The purpose of this study is to demonstrate the non-inferiority of pafuramidine maleate (DB289)versus trimethoprim-sulfamethoxazole (TMP-SMX)for the treatment of mild to moderately severe Pneumocystis pneumonia (PCP).  The gold standard treatment for PCP is trimethoprim-sulfamethoxazole (TMP-SMX). This drug is highly effective; however, a significant number of patients are unable to complete a course of therapy due to adverse events, some of which can be life-threatening. In addition, mutations that confer resistance to sulfa-based drugs in other microorganisms are increasingly being reported in P. jiroveci. A potential role of these mutations in conferring clinical resistance to PCP and in breakthroughs to prophylaxis regimens based on sulfa drugs is being actively studied. Second and third line agents or combinations for PCP treatment are also limited in efficacy and/or by adverse events.