled in a simulated Phase I study using midazolam to induce sedation-related AEs and cognitive dysfunction. For each subject, the magnitude of cognitive change between pre-dose and 1 hr post-dose assessments was calculated. Group and individual level cognitive outcome was compared between subjects who did and did not report sedation-related AEs following administration of 1.75 and 5.25 mg midazolam.

RESULTS

At both doses of midazolam, cognitive dysfunction was observed in both subject groups (i.e., those who did and did not report AEs). Analysis of individual outcomes identified consistent cognitive dysfunction among subjects who reported sedation-related AEs. Further, in the 5.25 mg condition a subset of individuals (66.7%) who did not report sedation-related AEs nevertheless displayed substantial cognitive dysfunction.

CONCLUSIONS

Following administration of oral midazolam, there is a dissociation between sedation-related AE recordings and performance on computerized cognitive tests of motor function, attention, strategy use and problem solving, learning and delayed recall. Inclusion of computerized cognitive tests in early phase trials may allow identification of subtle cognitive change, beyond that which is possible by self-report and clinical observation.

Pharmacological modulation of pain-related brain activity during normal and central sensitization states in humans

Abnormal processing of somatosensory inputs in the central nervous system (central sensitization) is the mechanism accounting for the enhanced pain sensitivity in the skin surrounding tissue injury (secondary hyperalgesia). Secondary hyperalgesia shares clinical characteristics with neurogenic hyperalgesia in patients with neuropathic pain. Abnormal brain responses to somatosensory stimuli have been found in patients with hyperalgesia as well as in normal subjects during experimental central sensitization. The aim of this study was to assess the effects of gabapentin, a drug effective in neuropathic pain patients, on brain processing of nociceptive information in normal and central sensitization states. Using functional magnetic resonance imaging (fMRI) in normal volunteers, we studied the gabapentin-induced modulation of brain activity in response to nociceptive mechanical stimulation of normal skin and capsaicin-induced secondary hyperalgesia. The dose of gabapentin was 1,800 mg per os, in a single administration. We found that (i) gabapentin reduced the activations in the bilateral operculoinsular cortex, independently of the presence of central sensitization; (ii) gabapentin reduced the activation in the brainstem, only during central sensitization; (iii) gabapentin suppressed stimulus-induced deactivations, only during central sensitization; this effect was more robust than the effect on brain activation. The observed drug-induced effects were not due to changes in the baseline fMRI signal. These findings indicate that gabapentin has a measurable antinociceptive effect and a stronger antihyperalgesic effect most evident in the brain areas undergoing deactivation, thus supporting the concept that gabapentin is more effective in modulating nociceptive transmission when central sensitization is present.

A potent, selective inhibitor of matrix metalloproteinase-3 for the topical treatment of chronic dermal ulcers

The pathology of chronic dermal ulcers is characterized by excessive proteolytic activity which degrades extracellular matrix (required for cell migration) and growth factors and their receptors. The overexpression of MMP-3 (stromelysin-1) and MMP-13 (collagenase-3) is associated with nonhealing wounds, whereas active MMPs-1, -2, -9, and -14 are required for normal wound healing to occur. We describe the synthesis and enzyme inhibition profile of (3R)-3-[([(1S)-2,2-dimethyl-1-(([(1S)-2-methoxy-1-phenylethyl]amino)carbonyl)propyl]amino)carbonyl]-6-(3-methyl-4-phenylphenyl)hexanoic acid (UK-370,106, 7), which is a potent inhibitor of MMP-3 (IC(50) = 23 nM) with >1200-fold weaker potency vs MMP-1, -2, -9, and -14. MMP-13, which may also contribute to the pathology of chronic wounds, was inhibited about 100-fold less potently by compound 7. Compound 7 potently inhibited cleavage of [(3)H]-fibronectin by MMP-3 (IC(50) = 320 nM) but not cleavage of [(3)H]-gelatin by either MMP-2 or -9 (up to 100 microM). Compound 7 had little effect, at MMP-3 selective concentrations, on keratinocyte migration over a collagen matrix in vitro, which is a model of the re-epithelialization process. Following iv (rat) or topical administration to dermal wounds (rabbit), compound 7 was cleared rapidly (t(1/2) = 23 min) from plasma, but slowly (t(1/2) approximately 3 days) from dermal tissue. In a model of chronic dermal ulcers, topical administration of compound 7 for 6 days substantially inhibited MMP-3 ex vivo. These data suggest compound 7 is sufficiently potent to inhibit MMP-3-mediated matrix degradation while leaving unaffected cellular migration mediated by MMPs 1, 2, and 9. These properties make compound 7 a suitable candidate for progression to clinical trials in human chronic dermal wounds, such as venous ulcers.

