Study to Investigate Effect of a New Drug (PF-00446687) in Males Suffering From Erectile Dysfunction

Inclusion Criteria: * Males aged 18-65 years who has given written informed consent to participate in the study and who suffers moderate to severe erectile dysfunction who has a current or recent successful response to treatment with phosphodiesterase type 5 inhibitors Exclusion Criteria: * Males with a history of significant cardiac disease; unstable angina or recent myocardial infarction. * Males suffering from treated or untreated hypo- or hypertension * Males currently receiving vasoactive medication * Males taking medications contraindicated, or cautioned with sildenafil or MC4 agonists