© 2017 CytomX Therapeutics, Inc.

An EGFR Targeting ProbodyTM T cell Bispecific

Induces Tumor Regressions While Reducing On-target

Toxicities in Preclinical Studies

Leila Boustany, Ph.D.

Sr. Scientist1

PROBODY is a trademark of CytomX Therapeutics, Inc.

Antibody Engineering and Therapeutics

12/14/17

• Bring cytotoxic T cells and cancer cells together

• Highly potent, but toxic modality

• Challenging to use for solid tumors: unforgiving for target expression on normal tissue

• Poor exposure

T-cell Engaging

Bispecific

Antibodies

• Potent anti-tumor activity

• Less systemic toxicity by avoiding T cell engagement outside of tumor

• Better exposure

• Expanded utility, especially for solid tumor targets

Potential

advantages of

T-cell Engaging

Probody

Bispecific

Therapeutics

T-Cell Engaging

Bispecifics

Target Cell

T Cell

Nature Biotechnology 23, 1065 - 1072 (2005)

T Cell Engaging Bispecific (TCB) Therapeutics:

Highly Potent, but Associated with Safety Liabilities

2

Probody Therapeutics are Antibody Prodrugs Designed

to be Activated in the Tumor Microenvironment

3

ANTI-CANCER

ANTIBODY

CLEAVABLE LINKER

MASKING PEPTIDE

PROTEASES

HEALTHY TISSUE TUMOR TISSUE

Probody Therapeutics:

• “Masked” to limit binding to normal tissue

• “Un-masked” by tumor-associated

proteases

• Linkers designed to be cleaved by

multiple proteases for utility across tumor

types

• Validated targets:

➢ Improve therapeutic window

• Difficult to drug targets:

➢ Create therapeutic window

• Applicable to all immunoglobulin formats

Normal Colon Primary Colon Cancer Metastatic Colon Cancer

IMAGING OF ACTIVE PROTEASE2

Activated Proteases are Prevalent in Tumors

But Not in Healthy Tissues

• Upregulated protease

activity is a hallmark of all

cancers

• Protease activity is tightly

controlled in healthy tissues

4

1. Sevenich, et. Al. Gene & Dev., 2014; 2. Matriptase: LeBeau, et al., PNAS 2012

PRIMARY TUMOR METASTASIS1

ANGIOGENESIS

INFLAMMATION

INTRAVASATIONINVASION

PROLIFERATION

AND SURVIVAL

EXTRAVASATION

COLONIZATION

AND OUTGROWTH

Preclinical Proof of Concept Achieved for Multiple

Antibody Modalities & Targets

5

Antibody

Drug Conjugates

T-Cell

Bispecifics

(TCBs)

CARs

Immune

Modulators/

Checkpoint

Inhibitors

EGFR as a Target for Probody TCB Modality

• Prevalent EGFR expression

– EGFR Probody TCB expected to be potent against both high and low

expressing cancers

• Expression in immunogenic indications

– Opportunity to combine with IO agents

• Unlock EGFR potential

– M.O.A. does not rely on EGFR signaling blockade

– Less concern with acquired resistance, e.g., activating mutations in NSCLC,

KRAS/BRAF mutations in CRC

6

CytomX Probody T Cell-engaging Bispecific (Pb-TCB)

• Full IgG bispecific format to maximize half-life, improve expression

• Fc-effector impaired to minimize cross-linking by FcR-bearing cells

• Format optimized for a-CD3 affinity, mask strength and cleavable substrates

a-EGFR

a-CD3

a-EGFR

a-CD3

Activated Bispecific (act-TCB)

masks

Protease

substrate

Fc effector

mutant

Probody Bispecific (Pb-TCB)

Cleavage by tumor

associated proteases

7

1 0 -3 1 0 -2 1 0 -1 1 0 0 1 0 1 1 0 2 1 0 3 1 0 4

0

5 0 0 0

1 0 0 0 0

1 5 0 0 0

2 0 0 0 0

2 5 0 0 0

C D 3 B in d in g (J u rk a t)

[A c t/P b -T C B ] n M

MF

I (c

orre

cte

d)

P b -T C B

a c t-T C B

1 0 -3 1 0 -2 1 0 -1 1 0 0 1 0 1 1 0 2 1 0 3 1 0 4

0

5 0 0 0

1 0 0 0 0

1 5 0 0 0

2 0 0 0 0

2 5 0 0 0

3 0 0 0 0

3 5 0 0 0

4 0 0 0 0

E G F R B in d in g (H T 2 9 )

[A c t/P b -T C B ] n M

MF

I (c

orre

cte

d)

P b -T C B

a c t-T C B

Pb-TCB Demonstrates Reduced Antigen Binding in vitro

8

Masking reduces binding to EGFR and CD3 on cells

Pb-TCB Attenuates Cytotoxicity and T cell Activation in vitro

9

1 0 -4 1 0 -3 1 0 -2 1 0 -1 1 0 0 1 0 1 1 0 2 1 0 3 1 0 4 1 0 5 1 0 6

0

2 0

4 0

6 0

8 0

1 0 0

H T 2 9 C y to to x ic ity

[A c t/P b -T C B ] p M%

Cy

to

to

xic

ity

P b -T C B

n o n E G F R b in d in g P b -T C B

a c t-T C B

• Pb-TCB protects >5 orders of magnitude in vitro

• High target density not required for potent cytotoxicity

• Masking shifts T cell activation curve

1 0 0 1 0 1 1 0 2 1 0 3 1 0 4 1 0 5 1 0 6 1 0 7

0

2 0

4 0

6 0

8 0

C D 8 T c e ll A c t iv a t io n

[A c t/P b -T C B ] p M

CD

69

(%

po

sit

ive

)

