Lantheus (LNTH) 424B3Prospectus supplement

Filed Pursuant to Rule 424(b)(3)
Registration No. 333-234627

JOINT PROXY STATEMENT/PROSPECTUS SUPPLEMENT

MAY 14, 2020

This is a supplement (this “supplement”) to the joint proxy statement/prospectus of Lantheus Holdings, Inc. (“Lantheus Holdings”) and Progenics Pharmaceuticals, Inc. (“Progenics”), dated March 19, 2020, that was mailed to stockholders of record as of the close of business on March 18, 2020 in connection with the solicitation of proxies by the board of directors of Lantheus Holdings and the board of directors of Progenics, as applicable, in connection with the merger of Progenics with a wholly-owned subsidiary of Lantheus Holdings pursuant to the terms of an amended and restated merger agreement entered into by Lantheus Holdings and Progenics on February 20, 2020. Capitalized terms used herein but not defined shall have the meanings ascribed to such terms in the joint proxy statement/prospectus.

The purpose of this supplement is, among other things:

To the extent information in this supplement differs from, updates or conflicts with information contained in the joint proxy statement/prospectus, the information in this supplement is the more current information, and shall supersede or supplement such other information. Lantheus Holdings and Progenics urge you to read carefully and in their entirety the joint proxy statement/prospectus and this supplement.

The joint proxy statement/prospectus and this supplement may also be found on the Internet at www.sec.gov. See “Where You Can Find More Information” beginning on page 260 of the joint proxy statement/prospectus and on page 58 of this supplement.

This supplement is being filed with the Securities and Exchange Commission and is first being mailed to Lantheus Holdings and Progenics stockholders on or about May 14, 2020.

THIS SUPPLEMENT SHOULD BE READ IN CONJUNCTION WITH THE JOINT PROXY

STATEMENT/PROSPECTUS.

NOTICE OF CHANGE OF DATE AND LOCATION OF

THE SPECIAL MEETING OF STOCKHOLDERS OF LANTHEUS HOLDINGS, INC.

TO BE HELD ON JUNE 16, 2020

To the Stockholders of Lantheus Holdings, Inc.:

Lantheus Holdings, Inc. (“Lantheus Holdings”) has changed the date of its upcoming special meeting of stockholders called for the purpose of approving certain proposals relating to the Amended and Restated Agreement and Plan of Merger, dated as of February 20, 2020, among Lantheus Holdings, Plato Merger Sub, Inc. and Progenics Pharmaceuticals, Inc. from April 28, 2020 to June 16, 2020 at 11:15 a.m. Eastern Time.

The special meeting of stockholders will be a virtual meeting conducted exclusively via live webcast due to public health concerns relating to theCOVID-19 pandemic and governmental actions related thereto. The Lantheus Holdings board of directors believes that this is the right choice for Lantheus Holdings and its stockholders at this time, as it permits stockholders to attend and participate in the special meeting from any location around the world while safeguarding the health and safety of all of our people, including our stockholders. Lantheus Holdings is committed to ensuring that Lantheus Holdings stockholders will be afforded the same rights and opportunities to participate as they would at anin-person meeting.

There is no change to the proposals to be presented to our stockholders for consideration at the special meeting of stockholders.

The Lantheus Holdings board of directors has fixed the close of business on May 12, 2020 as the record date for the determination of the stockholders entitled to vote at the Lantheus Holdings special meeting or any adjournment or postponement thereof. Only stockholders of record at the record date are entitled to notice of, and to vote at, the Lantheus Holdings special meeting or any adjournment or postponement thereof.

If you previously submitted a proxy with respect to the proposals for the Lantheus Holdings special meeting, your vote will not be counted. You must submit a new proxy (or proxies) in order for your vote to be counted or you must virtually attend and vote at the Lantheus Holdings special meeting. A new proxy card is supplied with this supplement and may be used to appoint a proxy to vote at the Lantheus Holdings special meeting. Lantheus Holdings urges you to discard any proxy cards originally sent to you. Only votes received using the new proxy card will be counted.

