Higher Dose of Rituxan Versus Standard Doses of Rituxan With Cyclophosphamide, Vincristine, and Prednisone in Subjects With Chronic ITP

The rationale for using chemotherapy in combination with Rituximab:

Since Rituximab is an anti-B cell therapy, in order to improve the rate of durable responses beyond the 32% (18 of 57) seen with Rituximab alone, it seems appropriate to combine it with a therapy that would also target T cells and/or macrophages. Our plan is therefore to combine Rituximab with a standard chemotherapy (CHOP)-like regimen as previously successfully tested in patients with follicular or diffuse large-B-cell lymphomas 13-15. The "CHOP" chemotherapy regimen is a combination of cyclophosphamide, doxorubicin, vincristine and prednisone (or prednisolone) that has been considered the "gold standard" for treating lymphomas for more than 20 years.

This combination of medications was used in a Hodgkin's patient with refractory ITP and became the template for developing the use of cyclophosphamide, vincristine, and prednisone for ITP as initially reported in 1993. Since reports on doxorubicin efficacy by itself in ITP are only anecdotal 16 and this drug has a potential cardiac toxicity, a CVP regimen (namely a combination of cyclophosphamide + vincristine and prednisone) should be similar in efficacy to CHOP in patients with ITP and have less toxicity. Indeed, the efficacy of such a chemotherapy12 as well as pulses cyclophosphamide therapy alone11 have already been reported in patients with refractory ITP.

Since attempts to increase the efficacy of CHOP by increasing the doses or adding other cytotoxic drugs have failed, a new therapeutic strategy combining CHOP with Rituxan has been successfully developed in the last few years in various types of B-cell lymphomas 13-15. In elderly patients with diffuse large-B-cell lymphoma, the addition of Rituxan to standard CHOP chemotherapy significantly reduced the risk of treatment failure and deaths without increasing toxicity 13. Moreover, in autoimmune disorders, there are few preliminary data suggesting that Rituxan and cyclophopshamide given in combination could be effective and relatively safe in patients with active rheumatoid arthritis 17.

Therefore, the rationale for combining Rituxan with a CHOP-like regimen in ITP is threefold:

Both Rituxan and CHOP or IV cyclophosphamide have efficacy in ITP The treatments have different mechanisms of action. They have minimally-overlapping toxicities.

The rationale for using higher doses of Rituxan in patients who had no response, or relapsed, to the drug at the standard dose:

The standard dose of Rituxan is arbitrary in that one dose of Rituxan has been used in the great majority of the clinical trials and virtually all patients since the FDA approval of Rituxan in 1997: 375 mg/m2 weekly x 4 weeks. To date, because Rituxan is a monoclonal antibody rather than a chemotherapeutic agent, it was recognized that a true maximum tolerated dose (MTD) might not be achieved. Among other factors that could influence the tolerance of higher doses of Rituxan are the rate of CD20 surface expression and the serum level of the antibody11. Limited trials of higher doses have been pursued in CLL18 (up to 2,250 mg/m2 per dose), but not in other types of lymphoma or in autoimmune diseases. In CLL, mild to severe toxicity was exclusively observed with the first dose (375 mg/m2) while toxicity on subsequent higher doses was minimal. In ITP, some of the few patients that have been retreated responded better to the second dose of rituximab than to the initial treatment (although the opposite is also true). Full depletion of the marrow and especially the lymph nodes is not achieved by the current dose regimen and B cells return in substantial number to the peripheral blood within 3-6 months in patients with ITP treated at the conventional dose. We therefore anticipate that in ITP patients who relapsed or did not respond after a previous course of rituximab, doubling the dose could lead to a deeper and more prolonged B cell depletion and to a better increase in the platelet count without enhancing toxicity.