Provided are capsules comprising N-tert-butyl-3-[(5-methyl-2-{ [4-(2-pyrrolidin-1-ylethoxy)phenyl]amino} pyrimidin-4-yl)amino]benzenesulfonamide (TG101348). In a preferred embodiment the capsule comprises TG101348 admixed with microcrystalline cellulose and sodium stearyl fumarate. The capsules are useful in the treatment of myelofibrosis.