Skeletal muscle tremor and the influence of adrenergic drugs

Sarcomeric deficits underlie MYBPC1-associated myopathy with myogenic tremor

Myosin binding protein-C slow (sMyBP-C) comprises a subfamily of cytoskeletal proteins encoded by MYBPC1 that is expressed in skeletal muscles where it contributes to myosin thick filament stabilization and actomyosin cross-bridge regulation. Recently, our group described the causal association of dominant missense pathogenic variants in MYBPC1 with an early-onset myopathy characterized by generalized muscle weakness, hypotonia, dysmorphia, skeletal deformities, and myogenic tremor, occurring in the absence of neuropathy. To mechanistically interrogate the etiologies of this MYBPC1-associated myopathy in vivo, we generated a knock-in mouse model carrying the E248K pathogenic variant. Using a battery of phenotypic, behavioral, and physiological measurements spanning neonatal to young adult life, we found that heterozygous E248K mice faithfully recapitulated the onset and progression of generalized myopathy, tremor occurrence, and skeletal deformities seen in human carriers. Moreover, using a combination of biochemical, ultrastructural, and contractile assessments at the level of the tissue, cell, and myofilaments, we show that the loss-of-function phenotype observed in mutant muscles is primarily driven by disordered and misaligned sarcomeres containing fragmented and out-of-register internal membranes that result in reduced force production and tremor initiation. Collectively, our findings provide mechanistic insights underscoring the E248K-disease pathogenesis and offer a relevant preclinical model for therapeutic discovery.

Insights into Pathophysiology from Medication-induced Tremor

Background

Medication-induced tremor (MIT) is common in clinical practice and there are many medications/drugs that can cause or exacerbate tremors. MIT typically occurs by enhancement of physiological tremor (EPT), but not all drugs cause tremor in this way. In this manuscript, we review how some common examples of MIT have informed us about the pathophysiology of tremor.

Methods

We performed a PubMed literature search for published articles dealing with MIT and attempted to identify articles that especially dealt with the medication's mechanism of inducing tremor.

Results

There is a paucity of literature that deals with the mechanisms of MIT, with most manuscripts only describing the frequency and clinical settings where MIT is observed. That being said, MIT emanates from multiple mechanisms depending on the drug and it often takes an individualized approach to manage MIT in a given patient.

Discussion

MIT has provided some insight into the mechanisms of tremors we see in clinical practice. The exact mechanism of MIT is unknown for most medications that cause tremor, but it is assumed that in most cases physiological tremor is influenced by these medications. Some medications (epinephrine) that cause EPT likely lead to tremor by peripheral mechanisms in the muscle (β-adrenergic agonists), but others may influence the central component (amitriptyline). Other drugs can cause tremor, presumably by blockade of dopamine receptors in the basal ganglia (dopamine-blocking agents), by secondary effects such as causing hyperthyroidism (amiodarone), or by other mechanisms. We will attempt to discuss what is known and unknown about the pathophysiology of the most common MITs.

Use of ICS/LABA (extra-fine and non-extra-fine) in elderly asthmatics

Age represents an exclusion criterion in randomized clinical trials designed to test the efficacy and safety of inhaled drugs in asthma. As a consequence, data on efficacy and safety of inhaled corticosteroid (ICS) and long-acting β2 agonist (LABA) combinations in elderly asthmatics are scanty. Older age is associated with an increased proportion of comorbid conditions; in addition, all organ functions undergo a process of senescence, thus reducing their ability to metabolize the agents. Overall, these age-associated conditions may variably, and often unpredictably, affect the metabolism and excretion of respiratory drugs. However, pharmacological treatment of asthma does not follow specific recommendations in the elderly. In the elderly, the ICS/LABA combinations may carry an increased risk of local indesiderable effects, primarily due to the lack of coordination between activation of the device and inhalation, and systemic adverse events, mainly due to the greater amount of active drug that is available because of the age-associated changes in organ functions as well as drug-to-drug and drug-to-concomitant disease interactions. The extra-fine formulations of ICSs/LABAs, which allow for a more favorable drug deposition in the lungs at a reduced dose, may contribute to overcome this issue. This review revises the efficacy and safety of treatment with ICSs/LABAs, focusing on the main pharmacodynamic and pharmacokinetic properties of the drugs and highlighting the potential risks in the elderly asthmatic population.

