A Single Dose of CD117 Antibody Drug Conjugate Enables Hematopoietic Stem Cell Based Gene Therapy in Nonhuman Primates

John F. Tisdale1, Naoya Uchida1, Robert E. Donahue1, Bradley R Pearse2, Sean M McDonough2, Jennifer

L Proctor2, Cindy Dunbar1, Allen E. Krouse1, Nathaniel S. Linde1, Aylin C. Bonifacino1, Sogun Hong1, Rajiv

Panwar2, Ganapathy Sarma2, Lena Kien2, Kellie Latimer2, Junia Dushime2, Sharon L Hyzy2, Melissa

Brooks2, Rahul Palchaudhuri2, Qing Li2, Pranoti Sawant2, Charlotte F McDonagh2, Anthony E Boitano2,

Michael P Cooke2

2 Magenta Therapeutics, Cambridge, MA

1 National Heart, Lung, and Blood Institute (NHLBI) and National Institute of Diabetes, Digestive, and Kidney Diseases (NIDDK), NIH. Bethesda, MD

HSC Based Gene Therapy: The Patient Journey to a Cure

ADC-mediated

conditioning

Opportunity:

• Less toxic conditioning

• Immune preservation

• Avoid secondary malignancy and infertility

• Improve risk benefit profile to broaden access

Targeted Conditioning Using Antibody Drug Conjugates

CD117 (C-kit) is an Ideal Target for ADC-Mediated Conditioning

Rapid internalization

Restricted expression profile

limited systemic toxicity

A single dose of CD117 ADC

enables robust engraftment in mice

Czechowicz, Palchaudhuri et al. Nat Comm 2019

Isotype

ADC

Anti-mCD117

ADC

PBS

Hematopoietic Stem

Cell

Common Myeloid Progenitor Common Lymphoid Progenitor

Megakaryocyte

Platelet

Erythrocyte Mast

Cell Myeloblast

Basophil

Neutrophil

EosinophilMonocyte

Macrophage

Natural killer cell

Small Lymphocyte

T cell B cell

Plasma cell

-16 -15 -14 -13 -12 -11 -100

25

50

75

100

125

Anti-CD117 ADC is Potent on Primary Human and NHP CD34+ Cells

Log [M]

% V

iab

le C

D34+

Human

EC50 = 0.2 pMPrimate

EC50 = 0.1 pM

Hematopoietic Stem Cell Killing Assay

Primate

Isotype

Human

Isotype

A Single Dose of Anti-CD117 ADC Enables Full Depletion of Human CD34+ Cells In Vivo

% Remaining

Relative to Control

0

5

10

15

20

100

200

0.3 1.0Control 3

Isotype

ADC

mg/kg

Human CD34+ Cell Depletion

1

Anti-

CD117Anti-CD117 ADC

3.0

Assess HSC depletion in

bone marrow

Humanized NSG Mice

CD117-ADC

(Single dose, i.v.)

Day 21Day 0

Single Dose of CD117 ADC Selectively Depletes HSCs in Primates

Hematopoietic Stem

Cell Depletion

CD

34

CD90

CD

3

CD20

Baseline

Maintenance of

Lymphocytes

Day 6 Post-

Dose

CD117-ADC Depletes Functional HSC

Pre-dose Day 60

250

500

CF

U (

x10

3)

/ m

L o

f

Bone

Marr

ow

Assess HSC

depletion in

bone marrow

Primates

CD117-ADC

(Single dose, i.v.)

Day 6Day 0

CD117 ADC Is Engineered for Rapid Clearance and Optimal Timing for Graft Infusion

0 1 2 3 4 5 6 70.001

0.01

0.1

1

10

100

1,000

10,000

Plasma ADC

Concentration (ng/ml)

Days Post-Dose

Engineered

CD117-ADC

Rapid Clearance Enables Graft Infusion Within

Days Post Dosing

Lower limit of

detection by ELISA

ADC in

circulation

Effective concentration that is not

cytotoxic to CD117 expressing cells

Wild Type

CD117-ADC

Modeled

Pharmacokinetics

Safe to

infuse graft

t1/2=10hrs

Treatment Schema For Gene-Marked Autologous Transplant in CD117-ADC Conditioned Primates

Single Dose

CD117-ADC

Cohort

Multi-Dose

Busulfan Cohort

(Clinical Regimen)Uchida et al.

