"Adult ITP is a serious chronic autoimmune disorder characterized by low platelet counts in the blood, a condition known as thrombocytopenia," said study investigator David J. Kuter, M.D., D. Phil., Chief of Hematology, Massachusetts General Hospital, Boston. "These study findings are encouraging and provide hope that Nplate may provide physicians with a new therapeutic option for adult patients with chronic ITP."
The 24-week pooled study was comprised of two parallel Phase 3 trials and included 63 splenectomized patients and 62 non-splenectomized patients with ITP and a mean of three platelet counts of 30,000 per microliter or less. Patients were randomly assigned 2:1 to subcutaneous injections of Nplate (n=42 in splenectomized study and n=41 in non-splenectomized study) or placebo (n=21 in both studies). The primary endpoints assessed the efficacy of Nplate as measured by a durable platelet response and treatment safety. Durable response was defined as a platelet count above 50,000 per microliter during six or more of the last eight weeks of treatment without rescue therapy ever being administered.
-- Durable Platelet Response: The durable response rate was significantly greater in patients treated with Nplate compared to those in the placebo group in both studies (difference in proportion of splenectomized patients responding 38 percent (95 percent CI (23.4-52.8 percent); p less than 0.0001); difference in proportion of non-splenectomized patients responding 56 percent (95 percent CI (38.7-73.7 percent); p less than 0.0001)). In the placebo groups, no splenectomized patients and only one non-splenectomized patient achieved a durable platelet response.
-- Overall Platelet Response: The overall platelet response (either durable or transient response, with transient defined as greater than or equal to 4 weekly platelet responses from week two to 25) was 88 percent (36/41) in non-splenectomized patients compared to 14 percent (3/21) in the placebo group; and 79 percent (33/42) in splenectomized patients given Nplate compared to no splenectomized patients given placebo (p less than 0.0001).
-- Platelet Counts: Nplate-treated patients achieved platelet counts of 50,000 per microliter or more for an average of 13.8 weeks in the splenectomized group and 15.2 weeks in the non-splenectomized group compared with 0.2 and 1.3 weeks for patients in the respective placebo groups.
-- Reduction or Discontinuation of Concurrent Therapies: In both studies, 23 (12 splenectomized and 11 non-splenectomized) patients with Nplate and 16 (six splenectomized and 10 non-splenectomized) patients in the placebo group received concurrent ITP therapy with corticosteroids, azathioprine, and/or danazol. During the first 12 weeks of the study, 52 percent of the Nplate patients and 19 percent of the placebo patients discontinued all of their concurrent ITP treatments. An additional 35 percent of the Nplate patients and 19 percent of the placebo patients reduced at least one of their concurrent ITP medicines by more than 25 percent.
-- Rescue Medications: Significantly more patients in the placebo group received rescue treatment to increase platelet counts to prevent or treat bleeding compared to the Nplate-treated patients (57 percent placebo vs. 26 percent Nplate-treated splenectomized group; 62 percent placebo vs. 17 percent Nplate-treated non-splenectomized group) (p less than 0.0001). Rescue medication was defined as an increased dose of concurrent ITP drug, or the use of any new drug to increase platelet counts.
Adverse event rates were similar between the Nplate and placebo groups. According to the study authors, a study analysis indicated no difference in the safety profile between splenectomized and non-splenectomized ITP patients treated with Nplate, and therefore the authors pooled safety data for all patients in the Nplate and placebo groups.
In one splenectomized Nplate-treated patient, an increase in bone marrow reticulin that returned to baseline three months after withdrawal of Nplate was reported as a treatment-related serious adverse event. The other treatment-related serious adverse event reported was a peripheral thrombosis that was successfully treated, allowing study continuation. Deaths on-study included two patients in the placebo group and one in the Nplate group.
Significant bleeding events (rated as severe, life-threatening or fatal) were reported in 12 percent of patients in the placebo group compared to seven percent of Nplate patients. No patient tested positive for neutralizing antibodies against thrombopoietin.
The most common adverse events reported in patients treated with Nplate were headache, fatigue, epistaxis, arthralgia, and contusion.
Amgen filed for regulatory approval of Nplate for use in the treatment of thrombocytopenia in adults with chronic ITP in the United States (U.S.) and has received priority review from the U.S. Food and Drug Administration (FDA). Additionally, Amgen has submitted regulatory filings for the same indication in the European Union, Canada and Australia.
Romiplostim is an investigational thrombopoiesis-stimulating protein Fc-peptide fusion protein ("peptibody") that contains two component regions. Peptibodies are engineered therapeutic molecules that can bind to human drug targets and contain peptides linked to the constant domains of antibodies. Nplate works similarly to thrombopoietin (TPO), a natural protein in the body. Nplate binds to the TPO receptor, which activates the pathway necessary for growth and maturation of bone marrow megakaryocyte cells, resulting in increased platelet production. In 2004, the FDA granted fast track designation for Nplate. Orphan designation for ITP was granted in 2003 by the FDA and in 2005 by the European Agency for the Evaluation of Medicinal Products (EMEA). Nplate also has received orphan designation for this proposed indication in Switzerland (2005) and Japan (2006).
About Adult ITP
Adult Immune (Idiopathic) Thrombocytopenic Purpura (ITP) is a chronic and potentially serious autoimmune disorder characterized by low platelet counts in the blood, a condition known as thrombocytopenia. A normal platelet range for a person without ITP is 150,000 - 400,000 platelets per microliter of blood. The risk of a bleeding event increases when platelet counts drop to less than 30,000 platelets per microliter.
With ITP, platelets are destroyed by the patient's own immune system. ITP has historically been considered a disease of platelet destruction; however, recent data also suggest that the body's natural platelet production processes are unable to compensate for low levels of platelets in the blood. Increasing the rate of platelet production may address low platelet levels associated with ITP.
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