| E.1 Medical condition or disease under investigation | 
| E.1.1 | Medical condition(s) being investigated  | 
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| E.1.1.1 | Medical condition in easily understood language  | 
| Hemophilia A is an inborn condition where the body does not create enough of a protein called clotting factor 8 (FVIII) in the blood. Without it, the blood cannot clot properly to control bleeding. | 
 
 
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| E.1.1.2 | Therapeutic area  | Diseases [C] - Blood and lymphatic diseases [C15] | 
| MedDRA Classification | 
| E.1.2 Medical condition or disease under investigation | 
| E.1.2 | Version  | 20.0 | 
 
| E.1.2 | Level  | LLT | 
 
| E.1.2 | Classification code  | 10053753 | 
 
| E.1.2 | Term  | Hemophilia A without inhibitors | 
 
| E.1.2 | System Organ Class  | 100000004850 | 
 
 
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| E.1.3 | Condition being studied is a rare disease  |  Yes  | 
| E.2 Objective of the trial | 
| E.2.1 | Main objective of the trial  | 
| Evaluate individual bleeding pattern and bleeding control in hemophilia A patients with clinically severe phenotype treated with FVIII products | 
 
 
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| E.2.2 | Secondary objectives of the trial  | 
| Collect information on patient-reported outcomes (PROs) | 
 
 
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| E.2.3 | Trial contains a sub-study  |  No  | 
| E.3 | Principal inclusion criteria  | 
- Participant must be ≥18 years of age at the time of signing the informed consent. - Patients with severe hemophilia A (FVIII:C ≤1% baseline FVIII activity [FVIII:C] as determined by measurement at the time of screening or from reliable prior documentation in clinical records of the patients). - Male. - Previously treated with FVIII concentrate(s) (plasma derived or recombinant) for a minimum of 150 exposure days (ED) as documented in reliable prior clinical records of the patients. - On regular prophylaxis with FVIII (defined as ≥ 45 weeks/year of treatment with an adequate dose as per label ) and on stable treatment for at least 6 months as documented in reliable prior clinical records of the patients - Well-managed patients with at least 1 documented visit at the hemophilia treatment center in the year prior to enrollment. - Willing to participate in the interventional Phase 3 gene therapy study with BAY2599023. - Capable of giving signed informed consent, which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol.   | 
 
 
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| E.4 | Principal exclusion criteria | 
Participants are excluded from the study if any of the following criteria apply. Exclusion criteria marked with an asterisk (*) will lead to exclusion of the participant from the subsequent Phase 3 study, if occurring at any time during the lead-in study. If the criteria marked with an asterisk are met during the study, the participant will withdraw from the study. 
 Medical Conditions: - *Current inhibitor to FVIII with a titer ≥ 0.6 BU, confirmed by more than 1 test. - History of inhibitor with a titer >1.0 BU (as documented in reliable prior clinical records of the patients) or with a repeated titer 0.6 to <1.0 BU in more than one subsequent occasion. - *Significant underlying liver disease, as evidenced by any of the following: portal hypertension, splenomegaly, ascites, esophageal varices, hepatic encephalopathy, reduction below normal limits of serum albumin or an advanced liver disease (Child-Pugh Grade B and C), suspicion of liver malignancy or fibrosis by ultrasound / Fibroscan. - Any of the following: -- Hemoglobin <11 g/dL -- Platelets <100,000 cells/µL -- Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) >1.5× ULN -- Alkaline phosphatase (AP) >2.5 × ULN -- Total bilirubin >1.5 × ULN -- Prothrombin time (PT) or international normalized ratio (INR) >1.0 × ULN -- Creatinine >1.5 mg/dL  - Another bleeding disorder that is different from hemophilia A (e.g., von Willebrand disease, hemophilia B) - *Active hepatitis B or C infection, as reflected by HBsAg or HCV-RNA viral load positivity - Serological evidence of active HIV-1 or HIV-2 as measured by CD4+ cell count <200 cells/mm3 and/or viral load >50 gc/mL. - *Pre-existing immunity against AAVhu37. - *Any current diagnosis of malignancy. - Known or suspected autoimmune diseases requiring immunosuppressive therapy. - Body mass index > 35 kg/m^2. - Contraindication for corticosteroid treatment. - *Any other significant medical condition that would be a risk to the patient or would affect patient’s ability to receive gene transfer following completion of his participation in this lead-in study. 
 Prior/Concomitant Therapy Listed therapies/medications are not allowed at enrollment, during the study and will also not allow transition to the Phase 3 study if introduced during the lead-in study: - Antiviral therapy for hepatitis B or C ,  - Pre-medication to tolerate FVIII treatment, - Immunomodulatory drugs (other than corticosteroids),  - Efavirenz,  - Emicizumab,  - Planned major surgery. 
 Prior/Concurrent Clinical Study Experience - Participation in any investigational hemophilia product study within 3 months before screening (participation in any other investigational product study will not be allowed throughout the study). - Has received the same or another gene therapy product. 
 
