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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. REFERENCES

Objective

The myopathy associated with anti–signal recognition particle (anti-SRP) is a severe necrotizing immune-mediated disease characterized by rapidly progressive proximal muscle weakness, markedly elevated serum creatine kinase (CK) levels, and poor responsiveness to traditional immunosuppressive therapies. Reports on the efficacy of B cell depletion therapy for anti-SRP–associated myopathy are mixed. We describe 8 patients with anti-SRP–associated myopathy and their response to treatment with the anti-CD20 monoclonal antibody rituximab.

Methods

We identified 8 patients with myopathy who tested positive for anti-SRP antibodies by immunoprecipitation and were treated with rituximab as part of clinical care. We reviewed their medical records to assess clinical, serologic, and histologic characteristics and response to therapy. In 5 patients, serum was collected before and after rituximab therapy. Autoantibodies were detected by immunoprecipitation and quantitated by densitometry, and the percent decreases in anti-SRP autoantibody levels were calculated.

Results

Six of 8 patients who had been refractory to standard immunosuppressive therapy demonstrated improved manual muscle strength and/or decline in CK levels as early as 2 months after rituximab treatment. Three patients sustained the response for 12–18 months after initial dosing. All of the patients were continued on adjunctive corticosteroids, but doses were substantially reduced after rituximab. Quantitative levels of serum anti-SRP antibodies also decreased after rituximab treatment.

Conclusion

B cell depletion therapy with rituximab is effective for patients with myopathy associated with anti-SRP. The substantial decrease in anti-SRP antibody levels after rituximab treatment also suggests that B cells and anti-SRP antibodies may play a role in the pathogenesis of this myopathy.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. REFERENCES

Myositis-specific or myositis-associated antibodies are detected in approximately 50% of patients with idiopathic inflammatory myopathies (IIMs) and help to define subgroups of patients with certain distinguishing clinical features (1, 2). Anti–signal recognition particle (anti-SRP) autoantibodies are myositis-specific antibodies found in 4–6% of patients with IIMs (2, 3). These antibodies are directed against SRP, a ribonuclear protein particle that regulates protein translocation across the endoplasmic reticulum membrane during protein synthesis. Studies have demonstrated that anti-SRP myopathy appears distinct from polymyositis (PM) and other IIMs by its clinical features and histopathology (3–6). Patients with anti-SRP antibodies often present clinically with a severe myopathy characterized by markedly elevated serum creatine kinase (CK) levels and rapidly progressive proximal muscle weakness leading to significant disability. On histopathology, anti-SRP patients demonstrate a necrotizing myopathy without primary inflammation; however, several studies have demonstrated class I major histocompatibility complex immunostaining and most histopathologic studies have found capillary pathology with deposition of the terminal components of complement C5b–9, or the membrane attack complex (4–6).

Anti-SRP myopathy also differs from other immune-mediated myopathies by its characteristically poor responsiveness to steroid monotherapy and conventional immunosuppressive therapies. Although the pathophysiologic role of B cells as causative agents in several autoimmune diseases is not entirely understood, several off-label studies have shown efficacy of the B cell depleting therapy rituximab, an anti-CD20 monoclonal antibody, in diseases that can be treatment refractory such as systemic lupus erythematosus (SLE) (7), rheumatoid arthritis (RA) (8), and systemic vasculitides (9). B cell depletion therapy has also been an encouraging option for patients with PM, dermatomyositis (DM), and juvenile DM in several case series (10–12). Thus far, reports of the efficacy of rituximab in the treatment of anti-SRP myopathy have been mixed. A recent case report described poor clinical response to rituximab in 2 anti-SRP patients (13). However, an earlier investigation by Arlet et al of 2 patients with refractory anti-SRP myopathy demonstrated marked and sustained clinical response to the combination of prednisone, plasma exchange, and repeated courses of rituximab (14).

In this case series, we report the characteristics of 8 patients with anti-SRP myopathy and their dramatic response to B cell depletion therapy when their disease was refractory to traditional therapeutic agents.

PATIENTS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. REFERENCES

Design.

This is a retrospective case series review of 8 patients with anti-SRP myopathy who were treated with rituximab at the Johns Hopkins Myositis Center.