Mol Ther 2019

Primate

Primate

Single Dose of Anti-CD117 ADC is Sufficient to Enable Autologous Gene Modified Transplant in Rhesus

Days Post-Transplant

Ne

utr

op

hils

(1

03/u

l)

Lym

ph

ocyte

s (

10

3/u

l)

Pla

tele

ts (

10

5/u

l)

NHP #1

NHP #2

Conditioning

RegimenAnimal Number

Neutrophil

Recovery

Platelet

RecoveryLymphocyte Counts

Days post Transplant Range (103/uL)

CD117 ADCNHP #1 8 10 0.9-4.2

NHP #2 10 11 0.9-4.0

Busulfan Busulfan Cohort*Uchida et al. Mol Ther 2019

8-10 10-15 0.6-20

Days Post-Transplant Days Post-Transplant

Neutrophil Recovery Platelet Recovery Lymphocytes Counts

ANC

500

50,000

-6 1 8 150

200

400

600

20 40 60 80 100-6 1 8 150.01

0.1

1

10

100

20 40 60 80 100 -6 1 8 150.1

1

10

20 40 60 80 100

CD117ADC

11

0 30 60 90 12010 -3

10 -2

10 -1

100

Peripheral Granulocyte Vector Copy Number is Stable Over Time and Comparable to Historical Data with Busulfan Conditioning

Conditioning

Regimen

Animal

Number

CD34 dose

(x106 cells/kg)

VCN of

infused cells

Peripheral VCN

@ 1-6 months

Anti-CD117 ADCNHP #1 3.3 5 0.01-0.04

NHP #2 1.1 4 0.01-0.05

Busulfan Busulfan Cohort*Uchida et al. Mol Ther 2019

4.1-4.2 8-10 0.004-0.08

Peripheral Granulocyte β-globin Vector Copy Number

Ve

cto

r C

op

y N

um

ber

Days Post-Transplant

NHP #1

NHP #2

VCN range with

busulfan conditioning

CD117-ADC is Well-Tolerated in Primates

-10 0 10 20 30 40 50 60 70 80 90 1000

100

200

300

400

500

Days Post-Transplant

AS

T (

U/L

)

Normal Range

No ADC-related changes outside normal

range for: ALT, LDH, Albumin, Cr, BUN, TBIL,

ALP, Glucose

NHP #1

NHP #2

No Changes in Liver Serum

Chemistries Including AST Busulfan Side EffectsOutcomes With CD117-

ADC

Emesis Not observed

Diarrhea Not observed

Mucositis Not observed

Wasting Syndrome Not observed

Seizures Not observed

Veno-occlusive Disease Not observed

Pulmonary Fibrosis Not observed

1-3x above upper

limit of Normal

3-5x above upper

limit of Normal

Conclusions

• The engineered CD117 ADC shows potent activity on human and NHP CD34+ cells ex vivo

• The CD117-ADC robustly depletes rhesus HSCs in vivo, has a favorable safety profile, spares the immune system and is cleared rapidly to allow for optimal timing of graft infusion

• A single dose of the CD117-ADC enables engraftment of β-globin gene modified HSCs in a primate model without chemotherapy

• These results validate the use of CD117-ADC for targeted stem cell depletion prior to transplant and supports development as a new conditioning agent for autologous HSC-based gene therapy approaches

• This targeted approach for safer conditioning could improve the risk benefit profile for patients undergoing transplant and enable more patients to benefit from these potentially curative therapies including gene therapy for sickle cell disease and severe combined immunodeficiency