FS222, a CD137/PD-L1 tetravalent bispecific antibody exhibits low … · 1 FS222, a CD137/PD-L1 tetravalent bispecific antibody exhibits low toxicity and anti-tumor activity in colorectal

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FS222, a CD137/PD-L1 tetravalent bispecific antibody exhibits low toxicity

and anti-tumor activity in colorectal cancer models

Authors

Matthew. A. Lakins*, Alexander Koers, Raffaella Giambalvo, Jose Munoz-Olaya, Robert

Hughes, Emma Goodman, Sylwia Marshall, Francisca Wollerton, Sarah Batey, Daniel

Gliddon, Mihriban Tuna, Neil Brewis

Affiliations

F-star Therapeutics Ltd, Cambridge, CB22 3AT, United Kingdom

Running Title

Characterisation of a CD137/PD-L1 bispecific antibody

*To whom correspondence should be addressed:

Dr Matthew A. Lakins

F-star Therapeutics Ltd, Eddeva B920, Babraham Research Campus, Cambridge, CB22 3AT,

UK

+44 (0) 1223 948159

[email protected]

Conflicts of Interest

All authors are current or former employees of F-star Therapeutics Ltd.

Research. on July 11, 2020. © 2020 American Association for Cancerclincancerres.aacrjournals.org Downloaded from

Author manuscripts have been peer reviewed and accepted for publication but have not yet been edited. Author Manuscript Published OnlineFirst on April 28, 2020; DOI: 10.1158/1078-0432.CCR-19-2958

http://clincancerres.aacrjournals.org/

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Translational Relevance

We developed a mAb2 bispecific antibody for targeting CD137 and PD-L1 in solid tumours

which potently activated CD8+ T cells in vitro only in the presence of PD-L1 expressing

cells. Our surrogate molecule activated intra-tumoural CD8+ T cells and effectively

controlled tumour growth in syngeneic mouse tumour models without toxicity. We found that

FS222 activates CD4+ and CD8

+ T cells in vitro with activity superior to the combination of

monospecific, monoclonal antibodies representative of those used in the clinic currently,

providing evidence that our tetravalent bispecific clinical candidate will provide greater

benefit to patients than a combination approach against both targets in solid tumours.

Considering the broad expression of PD-L1 in many solid tumours, FS222 may provide

therapeutic opportunities for cancer patients who remain challenging to treat.




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Abstract

Purpose: With the increased prevalence in checkpoint therapy resistance, there remains a

significant unmet need for additional therapies for relapsing or refractory cancer patients. We

have developed FS222, a bispecific tetravalent antibody targeting CD137 and PD-L1 to

induce T cell activation to eradicate tumours without the current toxicity and efficacy

limitations seen in the clinic.

Experimental Design: A bispecific antibody (FS222) was developed by engineering CD137

antigen binding sites into the Fc region of a PD-L1 IgG1 mAb. T cell activation by FS222

was investigated using multiple in vitro assays. The anti-tumour efficacy, survival benefit,

pharmacodynamics and liver pharmacology of a murine surrogate molecule were assessed in

syngeneic mouse tumour models. Toxicology and the pharmacokinetic/pharmacodynamic

profile of FS222 was investigated in a non-human primate dose-range finding study.

Results: We demonstrated simultaneous binding of CD137 and PD-L1 and showed potent T

cell activation across CD8+ T cell activation assays in a PD-L1-dependent manner with a

CD137/PD-L1 bispecific antibody, FS222. FS222 also activated T cells in a human primary

mixed lymphocyte reaction assay, with greater potency than the monospecific mAb

combination. FS222 showed no signs of liver toxicity up to 30 mg/kg in a non-human primate

dose-range finding study. A surrogate molecule caused significant tumour growth inhibition

and survival benefit, concomitant with CD8+ T cell activation, in CT26 and MC38 syngeneic

mouse tumour models.

Conclusions: By targeting CD137 agonism to areas of PD-L1 expression, predominantly

found in the tumour microenvironment, FS222 has the potential to leverage a focused, potent

and safe immune response augmenting the PD-(L)1 axis blockade.




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Introduction

Immunomodulatory monoclonal antibodies are a promising approach for cancer patients.

Immune checkpoint inhibitors targeting programmed cell death (PD)-1, PD-L1, and cytotoxic

T-lymphocyte-associated protein 4 (CTLA-4) are the most advanced immunotherapy agents

for oncology. However, only a subset of patients benefit from long term survival and there

remains an unmet clinical need (1). Whilst bispecific T cell engagers (BiTEs), such as

blinatumomab (BLINCYTO®) targeting CD3 and CD19, are the most advanced next

generation immuno-oncology modalities, their use is limited to haematological malignancies

and further limited by acute safety concerns (2). We believe agonist antibodies against

specific co-stimulatory receptors from the tumour necrosis factor receptor superfamily

(TNFRSF) may represent the next stage in solid cancer treatment.

CD137 (4-1BB) is a co-stimulatory molecule and widely known to be upregulated on CD8+ T

cells following activation (3). CD137 can also be expressed on activated CD4+ helper T cells,

B cells, regulatory T cells, natural killer (NK) cells, natural killer T (NKT) cells, and

dendritic cells (DCs) (4). Engagement of CD137 by its ligand CD137L results in receptor

trimer formation and subsequent clustering leads to CD137 signalling cascade activation.

This provides a survival signal to T cells, thereby sustaining effective T cell activation and

generation of immunological memory. The primary functional role of CD137 in enhancing T

cell cytotoxicity was first described in 1997 (5), and soon thereafter CD137 monoclonal

antibodies (mAbs) were proposed as anti-cancer therapeutics.

Clinical development of CD137 mAbs has been hampered by dose-limiting high-grade liver

inflammation associated with CD137 agonist antibody treatment. Urelumab (Bristol-Myers

Squibb, BMS-663513), a human IgG4 isotype antibody, was the first CD137 mAb to enter

clinical trials but these were halted after significant, on target, dose-dependent liver toxicity

was observed (6) (7) (8). This outcome was not predicted because urelumab failed to

preclinically identify liver inflammation due to its low affinity for the cynomolgus monkey

target molecule (9). More recently, clinical trials of urelumab in the treatment of solid cancers

were recommenced, however urelumab dosing in these trials had to be limited and efficacy

results were disappointing with no objective response reported in the 64 patients with solid

tumours treated with monotherapy (6).

No dose-limiting toxicity has been observed with CD137 mAb utomilumab (PF-05082566,

Pfizer), a human IgG2 isotype antibody, in dose escalation Phase I clinical trials dosing up to




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10 mg/kg in Phase I clinical trials of advanced cancer (6) (8). However, the overall objective

response rate with this antibody was only 3.8% in patients with solid tumours, potentially

indicating that utomilumab has a weaker potency and clinical efficacy than urelumab, whilst

having a more favourable safety profile (6) (8). Trials of utomilumab in combination with

radiotherapy or chemotherapy, as well as in combination with other antibody therapies, are

ongoing with early results showing no DLTs for doses up to 5 mg/kg and a 26% patient

response rate for the combination of utomilumab and pembrolizumab (10).

PD-1 and its ligands PD-L1 (CD274, B7-H1) and PD-L2 (B7-DC) deliver inhibitory signals

that regulate the balance between T cell activation, tolerance, and immunopathology.