Inhibition of cyclic GMP-dependent protein kinase-mediated effects by (Rp)-8-bromo-PET-cyclic GMPS

1. The modulation of the guanosine 3':5'-cyclic monophosphate (cyclic GMP)- and adenosine 3':5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase activities by the diastereomers of 8-bromo-beta phenyl-1, N2-ethenoguanosine 3':5'-cyclic monophosphorothioate, ((Rp)- and (Sp)-8-bromo-PET-cyclic GMPS) was investigated by use of purified protein kinases. In addition, the effects of (Rp)-8-bromo-PET-cyclic GMPS on protein phosphorylation in intact human platelets and on [3H]-noradrenaline release and neurogenic vasoconstriction in electrical field stimulated rat tail arteries were also studied. 2. Kinetic analysis with purified cyclic GMP-dependent protein kinase (PKG) type I alpha and I beta, which are expressed in the rat tail artery, revealed that (Rp)-8-bromo-PET-cyclic GMPS is a competitive inhibitor with an apparent Ki of 0.03 microM. The activation of purified cyclic AMP-dependent protein kinase (PKA) type II was antagonized with an apparent Ki of 10 microM. 3. In human platelets, (Rp)-8-bromo-PET-cyclic GMPS (0.1 mM) antagonized the activation of the PKG by the selective activator 8-(4-chlorophenylthio)-guanosine 3':5'-cyclic monophosphate (8-pCPT-cyclic GMP; 0.2 mM) without affecting the activation of PKA by (Sp)-5, 6-dichloro-1-beta-D-ribofurano-sylbenzimidazole- 3':5'-cyclic monophosphorothioate ((Sp)-5,6-DCl-cyclic BiMPS; 0.1 mM). 4. (Rp)-8-bromo-PET-cyclic GMPS was not hydrolysed by the cyclic GMP specific phosphodiesterase (PDE) type V from bovine aorta but potently inhibited this PDE. 5. The corresponding sulphur free cyclic nucleotide of the two studied phosphorothioate derivatives, 8-bromo-beta-phenyl-1, N2-ethenoguanosine-3':5'-cyclic monophosphate (8-bromo-PET-cyclic GMP), had no effect on electrically-induced [3H]-noradrenaline release but concentration-dependently decreased the stimulation-induced vasoconstriction. (Rp)-8-bromo-PET-cyclic GMPS (3 microM) shifted the vasoconstriction response to the right without affecting stimulation evoked tritium overflow. 6. The NO donor, 3-morpholinosydnonimine (SIN-1) relaxed rat tail arteries precontracted with phenylephrine (1 microM). The SIN-1 concentration-relaxation curve was shifted in a parallel manner to the right by (Rp)-8-bromo-PET-cyclic GMPS, suggesting that the relaxation was mediated by a cyclic GMP/PKG-dependent mechanism. 7. The [3H]-noradrenaline release-enhancing effect and stimulation-induced decrease in vasoconstriction of forskolin were unaffected by (Rp)-8-bromo-PET-cyclic GMPS. Moreover, the forskolin concentration-relaxation curve was not changed in the presence of the PKG inhibitor, suggesting a high selectivity in intact cells for PKG- over PKA-mediated effects. 8. The results obtained indicate that (Rp)-8-bromo-PET-cyclic GMPS presently is the most potent and selective inhibitor of PKG and is helpful in distinguishing between cyclic GMP and cyclic AMP messenger pathways activation. Therefore, this phosphorothioate stereomer may be a useful tool for studying the role of cyclic GMP in vitro.

