Summary of Significant Accounting Policies (Policies)	12 Months Ended
Dec. 31, 2023
Accounting Policies [Abstract]
Basis of Preparation and Presentation of Financial Information	Basis of Preparation and Presentation of Financial Information These Consolidated Financial Statements have been prepared in accordance with the accounting principles generally accepted in the U.S. (“U.S. GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). These Consolidated Financial Statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. The Consolidated Financial Statements of Prothena Corporation plc are presented in U.S. dollars, which is the functional currency of the Company and its consolidated subsidiaries. Monetary assets and liabilities denominated in foreign currency are remeasured at period-end exchange rates. Foreign currency gains and losses resulting from remeasurement are recognized in other expense, net in the Consolidated Statements of Operations.
Use of Estimates	Use of Estimates The preparation of the Consolidated Financial Statements in conformity with U.S. GAAP requires the Company to make judgments, estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosures. On an ongoing basis, management evaluates its estimates, including critical accounting policies or estimates related to revenue recognition and research and development expenses. The Company bases its estimates on historical experience and on various other market specific and other relevant assumptions that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Because of the uncertainties inherent in such estimates, actual results may differ materially from these estimates.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents, and Restricted Cash The Company considers all highly liquid investments held at financial institutions, such as commercial paper, money market funds, and other money market securities with original maturities of three months or less at date of purchase to be cash equivalents. Cash accounts that are restricted to withdrawal or usage are presented as restricted cash. As of December 31, 2023, the Company had $2.2 million of restricted cash held by a bank in certificates of deposit as collateral to standby letters of credit under certain operating leases. See Note 6, "Commitments and Contingencies" for additional information regarding the Company’s operating leases.
Accounts Receivable	Accounts Receivable The accounts receivable balance on the Consolidated Balance Sheets represents amounts receivable from the Company's collaboration partners. The Company monitors the financial performance and creditworthiness of customers so that it can properly assess and respond to changes in their credit profiles. The Company makes judgments as to its ability to collect outstanding receivables and provides an allowance for credit losses against the trade account receivables, when appropriate. As of December 31, 2023, the Company has not provided any allowance for credit losses against its outstanding accounts receivables.
Property and Equipment, net	Property and Equipment, net Property and equipment, net are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the related assets. Maintenance and repairs are charged to expense as incurred, and leasehold improvements where the Company is deemed the accounting owner are capitalized. When assets are retired or otherwise disposed of, the cost and accumulated depreciation are removed from the balance sheet and any resulting gain or loss is reflected in operations in the period realized.
Impairment of Long-lived Assets	Impairment of Long-lived Assets The Company periodically evaluates property and equipment for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable or the estimated useful life is no longer appropriate. If such events or changes in circumstances arise, the Company compares the carrying amount of the asset to the estimated undiscounted future cash flows expected to be generated by the asset. If the carrying amount of the long-lived asset is not recoverable on an undiscounted cash flow basis, an impairment charge is recognized to the extent that the carrying amount exceeds its fair value. The Company determines fair value using the income approach based on the present value of expected future cash flows. The Company’s cash flow assumptions consider historical and forecasted revenue and operating costs and other relevant factors.
Leases	Leases The Company leases both real property and certain equipment for use in its operations. A determination is made as to whether an arrangement is a lease at inception. If so, the Company evaluates the lease agreement to determine whether the lease is an operating or finance lease using the criteria in ASC 842. The Company does not recognize right-of-use assets and lease liabilities that arise from short-term leases for any class of underlying assets. When lease agreements also require the Company to make additional payments for taxes, insurance and other operating expenses incurred during the lease period, such payments are expensed as incurred. See Note 6, “Commitments and Contingencies,” which provides additional details on the Company's current lease arrangements. As of December 31, 2023 and 2022, the Company had no financing leases. Operating leases are included in the operating lease right-of-use (“ROU”) assets, lease liability, current and lease liability, non-current in the Company's Consolidated Balance Sheets. Operating lease ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at the lease commencement date based on the present value of all lease payments over the lease term. In determining the present value of lease payments, the Company uses its incremental borrowing rate based on information available at the lease commencement date. The operating lease ROU assets also include any lease prepayments made and exclude lease incentives such as rent abatements and/or concessions and rent holidays. The Company does not assume renewals in its determination of the lease term unless the renewals are deemed by management to be reasonably certain at lease inception. Tenant improvements made by the Company as a lessee in which they are deemed to be owned by the lessor is viewed as lease prepayments by the Company and included in the operating lease ROU assets upon commencement of the lease prior to which they are recorded as prepaid assets. Lease expense for operating leases is recognized on a straight-line basis over the expected lease term as an operating expense. For lease agreements that include lease and non-lease components, such components are generally accounted for separately.