Consequently, PD-L1 expression by cells can mediate protection against cytotoxic T

lymphocyte (CTL) killing. Cancer, as a chronic and pro-inflammatory disease, subverts this

immune-protective pathway through upregulation of PD-L1 expression to evade the host

immune response. PD-L1 expression has been shown in a wide variety of solid tumours (11),

and clinical trials have shown the benefit of targeting PD-L1 in patients leading to the

approval of three PD-L1 targeting mAbs to date: atezolizumab (MPDL3280A, Tecentriq™,

Hoffmann-La Roche, Genentech), a humanised IgG1 antibody; avelumab (MSB0010718C,

Bavencio™, Merck KGaA, Pfizer), a fully human IgG1 antibody; and durvalumab

(MEDI4736, Imfinzi™, AstraZeneca) a fully human IgG1 antibody. The PD-L1/PD-1

immune checkpoint is also being targeted by three approved PD-1 monoclonal antibodies,

namely pembrolizumab (Keytruda, Merck), nivolumab (Opdivo, BMS) and cemiplimab

(Libtayo, Regeneron Pharmaceuticals).

In mouse models resistant to single agent treatment with either CD137 agonists or PD-1/L1

blockade, significant synergistic effects have been observed when antibodies targeting both

pathways are combined. The mechanistic basis for this synergy, even in poorly immunogenic

mouse tumour models, is that tumour-infiltrating lymphocytes (TILs) co-express PD-1 and

CD137 and following combination treatment CD8+ T cells can now effectively respond to

tumour-associated neoantigens (12). The mechanistic basis for CD137 agonist antibodies

alone can be two-fold. Firstly, their ability to induce effector cell function can result in anti-

tumour activity in some preclinical models. Secondly, their alternative function to deplete

regulatory T cells (Treg cells) has been described as the most effective mechanism of action

for CD137 mAbs (13). However, an alternative approach is to direct potent CD137 agonist

activity to tumour-reactive T cells whilst not relying on Treg cell depletion. Avoiding Treg

cell depletion can be achieved by removing FcR-binding by L234A and L235A (LALA)




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mutation whilst retaining agonist activity via an alternative mechanism of crosslinking as

described in this paper.

As only a fraction of patients respond to monotherapies that block the PD-1/PD-L1 pathway

(14), and CD137-targeting agonistic molecules have yet to demonstrate significant responses

in cancer patients without toxicity (7) (8), we believe there remains a need to develop

treatments which combine PD-L1 blockade and provoke strong CD137 agonism in safe and

efficacious therapies which do not rely on a combination approach. An alternative to

combining CD137 and PD‐L1 monotherapies is the development of a bispecific antibody that

encompasses the two modalities. It is anticipated that such a bispecific monoclonal antibody

could deliver superior anti-tumour efficacy over combining monotherapies. There are

existing preclinical approaches combining CD137 mAb activity with PD-L1 mAb activity

into bispecific therapies. These can be subdivided in to two broad range categories; non-IgG-

and IgG-like molecules, both of which can be further divided by their binding valency for

each target.

Here, we describe a fully human, tetravalent, IgG bispecific antibody (mAb2, FS222)

comprising a PD-L1 specific mAb with 5 amino acid insertions and 7 amino acid

substitutions in the CH3 region of the Fc domain to create two binding sites forming an Fc

fragment antigen-binding (Fcab) for CD137 (15). FS222 blocked PD-L1 and activated

CD137+ tumour-reactive T cells in a PD-L1-dependent manner. It demonstrated similar

potency in primates and preliminary toxicity studies in this species showed significant

pharmacodynamic responses and a lack of toxicity. A surrogate mouse cross-reactive

CD137/PD-L1 mAb2 with homologous mechanisms of action to FS222 was observed to

provide a substantial survival benefit in multiple mouse tumour models with no toxicity and

showed potent in vivo pharmacodynamic changes related to anti-tumour immune responses.




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Materials and Methods

Production and characterisation of a CD137/PD-L1 mAb2, FS222

The CD137/PD-L1 mAb2 molecule named FS222 consisting of an IgG1 molecule comprising

the human CD137 Fcab was prepared by substituting part of the CH3 domain comprising the

AB, CD and EF loops (15), for the corresponding region of the CH3 domain of an PD-L1

mAb (E12v2). Fcab generation has been previously described (16). FS222 incorporates a

LALA mutation (leucine to alanine at positions 234 and 235 according to Eu numbering) in

the CH2 domain (AA) to reduce Fc receptor binding (17) (18). FS222 was expressed

transiently using HEK293 6E (National Research Council Canada (NRCC), Canada) cells.

Supernatants were purified on MabSelect SuRe LX Protein-A pre-packed columns using

ÄKTAxpress instrument (both GE Healthcare Life Sciences, Uppsala, Sweden). IgG protein

content was quantified by BioLayer Interferometry (BLI) using the Octet® QKe System

(FortéBio, CA, USA) platform with Protein A quantitation biosensors (FortéBio, CA, USA,

18-5013). FS222 was purified by Protein A affinity chromatography using mAb SelectSure

columns.

Biophysical characterisation of mAb2 by size exclusion chromatography (SEC) and

SDS-PAGE

Post-purification, SE-HPLC was performed on an Agilent 1100 series HPLC Value System

(Agilent Technologies, Inc, USA), fitted with a TSKgel® SUPERSW3000 HPLC 4.6 mm ID

x 30 cm column (Tosoh Bioscience, LLC, USA) using 20 mM sodium phosphate, 200 mM

sodium chloride, pH 6.8 as a mobile phase. Quantification of percentage monomer was

performed using ChemStation software (Agilent Technologies, Inc, USA). CE-SDS analysis

was performed on a 2100 Bioanalyzer Capillary Electrophoresis System (Agilent

Technologies, Inc, USA), according to manufacturer’s instructions. For reducing conditions,

DTT was added and samples were denatured at 70° C for 5 minutes.

Simultaneous binding of FS222 to human PD-L1 and human CD137 by SPR

His-tagged human PD-L1 antigen was coated on to a CM5 chip to ~1,100 RU and was used

to immobilise FS222 when injected at 100nM which resulted in ~300 RU of FS222 being

captured. Fc-tagged human CD137 antigen was then injected at a single concentration

(100nM), using the Biacore T200 (GE Healthcare Life Sciences, USA), to observe dual

binding.




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FS222 binding to cell-expressed receptors by flow cytometry

DO11.10 T cells overexpressing human CD137 were cultured in Dulbecco's modified eagle

medium (DMEM) containing 10% heat-inactivated Foetal Bovine Serum (FBS), 1mM

sodium pyruvate, and 50 µg/mL puromycin. HEK cells overexpressing human PD-L1 were

cultured in DMEM containing 10% FBS, 100 µg/mL hygromycin B, 15 µg/mL blasticidin

and 1 µg/mL doxycycline. Cells were resuspended in 40 µL of FS222, CD137/Ctrl(HelD1.3)

mAb2, CD137(MOR7480.1) mAb, Ctrl(4420) mAb, or PD-L1(E12v2) mAb (Table S1)

titrations prepared in DPBS and then washed and resuspended in a secondary human IgG

detection antibody (A-21445, ThermoFisher Scientific, USA) which had been diluted in

DPBS. Cells were then resuspended in the viability dye 7-AAD (A1310, ThermoFisher

Scientific, USA) and examined with either a BD FACSCanto™ II or BD LSRFortessa™ II

(BD Biosciences, USA) before being analysed using FlowJo V10 (TreeStar, Inc, USA).

FS222 binding in human primary T cell assay

Human primary T Cell Isolation and Activation

Peripheral Blood Mononuclear Cells (PBMCs) were isolated from leukocyte cones, obtained

from platelet donors, by Ficoll (GE17-1440-02, Sigma-Aldrich) gradient separation. Pan T

cells were isolated from the PBMCs present in the eluent using the Pan T Cell Isolation kit II

according to the manufacturer’s instructions (130-096-535, Miltenyi Biotec, Germany). Pan

T cells were incubated overnight at 37ᵒC and 5% CO2 in Roswell Park Memorial Institute

medium (RPMI) supplemented with 10% heat-inactivated FBS, 1 mM 2-mercaptoethanol,

Penicillin (100 U/mL) / Streptomycin (100 U/mL) and 1 µM sodium pyruvate. DynabeadsTM

Human T-Activator CD3/CD28 beads (11132D, ThermoFisher Scientific) were used to

activate T cells and upregulate CD137 and PD-L1 surface expression. Beads were washed

from the T cells using a DynaMagTM

-15 Magnet (12301, ThermoFisher Scientific) following

manufacturer’s instructions.