PKG-I alpha phosphorylates the alpha-subunit and upregulates reconstituted GKCa channels from tracheal smooth muscle

Modulation of Ca(2+)-dependent K+ channel (GKCa) activities in airway smooth muscles (ASM) by guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kinase (PKG) is thought to play a central role in mediating the effect of some bronchodilator agents that elevate cytoplasmic basal cGMP concentrations. However, no direct evidence supports this hypothesis in ASM. In the present work, we provide evidence that PKG-I alpha upregulates GKCa channels derived from bovine tracheal smooth muscle cells and reconstituted into planar lipid bilayers. In two different experimental approaches, PKG increased the open probability as well as the mean open time of GKCa channels, without any effect on unitary current amplitudes and unit conductance. Our results indicate that the kinetics of GKCa channels are controlled by a phosphorylation step mediated by PKG, and thus might be modulated by intracellular cGMP. Biochemical assays demonstrated that PKG phosphorylates several protein bands in the membrane fraction. Two of those proteins co-migrate with the same relative molecular mass as the 62- and 30-kDa components of the purified channel complex, identified as GKCa-alpha and -beta subunits, respectively. Our results also indicate that PKG phosphorylates the GKCa-alpha subunit with an apparent stoichiometry of 0.89, which would be consistent with the presence of a single PKG-sensitive phosphorylating site within its amino acid sequence. Furthermore, these results demonstrate for the first time that PKG directly phosphorylates GKCa from airway smooth muscle cells and thereby activates the channels at negative voltage or at low free Ca2+ concentrations.

Stimulation of cGMP-dependent protein kinase I alpha by a peptide from its own sequence. An investigation by enzymology, circular dichroism and 1H NMR of the activity and structure of cGMP-dependent protein kinase I alpha-(546-576)-peptide amide

The structure of cGMP-dependent protein kinase I alpha-(546-576)-peptide amide (peptide-546) and its effects on cGMP-dependent protein kinase I alpha (G-kinase) have been studied. By primary sequence analysis and analogy to a peptide that stimulates protein kinase C, peptide-546 was predicted to form part of the protein/peptide binding site of G-kinase, and it was proposed that it would stimulate the enzyme by interaction with an autoinhibitory site. The portion of cAMP-dependent protein kinase analogous to peptide-546 forms part of the peptide substrate binding site, interacting with the peptide inhibitor residues Argp-2 and Phep-11 (where p is the pseudophosphorylation site), through residues at positions corresponding to Glu4, Pro10 and Ser13 in peptide-546. Peptide-546 is a reasonably potent G-kinase activator, increasing the turnover number with the peptide substrate Arg-Lys-Arg-Ser-Arg-Lys-Glu by about threefold with an activation constant that is about fivefold lower than the Km value of this peptide substrate. Peptide-546 does not appear to change the affinity of the enzyme for the above substrate, ATP or cGMP and does not affect the binding of [3H]cGMP to G-kinase. The activation does not seem to result from an interaction between peptide-546 and peptide substrates, and a kinetic scheme is proposed which is compatible with an action of peptide-546 on G-kinase independent of substrates. The activation is additive with that given by cGMP and causes the enzyme to enter a hitherto unrecognised superactive state. Peptide conformation has been monitored in mixed 2,2,2-trifluoroethanol/H2O solvents by circular dichroism: helical structure is observed in these mixtures when the 2,2,2-trifluoroethanol content is above 25%. The structure is lost only gradually on raising the temperature to 80 degrees C with no clear melting transition. Assignment of the resonances in the 1H-NMR spectrum has allowed the identification of elements of secondary structure from detected nuclear Overhauser effects. In particular, a helical segment from Met18 to Arg26 is observed. The four proline residues (Pro10, Pro11, Pro15 and Pro17) are all seen to be in the trans conformation, although additional, weaker peaks in the spectra may correspond to a minor conformer in which one or more of the prolines is in a cis conformation. The N-terminal residues are less structured but show some helical character.(ABSTRACT TRUNCATED AT 400 WORDS)

