Side Letters to Master Collaboration Agreement, dated as of March 20, 2018, between Prothena Biosciences Limited and Celgene Switzerland LLC

Prothena Biosciences Limited

77 Sir John Rogerson’s Quay, Block C

Grand Canal Docklands, Dublin 2, D02 VK60, Ireland

Attn: Company Secretary

Dear Ms. Yvonne Tchrakian:

Reference is made to the Master Collaboration Agreement between Prothena Biosciences Limited, an Irish private limited company (hereinafter "Prothena"), and Celgene Switzerland LLC, a Delaware limited liability company and a subsidiary of Bristol-Myers Squibb Company (hereinafter "Celgene"), having an effective date of March 20, 2018 (the “MCA”). Celgene and Prothena are each referred to herein by name or as a “Party” or, collectively, as the “Parties.” The terms in this letter agreement with initial letters capitalized shall have the meaning set forth in this letter agreement, and if not defined in this letter agreement, shall have the meaning set forth in the MCA.

[***] The Parties mutually agree to amend the MCA [***] as follows:

1.)[***]

2.)[***]

The provisions of this letter agreement shall constitute an amendment to the MCA, and, to the extent that any term or provision of this letter agreement may be deemed expressly inconsistent with any term or provision in the MCA, this letter agreement shall govern and control. Except as expressly modified by the terms of this letter agreement, all of the terms, conditions, and provisions of the MCA shall remain unchanged, unmodified and in full force and effect.

This letter agreement shall be governed by Article 12 of the MCA and this letter agreement may be executed in counterparts with the same effect as if both Parties had signed the same document. All such counterparts shall be deemed an original, shall be construed together, and shall constitute one and the same instrument. Any such counterpart to the extent delivered by Electronic Delivery shall be treated in all manner and respects as an original executed counterpart and shall be considered to have the same binding legal effect as if it were the original signed version thereof delivered in person. No Party hereto shall raise the use of Electronic Delivery to deliver a signature or the fact that any signature or agreement or instrument was transmitted or communicated through the use of Electronic Delivery as a defense to the formation of a contract, and each Party forever waives any such defense, except to the extent that such defense relates to lack of authenticity.

Please have an authorized representative of Prothena sign below where indicated and return an executed copy to me at the address listed below. Upon the signing of this letter agreement by Prothena, this letter agreement shall be a binding agreement between and effective as of January 16, 2024.

cc: Prothena Bioscience Inc, 1800 Sierra Point Parkway, Brisbane, CA 94005, USA, Attn: Legal Department

Prothena Biosciences Limited

77 Sir John Rogerson’s Quay, Block C

Grand Canal Docklands, Dublin 2, D02 VK60, Ireland

Attn: Company Secretary

Dear Ms. Yvonne Tchrakian:

Reference is made to the Master Collaboration Agreement between Prothena Biosciences Limited, an Irish private limited company (hereinafter "Prothena"), and Celgene Switzerland LLC, a Delaware limited liability company and a subsidiary of Bristol-Myers Squibb Company (hereinafter "Celgene"), having an effective date of March 20, 2018 (the “MCA”). Celgene and Prothena are each referred to herein by name or as a “Party” or, collectively, as the “Parties.” The terms in this letter agreement with initial letters capitalized shall have the meaning set forth in this letter agreement, and if not defined in this letter agreement, shall have the meaning set forth in the MCA.

[***] On January 16, 2024, the Parties amended the MCA [***] (the “16Jan2024 Agreement”). The Parties now mutually agree to amend the MCA and the 16Jan2024 Agreement [***] as follows:

1.)[***]

2.)[***]

The provisions of this letter agreement shall constitute an amendment to the MCA and the 16Jan2024 Agreement, and, to the extent that any term or provision of this letter agreement may be deemed expressly inconsistent with any term or provision in the MCA or the 16Jan2024 Agreement, this letter agreement shall govern and control. Except as expressly modified by the terms of this letter agreement, all of the terms, conditions, and provisions of the MCA shall remain unchanged, unmodified and in full force and effect.

