Amifampridine for the Management of Lambert-Eaton Myasthenic Syndrome: A New Take on an Old Drug

A Unique Case of Non-paraneoplastic Lambert-Eaton Myasthenic Syndrome Treated With Subcutaneous Immunoglobulin: A Case Report and Review of Literature

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune neuromuscular disorder caused by pathogenic autoantibodies directed against voltage-gated calcium channels present on the presynaptic nerve terminal. For LEMS patients refractory to initial symptomatic treatment with amifampridine, immunomodulatory therapy with intravenous immunoglobulin (IVIG) is often utilized. However, in the authors' review of literature, the utility of subcutaneous immunoglobulin (SCIG) in the treatment of LEMS has been scarcely reported. Here, we present a unique case of non-paraneoplastic LEMS managed with SCIG with excellent clinical response and improvement on electromyography. SCIG therapy may be a reasonable alternative for patients with LEMS who do not tolerate the intravenous formulation.

Targeting neuromuscular junction to treat neuromuscular disorders

The integrity and preservation of the neuromuscular junction (NMJ), the interface between the motor neuron and skeletal muscle, is critical for maintaining a healthy skeletal muscle. The structural and/or functional defects in the three cellular components of NMJ, namely the pre-synaptic terminal, synaptic cleft, and post-synaptic region, negatively affect skeletal muscle mass and/or strength. Therefore, NMJ repair appears to be an appropriate therapy for muscle disorders. Mouse models provide a detailed molecular characterization of various cellular components of NMJ with relevance to human diseases. This review discusses different molecular targets on the three cellular components of NMJ for treating muscle diseases. The potential effects of these therapies on NMJ morphology and motor performance, their therapeutic efficacy, and clinical relevance are discussed. Collectively, the available data supports targeting NMJ alone or as an adjunct therapy in treating muscle disorders. However, the potential impact of such interventions on human patients with muscle disorders requires further investigation.

Neuromuscular junction transmission failure in aging and sarcopenia: The nexus of the neurological and muscular systems

Sarcopenia, or age-related decline in muscle form and function, exerts high personal, societal, and economic burdens when untreated. Integrity and function of the neuromuscular junction (NMJ), as the nexus between the nervous and muscular systems, is critical for input and dependable neural control of muscle force generation. As such, the NMJ has long been a site of keen interest in the context of skeletal muscle function deficits during aging and in the context of sarcopenia. Historically, changes of NMJ morphology during aging have been investigated extensively but primarily in aged rodent models. Aged rodents have consistently shown features of NMJ endplate fragmentation and denervation. Yet, the presence of NMJ changes in older humans remains controversial, and conflicting findings have been reported. This review article describes the physiological processes involved in NMJ transmission, discusses the evidence that supports NMJ transmission failure as a possible contributor to sarcopenia, and speculates on the potential of targeting these defects for therapeutic development. The technical approaches that are available for assessment of NMJ transmission, whether each approach has been applied in the context of aging and sarcopenia, and the associated findings are summarized. Like morphological studies, age-related NMJ transmission deficits have primarily been studied in rodents. In preclinical studies, isolated synaptic electrophysiology recordings of endplate currents or potentials have been mostly used, and paradoxically, have shown enhancement, rather than failure, with aging. Yet, in vivo assessment of single muscle fiber action potential generation using single fiber electromyography and nerve-stimulated muscle force measurements show evidence of NMJ failure in aged mice and rats. Together these findings suggest that endplate response enhancement may be a compensatory response to post-synaptic mechanisms of NMJ transmission failure in aged rodents. Possible, but underexplored, mechanisms of this failure are discussed including the simplification of post-synaptic folding and altered voltage-gated sodium channel clustering or function. In humans, there is limited clinical data that has selectively investigated single synaptic function in the context of aging. If sarcopenic older adults turn out to exhibit notable impairments in NMJ transmission (this has yet to be examined but based on available evidence appears to be plausible) then these NMJ transmission defects present a well-defined biological mechanism and offer a well-defined pathway for clinical implementation. Investigation of small molecules that are currently available clinically or being testing clinically in other disorders may provide a rapid route for development of interventions for older adults impacted by sarcopenia.

