Phase 2 study of MCLA-128-based combinations in metastatic breast cancer (MBC): MCLA-128/trastuzumab/chemotherapy in HER2-positive MBC and MCLA-128/endocrine therapy in estrogen receptor positive and low HER2 expression MBC

Preclinical Studies for MCLA-128 + Trastuzumab in HER2-Positive MBC
There is a strong rationale to combine trastuzumab with MCLA-128 for the
treatment of MBC. Trastuzumab is effective in blocking constitutive
ligand-independent signaling through the HER2:HER3 heterodimer in the context
of HER2 overexpression. However, it has very poor growth inhibition activity in
tumors and cell lines where HER2:HER3 signaling is driven by HER3 ligand
dependency (Wehrman et al., 2006; Junttila et al., 2009). Preclinical and
clinical evidence points to increased HER3 expression and/or HER3 ligand
upregulation as the main drivers leading to trastuzumab resistance (Sergina et
al., 2007; Ocana et al., 2013). The bispecific antibody MCLA-128 docks HER2
present on the cancer cells to subsequently lock HER3 in an inactive
confirmation. This results in potent inhibition of the HRG ligand-induced
HER2:HER3 dimer formation and may overcome the resistance mechanism observed
with trastuzumab.
Preclinical studies showed that the combination of MCLA-128 and trastuzumab was
more potent than single-agent MCLA-128 to block HRG-induced tumor
proliferation, supporting the addition of MCLA-128 to a trastuzumab-containing
regimen in the clinic. In addition, the bispecific anti-HER2xHER3 antibody
MCLA-128 presents enhanced ADCC. The monovalent interaction of the antibody to
HER2 results in a greater density of antibody per tumor cell. In turn, this
will result in greater recruitment of immune effector cells and, combined with
ADCC, superior tumor cell killing activity.
In light of the distinct mechanisms of action for MCLA-128 and trastuzumab,
these preclinical data suggest that administration of MCLA-128 and trastuzumab
in HER2-positive patients has the potential for additive or synergistic
activity in this setting.
1.3.2 Preclinical Studies for MCLA-128 + Endocrine Therapy in Hormone
Receptor-Positive/HER2-Negative MBC
Bidirectional crosstalk between ER and HER2/3 has been hypothesized to underlie
ET resistance via their activation (Houston et al., 1999; Thrane et al., 2013).
The HER2:HER3 heterodimer activates PI3K/AKT and MAPK downstream signaling
cascades but can also induce ER phosphorylation independent of estrogen
(Pietras et al., 1995; Curley et al., 2015) which may reduce the effectiveness
of endocrine therapies (Arpino et al., 2008). On the other hand, HER2 may be
preferentially upregulated by estrogen deprivation, while tamoxifen induces
expression of EGFR and HER2 (García-Becerra et al., 2012). The nature of the
HER2/ERα cross-talk seems to be conditioned by the context of endocrine
resistance; in a series of paired biopsies from patients relapsing on tamoxifen
evaluated in an in vivo model, conversion from HER2-negative to HER2-positive
was rare, however <20% of tamoxifen-resistant patients relapsed with adaptive
upregulation in HER2 and either suppressed or enhanced ERα expression and
signaling (Gutierrez et al., 2005).
Taken together, these data support a combined ER/HER2-targeted approach in
patients with acquired ET resistance in ERα-positive/HER2-negative breast
cancer. The EGF30008 trial comparing letrozole alone versus in combination with
lapatinib in hormone receptor-positive MBC suggests this may be translated into
the clinic, as although the addition of lapatinib did not confer benefit in the
overall hormone receptor-positive/HER2-negative group, there was a trend toward
a benefit in the endocrine-resistant hormone receptor-positive/HER2-negative
subset (Johnston et al., 2009).
A recent preclinical study took this concept a step further, demonstrating
direct cross-talk between HER2/HER3 and ER with phosphorylation of ER only
observed in cells expressing both HER2 and ER* or HER3 and ER**(Collins et al.,
2017). The authors used an in vivo mouse model of ER+/HER2-low (HER2 IHC 1+ or
2+ without gene amplification) human breast cancer to evaluate a triple therapy
targeting HER2 (pertuzumab), HER3 (lumretuzumab) and the ER (fulvestrant),
resulting in long-lasting tumor regression.
Preclinical studies support that adding MCLA-128 to hormonal treatment could
delay or reverse resistance to therapy in ER+ HER2- (HER2 IHC 1+ or 2+ by
HercepTest*) breast cancer. MCLA-128 with anti-ER drugs (tamoxifen and
fulvestrant) combined with the AI letrozole was evaluated in two in vivo
xenograft models of ER+ HER2-low breast cancer. All hormone therapies reduced
tumor growth, while MCLA-128 single agent had no efficacy. However, when
MCLA-128 was added to the hormone therapy regimen, it led to a further
decreased in tumor growth. Mechanistically, HER2 expression was increased with
fulvestrant and tamoxifen, which is supported by published studies (Osipo et
al., 2007; García-Becerra et al., 2012). When MCLA-128 was added to anti-ER
drugs, this increase in HER2 could be reversed. Most importantly, a proximity
ligation assay showed that fulvestrant treatment upregulated the formation of
HER2:HER3 dimers and enhanced activation of the Akt signaling pathway, which
could be inhibited by adding MCLA-128. Together, these data show that the
efficacy of hormonal therapies may be limited due to activation of the
HER2/HER3 signaling axis, which could be reversed by MCLA-128.
While the in vivo tumor models tested responded to hormonal therapy, there is
also evidence that in the context of ET-resistant tumors, the inhibition of
HER3 by MCLA-128 could be therapeutically beneficial, as suggested by Collins
et al. (2017). The MCF-7 cell line engineered to respond to AIs was initially
sensitive to letrozole, but tumors subsequently became resistant (Curley et
al., 2015). The addition of an anti-HER3 antibody at the start of treatment was
more effective than hormone therapy alone, as observed for MCLA-128.
Importantly, the combination of the anti-HER3 with hormone therapy after
resistance restored antitumor activity. This was not the case when the
anti-HER3 was given as single agent, demonstrating that hormone receptor and
HER3 inhibition work in synergy before and after the occurrence of HT
resistance.
In patients with ER-positive/low HER2 expression MBC, based on this strong
preclinical evidence, a rescue approach will be used to evaluate the effect of
the addition of MCLA-128 on the natural history of the disease when treated
with ET.