Revenue Recognition	Revenue Recognition The Company’s collaboration revenue includes revenue recognized for milestone payments and reimbursements under the Company’s License Agreement with Roche as well as revenue recognized under the Company’s Collaboration Agreement with BMS. The Company’s license and intellectual property revenue includes revenue from Novo Nordisk for the sale of intellectual property and related rights to the Company’s ATTR amyloidosis business and pipeline and milestones payments. The Company analyzes its collaboration arrangements to assess whether they are financing arrangement within the scope of ASC 730 or as a collaboration arrangement pursuant to ASC 808, or whether such arrangements are reflective of a vendor-customer relationship and therefore within the scope of Topic 606. As of December 31, 2023, the Company has not had any arrangements outside the scope of Topic 606. The following describes the Company’s accounting treatment pursuant to Topic 606: License, Option and Collaboration Revenue The terms of license, option and collaboration agreements entered into typically include payment of one or more of the following: non-refundable, up-front license fees; option exercise fees; development, regulatory and commercial milestone payments; payments for manufacturing supply and research and development services and royalties on net sales of licensed products. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. Amounts received prior to satisfying the revenue recognition criteria are recorded as contract liabilities recorded as deferred revenue in the Company’s Consolidated Balance Sheets. At contract inception, for contracts that contain multiple performance obligations, such as the Company’s License Agreement with Roche and the Collaboration Agreement with BMS, the Company accounts for the individual performance obligations separately if they are distinct. Factors considered in the determination of whether the license performance obligations are distinct included, among other things, the research and development capabilities of each of Roche and BMS and their respective sublicense rights, and for the remaining performance obligations the fact that they are not proprietary and can be and have been provided by other vendors. The transaction price is allocated to the separate performance obligation on a relative standalone selling price basis. Revenue is recognized only when the Company satisfies an identified performance obligation by transferring a promised good or service to a customer (in the Company’s case, Roche and BMS). An asset is transferred when, or as, the customer obtains control of that asset, which for a service is considered to be as the services are received and used. The Company recognizes revenue over time by measuring the progress toward complete satisfaction of the relevant performance obligation using an appropriate input or output method based on the nature of the service promised to the customer. Milestone Revenue The Company generally classifies each of its milestones into one of three categories: (i) clinical milestones; (ii) regulatory and development milestones; and (iii) commercial milestones. Clinical milestones are typically achieved when a product candidate advances into or completes a defined phase of clinical research. For example, a milestone payment may be due to the Company upon the initiation of a clinical trial for a new indication. Regulatory and development milestones are typically achieved upon acceptance of the submission for marketing approval of a product candidate or upon approval to market the product candidate by the FDA or other regulatory authorities. For example, a milestone payment may be due to the Company upon submission for marketing approval of a product candidate by the FDA. Commercial milestones are typically achieved when an approved product reaches certain defined levels of net royalty sales by the licensee of a specified amount within a specified period. At the inception of each arrangement that includes developmental, regulatory or commercial milestone payments, the Company evaluates whether achieving the milestones is considered probable and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the value of the associated milestone (such as a regulatory submission by the Company) is included in the transaction price. Milestone payments that are not within the control of the Company, such as approvals from regulators or where attainment of the specified event is dependent on the development activities of a third party, are not considered probable of being achieved until those approvals are received or the specified event occurs. The Company considers such milestone payments as variable consideration with constraint and therefore recognizes the revenue from such milestone payments as collaboration revenue at point in time when the Company can conclude it is probable that a significant revenue reversal will not occur in future periods. Taxes, Shipping and Handling The Company excludes from the measurement of the transaction price all taxes assessed by a governmental authority that are both imposed on and concurrent with a specific revenue-producing transaction and collected by the Company from a customer (e.g., sales, use, value added, some excise taxes). In addition, the Company accounts for shipping and handling as activities that are performed after its customers obtain control of the goods as activities to fulfill our performance obligation to transfer the goods.
