Pharmacological Heterogeneity Research Articles

Heteromerization of G Protein-Coupled Receptors: Relevance to Neurological Disorders and Neurotherapeutics

Because G protein-coupled receptors (GPCRs) are numerous, widely expressed and involved in major physiological responses, they represent a relevant therapeutic target for drug discovery, particularly regarding pharmacological treatments of neurological disorders. Among the biological phenomena regulating receptor function, GPCR heteromerization is an important emerging area of interest and investigation. There is increasing evidence showing that heteromerization contributes to the pharmacological heterogeneity of GPCRs by modulating receptor ontogeny, activation and recycling. Although in many cases the physiological relevance of receptor heteromerization has not been fully established, the unique pharmacological and functional properties of heteromers are likely to lead to new strategies in clinical medicine. This review describes the main GPCR heteromers and their implications for major neurological disorders such as Parkinson's disease, schizophrenia and addiction. A better understanding of molecular mechanisms underlying drug interactions related to the targeting of receptor heteromers could provide more specific and efficient therapeutic agents for the treatment of brain diseases.

Triptans for the Management of Migraine

Migraine is a chronic, recurrent, disabling condition that affects millions of people in the US and worldwide. Proper acute care treatment for migraineurs is essential for a full return of function and productivity. Triptans are serotonin (5-HT)(1B/1D) receptor agonists that are generally effective, well tolerated and safe. Seven triptans are available worldwide, although not all are available in every country, with multiple routes of administration, giving doctors and patients a wide choice. Despite the similarities of the available triptans, pharmacological heterogeneity offers slightly different efficacy profiles. All triptans are superior to placebo in clinical trials, and some, such as rizatriptan 10 mg, eletriptan 40 mg, almotriptan 12.5 mg, and zolmitriptan 2.5 and 5 mg are very similar to each other and to the prototype triptan, sumatriptan 100 mg. These five are known as the fast-acting triptans. Increased dosing can offer increased efficacy but may confer a higher risk of adverse events, which are usually mild to moderate and transient in nature. This paper critically reviews efficacy, safety and tolerability for the different formulations of sumatriptan, zolmitriptan, rizatriptan, naratriptan, almotriptan, eletriptan and frovatriptan.

Pharmacokinetic Drug Interaction Studies Must Consider Pharmacological Heterogeneity, Use of Repeated Dosing, and Translation Into a Message Understandable to Practicing Clinicians

de Leon, Jose MD*†; Spina, Edoardo MD, PhD‡§; Diaz, Francisco J. PhD∥ Author Information

Generation and characterisation of the first anti-human H3R445/453 isoform specific antibody probe

The H3 receptor subtype is a classic G-protein coupled receptor (GPCR) expressed almost exclusively in the CNS. The existence of multiple H3R isoform mRNA has opened up possibilities to account for the pharmacological heterogeneity inH3R, within and across species, which has long been recognised, reviewed in [1, 2]. The full length clone is found in most abundance in the CNS in all species studied so far, however there is regional variation in the distribution of the different isoform mRNAs [3, 4]. This has given rise to speculation that H3R heterogeneity could underlie different activities and functions of the H3 R in specific brain regions. Furthermore, there is growing evidence that heteroligomerisation of H3 isoforms may occur and yield a novel regulatory mechanism [5]. H3R ligands may interact with multiple H3 isoforms in vivo so the signalling and functional properties of the isoforms need to be characterised. In order to define the importance of human H3 receptor heterogeneity, isoform specific immunological probes are required. Herein, we provide information on a new anti-human H3R445/453 receptor isoform specific antibody.

It is a correlation between the pharmacological heterogeneity and clinical effect of atypicals?

It is a correlation between the pharmacological heterogeneity and clinical effect of atypicals?