The receptors for endothelins and their analogues in SK-N-MC neuroblastoma cells

The potency order of peptides to inhibit [125I]endothelin-1 binding and to stimulate phosphatidylinositol phosphate (PtdInsP) turnover in SK-N-MC cells was consistent with the presence of ETA-endothelin receptors. Divalent cations enhanced [125I]endothelin-1 binding by, in the case of Mn2+, increasing radioligand affinity. Mn2+ did not induce conformational changes in endothelin-1, and its effect was maintained in solubilized receptors. Hence, metal ions may directly interact with endothelin receptors. The effects of BQ-123 and [Ala1,3,11,15]endothelin-1 on PtdInsP turnover were investigated. Concentration-response curves of endothelins were modeled by a second-order equation that assumes pseudoirreversible ligand binding.

Modelling and modification of the binding site of endothelin and other receptors

From three-dimensional models of its receptors, residues which bind the carboxy-terminus of endothelin were predicted. This site is in a pocket consisting of five putative transmembrane helices and includes a histidine in the sixth helix. This residue is either phenylalanine or asparagine in cationic neurotransmitter receptors. The histidine alkylating agent diethylpyrocarbonate potently inhibited binding of [125I]endothelin-1 to its receptors in bovine cerebellum, where a single population of endothelin ETB receptors was shown to exist. From the absence of pH sensitivity of inhibition above pH 5 and the reversal by hydroxylamine of inhibition, diethylpyrocarbonate is concluded to inhibit by histidine modification. Diethylpyrocarbonate inhibited ligand binding to several receptors with the potency order endothelin ETB > or = bombesin > or = dopamine D2 > or = m2 muscarinic > alpha 1-adrenoceptor > or = m 1 muscarinic > 5-HT2. This is consistent with histidine in the binding site of endothelin (and some other peptidergic) receptors and the proposed model.

Endothelins--from receptors to medicine

Since the discovery of endothelins, peptides with exceptional vasoconstrictor potency that were originally suggested to act by causing the opening of Ca2+ channels, it has emerged that these agents are important in intercellular communication in many tissues. They exert their effects through G protein-coupled receptors, of which two classes have been cloned. Robert Miller, John Pelton and John Huggins review the progress made towards a molecular understanding of ligand recognition by endothelin receptors. Receptor-selective agonists and antagonists have emerged from attempts to understand the three-dimensional structure of the endothelin pharmacophore, from structure-activity studies and from rapid-screening programmes. From the nature of the secretion and action of endothelins, it would seem that these peptides are involved in long-term changes rather than in acute responses to stimuli, and that they are likely to be important in a number of pathological states. Evidence suggests that receptor antagonists with appropriate affinity and selectivity may be useful in the treatment of conditions as diverse as hypertension, ulcerogenesis and ciclosporin toxicity.

The structure and specificity of endothelin receptors: their importance in physiology and medicine

In addition to involvement in vascular endothelium-smooth muscle communication, the secretion of and receptors for, endothelins are widely distributed. Two cloned receptor subtypes are G-protein-coupled to several intracellular messengers, predominantly inositol phosphates. From a knowledge of structure-activity relationships and peptide conformations, details of receptor architecture and selective agents, including nonpeptides and antagonists, have been discovered. From the nature of the actions of endothelins, receptor distributions (including CNS) and plasma levels, it is concluded that they are paracrine factors normally involved in long-term cellular regulation, but which may be important in several pathologies, many of which are stress-related.