If you have any questions about the merger or about how to vote or direct a vote in respect of your shares of Lantheus Holdings common stock, you may contact Lantheus Holdings’ proxy solicitor, Innisfree M&A Incorporated, toll-free at (877)750-9497 or (412)232-3651 for international callers.

By order of the Board of Directors,

Brian Markison

Chairman of the Board of Directors

North Billerica, Massachusetts

May14, 2020

NOTICE OF CHANGE OF DATE AND LOCATION OF THE SPECIAL MEETING OF STOCKHOLDERS OF PROGENICS PHARMACEUTICALS, INC. ORIGINALLY SCHEDULED FOR APRIL 28, 2020

To the Stockholders of Progenics Pharmaceuticals, Inc.:

Progenics Pharmaceuticals, Inc. (“Progenics”) has changed the date of its upcoming special meeting of stockholders called for the purpose of approving certain proposals relating to the Amended and Restated Agreement and Plan of Merger, dated as of February 20, 2020, among Progenics, Lantheus Holdings, Inc. and Plato Merger Sub, Inc. from April 28, 2020 to June 16, 2020 at 10:00 a.m. Eastern Time.

The special meeting of stockholders will be a virtual meeting conducted exclusively via live webcast due to public health concerns relating to theCOVID-19 pandemic and governmental actions related thereto. The Progenics board of directors believes that this is the right choice for Progenics and its stockholders at this time, as it permits stockholders to attend and participate in the special meeting from any location around the world while safeguarding the health and safety of all of our people, including our stockholders.

To access the Progenics’ special meeting’s live webcast, you will need tolog-in to www.virtualshareholdermeeting.com/PGNX2020SM using the control number found on your proxy card or voting instruction form.

There is no change to the proposals to be presented to our stockholders for consideration at the special meeting of stockholders.

The Progenics board of directors has fixed the close of business on May 12, 2020 as the record date for the determination of the stockholders entitled to vote at the Progenics special meeting or any adjournment or postponement thereof. Only stockholders of record at the record date are entitled to notice of, and to vote at, the Progenics special meeting or any adjournment or postponement thereof.

If you previously submitted a proxy with respect to the proposals for the Progenics special meeting, such proxy is invalid. You must submit a new proxy (or proxies) in order for your vote to be counted or you must virtually attend and vote at the Progenics special meeting. A new proxy card is supplied with this supplement and may be used to appoint a proxy to vote at the Progenics special meeting. Progenics urges you to discard any proxy cards originally sent to you. Only votes received using the new proxy card will be counted.

For additional details on the virtual meeting, please seepages 4-9 of this supplement.

By order of the Board of Directors,

Ann MacDougall

Interim Chairman of the Board of Directors of Progenics

New York, New York

May 14, 2020

ADDITIONAL QUESTIONS AND ANSWERS

The section of the joint proxy statement/prospectus entitled “Questions and Answers about the Merger and the Special Meetings” is hereby supplemented as follows:

The following questions and answers are intended to address briefly some questions regarding attending the special meetings of Lantheus Holdings and Progenics by virtual means. These questions and answers may not address all questions that may be important to you as a Lantheus Holdings or Progenics stockholder. Please refer to the more detailed information contained elsewhere in this supplement and the joint proxy statement/prospectus, each of which Lantheus Holdings and Progenics urge you to read in its entirety. You may obtain the information incorporated by reference in this supplement without charge by following the instructions under “Where You Can Find More Information” on page 260 of the joint proxy statement/prospectus and on page 58 of this supplement.

Additional Questions and Answers for Both Lantheus Holdings and Progenics Stockholders

This supplement provides important information about virtual access to the Lantheus Holdings and Progenics special meetings, as applicable. Lantheus Holdings and Progenics are also filing this supplement to, among other things, update unaudited pro forma condensed combined financial statements reflecting the results for the three months ended March 31, 2020, to add additional risk factors to the joint proxy statement/prospectus and to supplement certain disclosures in the joint proxy statement/prospectus.