Characteristics of postural control among young adults with asthma

OBJECTIVE

We investigated whether young adults with asthma have impaired balance and whether this impairment is related to altered musculoskeletal function and/or psychological characteristics.

METHODS

21 participants with a self-reported asthma diagnosis but no known postural instability or history of falls, and 18 control participants were recruited from undergraduate psychology courses. Participants performed a postural control task of maintaining the center of pressure (COP) in a fixed position with visual feedback (feedback condition) and while standing as still as possible without visual feedback (no-feedback condition). COP variability, regularity and task performance were used to characterize the quality of balance. To document group differences in musculoskeletal function, we measured neck and lower back angles as well as range of motion (ROM) of the neck in the frontal and sagittal planes. To document group differences in psychological state, we administered self-report questionnaires to assess symptoms of anxiety and depression, anxiety sensitivity and negative effect.

RESULTS

COP variability and task performance were similar between the groups, but participants with asthma exhibited more regular anterior-posterior COP dynamics. Participants with asthma had smaller ROM of neck extension, a more forwardly bent neck, greater thoracic spine angle, and they reported greater levels of the physical concerns facet of anxiety sensitivity. These musculoskeletal and affective variables moderated COP differences between the groups.

CONCLUSIONS

Young adults with asthma showed a different postural control strategy in the absence of any obvious balance impairment. This change in strategy is related to musculoskeletal and affective characteristics of individuals with asthma.

Bilateral tremor responses to unilateral loading and fatiguing muscle contractions

Although physiological tremor has been extensively studied within a single limb, tremor relationships between limbs are not well understood. Early investigations proposed that tremor in each limb is driven by CNS oscillators operating in parallel. However, recent evidence suggests that tremor in both limbs arises from shared neural inputs and is more likely to be observed under perturbed conditions. In the present study, postural tremor about the elbow joint and elbow flexor EMG activity were examined on both sides of the body in response to unilateral loading and fatiguing muscle contractions. Applying loads of 0.5, 1.0, 1.5, and 3.0 kg to a single limb increased tremor and muscle activity in the loaded limb but did not affect the unloaded limb, indicating that manipulating the inertial characteristics of a limb does not evoke bilateral tremor responses. In contrast, maximal-effort unilateral isometric contractions resulted in increased tremor and muscle activity in both the active limb and the nonactive limb without any changes in between-limb tremor or muscle coupling. When unilateral contractions were repeated intermittently, to the extent that maximum torque generation about the elbow joint declined by 50%, different tremor profiles were observed in each limb. Specifically, unilateral fatigue altered coupling between limbs and generated a bilateral response such that tremor and brachioradialis EMG decreased for the fatigued limb and increased in the contralateral nonfatigued limb. Our results demonstrate that activity in the nonactive limb may be due to a "spillover" effect rather than directly coupled neural output to both arms and that between-limb coupling for tremor and muscle activity is only altered under considerably perturbed conditions, such as fatigue-inducing contractions.

