Signals of Toxicity Associated With Gene Therapy: The Case of Etranacogene Dezaparvovec in the Treatment of Hemophilia B

Abstract

Background: Etranacogene dezaparvovec is the first adeno-associated virus (AAV)–based gene therapy approved for the treatment of hemophilia B. Due to its recent market authorization, post-marketing surveillance is essential to characterize its safety profile in real-world clinical practice. Objective: To identify and characterize adverse drug reaction signals associated with etranacogene dezaparvovec using post-marketing pharmacovigilance data. Methods: A disproportionality analysis was performed using reports from the FDA Adverse Event Reporting System (FAERS) from Q1 2023 to Q3 2025, selecting cases in which etranacogene dezaparvovec was reported as the primary suspect drug. Signal detection was conducted with OpenVigil 2.1 using Reporting Odds Ratio (ROR), Proportional Reporting Ratio (PRR), Relative Reporting Ratio (RRR), and chi-square statistics. The Evans criteria were applied to identify probable associations. Results: A total of 26 reports including 35 suspected adverse drug reactions were identified. Strong and statistically significant disproportionality signals were observed for hepatic adverse events, particularly increased alanine aminotransferase, aspartate aminotransferase, and liver enzymes, all fulfilling the Evans criteria for probable adverse drug reactions. Moderate but significant signals were also detected for headache, fatigue, and influenza-like illness. Conclusion and Relevance: Post-marketing pharmacovigilance data confirm hepatic toxicity as the predominant safety signal associated with etranacogene dezaparvovec, consistent with clinical trial evidence and the known immunogenic profile of AAV-based gene therapies. These findings support the importance of careful hepatic monitoring in clinical practice and highlight the role of pharmacovigilance in evaluating the real-world safety of innovative gene therapies.

Background

Etranacogene dezaparvovec (Hemgenix, CSL Behring) represents a breakthrough in the treatment of hemophilia B, being the first gene therapy approved by the US Food and Drug Administration (FDA) for this inherited coagulation disorder. ¹ This innovative medicinal product is an adeno-associated virus serotype 5 (AAV5) vector containing a codon-optimized Padua variant of the human F9 gene under the control of a liver-specific promoter, enabling sustained endogenous expression of factor IX following a single intravenous infusion. ²

The approval of etranacogene dezaparvovec marks a major milestone in the field of gene therapy, as it was the first AAV-based gene therapy approved in Europe for the treatment of hemophilia, followed by US approval in November 2022. ³ The phase 3 HOPE-B clinical trial demonstrated that the treatment was superior to conventional factor IX prophylaxis in reducing the annualized bleeding rate and allowed 96% of participants to discontinue continuous prophylactic therapy. ⁴

The revolution represented by AAV-based gene therapies for hemophilia A and B highlights the need for an in-depth understanding of the immunogenic mechanisms and long-term safety associated with these viral vectors. ⁵ Although controlled clinical trials have provided valuable safety data, these studies are typically conducted in selected populations and over limited follow-up periods, which may not fully capture the entire spectrum of adverse events that emerge in real-world clinical practice or within specific subpopulations. ⁶

Post-marketing pharmacovigilance through the use of the FDA Adverse Event Reporting System (FAERS) database represents a crucial tool for the early detection of previously unrecognized safety signals. This approach allows the integration of data from a wide range of real-time spontaneous reports, providing an epidemiological overview of adverse drug reactions (ADRs) in the general population ⁷ and is therefore primarily used for post-marketing surveillance. Reports are submitted on a voluntary basis by health care professionals, consumers, and manufacturers, and each report contains information on patient characteristics, suspected drugs, reported adverse events, and reporter details.

The use of disproportionality analyses—employing measures such as the Reporting Odds Ratio (ROR), Proportional Reporting Ratio (PRR), and Relative Reporting Ratio (RRR)—has proven effective in identifying statistically significant associations between drugs and adverse events. ⁸

Given the novelty of this gene therapy and the clinical importance of continuous safety surveillance, the present study aims to conduct a systematic analysis of adverse reactions reported in the FAERS database for etranacogene dezaparvovec from the time of market launch to the present. Using standardized disproportionality analysis methodologies, including PRR, χ², RRR, and ROR, we intend to identify both known adverse reactions and potential unexpected safety signals. ⁹ This approach will provide a comprehensive overview of the drug’s safety profile in the context of real-world clinical practice, thereby deepening the assessment of the benefit-risk balance of etranacogene dezaparvovec.