Simulations of active zone structure and function at mammalian NMJs predict that loss of calcium channels alone is not sufficient to replicate LEMS effects

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune-mediated neuromuscular disease thought to be caused by autoantibodies against P/Q-type voltage-gated calcium channels (VGCCs), which attack and reduce the number of VGCCs within transmitter release sites (active zones; AZs) at the neuromuscular junction (NMJ), resulting in neuromuscular weakness. However, patients with LEMS also have antibodies to other neuronal proteins, and about 15% of patients with LEMS are seronegative for antibodies against VGCCs. We hypothesized that a reduction in the number of P/Q-type VGCCs alone is not sufficient to explain LEMS effects on transmitter release. Here, we used a computational model to study a variety of LEMS-mediated effects on AZ organization and transmitter release constrained by electron microscopic, pharmacological, immunohistochemical, voltage imaging, and electrophysiological observations. We show that models of healthy AZs can be modified to predict the transmitter release and short-term facilitation characteristics of LEMS and that in addition to a decrease in the number of AZ VGCCs, disruption in the organization of AZ proteins, a reduction in AZ number, a reduction in the amount of synaptotagmin, and the compensatory expression of L-type channels outside the remaining AZs are important contributors to LEMS-mediated effects on transmitter release. Furthermore, our models predict that antibody-mediated removal of synaptotagmin in combination with disruption in AZ organization alone could mimic LEMS effects without the removal of VGCCs (a seronegative model). Overall, our results suggest that LEMS pathophysiology may be caused by a collection of pathological alterations to AZs at the NMJ, rather than by a simple loss of VGCCs.NEW & NOTEWORTHY We used a computational model of the active zone (AZ) in the mammalian neuromuscular junction to investigate Lambert-Eaton myasthenic syndrome (LEMS) pathophysiology. This model suggests that disruptions in presynaptic active zone organization and protein content (particularly synaptotagmin), beyond the simple removal of presynaptic calcium channels, play an important role in LEMS pathophysiology.

Critical illness-associated weakness and related motor disorders

Weakness of limb and respiratory muscles that occurs in the course of critical illness has become an increasingly common and serious complication of adult and pediatric intensive care unit patients and a cause of prolonged ventilatory support, morbidity, and prolonged hospitalization. Two motor disorders that occur singly or together, namely critical illness polyneuropathy and critical illness myopathy, cause weakness of limb and of breathing muscles, making it difficult to be weaned from ventilatory support, commencing rehabilitation, and extending the length of stay in the intensive care unit, with higher rates of morbidity and mortality. Recovery can take weeks or months and in severe cases, and may be incomplete or absent. Recent findings suggest an improved prognosis of critical illness myopathy compared to polyneuropathy. Prevention and treatment are therefore very important. Its management requires an integrated team approach commencing with neurologic consultation, creatine kinase (CK) measurement, detailed electrodiagnostic, respiratory and neuroimaging studies, and potentially muscle biopsy to elucidate the etiopathogenesis of the weakness in the peripheral and/or central nervous system, for which there may be a variety of causes. These tenets of care are being applied to new cases and survivors of the coronavirus-2 disease pandemic of 2019. This chapter provides an update to the understanding and approach to critical illness motor disorders.

Paraneoplastic syndromes: A focus on pathophysiology and supportive care

DISCLAIMER

In an effort to expedite the publication of articles, AJHP is posting manuscripts online as soon as possible after acceptance. Accepted manuscripts have been peer-reviewed and copyedited, but are posted online before technical formatting and author proofing. These manuscripts are not the final version of record and will be replaced with the final article (formatted per AJHP style and proofed by the authors) at a later time.

PURPOSE

This article aims to increase awareness of, outline pathophysiology for, and offer guidance on supportive care strategies for specific endocrine, neurological, and immunological syndromes associated with paraneoplastic syndromes (PNSs).

