SHENZHEN Hemogen Therapeutic Co., Ltd.

Gene therapy works by introducing exogenous normal genes into target cells to correct/compensate for therapeutic diseases caused by defective and abnormal genes. In broad terms, gene therapy requires learning how an inherited medical condition occurs at the DNA molecule level, and then going in and fixing it. Gene therapy holds promise for treating a wide range of life-threatening while hard-to-cure diseases, such as hemophilia, thalassemia, cancer, heart disease, and AIDS.

β-thalassemia, an autosomal recessive disease, is caused by mutations resulting in a single nucleotide substitution, small deletions or insertions within the β-globin gene or its immediate flanking sequence, or in rare cases, gross deletions. β-thalassemia is closely tied to the degree of imbalance between α-globin and β-globin chains. Deficient production of β-globin chains leads to the accumulation of excess, unstable α-globin tetramers in erythroid cells. Patients with transfusion-dependent β-thalassemia (β-thalassemia major or severe HbE–β-thalassemia) require lifelong, regular transfusions for survival. Without appropriate management, the life expectancy of patients with transfusion-dependent β-thalassemia (TDT) is reduced. Iron overload caused by repeated blood transfusions often leads to dysfunction in the heart, endocrine glands, and liver.

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) from an HLA-matched sibling donor (MSD) had been the only curative therapy for TDT for decades. Several aspects though limit allogeneic HSCT as a curative therapy, including the difficulties in finding fully matched donors, graft-versus-host disease, immunosuppressive therapy and consequently serious infectious complications and other side effects. Regardless of which of these treatments are used, the long-term quality of life is unsatisfactory, and there is also a heavy financial and social burden.

The pathogenesis of β-thalassemia is well understood. Fixing the defective β globin gene which impairs the expression of hemoglobin can cure this disease. Current gene therapy for β-thalassemia either reactivates γ-globin expression using CRISPR-Cas9 technology or transfers a functional β-globin gene via lentiviral vector. Unlike traditional allo-HSCT, gene therapy works with patient’s own hematopoietic stem cells (HSCs), making it advantageous because it does not require finding compatible donor cells, unlikely to lead to unwanted immune reactions (graft rejection and graft-versus-host disease), and cause milder side effects as myeloablation is conditioned with a single agent (busulfan). Thus, gene therapy is considered to be a very promising modality of TDT treatment.

HGI has successfully developed a gene therapy (HGI-001 Injection). HGI-001 Injection is based on the concept of correcting defective production of β-globin chains by isolating HSCs from a person with β-thalassemia and transducing them with lentiviruses to introduce exogenous β-like–globin transgenes that allows functional β-globin expression.

Gene therapy in β-thalassemia can be roughly divided in to four steps.

Step 1: After a combined mobilization by Granulocyte Colony-Stimulating Factor and plerixafor, peripheral blood mononuclear cells (PBMC) are collected by apheresis.

Step 2: CD34⁺ cells are isolated and cultured in a clean GMP complied facility. Once cells are tested and pre-treated, autologous CD34⁺ hematopoietic stem cells are isolated.

Step 3: After CD34⁺ isolation, patient-derived autologous CD34⁺ HSCs are transduced ex vivo with an optimally designed lentiviral vector which contains a functional human β-globin gene. This step needs to be performed in a clean GMP complied facility as well. The engineered cells are cooled in a controlled rate and then the cells are preserved in liquid nitrogen tank. Once HGI-001 Injection is released by Quality Control Department, it is ready for injection.

Step 4: Before reinfusion, patients need to go through myeloablative conditioning in order to remove endogenous HSCs from bone marrow. Then HGI-001 Injection is reinfused to the patient intravenously. Gene therapy is now complete. Follow up tests are performed to the patient to evaluate the efficacy of the treatment.

Several clinical trials for gene therapy used in the treatment of transfusion-dependent beta (β)-thalassemia (TDT) were conducted in EU and US. The results of the clinical trials show that gene therapy can be the potential curative option for TDT. HGB-204 and HGB-205 were Phase I/II studies conducted by Bluebird Bio. A total of 13 non-β0/β0 patients were enrolled in the HGB-204/205 studies, while 12 of them achieved transfusion-independence (TI).