Patients.

All of the patients had been evaluated as part of routine clinical care in the outpatient myositis clinic at the Johns Hopkins University Hospital or Johns Hopkins Bayview Medical Center in Baltimore, Maryland, between 2006 and 2009. We identified and reviewed the medical records of 8 patients who tested positive for anti-SRP antibodies and had been treated with rituximab.

Sera.

Serum samples had previously been collected and banked at −80°C from all of the patients with probable or definite IIM according to the criteria of Bohan and Peter (15) and from patients with conditions suggesting the diagnosis of myositis. Informed consent from the study participants was obtained according to institutional review board protocol.

Assessment of muscle disease.

Strength had been assessed by 1 of 2 physicians (LC-S, AM) at the Johns Hopkins Myositis Center through manual muscle testing and graded by the Medical Research Council scale. All of the patients were subsequently reassessed by the same physician who performed the initial evaluation. Five patients had electromyographic and nerve conduction studies performed and interpreted by the Neuromuscular Division at Johns Hopkins Bayview Medical Center. Seven patients had undergone lower extremity magnetic resonance imaging (MRI) with T1- and T2-weighted STIR, which was reviewed by the radiology department at Bayview Medical Center. Seven patients in this series had muscle biopsies performed, and 6 of these were interpreted at Johns Hopkins Hospital by the Neuromuscular Pathology Laboratory.

Detection of anti-SRP 72, 54, and 19 antibodies by immunoprecipitation.

Complementary DNA (cDNA) clones encoding human SRP 54 and 19 were purchased from Origene; cDNA encoding human SRP 72 was cloned in the lab using HeLa cDNA, and the cDNA encoding histidyl–transfer RNA synthetase (Jo-1) was provided by the National Institutes of Health. All of the clones were sequence verified prior to use. 35S-methionine–labeled SRP proteins were generated from these cDNAs by coupled in vitro transcription and translation (IVTT) using a rabbit reticulocyte lysate-based system (Promega). The radiolabeled protein products were immunoprecipitated as previously described (16) with the patient serum samples described above (obtained pre– and post–rituximab treatment). The IVTT immunoprecipitation assay to detect anti-SRP antibodies was validated using reference sera known to be positive for those antibodies. Immunoprecipitates were electrophoresed on 10% or 12% sodium dodecyl sulfate–polyacrylamide gels visualized by fluorography, and the autoradiograms were scanned by densitometry. In each case, the percent decrease in autoantibody level after treatment was calculated.

Rituximab treatment.

The patients were administered a standard dosing protocol of 1,000 mg/m2 of intravenous (IV) rituximab infused once with a second dose repeated after 2 weeks. The patients were pretreated with 100 mg IV methylprednisolone prior to infusions. One of the 8 patients (patient 4) received only 1 of the 2 infusions.

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. REFERENCES

Patient 1.

A 20-year-old African American woman presented 6 months after a diagnosis of mononucleosis when she began to experience falls, lower extremity weakness, dysphagia, a scaly erythematous neck rash, and new-onset Raynaud's phenomenon. The weakness rapidly progressed to the point that she required the use of a wheelchair, with grade 2/5 strength in the proximal upper and lower extremities. Laboratory testing revealed a markedly elevated CK level at 56,000 IU/liter and the presence of anti-Ro antibodies. Lower extremity MRI showed extensive edema, and electromyography was consistent with an irritable myopathy. Muscle biopsy sample showed a necrotizing myopathy with patchy endomysial and perimysial infiltrate and focal fiber degeneration. The presence of autoantibodies to SRP 72, 54, and 19 in this serum was confirmed in our research laboratory by IVTT immunoprecipitation. The patient was treated with high-dose IV and oral corticosteroids, azathioprine, and methotrexate with minimal improvement in strength, and the CK level remained elevated at 2,710 IU/liter. She was then treated with 2 doses of rituximab and continued receiving adjunctive methotrexate and prednisone. Four months after treatment with rituximab, the CK level had decreased to 622 IU/liter and strength improved to grade 5/5 in the proximal and distal muscles. Eight months later, MRI showed mild muscle edema that was markedly improved. CD19 and CD20 counts remained suppressed.

