Single, Low-dose Course of Rituximab Treats MuSK-MG, Pilot Study Suggests

A single, two-day and low-dose course of rituximab can effectively treat patients with muscle-specific tyrosine kinase myasthenia gravis (MuSK-MG) over at least six months, according to a pilot study.

The low dose administered eased disease symptoms and measures of clinical severity, and enabled patients to lower their steroid doses.

The study, “Short Term Effect of Low‐dose Rituximab on Myasthenia Gravis with Muscle‐specific Tyrosine Kinase Antibody,” was published in the journal Muscle & Nerve.

Rituximab, sold under the brand name Rituxan (among others), has emerged as a promising therapy for difficult-to-treat autoimmune disorders. It is approved, for instance, for people with moderate-to-severe and treatment-resistant rheumatoid arthritis. But an optimal treatment protocol for MG has not been established.

The medication consists of an antibody that targets a protein called CD-20, which is found on the surface of antibody-producing B-cells. These immune cells influence the body’s immune and inflammatory responses, which are excessive in those with MG. By binding to CD-20, rituximab lowers B-cell levels, potentially slowing MG progression and reducing the need for other medications.

Previous work by these researchers found that rituximab effectively treated acetylcholine receptor (AChR)-positive MG patients who are resistant to other medications, when given by an intravenous (IV, or into-the-vein) infusion at a dose of 500–600 mg every six months.

However, few studies have explored the therapeutic effects of a similar, low-dose regimen in patients with MuSK-MG. Of note, self-reactive antibodies targeting either AChR and MuSK are the two major causes of MG; when these antibodies are found in the bloodstream, patients are said to have AChR-MG or MuSK-MG.

Scientists with Fudan University and the Jing’an District Centre Hospital of Shanghai treated 12 MuSK-MG patients with a low dose regimen of rituximab, defined as a single 600 mg dose given over two days: 100 mg on day one followed by 500 mg the next day.

They then analyzed and compared blood samples taken before and six months after treatment to assess rituximab’s effectiveness at treating the disease.

Enrolled patients (11 women and one man) had a mean age of 40 and a median disease duration of 14 months. Three joined the study after experiencing a severe functional decline following steroid and immunosuppressant treatments, and one due to an unwillingness to take either steroids or immunosuppressants, despite relapsing after a crisis.

Six months after their infusions, all 12 patients showed significant clinical improvements and six were asymptomatic.

Improvements were reported across five clinical severity scores: the Quantitative MG (QMG) score, the Manual Muscle Testing (MMT), the MG Activities of Daily Living (MG-ADL), the Quality-of-Life 15 Question (QOL‐15) survey, and the MG Composite (MGC) score.

Three patients saw their QMG scores drop by more than four points, although their MG classification — a measure of overall severity — remained unchanged.

Following treatment, patients were able to lower their daily prednisone doses from about 27.29 to 12.29 mg, with six individuals tapering to under than 10 mg daily.

No serious side effects occurred over the course of the study, the research team reported.

Levels of MuSK antibodies mildly declined in most patients, but slightly increased in three, despite their clinical improvements. This contrasted with other studies, and the investigators suggested it could have been due to dose differences or to the use of a single two-day treatment course, compared to more frequent dosing.

Interestingly, blood levels of B-cell activating factors, known as BAFFs, significantly increased at six months after treatment. Although this could potentially increase the risk of a relapse, the team wrote, higher BAFF levels nonetheless were found to be correlated to lower B-cell counts and clinical severity.

“This small, self-controlled pilot study appeared to show the clinical efficacy of a single low-dose [rituximab] in improving symptoms and permitting reductions in steroid doses in MuSK- MG,” the researchers concluded.