Cohort 1 (HER2-positive/amplified MBC): MCLA-128 + trastuzumab ± vinorelbine
Primary objective:
• Evaluate efficacy of MCLA-128 combined with trastuzumab ± vinorelbine in
terms of clinical benefit rate (CBR) at 24 weeks based on RECIST 1.1 (per
investigator review) in HER2-positive/amplified MBC patients who have
progressed on prior HER2-directed therapy that included trastuzumab,
pertuzumab, and an HER2 antibody drug conjugate (ADC) in any sequence and in
any setting
Secondary objectives:
• Evaluate CBR at 24 weeks based on RECIST 1.1 per central review
• Evaluate progression-free survival (PFS; per investigator and central review)
• Evaluate overall response rate (ORR) based on RECIST 1.1 (per investigator
and central review)
• Evaluate duration of response (DoR) based on RECIST v1.1 (per investigator
and central review)
• Evaluate overall survival (OS)
• Evaluate safety and tolerability of MCLA-128 in combination with trastuzumab
± vinorelbine
• Characterize pharmacokinetics (PK) of MCLA-128 in combination with
trastuzumab ± vinorelbine
• Characterize immunogenicity of MCLA-128 in combination with trastuzumab
Exploratory objective:
• Evaluate potential correlations between biomarkers in tumor or blood samples
and antitumor activity (including HER2, HER3, HER2:HER3 dimers, heregulin and
other potential biomarkers)
Cohort 2 (estrogen receptor [ER]-positive/low HER2 expression MBC): MCLA-128 +
endocrine therapy
Primary objective:
• Evaluate efficacy of MCLA-128 combined with endocrine therapy in terms of CBR
at 24 weeks based on RECIST 1.1 (per investigator review) in ER-positive and
low HER2 expression MBC patients who have previously progressed on the same
endocrine therapy
Secondary objectives:
• Evaluate CBR at 24 weeks based on RECIST 1.1 per central review
• Evaluate PFS (per investigator and central review)
• Evaluate ORR based on RECIST 1.1 (per investigator and central review)
• Evaluate DoR based on RECIST 1.1 (per investigator and central review)
• Evaluate OS
• Evaluate safety and tolerability of MCLA-128 combined with endocrine therapy
• Characterize PK of MCLA-128 combined with endocrine therapy
• Characterize immunogenicity of MCLA-128 combined with endocrine therapy
Exploratory objective:
• Evaluate potential correlations between biomarkers in tumor or blood samples
and antitumor activity (including HER2, HER3, HER2:HER3 dimers, heregulin and
other potential biomarkers).