P b -T C B

a c t-T C B

UT

10

20X mag

NSUB* Pb-TCB2Pb-TCB1 Act-TCB

Increasing Pb-TCB Protease Sensitivity Enhances Tumor

T cell Infiltration and Efficacy

PBS

T cell infiltration 7 days after 1 mpk dose

• Tumor T cell infiltration is associated with efficacy

• More TILs observed in tumors treated with

Pb-TCB2

Weekly dosing

0.3 mpk

(Brown staining is CD3+ cells)

(Substrate Cleavability: Pb-TCB2> Pb-TCB1> NSUB*)

*NSUB is a Pb-TCB devoid of a cleavable substrate0 5 1 0 1 5 2 0

0

2 0 0

4 0 0

6 0 0

H T 2 9 X e n o g ra f t

S tu d y D ay

Tu

mo

r V

olu

me

(m

m3)

N S U B *

P b -T C B 2

P B S

a c t- T C B

P b -T C B 1

EGFR-CD3 Pb-TCB Regresses Established Tumors In

KRAS or BRAF Mutated Xenograft Models

Human PBMCs engrafted into HT29 and HCT116 Tumor-bearing NSG mice

HT29 Xenograft HCT116 Xenograft

11

Dosed weekly: days 1, 8, 15

0 5 1 0 1 5 2 0 2 5

0

2 0 0

4 0 0

6 0 0

8 0 0

S tu d y D a y

Tu

mo

r V

olu

me

(m

m3

+ S

EM

)

P B S

P b -T C B 1 .5 m p k

P b -T C B 0 .5 m p k

0 5 1 0 1 5 2 0 2 5

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

S tu d y D ay

Tu

mo

r V

olu

me

(m

m3

+ S

EM

)

P b -T C B , 0 .3 m p k

P b -T C B , 1 m p k

P B S

a c t-T C B , 0 .3 m p k

EGFR-CD3 Pb-TCB Protects against Acute Toxicities and

Increases MTD in Non-Human Primates

12

• Observations at MTD include transient, mild to moderate T cell-mediated toxicities

• Pb-TCB is well tolerated in cynomolgus monkeys up to MTD of 4 mg/kg

• Masking enables >60 fold increase in MTD relative to the unmasked TCB

IL-6 @ 8 hrs IFNg @ 8 hrs T cell Proliferation @ 72 hrs AST @ 48 hrs

0 .0 1 0 .1 1 1 0

0

1 0 0 0 0 0

2 0 0 0 0 0

3 0 0 0 0 0

4 0 0 0 0 0

D o s e (m g /k g )

pg

/m

l

a c t-T C B

P b -T C B

0 .0 1 0 .1 1 1 0

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

D o s e (m g /k g )

pg

/m

l

0 .0 1 0 .1 1 1 0

0

2 0

4 0

6 0

8 0

D o s e (m g /k g )

% K

i-6

7+

CD

4+

0 .0 1 0 .1 1 1 0

0

2 0 0

4 0 0

6 0 0

8 0 0

D o s e (m g /k g )

U/L

Observed Cyno Activity is Largely Dependent on EGFR Binding

EGFR-Independent Activity of the Pb-TCB is Undetectable in Cynos at 2 mg/kg

13

Pre

do

se

1 h

r

4 h

r

8 h

r

24 h

r

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

IL -6

IL-6

(p

g/m

L)

Pre

do

se

72 h

r

7 d

ays

0

2 0

4 0

6 0

8 0

1 0 0

T c e ll a c tiv a tio n

% P

D-1

+C

D4

+

n o n E G F R b in d in g P b -T C B

P b -T C B

Pre

Day 3

Day 8

0 .0

0 .2

0 .4

0 .6

T o ta l B ili

mg

/dL

EGFR-CD3 Pb-TCB Extends PK in Non-Human Primates

14

Masking markedly extends PK of the Pb-TCB relative to the unmasked TCB

0 2 4 4 8 7 2 9 6 1 2 0 1 4 4 1 6 8 1 9 2

0 .0 1

0 .1

1

1 0

1 0 0

1 0 0 0

P la s m a C o n c e n tra tio n

H o u rs P o s t-D o s e

nM

a c t-T C B , 0 .1 8 m p k

a c t-T C B , 0 .0 6 m p k

P b -T C B , 2 m p k

EGFR-CD3 Pb-TCB Preclinical Summary

• Efficacy: Potent anti-tumor activity despite strong masking

– Greatly attenuates target binding and T cell mediated cytotoxicity in vitro

– Is highly effective in vivo, regressing established tumors in KRAS and BRAF

mutant xenograft models

• Safety: Increased tolerability in cynos despite much higher

exposure

– Probody TCB MTD is >60 times that of the corresponding unmasked TCB

– Tolerated Probody TCB serum concentration is significantly greater than that of

the unmasked TCB

15

Acknowledgements

16

Laurie Wong Hong Lu

Clayton White Shouchun Liu

Linnea Diep Jennifer Richardson

Yuanhui Huang Bryan Irving

Sherry La Porte Michael Kavanaugh