Research and Development	Research and Development Research and development costs are expensed as incurred. Such costs include, but are not limited to, salaries and benefits, share-based compensation, costs related to preclinical and clinical trial activities including fees paid to clinical research organizations and investigative sites, costs related to drug development and manufacturing prior to regulatory approval for commercial sale, and consulting fees. There can be judgment involved in measuring the research and development expenses to be recognized in a particular period. The level of judgment varies based on the nature of the services being performed and the underlying support obtained. The Company recognizes costs for certain development activities, such as clinical trials, based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, or information provided to the Company by our vendors on their actual costs incurred. For certain clinical trials, expense is recorded based on information obtained from vendors as an intermediary to those performing the underlying services, such as contract research organizations. These estimates are inherently more judgmental because the quality and availability of the underlying data may vary. The Company recognizes costs for contract manufacturing based on evaluation of the progress to completion of specific tasks. The objective of the Company’s accrual policy is to match the recording of the expenses in the Consolidated Financial Statements to the actual services the Company has received and efforts expended. As such, expense accruals related to clinical trials and contract manufacturing are recognized based on the Company’s estimate of the degree of completion of the events specified in the specific clinical study or trial contract or drug development and manufacturing contract, respectively. The Company does not make significant estimates where costs incurred are supported by invoices or reports of costs incurred are obtained from a vendor that is directly performing the underlying services, such as a consultant or contract manufacturing organization. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the Consolidated Financial Statements as prepaid or accrued research and development. Amounts due may be fixed fee, fee for service, and may include upfront payments, monthly payments, and payments upon the completion of milestones or receipt of deliverables. Nonrefundable advance payments for goods and services that will be used or received in future research and development activities are deferred and recognized as expense in the period in which the related goods are delivered or services are performed. The Company has acquired and may continue to acquire the rights to develop and commercialize new drug candidates from third parties. The upfront payments to acquire license, product or rights, as well as any future milestone payments, are immediately recognized as research and development expense provided that the drug has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use.
Share-based Compensation	Share-based Compensation The Company’s share-based compensation programs include options for the purchase of shares and restricted share units (RSUs). Such awards may be granted to employees, directors, and non-employee service providers. The Company measures compensation expense for all share-based awards at the grant date based on the fair value measurement of the award. Share-based compensation expense is recognized on a straight-line basis over the requisite service period, which is generally the vesting period, for each award. The fair value of RSUs is based on the closing market price of the Company’s ordinary shares on the date of grant. To determine the fair value of options for the purchase of shares, the Company uses the Black-Scholes option-pricing model. The determination of fair value using the Black-Scholes option-pricing model is affected by the Company’s share price as well as assumptions regarding a number of complex and subjective variables. Judgment is required in determining the assumptions used in these models which include the risk-free interest rate, expected term, expected volatility and expected dividend yield. The Company uses its historical volatility for the Company's shares to estimate expected volatility. The simplified method has been used to estimate the expected term of all options in previous years. Beginning January 1, 2023, expected term is estimated based on historical experience. Share-based compensation expense recognized in the Consolidated Statements of Operations is based on awards expected to vest and therefore the amount of expense has been reduced for estimated forfeitures which are based on historical experience. Share-based compensation expense is adjusted in subsequent periods for actual forfeitures. The Company records any excess tax benefits or tax shortfalls from its equity awards in its Consolidated Statements of Operations in the reporting periods in which options for the purchase of shares are exercised or RSUs vest.
Income Taxes	Income Taxes The Company files its own U.S. and foreign income tax returns and income taxes are presented in the Consolidated Financial Statements using the asset and liability method prescribed by the accounting guidance for income taxes. Deferred tax assets (“DTAs”) and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using the enacted tax rates projected to be in effect for the year in which the differences are expected to reverse. Net deferred tax assets are recorded to the extent the Company believes that these assets will more likely than not be realized. In making such determination, all available positive and negative evidence is considered, including scheduled reversals of deferred tax liabilities, recent cumulative earnings/losses by taxing jurisdiction, projected future taxable income, tax planning strategies and recent financial operations. Actual operating results in future years could differ from our current assumptions, judgments and estimates. The Company’s significant tax jurisdictions are Ireland and the United States. Estimates are required in determining the Company’s provision for income taxes. Some of these estimates are based on management’s interpretations of jurisdiction-specific tax laws or regulations. Various internal and external factors may have favorable or unfavorable effects on the future effective income tax rate of the business. These factors include, but are not limited to, changes in tax laws, regulations and/or rates, changing interpretations of existing tax laws or regulations, changes in estimates of prior years’ items, past and future levels of R&D spending, the impact of accounting for share-based compensation, and changes in overall levels of income before taxes. The Company did not recognize certain tax benefits from uncertain tax positions within the provision for income taxes. The tax benefit from an uncertain tax position is recognized only if it is more likely than not the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the financial statements from such positions are then measured based on the largest benefit that has a greater than 50% likelihood of being realized upon settlement. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. Interest and penalties related to unrecognized tax benefits are accounted for in income tax expense.
