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Days Post Implant

VehicleHPN424, 2 µg/kgHPN424, 10 µg/kgHPN424, 50 µg/kgHPN424, 250 µg/kg

HPN424, a half-life extended, PSMA/CD3-specific TriTAC for the treatment of metastatic prostate cancerBryan Lemon, Wade Aaron, Richard Austin, Patrick A. Baeuerle, Manasi Barath, Adrie Jones, Susan D. Jones, Kathryn Kwant, Che-Leung Law, Anna Muchnik, Kenneth Sexton, Laurie Tatalick, Holger Wesche, Timothy Yu. Harpoon Therapeutics, South San Francisco, CA.

PEGS Boston 2018 www.harpoontx.com

• Metastatic castration-resistant prostate cancer(mCRPC) kills 27,000 patients in the US each year

• Once mCRPC has metastasized beyond regionallymph nodes the 5-year survival rate is 30%

• Abiraterone and Enzalutamide have improvedthe treatment options for mCRPC but newcurative therapies are urgently needed

• PSMA is expressed in >90% of malignant lesionsof mCRPC patients

• In normal tissues outside the central nervoussystem PSMA expression is largely restricted tothe prostate

• HPN424 is a Tri-specific T cell ActivatingConstruct designed to direct T cells to killprostate cancer cells

HPN424 is a PSMA-targeting TriTAC

HPN424 has a half-life of ~ 3.3 days in cynomolgus monkeys

HPN424 dosing qd x 10, d5 to d14

Admixture model, d05 x 106 human PBMC

+ 22Rv1 prostate cancer cells

HPN424 potently inhibits growth of 22Rv1 xenografts

• Single dose pharmacokinetic study with two subjects per dose level• PK results suggest once or twice weekly dosing in humans

HPN424 potently directs T cell killing of PSMA expressing cells

No observed adverse effect level was 3 mg/kg, qw x 4• Similar results observed with 1 mg/kg and 0.1 mg/kg doses• CD3 and albumin binding domains cross-react with cyno

targets, modest binding of HPN424 to recombinant cyno PSMA• Pharmacodynamic effects consistent with T cell engagement

- Transient reduction in circulating T cells, NK cells, and monocytes- Upregulation of activation markers (CD25 & CD69) in the

remaining circulating T cells- Mild and transient increase in cytokines (IFNγ, IL-6, IL-10) with

first dose, changes after fourth dose much less pronounced

No adverse histopathology findings

• Rapid decline of circulating T cells within 8 hrs post dose• Much less lymphocyte margination after 4th and final dose

HPN424 induces transient T lymphocyte margination and activation

HPN424 induces limited cytokines,no evidence of cytokine release syndrome

HPN424 is highly tolerated with repeat dosing in cynomolgus monkeys

PSMA-dependent activation of T cells by HPN424

Dose(mg/kg)

Terminal t1/2 (h)

Cmax(nM)

AUC0-168 hr(hr*nM)

AUC0-inf(hr*nM)

CL (mL/hr/kg)

Vss(mL/kg)

0.1 79.9 65.5 3530 3570 0.533 51.10.03 80.9 25.4 1260 1270 0.450 46.7

• HPN424 induced CD69 expression on T cells in the presence of LNCaP cells not observed with an αGFP-TriTAC as measured by FACS

• HPN424 induced TNFαexpression by T cells in the presence of LNCaP cells but not with PSMA-negative HCT116 cells as measured by ELISA

CD45+/CD3+/CD4+/CD69+

CD45+/CD3+/CD4+/CD69+

Vehicle 8 h post first dose 3 mg/kg 8 h post first dose

Side

scat

ter

CD45+/CD3+/CD4+/CD69+ CD45+/CD3+/CD4+/CD69+

Side

scat

ter

CD69

Cancer cell killing

Cytolyticsynapse

Prostate cancer cell

PSMA/FOLH1

T cell

CD3

anti-PSMA sdAb

anti-ALB sdAb

anti-CD3ε scFv

RATIONALE

TriTAC~50 kDa

Solid Tumor

Activity

Safety

Small size for diffusion-controlled solid tumor penetration

Optimized CD3 binding to address T cell-mediated clearance

No potential for Fc receptor binding

Monovalent CD3 binding minimizes non-specific T cell activation

Albumin binding for extended serum half-life

Single polypeptide Protein A purification Very stable

Manufacturing

Convenience

TriTAC platform optimized for treatment of solid tumors

BIOPHYSICAL CHARACTERIZATION

Concentration Normalized DSCThermograms

Process

T0

5x F/T

40°C 2wk4°C 2wk

Bfr

GFstds

StabilityPurity

SDS - PAGE4 - 20% TRIS Glycine

1 Non-Reduced2 x3 MW Standards4 Reduced

Coomassie stain2.5ug / lane

-- 200

-- 116-- 97

-- 66

-- 45

-- 31

-- 21

-- 14-- 6

1 2 3 4

Differential Scanning CalorimetryHPN424 Tm1 (°C) Tm2 (°C)