For Lantheus Holdings Stockholders

The Lantheus Holdings special meeting will convene at 11:15 a.m. Eastern Time on June 16, 2020 in a virtual meeting format only. Because the Lantheus Holdings special meeting is completely virtual and being conducted via live webcast, Lantheus Holdings stockholders will not be able to attend the meeting in person.

Lantheus Holdings believes that hosting a virtual meeting will facilitate stockholder attendance and participation at the special meeting by enabling stockholders to participate remotely from any location around the world. Lantheus Holdings has designed the special meeting to provide the same rights and opportunities to participate as stockholders as they would at anin-person meeting.

1

Your vote is important. Accordingly, please submit your proxy promptly by Internet, telephone or mail whether or not you plan to virtually attend the Lantheus Holdings special meeting.

If you have previously voted against any of the Lantheus Holdings proposals, Lantheus Holdings encourages you to reconsider your vote and to vote“FOR” the stock issuance proposal and the Lantheus Holdings adjournment proposal.

2

We are committed to ensuring that stockholders will be afforded the same rights and opportunities to participate as they would at anin-person meeting. We will try to answer as many stockholder-submitted questions as time permits, provided that we reserve the right to edit profanity or other inappropriate language, or to exclude questions that are determined by the chairman of the Lantheus Holdings special meeting to not be pertinent to meeting matters or otherwise inappropriate. If substantially similar questions are received, we will group such questions together and provide a single response to avoid repetition. We may post questions and answers if applicable to our business on Lantheus Holdings’ investor relations website at https://investor.lantheus.com/investor-relations following the special meeting.

Innisfree M&A Incorporated

501 Madison Avenue, 20^th Floor

New York, New York 10022

Telephone (Toll-Free): (877)750-9497

International Callers: (412)232-3651

Banks and brokers may call collect: (212)750-5833

3

For Progenics Stockholders

Progenics believes that hosting a virtual meeting will facilitate stockholder attendance and participation at the special meeting by enabling stockholders to participate remotely from any location around the world.

Your vote is important. Accordingly, please submit your proxy promptly by Internet, telephone or mail whether or not you plan to virtually attend the Progenics special meeting.

If you have previously voted against any of the Progenics proposals, Progenics encourages you to reconsider your vote and to vote“FOR” the merger agreement proposal, the Progenics adjournment proposal and the compensation advisory proposal.

4

MacKenzie Partners, Inc.

1407 Broadway, 27^th Floor

New York, New York 10018

Toll-Free: (800)322-2885

or

(212)929-5500

Email: pgnx@mackenziepartners.com

5

SUPPLEMENT TO QUESTIONS AND ANSWERS ABOUT THE MERGER AND SPECIAL MEETINGS

On page 3 of the joint proxy statement/prospectus, the answer to the last question is amended and restated as follows (new language and changes underlined):

Based on the closing price of shares of Lantheus Holdings common stock on the Nasdaq Global Market on February 19, 2020, the last trading day prior to announcement of the amended transaction between Lantheus Holdings and Progenics, the stock consideration represented approximately $5.12 in value for each share of Progenics common stock. Based on the closing price of shares of Lantheus Holdings common stock on the Nasdaq Global Market onMay 13, 2020, the most recent trading day prior to the date of thissupplement for which this information was available, the stock consideration represented approximately $3.66 in value for each share of Progenics common stock.

Because Lantheus Holdings will issue 0.31 of a share of Lantheus Holdings common stock in exchange for each share of Progenics common stock, the value of the stock consideration will depend on the market price of shares of Lantheus Holdings common stock at the time the merger is completed. The market price of shares of Lantheus Holdings common stock when Progenics stockholders receive those shares after the merger is completed will not be known at the time of the special meetings and could be greater than, less than or the same as the market price of shares of Lantheus Holdings common stock on February 19, 2020, on the date ofthe joint proxy statement/prospectusor this supplement or at the time of the special meetings or any adjournment or postponement thereof. Because the exchange ratio is fixed and the market price of shares of Lantheus Holdings common stock has fluctuated and will continue to fluctuate, Progenics stockholders cannot be sure of the value of the stock consideration they will receive in the merger. See “Risk Factors—Risks Related to the Merger.”