Patient 2.

A 34-year-old African American woman, previously a competitive cheerleading coach, presented with rapidly progressive and severe proximal upper and lower extremity weakness, dysphagia, and nonspecific facial erythema. Strength testing was initially grade 4/5 and 3/5 in the proximal upper and lower extremities, respectively. Evaluation showed an elevated CK level to 21,333 IU/liter, MRI with profound muscle edema, and irritable myopathy on electromyography. Muscle biopsy sample showed a necrotizing myopathy with no primary inflammation with degenerating and regenerating fibers, and the patient was diagnosed with PM. Antibodies against SRP 72 and 19 were confirmed in this patient's serum by IVTT immunoprecipitation. She was started on high-dose prednisone at 80 mg orally twice daily along with methotrexate 25 mg weekly, monthly IV immunoglobulin (IVIG), and then azathioprine without benefit. Eventually, the patient was unable to get out of bed without assistance with proximal strength grade 2/5, and she developed respiratory distress and severe dysphagia. She was hospitalized and received pulse dose IV methylprednisolone, IVIG, and cyclophosphamide, but continued to deteriorate and was subsequently intubated for respiratory muscle weakness. She then received 5 plasmapheresis treatments and 2 doses of rituximab. Two months later, the CK level had decreased to 371 IU/liter from 1,000 IU/liter, and strength had improved to grade 3/5. She continued to improve in strength and was re-dosed with rituximab 6 months after her initial dose primarily to sustain her response. Eighteen months after the initial dosing, the CK level had normalized to 163 IU/liter and the patient's strength was grade 5/5 and 4/5 in the proximal upper and lower extremities, respectively. CD19 and CD20 counts were undetectable.

Patient 3.

A 42-year-old white woman presented with a 3-year history of fatigue and proximal muscle weakness that rapidly progressed over several months to an inability to feed herself with significant dysphagia. The initial CK level was 3,148 IU/liter. Muscle biopsy sample showed a severe necrotizing myopathy with no primary inflammation. IVTT immunoprecipitation testing showed antibodies against SRP 72, 54, and 19. The patient was given a diagnosis of PM and treated with high-dose oral prednisone, methotrexate, and mycophenolate mofetil, with no significant improvement in muscle strength. Azathioprine was tried briefly but caused severe gastrointestinal intolerance. Nine IVIG infusions provided no sustained effect and were discontinued when she developed a pulmonary embolus. Since no primary inflammation was seen on the biopsy sample, the patient's physicians discontinued steroid therapy and the patient's strength precipitously declined until she was unable to get out of bed without assistance and enrolled in hospice. Manual muscle strength was graded 1/5 in the proximal upper and lower extremities and the CK level was 550 IU/liter. She was then treated with IV pulse steroids and rituximab. Two months after rituximab, biceps and hip flexor strength improved to grade 3/5 and the CK level decreased to 237 IU/liter. Methotrexate was added as an adjunctive medication and after 6 months, CD19 and CD20 counts were still undetectable. To sustain the clinical response, rituximab was readministered for 2 doses. Eight months later, CD19 and CD20 counts remained suppressed, but the patient had increased weakness and was administered a fifth dose of rituximab. Hip flexor strength then improved to grade 4/5, whereas proximal upper extremity and knee flexor strength substantially improved to grade 5/5. Nineteen months after the initial rituximab dose, CD19 and CD20 counts remained suppressed and the CK level had normalized to 126 IU/liter.

Patient 4.

