CastBio Raises Pre-Seed Funding via J-KISS from Real Tech Fund

CastBio, Inc. (Headquarters: Chuo-ku, Tokyo; Representative Director: Masaaki Fujiwara; hereinafter "CastBio") announces that it has raised funds through the issuance of J-KISS type stock acquisition rights underwritten by the Real Tech Fund 4 Investment Limited Partnership (hereinafter "Real Tech Fund") managed by UntroD Capital Japan, Inc. (hereinafter "UntroD").

The funds raised in this round will be used to establish an R&D structure and accelerate the non-clinical development of an Antibody-Drug Conjugate (ADC) based on the company's proprietary VLK linker technology.

CastBio is an ADC drug discovery startup established in March 2026, based on proprietary technology inherited from Rin Institute, Inc. Under its unique CAST (Cancer Stromal Targeting) therapy concept, the company aims to commercialize a novel drug delivery strategy that has been difficult to achieve with existing ADCs by combining an antibody targeting insoluble fibrin (IF) present in the stroma of tumor tissues with its proprietary VLK linker.

In recent years, the ADC field has attracted significant attention as a next-generation drug discovery modality, with a series of major partnerships and acquisitions by global pharmaceutical companies. CastBio will promote the creation of innovative therapeutic drugs in the field of intractable cancers, leveraging its proprietary linker technology and tumor stroma targeting technology.

■ Purpose of Fundraising
The funds raised this time will mainly be allocated to the following purposes:
- Establishing the R&D structure for anti-IF-ADC
- Promoting non-clinical development of anti-IF-ADC
- Strengthening and preserving intellectual property

■ About Fundraising via J-KISS
For this fundraising, the company adopted "J-KISS (Japanese Keep It Simple Security)" type stock acquisition rights, which are widely used domestically as an early-stage fundraising method for startups.
J-KISS is a mechanism that enables rapid fundraising while deferring corporate valuation to future rounds, and is attracting attention as a flexible financing method suitable for R&D startups.
The underwriter this time, Real Tech Fund, has an extensive track record of startup investments centered on the deep tech sector. CastBio will deepen its collaboration with them to accelerate R&D and business growth in the future.

■ Future Fundraising Policy
Following this Pre-seed round, CastBio plans to conduct fundraising in a Seed round with a target closing in November 2026.
With the funds raised in the Seed round, the company will conduct GLP safety studies for anti-IF-ADC, manufacture investigational drugs for clinical trials, and proceed with preparations for initiating clinical trials through PMDA consultations and collaboration with the National Cancer Center.
Therefore, in this Pre-seed round, anticipating swift fundraising in the subsequent Seed round, the company continues to invite the participation of investors following UntroD.

■ Future Outlook
CastBio aims to further accelerate the R&D of its proprietary ADC drug discovery platform, promote collaborations with domestic and international pharmaceutical companies, investors, research institutions, and contract manufacturing organizations, and maximize corporate value.
In the future, while considering strategic alliances, joint research, out-licensing, and M&A in the ADC field, the company aims to contribute to healthcare through the creation of innovative cancer therapeutic drugs.

■ Comment
Masaaki Fujiwara, Representative Director of CastBio
"I am very pleased to have been able to raise funds this time with the Real Tech Fund 4 Investment Limited Partnership as the underwriter. ADC is an area attracting great global attention in the treatment of intractable cancers, and we believe that our proprietary technology platform, which targets the cancer stroma acting as a barrier in existing cancer treatments, holds significant value in next-generation ADC drug discovery. Through the planned Seed round, we will further strengthen our R&D structure and work towards creating innovative cancer therapeutic drugs."

■ Company Profile
CastBio, Inc.
Location: Success Ginza East Building 2F, 1-13-10 Tsukiji, Chuo-ku, Tokyo
Representative: Masaaki Fujiwara, Representative Director
Established: March 30, 2026
Main Business: R&D of pharmaceuticals including Antibody-Drug Conjugates (ADCs), R&D and licensing business of ADC-related technologies, R&D of medical devices, in-vitro diagnostics, and research reagents, consulting related to drug discovery research