This letter agreement shall be governed by Article 12 of the MCA and this letter agreement may be executed in counterparts with the same effect as if both Parties had signed the same document. All such counterparts shall be deemed an original, shall be construed together, and shall constitute one and the same instrument. Any such counterpart to the extent delivered by Electronic Delivery shall be treated in all manner and respects as an original executed counterpart and shall be considered to have the same binding legal effect as if it were the original signed version thereof delivered in person. No Party hereto shall raise the use of Electronic Delivery to deliver a signature or the fact that any signature or agreement or instrument was transmitted or communicated through the use of Electronic Delivery as a defense to the formation of a contract, and each Party forever waives any such defense, except to the extent that such defense relates to lack of authenticity.

Please have an authorized representative of Prothena sign below where indicated and return an executed copy to me at the address listed below. Upon the signing of this letter agreement by Prothena, this letter agreement shall be a binding agreement between and effective as of February 2, 2024.

cc: Prothena Bioscience Inc, 1800 Sierra Point Parkway, Brisbane, CA 94005, USA, Attn: Legal Department

Prothena Biosciences Limited

77 Sir John Rogerson’s Quay, Block C

Grand Canal Docklands, Dublin 2, D02 VK60, Ireland

Attn: Company Secretary

Dear Ms. Yvonne Tchrakian:

Reference is made to the Master Collaboration Agreement between Prothena Biosciences Limited, an Irish private limited company (hereinafter "Prothena"), and Celgene Switzerland LLC, a Delaware limited liability company and a subsidiary of Bristol-Myers Squibb Company (hereinafter "Celgene"), having an effective date of March 20, 2018 (the “MCA”). Celgene and Prothena are each referred to herein by name or as a “Party” or, collectively, as the “Parties.” The terms in this letter agreement with initial letters capitalized shall have the meaning set forth in this letter agreement, and if not defined in this letter agreement, shall have the meaning set forth in the MCA.

The Parties mutually agree to amend the MCA by striking Section 1.41 of the MCA, the definition of “Initial Research Term”, and replacing it as follows:

“1.41 “Initial Research Term” means the period beginning on the Effective Date and ending on [***].”

The provisions of this letter agreement shall constitute an amendment to the MCA, and, to the extent that any term or provision of this letter agreement may be deemed expressly inconsistent with any term or provision in the MCA, this letter agreement shall govern and control. Except as expressly modified by the terms of this letter agreement, all of the terms, conditions, and provisions of the MCA shall remain unchanged, unmodified and in full force and effect.

This letter agreement shall be governed by Article 12 of the MCA and this letter agreement may be executed in counterparts with the same effect as if both Parties had signed the same document. All such counterparts shall be deemed an original, shall be construed together, and shall constitute one and the same instrument. Any such counterpart to the extent delivered by Electronic Delivery shall be treated in all manner and respects as an original executed counterpart and shall be considered to have the same binding legal effect as if it were the original signed version thereof delivered in person. No Party hereto shall raise the use of Electronic Delivery to deliver a signature or the fact that any signature or agreement or instrument was transmitted or communicated through the use of Electronic Delivery as a defense to the formation of a contract, and each Party forever waives any such defense, except to the extent that such defense relates to lack of authenticity.

Please have an authorized representative of Prothena sign below where indicated and return an executed copy to me at the address listed below. Upon the signing of this letter agreement by Prothena, this letter agreement shall be a binding agreement between and effective as of March 15, 2024.

Prothena Biosciences Limited

77 Sir John Rogerson’s Quay, Block C

Grand Canal Docklands, Dublin 2, D02 VK60, Ireland

Attn: Company Secretary

Dear Ms. Yvonne Tchrakian:

Reference is made to the Master Collaboration Agreement between Prothena Biosciences Limited, an Irish private limited company (hereinafter "Prothena"), and Celgene Switzerland LLC, a Delaware limited liability company and a subsidiary of Bristol-Myers Squibb Company (hereinafter "Celgene"), having an effective date of March 20, 2018 (the “MCA”). Celgene and Prothena are each referred to herein by name or as a “Party” or, collectively, as the “Parties.” The terms in this letter agreement with initial letters capitalized shall have the meaning set forth in this letter agreement, and if not defined in this letter agreement, shall have the meaning set forth in the MCA.