1H NMR and circular dichroism studies of the N-terminal domain of cyclic GMP dependent protein kinase: a leucine/isoleucine zipper

Cyclic GMP dependent protein kinase exists as a dimer in its native form. A peptide corresponding to the dimerization domain in the N-terminal segment has been characterized by circular dichroism, ultracentrifugation, and 1H NMR spectroscopy. The peptide (G-kinase1-39 amide) is shown to be dimeric in solution. Determination of the molecular weight of the species in solution from the sedimentation coefficient and diffusion coefficient yields a value more than twice that of the monomeric species. Circular dichroism studies show G-kinase1-39 amide to be largely helical in aqueous solution and stable over a wide range of pH and temperature. The conformational stability is found to be concentration dependent, the peptide having a melting temperature of 75 degrees C (at 20 microM and pH 4.0). The assignment of the 1H NMR spectrum and analysis of the patterns of nuclear Overhauser enhancements confirm the helical nature of the conformation. Distance geometry calculations result in a well-defined helical structure containing a kink near Ser 26. The dimerization of G-kinase is most likely to occur through the hydrophobic interaction of leucine and isoleucine side chains located on one face of a helical structure with supporting electrostatic interactions between flanking side chains. The dimerization domain of G-kinase is clearly analogous to the "leucine zipper" motifs found in a number of DNA transcriptional activators.

Modulation by endothelium of the responses induced by endothelin-1 and by some of its analogues in rat isolated aorta

1. The contractile effects of endothelin-1 and various analogues were studied in rat isolated rings. The potency of the peptides studied was endothelin-1 greater than sarafotoxin S6b greater than [Ala3,11]endothelin-1 greater than [Ala1,15]endothelin-1. [Ala1,3,11,15]endothelin-1 was neither agonist nor antagonist. 2. The concentration of endothelin-1 required to induced contractions equal to 50% of those induced by 1 microM noradrenaline was reduced from 5.8 nM when the vascular endothelium was present to 1.4 nM after it had been mechanically removed. 3. Contractions elicited by [Ala3,11]endothelin-1, [Ala1,15]endothelin-1 and sarafotoxin S6b were not modulated by the endothelium. 4. Endothelin-1 increased the tissue content of guanosine 3',5'-cyclic monophosphate (cyclic GMP) in rat aortic segments with endothelium by about 4 fold, suggesting that it increased the release of endothelium-derived relaxing factor. Sarafotoxin S6b did not reduce, or significantly increase, tissue cyclic GMP levels and therefore had little effect on EDRF release. 5. The concentration of sodium nitroprosside required to relax half-maximally aortae denuded of endothelium was 430 nM if the aortae had been precontracted with 10 nM endothelin-1 and 35 nM if 10 nM sarafotoxin S6b was used as the spasmogen. This indicates that differential sensitivities of the smooth muscle to cyclic GMP cannot explain differences between responses to endothelin-1 and sarafotoxin S6b in the presence of endothelium. 6. It is concluded that endothelin-1 contractions of rat aorta are modified by the endothelium, probably by enhancing the release of endothelium-derived relaxing factor (EDRF) and not by affecting the sensitivity of the smooth muscle to EDRF. This suggests that a stimulated release of an adequate amount of EDRF is necessary to modulate contractile responses to these peptides.

Effects of cyclic GMP on the secondary structure of cyclic GMP dependent protein kinase and analysis of the enzyme's amino-terminal domain by far-ultraviolet circular dichroism

Far-UV circular dichroism spectra of bovine lung cyclic GMP dependent protein kinase (G-kinase) show that the enzyme contains alpha-helical and beta-pleated sheet elements. Binding of cyclic GMP changes the spectra in a way consistent with the induction of beta-sheet from random coil. Examination of the amino-terminal sequence of G-kinase indicates the presence of a strongly alpha-helical segment with several features in common with the leucine zipper motif. We propose that this sequence may be the important part of the dimerization domain of the enzyme. A synthetic peptide corresponding to amino acids 1-39 of G-kinase has a strongly alpha-helical CD spectrum, supporting the predicted secondary structure of this amino-terminal sequence. In contrast to the native enzyme, a structure reduced in alpha-helix was found when a constitutively active form of G-kinase, which lacks amino acids 1-77, was studied.

