Roflumilast Plus Antipsychotics Proof of Mechanism Study in Schizophrenia

The drug being tested in this study is called roflumilast. Roflumilast is being tested as an add-on treatment to second generation antipsychotics (SGA) to treat cognitive impairment in people with stable schizophrenia. This study will look at improvement in cognitive impairment associated with schizophrenia in people who take roflumilast as an add-on to SGA. The study will enroll approximately 22 participants. Participants will be randomly assigned (by chance, like flipping a coin) to one of three treatment groups-which will remain undisclosed to the patient and study doctor during the study (unless there is an urgent medical need) All participants will receive the following treatments at different periods throughout the study: * Roflumilast Dose A + SGA * Roflumilast Dose B +SGA * Placebo (dummy inactive pill) - this is a tablet that looks like the study drug but has no active ingredient + SGA. All participants will be asked to take one tablet at the same time each day throughout the study. This single-centre trial will be conducted in the United Kingdom. The overall time to participate in this study is up to 64 days. Participants will make 2 screening visits to the clinic and then must be brought to the clinic every day for dosing during each of 3 Treatment Periods. Each Treatment Period will be 8 days in duration. All participants will also make 1 final visit 14 days after last dose of study drug for a follow-up assessment.