Efficacy and safety of rozanolixizumab in patients with muscle-specific tyrosine kinase autoantibody-positive generalised myasthenia gravis: a subgroup analysis of the randomised, double-blind, placebo-controlled, adaptive phase III MycarinG study

Background: Muscle-specific tyrosine kinase (MuSK) autoantibody-positive (Ab+) generalised myasthenia gravis (gMG) is a rare and frequently severe subtype of gMG. Objectives: To assess the efficacy and safety of rozanolixizumab in the subgroup of patients with MuSK Ab+ gMG in the MycarinG study. Design: A randomised, double-blind, placebo-controlled phase III study. Methods: Patients with acetylcholine receptor (AChR) Ab+ or MuSK Ab+ gMG (aged ⩾18 years, Myasthenia Gravis Foundation of America Disease Class II–IVa, Myasthenia Gravis Activities of Daily Living [MG-ADL] score ⩾3.0 [non-ocular symptoms], Quantitative Myasthenia Gravis score ⩾11.0) were randomly assigned (1:1:1) to receive once-weekly subcutaneous infusions of rozanolixizumab 7 mg/kg, rozanolixizumab 10 mg/kg or placebo for 6 weeks, followed by an 8-week observation period. Randomisation was stratified by AChR and MuSK autoantibody status. The primary study endpoint was change from baseline to Day 43 in MG-ADL score. Treatment-emergent adverse events (TEAEs) were also assessed. Results: Overall, 200 patients were randomised, of whom 21 had MuSK Ab+ gMG and received rozanolixizumab 7 mg/kg (n = 5), 10 mg/kg (n = 8) or placebo (n = 8). In patients with MuSK Ab+ gMG, reductions from baseline to Day 43 in MG-ADL scores were observed: rozanolixizumab 7 mg/kg least squares mean (LSM) change (standard error), –7.28 (1.94); 10 mg/kg, –4.16 (1.78); and placebo, 2.28 (1.95). Rozanolixizumab 7 mg/kg LSM difference from placebo was −9.56 (97.5% confidence interval: −15.25, −3.87); 10 mg/kg, −6.45 (−11.03, –1.86). TEAEs were experienced by four (80.0%), five (62.5%) and three (37.5%) patients with MuSK Ab+ gMG receiving rozanolixizumab 7 mg/kg, 10 mg/kg and placebo, respectively. No patients experienced serious TEAEs. No deaths occurred. Conclusion: This subgroup analysis of adult patients with MuSK Ab+ gMG enrolled in the MycarinG study supports the use of rozanolixizumab as an effective treatment option for patients with gMG who have MuSK autoantibodies. Trial registration: ClinicalTrials.gov: NCT03971422 (https://clinicaltrials.gov/study/NCT03971422); EU Clinical Trials Register: EudraCT 2019-000968-18 (https://www.clinicaltrialsregister.eu/ctr-search/trial/2019-000968-18/GB).