Pharmacology and therapeutics of bronchodilators

Bronchodilators are central in the treatment of of airways disorders. They are the mainstay of the current management of chronic obstructive pulmonary disease (COPD) and are critical in the symptomatic management of asthma, although controversies around the use of these drugs remain. Bronchodilators work through their direct relaxation effect on airway smooth muscle cells. at present, three major classes of bronchodilators, β(2)-adrenoceptor (AR) agonists, muscarinic receptor antagonists, and xanthines are available and can be used individually or in combination. The use of the inhaled route is currently preferred to minimize systemic effects. Fast- and short-acting agents are best used for rescue of symptoms, whereas long-acting agents are best used for maintenance therapy. It has proven difficult to discover novel classes of bronchodilator drugs, although potential new targets are emerging. Consequently, the logical approach has been to improve the existing bronchodilators, although several novel broncholytic classes are under development. An important step in simplifying asthma and COPD management and improving adherence with prescribed therapy is to reduce the dose frequency to the minimum necessary to maintain disease control. Therefore, the incorporation of once-daily dose administration is an important strategy to improve adherence. Several once-daily β(2)-AR agonists or ultra-long-acting β(2)-AR-agonists (LABAs), such as indacaterol, olodaterol, and vilanterol, are already in the market or under development for the treatment of COPD and asthma, but current recommendations suggest the use of LABAs only in combination with an inhaled corticosteroid. In addition, some new potentially long-acting antimuscarinic agents, such as glycopyrronium bromide (NVA-237), aclidinium bromide, and umeclidinium bromide (GSK573719), are under development, as well as combinations of several classes of long-acting bronchodilator drugs, in an attempt to simplify treatment regimens as much as possible. This review will describe the pharmacology and therapeutics of old, new, and emerging classes of bronchodilator.

Third-generation long-acting β₂-adrenoceptor agonists: medicinal chemistry strategies employed in the identification of once-daily inhaled β₂-adrenoceptor agonists

Inhaled long-acting β(2)-adrenoceptor agonists (LABAs) are highly effective bronchodilators in the treatment of asthma and chronic obstructive pulmonary disease. There is significant interest in the development of third-generation compounds that improve upon the marketed twice-daily LABAs salmeterol and formoterol. A principal advantage sought from the next generation is duration of action that supports once-daily dosing, although improved efficacy, faster onset, and increased therapeutic index are also frequently cited as objectives. Recent publications detailing medicinal chemistry programs directed at the discovery of third-generation LABAs illustrate a wide variety of strategies that have been successfully employed towards these goals. Some recent scientific advances in the understanding of inhaled bronchodilators are discussed and the reported medicinal chemistry strategies are reviewed in the context of these advances.

Tremor and β(2)-adrenergic agents: is it a real clinical problem?

Tremor is one of the most characteristic adverse effects following administration of β(2)-adrenergic agonists. It is reported by around 2-4% of patients with asthma taking a regular β(2)-adrenergic agonist and is induced by both short-acting and long-acting agents. Tremor associated with β(2)-adrenergic agonists is dose-related and may occur more commonly with oral dosing. The exact mechanism for tremor induction by β(2)-adrenergic agonists is still unknown, but there is some evidence that β(2)-adrenergic agonists act directly on muscle. An early explanation of the tremor was that β(2)-adrenoceptor stimulation shortens the active state of skeletal muscle, which leads to incomplete fusion and reduced tension of tetanic contractions. More recently, tremor has been correlated closely with hypokalaemia. A possible diverse impact of different modes of administration of β(2)-adrenergic agonists on tremorogenic responses has been suggested but solid evidence is still lacking. In any case, the desensitization of β(2)-adrenoceptors that occurs during the first few days of regular use of a β(2)-adrenergic agonist accounts for the commonly observed resolution of tremor after the first few doses. Therefore, tremor is not a really important adverse effect in patients under regular treatment with a β(2)-adrenergic agonist.

A dose-ranging study of indacaterol in obstructive airways disease, with a tiotropium comparison