Net Income (Loss) per Ordinary Share	Net Income (Loss) per Ordinary Share Basic net income (loss) per ordinary share is calculated by dividing net income (loss) by the weighted-average number of ordinary shares outstanding during the period. Diluted net income per ordinary share is computed based on the treasury stock method by dividing net income by the weighted-average number of ordinary shares outstanding, plus potentially dilutive ordinary equivalent shares outstanding. However, where there is a net loss, no adjustment is made for potentially issuable ordinary shares because their effect would be anti-dilutive and therefore diluted net loss per share is equal to basic net loss per share.
Comprehensive Loss	Comprehensive Loss Comprehensive income (loss) is comprised of net income (loss) and other comprehensive income (loss). The Company has no components of other comprehensive income (loss). Therefore, net income (loss) equals comprehensive income (loss) for all periods presented and, accordingly, the Consolidated Statements of Comprehensive Income (Loss) is not presented in a separate statement.
Segment	The Company operates in one segment. The Company’s chief operating decision maker (the “CODM”), its Chief Executive Officer, manages the Company’s operations and evaluates the Company’s financial performance on a consolidated basis for purposes of allocating resources.
Concentration of Risks	Financial instruments that potentially subject the Company to concentration of credit risk consist of cash and cash equivalents and accounts receivable. The Company places its cash equivalents with high credit quality financial institutions and, by policy, limits the amount of credit exposure with any one financial institution. Deposits held with banks have exceeded, and will continue to exceed, federally insured limits. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash and cash equivalents. The Company has not experienced any losses on its deposits of cash and cash equivalents and its credit risk exposure is up to the extent recorded on the Company's Consolidated Balance Sheet. The Company’s business is primarily conducted in U.S. dollars except for its agreements with contract manufacturers for drug supplies which are primarily denominated in Euros. The Company recorded a loss on foreign currency exchange rate differences of approximately $458,000, $397,000 and $96,000 during the years ended December 31, 2023, 2022 and 2021, respectively. If the Company increases its business activities that require the use of foreign currencies, it may be exposed to losses if the Euro and other such currencies continue to strengthen against the U.S. dollar. As of December 31, 2023, and 2022, $3.8 million and $1.7 million, respectively, of the Company’s property and equipment, net were held in the U.S. and a nominal amount were in Ireland. The Company does not own or operate facilities for the manufacture, packaging, labeling, storage, testing or distribution of nonclinical or clinical supplies of any of its drug candidates. The Company instead contracts with and relies on third-parties to manufacture, package, label, store, test and distribute all preclinical development and clinical supplies of our drug candidates, and it plans to continue to do so for the foreseeable future. The Company also relies on third-party consultants to assist in managing these third-parties and assist with its manufacturing strategy.
Recent Accounting Pronouncements	Recent Accounting Pronouncements On November 27, 2023, the FASB issued Accounting Standards Update 2023-07 ("ASU 2023-07"), Segment Reporting- Improvements to Reportable Segment Disclosures, which requires public entities to provide disclosures on significant segment expenses that are regularly provided to the CODM and included within each reported measure of segment profit or loss and other segment items on an annual and interim basis. The guidance also requires public entities to provide all disclosures about reportable segment’s profit or loss and assets in interim periods that are currently required annually. Public entities with a single reportable segment have to provide all disclosures required by Accounting Standards Codification (ASC) 280, Segment Reporting including the significant segment expense disclosures. The guidance is applied retrospectively to all periods presented in financial statements and is effective for fiscal years beginning after December 15, 2023, and for interim periods beginning after December 15, 2024. Early adoption is permitted. In December 2023, the FASB issued ASU 2023-09, Improvements to Income Tax Disclosures, which requires public business entities to disclose a tabular reconciliation using both percentages and amounts, broken out into specific categories with certain reconciling items at or above 5% of the expected tax further broken out by nature and/or jurisdiction. The guidance also requires all entities to disclose income taxes paid, net of refunds, disaggregated by federal (national), state and foreign taxes for annual periods and to disaggregate the information by jurisdiction based on a quantitative threshold. All entities are required to apply the guidance prospectively, with the option to apply it retrospectively. The guidance is effective for the Company’s fiscal year beginning January 1, 2025. Early adoption is permitted.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2023

