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Five Prime Therapeutics Presents Bemarituzumab Trial-in-Progress Poster at the 2018 ASCO Annual Meeting

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Five
Prime Therapeutics, Inc.
(NASDAQ:FPRX), a clinical-stage
biotechnology company focused on discovering and developing innovative
immuno-oncology protein therapeutics, announced that a poster titled
"FIGHT: A Phase 3 Randomized, Double-Blind, Placebo Controlled Study
Evaluating Bemarituzumab (FPA144) and Modified FOLFOX6 (mFOLFOX6) in
Patients with Previously Untreated Advanced Gastric and Gastroesophageal
Cancer with a Dose Finding Phase 1 Lead-In" was presented today at the
2018 American Society of Clinical Oncology (ASCO) Annual Meeting in
Chicago. The poster (Abstract #TPS4135), is available at http://www.fiveprime.com/publications.

"Patients with advanced gastric or gastroesophageal junction cancer need
new treatment options, particularly those whose tumors overexpress
FGFR2b and whose prognosis is especially poor," said Helen Collins,
M.D., senior vice president and chief medical officer of Five Prime. "We
have seen encouraging monotherapy activity with FPA144 as a late-line
treatment for gastric cancer and we believe that combining with
chemotherapy in the front-line setting will provide the greatest patient
benefit. Our global Phase 1/3 FIGHT trial is studying bemarituzumab in
combination with mFOLFOX6 in patients with newly diagnosed gastric and
gastroesophageal junction cancer whose tumors overexpress FGFR2b with
the goal of providing a new treatment option for patients."

FIGHT Trial

In December 2017, Five Prime initiated the Phase 1 portion (NCT03343301)
of the Phase 1/3 FIGHT (FGFR2b Inhibition
in Gastric and Gastroesophageal
Junction Cancer Treatment) global
registrational trial. The Phase 1 safety lead-in portion of the trial is
designed to identify a recommended dose of bemarituzumab in combination
with the modified FOLFOX6 standard-of-care chemotherapy regimen
(mFOLFOX6) to support the initiation of the Phase 3 portion of the trial.

The Phase 3 portion of the FIGHT trial will evaluate bemarituzumab in
combination with mFOLFOX6 versus placebo plus mFOLFOX6 in approximately
550 patients with gastric cancer (GC) or gastroesophageal junction (GEJ)
cancer whose tumors overexpress FGFR2b. The Phase 3 portion of the trial
is expected to begin in the second half of 2018 and will include more
than 250 sites in the U.S., Europe and Asia, including China, South
Korea and Japan, where the incidence of gastric cancer is high.

The primary endpoint of the FIGHT trial is overall survival (OS) with
secondary endpoints of progression-free survival (PFS), objective
response rate (ORR), safety and pharmacokinetic (PK) parameters.

Previous Bemarituzumab Trial Results

Data from a Phase 1 clinical trial of single-agent bemarituzumab were
presented at the 2017 ASCO Annual Meeting. Bemarituzumab demonstrated
single-agent activity and an acceptable safety profile in heavily
pretreated patients with metastatic gastric cancer whose tumors
overexpress FGFR2b.

Efficacy:

  • In 21 treated patients with late-line GC/GEJ and high FGFR2b
    overexpression:
    • ORR was 19.0% with 4 confirmed PRs
    • Disease control rate was 54.9%
    • Median duration of response was 15.4 weeks

Overall safety:

  • Bemarituzumab was well tolerated
  • There were no dose-limiting toxicities
  • Maximum tolerated dose was not reached during dose escalation

Unmet Need in GC and GEJ

GC, including GEJ cancer, is the fifth most common cancer worldwide and
third leading cause of cancer death. More than 50 percent of GC cases
occur in eastern Asia.

Current first-line chemotherapy treatments prolong survival by
approximately 6 months compared to best supportive care but median OS
remains poor with literature-reported ranges of approximately 10 to 11
months and PFS from 5 to 5.6 months. An unmet medical need exists in the
treatment for GC/GEJ since few treatment options following progression
are available after first-line chemotherapy.

The presence of FGFR2 amplification or FGFR2b overexpression is
associated with a worse prognosis and is present in approximately 10% of
patients with GC/GEJ.

FGFs can stimulate transformation and proliferation of tumor cells
through signaling mediated by FGF receptors (FGFR 1-4). FGFR2 has 2
splice variants (b and c).

FGFR2b is expressed in tissues of epithelial origin and alterations in
FGF/FGFR2 pathway are associated with gastric, breast and other cancers.
As a result, targeting this pathway appears to be important in GC/GEJ
cancer treatment.

Rationale for Combination with Chemotherapy and Companion Diagnostics

Five Prime made a strategic decision to pursue a front-line
bemarituzumab plus chemotherapy combination trial based on preclinical
data that showed additive efficacy of bemarituzumab plus chemotherapy.
Mutational heterogeneity of GC/GEJ suggests that combining bemarituzumab
plus chemotherapy will result in improved activity by acting on
non-FGFR2b overexpressing tumor cells and by improving the activity of
bemarituzumab in FGFR2b overexpressing tumor cells.

Five Prime is developing companion diagnostics to identify FGFR2b
overexpression using an IHC test and FGFR2 gene amplification using
ctDNA analysis. Five Prime will use both assays to select patients for
the FIGHT trial to identify the estimated 10% of patients with gastric
and GEJ tumors that would qualify for the trial.

About Bemarituzumab

Bemarituzumab is a first-in-class, isoform-selective, humanized
monoclonal antibody in clinical development as a targeted immunotherapy
for tumors that overexpress FGFR2b, a splice variant of a receptor for
some members of the fibroblast growth factor (FGF) family. Bemarituzumab
has been engineered for enhanced antibody-dependent cell-mediated
cytotoxicity (ADCC) to increase direct tumor cell killing by recruiting
natural killer (NK) cells. Clinical results to date suggest that the
specificity of bemarituzumab avoids the dose-limiting toxicities that
have been seen with less selective pan-FGFR tyrosine kinase inhibitors
that act on multiple FGFRs, including FGFR2.

Bemarituzumab is being evaluated in the FGF2b
Inhibition in Gastric
and Gastroesophageal Junction Cancer Treatment
(FIGHT) Phase 1/3 clinical trial, a global registrational study in
patients with advanced gastric or gastroesophageal junction cancer whose
tumors overexpress FGFR2b or have FGFR2 gene amplification. The
Phase 3 portion of the trial is expected to begin in the second half of
2018. In December 2017, Five Prime and Zai Lab announced a collaboration
for the development and commercialization of bemarituzumab in Greater
China. Zai Lab will manage the Phase 3 portion of the FIGHT trial in
China.

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 timing of initiation,
progress and scope of clinical trials of bemarituzumab; (ii) the extent
of FGFR2b protein overexpression and FGFR2 gene amplification in
certain patient populations; and (iii) the prevalence and incidence of
GC and GEJ cancers. 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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