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| E.5 End points | 
| E.5.1 | Primary end point(s) | 
Number of bleeds (all bleeds) per observation time for each participant (annualized bleeding rate - ABR) 
 
 
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| E.5.1.1 | Timepoint(s) of evaluation of this end point | 
| Endpoint will be assessed over the course of the study, from enrollment into the study till the completion. The duration per single patient will be between a minimum of 6 months and a maximum of 12 months. | 
 
 
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| E.5.2 | Secondary end point(s) | 
1)Number of spontaneous bleeds per observation time for each participant (ABR) 2)Number of treated bleeds per observation time for each participant (ABR) 3)FVIII consumption 4)Haem-A-QoL (haemophilia quality of life questionnaire) total Score  5)Haem-A-QoL domains “Physical Health”, “Sports and Leisure”  6)EQ-5D-5L (The European quality of life-5 dimension-5-level questionnaire) Visual Analogue Scale  7)HJHS (haemophilia joint health score) score 
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| E.5.2.1 | Timepoint(s) of evaluation of this end point | 
1-3) Endpoints will be assessed over the course of the study, from enrollment into the study till completion. The duration per single patient will be between a minimum of 6 months to a maximum of 12 months. 4-6) On day 1. 7) At screening, week 26 and End of Study. | 
 
 
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| E.6 and E.7 Scope of the trial | 
| E.6 | Scope of the trial | 
| E.6.1 | Diagnosis |  No  | 
| E.6.2 | Prophylaxis |  No  | 
| E.6.3 | Therapy |  No  | 
| E.6.4 | Safety |  No  | 
| E.6.5 | Efficacy |  No  | 
| E.6.6 | Pharmacokinetic |  No  | 
| E.6.7 | Pharmacodynamic |  No  | 
| E.6.8 | Bioequivalence |  No  | 
| E.6.9 | Dose response |  No  | 
| E.6.10 | Pharmacogenetic |  No  | 
| E.6.11 | Pharmacogenomic |  No  | 
| E.6.12 | Pharmacoeconomic |  No  | 
| E.6.13 | Others |  Yes  | 
| E.6.13.1 | Other scope of the trial description | 
| This lead-in study will not have any investigational drug intervention, but will prospectively collect detailed information on bleeds, treatment with the prescribed medications, and quality-of-life data. This information will provide baseline data for the planned Phase 3 study and will be used for intra-individual comparison of the effectiveness of gene therapy product BAY 2599023 with current hemophilia treatment in Phase 3 study.  | 
 
 
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| E.7 | Trial type and phase | 
| E.7.1 | Human pharmacology (Phase I) |  No  | 
| E.7.1.1 | First administration to humans |  No  | 
| E.7.1.2 | Bioequivalence study |  No  | 
| E.7.1.3 | Other |  No  | 
| E.7.1.3.1 | Other trial type description |  | 
| E.7.2 | Therapeutic exploratory (Phase II) |  Yes  | 
| E.7.3 | Therapeutic confirmatory (Phase III) |  Yes  | 
| E.7.4 | Therapeutic use (Phase IV) |  No  | 
| E.8 Design of the trial | 
| E.8.1 | Controlled |  No  | 
| E.8.1.1 | Randomised |  No  | 
| E.8.1.2 | Open |  Yes  | 
| E.8.1.3 | Single blind |  No  | 
| E.8.1.4 | Double blind  |  No  | 
| E.8.1.5 | Parallel group |  No  | 
| E.8.1.6 | Cross over  |  No  | 
| E.8.1.7 | Other |  No  | 
| E.8.2 |  Comparator of controlled trial | 
| E.8.2.1 | Other medicinal product(s) |  No  | 
| E.8.2.2 | Placebo  |  No  | 
| E.8.2.3 | Other |  No  | 
| E.8.2.4 | Number of treatment arms in the trial | 1 | 
| E.8.3 | 
                                The trial involves single site in the Member State concerned
                             |  No  | 
| E.8.4 |  The trial involves multiple sites in the Member State concerned  |  Yes  | 
| E.8.4.1 | Number of sites anticipated in Member State concerned | 6 | 
| E.8.5 | The trial involves multiple Member States |  Yes  | 
| E.8.5.1 | Number of sites anticipated in the EEA | 58 | 
| E.8.6 Trial involving sites outside the EEA | 
| E.8.6.1 | Trial being conducted both within and outside the EEA |  Yes  | 
| E.8.6.2 | Trial being conducted completely outside of the EEA |  No  | 
| E.8.6.3 | If E.8.6.1 or E.8.6.2 are Yes, specify the regions in which trial sites are planned | 
| Canada | 
 
| Japan | 
 
| Taiwan | 
 
| Turkey | 
 
| United States | 
 
| Belgium | 
 
| Bulgaria | 
 
| Denmark | 
 
| France | 
 
| Germany | 
 
| Italy | 
 
| Netherlands | 
 
| Norway | 
 
| Spain | 
 
| Switzerland | 
 
| United Kingdom | 
 
 
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| E.8.7 | Trial has a data monitoring committee |  No  | 
| E.8.8 | 
                        Definition of the end of the trial and justification where it is not the last
                        visit of the last subject undergoing the trial
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| The end of the study is defined as the date of last visit of the last patient in this lead-in study.  | 
 
 
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| E.8.9 Initial estimate of the duration of the trial | 
| E.8.9.1 | In the Member State concerned years | 2 | 
| E.8.9.1 | In the Member State concerned months |  | 
| E.8.9.1 | In the Member State concerned days |  | 
| E.8.9.2 | In all countries concerned by the trial years | 2 |