Company: PRME
Filing Date: 2025-02-28
Form Type: 10-K
Source: 0001628280-25-008884
Chunk: 30

Company: Prime Medicine, Inc.
Filing Date: 2025-02-28
Form: 10-K
Item: Item 1
Chunk 30
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 that have been designed to correct a high percentage of cells containing the disease-causing mutation. PM359 is delivered as an autologous HSC transplant, and is designed to restore normal immunologic function in individuals with p47phox CGD, without the risks of graft-versus-host disease, or GVHD, graft rejection and post-transplant immunosuppression associated with allogeneic transplantation.

In order to develop PM359, a therapeutic product candidate to treat patients with p47phox CGD, we screened pegRNA and ngRNA to identify Prime Editing guides and guide pairs that have high activity and perform precise editing at the NCF1 locus. Because healthy donors have NCF1 pseudogenes bearing the same ΔGT mutation, we have been able to utilize healthy donor CD34+ HSCs to demonstrate precise editing of the ΔGT mutation. Using healthy donor CD34+ HSCs we also have been able to demonstrate a very low off-target potential, and robust engraftment and hematopoietic functionality of Prime Edited HSCs in mouse models. We have confirmed these findings using p47phox CGD patient-derived HSCs, and in addition have demonstrated that Prime Editing is able to restore NADPH oxidase activity in patient-derived cells.

The overall process for generating PM359 follows a similar paradigm to that employed by other investigational ex vivo HSC CRISPR-Cas9 therapeutics that have been validated in the clinic, with a few notable modifications. The Prime Editor components, comprised of the Prime Editor mRNA, pegRNA and ngRNA, are delivered to CD34+ HSCs using electroporation. 

We have developed Prime Editor components that are able to correct over 80% of healthy donor CD34+ cells or CGD patient CD34+ cells at at least one ΔGT locus (pseudogene in healthy donor; pseudogene or NCF1 in patient). In order to be able to understand the effects of Prime Editing on the functionality of modified CD34+ cells and to determine the ability of Prime Edited cells to persist in vivo, we administered Prime Edited CD34+ cells to a specialized immunocompromised mouse model, the NBSGW mouse, which is capable of supporting engraftment of human HSCs. As shown in the figure below, mice are typically followed for 16 weeks in these experiments to allow for engraftment of the long-term HSCs and repopulation of the immune system by long-term HSCs