Introduction
2][3] The predominant manifestation is fluctuating and fatigable muscle weakness, which can be life-threatening if the respiratory or bulbar muscles are affected. 1,4,5ile the majority of patients with myasthenia gravis (MG) have detectable antigen-specific autoantibodies directed against the acetylcholine receptor (AChR) on the postsynaptic membrane of the NMJ, 4,6 a small proportion of patients with MG have muscle-specific tyrosine kinase (MuSK) autoantibodies (5%-8%). 4,7MuSK plays a central role in NMJ organisation and maintenance by facilitating AChR clustering. 1,4uSK autoantibody-positive (Ab+) gMG is mainly immunoglobulin G4 (IgG4)-mediated; IgG4 autoantibodies prevent MuSK-low-density lipoprotein receptor-related protein 4 interaction, subsequently reducing AChR clustering at the NMJ, leading to impaired muscle contraction.1,4,8 The MuSK Ab+ gMG subtype exhibits a strong female predominance, and a higher overall prevalence is observed in Southern Europe compared with Northern Europe.In contrast to AChR Ab+ gMG, disease onset is early, with a peak incidence towards the end of the third decade.7,9,10 Patients with MuSK Ab+ gMG are considered to have a distinctive subtype of MG, which is frequently more severe than other subtypes.7 Onset of MuSK Ab+ gMG is usually acute, with rapid symptom progression within a few weeks, and typically affects the bulbar muscles.7,9 Due to the atypical onset and clinical features of the disease, including marked muscle atrophy, selective bulbar involvement and lack of symptom fluctuations, the diagnosis of MuSK Ab+ gMG can be challenging.7,9,10 These challenges further extend to the management of MuSK Ab+ gMG, with patients experiencing an often unsatisfactory response to some of the treatments typically used for AChR Ab+ gMG.For example, patients with MuSK Ab+ gMG show limited response to intravenous immunoglobulin (IVIg) and may experience worsening with acetylcholinesterase inhibitors.6,7,9 Consistent thymic abnormalities have not been reported in patients with MuSK Ab+ gMG; hence, thymectomy is not considered a therapeutic option. 6,11 thermore, since immunoglobulin G (IgG) autoantibodies of the IgG4 subclass do not activate complement, the use of complement inhibitors is presumed ineffective in the MuSK Ab+ population.6,9,12 The limited treatment options available for patients with MuSK Ab+ gMG include plasma exchange therapy and rituximab, a CD20 inhibitor.11,13 Rituximab has been included in recommendations for the management of patients with MuSK Ab+ gMG whose response to initial immunotherapy is unsatisfactory, despite it not currently being licensed for this indication.11,14 Conventional immunosuppression therefore remains the cornerstone of treatment for MuSK Ab+ gMG.7,9 In recent years, progress has been made in the development of antigen-specific immunotherapies directed against the cells and immune pathways involved in MG pathogenesis.4,6 The reduction of IgG autoantibodies via inhibition of the neonatal Fc receptor (FcRn) is one such target for the treatment of MG. 15,16 FcRn functions as a natural salvage and recycling mechanism that is responsible for prolonging the half-life of serum IgG molecules by preventing lysosomal IgG degradation.[16][17][18][19] Inhibition of FcRn thus allows for the targeted reduction of IgG antibodies, including pathogenic autoantibodies of the IgG4 subclass implicated in MuSK Ab+ gMG pathogenesis.19 Rozanolixizumab is a humanised IgG4 monoclonal antibody that reversibly binds to FcRn with high affinity.15,16 In June 2023, rozanolixizumab was approved by the United States Food and Drug Administration for the treatment of adult patients with AChR Ab+ or MuSK Ab+ gMG.20 Rozanolixizumab has since been approved in Japan for the treatment of patients with gMG who inadequately respond to corticosteroids or non-corticosteroid immunosuppressants, 21 and in Europe and the UK as an add-on to standard therapy for the treatment of gMG in adult patients who are AChR Ab+ or MuSK Ab+.22,23 The pivotal phase III MycarinG study (NCT03971422; EudraCT 2019-000968-18) established the efficacy and safety of rozanolixizumab in adults with AChR Ab+ or MuSK Ab+ gMG.15 Here, we report findings in the subgroup of patients with MuSK Ab+ gMG in the phase III MycarinG study.

Study design and patients
MycarinG was a randomised, double-blind, placebo-controlled, parallel-group, two-stage adaptive phase III study in patients with AChR or MuSK Ab+ gMG; the full study design has been reported previously. 15In brief, patients were randomised 1:1:1 to receive subcutaneous infusions of rozanolixizumab 7 mg/kg, rozanolixizumab 10 mg/kg or placebo once a week for 6 weeks on top of their current gMG treatment (where permitted by the study inclusion criteria).Randomisation was stratified by the presence of AChR or MuSK autoantibodies.The 6-week treatment period was followed by an observation period of 8 weeks.Patients were then eligible to roll over into either of the open-label extension (OLE) studies: MG0004 (completed; NCT04124965; EudraCT 2019-000969-21) or MG0007 (completed; NCT04650854; EudraCT 2020-003230-20). 15Rescue therapy (IVIg or plasma exchange) was permitted at the investigator's discretion for patients who experienced disease worsening.Patients who required and opted to receive rescue therapy during the treatment period stopped receiving the study drug and completed all remaining visits before moving into the observation period.If they subsequently required rescue therapy during the observation period, they were given the option to either enrol in an OLE study, providing at least 2 weeks had passed since receipt of rescue therapy, or receive rescue therapy and not be invited to enrol in an OLE study.Patients who completed the treatment period without receiving rescue therapy but required rescue therapy during the observation period could choose to enrol in an OLE study or receive rescue therapy and not be invited to enrol in an OLE study.

