STI 6991 is a cellular decoy vaccine comprised of non-replicating human erythroleukemia K562 cells that are engineered to express membrane-bound S1 protein of the SARS-CoV-2 virus, being developed by Sorrento Therapeutics for the prevention of COVID-2019 infection. The vaccine candidate was developed by leveraging the I-Cell™ platform technology of Sorrento Therapeutics. The K562 cells are irradiated to prevent their replication and are designed to be administered by intramuscular injection as a vaccine. The K562 cell line are HLA negative and are as a result considered safe. The cells then express the S1 protein of the virus on a decoy cell surface and elicit both T cell and B cell immune response. This response will be triggered again upon exposure to SARS-CoV-2 coronavirus which will prevent attachment of the S1 spike protein with the ACE2 (angiotensin converting enzyme 2) on the normal human cell surface, thereby potentially preventing infection. Preclinical development is ongoing in the US.
In March 2020, Sorrento Therapeutics entered into a research testing agreement with the University of Texas Medical Branch at Galveston for the preclinical testing of Sorrento’s COVID-19 therapeutic product candidates. 
Key Development Milestones
Sorrento Therapeutics, as of March 2020, are in discussion with the Center for Biologics Evaluation and Research of the US FDA for required IND-enabling studies. The company intends to initiate phase I trial by mid year of 2020  .
In March 2020, Sorrento Therapeutics announced that they are investigating the ability of STI 6991 to elicit T cell an B cell response in animal models