Human primary T cell binding assay

Stimulated pan human primary T cell suspensions were resuspended in 40 µL FS222,

CD137/Ctrl(HelD1.3) mAb2, PD-L1 mAb, CD137(MOR7480.1) mAb, or Ctrl(4420) mAb

(Table S1) titrations prepared in DPBS and treated with AF647 goat anti-human IgG (H + L)

[1:500] (A-21445, ThermoFisher Scientific), anti-hCD4 FITC [1:200] (550628, BD

Biosciences, USA) and anti-hCD8 eF450 [1:200] (48-0087-42, ThermoFisher Scientific)

prepared in DPBS. 7-AAD (A1310, ThermoFisher Scientific) was used as a viability dye and




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samples were then examined with a BD FACSCanto II before being analysed using FlowJo

V10 Prism software.

Human primary CD8+ T cell assay with HEK.hPD-L1 crosslinking

Human primary CD8+ T cells were isolated from PBMCs obtained from leucocyte depletion

cones using the CD8+ T cell isolation kit II (130-096-495, Miltenyi Biotec Ltd,) according to

the manufacturer’s instructions. For cell-based crosslinking HEK293 cells overexpressing

hPD-L1 (HEK.hPD-L1) or HEK wildtype cells or a mixture of the two populations were

plated on to CD3 mAb-coated (8 µg/mL, Clone UCHT1, R&D Systems, MAB100-SP) 96

well flat bottom plates in 100 µL T cell culture medium (RPMI medium with 10% FBS, 1X

Penicillin Streptomycin, 1mM Sodium Pyruvate , 10mM Hepes (Sigma-Aldrich, H0887) and

50µM 2-mercaptoethanol (Gibco, M6250). CD8+ T cells were added. Cells were treated with

a titration of FS222, CD137(20H4.9) mAb, or Ctrl(4420) mAb (Table S1). Supernatants were

assayed with human IL-2 ELISA Ready-SET-Go! kit (88-7025-88, Fisher Scientific)

following the manufacturer’s instructions. Plates were read at 450 nm using the plate reader

with the Gen5 Software. The concentration of human IL-2 (hIL-2) was plotted vs the log

concentration of antibody and the resulting curves were fitted using the log (agonist) vs

response equation in GraphPad Prism.

Human primary mixed lymphocyte reaction (MLR)

Generation of expanded CD4+ T cells

Human primary CD4+ T cells were isolated from leukocyte cones using a Human CD4

+ T

Cell Isolation Kit (130-096-533, Miltenyi Biotec Ltd) according to the manufacturer’s

instructions. Dynabeads™ Human T-Activator CD3/CD28 (11131D, ThermoFisher

Scientific) were used in the presence of 50 IU/mL recombinant human IL-2 (PeproTech, 200-

02-50μg) with 3:1 bead to cell ratio to expand cells for 7 days. Dynabeads were removed and

CD4+ T cells were rested overnight with reduced 10 IU/mL recombinant human IL-2.

Differentiation of iDCs

Monocytes were isolated from human PBMCs using a Human Pan Monocyte Isolation Kit,

(130-096-537, Miltenyi Biotec Ltd, UK) following the manufacturer’s instructions.

Monocytes were differentiated to iDCs using Human Mo-DC Differentiation Medium (130-

094-812, Miltenyi Biotec Ltd, UK) following the manufacturer’s instructions.




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Expanded T cells were cultured in AIM V™ Medium (12055091, ThermoFisher Scientific)

and incubated overnight. Titrations of FS222, PD-L1(E12v2) mAb and CD137(20H4.9)

mAb, PD-L1(E12v2) mAb, CD137/Ctrl(HelD1.3) mAb2 and PD-L1(E12v2) mAb,

CD137(20H4.9) mAb, CD137/Ctrl(HelD1.3) mAb2, or Ctrl(4420) mAb (Table S1) were used

to treat a 1:10 mix of iDC cells and expanded CD4+ T cells in AIM V Medium for 5 days.

Supernatants were analysed for interferon gamma (IFN-γ) using Human IFN gamma ELISA

Ready-SET-Go! Kit (88-7316-86, ThermoFisher Scientific). Plates were read at 450 nm

using the plate reader with the Gen5 Software. The concentration of human IFN-γ was

plotted vs the log concentration of antibody and the resulting curves were fitted using the log

(agonist) vs response equation in GraphPad Prism.

Murine primary OT-1 CD8+ T cell activation assay

CD8+ T cell activation was achieved by antigen stimulation of genetically modified OT-1 T

cells, isolated from C57BL/6 OT-1 mice (003831, The Jackson Laboratory) having a T cell

receptor specific for ovalbumin peptide 257-264, and was determined by the release of IFN-γ.

OT-1 T cells were incubated with B16-F10 melanoma cells, which had previously been

cultured in the presence of 20 ng/mL IFN- (AF-315-05-100UG, PeproTech) to induce PD-

L1 expression, and that were then pulsed with 500 nM SIINFEKL peptide for 1 hour at 37°C,

to drive T cell activation. The efficacy of surrogate FS222 was subsequently assessed by

ELISA for secreted mIFN- (88-7314-88, ThermoFisher Scientific) after 3 days. This assay

was also carried out utilising MC38.OVA cells that express ovalbumin, in an identical

protocol, except for peptide pulsing which was not necessary.

Surrogate FS222 in vivo characterisation in CT26.WT and MC38 syngeneic mouse

tumour models

The CT26.WT colon carcinoma cell line (ATCC) was initially expanded, stored, and then

pre-screened by IDEXX Bioresearch for pathogens using the IMPACT I protocol and shown

to be pathogen-free. Each BALB/c female mice (Charles River) aged 8-10 weeks and

weighing 18 to 22 g each received 1 x 105 CT26.WT cells injected subcutaneously in the left

flank in 100 µL DMEM serum-free culture medium.

The MC38 colon carcinoma cell line (ATCC) was initially expanded, stored, and then pre-

screened by IDEXX Bioresearch for pathogens using the IMPACT I protocol and shown to

be pathogen-free. Each C57BL/6 female mice (The Jackson Laboratory) aged 9-10 weeks




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and weighing 18 to 24 g each received 1 x 106 MC38 cells injected subcutaneously in the

right flank in 100 µL DMEM serum-free culture medium.

The surrogate FS222 and control antibodies were injected intraperitoneally into mice at

appropriate µg per mouse in DPBS, 100 mM arginine, 0.05% Tween 80 on days 7, 9, and 11

following tumour inoculation. Tumour volume measurements were taken with callipers to

determine the longest axis and the shortest axis of the tumour and the following formula was

used to calculate the tumour volume:

L X (S2) / 2

Where L = longest axis; S= shortest axis

Pharmacodynamic assessment and receptor occupancy in a CT26.WT syngeneic mouse

tumour model

A single dose pharmacodynamic study was run in the same CT26.WT syngeneic tumour

model as described above. This study comprised 3 dosing groups, receiving either control

antibody or surrogate FS222 at one of two doses. Samples from tumour tissue and blood were

analysed over 8 time points (2h, 6h, 24h, 48h, 72h, 96h, 120h, 192h). Each dosing cohort had

64 mice (8 mice per timepoint). Each animal received 1 x 105 CT26.WT cells injected

subcutaneously in the left flank in 100 μL DMEM. Eleven days following tumour cell

inoculation, each mouse received the test sample via a 100 μL intravenous injection.

