BOSTON, April 13, 2020 /PRNewswire/ --Proteostasis Therapeutics, Inc. (Nasdaq:PTI), a clinical stage biopharmaceutical company dedicated to the discovery and development of groundbreaking therapies to treat cystic fibrosis (CF), today announced it has received Scientific Advice from the Dutch Medicines Evaluation Board (MEB) on the CHOICES program for the treatment of people living with CF. The MEB is the Dutch regulatory agency responsible for assessing, monitoring and promoting the proper use of medicines. The Company believes that CHOICES is the first ever personalized medicine-based study in CF and will test PTI drug combinations in an ex vivo study and then in a clinical trial to assess the predictability of the organoid assay for clinical benefit. In the meeting, the Company and the MEB discussed the development plan for PTI's triple combination of dirocaftor, posenacaftor and nesolicaftor in the Netherlands and across Europe. The MEB expressed support for PTI's personalized medicine approach and the goal of delivering effective medicines to patients who currently have no treatment options. The MEB also supported the expansion of the Company's personalized medicine approach in more common genotypes, including F508del homozygous and heterozygous patients. "We are thrilled with the outcome of our meeting with the Dutch regulators who provided us with scientific advice as we move forward with clinical development of our proprietary triple combination in patients with rare genotypes," said Meenu Chhabra, President and Chief Executive Officer of Proteostasis. "This meeting marks an important step toward securing a clear path for the development and regulatory approval of the dirocaftor, posenacaftor and nesolicaftor combination throughout Europe." In addition to the positive scientific advice from the MEB, PTI received a High Strategic Fit score from the Clinical Trials Network (CTN) for the HIT-CF – CHOICES protocol. The CHOICES protocol was assessed and scored by CF medical experts, experienced study coordinators, statisticians and trained patient reviewers across multiple domains and it has received the maximum rating in the domain of 'Fit with ECFS Strategic Priorities'. The European Cystic Fibrosis Society formed a Clinical Trial Network (ECFS-CTN) to enhance clinical research across 58 participating sites in 17 countries. Participating sites conduct only those CF trials that have been reviewed and accepted after the ECFS-CTN protocol review process. About Proteostasis Therapeutics, Inc. Proteostasis Therapeutics, Inc. is a clinical stage biopharmaceutical company developing small molecule therapeutics to treat cystic fibrosis and other diseases caused by dysfunctional protein processing. Headquartered in Boston, MA, the Proteostasis Therapeutics team focuses on identifying therapies that restore protein function. For more information, visit www.proteostasis.com . Safe Harbor This release contains "forward-looking statements" within the meaning of the Private Securities Litigation Reform Act of 1995, including, but not limited to, statements regarding the expectations of the dirocaftor, posenacaftor and nesolicaftor combination as a treatment for CF; the expected development plan for our CHOICES Phase 3 clinical trial, including its final protocol design; whether the results from the CHOICES Phase 3 clinical trial together with other available clinical data for the dirocaftor, posenacaftor and nesolicaftor combination will be sufficient to support submission of a marketing application; the potential for future regulatory approval in the Netherlands and throughout Europe of the dirocaftor, posenacaftor and nesolicaftor combination; and the timing and potential outcome of any future discussions with the FDA or any other regulatory agency. Words such as "aim," "may," "will," "expect," "anticipate," "estimate," "intend," and similar expressions (as well as other words or expressions referencing future events, conditions or circumstances) are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. Any forward-looking statements are based on management's current expectations of future events and are subject to a number of risks and uncertainties that could cause actual results to differ materially from those expressed or implied by the forward-looking statements, and we, therefore cannot assure you that our plans, intentions, expectations or strategies will be attained or achieved. Such risks and uncertainties include, without limitation, the potential of our proprietary combination therapies for the treatment of CF, the potential benefit of our proprietary combination therapies to patients, expected completion of our clinical studies and cohorts for our clinical programs, initiation of a pivotal or registrational study, the possibility final or future results from our drug candidate trials (including, without limitation, longer duration studies) do not achieve positive results or are materially and negatively different from or not indicative of the preliminary results reported by us (noting that these results are based on a small number of patients and small data set), uncertainties inherent in the execution and completion of clinical trials (including, without limitation, the possibility that FDA or other regulatory agency comments delay, change or do not permit trial commencement, or intended label, or the FDA or other regulatory agency requires us to run cohorts sequentially or conduct additional cohorts or pre-clinical or clinical studies), in the enrollment of CF patients in our clinical trials in a competitive clinical environment, in the timing of availability of trial data, in the results of the clinical trials, in possible adverse events from our trials, in the actions of regulatory agencies, in the endorsement, if any, by therapeutic development arms of CF patient advocacy groups (and the maintenance thereof). In addition, the COVID-19 pandemic and the associated containment efforts have had a serious adverse impact on the economy, the severity and duration of which are uncertain. Government stabilization efforts will only partially mitigate the consequences. The extent and duration of the impact on our business and operations is highly uncertain, and that impact includes effects on our commercialization and marketing, manufacturing and supply chain, and clinical trial operations. Factors that will influence the impact on our business and operations include the duration and extent of the pandemic, the extent of imposed or recommended containment and mitigation measures, and the general economic consequences of the pandemic. The pandemic could have a material adverse impact on our business, operations and financial results for an extended period of time. For a discussion of other risks and uncertainties, and other important factors, any of which could cause our actual results to differ from those contained in the forward-looking statements, see the section entitled "Risk Factors" in our most recent Annual Report on Form 10-K, as well as discussions of potential risks, uncertainties, and other important factors in our subsequent filings with the Securities and Exchange Commission. We assume no obligation to update or revise any forward-looking statements, whether as a result of new information, future events or otherwise, unless required by law. CONTACTS: Investors:

David Pitts / Claudia Styslinger

Argot Partners

212.600.1902

david@argotpartners.com / claudia@argotpartners.com Media:

David Rosen

Argot Partners

212.600.1902

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