A 72-year-old white man presented with a 6-month history of generalized weakness in his arms and legs, muscle cramps and stiffness, and dysphagia to solids and liquids. Examination showed proximal muscle weakness in the neck, deltoids, and hip flexors grade 2/5, and diffusely atrophic muscles in the scapula, proximal arms, quadriceps, and pectoralis. Laboratory results showed an elevated CK level of 6,885 IU/liter and an aldolase level of 62 IU/liter. Electromyography revealed an irritable myopathy and nerve conduction studies showed a mixed sensorimotor polyneuropathy. Extremity MRI demonstrated diffuse edema in the medial, anterior, and posterior compartments with atrophy. Muscle biopsy sample showed a necrotizing myopathy with regenerating and degenerating fibers, myophagocytosis, and acute and chronic neurogenic atrophy with no primary inflammation. Sural nerve biopsy sample showed a mild active and chronic neuropathy with no inflammation. IVTT immunoprecipitation in our research laboratory showed antibodies against SRP 72 and 19. The patient was treated with high-dose oral corticosteroids, IVIG, and plasma exchange with no substantial improvement in strength. He was then treated with one dose of rituximab. Within 2 weeks, the CK level had normalized to 22 IU/liter. One month later, the patient developed pneumonia and congestive heart failure exacerbation and died in hospice.

Patient 5.

A 21-year-old African American woman presented with a 4-month history of rapidly progressive upper and lower extremity weakness and weight loss. The initial CK level was greater than 20,000 IU/liter and she had muscle strength grade 1/5 in her upper and lower extremities. Extremity MRI showed widespread atrophy and muscle edema. Electromyography revealed an irritable myopathy. Muscle biopsy sample showed a necrotizing myopathy with atrophy, perivascular inflammation, and no primary inflammation. Antibodies against SRP 72, 54, and 19 were detected in this patient's serum using IVTT immunoprecipitation. She was treated with IV and oral corticosteroids and the CK level decreased to 3,000 IU/liter, but strength minimally improved and she required the use of a wheelchair. Methotrexate and IVIG treatment yielded no improvement. She was then treated with rituximab. After 2 months, there was marked improvement in strength to grade 4/5 and 2/5 in the proximal upper and lower extremities, respectively, and the CK level decreased to 1,144 IU/liter. MRI showed a decrease in muscle edema. CD19 and CD20 counts were undetectable. Ten months later, CD19 and CD20 counts remained suppressed, but the CK level increased to 5,600 IU/liter. The patient was readministered 2 doses of rituximab, and the CK level improved to 3,000 IU/liter within 3 months.

Patient 6.

A 26-year-old African American woman presented with a 1-month history of progressively worsening proximal arm and leg weakness, dysphagia, scaling pruritic hand rash, new-onset Raynaud's phenomenon, and myalgias. Examination of muscle strength was initially graded 4/5 in the proximal upper and lower extremities. Laboratory data showed a markedly elevated CK level at 20,180 IU/liter and an elevated aldolase level of 46 IU/liter. Electromyography revealed an irritable myopathy. MRI showed significant edema in the anterior and medial thigh muscles. Muscle biopsy sample showed a necrotizing myopathy with regeneration and degeneration, but no inflammation. Commercial laboratory testing of myositis-specific antibodies showed weakly positive anti-Ku. We confirmed the presence of anti-SRP 72 antibodies using IVTT immunoprecipitation. The patient was treated with high doses of IV and oral corticosteroid therapy, IVIG, methotrexate, and mycophenolate mofetil with progression of weakness to grade 3/5 and 2/5 in the proximal upper and lower extremities, respectively, and a persistently elevated CK level at 2,500 IU/liter. She was then treated with 2 doses of rituximab. Within 4 months, the CK level had decreased to 1,300 IU/liter, half the value prior to rituximab, and by 6 months, strength had improved to grade 5/5 in the proximal upper extremities and 3/5 in the hip flexors. Repeat MRI showed decreased muscle edema. CD19 and CD20 counts continue to be suppressed 6 months after treatment.

Patient 7.

A 51-year-old white man with a history of inflammatory polyarthritis for 20 years treated with methotrexate developed lower extremity weakness with bilateral knee inflammatory arthritis. The initial CK level was 7,855 IU/liter and electromyography showed an irritable myopathy suggesting PM. The patient elected not to have a muscle biopsy performed. Treatment was started with prednisone 60 mg daily, methotrexate 25 mg weekly, and eventually mycophenolate mofetil, but the CK level remained elevated at 3,000 IU/liter and weakness persisted with grade 4/5 proximal upper and lower extremity strength. IVTT immunoprecipitation in our research laboratory showed antibodies against SRP 72 and 54. The patient was treated with 2 doses of rituximab. Three months after treatment, the CK level had decreased to 1,080 IU/liter.

