To the Editors:

We were pleased to see our results replicated in 2 additional patients as reported by Deligny et al in their letter. It is an intriguing observation, although not entirely clear mechanistically, that there may in fact be a differential response to rituximab therapy in patients with anti-SRP–associated myopathy who are of African descent. This may explain why Whelan and Isenberg's results are not as robust (Whelan BR, Isenberg DA. Poor response of anti-SRP-positive idiopathic immune myositis to B-cell depletion. Rheumatology [Oxford] 2009;48:594–5). They report the results of two patients, one white and one of African descent, with anti-SRP–associated myopathy who are given rituximab. The patient of African descent demonstrates a more impressive response to rituximab than the white patient. Although the authors report an incomplete rituximab response in both patients, the second patient in this description clearly improved with a biochemical response: CK falling from a high of >3,000 IU/liter prior to rituximab therapy to 570 IU/liter. Whereas previously this patient was unable to bear weight, at 6 months posttreatment she was mobile with a walking device and could stand up out of a chair unassisted. Additionally, it would be interesting to know whether the second patient had persistent weakness due to fixed muscle damage, which could be seen as fatty replacement on MRI. In such an instance, this should not be construed as a nonresponse to rituximab.

Of 14 reported cases, 10 have responded to rituximab and, to date, 9 (90%) of these responders are of African descent, suggesting that there may be a differential response to rituximab therapy among certain ethnic groups. Although further studies are needed to support this observation, it is nonetheless important to recognize that anti-SRP–associated myopathy has been historically very difficult to treat in many patients, and rituximab may offer a sustained benefit in some.

Ritu Valiyil MD*, Livia Casciola-Rosen PhD*, Grace Hong BA*, Andrew L. Mammen MD, PhD*, Lisa Christopher-Stine MD, MPH*, * Johns Hopkins University School of Medicine Baltimore, MD.