Company: PRME
Filing Date: 2025-02-28
Form Type: 10-K
Source: 0001628280-25-008884
Chunk: 31

Company: Prime Medicine, Inc.
Filing Date: 2025-02-28
Form: 10-K
Item: Item 1
Chunk 31
---
 and then evaluated for durability of effect.

When Prime Edited healthy donor or CGD patient CD34+ cells were administered to NBSGW mice, they were able to engraft, and edited cells proliferated to repopulate the hematopoietic system. The left and middle panels in the figure below demonstrate results from a representative experiment in which healthy donor CD34+ cells, either Prime Edited or mock treated, were administered to NBSGW mice. In the left panel of the figure below, human hematopoietic cells, distinguished by the human CD45+ surface marker, were able to successfully engraft at high efficiency, with no differences between mock treated and Prime Edited cells. In the middle panel, after 16 weeks in the mice, long-term engrafted human CD34+ HSCs retained a very high degree of editing: over 80% of cells exhibited correction of the ΔGT mutation at at least one locus, compared to 0% of mock edited cells. Based on natural history studies of carriers of CGD-causative mutations, restoration of p47phox function in a minimum of 20% of neutrophils is believed to be sufficient to confer protection from serious infection with CGD-associated pathogens. Together these data suggest that Prime Editing is able to correct the ΔGT mutation at an efficiency many-fold above the lower bound of the projected threshold for therapeutic benefit, and that Prime Edited cells may effectively home to and stably engraft in the marrow after infusion.

CD34+ HSCs from an individual with p47phox CGD were Prime Edited ex vivo, and these cells were differentiated ex vivo to allow assessment of NADPH oxidase activity in myeloid progeny. The panel on the right in the figure 

10

below demonstrates the results of one commonly used measure of NADPH oxidase activity, the dihydrorhodamine (DHR) assay. Mock edited CGD patient-derived and differentiated cells produced negligible amounts of NADPH oxidase, reflected in the absence of any DHR positive cells. In contrast, NADPH oxidase activity was restored in approximately 80 percent of Prime Edited CGD patient cells, very closely corresponding to the editing rate observed in healthy donor CD34+ cells in the middle panel. This level significantly exceeds the projected therapeutic minimum threshold of DHR activity in 20 percent of neutrophils thought to be sufficient to prevent serious infection based on natural history studies. Additionally, these results confirm that editing rates in CG