Company: RNAC
Filing Date: 2025-04-28
Form Type: DEFA14A
Source: 0001453687-25-000074
Chunk: 0

Company: Cartesian Therapeutics, Inc.
Filing Date: 2025-04-28
Form: DEFA14A
Chunk 0
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### UNITED STATES

### SECURITIES AND EXCHANGE COMMISSION

#### Washington, DC 20549

#### SCHEDULE 14A
<div align='center'>(Rule 14a-101)</div>

### INFORMATION REQUIRED IN PROXY STATEMENT

#### SCHEDULE 14A INFORMATION

#### Proxy Statement Pursuant to Section 14(a) of the

#### Securities Exchange Act of 1934
<div align='center'>(Amendment No. )</div>

| Filed by the Registrantþ |     | Filed by a Party other than the Registranto |

Check the appropriate box:

| o |     | Preliminary Proxy Statement                                                     |
| o |     | Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2)) |
| o |     | Definitive Proxy Statement                                                      |
| þ |     | Definitive Additional Materials                                                 |
| o |     | Soliciting Material Pursuant to § 240.14a-12                                    |

#### CARTESIAN THERAPEUTICS, INC.
<div align='center'>(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement if Other Than the Registrant)</div>

| Payment of Filing Fee (Check the appropriate box) |     |                                                                                                     |
| þ                                                 |     | No fee required.                                                                                    |
| o                                                 |     | Fee previously paid with preliminary materials                                                      |
| o                                                 |     | Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11 |