NMDA receptor subunits: function and pharmacology

N-methyl-D-aspartate receptors (NMDARs) are glutamate-gated ion channels widely expressed in the central nervous system that play key roles in excitatory synaptic transmission. Because of their involvement in numerous neurological disorders, NMDARs are also targets of therapeutic interest. NMDARs occur as multiple subtypes which differ in their subunit composition and in their biophysical and pharmacological properties. In particular, NMDARs contain a diversity of sites at which endogenous ligands or pharmacological agents can act to modulate receptor activity in a subunit-selective manner, and recent structural and functional data have started to reveal the molecular determinants for this subunit selectivity. These include the binding sites for glutamate, the ion-channel pore and the recently identified allosteric sites on the N-terminal domain. Other potential sites yet unexplored by medicinal chemistry programs are also considered, in particular at the interface between subunits. Given the growing body of evidence that diverse brain disorders implicate different NMDAR subtypes, such as NR2B in pain or NR3A in white matter injury, there is a growing interest in exploiting the pharmacological heterogeneity of NMDARs for the development of novel NMDAR subtype-selective compounds.

A Pooled Analysis of Two Large Randomized Trials Comparing Ongoing Pregnancy Rates With Highly Purified Menotropin and Recombinant FSH in IVF Cycles

A recent meta-analysis of IVF/ICSI studies concluded that more randomized controlled trials are needed to precisely estimate the difference in ongoing pregnancy rates between menotropins and recombinant FSH (Fertil Steril 2003, 80: 1086). Differences in study designs and populations as well as pharmacological heterogeneity of the preparations included in the meta-analysis could affect the magnitude and precision of the observed difference. A pooled analysis integrating the individual data obtained from two large randomized trials comparing identical preparations and following a similar stimulation protocol should provide a precise estimate of the treatment difference between the two preparations.

Challenges Associated with Estimating Minimal Clinically Important Differences in COPD—The NHLBI Perspective

Chronic Obstructive Pulmonary Disease (COPD) is a common lung disease that exemplifies the value, as well as the difficulties and challenges, of using minimal clinically important differences (MCID) in clinical research. Development and validation of better endpoints for clinical studies is critical to research progress in COPD. However, the clinical, genetic, and pharmacological heterogeneity of the COPD patient population complicates attempts to define and validate MCIDs for COPD. It is difficult to identify a single measurable outcome that reflects the many components of the COPD patient's health state. Acute exacerbations of symptoms, which COPD patients often experience, present another challenge in the development of MCIDs for this disease. Consequently, the NHLBI does not require the use of MCIDs in clinical research. This allows research on the causes, prevention and diagnosis of COPD and use of endpoints for which an MCID is not yet known. It is important for the scientific community to reach agreement on what is a meaningful MCID in therapeutic trials for COPD. Further research into the concept of the MCID and its application should enable therapeutic trials in COPD to yield knowledge that is more effectively translated into improved public health.

Biochemical and Pharmacologic Heterogeneity in Low Molecular Weight Heparins. Impact on the Therapeutic Profile

Ever since the introduction of low molecular weight heparins (LMWHs) for clinical use, one of the major questions raised relates to product interchangeability and the differences between each of the individual LMWH preparations. Although differences between various commercially available products have been described in terms of molecular weight profile and biologic properties, very limited information on the direct comparison of individual products in a defined clinical setting is available at this time. European Pharmacopeia (EP) and the World Health Organization (WHO) have developed guidelines to characterize these agents in terms of molecular weight and biologic profiles. On a gravimetric basis, these potency assignments differ for anti-Xa and anti-IIa activities in terms of U potency per mg. The relative distribution of various molecular weight components has also been reported to vary. The oligosaccharide composition, microstructural differences in terms of specific sugars and the presence of unique structural features and the interaction with endogenous mediators such as antithrombin (AT) and heparin cofactor II (HC II) also differ. At equivalent anti-Xa levels, the amount of the anti-IIa activity and anticoagulant activity differs. Since the bioavailability and relative pharmacokinetics of the anti-Xa and anti-IIa effects are different, the specific pharmacodynamic effects of these drugs also differ. A large preclinical data base is now available on the differences between various LMWHs. However, only limited clinical data is available in the current literature. To date, the LMWHs have been primarily used for the management of post-surgical DVT. Only smaller dosages (30-40 mg or 2,500 to 4,000 anti-Xa U total dose) have been used. In these studies, because of the low dose and subcutaneous route of administration, the differences in clinical effects are rather small. Since LMWHs are now developed for therapeutic use, where relatively higher doses are used, these pharmacokinetic/pharmacodynamic differences will become more apparent. The reported differences in the clinical efficacy of LMWHs in such indications as unstable angina may be due to their pharmacologic properties and molecular composition. There are also major differences in the non-anticoagulant actions of these agents such as their ability to interact with growth factors and antithrombotic effects. Based on the available literature, it can be concluded that each product exhibits individuality.