Tumour tissue and blood were tested for drug-bound positive T cells, T cell proliferation, and

free PD-L1. Blood (100 μL) was collected in to EDTA coated capillaries by tail vein bleeding

and were lysed twice in red blood cell lysis buffer (ThermoFisher, 00-4333-57) according to

manufacturer’s instructions. Tumour tissue was collected by dissection and was

disaggregated to single cell suspension by standard mechanical and enzymatic methods. Red

blood cells were lysed in red blood cell lysis buffer according to manufacturer’s instructions.

Cells were stained with Fixable Viability Dye eFluor™ 780 (65-0865-14, ThermoFisher

Scientific) following manufacturer’s instructions. Cells were stained with an antibody

staining panel (Table S4, all but Ki67 and FoxP3 antibodies) in the presence of CD16/CD32

mAb Fc block (1:50, 14-0161-86, ThermoFisher Scientific) and then fixed and permeabilized

with the eBioscience™ Foxp3 staining kit (00-5523-00, ThermoFisher Scientific) according




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to manufacturer’s instructions. Cells were stained with Ki67 and Foxp3 antibodies in the

presence of Fc block and then examined in a BD Fortessa flow cytometer. Data was analysed

with FlowJo, Excel and GraphPad Prism.

Preliminary toxicology study in cynomolgus monkeys

A preliminary dose range finding study was conducted to evaluate the

pharmacokinetic/pharmacodynamic (PK/PD) response to and tolerability of FS222 in

cynomolgus monkeys. The study was performed using Mauritian cynomolgus macaques at

Charles River Laboratories in line with Institutional Animal Care and Use Committee

(IACUC) guidelines and in accordance with the ‘‘Guide for the Care and Use of Laboratory

Animals’’ (1996) by the Institute of Laboratory Animals Research Commission on Life

Sciences (ILARCLS, National Research Council, Washington, DC.

Briefly, FS222 was administered to cynomolgus monkeys (1/sex/group) via intravenous

infusion at 3 mg/kg as a single dose on day 1 or at 0.1, 1, 10 or 30 mg/kg as repeat doses on

days 1, 8, 15 and 22. For the 3 mg/kg dose group, serial serum samples were collected for PK

assessment on day 1 (predose, 0.083, 0.5, 2, 6 and 12h post-dose) and days 2, 3, 4, 6, 8, 11,

15, 22, 29, 36 and 43. For the remaining groups PK serum samples were collected on day 1

(predose, 0.083, 0.5, 2, 6 and 12h post-dose) and days 2, 3, 7, 8 (predose and 0.083 h post-

dose), day 15 (predose, 0.083, 0.5, 2, 6 and 12h post-dose), days 16, 17, 21, 22 (predose and

0.083 h post-dose) and day 25. Serum levels of FS222 were measured using a qualified Gyros

based immunoassay developed in-house to specifically detect free drug (human biotinylated

PD-L1 was used as a capture reagent and human Alexa Fluor labelled CD137 as a detection

reagent).

For the evaluation of tolerability, standard toxicology parameters such as body weight, food

consumption, clinical observations, haematology and blood chemistry were evaluated over

the duration of the study. The study was terminated 25 days after administration of first dose

in repeat dose animals and 43 days after administration of single dose (PK group of animals).

For evaluation of the PD response to FS222, immunophenotyping of peripheral blood was

performed to assess peripheral lymphocyte populations (monocyte, T cells, B cells and

natural killer (NK) cells) as well as the induction of proliferation and activation of central

memory (CM) and effector memory (EM) CD4+ and CD8

+ T cell subpopulations. Serial

blood samples (on EDTA) were collected prior to first dosing of FS222 (pre-dose) and on the

indicated study days over the course of the study, stained with antibodies against CD45, CD3,




13

CD4, CD8, CD16, CD28, CD25, FoxP3, CD95, CD69 and Ki67 (Table S3), and analysed

using a FACSCanto II flow cytometer (BD Biosciences, USA) and FACSDiva (BD

Biosciences, USA), Excel and GraphPad Prism software.




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Results

Creation of a CD137/PD-L1 bispecific antibody (mAb2, FS222)

FS222 was created by incorporating into a proprietary PD-L1 mAb, an engineered IgG1 Fc

region, termed Fcab™ (Fc-region with antigen binding), where high affinity antigen binding

for CD137 was introduced in the C-terminal region (Figure 1A). Binding to FcR was

removed by the introduction of L234A and L235A (LALA) mutations (Figure S1A), whilst

binding to human FcRn was maintained (Figure S1B). The mAb2 is tetravalent, with two

binding sites for PD-L1 (one in each Fab region) and two binding sites for CD137 (one in

each CH3, due to the homodimeric nature of the Fc region) and maintains the IgG1 structure.

Binding affinities of FS222 are equivalent to individual component antibodies

The simultaneous binding of FS222 to human CD137 and human PD-L1 was tested via SPR

and then individually to cells expressing either human CD137 or human PD-L1.

FS222 simultaneously bound to both targets as observed by SPR (Figure 1B) with affinities

of 0.66 nM to dimeric human CD137 and 0.19 nM to monomeric human PD-L1 (Figure S1C-

E). The cell binding assays demonstrated that FS222 bound human CD137 to an equivalent

level as the component CD137 Fcab (CD137/Ctrl(HelD1.3) mAb2) alone (EC50 6.2 nM)

(Figure 1C). FS222 also bound human PD-L1 to an equivalent level as the component Fab

(PD-L1(E12v2) mAb) alone (EC50 3.7nM) (Figure 1D). PD-L1(E12v2) mAb was found to be

just as effective as the PD-L1(S70) mAb (YW243.55.S70) at blocking PD-L1 binding to PD-

1 as investigated in a DO11.10 human PD-L1 T cell activation assay (data not shown). As

shown in Figure 1E, FS222 bound to activated CD4+ and CD8

+ primary T cells with an EC50

of 0.8 nM and 0.9 nM respectively. This was equivalent to the PD-L1(E12v2) mAb binding

characteristics but not equivalent to the CD137/Ctrl(HelD1.3) mAb2 showing that this cell

binding was driven primarily through PD-L1 rather than CD137. The positive control CD137

mAb, CD137(MOR7480.1) mAb, bound to activated human primary T cells as expected,

whereas the CD137/Ctrl(HelD1.3) mAb2 had minimal binding to the same cells. Therefore,

FS222 bound to PD-L1 with high affinity and, through design, bound CD137 with high

avidity, which are two features of the molecule critical for the crosslinking-dependent activity

as described below.

T cell activation through CD137 agonism was dependent upon crosslinking via PD-L1




15

Subsequent investigations were aimed to dissect the nature of FS222-mediated CD137

agonism. To demonstrate that FS222 can be crosslinked to mediate human CD137 signalling

only in the presence of cells expressing human PD-L1, human primary CD8+ T cells

stimulated by plate-bound CD3 mAb, were co-cultured with wild type HEK 293 cells, HEK

293 cells engineered to overexpress human PD-L1 or mixtures of the two cell lines in

different proportions. This allowed the investigation of varying ratios of human-PD-L1

expressing cells to wildtype cells to model the likely heterogeneity of expression present

within different human tumours.

FS222 showed maximum activity, measured by human IL-2 release, from activated human

primary CD8+ T cells, when 100% of HEK 293 cells expressed PD-L1 (Figure 2A). The

maximum IL-2 release (Emax) reduced in proportion to the reduction in the percentage of cells

expressing human PD-L1 present, however, the EC50 value remained broadly the same at

0.05 nM (Figure 2A).