Patient 8.

A 32-year-old white woman presented with progressively worsening weakness of the proximal upper and lower extremities, stiffness and myalgias, new-onset Raynaud's phenomenon, fevers, weight loss, and nonspecific erythema at the neck and malar regions. Initial CK and aldolase levels were elevated at 8,495 IU/liter and 147 IU/liter, respectively. Electromyography revealed an irritable myopathy. MRI demonstrated muscle edema of the anterior and posterior compartments. Muscle biopsy sample showed an inflammatory myopathy with scattered myophagocytosis, myofiber degeneration, and primary inflammation. Autoantibody testing using IVTT immunoprecipitation confirmed the presence of antibodies against SRP 54. The patient was started on prednisone 60 mg daily with a minimal improvement in strength. Azathioprine, methotrexate, and monthly IVIG treatments were administered, but the CK level remained elevated at 3,000 IU/liter with continued weakness in the proximal upper extremities grade 4/5 and hip flexors grade 2/5. The patient was then administered 2 doses of rituximab. Six months after rituximab therapy, the CK level decreased to 2,100 IU/liter and the CD19 and CD20 counts remained suppressed. Although proximal upper extremity weakness improved slightly to grade 5/5, lower extremity weakness persisted, which necessitated alternative therapy.

Adverse events.

Rituximab was well tolerated in all of the patients. Patient 5 developed a herpes zoster infection 3 months after rituximab treatment that healed within 2 weeks. Patient 3 developed a facial abscess 1 month after rituximab that resolved after antibiotic treatment. One death occurred in patient 4, who developed pneumonia and a congestive heart failure exacerbation 1 month after rituximab treatment. It is unclear whether rituximab contributed to the patient's infection in the setting of older age and multiple comorbidities.

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. REFERENCES

To our knowledge, the present study is the largest case series to date of patients treated for anti-SRP–associated myopathy with B cell depletion therapy. We have identified 8 patients with anti-SRP antibodies who developed severe myopathies unresponsive to other agents, and the majority showed a robust clinical response to rituximab treatment. This series suggests that B cell depletion therapy may be effective in patients with anti-SRP–associated myopathy and especially in those with refractory disease.

The 8 patients represented in this case series demonstrated similar clinical characteristics to anti-SRP patients described by others (3, 4, 6) (Table 1). The mean age of this cohort was younger at 37 years, and was comprised of a larger proportion of African Americans (50%) than has been reported in prior studies. The higher percentage of African American subjects in our cohort could be due to our particular geographic area and referral base. Alternatively, our findings could suggest that African Americans manifest a particularly severe myopathy and may be more responsive to rituximab treatment when compared with whites, but further prospective studies are needed to support this. Clinically, all of the patients in our study presented similarly with rapidly progressive and severe proximal muscle weakness accompanied with myalgias and dysphagia. Several also had signs of distal or asymmetric weakness that is atypical of PM or DM. Generally, patients also had extremely high CK levels at presentation (mean maximum CK level 18,987 IU/liter) with either mild or no elevation in inflammatory markers (Table 2). Only 2 patients demonstrated autoantibodies other than anti-SRP, but none manifested symptoms of overlapping autoimmune disease. All of the patients demonstrated an intense and widespread increase in STIR signal on muscle MRI reported as diffuse muscle or fascial edema, a finding that we have observed in patients with anti-SRP myopathy. Despite the MRI findings of a highly inflammatory process, the histologic appearance of their muscle biopsy samples consistently demonstrated significant muscle necrosis with complete absence of primary inflammation. Consequently, the diagnosis of anti-SRP myopathy was often delayed in many patients due to the absence of inflammation on muscle biopsy sample.

Table 1. Patient demographics and presenting characteristics*
PatientAge at onset, yearsSexRaceILDRaynaud's phenomenonRashMyalgiaArthritisDysphagiaWeakness on examination
PUDUPLDLA
  • *

    ILD = interstitial lung disease; PU = proximal upper extremity; DU = distal upper extremity; PL = proximal lower extremity; DL = distal lower extremity; A = asymmetrical weakness.

