This is a randomized, double-blind, placebo-controlled, multicenter study. The key measure of effectiveness of the study drug will be determined using a 6-minute walk test. Eligible patients will be treated for 16 weeks and may be eligible to enter a 52-week extension phase study (PHIRST-2). Study procedures for both studies (PHIRST-1 and PHIRST-2) will include routine blood tests, medical history, physical exams, questionnaire responses, and exercise tests.