Full inclusion and exclusion criteria of the
MycarinG study have been reported previously. 15riefly, patients aged ⩾18 years with a diagnosis of gMG (Myasthenia Gravis Foundation of America Disease Class II-IVa), a Myasthenia Gravis Activities of Daily Living (MG-ADL) score ⩾3.0 (for non-ocular symptoms) and a Quantitative Myasthenia Gravis (QMG) score ⩾11.0 who had been considered by the investigator for additional therapy such as IVIg or plasma exchange were eligible for enrolment.
At screening, all patients were required to have a previously documented positive record of autoantibodies against MuSK or AChR from historical diagnostic tests.The presence of autoantibodies against MuSK or AChR was also assessed at study baseline using a clinical laboratory radioimmunoassay.Baseline assessment was for exploratory quantification and did not inform eligibility.MG-specific autoantibody status from the historical diagnostic tests was used to define MuSK autoantibody positivity for this subgroup analysis.

Study endpoints
The primary efficacy endpoint was the change from baseline to Day 43 in MG-ADL score.Secondary efficacy endpoints included change from baseline to Day 43 in Myasthenia Gravis Composite (MGC), QMG and MG Symptoms Patient-Reported Outcomes (PRO) Muscle Weakness Fatigability, Physical Fatigue and Bulbar Muscle Weakness 24 scores.MG-ADL response (based on the established clinically meaningful individual patient-level improvement from baseline of ⩾2.0 points) 25 at Day 43 was also assessed.Other efficacy endpoints included MGC and QMG response (based on the clinically meaningful improvement of ⩾3.0 points) 26,27 at Day 43, change from baseline in MG-ADL, MGC and QMG at each scheduled assessment during treatment and observation periods and minimal symptom expression (MSE; MG-ADL score of 0 or 1) during the treatment period.
Safety and tolerability outcomes included the occurrence of treatment-emergent adverse events (TEAEs) and TEAEs leading to study drug discontinuation.Pharmacodynamic outcomes were change from baseline in MG-specific autoantibodies, serum total IgG and IgG subclass concentrations.Other outcomes, including pharmacokinetic outcomes, have been described previously. 15atistical analysis Full details of the statistical analyses conducted in the overall study population, including the calculation and justification of the sample size, have been described previously. 15SK Ab+ and AChR Ab+ gMG subgroup analyses were performed according to the randomly assigned treatment (randomised set).Evaluation of the primary and secondary efficacy endpoints in the subgroups was pre-specified and the endpoints were analysed using a mixed model for repeated measures, which included treatment group, baseline MG-ADL score, geographical region and treatment group by day as fixed factors, with study patient as a random effect.The model utilised an unstructured covariance pattern for the repeated measures.Based on the model, 97.5% confidence intervals (CIs) were reported.Safety analyses for the subgroups were carried out in the safety set, which included all randomly assigned patients who received at least one dose of rozanolixizumab, analysed according to the actual treatment received.All subgroup analyses and comparisons between the overall population and the subgroups were descriptive.

Baseline demographics and characteristics
The MycarinG study took place over 29 months, with patients randomised between 3 June 2019 and 30 June 2021.In total, 200 patients received rozanolixizumab 7 mg/kg (66 [33.0%]), rozanolixizumab 10 mg/kg (67 [33.5%]) or placebo (67 [33.5%]; Figure 1).A total of 21 (10.5%)patients had a documented history of autoantibodies against MuSK (rozanolixizumab 7 mg/kg: n = 5 [7.6%]; rozanolixizumab 10 mg/kg: n = 8 All patients with MuSK Ab+ gMG treated with rozanolixizumab had Myasthenia Gravis Foundation of America Disease Class II or III at baseline (Table 1).Compared with the overall population, mean MG-ADL score at baseline was numerically greater in patients with MuSK Ab+ gMG.Within the MuSK Ab+ gMG subgroup, mean baseline MG-ADL score was numerically greater in those treated with rozanolixizumab 7 mg/kg compared with those receiving rozanolixizumab 10 mg/kg.A greater proportion of patients with MuSK Ab+ gMG were female and had experienced prior MG crisis, while a lower proportion had thymectomy compared with the overall population at baseline.