A phase 2, open-label, multicenter international study will be performed to
evaluate the efficacy of MCLA-128-based combinations in two metastatic breast
cancer (MBC) populations, HER2-positive/amplified (Cohort 1) and
ER-positive/low HER2 expression (Cohort 2). Three combination treatments will
be evaluated, two in Cohort 1 and one in Cohort 2, in 20-30 sites in 7
countries in Europe and the USA.
Cohort 1: Patients with HER2-positive/amplified MBC, having confirmed HER2
overexpression by immunohistochemistry (IHC) 3+ or IHC 2+ combined with
positive fluorescence in situ hybridization (FISH), who have been treated with
up to a maximum of 5 lines of HER2-directed therapy in the metastatic setting
and have progressed on the most recent line as per RECIST v1.1, are eligible.
Patients must have been treated previously with trastuzumab, pertuzumab and an
HER2 ADC in any sequence and in any setting. For enrollment, HER2 status will
be based on medical records, and eligibility will be confirmed subsequently as
soon as possible, by central lab review. Patients found to be ineligible
retrospectively will not be evaluable for the primary objective and may be
replaced. Documented imaging proof of disease progression on the last prior
line of therapy should be made available when possible.
Initially MCLA-128 will be administered with trastuzumab (doublet combination).
Safety will be reviewed by an Independent Data Monitoring Committee (IDMC).
After the safety of the doublet has been assessed, MCLA-128 + trastuzumab +
vinorelbine (triplet combination) will be evaluated in parallel with the
doublet combination (see figure below).
The doublet and triplet Cohort 1 combinations will both be evaluated in two
steps with an initial safety run-in in 4 to 6 patients who will be reviewed by
the IDMC, followed by a cohort efficacy expansion, as described below. The
triplet combination go/no-go decision will be made by the IDMC after evaluation
of the doublet safety run-in patients. If the triplet combination is considered
safe, the expansion of the doublet and triplet combinations will be performed
in parallel. Patients with be included with a 3:1 ratio for triplet or doublet
respectively, favoring the triplet combination and taking into account previous
exposure to vinorelbine.