FS222 elicited superior activity in a CD4+ mixed lymphocyte reaction by FS222

compared to a combination of monospecific antibodies

The activity of FS222 was tested in a mixed lymphocyte reaction (MLR) which utilises

human primary CD4+ T cells and immature monocyte-derived dendritic cells (iDC)

expressing endogenous levels of both targets. The PD-L1 specific antibody (PD-L1(E12v2)

mAb) showed potent activity in the MLR assay (EC50 0.08 nM). The human CD137 mAb

CD137(20H4.9) mAb, even when crosslinked with hCH2 mAb, did not elicit activity. This

suggested that CD137 signalling alone is ineffective in this assay. However, a combination of

PD-L1(E12v2) and CD137(20H4.9) mAbs crosslinked with hCH2 mAb showed potent

activity (EC50 0.14 nM) with a higher maximum IFN- release compared to PD-L1(E12v2)

mAb alone, which indicated a synergistic effect of the two mAbs (Figure 2B). FS222 showed

similarly potent activity to the combination of the two separate monospecific antibodies with

an EC50 Figure 2B). FS222 was

also tested against each component part of the mAb2 that makes up FS222; the

CD137/Ctrl(HelD1.3) mAb2 and the PD-L1(E12v2) mAb, alone and in combination.

CD137(HelD1.3) mAb2 showed no activity in this assay indicating that the CD137 Fcab

component had an inability to activate T cells in an MLR. There was no additional effect on

activation by treating with a CD137/Ctrl(HelD1.3) mAb2 plus PD-L1(E12v2) mAb

combination above that already seen with PD-L1(E12v2) mAb alone. FS222 showed a




16

similar potency to the combination of its component parts CD137/Ctrl(HelD1.3) mAb2 and

PD-L1(E12v2) mAb (EC50 of 0.07 nM and 0.06 nM respectively) with superior efficacy as

max) (Figure 2B).

Surrogate FS222, a mouse CD137/PD-L1 tetravalent bispecific antibody, demonstrated

potent in vitro activity greater than a mAb combination approach

FS222 did not bind to mouse CD137 (data not shown) so it was not possible to evaluate

FS222 in mouse syngeneic tumour model systems in vivo. Therefore, a mouse surrogate of

FS222 was created using an Fcab targeting mouse CD137 selected using yeast display. The

Fcab against mouse CD137 was selected based on affinity measurements, crosslink-

dependent activation of CD137 and was tested in similar mouse systems to those used to

determine the function of FS222 in human systems. Surrogate FS222 was tested for cell

binding similar to FS222 and was found to have an EC50 of 2.7 nM and 2.6 nM to cells

engineered to overexpress mouse CD137 and mouse PD-L1 respectively.

The functional activity of surrogate FS222 was determined in a primary assay where CD137

and PD-L1 are endogenously expressed on activated T cells and B16-F10 tumour cells

respectively. B16-F10 mouse melanoma cells that had previously been pulsed with OVA

peptide (SIINFEKL) were co-cultured with CD8+ antigen-specific OT-1 T cells. CD8

+ T cell

activation (IFN-γ release) was increased after treatment with either mCD137(Lob12.3) mAb,

PD-L1(S70) mAb, or a combination approach. However, the greatest potency was seen upon

treatment with the surrogate FS222 (EC50 0.003 nM) (Figure 3A). The same result was

achieved using a similar assay described above but utilising MC38 cells expressing

ovalbumin as the source of both OVA peptide and PD-L1 (Figure S2A). As with FS222, no

activation was detected in the absence of PD-L1 crosslinking which indicated a similar mode

of action between the surrogate and human specific molecules.

Surrogate FS222 controlled tumour growth in syngeneic mouse tumour models

To evaluate the anti-tumour effects of surrogate FS222 in vivo, CT26.WT or MC38 tumour

cell lines were injected subcutaneously into the flank of BALB/c and C57BL/6 mice,

respectively. In the CT26.WT tumour model, mice were injected intraperitoneally on day

seven, nine and eleven post tumour implantation with approximately 10 mg/kg surrogate

FS222, PD-L1(S70) mAb, mCD137(Lob12.3) mAb or Ctrl(HelD.13) isotype control mAb.

Surrogate FS222 was shown to substantially reduce tumour growth (Figure 3B) and 42% of

mice remained tumour free up to 68 days post treatment (Figure 3C and Table 1). In contrast,




17

treatment with PD-L1(S70) mAb or mCD137(Lob12.3) mAb failed to show significant

survival advantage (Figure 3C and Table 1 and Table S8 and S9).

In the MC38 tumour model, surrogate FS222 was able to eradicate all tumours at a lower

dose of 1 mg/kg. Mice were injected intraperitoneally seven, nine and eleven days after

tumour cell inoculation with a dose of approximately 1 mg/kg of either surrogate FS222, PD-

L1(S70) mAb, mCD137(Lob12.3) mAb, a combination of both or Ctrl(4420) isotype control

mAb. Surrogate FS222 treated mice showed full tumour regression in all mice which

remained tumour free until day 49 when the study was ended. In contrast, single and

combined treatment of PD-L1(S70) mAb and mCD137(Lob12.3) mAb resulted in durable

tumour regression in only in a fraction (4/12 or less) animals (Figure 3D, 3E and Table 1 and

Table S10 and S11).

Surrogate FS222 produced dose-dependent survival benefit in CT26.WT syngeneic

mouse tumour model

In the syngeneic CT26.WT tumour model we assessed surrogate FS222 dose-dependency in

vivo and showed dose-dependent survival benefit between doses of ~0.1 mg/kg to ~10 mg/kg.

To evaluate dose-dependent efficacy, dose levels equivalent to approximately 0.1 mg/kg, 0.3

mg/kg, 1 mg/kg and 10 mg/kg or dosed at 10 mg/kg with Ctrl(4420) mAb isotype control

were administered intraperitoneally using the same study design described above. Surrogate

FS222 showed an anti-tumour efficacy from 0.3 mg/kg and durable tumour regression in 21%

of treated animals at 1 mg/kg and 40% at 10 mg/kg (Figure S2B). Using the Log-Rank

(Mantel-Cox) test, a significant survival dose-dependency was shown for surrogate FS222 at

0.3 mg/kg up to 1 mg/kg compared to Ctrl(4420) mAb treatment, but no significant benefit

raising from 1 mg/kg to 10 mg/kg surrogate FS222 despite the median survival extending

from 29 to 39 days, respectively (Figure 4 and Table 2).

Surrogate FS222-regulated dose-dependent pharmacodynamic changes in tumour and

blood

CT26.WT tumour-bearing mice were treated with a single intravenous dose of surrogate

FS222 (~1 mg/kg and ~10 mg/kg) eleven days after subcutaneous inoculation of CT26.WT.

Tumour tissue and blood were tested for T cell bound surrogate FS222, T cell proliferation,

and PD-L1 receptor occupancy over time (between 2h and 192h). Total CD137 receptor

expression was also assessed.




18

A high percentage of peripheral and tumour resident T cells showed bound surrogate FS222

as early as 2 hours after intravenous administration (Figure 5A). There was a dose-dependent

correlation in the longevity of binding, with surrogate FS222 no longer detected after 96h on

T cells isolated from mice administered with 1 mg/kg, whereas surrogate FS222 was still

detected between 120h and 192h after administration of 10 mg/kg.

Ki67 expression was used as a marker for T cell proliferation on CD4+ and CD8

+ T cells. T

cells isolated from tumour tissue exhibited a higher frequency of Ki67+ T cells as expected in

an inflammatory tumour microenvironment. At both dose levels, surrogate FS222 resulted in

increases in the frequency of Ki67+ peripheral blood T cells when compared to Ctrl(4420)

mAb isotype control, indicating a pharmacodynamic (PD) response (Figure 5B). The effect

appeared stronger for CD8+ T cells which is in line with CD8

+ T cells expressing higher

CD137 levels than CD4+ T cells.

