Company: ARWR
Filing Date: 2025-12-11
Form Type: S-3ASR
Source: 0001193125-25-314755
Chunk: 1

Company: ARROWHEAD PHARMACEUTICALS, INC.
Filing Date: 2025-12-11
Form: S-3ASR
Chunk 1
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 to Rule 462(e) under the Securities Act, check the following box. ☒ If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box. ☐ Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-acceleratedfiler, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2of the Exchange Act:

| Large accelerated filer |     | ☒ |     | Accelerated filer         |     | ☐ |
| Non-accelerated filer   |     | ☐ |     | Smaller reporting company |     | ☐ |
|                         |     |   |     | Emerging growth company   |     | ☐ |

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of Securities Act. ☐

EXPLANATORY NOTE This registration statement contains the following documents:

| • |     | a base prospectus which covers the offering, issuance and sale by us of an indeterminate amount of common stock,                                     
 preferred stock, debt securities, warrants or a combination of any of these securities, in each case from time to time in one or more offerings; and |

| • |     | a sale agreement prospectus supplement covering the offering, issuance and sale by us of up to a maximum                                                                                          
 aggregate offering price of $500,000,000 of our common stock that may be issued and sold under an amended and restated sale agreement, dated December 10, 2025, with Jefferies LLC (“Jefferies”). |

The base prospectus immediately follows this explanatory note. The specific terms of any securities to be offered pursuant to the base prospectus other than the shares of common stock offered under the Sale Agreement (defined below) will be specified in a prospectus supplement to the base prospectus. The specific terms of the shares of common stock to be issued and sold under the Sale Agreement are specified in the sale agreement prospectus supplement that immediately follows the base prospectus.

Prospect