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

Availability of this study's data will later be determined according to Bayer's commitment to the EFPIA/PhRMA "Principles for responsible clinical trial data sharing". This pertains to scope, timepoint and process of data access. As such, Bayer commits to sharing upon request from qualified researchers patient-level clinical trial data, study-level clinical trial data, and protocols from clinical trials in patients for medicines and indications approved in the US and EU as necessary for conducting legitimate research. This applies to data on new medicines and indications that have been approved by the EU and US regulatory agencies on or after January 01, 2014.

Interested researchers can use www.vivli.org to request access to anonymized patient-level data and supporting documents from clinical studies to conduct research. Information on the Bayer criteria for listing studies and other relevant information is provided in the member section of the portal.