This dose-ranging study assessed the bronchodilator efficacy and tolerability of indacaterol, a novel once-daily inhaled beta2-agonist, in subjects clinically diagnosed with COPD. Comparative data with tiotropium were collected. In the double-blind, core period of the study, 635 subjects with COPD (prebronchodilator FEV(1)40% of predicted and > or =1.0L; FEV1/FVC <70%) were randomized to receive indacaterol 50, 100, 200 or 400microg or placebo via multi-dose dry powder inhaler, or indacaterol 400microg via single-dose dry powder inhaler, once daily for 7 days. After completing double-blind treatment and washout, a subset of subjects from each treatment group entered an open-label extension and received tiotropium 18microg once daily for 8 days. The primary efficacy variable was the trough bronchodilator effect: standardized area under the FEV1 curve between 22 and 24h post-dose (FEV1 AUC(22-24h)) on Day 1. Clinically relevant improvements versus placebo in FEV1 AUC(22-24h) were seen for 400 and 200microg doses on Day 1 and all doses on Day 7. All indacaterol doses significantly (P<0.05) increased FEV1 from 5min to 24h post-dose; the 400 and 200microg doses were most effective. All doses were well tolerated. Indacaterol trough FEV1 levels compared favorably with the improvement seen by Day 8 in subjects treated with tiotropium in the open-label extension. The results confirm that indacaterol has a 24-h duration of bronchodilator effect and a fast onset of action in COPD and suggest that indacaterol could be an effective once-daily inhaled beta2-agonist bronchodilator. Indacaterol demonstrated a good overall safety and tolerability profile.

Beta-adrenoceptor responses of the airways: for better or worse?

Beta2-adrenoceptor agonists are the first-line treatment of asthma and chronic obstructive pulmonary disease (COPD), in which a short-acting beta2-adrenoceptor agonist is used as required for relief of bronchoconstriction. A long-acting beta2-adrenoceptor agonist may be added to an inhaled corticosteroid as step 3 in the management of chronic asthma. Long-acting beta2-adrenoceptor agonists may also be added in treatment of COPD. This review examines the beneficial and detrimental effects of beta2-adrenoceptor agonists. The beneficial effects of beta2-adrenoceptor agonists are mainly derived from their bronchodilator activity which relieves the bronchiolar narrowing and improves air flow. The potential anti-inflammatory actions of stabilizing mast cell degranulation and release of inflammatory and bronchoconstrictor mediators, is considered. Other potential beneficial responses include improvements in mucociliary clearance and inhibition of extravasation of plasma proteins that is involved in oedema formation in asthma. The side effects of beta2-adrenoceptor agonists are primarily related to beta2-adrenoceptor-mediated responses at sites outside the airways. Of major concern has been the development of tolerance and this is discussed in relation to incidence of increased morbidity and mortality to asthma over the past three decades. A clinical aspect of beta2-adrenoceptor pharmacology in recent years has been the recognition of genetic polymorphism of the receptor and how this affects responses to and tolerance to beta2-adrenoceptor agonists. A controversial feature of beta2-adrenoceptor agonists is their stereoisomerism and whether the inactive (S)-isomer of salbutamol had detrimental actions in the commercially used racemate. The consensus is that despite these adverse properties, beta2-adrenoceptor agonist remains the most useful pharmacological agents in the management of asthma and COPD.

Pharmacokinetics of bambuterol in healthy subjects

AIMS

To study the pharmacokinetics and bioavailability of the prodrug bambuterol and its bronchodilator moiety terbutaline in healthy subjects.

METHODS

Eight healthy subjects (four women) received intravenous doses of bambuterol and terbutaline. On a third occasion, they, plus another four subjects, ingested oral bambuterol as a single dose followed by repeated doses once daily for 7 days. Plasma concentrations and urinary excretion of bambuterol and terbutaline were measured.

RESULTS

After intravenous administration, renal clearances of bambuterol and terbutaline were similar (about 140 ml min(-1)), but there was a five-fold difference in total clearance (bambuterol 1.25 l min(-1), terbutaline 0.23 l min(-1)). Volume of distribution (Vss) was 1.6 l kg(-1) b.w. for both substances. A similar renal clearance of bambuterol was found during oral administration but that of terbutaline decreased (to about 120 ml min(-1)). Mean terminal half-life of bambuterol was 2.6 h after intravenous and 12 h after oral administration, implying that uptake was rate-limiting. Mean residence time of terbutaline generated from oral bambuterol was 34 h compared with 8.0 h when terbutaline as such was infused. Generated terbutaline had a bioavailability of 36% (28-46) after intravenous and 10.2% (6.1-13.2) after oral administration of the prodrug. Bambuterol was well tolerated. The mean activity of plasma cholinesterase, an enzyme catalyzing bambuterol metabolism, was inhibited between 30-60% during repeated oral dosing. It virtually regained original activity within 48 h after the last dose.