Materials and Methods

For the present study, FAERS data covering the period from Q1 2023 to Q3 2025 were extracted. The search strategy focused on reports in which etranacogene dezaparvovec was identified as the primary suspect drug. A disproportionality analysis was performed using OpenVigil version 2.1.10 ¹⁰ , a pharmacovigilance platform designed for signal detection within the FAERS database. ¹¹ This tool supports the evaluation of drug-event associations and enables the calculation of several disproportionality metrics, including the ROR, PRR, and RRR. ¹¹

Using OpenVigil 2.1, the ROR was calculated as a measure of the association between a specific drug and an adverse event, defined as the odds of reporting the event with the drug of interest compared with the odds of reporting the same event with all other drugs in the database. An ROR value greater than 1 was considered suggestive of a potential safety signal.^12,13 The PRR was used to compare the proportion of reports for the drug of interest that included the event under investigation with the corresponding proportion observed for all other drugs. A PRR greater than 1 was interpreted as indicative of a potential signal.^{12 -14}

The RRR, which is conceptually similar to the ROR but based on a different computational approach, was calculated as the reporting rate of a given event for a specific drug relative to the reporting rate of the same event for all other drugs. For the ROR, 95% confidence intervals (CIs) were estimated.^12,13 A signal was considered statistically significant when the lower bound of the 95% CI exceeded 1. ¹³ Only adverse events were selected in the reports generated by OpenVigil 2.1.

In this analysis, the Evans criteria were applied to assess the likelihood of an association between adverse reactions and etranacogene dezaparvovec. The Evans criteria, first described in 2001, provide a structured approach to evaluating the probability that an adverse event is causally related to a drug. ¹⁵ These criteria take into account the strength and consistency of the association, the temporal relationship between drug exposure and event onset, and the exclusion of alternative explanations.

Specifically, the association was evaluated based on the presence of more than 2 reported cases, a chi-square value greater than 4 derived from the disproportionality analysis, and a PRR greater than 2. When all 3 conditions were met, the relationship between the adverse reaction and etranacogene dezaparvovec was classified as “probable.” ¹⁶

Results

Based on the analysis of the OpenVigil 2.1 database covering the period from Q1 2023 to Q3 2025, a total of 26 reports and 35 suspected ADRs associated with etranacogene dezaparvovec were identified.

Table 1 and Figure 1 illustrate the collected adverse reactions along with their corresponding disproportionality metrics. According to the Evans criteria (2001)—defined as more than 2 reported cases (n > 2), a chi-square value greater than 4 (χ² > 4), and a PRR greater than 2 (PRR > 2)—this combination of drug and adverse events was classified as a probable ADR.

Table 1.

Disproportionality Analysis of Adverse Events Using Multiple Statistical Measures.

Adverse event	PRR	RRR	ROR	ROR (95% CI)		Chi squared	Number of adverse drug reactions
Alanine aminotransferase increased	143.385	143.347	218.765	97.51	490.803	1136.57	9
Aspartate aminotransferase increased	75.077	75.067	88.546	30.511	256.969	223.404	4
Fatigue	7.136	7.136	8.977	3.605	22.355	26.682	6
Headache	11.68	11.679	16.426	7.142	37.777	69.64	8
Hepatic enzyme increased	69.745	69.736	86.113	32.469	228.384	274.259	5
Influenza-like illness	32.727	32.725	36.865	11.068	122.786	63.492	3

Abbreviations: CI, confidence interval; PRR, proportional reporting ratio; ROR, reporting odds ratio; RRR, relative reporting ratio.

Figure 1.

Forest plot of adverse event reporting odds ratios with 95% confidence intervals.

All events showed statistically significant associations with the drug (P < 0.05). The vertical red line represents ROR = 1. Hepatic enzyme elevations demonstrated the strongest safety signals. Data are displayed on a logarithmic scale.

An increase in alanine aminotransferase (ALT) showed the strongest association classified as a probable ADR, with a PRR of 143.385 and an ROR of 218.765, with a 95% CI well above unity (97.51-490.803). The chi-square value was extremely high (1136.57), supporting the statistical robustness of the signal, despite the limited absolute number of adverse events (n = 9).