On page 4 of the joint proxy statement/prospectus, the answer to the last question is amended and restated as follows (new language and changes underlined):

In addition, payments under the CVRs are subject to an aggregate payment cap. The sum of (i) the aggregate amount of payments paid or payable pursuant to the CVR agreement (including any interest on such amounts paid or payable to the rights agent or any CVR holder) and (ii) the amount of any other cash or the fair market value of any property (other than Lantheus Holdings common stock or the CVRs) paid or payable to Progenics stockholders as consideration pursuant to the merger agreement will not (A) exceed 19.9% of the aggregate amount of consideration paid or payable to Progenics stockholders in the merger or (B) constitute an amount the payment of which, in the opinion of nationally recognized tax counsel, would more likely than not prevent the merger from satisfying the requirement of Section 368(a)(2)(E)(ii) of the

6

U.S. Internal Revenue Code of 1986, as amended, which is referred to inthe joint proxy statement/prospectus as the Code. Such amount is referred to inthe joint proxy statement/prospectus as the CVR cap.

In order to satisfy the requirement of Section 368(a)(2)(E)(ii) of the Code, the aggregate amount of payments paid or payable pursuant to the CVR agreement could be reduced depending on the amount of any other cash or the fair market value of any property (other than Lantheus Holdings common stock or the CVRs) paid or payable to Progenics stockholders as consideration received in the merger, including the amount payable to Progenics stockholders exercising their appraisal rights under Section 262 of the DGCL and the reimbursement amount paid by Progenics to the Velan stockholders pursuant to the expense reimbursement agreement, which is referred to in this supplement as the Velan reimbursement amount, in each case, to the extent treated as paid by Lantheus Holdings for U.S. federal income tax purposes.

On page 7 of the joint proxy statement/prospectus, the answer to the second question is amended and restated as follows (new language and changes underlined):

On page 7 of the joint proxy statement/prospectus, the answer to the third question is amended and restated as follows (new language and changes underlined):

On page 10 of the joint proxy statement/prospectus, the answer to the first question is amended and restated as follows (new language and changes underlined):

If you are a stockholder of record at the record date for the Lantheus Holdings special meeting (the close of business onMay 12, 2020), you can revoke your proxy or change your vote by:

7

If you hold your shares in “street name” through a broker, bank or other nominee, you must contact your broker, bank or other nominee to change your vote or obtain a written legal proxy to vote your shares if you wish to cast your votevia virtual means at the Lantheus Holdings special meeting.

On page 10 of the joint proxy statement/prospectus, the answer to the second question is amended and restated as follows (new language and changes underlined):

On page 13 of the joint proxy statement/prospectus, the answer to the first question is amended and restated as follows (new language and changes underlined):

On page 13 of the joint proxy statement/prospectus, the answer to the second question is amended and restated as follows (new language and changes underlined):

8

On page 15 of the joint proxy statement/prospectus, the answer to the fourth question is amended and restated as follows (new language and changes underlined):

If you are a stockholder of record at the record date for the Progenics special meeting (the close of business onMay 12, 2020), you can revoke your proxy or change your vote by:

On page 16 of the joint proxy statement/prospectus, the answer to the first question is amended and restated as follows (new language and changes underlined):

9

SUPPLEMENT TO RISK FACTORS

The section of the joint proxy statement/prospectus entitled “Risk Factors” is hereby supplemented as follows:

In addition to the other information contained in or incorporated by reference into this supplement and the joint proxy statement/prospectus, including the matters addressed in “Cautionary Statement Regarding Forward-Looking Statements” beginning on page 42 of this supplement and on page 74 of the joint proxy statement/prospectus, Lantheus Holdings and Progenics stockholders should carefully consider the following risk factors and the risk factors contained in the joint proxy statement/prospectus before deciding whether to vote for the stockholder proposals. You should also read and consider the risk factors associated with each of the businesses of Lantheus Holdings and Progenics because these risk factors may affect the operations and financial results of the combined company. These risks may be found under Part I, Item 1A, “Risk Factors” in each company’s Annual Report on Form10-K for the year ended December 31, 2019 and in Part II, Item 1A, “Risk Factors” in each company’s Quarterly Report on Form10-Q for the quarterly period ended March 31, 2020, and future filings with the SEC, each of which is on file or will be filed with the SEC and all of which are incorporated by reference in this supplement.

