January 26, 2024

Dear Huntington’s Disease Community,

We hope 2024 is off to a great start for you and your loved ones. To build on Prilenia’s presentations at patient conferences near the end of 2023, we also wanted to share an update on our HD program. Our goal is to keep you informed, as we continue to evaluate the data and work diligently to prepare for next steps.

We are pleased to let you know we are offering PROOF-HD study participants continued access to pridopidine through an expanded access/compassionate use program (EAP/CUP) while it remains an investigational product in clinical development. Under the EAP/CUP, the study participants will only need a check-in visit at their clinic every 6 months, similarly to an appointment with your HD neurologist. There are also fewer assessments in the EAP program than during the clinical trial. We hope that the reduction in the number of clinic visits and assessments will be less burdensome for patients and families. If you are a current PROOF-HD study participant, please contact your physician for more information.

We also wish to remind the community that programs are in place for access to pridopidine outside of clinical trials. For more information, please visit our website (https://www.prilenia.com/expanded-access/) or talk to your doctor to learn more.

We understand that there is an urgent need to develop treatment options for the HD community, and we are working tirelessly on our HD program. On behalf of our entire team, please accept our sincere thank you to the study participants and their families, along with the broader HD community, for your collaboration on PROOF-HD. We will be sure to keep the community informed as we learn more about our PROOF-HD Phase 3 results. We appreciate your dedication and commitment and are confident that the results of this study will advance HD research.

Sincerely, Seth Rotberg, Senior Manager of Patient

Pridopidine is an investigational drug that is not approved for the treatment of HD by health authorities.

Q&A

Who can I reach out to for help or if I have any questions? For study participants and their family members, we encourage you to reach out to your study physician for more information about the EAP. For members of the larger HD community, please reach out to your local HD care center or patient advocacy organization.

What is pridopidine? Pridopidine (45 mg twice daily) is an oral, highly selective and potent investigational S1R agonist that has exhibited a safety and tolerability profile similar to placebo in clinical studies to date. The S1R protein is highly expressed in the brain and spinal cord where it regulates several key processes that are commonly impaired in various neurodegenerative diseases. Activation of the S1R by pridopidine may lead to neuroprotective effects.

It is an investigational drug, and its safety and efficacy have not been determined by the FDA or EMA.

When will study participants know whether they received the study drug or placebo? We are planning to unblind the treatment group (pridopidine or placebo) by April 2024.

What are the next steps for Prilenia? As we continue analyzing the results of our PROOF-HD Phase 3 trial, we will be sure to communicate this with the HD community.