https://imcjms.com Ibrahim Medical College Journal of Medical Science https://imcjms.com/registration/journal_full_text/576 Sun, 07 Sep 2025 12:13:20 +0000 Background and objective: Lentivirus gene therapy (LGT) is an emerging therapy for sickle cell disease (SCD), although its efficacy and safety are under evaluation in clinical trials. This review assessed the efficacy and safety of LGT in relation to hydroxyurea (HU). Results: There was a significant increase (p-value<0.00001) in haemoglobin (Hb) level after LGT and production of HbAT87Q and foetal haemoglobin (HbF). Clinical outcome decreased significantly, and no hospitalization was required following LGT. A significant age-related difference in the LGT outcome was observed. Mode 1 treatment had significantly higher (p=0.004) outcome compared to mode 2 treatment. There was a significant increase (p<0.00001) in treatment outcome in SCD patients treated with LGT compared to those treated with HU. Gastroenteritis and leucopenia were the most reported adverse effects. July 2025; Vol. 19(2):007.  DOI: https://doi.org/10.55010/imcjms.19.018 *Correspondence: Chikadibia Fyneface Amadi, Department of Medical Laboratory Science, PAMO University of Medical Sciences, Rivers State, Nigeria. Email: worldwaiting@yahoo.com. © 2025 The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License(CC BY 4.0). Introduction The pathophysiology of SCD is intricate, involving several factors, such as hemolysis, vaso-occlusion, inflammation, oxidative stress, endothelial dysfunction, and hypercoagulability [6-8]. Treatment of SCD aims to prevent or mitigate the frequency and severity of complications, enhance quality of life, and extend lifespan of the patient. Existing therapeutic approaches encompass supportive care, pharmacological agents, and hematopoietic stem cell transplantation (HSCT) [9,10]. Various supportive care approaches include hydration, analgesics and antibiotics administration, blood transfusions, and immunization modalities [11,12]. Among pharmacological agents, hydroxyurea stands out—an agent boosting fetal hemoglobin production, thereby reducing the polymerization of hemoglobin S and the sickling of RBCs [12]. Demonstrating efficacy, hydroxyurea has been linked to a decrease in pain crises, incidents of acute chest syndrome, hospitalizations, and increased mortality rate in SCD patients [13]. Nevertheless, challenges such as variable response, adverse effects, and compliance issues temper its utility [14]. At present, a cutting-edge alternative in SCD intervention is gene therapy, aiming to rectify the underlying genetic anomaly at its source. This innovative approach involves the introduction of a functional gene into specific target cells, notably hematopoietic stem cells (HSCs), to bring about modifications in their gene expression and phenotype character [16]. Gene therapies broadly fall into two categories: gene addition and gene editing. Gene addition involves incorporating a therapeutic gene into the genome of the target cells without altering existing genes [16]. On the other hand, gene editing entails the precise modification or correction of the target gene, employing advanced tools such as zinc finger nucleases, transcription activator-like effector nucleases, or the CRISPR-Cas9 system [17,18]. One of the most exciting advances in sickle cell disease (SCD) treatment is the use of CRISPR/Cas9 gene editing, a technology that allows scientists to make precise changes to DNA. Generally, CRISPR is palindromic sequence in bacterial genome which can be excised by the cas9 enzyme, allowing scientists to modify, edit, insert or delete genes according to convenience. This breakthrough has led to CASGEVY™ (exagamglo gene autotemcel, or exa-cel), the first FDA-approved CRISPR-based therapy for SCD, developed by Vertex Pharmaceuticals and CRISPR Therapeutics. CASGEVY works by editing a patient’s own stem cells to boost the production of fetal hemoglobin (HbF), which helps counteract the harmful effects of sickle hemoglobin [19,20]. The FDA approval of CASGEVY in late 2023 was a landmark moment not just for SCD patients, but for the entire field of gene therapy. For decades, researchers have been working toward a true cure for SCD, and this therapy represents a major step forward. Clinical trials have shown that CASGEVY can dramatically reduce or even eliminate pain crises in many patients, offering hope for a life free from the most debilitating symptoms of SCD [21]. Beyond its clinical success, CASGEVY’s approval also sets a precedent for future gene-editing treatments, proving that CRISPR technology can be both safe and effective in treating genetic disorders. While challenges like cost and accessibility remain, this therapy opens a new era of personalized medicine for SCD patients [22,23].   (B) In Vivo Gene Therapy: Systemic delivery of a gene-modifying agent with affinity for HSCs directly targets cells within the patient's body, providing a streamlined and less invasive gene therapy approach. The efficacy of LGT hinges on the type of therapeutic gene delivered by the lentiviral vector [29]. In the context of sickle cell disease (SCD), there are two primary strategies for LGT: anti-sickling gene therapy and globin gene therapy [16,25-34]. Anti-sickling gene therapy entails the delivery of a gene encoding a modified hemoglobin variant capable of preventing or reducing the polymerization and sickling of hemoglobin S [32-34]. Examples of anti-sickling genes include hemoglobin F (HbF), the fetal form of hemoglobin typically silenced after birth, and hemoglobin A (HbA), the normal adult form of hemoglobin mutated in SCD [32-34]. Other examples involve hemoglobin A2 (HbA2), a minor adult hemoglobin form, and hemoglobin mutants like hemoglobin E (HbE) and hemoglobin G (HbG), both possessing reduced affinity for hemoglobin S [32-35].     