  • Weakness was defined as ≤4 on manual muscle strength testing.

120FAfrican American+++++++++
234FAfrican American+++++
342FWhite++++
472MWhite+++++++
521FAfrican American+++++++
626FAfrican American++++++
751MWhite+++++
832FWhite+++++
Table 2. Laboratory, EMG, and histologic features of anti-SRP myopathy*
PatientMax CK level, IU/literMax aldolase level, IU/literElevated ESR/CRP levelANA titerSRP+ (bands)Positive autoantibodiesEMG irritableMuscle biopsy features
PIPVIDRN
  • *

    EMG = electromyography; anti-SRP = anti–signal recognition particle; CK = creatine kinase; ESR = erythrocyte sedimentation rate; CRP = C-reactive protein; ANA = antinuclear antibody; PI = primary inflammation; PVI = perivascular inflammation; D = degeneration; R = regeneration; N = necrosis; N/A = information not available.

156,0008.8+/−+ (72, 54, 19)+Ro++
221,33355.6−/−+ (72, 19)+++
33,14814.6−/++ (72, 54, 19)N/A+++
46,88562−/++ (72, 19)+++
528,00030+/−+ (72, 54, 19)++++
620,18046+/−+ (72)+Ku+++
77,855N/A+/−N/A+ (72, 54)N/AN/AN/AN/AN/A
88,495147−/−+ (54)+++++

There has been substantial interest in the role of B cells in autoimmune disease, given the success of the monoclonal anti-CD20 antibody rituximab in the treatment of SLE, RA, and other processes that can be treatment refractory. Recently, B cell depletion therapy has also been effective in diseases with myositis-specific antibodies such as Jo-1–positive myopathy (11). Several studies have demonstrated that the levels of Jo-1 autoantibodies correlate with disease activity and may moderately decrease or disappear with successful therapy (4, 11). Similarly, in our case series, we demonstrate that the levels of anti-SRP antibodies can be reduced substantially with rituximab therapy (Table 3). This suggests that B cells and anti-SRP antibodies may have a pathogenic role in the inflammatory process of these particular myopathies, and rituximab as B cell depletion therapy may serve to directly target this immune response.

Table 3. Percent decrease in SRP autoantibody levels after rituximab treatment*
PatientAnti-SRP 54Anti-SRP 19Anti-SRP 72Anti–Jo-1Time from last rituximab treatment to autoantibody analysis, months
  • *

    Levels of autoantibodies were detected by immunoprecipitation using 35S-methionine–labeled in vitro transcription and translation products, and quantitated as described in the Patients and Methods section. Values of 100 and 0 represent complete disappearance and no disappearance of autoantibodies, respectively. Sera from patients 4, 7, and 8 were not tested for this analysis. SRP = signal recognition particle; NP = antibody not present in the patient's serum sample prior to rituximab treatment.

  • In this patient, there was no loss of antibodies to SRP 54; rather, there was a 10% increase.

110010096NP4
2NP1368NP2
3505747NP3
50180NP9
6NPNP42NP1
Jo-1 controlNPNPNP018

In this study, treatment with rituximab resulted in a dramatic and sustained clinical improvement in the majority of patients with anti-SRP myopathy (Table 4). Prior to rituximab therapy, many of these patients had continued to clinically deteriorate with marked weakness and severe disability. Several required the use of a wheelchair or could not get out of bed without assistance, and one had entered hospice despite maximal immunosuppression with multiple agents. One patient in particular (patient 2) had developed severe respiratory muscle weakness requiring prolonged intubation even after treatment with high-dose corticosteroids, methotrexate, azathioprine, cyclophosphamide, and plasma exchange. Now, 18 months after receiving 4 total doses of rituximab with adjunct corticosteroid therapy, this patient is able to walk, independently perform all of her activities of daily living, and has returned to work. Her response is similar to the other patients we have reported, as 6 of 8 patients showed a marked improvement in strength and/or decline in CK levels as early as 2 months after receiving 2 doses of rituximab. Three patients were readministered rituximab 6–8 months after initial dosing primarily to sustain the benefits achieved with the initial doses. Three patients have maintained this clinical improvement for as long as 12–18 months since the initial dose. Manual muscle strength data were incomplete for 2 patients who were either lost to followup (patient 7) or died (patient 4); in these patients, clinical response to rituximab was demonstrated by a decline in CK levels. Despite a lack of inflammation on histology, all of the patients were continued on adjunctive corticosteroid treatment after rituximab dosing since the anti-SRP myopathy can be responsive to steroids. However, in all 8 patients, the doses of corticosteroids were able to be substantially reduced after treatment with rituximab (Table 4).

