Bolt Biotherapeutics : CEA BDC-2034 AACR Poster

CEA-targeted Boltbody™ ISAC, BDC-2034, drives preclinical efficacy associated with innate immune activation, phagocytosis, and myeloid reprogramming

ABSTRACT

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Lisa K. Blum, Cecelia I. Pearson, Laughing Bear Torrez Dulgeroff, Rishali Gadkari, Angela Luo, Andrew Luo, Jennifer E. Melrose, Jess L. Nolin, Hai Li, Arthur Lee, Matthew N. Zhou, Puneet Anand, Ganapathy Sarma,

Karla A. Henning, Steven J. Chapin, Shelley E. Ackerman, Romas Kudirka, Yuyi Shen, Marcin Kowanetz, David Dornan, Bruce Hug, Edith A. Perez, Michael N. Alonso, Brian S. Safina, William G. Mallet

Bolt Biotherapeutics, 900 Chesapeake Drive, Redwood City, CA, USA

CEA (CEACAM5) is a well-validated cell-surface antigen that is highly expressed in multiple solid tumors. Bolt Bio's pioneering immune-stimulating antibody conjugates (ISACs) direct proprietary TLR7/8 agonists into tumors to activate tumor-infiltrating myeloid cells, initiating a broad innate and adaptive anti-tumor immune response.1 The favorable properties of CEA, including robust cell surface expression, low internalization rate, and limited normal tissue expression, support the antigen's suitability as an ISAC target. We are evaluating the CEA-targeting ISAC BDC‐2034 as a multi-functional approach to treat CEA-expressing cancers.

Boltbody™ ISAC mechanism targets the innate immune system

Stimulates adaptive immunity for optimal anti-tumor response

Innate Immune Response

Adaptive Immune Response

Myeloid Cells Kill Tumor Cells via ADCP

Engages T Cell-driven Tumor Killing

Boltbody™

ISACS

Boltbody™ISAC

Tumor AntigenTumor CellDying Tumor CellMyeloid APC

T Cell

Activated T CellMHC-Tumor Peptide ComplexGranzymes,IFNγ, TNFα

BDC-2034: Structure and critical properties

CEA1: Novel CEA-targeting mAb

CEA1 key attributes for ISAC:

• • Binds to cell-surface tumor antigen

• • Monovalent KD=23 nM

• • Slowly internalizing: 60% of CEA1 remains on cell surface 5 hours after in vitro binding (two cell lines)TLR7/8 agonist

• • Potent stimulator of innate immune systemFc effector function

• • Non-cleavable linker

• • Average DAR ≈ 2.5

1. Ackerman S, Pearson C, Gregorio J, et al. Immune-stimulating antibody conjugates elicit robust myeloid activation and durable antitumor immunity. Nature Cancer. 2021;2:18-33.https://doi.org/10.1038/s43018-020-00136-x

BDC-2034 binds differentially to surface-expressed CEA on a panel of cell lines

[ND = not determined]

BDC-2034 binding sites per cell for each tumor cell line were quantified by flow cytometry using anti-human IgG Quantum Simply Cellular (QSC) beads standards (Bangs Laboratories). Tumor cell lines and QSC beads standards were stained with Alexa Fluor 488 labelled BDC-2034 and analyzed by flow cytometry to obtain geometric mean fluorescent intensity (gMFI). BDC-2034 binding sites were calculated using QSC beads standard curve.

Cell line	Lineage	BDC-2034 sites per cell	Binding EC50	IHC H-score
MKN-45	Gastric	>2,000,000	HIGH	30.9 nM	300
HPAF-II	Pancreatic	1,760,000	HIGH	19.5 nM	220
LoVo	Colorectal	166,000	MEDIUM	25.0 nM	110
LS-174T	Colorectal	38,400	LOW	4.7 nM	ND
MDA-MB-231	Breast	0	NEGATIVE	ND	ND

Mouse surrogate CEA ISAC3 inhibits growth of CEA+ xenograft tumors

ISAC3: CEA1 ISAC with chemical analog of the BDC-2034 payload

• • Improved stability in rodents

• • Similar profile in TLR7/8 reporterassays and myeloid activation assays

HPAF-II Pancreatic CEAHigh

TumorVolume(mm3)

% TGI

• 0CEA1 / 5 mg/kg

• 2.1Isotype ISAC / 5 mg/kg

• 57.7CEA ISAC3 / 0.5 mg/kg

• 85.5CEA ISAC3 / 1 mg/kg

• 96.6CEA ISAC3 / 2.5 mg/kg

• 100CEA ISAC3 / 5 mg/kg

0

10

20

30

Day Post Initial TreatmentTumorVolume(mm3)

NCI-H2122 Lung CEAMedium 800

MKN-45 Gastric CEAHigh

% TGI

750

% TGI

0CEA1 / 10 mg/kg 0

600

-1.9Isotype ISAC / 10 mg/kg 12.7

400

***

-3.9CEA ISAC3 / 2.5 mg/kg 33.6

10.8CEA ISAC3 / 5 mg/kg 60.6

200

36.7CEA ISAC3 / 10 mg/kg 68.8

00

TumorVolume(mm3)

500

250

0

10

20 0

10

20 30

Day Post Initial Treatment

Day Post Initial Treatment

SCID/beige mice were implanted subcutaneously with 5 x 106 tumor. Once tumors reached 80 mm3 (HPAF-II; MKN-45) or 100 mm3 (NCI-H2122) in size, test articles were administered at the indicated doses q5d x 4 i.p. N = 5 mice per group (HPAF-II) or 8 mice per group (NCI-H2122; MKN-45). Percent tumor growth inhibition (% TGI) was calculated relative to CEA1 at 5 mg/kg (HPAF-II) or 10 mg/kg (MKN-45; NCI-H2122) five days following the last dose using the formula ((Average Control-Average Treated)/Average Control)*100. Data reported are from one experiment per tumor model. Data are presented as mean ± standard error of measurement (SEM). **P<0.01, ***P<0.001, **** P<0.0001 by two-way ANOVA relative to the CEA1 group.

• • ISAC BDC-2034 created by conjugation of CEA1 with dual TLR7/8 adjuvant via a non-cleavable linker

• • BDC-2034 and surrogates exhibit promising activity in preclinical models

  • - Tumor-dependent induction of immune-stimulating cytokine secretion by primary human innate effector cells

  • - Innate immune activation with CEA-medium models (CEA expression levels comparable to human cancers)

  • - Anti-tumor efficacy in xenograft models at dose levels as low as 0.5 mg/kg

  • - Dose-dependent tumor recruitment of innate effector cells and induction of immune-stimulating cytokines

  • - Intra-tumor myeloid reprogramming, including upregulation of antigen presentation and down-modulation of pro-tumor M2 phenotype

• • Bolt Bio's preclinical data support further development of BDC-2034 as a therapeutic option for patients with CEA-expressing cancers

• • We expect BDC-2034 to enter clinical development in the second half of 2022