CASES OF THE MONTH

EXACERBATION OF MYASTHENIA GRAVIS WITH VORICONAZOLERAED AZZAM, MD, AASEF G. SHAIKH, MD, PhD, ALESSANDRO SERRA, MD, PhD, and BASHAR KATIRJI, MD, FACP

Neuromuscular Center, Department of Neurology and Neurological Institute, University Hospitals Case Medical Center,11100 Euclid Avenue, Cleveland, Ohio 44106-5098, USA

Accepted 10 December 2012

ABSTRACT: Introduction: We describe a patient with stablegeneralized myasthenia gravis who presented with new onsetsevere ophthalmoplegia and ptosis after initiation of voricona-zole for aspergillosis. Methods: Ligand-protein docking softwarewas used to simulate the interaction of voriconazole with theacetylcholine receptor (AChR). We tested voriconazole bindingto AChR in comparison to high affinity and neutral compounds.Results: There was no clinical improvement after intravenousimmunoglobulin infusion and plasmapheresis. However, thepatient improved slowly after withdrawal of voriconazole. Basedon our results, voriconazole binds favorably to AChR and mayputatively block muscle nicotinic AChRs. Other theoreticalexplanations include blocking potassium channels and reducingtheir intracellular trafficking. Conclusions: The mechanismsinvolved in ocular exacerbation may be multi-factorial, reflectingthe intricate dynamics of the neuromuscular junction. It is im-portant to consider medications that harbor pyridine or pyrimi-dine moieties as potential causes of exacerbation in myasthenicpatients, especially those who present with ocular symptoms.

Muscle Nerve 47:928930, 2013

Numerous drugs have experimental and clinicalevidence of neuromuscular blockade. These drugsinclude anesthetic, antibiotic, anticonvulsant, anti-rheumatic, cardiovascular, and psychotropicdrugs.1 Antifungal agents were not previouslyreported to cause blockade of the NMJ or myasthe-nia gravis (MG) exacerbation. We used dockingsoftware to demonstrate the binding of voricona-zole to the AChR.

CASE REPORT

A 52-year-old right-handed woman with seropos-itive generalized MG presented with new onsetsevere bilateral ophthalmoplegia and ptosis. Theoriginal diagnosis was made 14 years prior whenshe presented with dysarthria, dysphagia, weaknessof both upper extremities, and an elevated AChRantibody level (36.4 mmol; normal

We also compared this with the affinity of 3 othercommon compounds as negative controls (acetamin-ophen, salicylic acid, and water) and 2 as positivecontrols (gentamicin and curare). Negative controlsare the molecules where no interaction is expected.They ensure that there is no effect when thereshould not be one. Positive controls are moleculeswhere an interaction is expected. That is, theyensure that there is an effect when there should beone. In this example, gentamicin and curare werechosen, because they are known to affect the neuro-muscular junction. We predicted that the negativecontrols would have low affinity for the AChR asopposed to the positive controls which would havehigh affinity for the AChR. We were not able to mea-sure statistical significance using this software.

RESULTS

Voriconazole, acetylcholine, nicotine, and cu-rare had low binding energies with the fetal-typeAChR and the adult-type AChR suggesting analo-gous high affinity for the receptors. All bindingenergies were contrasted with the negative controlmolecules (acetaminophen, salicylic acid, andwater) and the positive control molecules (genta-micin and curare). The positive binding energy ofgentamicin with the AChR is further discussedbelow. We also found that voriconazole binds tothe fetal-type AChR more favorably than to theadult-type one, although this was not tested for sta-tistical significance (Table 1).

DISCUSSION

Our patient, with seropositive generalized MGand thymoma, was minimally symptomatic formany years. She had a severe ocular exacerbation,which was temporally correlated with the initiationof treatment with voriconazole. Ocular symptomswere not part of her initial MG, which consisted ofbulbar and generalized manifestations. Becauseliterature suggesting an association of antifungalswith MG exacerbation was lacking, we asked

whether voriconazole could exacerbate MG andinvestigated the possible mechanisms by which itcould have interfered with the NMJ.

An antagonistic interaction between voricona-zole and the nicotinic AChR causing reduced ace-tylcholine-evoked current during in vitro prepara-tions was recently reported.3 We simulated theinteraction of voriconazole with the AChR and cal-culated the binding energies using the softwaredescribed in the methods.2 The binding freeenergy of a protein-ligand complex is the energeticdifference relative to the uncomplexed state andintuitively indicates how much the ligand likes tobe situated at the active site of the protein asopposed to somewhere else in the solvent far awayfrom the protein. Theoretically, the Gibbs freeenergy, according to the laws of thermodynamics,gives the free energy of binding in which a nega-tive value indicates that a reaction is being favored.Lower free energy values correspond to morefavorable reactions.4 Voriconazole, acetylcholine,and nicotine had low and comparable bindingenergies with the AChR, suggesting analogoushigh affinity to the receptor. Such binding ener-gies were contrasted with the negative control mol-ecules (acetaminophen, salicylic acid, and water)and positive control molecules (gentamicin andcurare) (see Table 1). We must emphasize that alow binding energy only indicates a favorable mo-lecular interaction and does not specify an agonis-tic or antagonistic nature of the outcome. Theconverse is also true, where a high or positive bind-ing energy indicates an unfavorable interaction.Curare, a known antagonist of the nicotinic AChR,binds the receptor with a highly negative or favor-able interaction.5 In the case of gentamicin, whichis well known to exacerbate myasthenia gravis, thepositive binding energy is possibly explained bythe lack of interaction of gentamicin at the postsy-naptic nicotinic AChR and possible interaction atpresynaptic sites6 making the interaction unfavora-ble. In an MG patient, the safety factor is reduced,because the AChRs are compromised in functionand quantity due their binding to the AChR anti-bodies and their destruction. Therefore, it is plau-sible that voriconazole might have blocked theAChR further and exacerbated MG in our patient.