Basis of Preparation and Presentation of Financial Information

These Consolidated Financial Statements have been prepared in accordance with the accounting principles generally accepted in the U.S. (“U.S. GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”). These Consolidated Financial Statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. The Consolidated Financial Statements of Prothena Corporation plc are presented in U.S. dollars, which is the functional currency of the Company and its consolidated subsidiaries. Monetary assets and liabilities denominated in foreign currency are remeasured at period-end exchange rates. Foreign currency gains and losses resulting from remeasurement are recognized in other expense, net in the Consolidated Statements of Operations.

Use of Estimates

The preparation of the Consolidated Financial Statements in conformity with U.S. GAAP requires the Company to make judgments, estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosures. On an ongoing basis, management evaluates its estimates, including critical accounting policies or estimates related to revenue recognition and research and development expenses. The Company bases its estimates on historical experience and on various other market specific and other relevant assumptions that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that

are not readily apparent from other sources. Because of the uncertainties inherent in such estimates, actual results may differ materially from these estimates.

Cash, Cash Equivalents and Restricted Cash

Cash, Cash Equivalents, and Restricted Cash

The Company considers all highly liquid investments held at financial institutions, such as commercial paper, money market funds, and other money market securities with original maturities of three months or less at date of purchase to be cash equivalents.

Cash accounts that are restricted to withdrawal or usage are presented as restricted cash. As of December 31, 2023, the Company had $2.2 million of restricted cash held by a bank in certificates of deposit as collateral to standby letters of credit under certain operating leases. See Note 6, "Commitments and Contingencies" for additional information regarding the Company’s operating leases.

Accounts Receivable

The accounts receivable balance on the Consolidated Balance Sheets represents amounts receivable from the Company's collaboration partners. The Company monitors the financial performance and creditworthiness of customers so that it can properly assess and respond to changes in their credit profiles. The Company makes judgments as to its ability to collect outstanding receivables and provides an allowance for credit losses against the trade account receivables, when appropriate. As of December 31, 2023, the Company has not provided any allowance for credit losses against its outstanding accounts receivables.

Property and Equipment, net

Property and equipment, net are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the related assets. Maintenance and repairs are charged to expense as incurred, and leasehold improvements where the Company is deemed the accounting owner are capitalized. When assets are retired or otherwise disposed of, the cost and accumulated depreciation are removed from the balance sheet and any resulting gain or loss is reflected in operations in the period realized.

Impairment of Long-lived Assets

The Company periodically evaluates property and equipment for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable or the estimated useful life is no longer appropriate. If such events or changes in circumstances arise, the Company compares the carrying amount of the asset to the estimated undiscounted future cash flows expected to be generated by the asset. If the carrying amount of the long-lived asset is not recoverable on an undiscounted cash flow basis, an impairment charge is recognized to the extent that the carrying amount exceeds its fair value. The Company determines fair value using the income approach based on the present value of expected future cash flows. The Company’s cash flow assumptions consider historical and forecasted revenue and operating costs and other relevant factors.

Leases

The Company leases both real property and certain equipment for use in its operations. A determination is made as to whether an arrangement is a lease at inception. If so, the Company evaluates the lease agreement to determine whether the lease is an operating or finance lease using the criteria in ASC 842. The Company does not recognize right-of-use assets and lease liabilities that arise from short-term leases for any class of underlying assets.

When lease agreements also require the Company to make additional payments for taxes, insurance and other operating expenses incurred during the lease period, such payments are expensed as incurred. See Note 6, “Commitments and Contingencies,” which provides additional details on the Company's current lease arrangements. As of December 31, 2023 and 2022, the Company had no financing leases.

Operating leases are included in the operating lease right-of-use (“ROU”) assets, lease liability, current and lease liability, non-current in the Company's Consolidated Balance Sheets. Operating lease ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at the lease commencement date based on the present value of all lease payments over the lease term. In determining the present value of lease payments, the Company uses its incremental borrowing rate based on information available at the lease commencement date. The operating lease ROU assets also include any lease prepayments made and exclude lease incentives such as rent abatements and/or concessions and rent holidays. The Company does not assume renewals in its determination of the lease term unless the renewals are deemed by management to be reasonably certain at lease inception. Tenant improvements made by the Company as a lessee in which they are deemed to be owned by the lessor is viewed as lease prepayments by the Company and included in the operating lease ROU assets upon commencement of the lease prior to which they are recorded as prepaid assets. Lease expense for operating leases is recognized on a straight-line basis over the expected lease term as an operating expense. For lease agreements that include lease and non-lease components, such components are generally accounted for separately.

