Which patients should be considered for gene therapy

Gene therapy for haemophilia, utilizing adeno‐associated viral vectors (AAVs) and coagulation factor genes, have demonstrated promising results, leading to recent approvals and introduction of the first gene therapy products into clinical practice. For successful and safe use, there are predefined inclusion and exclusion criteria, and the treatment process and associated risks should be thoroughly understood and long‐term safety and efficacy carefully evaluated during follow up. As gene therapy becomes more accessible outside of clinical study centers, continuous evaluation of patient eligibility for subsequent AAV‐based treatments becomes essential. Thorough evaluation of factors such as liver condition, anti‐AAV status, and medical history ensures that gene therapy maximizing benefits while minimizing risks. Apart from fulfilling the established inclusion and exclusion criteria, the success of gene therapy is greatly influenced by the motivation and willingness of patients to accept temporary constraints, such as regular laboratory monitoring, potential use of immunosuppressants, and thorough documentation. Furthermore, various patient‐related factors play a significant role in the management and outcomes of gene therapy, making a comprehensive evaluation essential. With the accumulation of more data, there is potential for the expansion of certain inclusion criteria, which may allow for a larger number of eligible patients to benefit from gene therapy. Empowering patients through shared decision‐making enables them to thoroughly consider the therapy's potential benefits and risks.

Gene therapy using has emerged as a promising approach to address these limitations offering the potential for sustained and adequate production of the missing clotting factor after one single intravenous administration of the gene therapy product, reducing or eliminating bleeding episodes and decreasing the reliance on external coagulation factor replacement. 3 This therapy involves delivering therapeutic genes into hepatocytes using recombinant adeno-associated viral vectors (AAVs) to induce the production of the deficient clotting factor.Recombinant AAVs have shown favourable safety profiles, strong liver tropism, and limited integration into the host cell genome.
While the efficacy of gene therapy can be assessed by coagulation factor measurements, there are treatment-related adverse events that can occur immediately or shortly after administration (infusion-related adverse events) and short-term elevations of liver transaminases (mainly alanine aminotransferase, ALT), with potential long-term effects on factor expression.Managing these adverse events can be achieved by early administration of immunosuppressive agents. 3Clinical trials necessitate assessing cellular and humoral immunity against the AAV capsid.Monitoring vector shedding is also required although germline and horizontal transmission has not been documented.
Several clinical studies have demonstrated the proof of concept for gene therapy in treating haemophilia using various AAVs and gene constructs.With recent approvals of valoctocogene roxaparvovec and etranocogene dezaparvovec in the EU and USA for gene therapy of haemophilia A and haemophilia B, gene therapies are now starting to be used in clinical practice.In light of these significant milestones, there is an urgent need for a practical approach and clearly defined guidelines to identify suitable patients.The well-established inclusion and exclusion criteria from gene therapy trials are crucial for ensuring the therapy's safety and effectiveness.Strict adherence to these criteria is of utmost importance to minimize the risk of gene therapy failure, as there seems to be only one chance for AAV-based gene therapy due to the regular development of anti-AAV antibodies after treatment.
Continuous evaluation of patient eligibility for subsequent AAVbased gene therapies is important as the treatment becomes accessible to a larger population and gene therapy would not only be carried out by well-educated and monitored clinical study centres.

