• Top-line data from ACTIV-5/BET-B expected to be announced late in the first quarter or early in the second quarter of 2022
• ACTIV-5/BET-B results to be shared with regulatory authorities in the US, UK, and EU as part of the ongoing review process
• Lenzilumab is a variant-agnostic therapeutic that targets the dysregulated host immune response, an approach which may be of greater value than targeting the virus in hospitalized patients¹

Humanigen, Inc. (Nasdaq: HGEN) (“Humanigen”), a clinical-stage biopharmaceutical company focused on preventing and treating an immune hyper-response called ‘cytokine storm’, announced that target enrollment in the Phase 2/3 ACTIV-5/BET-B study has been achieved. The study, sponsored by the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health, enrolled over 400 patients in the primary analysis population at approximately 55 recruiting clinical sites including international sites.

The ACTIV-5/BET-B study was designed to align with the Company’s Phase 3 study of its lead investigational product candidate, lenzilumab, in hospitalized patients with COVID-19 (“LIVE-AIR”), results of which recently were published in Lancet Respiratory Medicine, https://www.thelancet.com/journals/lanres/article/PIIS2213-2600(21)00494-X/fulltext. The Lancet publication concluded that lenzilumab treatment of hospitalized patients with COVID-19 can improve the likelihood of survival without the need for mechanical ventilation, with a safety profile similar to that of placebo.

Hospitalized COVID-19 patients enrolled in ACTIV-5/BET-B were randomized to receive either lenzilumab and remdesivir or placebo and remdesivir. Patients in both arms also received current standard of care for hospitalized COVID-19 patients, including corticosteroids. The primary analysis population in ACTIV-5/BET-B comprises all randomized patients with a baseline C-reactive protein (“CRP”) less than 150 mg/L, age < 85, and do not require mechanical ventilation at the time of enrollment. The primary endpoint of ACTIV-5/BET-B is the incidence of invasive mechanical ventilation or death through day 29. Analysis of the LIVE-AIR results demonstrated that patients with baseline CRP levels below 150 mg/L had even greater response to lenzilumab therapy, with a 62% relative reduction in the incidence of invasive mechanical ventilation or death (OR:0.38, 95%CI, 0.19-0.75, p = 0.005).

“Completion of target enrollment in ACTIV-5/BET-B is a significant milestone in the development of lenzilumab and we thank our colleagues at the NIH for their on-going efforts conducting this trial,” stated Cameron Durrant, Chairman & CEO of Humanigen. “We have alignment with the FDA that, if the trial is successful, we can include the results from ACTIV-5/BET-B in an amended Emergency Use Authorization (“EUA”) submission for lenzilumab for hospitalized patients with COVID-19. We look forward to sharing the topline results from ACTIV-5, when available, and submitting an amended EUA.”

Additional information about the ACTIV-5/BET-B trial can be found at clinicaltrials.gov using Identifier NCT04583969.

Lenzilumab is an investigational product and is not approved or authorized in any country.

About Humanigen

Humanigen, Inc. (Nasdaq: HGEN) (“Humanigen”), is a clinical-stage biopharmaceutical company focused on preventing and treating an immune hyper-response called ‘cytokine storm’. Lenzilumab is a first-in class antibody that binds to and neutralizes granulocyte-macrophage colony-stimulating factor (GM-CSF). Results from preclinical models indicate GM-CSF is an upstream regulator of many inflammatory cytokines and chemokines involved in the cytokine storm. Early in the COVID-19 pandemic, investigation showed high levels of GM-CSF secreting T cells were associated with disease severity and intensive care unit admission. Humanigen’s Phase 3 LIVE-AIR study suggests early intervention with lenzilumab may prevent consequences of a full-blown cytokine storm in hospitalized patients with COVID-19. Humanigen is developing lenzilumab as a treatment for cytokine storm associated with COVID-19 and certain CD19-targeted CAR-T cell therapies and is also exploring the effectiveness of lenzilumab in other inflammatory conditions such as acute Graft versus Host Disease in patients undergoing allogeneic hematopoietic stem cell transplantation, eosinophilic asthma, and rheumatoid arthritis. For more information, visit www.humanigen.com and follow Humanigen on LinkedIn, Twitter, and Facebook.

Forward-Looking Statements

All statements other than statements of historical facts contained in this press release are forward-looking statements. Forward-looking statements reflect management's current knowledge, assumptions, judgment, and expectations regarding future performance or events. Although management believes that the expectations reflected in such statements are reasonable, they give no assurance that such expectations will prove to be correct, and you should be aware that actual events or results may differ materially from those contained in the forward- looking statements. Words such as "will," "expect," "intend," "plan," "potential," "possible," "goals," "accelerate," "continue," and similar expressions identify forward-looking statements, including, without limitation, statements regarding the anticipated time for release of topline data from the ACTIV-5/BET-B study and, if favorable, submitting an amended EUA request, and other statements regarding our plans relating to lenzilumab.

Forward-looking statements are subject to a number of risks and uncertainties including, but not limited to, the risks inherent in our lack of profitability and need for additional capital to grow our business; our dependence on partners to further the development of our product candidates; the uncertainties inherent in the development, attainment of the requisite regulatory authorizations and approvals and launch of any new pharmaceutical product; the outcome of pending or future litigation; and the various risks and uncertainties described in the "Risk Factors" sections of our latest annual and quarterly reports and other filings with the SEC.

All forward-looking statements are expressly qualified in their entirety by this cautionary notice. You should not rely upon any forward-looking statements as predictions of future events. We undertake no obligation to revise or update any forward-looking statements made in this press release to reflect events or circumstances after the date hereof, to reflect new information or the occurrence of unanticipated events, or to update the reasons why actual results could differ materially from those anticipated in the forward-looking statements, in each case, except as required by law.

1. Leavis, H. et al. (2021). Comment: Stimulating severe COVID-19: the potential role of GM-CSF antagonism. The Lancet Respiratory Medicine. https://doi.org/10.1016/S2213-2600(21)00539-7

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