Because almost 30% of individuals with ITP do not respond to the first- and second-line therapies, they live with low platelet counts, a situation that carries a high risk of hemorrhage and, eventually, a short life expectancy. It has been largely demonstrated that ITP with persistent low platelet counts carries a grave prognosis [6, 13]. Efforts to increase the platelet count to at least 30 × 109/l include IVIg, anti-D immunoglobulin, cyclophosphamide, polychemotherapy such as the CHOP regimen, thrombopoietin, interleukin-11, dapsone, alpha interferon, plasma exchange, and bone marrow transplantation . Using these therapies, the response rate is low and the patient is exposed to unnecessary risks . When the first ITP patient successfully treated with rituximab was informed, it drew the attention of physicians because this regimen offered high response rate and low toxicity. In subsequent reports, researchers have found similar results.
The aim of this study was to evaluate the response rate achieved in 18 patients with chronic and refractory ITP after rituximab treatment as well as the evolution of these patients to learn about the possible long-term side effects associated with the use of this drug, information that is almost inexistent in the literature. We observed 12 favorable responses (67%) and at least a stable clinical evolution in two additional patients (11%), for a global response rate of 78%. Based on the characteristics of the evolution of our group of patients, we may emphasize some points of interest: (1) Response rate obtained was quite satisfactory considering the history of chronicity and refractoriness of the patients; (2) the monoclonal antibody was well tolerated and caused only mild allergic reactions that could be easily managed with antihistaminics and paracetamol; (3) most of the patients achieve SR; and (4) increase of platelet count was not always immediate (median time to response = 14 weeks), a fact that slightly disagrees with previous reports that found shorter time to response (3 to 8 weeks; Table 2) [4, 8, 20]. In fact, in a recent systematic review about the efficacy of rituximab in adult patients with ITP, it was found that median time to response was 5.5 weeks (ranges 2 to 18 weeks) . We do not have an explanation for this discrepancy. After a careful analysis, we did not find significant differences between the results obtained in the systematic review and our results in terms of the variables that may affect the time to response to rituximab, namely, age, sex, previous use of corticosteroids, number of treatments before rituximab, rituximab dose and schedule, period between diagnosis and rituximab administration, duration of ITP before rituximab, and pretreatment platelet counts. Although splenectomy may not be a significant predictor of response to rituximab as previously suggested [2, 4, 8], it should be noted that 83% of the patients included in our report had had splenectomy as compared with 50.5% of the patients in the systematic review. Therefore, history of splenectomy seems to be the only factor related to a different time to response to rituximab in our series.
Of course, we have no data about all possible variables influencing the pattern of response to rituximab. For example, we do not show information about B-cell counts and platelet autoantibodies before and after rituximab therapy, two variables that may influence the response as observed in other trials. The expected therapeutic effect of rituximab is a reduction in specific platelet-associated autoantibodies and the consequent increase in platelet counts. Although rituximab has not been always associated with a reduced load of platelet autoantibodies and most of the publications reporting this effect are small series of cases , the high overall response rate obtained in our study allows us to believe that rituximab effectively decreased the B-cell counts as well as the levels of platelet autoantibodies.
Three patterns of response to rituximab have been proposed: early (before the fourth dose of rituximab), intermediate (7 to 11 weeks after rituximab), and delayed (>13 weeks after rituximab) . In our study, the probability to achieve a platelet count >50 × 109/l occurred at a median of 5 months, so we speculate that the complete inhibition of antibody formation and restoration of platelet counts with rituximab may occur after at least 5 months (95%CI = 0.5 to 11.6 months) from the first dose of the antibody (Fig. 2). However, some patients may achieve a quite delayed response (as long as 1 year after therapy), a situation in which it is important to wait a reasonable time period before another treatment is planned.
A previous report informed the results obtained in patients treated with the same regimen as we used . After a median follow-up of 47 weeks, the authors observed a lower overall response rate of 44% (CR = 18%, PR = 15%, MR = 10%); most of the responses were sustained. They observed two response patterns: (1) an early-response group in which responses appeared within the first 2 weeks after the first dose of rituximab; (2) a late-response group characterized by an increase in platelets several weeks after rituximab. Finally, after a median follow-up of 72.5 weeks, the response rate was 54% with a majority of SR . In a prospective trial performed in pediatric patients with chronic ITP, treatment with rituximab produced an increase of >50 × 109/l platelets in 11 of 36 children (31%) . Median time to response was 1 week (range, 1 to 7 weeks); however, a 6% incidence of serum sickness was observed. More recently, one of the largest experiences in ITP patients treated with rituximab was published . An increase of >50 × 109/l platelets was observed in 55% of the patients (CR = 46%, SR = 35%). The only predictive factor for SR was to achieve CR. Patients who were treated more intensively (more than three different treatments) and those with a longer ITP duration (>10 years from diagnosis) had the worse response. In this study, non-splenectomized patients had a higher early response rate than those splenectomized. Although some of these results are similar to those reported in our study, we believe that contrasting results seen in our study are partially explained by differences in the inclusion and response criteria used, in the evolution time of the disease, and in the number and type of treatments given before rituximab.
As early relapses may be seen in ITP patients treated with rituximab and because re-treatment with this monoclonal antibody offers good results , this drug has been used as a long-term maintenance treatment. Based on long-lasting remissions achieved using one or two doses of rituximab  and the results of late responders, we feel that a single dose of 375 mg/m2 every 6 months could be an alternative for this purpose.
On the other hand, because of the mechanism of action of rituximab and its interference with the immune system, we searched for possible long-term complications associated with its use, namely, neoplasias, chronic infections, or autoimmune diseases. Immune status is a major concern in patients treated with rituximab being the induction of low CD20+ counts and hypogammaglobulinemia (specifically low levels of immunoglobulin M), two previously reported secondary effects. Although we do not have data about these two immunological variables, the lack of severe infectious diseases during the long-term follow-up of our patients allows us to speculate that no severe immune abnormalities were developed in our series. As we previously stated, during the follow-up period, no associated illnesses or pathological phenomena were observed. To our knowledge, this seems to be the first report about the lack of long-term complications associated with rituximab.
In conclusion, rituximab is a long-term, safe, and effective alternative treatment of chronic and refractory ITP patients. Prospective randomized clinical trials are needed to elucidate the efficacy of the drug in comparison to splenectomy in early stages of the disease