Myogenic tone and reactivity of the rat ophthalmic artery.

To study and quantify myogenic behavior and reactivity of the rat ophthalmic artery to pressure and different vasoactive substances in vitro. Rat ophthalmic arteries (diameter of 217 +/- 6 microm, n = 22) were isolated, cannulated with glass pipettes in an arteriograph and pressurized in a physiological buffer. Internal diameter was continuously monitored. The effect of intraluminal pressure on the diameter was assessed and concentration-response curves to different constrictor and dilator agonists were obtained at an intraluminal pressure of 70 mm Hg. Myogenic tone developed at an intraluminal pressure of 30 to 40 mm Hg, continued to increase, and was maintained up to a pressure of 199 mm Hg in these arteries. Arteries dilated and constricted in response to 16 and 60 mM potassium, respectively. Endothelin-1 was the most potent and efficacious constrictor, with a biphasic concentration-response curve, followed by vasopressin, serotonin, U-46619 and phenylephrine. Carbachol was the most efficacious dilator, followed by isoprenaline. The peptide dilators calcitonin gene-related peptide (CGRP) and vasoactive intestinal peptide (VIP) were potent but less efficacious than carbachol and isoprenaline. Histamine and adenosine were even less potent and less efficacious dilators. NG-nitro-L-arginine methyl ester (L-NAME) constricted and indomethacin dilated the arteries. This study provides the first direct evidence for myogenic autoregulatory properties and pharmacological heterogeneity in the rat ophthalmic artery.

5-HT 3 receptors in the CNS: 3B or not 3B?

5-HT 3 receptors are widely distributed in the CNS and PNS where they participate in a variety of physiological processes. Native 5-HT 3 receptors in the CNS display functional and pharmacological heterogeneity, suggesting the existence of multiple receptor subunits. However, recent evidence suggests that of the two known subunits only the 5-HT 3A receptor subunit (and not the 5-HT 3B receptor subunit) is functionally present in the CNS. The molecular makeup of the 5-HT 3 receptor therefore remains an open question.

Antiadrenergic therapy of chronic heart failure: surprises and new opportunities.

The great British-American scientist-philosopher Alfred North Whitehead divided progress into three stages, the second of which was termed precision, in which the “right ways and wrong ways” of an original idea are elucidated.1 During this period, reinterpretation of the basic idea occurs and is essential to progress. Whitehead’s message is that important ideas are dynamic instruments that are constantly changing as new and usually unexpected information becomes available. This is why tests of particular hypotheses, including those tested in Phase III clinical trials, are often unsupportive, and why therapeutic paradigms constantly change. Within such an ephemeral milieu, the key to ultimate success is to view each unexpected or negative result as an opportunity for constructing and testing even more novel and valuable hypotheses within the framework of the general idea, to ascend to the final stage where progress can be “generalized.”1 Whitehead’s philosophical legacy is impressively in play in the area of antiadrenergic therapy of chronic heart failure (CHF). The unarguable basic idea is that the biologically powerful adrenergic compensatory mechanism plays a critical role in the natural history of CHF. It is the details or nuances within the general paradigm that continue to change, and lately, surprisingly so. As recently reviewed,2 the importance of dysfunctional adrenergic activation in CHF was first elucidated by work performed by Braunwald’s group at the National Institutes of Health in the 1960s. Among other things, this early work provided the first evidence of marked adrenergic activation in CHF. However, on the basis of reduction in myocardial tissue norepinephrine, and the short-term effects of large doses of antiadrenergic agents,3 the overall interpretation was that adrenergic support was deficient in CHF. This view prevailed for 10 to 15 years, and it contributed to the original logic behind developing Type III phosphodiesterase inhibitors …