Revenue Recognition

The Company’s collaboration revenue includes revenue recognized for milestone payments and reimbursements under the Company’s License Agreement with Roche as well as revenue recognized under the Company’s Collaboration Agreement with BMS. The Company’s license and intellectual property revenue includes revenue from Novo Nordisk for the sale of intellectual property and related rights to the Company’s ATTR amyloidosis business and pipeline and milestones payments.

The Company analyzes its collaboration arrangements to assess whether they are financing arrangement within the scope of ASC 730 or as a collaboration arrangement pursuant to ASC 808, or whether such arrangements are reflective of a vendor-customer relationship and therefore within the scope of Topic 606. As of December 31, 2023, the Company has not had any arrangements outside the scope of Topic 606. The following describes the Company’s accounting treatment pursuant to Topic 606:

License, Option and Collaboration Revenue

The terms of license, option and collaboration agreements entered into typically include payment of one or more of the following: non-refundable, up-front license fees; option exercise fees; development, regulatory and commercial milestone payments; payments for manufacturing supply and research and development services and royalties on net sales of licensed products.

In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under each of its agreements, the Company performs the following steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation.

Amounts received prior to satisfying the revenue recognition criteria are recorded as contract liabilities recorded as deferred revenue in the Company’s Consolidated Balance Sheets.

At contract inception, for contracts that contain multiple performance obligations, such as the Company’s License Agreement with Roche and the Collaboration Agreement with BMS, the Company accounts for the individual performance obligations separately if they are distinct. Factors considered in the determination of whether the license performance obligations are distinct included, among other things, the research and development capabilities of each of Roche and BMS and their respective sublicense rights, and for the remaining performance obligations the fact that they are not proprietary and can be and have been provided by other vendors. The transaction price is allocated to the separate performance obligation on a relative standalone selling price basis.

Revenue is recognized only when the Company satisfies an identified performance obligation by transferring a promised good or service to a customer (in the Company’s case, Roche and BMS). An asset is transferred when, or as, the customer

obtains control of that asset, which for a service is considered to be as the services are received and used. The Company recognizes revenue over time by measuring the progress toward complete satisfaction of the relevant performance obligation using an appropriate input or output method based on the nature of the service promised to the customer.

Milestone Revenue

The Company generally classifies each of its milestones into one of three categories: (i) clinical milestones; (ii) regulatory and development milestones; and (iii) commercial milestones. Clinical milestones are typically achieved when a product candidate advances into or completes a defined phase of clinical research. For example, a milestone payment may be due to the Company upon the initiation of a clinical trial for a new indication. Regulatory and development milestones are typically achieved upon acceptance of the submission for marketing approval of a product candidate or upon approval to market the product candidate by the FDA or other regulatory authorities. For example, a milestone payment may be due to the Company upon submission for marketing approval of a product candidate by the FDA. Commercial milestones are typically achieved when an approved product reaches certain defined levels of net royalty sales by the licensee of a specified amount within a specified period.

At the inception of each arrangement that includes developmental, regulatory or commercial milestone payments, the Company evaluates whether achieving the milestones is considered probable and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the value of the associated milestone (such as a regulatory submission by the Company) is included in the transaction price. Milestone payments that are not within the control of the Company, such as approvals from regulators or where attainment of the specified event is dependent on the development activities of a third party, are not considered probable of being achieved until those approvals are received or the specified event occurs. The Company considers such milestone payments as variable consideration with constraint and therefore recognizes the revenue from such milestone payments as collaboration revenue at point in time when the Company can conclude it is probable that a significant revenue reversal will not occur in future periods.

Taxes, Shipping and Handling

The Company excludes from the measurement of the transaction price all taxes assessed by a governmental authority that are both imposed on and concurrent with a specific revenue-producing transaction and collected by the Company from a customer (e.g., sales, use, value added, some excise taxes). In addition, the Company accounts for shipping and handling as activities that are performed after its customers obtain control of the goods as activities to fulfill our performance obligation to transfer the goods.

Research and Development

Research and development costs are expensed as incurred. Such costs include, but are not limited to, salaries and benefits, share-based compensation, costs related to preclinical and clinical trial activities including fees paid to clinical research organizations and investigative sites, costs related to drug development and manufacturing prior to regulatory approval for commercial sale, and consulting fees.

