Merrimack Pharmaceuticals (MACK) shares have edged higher in early trading after the company announced that the FDA has granted its MM-121 (seribantumab) Fast Track review for the treatment of patients with heregulin-positive, locally advanced or metastatic non-small cell lung cancer (NSCLC). Seribantumab is a fully human monoclonal antibody that binds to a receptor on tumor cells called ErbB3, which is activated by heregulin.