SUMMARY

PNS refers to remote effects that cannot be attributed to the direct or invasive effects of a malignancy. These syndromes are considered clinically important because they may provide early recognition, diagnosis, and management of the malignancy in a timely manner. Many of their presenting symptoms such as ectopic Cushing's syndrome, hypercalcemia of malignancy (HCM), syndrome of inappropriate secretion of antidiuretic hormone (SIADH), neurological dysfunctions, and paraneoplastic autoimmune thrombocytopenia overlap with those of nonneoplastic disorders, yet their pathogenesis and responses to treatment differ. Treatment for ectopic Cushing's syndrome due to a PNS consists of treatment of the underlying malignancy and its comorbidities. Drug therapies may include ketoconazole, mitotane, metyrapone, somatostatin analogs, and dopamine agonists. Hypercalcemia may be classified into cases with parathyroid hormone (PTH)-dependent causes or PTH-independent causes such as HCM, in which osteoclast inhibitors may be deployed. Treatments of PNS-mediated SIADH include treatment of the underlying malignancy and strategies to increase serum sodium levels. Amifampridine is now considered the first-line agent for paraneoplastic Lambert-Eaton myasthenic syndrome, whereas steroids, intravenous immune globulin, thrombopoietin receptor agonists (eg, romiplostim, eltrombopag, and avatrombopag), fostamatinib, and rituximab may find their niche in treatment of PNS-mediated autoimmune thrombocytopenia.

CONCLUSION

Supportive care for PNSs lends opportunities to pharmacists to add quality, value, and safety.

Recognition and Management of Neuromuscular Emergencies

Acute neuromuscular disorders represent an important subset of neurologic consultation requests in the inpatient setting. Although most neuromuscular disorders are subacute to chronic, hospital-based neurologists encounter neuromuscular disorders presenting with rapidly progressive or severe weakness affecting limb movement, respiratory, and bulbar function. Recalling fundamentals of neurologic localization assists in prompt recognition and diagnosis. Despite the differing localizations and the causal diagnoses, the initial management principles of acute myopathies, neuropathies, and neuromuscular junction disorders are similar.

Lambert-Eaton myasthenic syndrome revealed after surgery of lung adenocarcinoma

We report a very rare case of cT1N0M0 lung adenocarcinoma reveling Lambert-Eaton myasthenic syndrome (LEMS). A 69-year-old nonsmoking woman, with several comorbidities consulted for cough and dyspnea. Chest radiograph and CT scanning detected a left lower lobe mass; Needle biopsy confirmed differentiated adenocarcinoma; 18FDG-PET scan and Brain MRI eliminated metastatic disease dissemination. Our patient underwent a left lower lobectomy with mediastinal lymphadenectomy (pT1N0M0), no adjuvant chemotherapy was administrated. One month later patient present a muscle weakness in both lower limbs and fatigability followed by an inability to walk. The diagnosis of LEMS was made from the distinctive electromyogram (EMG) findings and a treatment with Amifampridine (3, 4-diaminopyridine phosphate [3, 4-DAP]) was prescribed with evident efficacy for symptoms.

A high-affinity, partial antagonist effect of 3,4-diaminopyridine mediates action potential broadening and enhancement of transmitter release at NMJs

