Study to Test the Safety and How Well Patients With Severe Hemophilia A Respond to Treatment With BAY 2599023 (DTX 201), a Drug Therapy That Delivers a Healthy Version of the Defective Factor VIII Gene Into the Nucleus of Liver Cells Using an Altered, Non-infectious Virus (AAV) as a "Shuttle"

In this study researchers want to gather more information about safety and effectiveness of BAY 2599023 (DTX201), a drug therapy that delivers the human factor VIII gene into the human body by use of a viral vector to treat the disease. By replacing the defective gene with a healthy copy the human body may produce clotting factor on its own. Hemophilia A is a bleeding disorder in which the human body does not have enough clotting factor VIII, a protein that controls bleeding. Researcher want to find the optimal dose of BAY 2599023 (DTX201) so that the body may produce enough clotting factor on its own.

Inclusion Criteria: * Males age 18 years or older. * Confirmed diagnosis of hemophilia A as evidenced by their medical history with plasma FVIII activity levels \< 1% of normal or at screening. * Have \>150 exposure days (EDs) to FVIII concentrates (recombinant or plasma-derived). If on prophylaxis, are required to be willing to stop prophylactic treatment at specified time points throughout the study or If on-demand: should have had \> 4 bleeding events in the last 52 weeks \- Agree to use reliable barrier contraception. Exclusion Criteria: * History of allergic reaction to any FVIII product. * Clinically relevant findings in the physical examination considered critical by the treating physician, including obesity with BMI \> 35 kg/m\*2 * Current evidence of measurable inhibitor against factor VIII, prior history of inhibitors to FVIII protein or clinical history suggestive of inhibitor. * Evidence of active hepatitis B or C. * Currently on antiviral therapy for hepatitis B or C. * Significant underlying liver disease. * Serological evidence of HIV-1 or HIV-2 with CD4 counts ≤200/mm\*3; HIV+ and stable participants with CD4 count \>200/mm\*3 and undetectable viral load are eligible to enroll. * Detectable antibodies reactive with AAVhu37capsid. * Participant with another bleeding disorder that is different from hemophilia A (e.g., von Willebrand disease, hemophilia B). * Participated in a gene transfer trial within the last 52 weeks or in a clinical trial with an investigational product within the last 12 weeks. * Known or suspected hypersensitivity or allergic reaction to trial product(s) or related FVIII products or any component of BAY2599023 (DTX201), or a contraindication to prednisolone