4. CD137/PD-L1 mAb² immunopharmacology does not result in hepatotoxicity

ConclusionsA CD137/PD-L1 mAb² was developed, which binds mouse PD-L1 and which, uponbinding to mouse CD137, elicits potent T cell stimulation in vitro (EC50: 3 pM inprimary antigen-specific OT-1 assay). The mAb2 significantly reduced tumourgrowth in MC38 and CT26 syngeneic colon carcinoma models, as well as in the B16-F10 syngeneic melanoma model. In CT26 this activity was dose-dependentresulting in a significant survival benefit at concentrations of 0.3 mg/kg or abovewith minimal, non-adverse changes in liver histology following treatment.

We report potent in vitro CD137-mediated activation only upon engagement of PD-L1 using the CD137/PD-L1 bispecific mAb2 ie. conditional agonism. This activityoutperforms monospecific antibodies on their own and in combination in multiplesyngeneic mouse tumour models, in a dose-dependent manner. None of the liverpharmacology and resultant toxicity reported with other CD137 agonist mAbs areobserved with the CD137/PD-L1 mAb2. This data supports the rationale fordevelopment of FS222, a first-in-class human CD137/PD-L1 mAb² with a novelmode of action and improved therapeutic index for the treatment of humancancer.

Matthew A. Lakins, Alexander Koers, Raffaella Giambalvo, Jose Munoz-Olaya, Daniel Jones, Clinton Hall, Anne Knudsen, Neus Masque Soler, Sarka Pechouckova, Emma Goodman, Cristian Gradinaru, Sylwia Marshall, Mateusz Wydro, Francisca Wollerton, Sarah Batey, Dan Gliddon, Jacqueline Doody, Mike Davies, Michelle Morrow, Mihriban Tuna, Neil Brewis

F-star Biotechnology Ltd, Cambridge, UK

A Novel CD137/PD-L1 Bispecific Antibody Modulates the Tumour Microenvironment by Activating CD8+ T cells and Results in Tumour Growth Inhibition

5. Optimum agonist activity in the right tissue

Figure 5. F-star is developing a first-in-class human bispecific CD137/PD-L1 mAb² named FS222. The data presented here demonstrates thetherapeutic concept with a mouse surrogate molecule. The FS222 surrogate mAb² potently activates anti-tumour immunity and has a widetherapeutic window due to little off-tumour on-target activity and toxicity. Efficacy is achieved through its mechanisms of action: both PD-1/L1 blockade and potent CD137 agonism following PD-L1 binding and crosslinking (conditional agonism). Unlike other CD137 agonists indevelopment it does not require FcgR binding. Both these design features are hypothesised to enable a wide therapeutic window.* representative of CD137 mAbs in clinical development

Figure 3. Mice were inoculated with 0.1x10E6 CT26 tumour cells subcutaneously and treated with 2, 6, 20, or 200 µg/mouse(equivalent to approximately 0.1, 0.3, 1, or 10 mg/kg) CD137/PD-L1 mAb² or IgG control Q2D starting on day 7 for 3 doses.A - Kaplan-Meier B – Table of median survival (days) and p-values for Kaplan-Meier graph C - Mixed model statistical testing showsa significant difference in tumour growth rate between the different CD137/PD-L1 mAb² dose levels and control treated mice.

Statistical significance in group means are shown pairwise for growth rates over the full time of study using the mixed modelanalysis. Mean tumour volume shown as geometric. * p ≤ 0.05; ** p ≤ 0.01; *** p ≤ 0.001; **** p ≤ 0.0001

Fc

Phage library

Yeastdisplay library

mAb2

mAb2™

mAb2Fcab™CD137

PD-L1

Bivalent binding for both antigens

Limited number of amino acid substitutions

in CH3 domain

Fc region with antigen binding

Effector functions retained

Plug-and-play insertion into any mAb

Virtually limitless possibilities

SITC 2018 | 7-11 Nov | Washington, D.C. | Poster Number: P631 Strictly for personal use - DO NOT POST ONLINE