During EHA 2021 Virtual, the 26th Annual Congress ofthe European Hematology Association (2021), Bluebird Bio presented the data of its Phase III study (Northstar-2 and Northstar-3). As of March 9, 2021, 41 patients were treated in the Phase III studies HBG-207 (Northstar-2; n=23; median follow-up 24.3 months [min-max: 13.0 - 27.5]); and HGB-212 (Northstar-3; n=18; median follow-up 23 months[min-max: 4.1 – 26.8]). Following treatment with beti-cel, 89% (32/36) of evaluable patients across ages and genotypes in both Phase 3 studies achieved transfusion independence. As of the data cut-off date, these patients continue to be free of transfusions for a median duration of 25 months (min-max: 12.5 – 38.5), with median weighted average total hemoglobin levels during TI of 116 g/L (min-max: 93 – 137).

After participating in and completing the two years of follow-up in any of the Phase I/II or Phase III studies (HGB-207,HGB-212), patients treated with beti-cel were invited to enroll in a 13-year long-term follow-up study, LTF-303. As of March 9, 2021, 51 of 63 beti-cel-treated patients across age groups and genotypes spanning a broad range of TDT severity have completed two years of follow-up in the parent study and were enrolled in LTF-303 (22 treated in Phase I/II studies, 29 treated in Phase III studies) with a median post-infusion follow-up of 44.2 months (min-max: 22.9 – 86.5)

Of the 51 patients enrolled in LTF-303, 40 patients achieved transfusion independence (TI): 15/22 (68%) patients treated in Phase I/II and 25/29 (86%) patients treated in Phase III. All patients achieved TI in the parent studies and maintained it through last follow-up in LTF-303. Weighted average hemoglobin (Hb) in patients who achieved TI reached normal or near-normal levels in the Phase I/II studies (10.3 g/dL; min-max: 9.1– 13.2) and in the Phase III studies (11.8 g/dL; min-max: 9.4 – 13.7).

No drug-related adverse events (AEs) were reported in these clinical trials. All patients achieved TI in the parent studies and maintained it through last follow-up in LTF-303.

HGI-001 Injection is a gene therapy indicated for the treatment of TDT developed by HGI. HGI-001 Injection is based on lentiviral vector, which is similar to that of Bluebird’s. As of 2022, a total of 4 patients were treated with HGI-001 Injection. All of them achieved transfusion-independence and maintained it through last follow-up. As of August 30 2022, 4 patients have achieved TI for 18 months, 14 months, 3 months and 2 months, with median weighted average total hemoglobin levels of 95g/L,104g/L, 136g/L and 115g/L, respectively. No drug-related serious adverse events (SAE) were reported in the study yet.

Gene therapy, in which a functional β-globin gene is inserted into the patient’s HSCs, can be used to treat TDT and prevent patients from receiving repeated blood transfusion. It also provides a possible cure for those who is not suitable to receive allo-HSCT and for those who cannot find suitable donors, with the added benefits of no risks of graft rejection and graft-versus-host disease. In fact, gene therapy for β-thalassemia treatment has long been listed as one of the most promising fields of gene therapy by the American Society of Gene and Cell Therapy. After years of research, a new era for thalassemia treatment has begun, and gene therapy will soon become an alternative for TDT patients. Long-term follow-up data is not available for treatment using gene editing technology yet, thus we cannot say that gene editing technology is safer compared to lentiviral vector technology. Therefore, the lentivirus vector technology is still the most promising strategy. Currently, Zynteglo from Bluebird bio is on US market, but with a price of $2.8 million. Once the cost is reduced and the long-term safety and efficacy is further confirmed, gene therapy may be adopted by a large number of TDT patients. Gene therapies not only brings new hope to thalassemia patients and their families, but also makes it possible to cure other rare genetic diseases using the same technology.

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