CONCLUSIONS

The plasma concentration ofterbutaline fluctuated little during repeated oral administration (mean peak: trough ratio 1.9), as a result of prolonged absorption of bambuterol and slow formation of terbutaline. Thus, the pharmacokinetic properties of bambuterol make it suitable for oral once-daily dosage.

Trimethoprim/sulfamethoxazole-induced tremor in a patient with AIDS

OBJECTIVE

To report a case of trimethoprim/sulfamethoxazole (TMP/SMX)-induced tremor responsive to a reduction in dosage.

CASE SUMMARY

A 55-year-old white man with AIDS and Pneumocystis carinii pneumonia (PCP) developed a tremor after receiving 5 days of therapy with TMP/SMX 19.4 mg/kg/d (TMP). The tremor resolved completely 3 days after a dosage reduction to TMP/SMX 15.1 mg/kg/d.

DISCUSSION

Central nervous system adverse reactions to TMP/SMX have been reported in both the AIDS and non-AIDS populations. To our knowledge, this is the first reported case of TMP/SMX-induced tremor responsive to a reduction in dosage. Pharmacokinetic and clinical data suggest a concentration-dependent etiology for various adverse effects, including tremor. The mechanism of the tremor is unknown; however, toxic metabolites of SMX and disruptions of biogenic amine neurotransmission by TMP have been hypothesized.

CONCLUSIONS

TMP/SMX remains the drug of first choice for treating PCP, but it is clearly not well tolerated by patients with AIDS. Concentration-dependent toxicities such as tremor may lead to premature discontinuation of proven, effective TMP/SMX therapy. Using the lower end of the recommended dosing range for TMP/SMX (TMP 15 mg/kg/d) may reduce the incidence of these toxicities while still achieving acceptable TMP concentrations and antimicrobial efficacy.

Comparison of the efficacy, tolerability and patient acceptability of once-daily bambuterol tablets against twice-daily controlled release salbutamol in nocturnal asthma. ACROBATICS Research Group

The aim of the study was to compare the efficacy, tolerability and patient acceptability of bambuterol (Bambec) against controlled release (CR) salbutamol (Volmax) in the treatment of nocturnal asthma. One hundred and fifty two asthmatic patients aged 17-78 years, using > or = 800 micrograms/day of an inhaled steroid, with nocturnal asthma symptoms, openly received three weeks of bambuterol 20 mg nocte and three weeks of salbutamol CR 8 mg b.i.d. in a randomised, cross over sequence. Both bambuterol and salbutamol CR treatment produced a significant 63% decrease in the severity of baseline nocturnal asthma symptoms. This improved control of nocturnal asthma was reflected by significant improvements in baseline lung function. Both the severity and number of days of tremor during the first week of treatment was significantly lower with bambuterol than with salbutamol CR. Patients considered bambuterol to cause less shakiness and treatment preference was bambuterol 49%, salbutamol CR 36%, no preference 15%. The predominant reason for patient treatment preference was control of asthma symptoms, however a significant sub-group of patients (27%) chose bambuterol because of fewer adverse effects compared to 11% choosing salbutamol CR. Fifty six percent of patients preferred taking their medication once-daily and 7% preferred twice-daily. This study shows that both bambuterol and salbutamol CR are equally effective treatments for nocturnal asthma in patients already receiving inhaled steroid. The most important factor in terms of patient treatment acceptability appears to be control of symptoms. Sub-groups of patients may chose bambuterol due to its better adverse effect profile and once-daily regimen.