Figure 1. OT-1 mouse spleens were harvested and CD8+ T cells were activated with ovalbumin peptide in vitro, expanded and isolatedbefore being used in an in vitro T cell activation assay co-culturing them with ovalbumin-loaded B16-F10 cells expressing mouse PD-L1in the presence of CD137/PD-L1 mAb² or controls. Interferon-gamma production after 3 days was used as a readout of T cell activity.This assay shows the superior potency of the CD137/PD-L1 mAb², when compared to either of the monoclonal antibodies alone or incombination. The CD137 Fcab component by itself has no activity in this assay demonstrating the need for PD-L1 crosslinking forCD137 agonist activity.

0 .0 0 0 1 0 .0 0 1 0 .0 1 0 .1 1 1 0 1 0 0

0

1 0 0 0 0

2 0 0 0 0

3 0 0 0 0

A n t ib o d y c o n c (n M )

mIF

Ng

pg

/m

l

P D -L 1 m A b C D 1 3 7 m A b P D -L 1 m A b + C D 1 3 7 m A b C D 1 3 7 / P D -L 1 m A b2

C D 1 3 7 / m o c k m A b2

1. CD137/PD-L1 mAb² demonstrates superior activity to combinations and a requirement for PD-L1 crosslinking

2. CD137/PD-L1 mAb² anti-tumour activity is superior to combinations in a PD-L1 insensitive MC38 model. Potent activity is also seen in the CT26 and B16-F10 models.

Figure 2. A – Mice were inoculated with relevant tumour cell lines subcutaneously and treated with 1 or 10 mg/kg CD137/PD-L1mAb² as stated, or controls, Q2D starting on day 7 for 3 doses (MC38 n=12, CT26 n=12; B16-F10 n=15 mice/group). Mixedmodel statistical testing shows a significant difference in tumour growth between CD137/PD-L1 mAb² and control treated mice.B & C – Mice inoculated on day 0 with MC38 tumour cell line and subsequently treated on day 7, 9 and 11 with 1mg/kgCD137/PD-L1 mAb² resulted in 100% tumour-free animals by study end.

Statistical significance in group means are shown pairwise for growth rates over the full time of study using the mixed modelanalysis. Mean tumour volume shown as geometric mean. * p ≤ 0.05; ** p ≤ 0.01; *** p ≤ 0.001; **** p ≤ 0.0001

1 0 2 0 3 0 4 0 5 0

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D a y s p o s t in o c u la t io n

C D 1 3 7 / P D -L 1 m A b ²

A

3. CD137/PD-L1 mAb² is a highly potent molecule with activity detectable at low doses and anti-tumour activity related to dose

B

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-1

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H e p a to c e llu la r N e c ro s is P o rta l T ra c t M ix e d In fla m m a tio n H e p a t ic M ito se s

T cell

PD-L1+ cell

BackgroundBlockade of the PD-1/L1 axis has shown durable responses and extended overall survival inmultiple cancer types. However, there is still a significant unmet need for patients who relapse andactivation of the Tumour Necrosis Factor Receptor (TNFR) superfamily embodies the next stage ofcancer immunotherapy. CD137-mediated co-stimulation directs the fate of antigen-stimulated Tand NK cells. Upon interaction with its ligand, CD137 signalling supports cell activation, survivaland proliferation. Current agonist therapeutic interventions aimed at CD137 hold great promise forcancer immunotherapy with the caveat of being associated with safety concerns.

Here, we show how a novel bispecific antibody (mAb²™) bivalently binding to both CD137 and PD-L1 induces potent in vitro T cell activity in a PD-L1-dependent manner and results in significanttumour control across three syngeneic tumour models without toxicity.