[***] On January 16, 2024, the Parties amended the MCA [***] (the “16Jan2024 Agreement”). On February 2, 2024, the Parties amended the MCA and the 16Jan2024 Agreement [***] (“02Feb2024 Agreement”). The Parties now mutually agree to amend the MCA, the 16Jan2024 Agreement, and the 02Feb2024 Agreement [***] as follows:

1.)[***]

2.)[***]

The provisions of this letter agreement shall constitute an amendment to the MCA, 16Jan2024 Agreement, and the 02February2024 Agreement, and, to the extent that any term or provision of this letter agreement may be deemed expressly inconsistent with any term or provision in the MCA,16Jan2024 Agreement, or the 02February2024 Agreement, this letter agreement shall govern and control. Except as expressly modified by the terms of this letter agreement, all of the terms, conditions, and provisions of the MCA shall remain unchanged, unmodified and in full force and effect.

This letter agreement shall be governed by Article 12 of the MCA and this letter agreement may be executed in counterparts with the same effect as if both Parties had signed the same document. All such counterparts shall be deemed an original, shall be construed together, and shall constitute one and the same instrument. Any such counterpart to the extent delivered by Electronic Delivery shall be treated in all manner and respects as an original executed counterpart and shall be considered to have the same binding legal effect as if it were the original signed version thereof delivered in person. No Party hereto shall raise the use of Electronic Delivery to deliver a signature or the fact that any signature or agreement or instrument was transmitted or communicated through the use of Electronic Delivery as a defense to

the formation of a contract, and each Party forever waives any such defense, except to the extent that such defense relates to lack of authenticity.

Please have an authorized representative of Prothena sign below where indicated and return an executed copy to me at the address listed below. Upon the signing of this letter agreement by Prothena, this letter agreement shall be a binding agreement between and effective as of May 1, 2024.

cc: Prothena Bioscience Inc, 1800 Sierra Point Parkway, Brisbane, CA 94005, USA, Attn: Legal Department

Prothena Biosciences Limited

77 Sir John Rogerson’s Quay, Block C

Grand Canal Docklands, Dublin 2, D02 VK60, Ireland

Attn: Company Secretary

Dear Ms. Yvonne Tchrakian:

Reference is made to the Master Collaboration Agreement between Prothena Biosciences Limited, an Irish private limited company (hereinafter "Prothena"), and Celgene Switzerland LLC, a Delaware limited liability company and a subsidiary of Bristol-Myers Squibb Company (hereinafter "Celgene"), having an effective date of March 20, 2018, and as amended by Prothena and Celgene on March 15, 2024 (the “MCA”). Celgene and Prothena are each referred to herein by name or as a “Party” or, collectively, as the “Parties.” The terms in this letter agreement with initial letters capitalized shall have the meaning set forth in this letter agreement, and if not defined in this letter agreement, shall have the meaning set forth in the MCA.

The Parties mutually agree to amend the MCA by striking Section 1.41 of the MCA, the definition of “Initial Research Term”, and replacing it as follows:

“1.41 “Initial Research Term” means the period beginning on the Effective Date and ending on May 24, 2024.”

The provisions of this letter agreement shall constitute an amendment to the MCA, and, to the extent that any term or provision of this letter agreement may be deemed expressly inconsistent with any term or provision in the MCA, this letter agreement shall govern and control. Except as expressly modified by the terms of this letter agreement, all of the terms, conditions, and provisions of the MCA shall remain unchanged, unmodified and in full force and effect.

This letter agreement shall be governed by Article 12 of the MCA and this letter agreement may be executed in counterparts with the same effect as if both Parties had signed the same document. All such counterparts shall be deemed an original, shall be construed together, and shall constitute one and the same instrument. Any such counterpart to the extent delivered by Electronic Delivery shall be treated in all manner and respects as an original executed counterpart and shall be considered to have the same binding legal effect as if it were the original signed version thereof delivered in person. No Party hereto shall raise the use of Electronic Delivery to deliver a signature or the fact that any signature or agreement or instrument was transmitted or communicated through the use of Electronic Delivery as a defense to the formation of a contract, and each Party forever waives any such defense, except to the extent that such defense relates to lack of authenticity.

Please have an authorized representative of Prothena sign below where indicated and return an executed copy to me at the address listed below. Upon the signing of this letter agreement by Prothena, this letter agreement shall be a binding agreement between and effective as of May 1, 2024.

cc: Prothena Biosciences Inc, 1800 Sierra Point Parkway, Brisbane, CA 94005, USA, Attn: Legal Department