Cellectis Announces First Dosing of a Patient with its In-house Manufactured Product Candidate UCART22 for the treatment of r/r B-cell ALL

o The first patient completed the 28-day Dose Limiting Toxicity (DLT) observation period without complication

New York, NY – December 22, 2022 - Cellectis (the “Company”) (Euronext Growth: ALCLS - NASDAQ: CLLS), a clinical-stage biotechnology company using its pioneering gene-editing platform to develop life-saving cell and gene therapies, today announced that for the first time, a patient was dosed in the United States with its in-house manufactured product candidate UCART22, and completed the 28 day DLT period on December 14th, 2022, without complication.

“First dosing of a patient with a product candidate manufactured in-house is a major milestone for Cellectis. UCART22 has been developed to potentially offer a therapeutic alternative for patients with r/r B-ALL, including patients that have relapsed from or unable to receive CD19-directed therapy. The ability to have our manufacturing completely in-house maximizes the chances that eligible patients can be treated without delay.” said Mark Frattini, M.D., Ph.D., Chief Medical Officer at Cellectis.

“This is a transformational step forward for Cellectis: our in-house manufacturing capabilities would allow us to move product candidates like UCART22 from R&D to development to a finished UCART product on a timeline that would not have been possible working with a contract manufacturer,” said Steven Doares, Senior Vice President, US Manufacturing & Raleigh Site Head. “We believe that having this capability in-house is a great competitive advantage as it would give us the ability to swiftly version our product candidates as we monitor clinical responses, resulting in what we expect to be the best product possible.”

UCART22 is an allogeneic CAR T-cell product candidate that targets CD22 and is evaluated in the BALLI-01 clinical study, a Phase 1/2a open-label dose-escalation study designed to evaluate the safety and clinical activity of the product candidate in patients with relapsed or refractory B-cell acute lymphoblastic leukemia (r/r B-ALL).

Three years ago, Cellectis made the decision to build its proprietary GMP manufacturing facilities in both Raleigh (North Carolina) and Paris to take control of its production and manufacturing timelines. Cellectis’ facilities are fully operational, showcasing the Company’s transformation into an end-to-end cell and gene therapy company, from discovery & product development, transfer, and cGMP manufacturing to clinical development.

As of now, Cellectis is one of the few end-to-end gene editing, allogeneic CAR T-cell companies that control its gene and cell therapy process from start to finish.

BALLI-01 is actively enrolling patients with relapsed or refractory B-ALL.

For more information, eligibility criteria and trial locations, please visit www.clinicaltrials.gov (NCT04150497) or contact clinicaltrials@cellectis.com.

About Acute Lymphoblastic Leukemia

Acute lymphoblastic leukemia (ALL) is a cancer of the lymphoid line of blood cells characterized by the development of large numbers of immature lymphocytes. ALL accounts for 0.3% of all new cancer cases, and 0.3% of all cancer deaths. It is estimated that 6,660 new cases of ALL and 1,560 deaths related to the disease occurred in the US in 2022. ALL represents 12% of all leukemia cases, progresses rapidly, and is typically fatal within weeks or months if left untreated [1].

[1] SEER reference3