STI 4398, a proprietary ACE2 (angiotensin-converting enzyme 2)-Fc fusion protein is being developed by Sorrento Therapeutics for the treatment of COVID-2019 infections. The ACE2-Fc protein binds to the S1 domain of spike protein of the SARS-CoV-2 virus thereby blocking its binding to the ACE2 receptors present on the target respiratory epithelial cells and inhibiting viral replication. STI-4398 preserves the ACE2 enzyme activity by converting angiotensin II to angiotensin 1-7, resulting in reduced vasoconstriction and increased blood flow to the infected lung tissue. The drug potentially also confers organ protection and reduces severe acute respiratory distress syndrome in COVID-19 patients. This indicates therapeutic potential for late-stage COVID-19. The recombinant fusion protein is expected to demonstrate better efficacy and longer half-life which may allow it to be used prophylactically to confer short-term and durable passive immunity (with repeated doses) to those that are regularly exposed to the virus (healthcare workers) or are at particular risk following an exposure event (elderly, patients with hypertension or immune-compromised patients). Early research is underway 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
Early research is underway in the US. Sorrento plans to commence preclinical studies for STI 4398 in the first half of 2020, to test its neutralization and blocking activity in preventing SARS-CoV-2 virus from infecting ACE2-expressing cells  .