3,4-Diaminopyridine (3,4-DAP) increases transmitter release from neuromuscular junctions (NMJs), and low doses of 3,4-DAP (estimated to reach ∼1 μM in serum) are the Food and Drug Administration (FDA)-approved treatment for neuromuscular weakness caused by Lambert–Eaton myasthenic syndrome. Canonically, 3,4-DAP is thought to block voltage-gated potassium (Kv) channels, resulting in prolongation of the presynaptic action potential (AP). However, recent reports have shown that low millimolar concentrations of 3,4-DAP have an off-target agonist effect on the Cav1 subtype (“L-type”) of voltage-gated calcium (Cav) channels and have speculated that this agonist effect might contribute to 3,4-DAP effects on transmitter release at the NMJ. To address 3,4-DAP’s mechanism(s) of action, we first used the patch-clamp electrophysiology to characterize the concentration-dependent block of 3,4-DAP on the predominant presynaptic Kv channel subtypes found at the mammalian NMJ (Kv3.3 and Kv3.4). We identified a previously unreported high-affinity (1–10 μM) partial antagonist effect of 3,4-DAP in addition to the well-known low-affinity (0.1–1 mM) antagonist activity. We also showed that 1.5-μM DAP had no effects on Cav1.2 or Cav2.1 current. Next, we used voltage imaging to show that 1.5- or 100-μM 3,4-DAP broadened the AP waveform in a dose-dependent manner, independent of Cav1 calcium channels. Finally, we demonstrated that 1.5- or 100-μM 3,4-DAP augmented transmitter release in a dose-dependent manner and this effect was also independent of Cav1 channels. From these results, we conclude that low micromolar concentrations of 3,4-DAP act solely on Kv channels to mediate AP broadening and enhance transmitter release at the NMJ.

Probing the Strength and Mechanism of Binding Between Amifampridine and Calf Thymus DNA

In this work, we have investigated the strength and mechanism of amifampridine (3,4-Diaminopyridine/3,4-DAP) interaction with calf thymus DNA (ct-DNA). The existence and the strength of interaction are evaluated using circular dichroism (CD), UV-vis absorption, and differential pulse voltammogram studies. Results from UV-vis absorption technique indicate that amifampridine can significantly interact with DNA through a binding constant of Kb = 1.66 × 105 M-1 at 298 K. The mechanism of the interaction between amifampridine and DNA is also studied using ionic effect investigations, competitive fluorescence experiments, viscosity measurements, and molecular docking studies. The viscosity results indicate that amifampridine can bind to DNA via intercalation binding mode. Competitive fluorescence experiments using Acridine Orange (AO) and Hoechst 33258 (HO) probes also reveal that amifampridine binds to DNA via an intercalation mode of binding. Finally, the molecular docking studies also suggest that amifampridine tends to bind with the G-C rich region of DNA.

Management of Juvenile Myasthenia Gravis

Juvenile Myasthenia Gravis (JMG) is a rare disorder, defined as myasthenia gravis in children younger than 18 years of age. While clinical phenotypes are similar to adults, there are a number of caveats that influence management: broader differential diagnoses; higher rates of spontaneous remission; and the need to initiate appropriate treatment early, to avoid the long-term physical and psychosocial morbidity. Current practice is taken from treatment guidelines for adult MG or individual experience, with considerable variability seen across centers. We discuss our approach to treating JMG, in a large specialist JMG service, and review currently available evidence and highlight potential areas for future research. First-line treatment of generalized JMG is symptomatic management with pyridostigmine, but early use of immunosuppression, where good control is not achieved is important. Oral prednisolone is used as first-line immunosuppression with appropriate prevention and monitoring of side effects. Second-line therapies including azathioprine and mycophenolate may be considered where there is: no response to steroids, inability to wean to a reasonable minimum effective dose or if side-effects are intolerable. Management of ocular JMG is similar, but requires close involvement of ophthalmology in young children to prevent amblyopia. Muscle-specific tyrosine kinase (MuSK)-JMG show a poorer response to pyridostigmine and anecdotal evidence suggests that rituximab should be considered as second-line immunosuppression. Thymectomy is indicated in any patient with a thymoma, and consideration should be given in acetylcholine receptor (AChR) positive JMG allowing time for spontaneous remission. The benefit is less clear in ocular JMG and is not advised in MuSK-JMG. Children experiencing a myasthenic crisis require urgent hospital admission with access to the intensive care unit. PLEX is preferred over IVIG due to rapid onset of action, but this needs to be balanced with feasibility in very young children. Key questions remain in the management of JMG: when to initiate both first- and second-line treatments, choosing between steroid-sparing agents, and determining the optimal dose and treatment duration. We feel that given the rarity of this disease, the establishment of national registries and collaboration across groups will be needed to address these issues and facilitate future drug trials in JMG.