On October 25, Christine Brown, PhD, of City of Hope’s T-Cell Therapeutics Research Laboratory, will present results on Mustang Bio’s Phase 1 clinical-stage product, MB-101 (IL13Rα2‐targeted CAR T cells) at a virtual conference. Mustang collaborates with City of Hope on several chimeric antigen receptor T cell clinical trials, including MB‐101 trials for brain cancer.

IL13Rα2 is an attractive CAR T target as its expression is elevated on surfaces of most malignant glioma cells. MB-101 CAR T cells express a membrane‐tethered IL‐13 receptor ligand (IL‐13) incorporating a single‐point mutation that provides high affinity for IL13Rα2 and reduces binding to IL13Rα1 to minimize off-target effects. City of Hope designed the therapy for improved antitumor potency and T cell persistence.

MB‐101 incorporates a second‐generation hinge-optimized CAR containing mutations in the IgG4 linker/hinge domain to reduce off‐target Fc interactions, the 4-1BB (CD137) co‐stimulatory signaling domain for improved persistence of CAR T cells, and the extracellular domain of CD19 as a selection/safety marker. To further improve persistence, central memory T cells are enriched and genetically engineered using a manufacturing process that limits ex vivo expansion to reduce T cell exhaustion and maintain a memory T cell phenotype.

“Second-generation” refers to a co-stimulatory domain to enhance signaling and CAR-T cell activation after the cell binds to its tumor target.

“Generally, the incorporation of a costimulatory domain results in greater efficacy over first-generation CAR molecule,” explains Mustang CTO Knut Niss, PhD. Both first- and second-generation CAR molecules carry the CD3-zeta signaling domain, although as Niss notes first-gen treatments are no longer in clinical development.

Establish optimal distance between CAR-T cell and tumor cell

The CAR hinge domain provides separation between the scFv binding moiety and the T cell membrane. “The hinge must be optimized for the target in question in order to establish an optimal distance between the CAR-T cell and the tumor cell so as to elicit optimal CAR-T cell activation,” Niss says.

There is a reason why every news item regarding cell and gene therapies seems to focus on manufacturing. Commercialization challenges, according to Niss, can be reduced to “a capacity problem,” which Mustang solved by optimizing facility utilization while minimizing operator time.

“We developed our process with these factors in mind. In addition, we designed a manufacturing facility that addresses unique aspects of autologous cell therapies,” he says. “For example, for safety reasons only one product will be manufactured in a cleanroom at any one time, thus tying up valuable cleanroom space while cells are expanding. Incubation can easily be over 90% of the manufacturing time. We developed a facility that allows for faster cleanroom turnover, which expands facility capacity without increasing its physical dimensions.”

Treating brain tumors involves delivering therapy through the blood-brain-barrier, so intravenous delivery is unlikely to succeed. City of Hope has developed a dual delivery method to reach the brain directly through both the cavity left by the tumor resection, as well as through the cerebrospinal fluid (CSF).

“Delivery into the CSF ensures optimal circulation of CAR-T cells throughout the brain and spinal canal, where metastatic spread often occurs,” Niss tells GEN.