Efficacy
In patients with MuSK Ab+ gMG who received rozanolixizumab, reductions from baseline to Day 43 in MG-ADL scores were more pronounced than in the overall population (Table 2 and Figure 2(a)).In the overall population at Day 43, both rozanolixizumab dose groups achieved a clinically meaningful and statistically significant least squares mean (LSM) difference from placebo for the change from baseline in MG-ADL score (primary efficacy endpoint). 15The rozanolixizumab LSM differences from placebo in MG-ADL scores were numerically greater among patients with MuSK Ab+ gMG compared with the overall population (Table 2).
Rozanolixizumab-treated patients in the overall population also achieved clinically meaningful and statistically significant LSM differences from placebo for the change from baseline to Day 43 in MGC and QMG scores.Statistically significant improvements from baseline in MG Symptoms PRO scale scores were also observed with rozanolixizumab versus placebo (Table 2). 15Compared with the overall population, rozanolixizumab-treated patients with          15 The safety profile of rozanolixizumab in patients with AChR Ab+ gMG was similar to that observed in the overall population (Table 4).

Pharmacodynamics
Within the overall population, reductions in total IgG were observed in the rozanolixizumab groups from the first post-baseline measurement, Day 8, with levels gradually returning to baseline by the end of the observation period. 15 Myasthenia gravis 0 0 0 Myasthenia gravis crisis 0 0 0 Permanent discontinuation from study due to TEAE, n (%) 0 0 1 ( Severe TEAEs, n (%) 0 0 0 Deaths, n (%) 0 0 0 0 0 0 0 0 0 The safety set consisted of all randomised patients who received at least one dose of rozanolixizumab, analysed according to the actual treatment received.*Two patients in the 7 mg/kg group who incorrectly received 10 mg/kg were analysed in the 10 mg/kg group for safety and PK/PD analyses.† Specific TEAEs listed are those occurring in ⩾5% of patients in any treatment group in the overall population.‡ Specific serious TEAEs listed are those occurring in more than one patient in any treatment group in the overall population.