To gauge the effectiveness of this therapeutic approach, a range of assessment methods is employed. In vitro analyses are conducted to scrutinize alterations in vector titers and transduction efficacy [42]. In vivo studies entail the transplantation of vector- or mock-transduced cells into animal models to evaluate therapeutic effectiveness [42]. Rigorous clinical trials are undertaken to assess the safety and efficacy of the lentiviral vector, the in vivo gene transfer clinical protocol, and the sustained correction of associated pathological symptoms [43]. The evaluation of vector integration sites is crucial to ensure the safety of the gene therapy [43]. Additionally, measuring degradative metabolite levels in patients during treatment aids in evaluating therapeutic efficacy [43]. The monitoring of clinical endpoints involves observing changes in disease symptoms, the frequency of disease-related complications, and the overall health and quality of life of patients [44]. These outcomes aim to improve oxygen-carrying capacity, minimize painful episodes, prevent life-threatening complications, and enhance the overall well-being of individuals with SCD. While LGT has shown promising outcomes, it is essential to acknowledge certain limitations that warrant attention. These limitations are limited patient numbers, short follow-up periods, and a deficiency in long-term data [47]. To strengthen the robustness of LGT's safety and efficacy profile, further studies are imperative. These studies should delve into critical parameters such as lentiviral vector design, conditioning regimens, transduction protocols, and comparative analyses with alternative gene therapy strategies [47]. Additionally, the optimization of clinical endpoints and the resolution of practical challenges, including cost, accessibility, ethics, and regulation, are pivotal for propelling LGT toward becoming a viable treatment for Sickle Cell Disease [47].   The Preferred Reporting Items for Systematic Review and Meta-analysis (PRIMSA) protocol of 2015 [49] was followed in the step-by-step development of the review to ensure reproducibility and transparency in the review process. The summary of the PRISMA protocol was reported using a PRISMA flowchart. Exclusion criteria: Studies not relevant to LGT in SCD such as other haemoglobinopathies like thalassemia were excluded. Animal studies, editorials and review articles, original studies using other forms of gene therapy were also excluded. Search strategy: A comprehensive search strategy was developed using a combination of Boolean function [50] and filters to narrow the study to original articles and clinical trials (randomized and non-randomized clinical trials) with advanced search including specific keywords like “lentivirus sickle cell disease” particularly for ScienceDirect. It is important to mention that the search strategy was adjusted to the specific provisions of each database.   Data management: All search results from the listed databases were first imported and managed by EndNote software, after which they were exported as XML files to Covidence for screening, selection, extraction and quality assessment of the included studies. Leveraging on the features of the software (EndNote), streamlining the process of reference formatting of included studies to the desired citation style was possible [51]. Covidence is a web-based tool for systematic review management [52,53] following PRISMA guideline, including title and abstract screening, full text screening, quality assessment. The Covidence tool was also used for data extraction and PRISMA flowchart generation [52,53]. Quality assessment: Virtually all the studies included were in the 1/2 phase of clinical trial. Since these phases of studies are typical of pilot studies, the checklist tool used was put into consideration to capture the peculiarity of such studies because studies in early phase clinical trials may require modifications in their quality assessment due to their uniqueness. In this case the studies were neither a full-scale clinical nor randomized clinical trial. To fulfill this purpose, the University of Chicago checklist for pilot studies was used to judge the quality of each study. It reflected at parameters such as the study’s goal, the reason for doing it, whether the way data was collected matched the goal, the number of participants (though not in a strict statistical way), if the data collection method would work in a larger study, and whether there was a good reason to move forward with a full-scale study.