Company: NCNA
Filing Date: 2025-07-11
Form Type: 6-K
Source: 0001193125-25-157995
Chunk: 0

Company: NuCana plc
Filing Date: 2025-07-11
Form: 6-K
Chunk 0
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### UNITED STATES

### SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM 6-K

REPORT OF FOREIGN ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16

OF THE SECURITIES EXCHANGE ACT OF 1934

For the month of July 2025

(Commission File No. 001-38215)

### NUCANA PLC
(Translation of registrant’s name into English)

3 Lochside Way

Edinburgh EH12 9DT

United Kingdom

(Address of registrant’s principal executive office)

Indicate by check mark whether the registrant files or will file annual reports under
cover Form 20-F or Form 40-F.

Form 20-F ☒ Form 40-F ☐

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101 (b) (1): ☐

Indicate by check mark if the registrant is
submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101 (b) (7): ☐

NuCana plc announces plan to implement ADS ratio change On July 11, 2025, NuCana plc (the “Company”) announced its plan to change the ratio of its American Depositary Shares (“ADSs”) to its ordinary shares from one (1) ADS representing twenty-five (25) ordinary shares to one (1) ADS representing five thousand (5,000) ordinary shares. For the Company’s ADS holders, the change in the ADS ratio has the same effect as a one-for-twohundred reverse ADS split and will have no impact on an ADS holder’s proportional equity interest in the Company. The effect of the ratio change on the ADS trading price on the Nasdaq Capital Market is expected to take place at the opening of trading on August 8, 2025 (U.S. Eastern Time). Further information is set out in the press release attached hereto as Exhibit 99.1 and which is incorporated by reference herein. The information contained in this Form 6-Kis hereby incorporated by reference into the Company’s Registration Statement on Form F-3,as amended (File No. 333-281576),Registration Statement on Form S-8(File No. 333-223476)and Registration Statement on Form S-8(File No. 333-248135). Forward Looking Statements This Form 6