There can be judgment involved in measuring the research and development expenses to be recognized in a particular period. The level of judgment varies based on the nature of the services being performed and the underlying support obtained. The Company recognizes costs for certain development activities, such as clinical trials, based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, clinical site activations, or information provided to the Company by our vendors on their actual costs incurred. For certain clinical trials, expense is recorded based on information obtained from vendors as an intermediary to those performing the underlying services, such as contract research organizations. These estimates are inherently more judgmental because the quality and availability of the underlying data may vary. The Company recognizes costs for contract manufacturing based on evaluation of the progress to completion of specific tasks. The objective of the Company’s accrual policy is to match the recording of the expenses in the Consolidated Financial Statements to the actual services the Company has received and efforts expended. As such, expense accruals related to clinical trials and contract manufacturing are recognized based on the Company’s estimate of the degree of completion of the events specified in the specific clinical study or trial contract or drug development and manufacturing contract, respectively. The Company does not make significant estimates where costs incurred are supported by invoices or reports of costs incurred are obtained from a vendor that is directly performing the underlying services, such as a consultant or contract manufacturing organization. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the Consolidated Financial Statements as prepaid or accrued research and development. Amounts due may be fixed fee, fee for service, and may include upfront payments, monthly payments, and payments upon the completion of milestones or receipt of deliverables. Nonrefundable advance payments for goods and services that will be used or received in future research and development activities are deferred and recognized as expense in the period in which the

related goods are delivered or services are performed.

The Company has acquired and may continue to acquire the rights to develop and commercialize new drug candidates from third parties. The upfront payments to acquire license, product or rights, as well as any future milestone payments, are immediately recognized as research and development expense provided that the drug has not achieved regulatory approval for marketing and, absent obtaining such approval, has no alternative future use.

Share-based Compensation

The Company’s share-based compensation programs include options for the purchase of shares and restricted share units (RSUs). Such awards may be granted to employees, directors, and non-employee service providers.

The Company measures compensation expense for all share-based awards at the grant date based on the fair value measurement of the award. Share-based compensation expense is recognized on a straight-line basis over the requisite service period, which is generally the vesting period, for each award. The fair value of RSUs is based on the closing market price of the Company’s ordinary shares on the date of grant. To determine the fair value of options for the purchase of shares, the Company uses the Black-Scholes option-pricing model. The determination of fair value using the Black-Scholes option-pricing model is affected by the Company’s share price as well as assumptions regarding a number of complex and subjective variables. Judgment is required in determining the assumptions used in these models which include the risk-free interest rate, expected term, expected volatility and expected dividend yield. The Company uses its historical volatility for the Company's shares to estimate expected volatility. The simplified method has been used to estimate the expected term of all options in previous years. Beginning January 1, 2023, expected term is estimated based on historical experience.

Share-based compensation expense recognized in the Consolidated Statements of Operations is based on awards expected to vest and therefore the amount of expense has been reduced for estimated forfeitures which are based on historical experience. Share-based compensation expense is adjusted in subsequent periods for actual forfeitures.

The Company records any excess tax benefits or tax shortfalls from its equity awards in its Consolidated Statements of Operations in the reporting periods in which options for the purchase of shares are exercised or RSUs vest.

Income Taxes

The Company files its own U.S. and foreign income tax returns and income taxes are presented in the Consolidated Financial Statements using the asset and liability method prescribed by the accounting guidance for income taxes. Deferred tax assets (“DTAs”) and liabilities are determined based on the difference between the financial statement and tax basis of assets and liabilities using the enacted tax rates projected to be in effect for the year in which the differences are expected to reverse. Net deferred tax assets are recorded to the extent the Company believes that these assets will more likely than not be realized. In making such determination, all available positive and negative evidence is considered, including scheduled reversals of deferred tax liabilities, recent cumulative earnings/losses by taxing jurisdiction, projected future taxable income, tax planning strategies and recent financial operations. Actual operating results in future years could differ from our current assumptions, judgments and estimates.

The Company’s significant tax jurisdictions are Ireland and the United States. Estimates are required in determining the Company’s provision for income taxes. Some of these estimates are based on management’s interpretations of jurisdiction-specific tax laws or regulations. Various internal and external factors may have favorable or unfavorable effects on the future effective income tax rate of the business. These factors include, but are not limited to, changes in tax laws, regulations and/or rates, changing interpretations of existing tax laws or regulations, changes in estimates of prior years’ items, past and future levels of R&D spending, the impact of accounting for share-based compensation, and changes in overall levels of income before taxes.