Table 4. Summary of response to B cell depletion therapy and outcome*
PatientCK level prior to therapy, IU/literHighest prednisone dosage, mg/dayPrevious treatmentsDoses of B cell depletion therapyLowest prednisone dosage posttherapy, mg/dayLowest CK level posttherapy, IU/literOutcome post–B cell depletion and duration of remission
  • *

    CK = creatinine kinase; AZA = azathioprine; MTX = methotrexate; IV = intravenous; IVIG = IV immunoglobulin; MMF = mycophenolate mofetil; CHF = congestive heart failure; N/A = information not available.

12,71060AZA, MTX1,000 mg IV × 2 doses20622Decline in CK and improvement in strength for 10 months
21,000160MTX, AZA, IVIG, plasma exchange1,000 mg IV × 4 doses5163Normalization of CK and improvement in strength for 18 months
355040MTX, AZA, IVIG, MMF1,000 mg IV × 5 doses15126Normalization of CK and improvement in strength for 19 months
41,06380IVIG, plasma exchange1,000 mg IV × 1 dose5022No outcome data; died 1 month later from pneumonia and CHF
52,90080MTX, IVIG1,000 mg IV × 4 doses10963Decline in CK for 12 months, re-dosed for increased CK
62,10060MTX, MMF, IVIG1,000 mg IV × 2 doses301,144Improvement in strength and decline in CK for 9 months
71,25060MTX, MMF1,000 mg IV × 2 dosesN/A1,080Decline in CK for 5 months
83,11060AZA, MTX, IVIG, plasma exchange1,000 mg IV × 2 doses152,100Modest decline in CK after 6 months with persistent proximal lower extremity weakness

Anti-SRP antibody levels were quantitated pre– and post–rituximab treatment in 5 patients (Table 3). Similar initial levels of anti-SRP autoantibodies were detected in all 5 patients prior to rituximab treatment. In 4 of the 5 patients, anti-SRP antibody levels decreased significantly after rituximab therapy. In those patients, the time between treatment and autoantibody analysis ranged from 1–4 months. Clinical treatment response in terms of manual muscle strength testing and/or decline in CK level was robust and sustained in those 4 patients. Rituximab had no effect on anti-SRP antibody levels in 1 of the 5 patients (patient 5); however, this patient also had the longest time period between autoantibody analysis and rituximab treatment (9 months). Although patient 5 demonstrated an initial clinical response to rituximab, she began to manifest increased CK levels 2 months after autoantibody analysis, which prompted retreatment with rituximab 12 months after the initial dose. This suggests that the effect of rituximab may wane over time, and decreases in SRP autoantibody levels may be the most accurately quantified when performed soon after rituximab administration.

We conclude that B cell depletion with rituximab may be an effective and often life-saving therapy for patients with anti-SRP myopathy. Given that the manifestations of this myopathy can be quite severe and refractory to standard medications, prompt recognition of the disease and aggressive treatment has become increasingly important to prevent irreversible muscle damage and atrophy. Further studies are needed to elucidate the underlying disease pathogenesis and the precise role of anti-SRP antibodies in this unique subset of myopathies.

AUTHOR CONTRIBUTIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. REFERENCES

All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be submitted for publication. Dr. Christopher-Stine had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study conception and design. Valiyil, Casciola-Rosen, Christopher-Stine.

Acquisition of data. Valiyil, Casciola-Rosen, Hong, Mammen, Christopher-Stine.

Analysis and interpretation of data. Valiyil, Casciola-Rosen, Mammen, Christopher-Stine.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. PATIENTS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. AUTHOR CONTRIBUTIONS
  8. REFERENCES