| IN CLUS I ON AND E XCLUS I ON CRITERIA IN CLINI C AL TRIAL S FOR G ENE THER APY OF HAEMOPHILIA
Gene therapy eligibility criteria can vary depending on the specific type of gene therapy of haemophilia and the individual patient's characteristics.In both haemophilia A and haemophilia B, patient's eligibility can be categorized based on medical history and laboratory test results (Table 1).Clinical trials have predominantly focused on adult patients whereas the first trials for adolescents recently have started.As an example, in the HOPE-B trial, the age of participants varied from 19 to 75 years, encompassing all adult age groups. 4It's important to note that in patients younger than 18 years, the higher turnover of a growing liver might lead to a weaker response and potentially shorter duration of transgene expression.Safety considerations for children and adolescents undergoing gene therapy may differ from those for adults.Caution is warranted based on previous gene therapy attempts in children with other inherited disorders, which resulted in cases of severe liver toxicity and fatal outcomes. 5deed, the severity of haemophilia plays a crucial role in patient selection for gene therapy, as individuals with severe haemophilia may potentially benefit the most from the phenotypic transformation that gene therapy can offer.Most studies focus on patients with severe haemophilia, characterized by very low clotting factor activity levels, typically below 1% (compared to normal levels around 60%).Patients with severe haemophilia are more prone to spontaneous bleeding episodes, which can lead to severe joint damage, making them potential beneficiaries of gene therapy.In some cases, a few studies have also included patients with moderate haemophilia B and factor activity levels can be up to 2%. 4 Liver-related considerations play a significant role in various phases of gene therapy, including pretherapy assessment of liver health, patient selection and follow up, post-gene therapy liver health maintenance and management of potential short-and long-term adverse events.Recently, a collaboration between haemophilia treaters and hepatologists has been proposed to comprehensively examine liver-specific aspects throughout all phases of gene therapy in an interdisciplinary manner to assess safety concerns and ensure the success of long-term gene therapy. 6,7ecial attention should be given to patients with conditions that are not well-defined, such as liver fibrosis or fatty liver disease, history of hepatitis C and hepatitis B infection, HIV infection and use of medications that may impact liver function.A successfully treated hepatitis C infection or well-controlled HIV infection is not a contraindication for gene therapy in many cases.However, the risk of hepatitis B reactivation under possible immunosuppression should be considered if the patient has a history of hepatitis B infection.Adverse events such as increased ALT levels are common after gene therapy and require prompt evaluation and potentially immunosuppressive interventions.Proper assessment and monitoring of liver function can help mitigate potential risks and ensure optimal outcomes for patients undergoing gene therapy for haemophilia.
TA B L E 1 Gene therapy-specific eligibility criteria.

MIESBACH
In the majority of trials, HIV infection is not considered an exclusion criterion for gene therapy.However, caution should be taken in patients receiving retroviral HIV medication, as this treatment can lead to an increase in liver transaminases.As a result, there may be potential misinterpretations of elevated ALT levels, which could be attributed to the HIV medication rather than the gene therapy itself.
Patients with active or previous presence of inhibitors to coagulation factors were excluded so far from gene therapy and considered ineligible for treatment.This exclusion was due to the concern that their immune response to the clotting factor could interfere with the success of gene therapy.However, there is now a growing interest in addressing this patient population, and clinical studies have begun to explore the feasibility and safety of gene therapy in individuals with inhibitors. 8The results of these studies have not been published yet, leaving the potential benefits and risks for this specific patient group still to be fully understood.Testing for antibodies against AAV is also specific to the gene therapy product and is typically carried out by specialized laboratories, following the manufacturer's guidelines.Notably, certain trials, such as the HOPE-B trial for haemophilia B, may not consider anti-AAV titers as an exclusion criterion, possibly allowing individuals with pre-existing anti-AAV immunity to participate in these studies. 4The understanding of AAV immunity and its impact on gene therapy outcomes continues to be an important area of research to enhance the success and accessibility of gene therapy approaches for a broader population of patients.
One potential contraindication in the future, although not yet established, is a history of venous or arterial thrombosis or an elevated laboratory thrombotic risk.Thus far, two thrombotic events have been reported in haemophilia patients after gene therapy.In an asyet-unpublished Phase 3 study (SB-525, AFFINE Trial), elevated FVIII levels (>150 IU/dL) were measured in some treated patients.One patient experienced a deep venous thrombosis of the lower leg. 10other thromboembolic event was reported in a study on gene therapy for haemophilia B using the FIX Padua variant. 11In some cases of gene therapy for haemophilia A, transiently elevated FVIII levels have been observed.For example, in a total of 134 patients, 7 individuals had FVIII levels exceeding the upper limit of normal at 150 IU/dL. 12Therefore, caution should be taken and closely monitoring is required when considering gene therapy in individuals with a history of thrombosis or the presence of thrombotic risk markers, particularly when the gene therapy results in supraphysiological elevated factor levels.

| PATIENT REL ATED FAC TOR S A S ELI G IB ILIT Y CRITERIA FOR G ENE THER APY
Apart from the clinical study's inclusion and exclusion criteria, patient-related characteristics can significantly impact the results of gene therapy and, thus, require thorough evaluation before considering this treatment.Patients must be willing to accept temporary limitations, including committing to regular, initially weekly, monitoring of laboratory values, potential intake of immunosuppressants and extensive documentation.Safe contraception is mandatory during the first year after gene therapy due to the possibility of vector components being present in sperm.
Consequently, patients undergoing gene therapy may experience higher temporary burdens after the treatment compared to those who continue with the traditional prophylaxis using clotting factor concentrates or emicizumab.High levels of motivation and reliability are therefore essential, as gene therapy is irreversible once performed.
Additionally, various patient-related factors can significantly influence the management and results of gene therapy, necessitating thorough evaluation (