The Company did not recognize certain tax benefits from uncertain tax positions within the provision for income taxes. The tax benefit from an uncertain tax position is recognized only if it is more likely than not the tax position will be sustained on examination by the taxing authorities, based on the technical merits of the position. The tax benefits recognized in the financial statements from such positions are then measured based on the largest benefit that has a greater than 50% likelihood of being realized upon settlement. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. Interest and penalties related to unrecognized tax benefits are accounted for in income tax expense.

Net Income (Loss) per Ordinary Share

Basic net income (loss) per ordinary share is calculated by dividing net income (loss) by the weighted-average number of ordinary shares outstanding during the period. Diluted net income per ordinary share is computed based on the treasury stock

method by dividing net income by the weighted-average number of ordinary shares outstanding, plus potentially dilutive ordinary equivalent shares outstanding. However, where there is a net loss, no adjustment is made for potentially issuable ordinary shares because their effect would be anti-dilutive and therefore diluted net loss per share is equal to basic net loss per share.

Comprehensive Loss

Comprehensive income (loss) is comprised of net income (loss) and other comprehensive income (loss). The Company has no components of other comprehensive income (loss). Therefore, net income (loss) equals comprehensive income (loss) for all periods presented and, accordingly, the Consolidated Statements of Comprehensive Income (Loss) is not presented in a separate statement.

Segment

The Company operates in one segment. The Company’s chief operating decision maker (the “CODM”), its Chief Executive Officer, manages the Company’s operations and evaluates the Company’s financial performance on a consolidated basis for purposes of allocating resources.

Concentration of Risks

Financial instruments that potentially subject the Company to concentration of credit risk consist of cash and cash equivalents and accounts receivable. The Company places its cash equivalents with high credit quality financial institutions and, by policy, limits the amount of credit exposure with any one financial institution. Deposits held with banks have exceeded, and will continue to exceed, federally insured limits. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash and cash equivalents. The Company has not experienced any losses on its deposits of cash and cash equivalents and its credit risk exposure is up to the extent recorded on the Company's Consolidated Balance Sheet.

The Company’s business is primarily conducted in U.S. dollars except for its agreements with contract manufacturers for drug supplies which are primarily denominated in Euros. The Company recorded a loss on foreign currency exchange rate differences of approximately $458,000, $397,000 and $96,000 during the years ended December 31, 2023, 2022 and 2021, respectively. If the Company increases its business activities that require the use of foreign currencies, it may be exposed to losses if the Euro and other such currencies continue to strengthen against the U.S. dollar.

As of December 31, 2023, and 2022, $3.8 million and $1.7 million, respectively, of the Company’s property and equipment, net were held in the U.S. and a nominal amount were in Ireland.

The Company does not own or operate facilities for the manufacture, packaging, labeling, storage, testing or distribution of nonclinical or clinical supplies of any of its drug candidates. The Company instead contracts with and relies on third-parties to manufacture, package, label, store, test and distribute all preclinical development and clinical supplies of our drug candidates, and it plans to continue to do so for the foreseeable future. The Company also relies on third-party consultants to assist in managing these third-parties and assist with its manufacturing strategy.

Recent Accounting Pronouncements

On November 27, 2023, the FASB issued Accounting Standards Update 2023-07 ("ASU 2023-07"), Segment Reporting- Improvements to Reportable Segment Disclosures, which requires public entities to provide disclosures on significant segment expenses that are regularly provided to the CODM and included within each reported measure of segment profit or loss and other segment items on an annual and interim basis. The guidance also requires public entities to provide all disclosures about reportable segment’s profit or loss and assets in interim periods that are currently required annually. Public entities with a single reportable segment have to provide all disclosures required by Accounting Standards Codification (ASC) 280, Segment Reporting including the significant segment expense disclosures. The guidance is applied retrospectively to all periods presented in financial statements and is effective for fiscal years beginning after December 15, 2023, and for interim periods beginning after December 15, 2024. Early adoption is permitted.

In December 2023, the FASB issued ASU 2023-09, Improvements to Income Tax Disclosures, which requires public business entities to disclose a tabular reconciliation using both percentages and amounts, broken out into specific categories with certain reconciling items at or above 5% of the expected tax further broken out by nature and/or jurisdiction. The guidance also requires all entities to disclose income taxes paid, net of refunds, disaggregated by federal (national), state and foreign taxes for annual periods and to disaggregate the information by jurisdiction based on a quantitative threshold. All entities are required to apply the guidance prospectively, with the option to apply it retrospectively. The guidance is effective for the Company’s fiscal year beginning January 1, 2025. Early adoption is permitted.