The first risk factor on page 41 of the joint proxy statement/prospectus is amended and restated as follows (new language and changes underlined):

Because the exchange ratio is fixed and will not be adjusted in the event of any changes in either Lantheus Holdings’ or Progenics’ stock price, and because of the uncertainty of the fair market value of, and the ultimate realization on, the CVRs, Progenics stockholders cannot be sure of the value of the merger consideration they will receive in the merger.

Upon completion of the merger, each share of Progenics common stock outstanding immediately prior to the completion of the merger (other than excluded shares and dissenting shares) will be converted into the right to receive 0.31 of a share of Lantheus Holdings common stock without interest and one CVR representing the right to receive, subject to the CVR cap, such stockholder’s pro rata share of aggregate cash payments equal to 40% of U.S. net sales generated by PyL in calendar years 2022 and 2023 in excess of $100 million and $150 million, respectively, as set forth in the CVR agreement. The exchange ratio is fixed and will not be adjusted for changes in the market price of either Lantheus Holdings common stock or Progenics common stock between the date the merger agreement was signed and completion of the merger. As a result, changes in the price of Lantheus Holdings common stock prior to the completion of the merger will affect the value of the Lantheus Holdings common stock that Progenics stockholders will receive upon completion of the merger.

The market price of shares of Lantheus Holdings common stock has fluctuated since the date of the announcement of the merger agreement and may continue to change through the date of each of the Lantheus Holdings and the Progenics special meetings and the date the merger is completed. For example, based on the range of closing prices of Lantheus Holdings common stock during the period from February 19, 2020, the last full trading day before the public announcement of the merger, throughMay 13, 2020, thelast practicable trading dateprior to the date of thissupplement, the exchange ratio resulted in an implied value of the stock consideration ranging from a high of approximately $5.17 to a low of approximately $2.82 for each share of Progenics common stock. The actual market value of the Lantheus Holdings common stock received by holders of Progenics common stock may result in an implied value of the stock consideration outside this range. There is also uncertainty regarding the fair market value of the CVRs and whether any payment will ultimately be realized on the CVRs.

Stock price changes may result from a variety of factors, including, among others, general market and economic conditions, changes in Lantheus Holdings’ and Progenics’ respective businesses, operations and prospects, risks inherent in their respective businesses,reductions or changes in U.S. government spending or budgetary policies, the effects of pandemics and viruses including the recent worldwide pandemic of the novel strain ofCOVID-19, catastrophic events, both natural andman-made, changes in market assessments of the likelihood that the merger

10

will be completed and/or the value that may be generated by the merger, and changes with respect to expectations regarding the timing of the merger and regulatory considerations.Many of these factors are outside the control of Lantheus Holdings and Progenics.Due to the negative effect of theCOVID-19 pandemic and other factors, the financial markets have experienced significant volatility since the announcement of the merger. Stock prices in the broader equity market, including the prices of the Lantheus Holdings common stock and the Progenics common stock, have generally decreased, experienced fluctuations and may further fluctuate through the date of each of the Lantheus Holdings and the Progenics special meetings and the date the merger is completed. At the time of the Progenics special meeting, Progenics stockholders will not know with certainty the value of the shares of Lantheus Holdings common stock that they will receive upon completion of the merger.

The second risk factor on page 42 of the joint proxy statement/prospectus is amended and restated as follows (new language and changes underlined):

The merger is subject to conditions, some or all of which may not be satisfied, or completed on a timely basis, if at all. Failure to complete the merger could have material adverse effects on Lantheus Holdings’ and Progenics’ businesses.