TA B L E 2
The impact of patient characteristics on the management of gene therapy.
drugs, particularly in the early stages after gene therapy to preserve liver health.
The assessment of liver health before gene therapy also plays a role in evaluating co-morbidities and determining the suitability for gene therapy in collaboration with hepatologists.It is essential to identify and treat all co-existing medical conditions adequately, with special attention given to those that could potentially worsen with the use of immunosuppressants, like prednisolone, which may be necessary due to ALT elevation, including conditions like diabetes mellitus or arterial hypertension.
For patients with a thrombotic risk, cautious control of coagulation factors is necessary, and preventive anticoagulation may be required if post-therapy coagulation levels become elevated beyond normal physiological levels.
Addressing the patients' psychological and mental state is also crucial.Gene therapy introduces uncertainties and unknowns for patients.Currently, predicting the exact therapy outcome and duration of response, as well as the likelihood of side effects like ALT elevation and the need for immunosuppressive treatment, remains challenging.
Despite existing data suggesting that recombinant AAVs integrate into the genome minimally, there remains a theoretical risk of cells undergoing consecutive malignant transformations.However, it is important to note that the four observed cancer cases showed no association with gene therapy based on histopathologic and genetic analyses. 13rthermore, it's important for patients to be aware that gene therapy for haemophilia may not be everywhere available at present.As a result, some patients may need to travel to different haemophilia centers where this treatment is offered.To facilitate this, a hub and spoke model has been proposed to efficiently organize various responsibilities related to gene therapy. 14This model aims to centralize expertise and resources at specific hub centers, while establishing a network of spoke centers to ensure broader access to the treatment for eligible patients.
Finally, gene therapy holds the potential to bring about substantial life improvements and enhance the quality of life for patients.
However, this transformation may also pose challenges as individuals adjust to their new circumstances.Therefore, it becomes crucial to offer psychological support to patients undergoing gene therapy.
In certain cases, assessing the psychological status of patients before the therapy begins could prove beneficial. 15A case study highlighted the difficulties patients faced in comprehending their altered life situation, particularly when they no longer required the lifelong regular therapy they were accustomed to. 16Additionally, they had to navigate the possibility of increased physical activity without experiencing bleeding episodes.
Managing expectations post gene therapy is crucial, as it is not a cure but can reduce bleeding frequency and the need for coagulation factor replacement.Comprehensive education about the treatment is essential to ensure patient compliance.Adopting a 'shared decision' approach empowers patients to actively participate in treatment choices. 17Thorough conversations with patients and their families should address assessments during the pre-treatment period, including frequent follow up visits that may become less frequent over time.

| CON CLUS ION
In conclusion, a collaborative and informed approach is crucial for achieving successful gene therapy outcomes in haemophilia patients, with strict adherence to initial inclusion and exclusion crite-

ACK N OWLED G EM ENT
Open Access funding enabled and organized by Projekt DEAL.

Finally, the presence
of pre-existing anti-AAV immunity poses a significant limitation to gene therapy.These neutralizing antibodies (nAbs) against various AAV serotypes can hinder the success of AAV-based gene therapy approaches and restrict the suitability of certain treatments for specific patients.The prevalence of wild-type AAV infections and their cross-reactivity vary among individuals in the human population, with exposure often occurring early in life and becoming more common with age.Due to the impact of preexisting nAbs on gene therapy outcomes, individuals with such antibodies are frequently excluded from participating in clinical trials for gene therapy.This exclusion affects a considerable portion of potential candidates, as up to 50% of individuals may have nAbs to wild-type AAV.9 ria.Patient selection for gene therapy necessitates a comprehensive evaluation of factors such as liver condition, anti-AAV status, and medical history.This approach ensures that gene therapy aligns with each patient's individual needs, maximizing potential benefits while minimizing risks.In addition, motivation and willingness of patients to accept temporary limitations, such as regular laboratory monitoring, potential use of immunosuppressants, and comprehensive documentation, also plays a crucial role in determining gene therapy success.As more data becomes available, some inclusion criteria may be expanded, potentially making a larger number of patients eligible for gene therapy.Shared decision-making between patients and physicians is essential to ensure patient compliance and a clear understanding of the potential benefits and risks associated with gene therapy.Hence, personalized care and consideration of individual characteristics are crucial for the successful implementation and outcomes of gene therapy.