4. Discussion
Current haemophilia treatment consists of parenterally administered plasma-derived or recombinant clotting factor concentrates. Patients have significantly benefited from replacement therapy, as it greatly improves their quality of life and prolongs life-expectancy. It has been observed that the recently introduced into the market modified release formulations which can be applied sub-cutaneously have improved the dosing regimen for these patients.
Although current coagulation factor replacement prophylaxis regimens are effective, they are expensive, challenging to adhere to, result in regular sub-therapeutic trough factor levels, and are potentially immunogenic. As such, nearly all haemophilia patients would benefit from an intervention that induces stable factor expression. There is the need to develop such a therapy as the haemophilia population ages and develops medical co-morbidities that make effective prophylaxis administration more difficult.
In recent studies, gene therapies have induced sustained levels of FVIII and FIX, prevented bleeding, and reduced or eliminated the use of factor replacement products. This approach could also alleviate the heavy burden of the need for frequent factor infusions, concerns about inhibitor development, and limited global access to factor products. However, while short-term risks appear to be low, it has been noted that long-term safety and efficacy are not yet established.
Two separate AAV-based gene therapy phase 1/2 clinical trials for haemophilia A have now reported stable FVIII levels after a single vector dose, demonstrating that gene therapy is an effective investigational treatment of patients with adult haemophilia A. In haemophilia B, even though multiple phase 1/2 gene therapy clinical trials, have now reported stable therapeutic FIX expression after a single treatment using an AAV vector.
Some additional considerations are related to the type of vector used (namely AAV-based or lentiviral) and their applicability in different age groups of patients with haemophilia. Humoral immunity to wild-type AAV represents one of the most important limitations to successful systemic transduction with AAV vectors. Pre-existing and recall responses to the WT virus from which the vector is engineered, or to the transgene product, can interfere with therapeutic efficacy [30-31]. Humans are naturally infected with wild-type AAV during childhood, and thus may develop neutralising antibodies that prevent gene transfer with AVV vectors. Due to the high level of conservation in the amino acid sequence among AAV capsids, anti-AAV antibodies show cross- reactivity with multiple serotypes. This has resulted in exclusion of many patients from recent studies. The main toxicity seen in clinical trials has been dose-related elevation of liver transaminase following vector infusion and, in some studies, this coincides with the demonstration of cell-mediated AAV capsid immunity. Early intervention or prophylaxis with corticosteroids with the aim of protecting transduced hepatocytes have been used in most studies, but not all episodes have been managed effectively and partial/complete loss of transgene expression occurred. Pre-existing AAV humoral immunity and the potential loss of vector transduction with time represent important and limiting issues in the AAV-based gene therapy and can render the therapy ineffective. Thus, there is a requirement for other viral and non-viral-based vectors for transgene delivery. The predominant methods studied in Haemophilia have used lentiviral vectors, for which there is a lower incidence of pre-existing humoral immunity and which have a greater packaging capacity than AAV vectors.
A specific concern is the application of gene therapies in children. It is still unclear how effective AAV gene therapy will be in young children lacking data on long term exposure, particularly in terms of the risk for inhibitor and potential loss of factor expression over time as a result of dilution of AAV-transduced hepatocytes during liver growth [5]. Integrating Lentiviral Vector may provide an alternative vector platform for treating children with haemophilia with in vivo liver gene delivery of immune stealth and tolerogenic LV vectors to hepatocytes. In children and adults with either high-risk mutations for inhibitors, history of inhibitor, or presently with inhibitor, it is unclear whether the robust AAV mediated liver tolerance and ITI observed in haemophilia animal models will translate in humans [5].
Lentiviral vectors (LV) potentially offer certain advantages. They are able to efficiently integrate into the target genome, even when the cells are not actively dividing. If the liver is the target organ, this can lead to transduction of nondividing hepatocytes with the potential of long-term transgene expression. The liver also contains an abundance of antigen-presenting cells (APCs) that can also be transduced. Transduction of APCs might increase the risk of an immune response to FVIII or FIX and thereby inhibit long-term expression. The use of hepatocyte-specific promoters and incorporation of hematopoietic-specific microRNA target sequences into the vector may reduce this immune risk.
Another point of discussion is the external validity of the results obtained from the clinical trials with a selected patient population. Although the clinical data supporting gene therapy for haemophilia have been rather positive, the enrolled subjects bias are a selected group, and thus some caution is advised in extending the benefits of gene therapy to the general population of patients with haemophilia due to potential selection bias of the results from the clinical studies.
Finally, the many factors playing a role in the decision-making process for patients with haemophilia need to be considered carefully. Despite some very promising data provided on the novel treatments for haemophilia A and B, patients still have needs that require clarifications in consideration of the impact that these new therapies can have on their lives. The use of Emicizumab in Haemophilia A patients with inhibitors constitutes a remarkable achievement as this novel medicine, approved in EU on January 2018, allow the patients to have one subcutaneous administration per week, rather than one every 2/3 days. This is a great improvement in the treatment of Haemophilia A, especially in terms of QoL, but there’s still the need to know how and when it will be possible to switch the patients with inhibitors to this novel approach. Gene therapies using the AAVs look very promising in reducing the need for replacement therapy very close to 0 as the ABR as well. But in this context, patients still have some concerns that need to be elucidated, particularly many unanswered questions about the AAVs, the increase of the liver enzymes level and hepatotoxicity, the reduction in the FVIII expression during the time and the possibility to be re-treated with the vector if a drop in the FVIII expression occurs. Further, some ethical considerations have to be done concerning the use of gene therapies in children. The hot topic for the patients’ community is the current lack of long-term data on the treatment use of gene replacement and splicing therapies. The latter is an urgent challenge to be addressed in the upcoming months in order to allow haemophilia patients to be much more aware about the therapeutic options available for the whole community. Considering 1 million persons with haemophilia worldwide, of whom about 418 000 have severe and mostly undiagnosed disease, this constitutes a challenging task for researchers and health care systems, especially because only 210,000 patients have been identified and reported globally (WHF report 2018). More efficient diagnostic approaches are needed in less developed countries to take advantage of current and future treatment modalities, including gene therapy.