Safety run-in: After 4-6 patients have received at least 2 complete cycles (6
weeks) of MCLA-128 + trastuzumab, a safety review will be performed by the
IDMC. If the doublet combination is considered safe, the safety run-in for the
triplet combination will be initiated in the next 4 to 6 successive eligible
patients. Safety of the triplet will be evaluated by the IDMC after these 4-6
patients have received at least 2 complete cycles (6 weeks) of MCLA-128 +
trastuzumab + vinorelbine.
Based on the observed safety in the first 4-6 patients (adverse events [AEs],
serious adverse events [SAEs], relationship to study drug, and other clinically
relevant parameters [e.g. laboratory parameters], available PK, immunogenicity,
and cytokine data) the IDMC, investigators and Sponsor will decide on a
potential additional run-in period for each combination (i.e. doublet and
triplet).
Expansion: After the safety run-in, each Cohort 1 combination therapy
considered tolerable by the IDMC will be expanded to a total of up to 40
patients evaluable for efficacy.
If the doublet combination regimen is not tolerated, Cohort 1 will be closed.
If the triplet combination is not well tolerated but the doublet is acceptable,
the doublet expansion will be continued.
Cohort 2: Patients with ER-positive and low HER2 expression MBC (IHC 1+, or IHC
2+ combined with negative FISH) who have radiologic or photographic evidence of
disease progression on the last line of prior endocrine therapy (administered
for at least 12 weeks) that included an aromatase inhibitor or fulvestrant.
Patients who have received up to 3 prior endocrine therapies in the metastatic
setting and have progressed on a cyclin-dependent kinase inhibitor (in any
line) are eligible. For enrollment, HER2 and HR status and
radiologic/photographic documentation of prior progression will be based on
medical records. Eligibility will be confirmed as soon as possible for HER2/HR
status by central lab review and for prior disease progression by central
imaging review. Patients found to be ineligible retrospectively will not be
evaluable for the primary objective and may be replaced.
MCLA-128 will be administered in combination with the same previous endocrine
therapy on which progressive disease is radiologically/photographically
documented. A total of up to 40 patients evaluable for efficacy will be
included.

An Independent Data Monitoring Committee (IDMC), composed of at least 2
physicians expert in the domain of early clinical development in MBC, will
review safety and efficacy throughout the study and decide on the addition of
extra patients in the expansion parts, opening of the triple combination
cohorts, and any ad hoc safety decisions. The principal investigators, the
Sponsor*s medical expert(s) and other representatives may be called upon to
assist as observers.

• MCLA-128 risks
MCLA-128 has been given to ~50 patients in an ongoing clinical study of
different tumor types. The preliminary data from this clinical study shows
that when MCLA-128 was administered alone (without other cancer therapies) it
was relatively well tolerated at the same dose you will receive in this
clinical study.
The most frequent side effects observed at this dose include infusion-related
reactions (allergic reaction, nausea, vomiting, and fever, which were reported
25% of patients), diarrhea (19% of patients) and fatigue (13% of patients). At
other doses (lower or higher), some patients also experienced skin disorders
such as rash (21% of patients), stomatitis (inflammation in the mouth mucosa,
14% of patients) and abdominal pain (7% of patients). When they occurred, these
toxicities were mild or moderate and patients usually recovered completely from
them before the next planned treatment. However, in one patient with associated
cardiac disease, allergic reaction resulted in death.
• Trastuzumab risks
As trastuzumab is an approved drug for the treatment of breast cancer and has
been administered to many patients worldwide, its tolerance is better known.
The most common side effects (reported in > 10% of patients) are: infections,
decreased number of blood cells (white blood cells, red blood cells,
platelets), weight decrease, anorexia, insomnia, tremor, dizziness, headache,
paresthesia (feeling of tingling or pricking), taste disturbance,
conjunctivitis, modification of blood pressure (increase or decrease),
palpitations, decrease of cardiac function, hot flush, wheezing, respiratory
difficulties, cough, nose hemorrhage, rhinorrhea, diarrhea, vomiting, nausea,
abdominal pain, digestion difficulties, constipation, stomatitis, skin
eruption, hair loss, nails disorders, joint pain, muscles pain, fatigue, chest
pain, chills, flu-like syndrome, infusion-related reactions, pain, fever
(associated or not with low white blood cells), mucosal inflammation and
peripheral edema.
• Vinorelbine risks
Vinorelbine is also an approved drug with a well-known safety profile which has
been administered to many patients worldwide. The most common side effects
(reported in > 10% of patients) are: decreased number of blood cells (white
blood cells, platelets), neurologic disorders including loss of deep tendon
reflexes, stomatitis, nausea, vomiting, constipation, transient elevation of
liver tests without clinical symptoms, hair loss, reactions at injection site
(erythema, burning pain).
• Hormone therapy risks
The side effects of the hormone therapy agent that you are receiving were
already explained to you by your investigating doctor when the treatment was
initiated.
However, any of these side effects for both drugs (MCLA-128 and hormone therapy
agent) may occur with greater intensity, or other effects, not yet known, could
occur given that this clinical study is the first time that these drugs have
been administered together.
Your investigating doctor will assess you for these side effects regularly and
inform you of their intensity or of any potential risk. You should be sure to
inform your doctor if any side effects occur. If necessary, he or she will give
you treatment to prevent or relieve symptoms.
Your study treatment (MCLA-128 and your hormone therapy) could be interrupted,
delayed or even stopped altogether, if any side effects occur and are severe.

