Five Prime Therapeutics Initiates Patient Dosing in a Phase 1 Clinical Trial of FPT155, a First-in-Class CD80 Fusion Protein

SOUTH SAN FRANCISCO, Calif.

Five Prime Therapeutics, Inc. (Nasdaq: FPRX), a biotechnology company discovering and developing innovative immuno-oncology protein therapeutics, today announced it initiated patient dosing in a Phase 1 clinical trial of FPT155, a first-in-class CD80 fusion protein. The trial was initiated in Australia.

“We are pleased to initiate patient dosing for FPT155, our first-in-class CD80-Fc fusion protein, which is engineered to activate T cells through multiple pathways,” said Helen Collins, M.D., Senior Vice President and Chief Medical Officer of Five Prime. “In vivo, FPT155 demonstrates strong single-agent activity in multiple preclinical models, including some that are generally resistant to immune-modulating therapy. We look forward to testing FPT155 in the clinic to see if it has the potential to improve outcomes across multiple tumor indications where new treatment options are needed.”

“FPT155 is further evidence of Five Prime executing on the strategy of leveraging our IND engine as a source of competitive advantage in developing first-in-class agents that have the potential to demonstrate single-agent activity or activity in tumors that typically do not respond to checkpoint inhibitors,” said Aron Knickerbocker, Chief Executive Officer of Five Prime Therapeutics. “Beyond FPT155, this strategy can be seen across our robust pipeline, including with cabiralizumab, bemarituzumab and FPA150. These drug candidates target or engage multiple cells in the tumor microenvironment, including tumor cells, effector T cells, macrophages and NK cells.”

The Phase 1a/1b open-label, multicenter, dose escalation, dose exploration and dose expansion study will evaluate the safety and tolerability of FPT155 in patients with advanced solid tumors. The Phase 1a dose escalation portion of the trial will characterize the safety and pharmacokinetic (PK)/pharmacodynamic (PD) profile of FPT155 and will identify a recommended dose for the Phase 1b portion of the trial. During the dose escalation, Five Prime will also open an exploratory cohort to investigate FPT155 monotherapy in select tumor types with high unmet need. The Phase 1b portion of the trial is intended to further characterize the safety, PK/PD profile, and preliminary efficacy of FPT155.

About FPT155

FPT155 is a first-in-class CD80 fusion protein that (i) directly engages CD28 to enhance its co-stimulatory T-cell activation activity without inducing super agonism, and (ii) blocks CTLA-4 from competing for endogenous CD80, allowing CD28 signaling to prevail in T-cell activation in the tumor microenvironment. FPT155 has also demonstrated its ability to retain anti-tumor activity independent of its engagement with CTLA-4, suggesting a differentiated mechanism of action from CTLA-4-blocking antibodies. Studies in preclinical models suggest FPT155 has the potential to be a potent T-cell co-stimulator with strong monotherapy anti-tumor activity.

About Five Prime

Five Prime Therapeutics, Inc. discovers and develops innovative therapeutics to improve the lives of patients with serious diseases. Five Prime’s comprehensive discovery platform, which encompasses virtually every medically relevant extracellular protein, positions it to explore pathways in cancer, inflammation and their intersection in immuno-oncology, an area with significant therapeutic potential and the focus of the company’s R&D activities. Five Prime has entered into strategic collaborations with leading global pharmaceutical companies and has promising product candidates in clinical and late preclinical development. For more information, please visit www.fiveprime.com or follow us on LinkedIn, Twitter and Facebook.

Cautionary Note on Forward-looking Statements

This press release contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. Words such as “may,” “will,” “expect,” “plan,” “anticipate,” “estimate,” “intend” and similar expressions (as well as other words or expressions referencing future events, conditions or circumstances) are intended to identify forward-looking statements. These forward-looking statements are based on Five Prime’s expectations and assumptions as of the date of this press release. Each of these forward-looking statements involves risks and uncertainties. Actual results may differ materially from these forward-looking statements. Forward-looking statements contained in this press release include statements about (i) the progress and scope of the Phase 1 clinical trial of FPT155; and (ii) the potential use of FPT155 to treat patients with cancer. Many factors may cause differences between current expectations and actual results, including unexpected safety or efficacy data observed during non-clinical or clinical studies, clinical site activation rates or clinical trial enrollment rates that are lower than expected and changes in expected or existing competition. Other factors that may cause actual results to differ from those expressed or implied in the forward-looking statements in this press release are discussed in Five Prime’s filings with the U.S. Securities and Exchange Commission, including the “Risk Factors” contained therein. Except as required by law, Five Prime assumes no obligation to update any forward-looking statements contained herein to reflect any change in expectations, even as new information becomes available.

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