A proportion of cells isolated from the blood and tissue of mice dosed with Ctrl(4420) mAb

isotype control were saturated with 100 nM surrogate FS222 ex vivo which acted as control

for 100% PD-L1 receptor engagement and was confirmed by fully blocking binding with a

competing mouse PD-L1 mAb (clone 10F.9G2). Cells isolated from tumour tissue and the

blood of mice treated with surrogate FS222 at the 10 mg/kg dose showed near complete PD-

L1 blockade for 8 days, as shown in Figure 5C represented by 100% PD-L1 receptor

occupancy. At 1 mg/kg surrogate FS222 achieved near complete PD-L1 receptor occupancy

for approximately 72h on peripheral T cells. PD-L1 receptor engagement on T cells present

in blood showed an accelerated decrease compared to T cells present in the tumour tissue

which retained a greater PD-L1 receptor occupancy with greater longevity.

Serum cytokines were analysed by multiplex electrochemiluminescent immunoassay (Meso

Scale Discovery, MSD) to assess cytokine production after surrogate FS222 treatment in the

same model. Surrogate FS222, when dosed at 10 mg/kg, resulted in increased serum pro-

inflammatory cytokines IFN-γ, TNF- and IL-6. The immunosuppressive cytokine IL-10

likewise shows increase in serum post-dosing, presumably to counter the proinflammatory

response. This effect was dose-dependent and serum cytokine levels remained similar in mice

treated with 1 mg/kg surrogate FS222 compared to Ctrl(4420) mAb treated mice (Figure 5D).

Anti-tumour activity observed via tumour growth inhibition, tracked by measuring excised

tumours at indicated timepoints, still showed highly significant tumour growth inhibition in

mice treated with a single dose of 1 mg/kg surrogate FS222 (Figure S3A). This indicated




19

localised anti-tumour cytotoxic activity without systemic exposure to inflammatory

cytokines.

Surrogate FS222 immunopharmacology did not result in hepatotoxicity

In 2008, clinical trials investigating urelumab in solid tumours were halted due to severe

treatment-related immune events which manifested in the liver as severe hepatotoxicity

resulting in patient deaths (7). More recently, urelumab has been administered at far reduced

dose levels to mitigate this toxicity. Preclinical mechanistic work undertaken in mice wherein

animals dosed with CD137 agonist mAbs showed similar hepatotoxicity. These studies

showed a requirement for T cells and CD137 expression in the resultant hepatotoxicity (19)

(9). Therefore, these animal models have some translational relevance for predicting the risk

of hepatoxicity in the clinic in human patients following administration of other CD137

agonists such as FS222. Mice from our CT26.WT syngeneic tumour studies showed no overt

signs of toxicity following repeated dosing with surrogate FS222 and maintained normal

bodyweight throughout. To determine whether immune activation and anti-tumour activity

observed as a result of treatment with 1 mg/kg surrogate FS222 correlated with

hepatotoxicity, liver samples were taken at necropsy for histological assessment. Surrogate

FS222 treated and control mice were necropsied 4, 7 and 14 days after the last administration

whereby liver samples were excised and examined.

Each liver section was scored for pathology and the frequency of mice showing zero,

minimal, slight and moderate effects within each group are shown in Table S2 (0=zero,

1=minimal, 2=slight, 3=moderate). Surrogate FS222 treated animals showed minimal liver

pathology (Figure S3B and Table S2). Specifically, the livers showed minimal to slight

hepatocellular necrosis with mixed lymphocyte infiltrate in the parenchyma, minimal to slight

mixed inflammatory cells in periportal tracts, no degenerative hepatocytes and minimal to

slight increased mitoses (Table S2). These findings are not deemed to represent adverse

hepatoxicity, as observed with other examples of CD137 agonist mAbs.

In a similar liver pharmacology mouse study, which included dosing CT26.WT tumour-

bearing mice with 10 mg/kg of a CD137 targeting mAb clone 3H3 known to induce liver

toxicity (20), both surrogate FS222 treated (also at 10 mg/kg) and CD137(3H3) mAb treated

animals showed CD3+ T cell liver infiltration from day 13 onwards to similarly high levels

above control mice (Figure 5E). However, activated CD8+ T cells remained at significantly

higher levels with greater longevity in CD137(3H3) mAb treated animals compared to




20

animals treated with surrogate FS222 (Figure 5F). The CD8+ T cell response for both

CD137(3H3) mAb and surrogate FS222 peaks at ~95% positive for Ki67 across days 8 and

13 post first dose. However, for surrogate FS222 this returned to baseline by day 16 whereas

for CD137(3H3) mAb this did not happen by the end of the study at day 28 post first dose

(Figure 5F). This indicated a difference in the mode of action of these two CD137 targeting

agents; CD8+ T cells activated by surrogate FS222 proliferated less so and for a shorter

period compared to CD8+ T cells activated by CD137(3H3) mAb. CD137(3H3) mAb has

been shown to lead to hepatic degeneration previously (20). CD4+ FoxP3

+ regulatory T cells

were present at higher levels in surrogate FS222 treated livers compared to CD137(3H3)

mAb treated livers at day 8 and day 13 post first dose (Figure S3C). Approximately 30% of

CD4+ T cell were positive for FoxP3 after surrogate FS222 treatment, whereas the level after

CD137(3H3) mAb remained nearer baseline at 10% (Figure S3C). This indicated a

potentially more immunosuppressive environment which could dampen the damaging effect

of activated CD8+ T cell accumulation, shown to otherwise lead to hepatocyte death (21).

Due to the potential preclinical limitations of surrogate molecules and mouse models for

predicting CD137 induced liver toxicity, we ran a non-GLP PK/PD toxicity study of single

and repeat dosing of FS222 in cynomolgus monkey.

FS222 elicited immune activation with no liver toxicity in a preliminary toxicity study in

cynomolgus monkeys

FS222 was shown to be fully cross-reactive in cell binding assays and primary immune cell

functional assays using PBMCs from human or cynomolgus blood (Figure S4A-D).

Therefore, the pharmacokinetic (PK) behaviour of FS222 was characterised in cynomolgus

monkeys after intravenous (IV) administration of FS222 in a non-GLP dose-range finding

study (Figure 6A). FS222 displayed a dose proportional increase in Cmax and AUC (0-

168hr) (Table S5) and linear plasma clearance (at doses ≥ 1 mg/kg) (Figure 6A). FS222 had a

mean terminal half-life of approximately 148 h which is in line with human antibodies in

monkeys targeting PD-L1 (atezolizumab BLA #761034 Pharmacology Review). In general,

FS222 PK followed a linear dose response (at dose levels ≥ 1 mg/kg) and clearance rates

(CLp) and the volumes of distribution were similar between animals (Table S6 and S7).

FS222 was generally well tolerated up to 30 mg/kg dosed weekly as determined by clinical

chemistry and histopathology results (Table 3).




21

As shown in Figure 6B the levels of serum sPD-L1 were quantified as a measure of direct

target engagement and indicative of downstream cell activation (22). Increased serum sPD-

L1 levels were observed in all animals on day 1, with an apparent peak at 168 hours post end

of infusion, following which the levels declined in line with the decline in the systemic levels

of FS222. Repeat administration of FS222 resulted in prolonged increase in serum sPD-L1 in

animals that were shown to have no or low levels of anti-drug antibodies (ADA). Consistent

with the findings of the study to assess the PD response of the surrogate FS222 in a syngeneic

mouse tumour model described previously, a drug-related increase in cell proliferation and

activation was also observed in NK cells (Figure 6C) and CD4+ and CD8

+ central memory T

cells (Figure 6D and 6E). In many animals, Ki67 expression reached plateau at Day 11,

remained high at Day 15, and deceased progressively to reach baseline expression between

Day 18 and 22 with a maximum response being observed between 3 and 10 mg/kg. A

moderate but transient increase in the relative percentage and absolute counts of CD4+

FoxP3+ regulatory T cells was also seen (Figure S4E).