Pharmacological heterogeneity of release-regulating presynaptic AMPA/kainate receptors in the rat brain: study with receptor antagonists

Presynaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptors mediating hippocampal [ 3H]noradrenaline or [ 3H]serotonin release, striatal [ 3H]dopamine release and cortical [ 3H]acetylcholine release were pharmacologically characterized using several AMPA/kainate receptor antagonists. The releases of the four transmitters elicited by exposing synaptosomes to AMPA were antagonized by NBQX, indicating that they reflect AMPA/kainate receptor activation. GYKI52466 did not inhibit the AMPA-induced release of [ 3H]noradrenaline, [ 3H]dopamine or [ 3H]serotonin, while it weakly affected the AMPA-mediated release of [ 3H]acetylcholine. On the contrary, LY300164 and LY303070 were potent antagonists able to discriminate among AMPA/kainate receptor subtypes. Both compounds blocked the AMPA receptors mediating [ 3H]dopamine and [ 3H]acetylcholine release. However, LY303070, but not LY300164, inhibited the AMPA-induced release of [ 3H]noradrenaline, while the AMPA-mediated [ 3H]serotonin release was sensitive to LY300164 but not to LY303070. SYM2206 mimicked LY300164 and prevented the AMPA-induced release of [ 3H]dopamine, [ 3H]acetylcholine and [ 3H]serotonin, but not that of [ 3H]noradrenaline. NS102 failed to antagonize the AMPA-induced release of all four transmitters. LY293558 prevented the AMPA-mediated release of [ 3H]noradrenaline, [ 3H]dopamine, [ 3H]acetylcholine or [ 3H]serotonin. Differently, LY377770 did not inhibit the AMPA-mediated release of [ 3H]noradrenaline and [ 3H]acetylcholine, but it potently blocked the AMPA-induced release of [ 3H]serotonin and, less so, of [ 3H]dopamine. AMOA inhibited the AMPA-induced release of [ 3H]serotonin or [ 3H]acetylcholine, but not that of [ 3H]noradrenaline or [ 3H]dopamine. GAMS prevented the AMPA-mediated release of [ 3H]acetylcholine and, more weakly, that of [ 3H]dopamine, but it failed to inhibit the release of [ 3H]noradrenaline or [ 3H]serotonin elicited by AMPA. γ-DGG did not affect the AMPA-mediated release of any of the four transmitters studied. In conclusion, based on the antagonist profiles obtained, the four receptors here analyzed all belong to the AMPA-preferring subclass of glutamate receptors; however, they appear to differ from each other, probably due to differences in subunit composition. The compounds LY300164, LY303070, LY377770, AMOA and GAMS may be useful to discriminate among AMPA-preferring receptor subtypes.

Treatments conferring renoprotection in patients with Nephrotic Syndrome

SUMMARY: Nephrotic Syndrome that fails to respond to specific treatment presents a major therapeutic conundrum. Management strategies aim to normalize glomerular filtration barrier dysfunction, optimize intrarenal haemodynamics to limit stress on capillary walls and limit adverse signalling profiles in the tubulo‐interstitial compartment. Angiotensin converting enzyme (ACE) inhibitors are currently the only proven therapeutic option for patients with Nephrotic Syndrome, although several other agents have demonstrated potential. However, these therapies have not been evaluated in well‐designed trials and their efficacy is largely untested. The angiotensin‐II receptor (AII) antagonists, that interrupt the renin‐angiotensin cascade at a different point to ACE inhibitors, may also have renoprotective effects although few long‐term data are available. The efficacy of another class of antihypertensive drugs, the calcium channel blockers, is unclear, perhaps because of the pharmacological heterogeneity among the group. Rigorous comparative studies of these different agents in patients with Nephrotic Syndrome have not been conducted. There are even less data about non‐antihypertensive agents like Cyclosporin A (CsA), dipyridamole and indomethacin. Dietary manipulations to restrict protein and sodium intake and reducing serum lipid levels with diet and drug intervention may have additional renal benefits in selected patients.