The expression of canine cardiac phospholamban in heterologous systems

A synthetic phospholamban gene has been cloned and expressed in Escherichia coli, producing both native phospholamban and a fusion protein with 81 amino acids of the influenza virus NS1 protein. Both the native phospholamban and fusion proteins produced extensive cell lysis upon induction of gene expression, but only the native protein underwent spontaneous pentamer formation in E. coli. Translation in vitro of mRNA produced by transcription in vitro of phospholamban cDNA was used to demonstrate the spontaneous aggregation of phospholamban to form pentamers in this system also, both in the presence and absence of exogenous microsomes from canine pancreas or heart. Phospholamban produced by translation in vitro was apparently susceptible to proteolysis by enzymes present in the particulate fractions in these experiments.

Phospholamban is a good substrate for cyclic GMP-dependent protein kinase in vitro, but not in intact cardiac or smooth muscle

1. Cyclic GMP-dependent protein kinase phosphorylates purified phospholamban. It also phosphorylates phospholamban present in vesicles of cardiac sarcoplasmic reticulum and smooth muscle microsomal fractions, and in transformants of Escherichia coli which contain a plasmid into which a gene encoding phospholamban has been inserted. 2. In vitro the phospholamban present in cardiac sarcoplasmic reticulum membranes is a better substrate for cyclic GMP-dependent protein kinase than for cyclic AMP-dependent protein kinase. 3. Studies using [32P]Pi to label the cellular ATP in intact cardiac or smooth muscle failed to demonstrate that phosphorylation of phospholamban occurs in response to stimuli which increase intracellular cyclic GMP. Possible reasons for this functional separation between increased cyclic GMP and phosphorylation of phospholamban are discussed.

The effects of calcium, temperature and phospholamban phosphorylation on the dynamics of the calcium-stimulated ATPase of canine cardiac sarcoplasmic reticulum

Highly purified sarcoplasmic reticulum (SR) has been prepared from dog hearts and has been incubated with the triplet probe erythrosinyl isothiocyanate to specifically label the Ca2+-stimulated ATPase (Ca2+-ATPase) of the SR. The rotational mobility of the Ca2+-ATPase has been studied in this erythrosin-labelled SR using time-resolved phosphorescence polarization. Qualitatively, the mobility of the cardiac Ca2+-ATPase resembles that of skeletal muscle SR Ca2+-ATPase. Addition of Ca2+ to SR affects the mobility of the Ca2+-ATPase in a way consistent with a segment of the ATPase altering its orientation relative to the plane of the membrane. Phosphorylation of phospholamban in cardiac SR by the purified catalytic subunit of cAMP-dependent protein kinase, which is known to increase the activity of the Ca2+-ATPase by deinhibition, also alters measured anisotropy. The changes observed are not compatible with dissociation of the Ca2+-ATPase from phospholamban after the latter is phosphorylated. The data are more consistent with phospholamban associating with the Ca2+-ATPase following phosphorylation, or more complex models in which only the hydrophilic domain of phospholamban binds with and dissociates from the Ca2+-ATPase.

Evidence for a phosphorylation-induced conformational change in phospholamban from the effects of three proteases

The effect of the proteases trypsin, thermolysin and papain on the cardiac membrane protein phospholamban was examined before or after phosphorylating the protein with the catalytic subunit of cyclic AMP-dependent protein kinase. The sensitivity of phospholamban to digestion by trypsin and thermolysin was greatly reduced by phosphorylation, suggesting that phospholamban undergoes a conformational change upon phosphorylation. It is suggested that this change in conformation is the mechanism by which phospholamban phosphorylation relieves its inhibition of the sarcoplasmic reticulum Ca2+-ATPase pump.

Sarcolemmal phospholamban is phosphorylated in isolated rat hearts perfused with isoprenaline

Phosphorylation of phospholamban in cardiac sarcolemma is implicated in the increased influx of Ca2+ through the slow calcium channel induced by catecholamines. A method is described for the preparation of highly purified sarcolemmal vesicles from rat heart, and this has been used to examine the phosphorylation of phospholamban in 32Pi-perfused rat hearts. Phospholamban phosphorylation is increased 3-fold after 30 s of perfusion with 0.1 microM isoprenaline. The time course of this increase precedes the inotropic response by 5-10 s.