22

Discussion

FS222, a CD137/PD-L1 tetravalent bispecific antibody, exhibited potent in vitro CD137-

mediated T cell activation upon engagement of PD-L1. No cross-reactivity was observed to

mouse CD137, therefore a mouse surrogate molecule was developed. Surrogate FS222

outperformed CD137 and PD-L1 monospecific mAbs as monotherapies or in combination in

multiple syngeneic mouse tumour models.

No liver pharmacology or toxicity, previously reported with other CD137 agonist mAbs, was

observed with FS222 or the mouse surrogate. Contrasting observations of liver toxicity in the

clinic, with CD137 mAbs urelumab and utomilumab, suggest that targeting of CD137 is not

an intrinsically toxic pathway for therapy, but the way it is targeted is crucial. Urelumab is a

potent fully human IgG4 antibody but causes dose dependent and on-target liver toxicity

whereas utomilumab demonstrates no dose limiting toxicity but weaker potency on a human

IgG2 backbone. FS222, although a human IgG1, had no Fc-mediated effector function and its

potent CD137 activity was dependent upon PD-L1 expression. This resulted in a highly

active molecule as seen in vitro and in vivo in multiple syngeneic tumour models, with no

liver toxicity. Furthermore, the results from our preliminary toxicology study indicated that

FS222, which is cross-reactive with cynomolgus monkey and has the same in vitro potency in

this species and human, had potent in vivo pharmacological activity in the cynomolgus

monkey and is well tolerated up to 30mg/kg.

Despite being able to bind cell-expressed human CD137, the Fcab component of FS222 was

unable to cluster and activate CD137 in the absence of PD-L1-mediated crosslinking, a

significant safety feature of the molecule. Coupled with directing CD137 activity to areas of

PD-L1 expression, for example tumour microenvironments, FS222 is designed to overcome

the adverse side effects associated with CD137 agonists currently in the clinic. This is further

strengthened by reducing FcR binding and allows FS222 to not rely on FcR-expressing

cells to provide the crosslinking necessary for CD137 clustering in current monoclonal

antibody therapies. The combination approach adopted for in vitro experiments of using two

separate monospecific antibodies relied not only on releasing the PD-1/PD-L1 blockade via

one antibody, but also on maximum crosslinking of CD137(20H4.9) mAb by a hCH2 specific

mAb. For this combination in the clinic, the natural crosslinking mechanism would be via

FcR crosslinking via the Fc region of IgG4-based urelumab (20H4.9). Not only are FcR-

expressing cells found throughout the body, therefore bringing another significant safety




23

concern for aberrant IgG crosslinking and CD137 agonism, they are also varied in prevalence

with diverse FcR expression levels making them an unreliable source of crosslinking-

dependent activation within a tumour (23). Therefore, FS222 mitigated high systemic toxicity

and variable anti-tumour activity by not relying on FcR crosslinking for potent site-specific

activity. FS222 also did not rely on Fc-mediated cell killing as a mechanism of action on

account of significantly reduced FcR binding. This resulted in highly potent activity which

did not come at the loss of important CD137- or PD-L1-expressing immune cells that can

then potentiate the cell-mediated tumour killing action.

FS222 showed potent activity in human primary T cell assays, but only when PD-L1-

expressing cells were present. There was also no activity when the CD137 Fcab was paired

with an irrelevant, non-PD-L1 binding Fab domain, HelD1.3. Surrogate FS222 had

comparable in vitro activity to FS222, therefore justifying its use in syngeneic mouse tumour

models. We believe the tumour control activity shown by surrogate FS222 addresses one of

the hurdles of treating a PD-L1 insensitive tumour, or one that has become refractory to PD-

L1 therapy. It does this by harnessing PD-L1 target expression in an alternative way to exert

direct cytotoxic T cell activation through CD137 engagement and clustering. The highly

immunogenic MC38 tumour model demonstrated insensitivity to PD-L1 mAb treatment and

Fc-disabled CD137 mAb treatment as monotherapies at the dose levels employed in this in

vivo study. However, despite also being Fc-disabled, treatment with surrogate FS222 resulted

in complete tumour eradication and 100% animal survival. In this model, PD-L1 expression,

presumably in the tumour microenvironment, provided a setting in which surrogate FS222

can exert superior activity and efficacy to either monotherapy. This is clearly through PD-L1-

dependent crosslinking of FS222 and CD137 receptor clustering on T cells resulting

ultimately in enhanced tumour-specific CD8+ T cell cytotoxic activity. Surrogate FS222 and

therefore FS222 could be bridging a PD-L1-expressing tumour cell and tumour infiltrating T

cell, localising T cell cytotoxic activation to the tumour cell/T cell interface. The same is true

for the significant activity and superiority over monotherapy of surrogate FS222 in the less

immunogenic CT26.WT model (24), which in our hands is also insensitive to PD-L1 mAb

treatment.

The pharmacodynamic (PD) changes after treatment with surrogate FS222 in a CT26.WT

tumour-bearing mouse model were investigated after administration of a single high (10

mg/kg) and a single low (1 mg/kg) dose. Strikingly, T cells with bound drug were present in

the tumour at 2h post-administration for both dose levels. These T cells had prolonged PD-L1




24

occupancy specifically in the tumour, the longevity, but not magnitude, of which was

correlated with dose. By the end of the study (8 days post drug administration) intra-tumoural

PD-L1 occupancy was still ~80% on T cells at 10 mg/kg, whereas peripheral PD-L1

occupancy on T cells had decreased substantially. This highlights the potential of surrogate

FS222 to locate to the tumour microenvironment, in preference to remaining in the periphery.

Evidence of cytokine production as a consequence of surrogate FS222 treatment was also

observed in the serum of mice treated with 10 mg/kg, whereas this was less pronounced with

the lower dose level perhaps indicating efficacy without systemic cytokine exposure.

Given the relevance of preclinical studies in mice for risk assessment of severe hepatoxicity

in human patients treated with CD137 agonist agents, the lack of hepatotoxicity in mice in

these studies indicates that a mAb2 agonising CD137 via PD-L1-mediated crosslinking has a

significantly reduced risk of inducing hepatoxicity in human patients treated at therapeutic

doses. FS222 had a mean terminal half-life of approximately 6 days in cynomolgus monkey,

in line with PD-L1 targeting antibodies such as atezolizumab and followed a linear dose

response at dose levels ≥ 1 mg/kg. FS222 was generally well tolerated up to 30 mg/kg dosed

weekly. With comparable potency between the cynomolgus and human immune systems, we

believe these findings will translate successfully to a human setting. Both the surrogate

molecule in mouse, and FS222 in cynomolgus monkeys caused a drug-related increase in T

cell proliferation and activation as measured by Ki67 expression. This would indicate that the

PD responses seen in our mouse models translate to the effect of FS222 on cynomolgus

monkey T cells.

In summary, we have developed FS222, a CD137/PD-L1 tetravalent bispecific antibody with

a novel mode of action and potentially improved therapeutic index for the treatment of human

cancer. FS222 did not cause evident toxicity in cynomolgus monkey upon repeated dosing

which we believe further encourages clinical development targeted at tumours where a

significant unmet medical need exists in immunotherapy. Checkpoint inhibitors are failing or

only providing modest clinical benefit in many tumour settings and for many of those we feel

there is a mechanistic rationale for improvement in clinical outcomes with FS222.