Replicate 1 58.78 61.12Replicate 2 59.18 61.02

Average 58.98 61.07

HPN424 Analytical SEC

Condition%

HMW%

Main%

LMWT0 1.6 94 4.4

5x F/T 1.9 94.4 3.740˚C 2 wk 2 93.4 4.64˚C 2 wk 1.8 94.5 3.8

Sedimentation VelocityDistribution

Analytical Ultra Centrifugation

HPN424

Main Peak HMW Peak Frictional Ratio (ffo)

s-value

% Area

MW (kDa)

s-value

% Area

MW (kDa)

Average 2.63 98.5 45.3 4.43 2.5 102 1.73

Depthfiltration

Protein Aaffinity

Low pH viralinactivation

Ion exchangestep 1

Ion exchangestep 2

Viralfiltration

Ultrafiltrationdiafiltration

HPN424 binds to cells expressing PSMA and to T cells

HPN424

Control

Biolayer Interferometry PSMA* ALB CD3ε*

Human KD (nM) 0.5 8 12

Cyno KD (nM) indeterminate 7.7 10

0.3

0.2

0.1

0

nm

0 200 400 600Time (sec)

Binding to human PSMA

HPN424 affinity for human and cyno PSMA, ALB, and CD3ε

Binding to MDAPCa2b cells

Rela

tive

Cell

Num

ber

FACS FITC

Binding to human T cells

Rela

tive

Cell

Num

ber

FACS Alexa fluor 647

1

0.5

00 100 200 400

Time (sec)300

Binding to human albumin

0.5

1

00 100 200 400

Time (sec)300

Binding to human CD3ε

* Binding measured in the presence of 15 mg/ml human serum albumin

IN VITRO BINDING

IN VITRO PHARMACOLOGY

• Prostate cancer cell lines incubated with human T cells and HPN424 or control • Viability of cancer cells assessed by measuring luciferase activity at 48 hours

HPN424 directed killing of four prostate cancer cell linespotencies determined with four T cell donors

Cell LinePSMA

Expression

EC50 Values (pM)Donor

24Donor 8144

Donor 72

Donor 41

LNCaP Positive 1.5 0.22 0.36 0.43MDAPCa2b Positive 4.8 0.41 0.49 0.65

VCaP Positive 0.64 0.16 0.2 0.3522Rv1 Positive n/a 0.72 1.4 1.3

HCT116 Negative >10,000 >10,000 >10,000 >10,000NCI-1563 Negative >10,000 >10,000 >10,000 >10,000

-16 -14 -12 -10 -8 -60

50

100

TriTAC Concentration log(M)N

orm

aliz

ed V

iabi

lity

(%) Donor 24 HPN424

Donor 24 αGFP TriTAC

Donor 8144 HPN424

Donor 8144 αGFP TriTAC

Donor 72 HPN424

Donor 72 αGFP TriTAC

Donor 41 HPN424

Donor 41 αGFP TriTAC

Killing of VCaP cells with T cells from four different donors

IN VIVO PHARMACOLOGY

PHARMACOKINETICS SUMMARY

SAFETY

• HPN424 TriTAC is a stable, manufacturable, single chain moleculethat binds with high affinity and specificity to PSMA, CD3 andserum albumin

• HPN424 potently activates and redirects T cells to kill PSMAexpressing cells in both in vitro and in vivo prostate cancermodels

• HPN424 has a long serum half life and was very well tolerated,even at high doses, in cynomolgus monkeys

• With its small size, HPN424 is anticipated to be more able topenetrate solid tumors than antibodies

• HPN424 is expected to be a safe, effective, and convenienttreatment for patients with metastatic castration resistantprostate cancer

• HPN424 is anticipated to enter the clinic in 2018

CD69

• Transient, dose-dependent increases in IL-6 and IL-10 as similarly reported with other bispecific T cell engagers; no observable trend of increase in IL-2, -4, -5, TNFα, or IFNγ

• First dose effect; no cytokine increase after the fourth and last dose

-16 -14 -12 -10 -80

10

20

30

40

50

TriTAC log(M)

%CD

69P o

siti v

eC e

ll s

HPN424 Donor 24

HPN424 Donor 41

HPN424 Donor 72

HPN424 Donor 8144

αGFP TriTAC Donor 24

αGFP TriTAC Donor 41

αGFP TriTAC Donor 72

αGFP TriTAC Donor 8144

-15 -14 -13 -12 -11 -10 -9 -8 -70

40000

80000

120000

TriTAC log(M)

TNFα

( RLU

) LNCaP Donor 41

LNCaP Donor 72

HCT116 Donor 72

HCT116 Donor 41

0 7 14 210.01

0.1

1

10

100

Time (days)

Seru

mH

PN42

4(n

M)

0.03 mg/kg0.1 mg/kg

0

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8 0 0 0

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v e h i c l e

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Vehicle

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1 mg/kg

3 mg/kg

CD45

+/CD

3+ T

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ls/µ

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Day 1Pre-dose

Day 18 h

Postdose

Day 124 hPostdose

Day 22Pre-dose

Day 228 h

Postdose

Day 2224 hPostdose

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

LLOQ

Pre-dose

4 hPostdose

8 hPostdose

24 hPostdose

48 hPostdose

168 hPostdose

IL10

(pg/

ml)

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Vehicle

0.1 mg/kg

1 mg/kg

3 mg/kg

LLOQ

Pre-dose

4 hPostdose

8 hPostdose

24 hPostdose

48 hPostdose

168 hPostdose

IL6

(pg/

ml)