Discussion
The MycarinG study is the largest phase III clinical study of patients with gMG to date.The proportion of patients with MuSK Ab+ gMG enrolled in the study (10.5% [n = 21]) was representative of the proportion of patients with this subtype of MG in the real world (5%-8% of patients). 7Further, the number of patients with MuSK Ab+ gMG is the largest to be enrolled in a randomised controlled phase III study.In this subgroup analysis, treatment with rozanolixizumab demonstrated improvements from baseline in multiple MG-specific endpoints in patients with MuSK Ab+ gMG, consistent with those observed in the overall population.Both rozanolixizumab doses were well tolerated.
Numerically greater improvements from baseline in MG-ADL, MGC, QMG and MG Symptoms PRO scale scores at Day 43 were observed with rozanolixizumab 7 mg/kg compared with rozanolixizumab 10 mg/kg in patients with MuSK Ab+ gMG.Conversely, in the overall population, improvements in several efficacy endpoints appeared to be greater with rozanolixizumab 10 mg/kg than with 7 mg/kg. 15This may be explained by the imbalance in baseline characteristics within the MuSK Ab+ gMG subgroup; patients treated with rozanolixizumab 7 mg/kg had higher MG-ADL, MGC, QMG and MG Symptoms PRO scale scores at baseline (data not presented for MGC and MG Symptoms PRO scale baseline scores) compared with those receiving rozanolixizumab 10 mg/kg, allowing more scope for improvement in scores.However, due to the small number of patients with MuSK Ab+ gMG in each dosing group, the apparent larger effect of the lower dose in MuSK patients may simply be due to chance.
Rozanolixizumab treatment improved physical fatigue in patients with MuSK Ab+ gMG as measured by a reduction from baseline to Day 43 in MG Symptoms PRO Physical Fatigue scores.Physical fatigue has been identified as an important symptom for patients with MG. 28 In the MycarinG study, use of the MG Symptoms PRO measure facilitated comprehensive assessment of physical fatigue and its manifestations, such as lack of energy, muscle weakness and heaviness in the body and limbs. 29Hence, inclusion of the MG Symptoms PRO complemented other MG-specific patient-and clinician-reported outcome measures used in the study that do not fully capture physical fatigue. 24,29tients were required to have a previously documented positive record of autoantibodies against MuSK or AChR at screening.Six patients with a documented history of MuSK Ab+ gMG tested negative for the presence of MuSK autoantibodies at baseline.Autoantibodies of low abundance may not be detectable by classical assays, and it is known that immunosuppressive treatment influences autoantibody titres.1][32] Autoantibody levels may also be impacted by other treatments, for example those that suppress B-cell proliferation. 33However, patients who had received treatment with rituximab 6 months prior to baseline, or 12 months prior if B cells had not returned to the normal range, were excluded from the MycarinG study.Further, no patterns in prior treatment with IVIg or plasma exchange were observed that could be deemed responsible for the changes in autoantibody status.Patients who subsequently test negative for MG-specific autoantibodies may still be considered positive for autoantibodies against MuSK or AChR, and show a response to immunotherapy. 34This may explain why all five of the patients in this group who received rozanolixizumab were MG-ADL responders.
Patients with MuSK or AChR Ab+ gMG have shown high rates of response to plasma exchange; 35 therefore, it can be reasonably assumed that they are both responsive to other IgG-lowering therapies.Rozanolixizumab led to rapid reductions in total IgG concentrations, with a robust lowering of IgG4 observed in patients with MuSK Ab+ gMG.This was in line with total IgG lowering observed following rozanolixizumab treatment in the overall population.
A limitation of this analysis was the small patient numbers within the MuSK Ab+ gMG treatment groups, which may limit generalisability of the results to the wider MuSK Ab+ gMG population.While the proportion of patients with MuSK Ab+ gMG in the study was reflective of the lower prevalence of MuSK Ab+ gMG in the real-world gMG population, the small patient numbers led to imbalances in the baseline disease characteristics between treatment groups.Despite this, the observed baseline demographics and characteristics were broadly reflective of those described in the literature for patients with MuSK Ab+ gMG. 5,36For example, a high proportion of patients with MuSK Ab+ gMG in the study were female and had prior MG crisis, and as expected, a low proportion had prior thymectomy.
Due to nuances in the clinical characteristics and poor response to standard treatments considered for gMG, MuSK Ab+ gMG presents a high burden of disease for patients. 5,7,9The disease phenotype is often associated with greater bulbar symptom severity and more frequent myasthenic crises than AChR Ab+ gMG, and standard therapies for gMG are not always effective. 5,7herefore, there is an urgent need for effective targeted treatments in this patient population.
Primary results reported for the MycarinG study provided support for FcRn inhibition in gMG, 15 and when combined with the results of this subgroup analysis, provide evidence for rozanolixizumab treatment in patients with MuSK Ab+ gMG.However, longer follow-up of these patients is required to demonstrate continuous benefits of rozanolixizumab treatment in the MuSK Ab+ gMG population.The safety and efficacy of repeated 6-week treatment cycles was investigated in the OLE study, MG0007.

Figure 1 .
Figure 1.Trial profile and patient disposition.*Required rescue therapy (investigator judgement) during the observation period of MycarinG.† Completed both the treatment and observation periods; 64 patients completed the treatment period in each of the placebo and rozanolixizumab 7 mg/kg groups and 62 patients completed the treatment period in the rozanolixizumab 10 mg/kg group.AE, adverse event; MuSK Ab+, muscle-specific tyrosine kinase autoantibody positive; ITT, intention-to-treat; OLE, open-label extension.

Figure 3 .
Figure 3. Mean percentage change from baseline in IgG4 concentration in patients with MuSK Ab+ gMG and the overall population (safety set).

Table 1 .
Baseline demographic and clinical characteristics for patients with MuSK Ab+ gMG, AChR Ab+ gMG and the overall population (randomised set). 6journals.sagepub.com/home/tan

Table 2 .
Change from baseline at Day 43 in MG-ADL, MGC, QMG and MG Symptoms PRO scale scores in patients with MuSK Ab+ gMG, AChR Ab+ gMG and in the overall population (randomised set).