25

Acknowledgments

The authors would like to thank the F-star Protein Sciences, in vivo, Assay Development and

Drug Discovery team; Cristian Gradinaru for statistical analyses; Jacqueline Doody for

scientific contributions; Alison McGhee for critical review; Babraham BSU staff members

for animal husbandry and technical assistance; Dr Sarah Taplin for pathology assessment; Dr

Sarah Burl and Natalie Allen for manuscript editing and review.




26

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Tables

Table 1. Summary table of tumour free animals by end of study

Compound

IgG

Contr

ol

PD

-L1(S

70)

CD

137(L

ob

12.3

)

PD

-L1(S

70)

+

CD

137(L

ob

12.3

)

Surr

ogat

e F

S222

CT26 tumour-

free animals

Number 0/12 0/12 1/12 N/A 5/12

Percent 0 % 0 % 8 % N/A 42 %

MC38 tumour-

free animals

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

Percent 0 % 17 % 17 % 33 % 100 %

Table 2. Increased dose of surrogate FS222 correlated with increased survival

Surrogate FS222 Median Survival

(days)

p-values Log-rank (groupwise

comparison with lower dosed group)

10 mg/kg 39 0.2

1 mg/kg 29.5 0.02

0.3 mg/kg 24 0.007

0.1 mg/kg 21 0.6

0 mg/kg (IgG Control) 21

Table 3. Changes in clinical chemistry parameters relating to liver function of cynomolgus

monkey in FS222 repeat dose phase

Lower limit of

normal range FS222

Upper limit of

normal range

AST (U/L) 20 23-69 94




30

ALT (U/L) 21 19-111 112

ALP (U/L) 140 485-1310 1350

TBIL (mg/dL) 0.06 0.07-0.38 0.43




31

Figure Legends

Figure 1. mAb2 structure and concurrent high affinity binding of CD137/PD-L1 mAb

2

equivalent to individual component antibodies

(A) Representation of the bispecific CD137/PD-L1 mAb2 FS222 on a human IgG1

backbone with FcR-binding removed by L234A and L235A (LALA) mutations

highlighted in green. The PD-L1 complementarity-determining regions (CDR) of

heavy and light chains are highlighted in orange. The CD137 CH3 domain AB and EF

binding loops are highlighted in cyan.

(B) FS222 simultaneous binding to both human PD-L1 and human CD137 as determined

by SPR.

(C) FS222 binding to DO11.10 T cells expressing human CD137 as determined by flow

cytometry.

(D) FS222 binding to HEK cells expressing human PD-L1 as determined by flow

cytometry.

(E) FS222 binding to in vitro activated human primary CD4+ and CD8

+ T cells as

determined by flow cytometry.

Figure 2. CD137 agonism via FS222 is dependent upon crosslinking via PD-L1 in a

human primary T cell assay and has activity superior to monoclonal antibody

combinations in a mixed lymphocyte reaction

(A) FS222 activity in a human primary CD8+ T cell activation assay with varying ratios of

HEK cells that are positive for PD-L1 to HEK cells that are negative for PD-L1.

Significance determined by extra sum-of-squares F test. ***, P < 0.001.

(B) FS222 activity in mixed lymphocyte reaction against monospecific component parts

that make up the complete FS222 mAb2 either alone or in combination with each

other. Significance determined by extra sum-of-squares F test. ***, P < 0.001.

Figure 3. In vitro characterisation of surrogate FS222 and in vivo efficacy and survival

in two syngeneic mouse tumour models




32

(A) Surrogate FS222 activity in an OT-1 CD8+ mouse T cell activity assay with cell-based

crosslinking provided by B16-F10 tumour cells expressing mouse PD-L1.

Significance determined by extra sum-of-squares F-test. ***, P < 0.001.

(B) Individual tumour growth spaghetti plots for CT26.WT tumour-bearing mice treated

on day 7, 9, and 11 post-tumour inoculation with 10 mg/kg Ctrl(HelD.13), PD-

L1(S70), CD137(Lob12.3), or surrogate FS222.

(C) Survival data for CT26.WT tumour-bearing mice treated with 10 mg/kg

Ctrl(HelD.13), PD-L1(S70), CD137(Lob12.3), or surrogate FS222.

(D) Survival data for MC38 tumour-bearing mice treated with 1 mg/kg Ctrl(4420), PD-

L1(S70), CD137(Lob12.3), PD-L1(S70) + CD137(Lob12.3), or surrogate FS222.

(E) Individual tumour growth spider plots for MC38 tumour-bearing mice treated on day

7, 9, and 11 post-tumour inoculation with 1 mg/kg Ctrl(4420), PD-L1(S70),

CD137(Lob12.3), PD-L1(S70) + CD137(Lob12.3), or surrogate FS222.

Figure 4. Surrogate FS222 was tested for efficacy and survival in a CT26.WT syngeneic

mouse tumour model dose-range finding study

Kaplan-Meier survival plot of dose-range finding study in CT26.WT of surrogate

FS222 dosing in the range 0.1 to 10 mg/kg showing significant survival benefit of

increasing doses of surrogate FS222 above 0.1 mg/kg compared to IgG control.

Figure 5. Surrogate FS222 was tested for pharmacodynamics and liver pharmacology in

a CT26.WT syngeneic mouse tumour model

(A) The percentage of CD8+ and CD4

+ T cell populations in the tumour or blood

determined by flow cytometry to be positive for bound surrogate FS222 in a

pharmacodynamic study for CT26.WT tumour-bearing mice upon treatment with 1

dose of surrogate FS222.

(B) The frequency of Ki67+ CD8

+ T cells or Ki67

+ CD4

+ T cells in the tumour or blood as

determined by flow cytometry similarly to (A)

(C) PD-L1 receptor occupancy of CD8+ and CD4

+ T cells in the tumour or blood as

determined by normalising to a negative control (cells isolated from Ctrl(4420) mAb

treated mice set to 0% receptor occupancy at each time point, black circles) and a

positive control (cells isolated Ctrl(4420) mAb treated mice which were then




33

saturated with surrogate FS222 set to 100% receptor occupancy at each time point,

black triangles).

(D) Serum cytokine levels as determined by MSD analysis, significance determined by

two-way ANOVA and shown for surrogate FS222 10 mg/kg group vs Ctrl(4420)

mAb group. *, P < 0.05; **, P < 0.001.

(E) CD3+ T cells (as a percentage of total CD45

+ immune cells) in the liver of treated

mice as determined by flow cytometry.

(F) Proliferating CD8+ T cells present in the liver, using Ki67 expression as a marker of

proliferation, as determined by flow cytometry.

Figure 6. Non-GLP PK/PD toxicity study of single and repeat dosing of FS222 in

cynomolgus monkey

(A) Pharmacokinetic profile of FS222 in cynomolgus monkeys (SD = single dose).

(B) Kinetic changes in serum sPD-L1 levels after repeat dosing with FS222.

(C) Kinetic changes in peripheral NK cell frequency expressing Ki67 after repeat dosing

with FS222.

(D) Kinetic changes in peripheral CD4+ central memory cell frequency expressing Ki67

after repeat dosing with FS222.

(E) Kinetic changes in peripheral CD8+ central memory cell frequency after repeat dosing

with FS222.






















Published OnlineFirst April 28, 2020.Clin Cancer Res Matthew A Lakins, Alexander Koers, Raffaella Giambalvo, et al. modelslow toxicity and anti-tumor activity in colorectal cancer FS222, a CD137/PD-L1 tetravalent bispecific antibody exhibits

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FS222, a CD137/PD-L1 tetravalent bispecific antibody exhibits low … · 1 FS222, a CD137/PD-L1 tetravalent bispecific antibody exhibits low toxicity and anti-tumor activity in colorectal