Treatments conferring renoprotection in patients with Nephrotic Syndrome

SUMMARY: Nephrotic Syndrome that fails to respond to specific treatment presents a major therapeutic conundrum. Management strategies aim to normalize glomerular filtration barrier dysfunction, optimize intrarenal haemodynamics to limit stress on capillary walls and limit adverse signalling profiles in the tubulo‐interstitial compartment. Angiotensin converting enzyme (ACE) inhibitors are currently the only proven therapeutic option for patients with Nephrotic Syndrome, although several other agents have demonstrated potential. However, these therapies have not been evaluated in well‐designed trials and their efficacy is largely untested. the angiotensin‐II receptor (All) antagonists, that interrupt the renin‐angiotensin cascade at a different point to ACE inhibitors, may also have renoprotective effects although few long‐term data are available. the efficacy of another class of antihypertensive drugs, the calcium channel blockers, is unclear, perhaps because of the pharmacological heterogeneity among the group. Rigorous comparative studies of these different agents in patients with Nephrotic Syndrome have not been conducted. There are even less data about non‐antihypertensive agents like Cyclosporin A (CsA), dipyridamole and indomethacin. Dietary manipulations to restrict protein and sodium intake and reducing serum lipid levels with diet and drug intervention may have additional renal benefits in selected patients.

Species-related pharmacological heterogeneity of histamine H 3 receptors

We compared radioligand binding and functional data for histamine H 3 receptor ligands across different tissues or species to evaluate the basis for pharmacological evidence of receptor heterogeneity previously reported. Agonist binding affinities showed correlation coefficients near unity in comparing human, dog, rat, and guinea pig cerebral cortical histamine H 3 receptors. Antagonist binding affinities revealed lower correlations for human compared to dog, rat, or guinea pig, suggesting species-based pharmacological differences. The functional potencies of histamine H 3 receptor antagonists in field-stimulated guinea pig ileum were highly correlated to binding affinities for guinea pig, dog, and, to a lesser extent, rat cerebral cortex. However, antagonist binding affinity at human cerebral cortex did not correlate well with guinea pig ileum functional potency. These results suggest significant interspecies histamine H 3 receptor heterogeneity, consistent with recent receptor gene sequence data. Therefore, genetic heterogeneity, rather than peripheral and central histamine H 3 receptor diversity, is responsible for the pharmacological differences observed.

Pharmacological heterogeneity of γ-aminobutyric acid receptors during development suggests distinct classes of rat cerebellar granule cells in situ

The γ-aminobutyric acid receptor (GABA AR) represents a ligand-gated Cl −-channel assembling as heteropentamere from 19 known subunits. Cerebellar granule cells contain a unique subset, namely the α1-, α6-, β2-, γ2- and δ-subunits. We studied their GABAergic pharmacology in situ using whole-cell patch-clamp recordings in brain slices and a modified Y-tube application system. The distribution of the EC 50s for GABA in young (P8–P14) and medium aged animals (P15–P28) could be fitted with the sum of two Gaussian distributions with means of 60 and 185 μM and 27 and 214 μM, respectively. In older animals (P29–P48) the observed homogeneous range of sensitivities fitted a single Gaussian distribution (11 μM). In young animals (≤P14) GABA-responses were largely insensitive towards 300 μM of the α6-specific inhibitor furosemide (82% of control response). The sensitivity increased in older animals at the EC 5–20 of GABA (31% of control responses), supporting an increased expression of α6-subunits as molecular basis for the observed developmental changes. Approximately 50% of cells in the age range P15–P48 were potentiated by 1 μM diazepam and by 3 μM methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM), suggesting the concurrent presence of α1- and α6-containing receptors, whereas the remaining of cells were neither potentiated by diazepam nor did they show the α6-typical DMCM potentiation, though they were potentiated by loreclezole. These properties indicate unknown pharmacological characteristics of cerebellar receptor-subunit combinations in approximately 50% of granule cells in situ.

NMDA receptor pharmacology: perspectives from molecular biology.