Similarly, an increase in aspartate aminotransferase (AST) demonstrated a strong association with etranacogene dezaparvovec, with a PRR of 75.077 and an ROR of 88.546 (95% CI = 30.511-256.969), and a chi-square value of 223.404, based on 4 ADRs. A relevant signal was also observed for the broader category of increased liver enzymes, with a PRR of 69.745 and an ROR of 86.113 (95% CI = 32.469–228.384), supported by a chi-square value of 274.259 and 5 reported ADRs. Overall, these findings indicate a strong association between etranacogene dezaparvovec and hepatic toxicity. Among non-hepatic adverse reactions, headache showed a PRR of 11.68 and an ROR of 16.426 (95% CI = 7.142-37.777), with a chi-square value of 69.64 and 8 events, suggesting a potentially clinically relevant association.

Fatigue demonstrated a more moderate but statistically significant disproportionality signal, with a PRR of 7.136 and an ROR of 8.977 (95% CI = 3.605-22.355), and a chi-square value of 26.682, based on a total of 6 ADRs.

Finally, influenza-like syndrome exhibited a PRR of 32.727 and an ROR of 36.865 (95% CI = 11.068-122.786), with a chi-square value of 63.492, although this signal was based on a limited number of events (n = 3).

Discussion

The present pharmacovigilance analysis based on the FAERS database identified statistically significant disproportionality signals associated with etranacogene dezaparvovec, with a clear predominance of adverse reactions affecting hepatic function. In particular, increases in transaminases (ALT and AST) and the broader category of elevated liver enzymes showed very high PRR, RRR, and ROR values, with CIs well above unity and high chi-square values, fully meeting the Evans criteria for classification as a “probable” ADR.

These findings are consistent with the known mechanism of action of AAV-based gene therapies, which are associated with hepatic toxicity ¹⁷ and immune activation.

In the phase 3 HOPE-B study, ¹⁸ increases in transaminases (20 cases) were indeed reported as a common adverse event and were generally manageable with corticosteroid therapy. However, in the real-world context represented by FAERS, hepatic reactions emerge as the dominant signal compared with other events more frequently observed in clinical trials, such as headache, arthralgia, nasopharyngitis, and fatigue. Specifically, in the HOPE-B study of 54 treated patients, the most frequently reported adverse reactions were headache (n = 31), arthralgia (n =31), nasopharyngitis (n = 20), and fatigue (n = 16). ¹⁸

For the non-hepatic adverse reactions identified, it can therefore be stated that the disproportionality signals derived from FAERS are more moderate but still statistically significant.

This discrepancy can be explained by several factors, including the limitations of this study. First, clinical trials are conducted in highly selected populations, with strict exclusion criteria and intensive monitoring, whereas post-marketing pharmacovigilance reflects a more heterogeneous population and less controlled conditions of use. Second, spontaneous reporting systems tend to preferentially capture events perceived as clinically relevant or potentially serious, such as hepatic abnormalities, while more common and less clinically impactful reactions may be underreported. Finally, the limited absolute number of reports (n = 26) warrants caution in interpreting the results, even in the presence of statistically robust signals.

Other inherent limitations of this study stem from the voluntary nature of FAERS reporting, the potential for underreporting, the lack of a defined denominator for incidence calculations, and the impossibility of establishing a definitive causal relationship. Furthermore, the recent introduction of the therapy limits the number of available reports and long-term follow-up.

An additional limitation should be acknowledged. Because hemophilia B is a rare disease ¹⁹ and etranacogene dezaparvovec is administered as a single, ²⁰ 1-time infusion rather than as a repeated treatment, these 2 characteristics may have contributed to an underestimation of reported cases. The rarity of the condition limits overall exposure, while the single-dose administration may make it more difficult to attribute subsequent adverse events to the therapy and to consistently identify it as the primary suspect drug in spontaneous reports, potentially affecting signal detection.

The events analyzed in FAERS are consistent with those already observed in clinical trials and with the immunogenic profile of gene therapies, and they do not appear to introduce any new unexpected risks. Overall, the absence of emerging serious or unanticipated signals supports the conclusion that the safety profile observed in real-world clinical practice is largely consistent with that seen during clinical development.

Conclusion and Relevance

Post-marketing pharmacovigilance analysis using FAERS and disproportionality tools identified statistically significant safety signals for etranacogene dezaparvovec, primarily affecting hepatic function. These signals are consistent with clinical trial data and the mechanism of action of AAV-mediated gene therapies, without revealing any new unexpected adverse reactions.

These findings underscore the importance of careful hepatic monitoring in treated patients and highlight the crucial role of pharmacovigilance in complementing evidence from clinical trials, particularly for innovative therapies with high clinical impact, while still advising caution in the interpretation of results.

Future studies, based on longer follow-up and dedicated clinical registries, will be essential to more accurately define the long-term benefit-risk profile of etranacogene dezaparvovec in real-world clinical practice.