CD137

PD-L1

MHC I+ OVA

TCR

CD137

PD-L1

T cell

Tumour cell

+ +

Active only in the presence of FcgR crosslinking

CD137

T cell

+-FcgRno FcgR

FcgR+ cell

Activity boosted only in presence of FcgR crosslinking

CD137

T cell

+FcgRno FcgR

FcgR+ cell

+ +

No T cell activation

-

PD-L1+ cell PD-L1+ cell

Checkpoint blockade with strong T cell activation

upon binding of both targets

T cell T cell

CD137

PD-L1 PD-L1

CD137

+ +Checkpoint blockade

T cell activation

T cell

CD137/PD-L1 mAb²

Figure 4. Mice from a CT26 syngeneic tumour study showed no signs of hepatotoxicity following repeated dosing of CD137/PD-L1mAb². To determine if hepatic toxicity as seen with other CD137 agonists was observed upon treatment with CD137/PD-L1 mAb², liversamples were taken at necropsy for histological assessment. Mice were necropsied 4, 7, and 14 days after the last administration andliver samples were formalin fixed and paraffin embedded. Sections were subjected to H&E and histopathological evaluation. A scoringsystem was used to assess liver pathology (1=minimal, 2=slight, 3=moderate, 4=marked) and the treated animals were observed tohave minimal liver changes. Unlike with other examples of CD137 agonist monoclonal antibodies, the findings with CD137/PD-L1 mAb²are not representative of hepatoxicity. These results indicate that a mAb² agonising CD137 via PD-L1-mediated crosslinking has a lowrisk of inducing hepatoxicity in human patients treated at therapeutic doses.

Tum

ou

rLi

ver

FcgR CD137 PD-L1

Activity boosted

through FcgRcrosslinking

Dose limitingliver toxicity

Moderate anti-tumour activity since

TME has variable levels of FcgR

Acceptabletoxicity

+

IgG4IgG2

+ ++

High activityonly upon

PD-L1 crosslinking

Acceptable toxicity

IgG1 LALA

+ + +

+++ + + ++ + +

Active only in the presence of

FcgR crosslinking*

Activity boosted only in presence of

FcgR crosslinking*

CD137 mAb in clinical development

CD137/PD-L1 mAb² highly differentiated MoA

CD137/PD-L1 mAb2

Median survival (days)

p-values Log-rank (groupwise

comparison with lower dosed group)

10mg/kg 39 0.2

1mg/kg 29 0.02

0.3mg/kg 24 0.007

0.1mg/kg 21 0.6

IgG ctrl 21

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1 0 m g / k g

1 m g / k g

0 .3 m g / k g

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ns

***

ns

p-valuesCD137/PD-L1 mAb2

10mg/kgCD137/PD-L1 mAb2

1mg/kgCD137/PD-L1 mAb2

0.3mg/kgCD137/PD-L1 mAb2

0.1mg/kg

CD137/PD-L1 mAb2 1mg/kg 0.0057

CD137/PD-L1 mAb2 0.3mg/kg <0.00001 <0.00001

CD137/PD-L1 mAb2 0.1mg/kg <0.00001 <0.00001 0.0001

IgG ctrl 10mg/kg <0.00001 <0.00001 <0.00001 0.4985

A

+

CCompound IgG ctrl PD-L1 mAb CD137 mAb

PD-L1 mAb + CD137 mAb

CD137/PD-L1 mAb2

Tumour-free animals

Number 0/12 2/12 2/12 4/12 12/12

Percentage 0 % 16 % 16 % 33 % 100 %

PD-1

B

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*

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****

****

****

M C 3 8 (1 m g / k g )

C D 1 3 7 / P D -L 1 m A b ²

P D -L 1 m A b

C D 1 3 7 m A b + P D -L 1 m A b

C D 1 3 7 m A b

Ig G c t r l

1 0 1 5 2 0 2 50

2 0 0

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8 0 0

1 0 0 0

D a y s p o s t in o c u la t io n

*******

C T 2 6 (1 0 m g / k g )

C D 1 3 7 / P D -L 1 m A b ²

Ig G c t r l

C D 1 3 7 m A b

P D -L 1 m A b

1 0 1 5 2 0 2 50

2 0 0

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* * * *

B 1 6 -F 1 0 (1 m g / k g )

C D 1 3 7 / P D -L 1 m A b ²

Ig G c t r l

+