Patients with the cutaneous eruption and at least three of the other four criteria met the requirements for definite DM according to these criteria, while requirements for definite PM were met by those with all four criteria other than the cutaneous features [ 3 ]. Patients with findings indicating the presence of other disorders that may present similarly were excluded. Those disorders included central or peripheral neurologic disease; muscular dystrophy; granulomatous myositis, such as sarcoidosis; infectious muscle disease (eg, trichinosis or toxoplasmosis); recent exposure to myopathic drugs or toxins; rhabdomyolysis, with history of a trigger; metabolic muscle disease; myopathic endocrinopathy; and myasthenia gravis. Patients who did not meet these criteria but who lacked any of the exclusions could potentially have been diagnosed with possible or probable DM or PM, depending upon the number of criteria met.
The presence of anti-natalizumab antibodies was correlated with a reduction in serum natalizumab levels. In Study MS1, the Week 12 pre-infusion mean natalizumab serum concentration in antibody-negative patients was 15 mcg/mL compared to mcg/mL in antibody-positive patients. Persistent antibody-positivity resulted in a substantial decrease in the effectiveness of TYSABRI. The risk of increased disability and the annualized relapse rate were similar in persistently antibody-positive TYSABRI-treated patients and patients who received placebo. A similar phenomenon was also observed in Study MS2.