The NMDA receptor is an important target for drug development, with agents from many different classes acting on this receptor. While the severe side effects associated with complete NMDA receptor blockade have limited clinical usefulness of most antagonists, the understanding of the multiple forms of NMDA receptors provides an opportunity for development of subtype specific agents with potentially fewer side effects. Different NMDA receptor subtypes are assembled from combinations of NR1 and NR2 subunits with each subunit conveying distinct properties. The NRI subunit is the glycine binding subunit and exists as 8 splice variants of a single gene. The glutamate binding subunit is the NR2 subunit, which is generated as the product of four distinct genes, and provides most of the structural basis for heterogeneity in NMDA receptors. Pharmacological heterogeneity results from differences in the structure of ligand binding regions, as well as structural differences between subtypes in a modulatory region called the LIVBP-like domain. This region in NR1 and NR2B controls the action of NR2B-selective drugs like ifenprodil, while this domain in receptors containing the NR2A subunit controls the action of NR2A-selective drugs such as zinc. This suggests that NMDA receptor subtype selective drugs can be created, and further understanding of subtype specific mechanisms ultimately may allow successful use of NMDA receptor antagonists as therapeutic agents.

Polyamine and Redox Modulation of [3H]MK‐801 Binding to N‐Methyl‐d‐aspartate Receptors in the Spinal Cord and Cerebral Cortex

The pharmacology of N-methyl-D-aspartate (NMDA) receptors shows regional differences in affinity for various agonists and antagonists. We have investigated the modulatory mechanisms acting via the polyamine, redox and proton sites in the cerebral cortex and the spinal cord of adult, male rats using [3H]MK-801 binding. The affinity for glycine-independent spermine stimulation was one magnitude higher in cerebrocortical than in spinal cord membranes while the affinity for the spermine antagonist arcaine was similar. Spermine abolished the inhibiting effect of low pH in both regions. Thus, the difference in the polyamine site between the two regions seems to be restricted to agonist binding. The proportion of high affinity/total ifenprodil binding was approximately 35% both in the spinal cord and the cerebral cortex, suggesting similar relative amounts of the NMDA receptor subunit 2B. The affinity of ifenprodil to the high affinity site was however significantly higher in the cerebral cortex. Redox modulatory agents had similar effects in the two regions but spermine fully counteracted the inhibiting effect of 0.2 mM 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB) in the cerebral cortex while there was only a partial effect in the spinal cord. These data show that the regional pharmacological heterogeneity involves several of the mechanisms regulating the function of the NMDA receptor. The data also indicate that the NMDA receptor subunit 2B is much more common in spinal cord than previously suggested.

Antithrombotic therapy in acute coronary syndromes: Key notes from ESSENCE and TIMI 11B

Platelet activation and thrombus formation are key events in the pathogenesis of acute coronary syndromes (unstable angina and non-Q-wave myocardial infarction). Therefore, current management of these conditions consists of antithrombotic and antiplatelet therapy, principally heparin and oral aspirin. However, such treatment is unsuccessful in a significant proportion of patients. Two recent large studies with the low-molecular-weight heparin (LMWH) enoxaparin have shown that this agent significantly reduces the risk of major ischemic events compared with unfractionated heparin. In the Efficacy and Safety of Subcutaneous Enoxaparin in Non-Q-Wave Coronary Events (ESSENCE) study, treatment with enoxaparin for 2 to 8 days reduced the risk of death, myocardial infarction, or recurrent angina by 20% at 14 days compared with treatment with unfractionated heparin. In the Thrombolysis in Myocardial Infarction (TIMI) 11B study, enoxaparin was associated with a significant reduction in the risk of death, myocardial infarction, or urgent revascularization compared with unfractionated heparin, which became apparent within 48 hours; this benefit was maintained during outpatient treatment for 5 weeks. A meta-analysis of these two studies showed that the risk of death or myocardial infarction was consistently approximately 20% lower in enoxaparin-treated patients than in heparin-treated patients. In contrast, studies with other LMWHs have not shown consistent superiority over unfractionated heparin. This may reflect the pharmacologic heterogeneity of LMWH and/or differences in trial design.