However, any of these side effects for all three drugs may occur with greater
intensity, or other effects, not yet known, could occur given that this
clinical study is the first time that these drugs have been administered
together in the clinical setting.
Your investigating doctor will assess you for these side effects regularly and
inform you of their intensity or of any potential risk. You should be sure to
inform your doctor if any side effects occur. If necessary, he or she will give
you treatment to prevent or relieve symptoms.
Your study treatment (MCLA-128, trastuzumab and vinorelbine) could be
interrupted, delayed or even stopped altogether, if any side effects occur and
are severe. In addition, the dose of vinorelbine can be reduced in the event of
certain side effects.
• Pregnancy and reproductive risks
The potential risks of MCLA-128 on human reproductive function and on the human
fetus have not yet been studied. You must not therefore become pregnant or
breast-feed while you are taking part in the present clinical study. As a
result, if you (or your sexual partner) are of child-bearing potential, you
must use an effective method of contraception throughout the clinical study and
for 7 months after the last dose of MCLA-128. Effective methods of
contraception include: true abstinence, or a sole partner who is vasectomized,
or a combination of two of the following: intrauterine device/system, a condom
with spermicidal foam, gel, film, cream, or suppository, and an occlusive cap
(diaphragm or cervical/vault caps) with spermicidal foam, gel, film, cream or
suppository. The use of hormonal methods of contraception is not considered
effective enough for this clinical study. If you fall pregnant during the
clinical study, you must inform your investigating doctor and you will no
longer be able to take part in this clinical study.

As MCLA-128 is a new drug being developed, there may be other side effects that
are not yet known especially since MCLA-128 has not previously been
administered in combination with other anticancer drugs.
5. Other side effects associated with medical procedures used during the
clinical study
Blood draws are standard practice in your illness and are not specific to this
clinical study. However, there may be more blood draws than usual which may
make you more susceptible to side effects (pain, bruising, inflammation and
swelling of the vein, bleeding or even an infection at the puncture site).
Minimal risks and side effects are possible after tumor biopsies, such as pain,
bleeding, bruising or infection.
CT scans, magnetic resonance imaging (MRI) and bone scans normally carried out
to diagnose and assess your disease will not be different from routine practice
in this clinical study.
A CT scan is a series of x-rays put together by a computer. CT scans will
expose you to small amounts of radiation. Although repeated radiation may
damage your body tissues and slightly increase your chances of having cancer,
you should not expect an increased risk from the imaging being done for this
clinical study.
Magnetic resonance imaging (MRI) is a technique that uses a magnetic field and
radio waves to create detailed images of the organs and tissues within your
body.

CT scans and MRI scans involve dyes (called *contrast medium*) being injected
into one of your veins to help show up the organs in your body and your cancer.
There is a risk that you may have an allergic reaction to the dye. This
reaction may be mild (such as a skin rash or hives) or severe (such as
breathing difficulties and shock. In rare instances severe or fatal allergic
reactions have been reported.
A bone scan is a nuclear medicine test which helps find cancer that has started
in or has spread to the bones. This means that the procedure uses a very small
amount of a radioactive substance, called a tracer. The tracer is injected into
a vein. A bone scan carries no greater risk than conventional X-rays. The
tracers in the radioactive dye used in a bone scan produce very little
radiation exposure. Even the risk of having an allergic reaction to the tracers
is low.