[54]. Ethical consideration: Following the fact that the review depended on already published data (secondary data) available in the public domain for public use, ethical clearance was not required for the commencement of the study. However, all used data from the secondary sources were duly cited. Results   Figure-3: PRISMA Flowchart Figure-3 above shows the PRISMA flowchart illustrating the review process. Out of449 studies, 10 were considered eligible for quality assessment and onward data extraction. Study Design Treatment Summary of the findings Studies on Hydroxyurea therapy Lad et al., 2022 [65] Clinical trial SCD patients Sample size: 138 Mean age: ≤14 yrs Hydoxyurea; Dose(CD34+) (cells/Kg):18.7 24 months The study showed that post-treatment hemoglobin levels averaged 9.2 g/dL with 25.6% HbF production. Clinical outcomes showed minimal vaso-occlusive pain (3.6%) and no chest pain, though non-cardiac pain remained prevalent at 54.3%. Hospitalization data was not reported Hoppe et al., 1999 [66] Clinical trial Severe SCD patients Sample size: 8 Mean age: 3.7 yrs Hydroxyurea; Dose(CD34+) (cells/Kg): 27 137 weeks The study demonstrated the highest hemoglobin improvement among hydroxyurea studies (10.7 g/dL) with 19% HbF. Notably eliminated all vaso-occlusive and non-cardiac pain, but reported a 20% hospitalization rate post-treatment Ofakunrin et al., 2018 [67] Quasi-experimental study SCA patients Sample size: 54 Mean age: 8.4 yrs Hydroxyurea; Dose(CD34+) (cells/Kg): n/m     12 months The study achieved hemoglobin levels of 9.3 g/dL, though HbF percentages were not documented. The study reported complete resolution of both vaso-occlusive and non-cardiac pain (0% for both), with no reported hospitalizations. PTA interval: Post-treatment assessment interval; SCD: Sickle cell disease; SCA: Sickle cell anaemia; n/m: Not mentioned   Meta-analysis of the efficacy of a treatment (Lentivirus gene therapy) in managing sickle cell disease based on Haemoglobin     Figure-4 shows that among the seven studies, there was statistical difference among the groups of the studies (Z=2.20, P<0.03). This implies significant reduction in Hemoglobin before gene therapy (MD= -3.50, 95% C.I [-6.63, -0.38]). Also, significant heterogeneity was seen across the studies (I2= 99%; P<0.00001). Table-3: Summary of the efficacy of a treatment (lentivirus gene therapy) in managing sickle cell disease     Table-4: Proportion of HBAT87Q and HbF among the studies       Age dependent variation in treatment outcome     Table-6 provides a summary of descriptive statistics related to age-dependent variation in treatment outcomes for sickle cell disease across different studies. The table includes data on the ages of individuals who participated in the studies. The average age of the participants across all studies is approximately 22 years, with an age interval spanning from 14 to 32 years. The table presents various treatment outcomes, including the percentage of HbAT87Q treatment, the percentage of HbF treatment, absolute reticulocyte count (ARC) after treatment, and lactate dehydrogenase (LD) levels after treatment. Table-6: Summary of descriptive statistics on age dependent variation in treatment outcome   Mode 2: represents treatment targeted at improving HbF level. Figure-6 below shows the meta-analysis of treatment outcome based on mode of action of lentiviral gene therapy across the studies. It was seen that there was a significant mean difference between the mode 1 and mode 2 groups and Lentiviral gene therapy favoured mode 1 action (IV=-14.69, 95% CI [-24.61, -4.77], Z=2.90, p=0.004). Significant heterogeneity existed among the groups (I2= 100%, P<0.00001). Treatment outcome based on disease severity Figure-7: Forest Plot showing treatment outcome based on disease severity (SSCD versus SCD)       Based on Figure-8 as illustrated, the meta-analysis of treatment outcome based on duration of treatment assessment revealed no effect for duration of treatment assessment at 1 year duration term and <1 year duration term (OR, 0.78, 95% [0.34, 1.79), Z=0.59, p=0.55). Meta-analysis of treatment outcome between lentivirus gene therapy and hydroxyurea for SCD     Table-7 presents a summary of descriptive statistics comparing treatment outcome (HbF) between two different approaches for managing sickle cell disease (SCD): lentivirus gene therapy and hydroxyurea treatment.     Comparison clinical outcome between lentivirus gene therapy and Hydroxyurea for SCD Figure-10: Forest plot showing the comparison of clinical outcomes between lentivirus gene therapy and Hydroxyurea for SCD The meta-analysis of the comparison of clinical outcomes between lentivirus gene therapy and hydroxyurea for SCD (Figure-10) showed that there was no significant difference between lentivirus gene therapy and hydroxyurea for SCD (IV=1.88, 95%, [-10.50, 14.26], Z=0.30, P=0.77). There wassignificant heterogeneity among the studies (I2=100%, P<0.00001). Discussion The substantial increase in Hb levels indicated a positive response to the therapy, as higher Hb levels are generally desirable in managing sickle cell disease. A reduction in absolute reticulocyte count (ARC) is typically seen as a positive response to treatment in sickle cell disease as well as a decrease in lactate dehydrogenase (LD) levels. All these changes in the parameters are often indicative of improved red blood cell health. These findings are in consonance with the study conducted by Abraham in 2021 who reported that increase in Hb level is an indication of improvement of red blood cell health and treatment success [25,68]. This is to say that the decrease in ARC and LD levels reported in this review were suggestive of therapeutic success of LGT in SCD patients whose red blood cells were often destroyed or lysed due to their sickle shape. In general, the increase in Hb, and decrease in ARC and LD levels suggested that the therapy was effective in improving the health of individuals with SCD [69]. Clinical outcomes such as vaso-occlusive pain, chest pain syndrome, hospitalization and non-cardiac pain were assessed among the studies. The findings revealed that there were no reported cases of hospitalization after treatment although there were few reported cases of vaso-occlusive pain [58,60,63], chest pain syndrome [60,64], and non-cardiac pain [58,60,63]. These findings support the fact that LGT improves the quality of life as reported by other studies [58,72-74]. It is noteworthy that LGT provides therapeutic intervention via any of the two mechanisms: gene addition [16] and promoting HbF production [25-33]. In comparing between modes of treatment, since the results showed that there was significant improvement in the treatment outcome in mode 1 compared to mode 1I, it implies that the LGT was more effective when the treatment was targeted towards correcting the mutant gene than when treatment was targeted towards improving HbF level. This means that whether the LGT was made to fix the faulty gene or to increase fetal hemoglobin levels, both approaches showed better treatment result, although correcting the mutant gene provided better therapeutic achievement than improving foetal haemoglobin level.Till now, no study has compared the treatment outcomes between these two modes. The study conducted by Demirci and Germino-Watnick who reported improvements in total Hb levels in lentiGlobin gene and BCL11A shmiR gene infusion [33,76] supports the fact that both modes of treatments achieved therapeutic success. The result presented in Figure-6 highlighted the diversity in the duration of treatment assessment across the studies, however, the duration of the treatment (whether long term or short term) did not make any difference in the success achieved. This may be due to the sustained presence of the corrected gene in the haematopoietic stem cell infused in the treatment process, resulting in continuous production of healthy red blood cells and improved treatment outcome. This is supported by the works conducted by Kanter and Drakopoulou in 2021 and 2022 respectively who reported long term effectiveness of LGT [16,78]. When it comes to the percentage of HbF, it appears that lentivirus gene therapy, as seen in Esrick et al. [59], and Malik et al. [62] resulted in slightly higher percentages compared to HU treatment. However, it is important to note that these changes may be due to chance, or on various factors, including individual patient characteristics, the specific protocol used in each study, mechanism of drug action and the duration of treatment. Some safety concerns were identified in course of this review. One study identified some safety concerns such as occurrence of Type 1 diabetes and respiratory infection, but he reported those adverse effects were not necessarily related to the effect of the administered treatment (LGT) [59]. Hydroxyurea treatment was reported to have a few safety concerns also such as leucopenia, myelosuppression, brain infarction. However, although there was no leading or most frequent safety issue identified, leucopenia was consistent in both HU treatment and LGT. This may be due to the impact the treatments have on haematopoietic system and bone marrow. Studies have established a dose-dependent relationship of leucopenia occurrence in HU [80]. Based on previous reports by Kanter and Ofakunrin, the leucopenia may be due to neutropenia which gave rise to the condition febrile neutropenia reported by them [16,61]. Contrarily, Lad and his colleagues did not identify any adverse effect after the administration of HU [67].   This study comprehensively examined the efficacy, clinical outcomes, and safety of LGT for sickle cell disease (SCD) in comparison with HU. This review has revealed that although both treatment interventions provided improvement in the laboratory data like haemoglobin level, LGT had better treatment achievement compared to HU. While both treatments had improvements in the clinical outcomes, there was no significant difference in the improvement levels between both treatments. This suggests that both treatment approaches have comparable outcomes in terms of managing these clinical manifestations of SCD.   To gain better comprehensive understanding of the comparative effectiveness of these treatments and their long-term impact on the quality of life for individuals with SCD, further research, including large-scale clinical trials and extended follow-up studies is imperative. Since LGT has better efficacy and comparable safety concerns with HU, LGT may be considered a better treatment option for SCD patients. Owing to the fact there were limited studies in LGT, most studies on LGT were currently non-randomized clinical trials, and therefore, it is recommended that future studies should be designed as randomized controlled clinical trials. Further research should build upon the lessons learned from early clinical trials and preclinical models to refine treatment protocols and enhance the safety profile of LGT. Limitation   We extend our acknowledgments to family members, friends and academic colleagues who provided various kinds of support on this research journey. Very importantly, we extend our appreciation to Department of Biomedical Science, College of Medicine, University of Chester for providing the platform and approval for this research. Author’s contributions Conflict of interest Funding   2026 Ibrahim Medical College. All rights reserved.