GlaxoSmithKline reported positive results from two phase II trials of Promacta -- a non-peptide thrombopoietin receptor agonist (platelet growth factor) which can stimulate the growth and differentiation of bone marrow cells that priduce blood platelets -- for the treatment of chronic hepatitis C-associated thrombocytopenia and chronic Idiopathic thrombocytopenic purpura (ITP). The first study was an international, multicenter, double-blind, randomized, placebo-controlled, dose-ranging study. A total of seventy-four HCV-infected subjects with platelet counts between 20,000 and 70,000/mL were enrolled. The subjects received Promacta (30mg, 50mg, or 75mg daily) or placebo for four weeks (pre-antiviral phase).

The primary endpoint was platelet count increase to greater than or equal to 100,000/mL at week four. The subjects could then initiate antiviral therapy and continue Promacta or placebo for 12 additional weeks (antiviral phase).

The primary endpoint was reached by 75%, 79% and 95% of subjects in the Promacta 30mg, 50mg and 75 mg groups respectively, compared to no platelet elevations in the placebo group (p<0.001). Treatment was generally well tolerated, with all adverse events mild to moderate in nature. The second trial was a multicenter, randomized, double-blind, placebo-controlled study.

A total of 118 subjects with chronic ITP and platelet counts <30,000/mL and who had relapsed or were refractory to at least one ITP treatment were enrolled. The subjects received once-daily oral Promacta (30mg, 50mg, or 75 mg) or placebo. The primary endpoint was the proportion of subjects with a platelet count greater than or equal to 50,000 per cubic millimeter after up to six weeks of therapy.

The primary endpoint was achieved in 28%, 70% and 81% of subjects who received Promacta (30mg, 50mg and 75mg, respectively) compared to 11% of the  placebo group <0.001). Platelet counts rose to greater than 200,000/µL in 4% of the placebo-treated subjects and in 14%, 37% and 50% of the Promacta 30mg, 50mg and 75mg-treated subjects, respectively. Treatment was generally well tolerated.

Phase III trials of Promacta are currently underway.