Table 3 .
MG-ADL, MGC and QMG responders at Day 43 in patients with MuSK Ab+ gMG, AChR Ab+ gMG and the overall population (randomised set).Observed values.Percentages are based on the number of patients with non-missing data on Day 43.MG-ADL responders are defined as having a ⩾2.0-point improvement from baseline; MGC and QMG responders are defined as having a ⩾3.0-point improvement from baseline.AChR Ab+, acetylcholine receptor autoantibody positive; gMG, generalised myasthenia gravis; MG-ADL, Myasthenia Gravis Activities of Daily Living; MGC, Myasthenia Gravis Composite; MuSK Ab+, muscle-specific tyrosine kinase autoantibody positive; QMG, Quantitative Myasthenia Gravis; RLZ, rozanolixizumab.

Table 4 .
TEAEs in patients with MuSK Ab+ gMG, AChR Ab+ gMG and the overall population (safety set). 37 UCB Pharma and Viela Bio (now Amgen).He has received honoraria from Alexion Pharmaceuticals, Alpine Immune Sciences, argenx, Genentech/Roche, Immunovant, Inhibrx, Regeneron Pharmaceuticals, NMD Pharma and UCB Pharma.SS has nothing to disclose.GA has received funding for travel, conference and meeting attendance or advisory board participation from Alexion Pharmaceuticals, argenx, Janssen Pharmaceuticals (now Johnson & Johnson Innovative Medicine), Kedrion and UCB Pharma.EC-V receives public speaking honoraria and compensation for advisory boards and/ or consultation fees from Alexion Pharmaceuticals, argenx, Janssen Pharmaceuticals (now Johnson & Johnson Innovative Medicine) and UCB Pharma.JG has served as a Consultant for Alexion Pharmaceuticals, Biogen and UCB Pharma, and his institution has received research support from the Boris Canessa Foundation.ZKM has received funding for advisory board participation from Alexion Pharmaceuticals.RM has received funding for travel and meeting attendance or advisory board participation from Alexion Pharmaceuticals, argenx, BioMarin, Catalyst, Sanofi, Regeneron Pharmaceuticals and UCB Pharma.RMP has nothing to disclose.KU has served as a paid Consultant for argenx, Chugai Pharmaceutical, HanAll Biopharma, Janssen Pharmaceuticals (now Johnson & Johnson Innovative Medicine), Merck, Mitsubishi Tanabe Pharma, UCB Pharma and Viela Bio (now Amgen); he has received speaker honoraria from Alexion Pharmaceuticals, argenx, the Japan Blood Products Organization and UCB Pharma.JV has been a Consultant on advisory boards for Amicus Therapeutics, Roche, Sanofi Genzyme (now Sanofi), Sarepta Therapeutics and UCB Pharma.He has received research, travel support and/or speaker honoraria from Alexion Pharmaceuticals, argenx, Biogen, Edgewise Therapeutics, Fulcrum Therapeutics, Lupin, Sanofi Genzyme (now Sanofi) and UCB Pharma.He is a Principal Investigator in clinical trials for Alexion Pharmaceuticals, argenx, Genethon, Horizon Regeneron Pharmaceuticals and UCB Pharma, and has served as a speaker for Alexion Pharmaceuticals, argenx, and CSL Behring.He performed consulting work for Alexion Pharmaceuticals, argenx, Dianthus Therapeutics, ImmunAbs and UCB Pharma.HW is a Scientific Advisor for AbbVie, Alexion Pharmaceuticals, argenx, Bristol Myers Squibb/Celgene, Janssen Pharmaceuticals (now Johnson & Johnson Innovative Medicine), Merck, Novartis and Sandoz.He has received speaker honoraria and travel support from Alexion Pharmaceuticals, Biogen, Bristol Myers Squibb, Genzyme, Merck, Neurodiem, Novartis, Ology, Roche, TEVA and WebMD Global and is a paid Consultant for AbbVie, Actelion, argenx, BD, Biogen, Bristol Myers Squibb, EMD Serono, Fondazione Cariplo, Gossamer Bio, Idorsia, Immunic, Immunovant, Janssen Pharmaceuticals (now