The pyrimidine moiety of voriconazole is proneto interact with the AChR.3 In fetal muscle prior toinnervation or after denervation, the nicotinicAChR subunit stoichiometry is (a1)2b1dc,

7 whereas,in adult muscle, the c subunit is replaced by e togive the (a1)2b1de stoichiometry. In adults, the fe-tal-type nicotinic AChR continues to be expressedin the thymus and in some extra-ocular muscles.In our simulation experiment, voriconazoleappears to bind more favorably to fetal AChR than

Table 1. Simulated binding energies of the ligands water,acetaminophen, salicylic acid, acetylcholine, nicotine, curare,

gentamicin, and voriconazole to the fetal and adult muscle-typenAChR (nicotinic AcetylCholine Receptor).

Ligand

Binding energyto fetal nAChR

(kcal/mol)

Binding energyto adult nAChR

(kcal/mol)

Water -1.19 -1.00Acetaminophen -1.00 -1.13Salicylic acid -1.03 -1.05Acetylcholine -4.18 -4.16Nicotine -6.31 -6.05Curare -50.08 -65.80Gentamicin 14.60 14.58Voriconazole -5.04 -4.01

Voriconazole Exacerbates Myasthenia MUSCLE & NERVE June 2013 929

to the adult-type.8 Normally, the binding of 2 mole-cules of acetylcholine to specific sites at the inter-face of the alpha subunits with the gamma anddelta subunits causes a conformational change inthe receptor protein, allowing ion permeation.9

Antibodies to the gamma subunit have been shownto trigger humoral and cellular immune responses.In due course this activates the complement cas-cade and presumably contributes to the degrada-tion of AChRs at the NMJs of extra-ocular muscles,mimicking ocular myasthenia.10 In our patient andbased on our simulation method, we suspect thatvoriconazole may preferentially bind to the gammasubunit binding site in the extra-ocular NMJ ratherthan at the epsilon subunit binding site. The resultis severe ophthalmoplegia with sparing of the restof the skeletal muscles (see Table 1). However, thisis only speculation, as sera from ocular myasthenicswere more often seropositive with assays that usedmixtures of fetal and adult receptor isoforms thanwith traditional assays using denervated muscle,which have mainly fetal isoforms.11 Therefore, thepresence of fetal AChR antibodies does not fullyaccount for the susceptibility of extraocular musclesin ocular myasthenia, and a reduced safety factorassociated with the high motor unit firing fre-quency remains a feasible explanation.11

Many antibiotics, such as the aminoglycosides,are known to exacerbate MG.12 The precise mecha-nisms for this adverse effect are not clear, but themajority of studies point to dominant presynaptic in-hibition by competing with calcium ions for recep-tors and inhibiting the release of acetylcholine.13

The underlying mechanisms may also include pro-moting the immune response to AChR, increasingthe level of AChR antibodies in serum, blocking thebinding of acetylcholine to the receptor, accelerat-ing the loss of AChR on the NMJ, or destroying thestructure of presynaptic and postsynaptic mem-branes.14 Macrolides, such as telithromycin, are alsoknown to exacerbate MG.15 It is speculated that theirpyridine moiety is responsible for the interactionwith the AChR.16 In nearly all instances of NMJ neu-rotoxicity, there is a reduction in the safety factor ofneuromuscular transmission by one of severalmechanisms. These neurotoxins may affect eitherthe presynaptic or the postsynaptic elements of theNMJ.

The exacerbation of MG in our patient wasunlikely to have been spontaneous, because shehad not had any ocular symptoms since the diag-nosis of MG 14 years earlier. Also, a rapid favorableclinical outcome following plasmapheresis andIVIG, which is common in the majority of MGpatients, was not observed here.17 The slowimprovement of this patients eye movements overseveral months suggests that voriconazole not only

blocked AChR, but could have interfered withother channels including hERG (human Ether-a-go-go-Related gene that codes for a protein knownas Kv11.1 potassium ion channels) and KCa (cal-cium-regulated potassium channels). In addition,impaired intracellular trafficking of these channelsmay have slowed the receptor turnover time.1820

We conclude that voriconazole, and possiblyother antifungals, may exacerbate MG. The mecha-nism involved in the exacerbation is likely relatedto nicotinic AChR blockade, but it may be multi-factorial due to the intricate dynamics of the NMJ.It is important to consider all medications that har-bor the pyridine or pyrimidine moieties as poten-tial causes of MG exacerbation or crisis.

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