The completion of the merger is subject to a number of conditions in addition to the termination or expiration of any applicable waiting period (and any extension thereof) under the HSR Act(which has been obtained by grant of early termination of the HSR Act waiting period on October 25, 2019), including, among other conditions: (i) approval of the merger agreement proposal by Progenics stockholders at the Progenics special meeting; (ii) approval of the stock issuance proposal by Lantheus Holdings stockholders at the Lantheus Holdings special meeting; (iii) approval for the listing on the Nasdaq Global Market of the shares of Lantheus Holdings common stock to be issued in the merger and such other shares to be reserved for issuance in connection with the merger; (iv) the absence of any law enacted or order issued by a governmental authority of competent jurisdiction having the effect of making the merger illegal or otherwise prohibiting completion of the merger; (v) the accuracy of the representations and warranties made in the merger agreement by the other party, subject to the applicable materiality standards set forth in the merger agreement; (vi) the absence of any fact, circumstance, condition, development, change, event, occurrence or effect that has had or would reasonably be expected to have, individually or in the aggregate with all other facts, circumstances, conditions, developments, changes, events, occurrences or effects, a material adverse effect on Progenics since the date of the original merger agreement; (vii) performance or compliance in all material respects by the other party with all the agreements and covenants required to be performed by such party under the merger agreement on or prior to the closing date; and (viii) the effectiveness of the registration statement on FormS-4 relating to the merger and no stop order suspending the effectiveness of the registration statement and no proceedings for such purpose initiated or threatened by the SEC.The impact of theCOVID-19 pandemic and the resulting social and economic disruption may increase the risk that one or more of the closing conditions will not be satisfied and the merger will not occur or that the completion of the merger will be delayed.

11

In addition, the merger agreement contains certain termination rights for both Lantheus Holdings and Progenics, including, among other things, (i) if the merger has not been completed by July 1, 2020, (ii) if the required approval of Lantheus Holdings stockholders or the Progenics stockholders is not obtained, (iii) if the other party willfully breaches itsnon-solicitation obligations in the merger agreement, (v) if the other party materially breaches its representations, warranties or covenants which breach or failure to perform would result in a failure to satisfy any condition to completion of the merger related to the accuracy of its representations and warranties or the performance of its covenants and agreements, and fails to cure such breach, (vi) if any law or order prohibiting the merger or the stock issuance has become final and nonappealable, or (vii) if the board of directors of the other party changes its recommendation in connection with the merger. If the merger is not completed, the ongoing business of Lantheus Holdings and Progenics may be materially adversely affected and, without realizing any of the benefits that they could have realized had the merger been completed, they will be subject to a number of risks, including the following:

In addition, if the merger is not completed, Lantheus Holdings and Progenics could be subject to litigation related to any failure to complete the merger or related to any enforcement proceeding commenced against either Lantheus Holdings or Progenics to perform its obligations under the merger agreement. If any such risk materializes, it could adversely impact either Lantheus Holdings’ or Progenics’ ongoing business. Similarly, delays in the completion of the merger could, among other things, result in additional transaction costs, loss of revenue or other negative effects associated with uncertainty about completion of the merger and cause them not to realize some or all of the benefits that they expect to achieve if the merger is successfully completed within its expected timeframe. We cannot assure you that the conditions to the closing of the merger will be satisfied or waived or that the merger will be completed.

12

In addition, Progenics anticipates it being necessary to obtain additional financing in order to meet its cash requirements if the merger is not consummated. However, there can be no assurances that Progenics will be able to obtain additional capital on acceptable terms and in the amounts necessary to fully fund Progenics’ current operating expenses or reduced operating expenses reflecting delays or reductions in the scope of Progenics’ product development programs beyond 2020. These and other circumstances raise substantial doubt about Progenics’ ability to continue as a going concern if the merger is not consummated. Progenics’ independent registered public accounting firm has issued an opinion on Progenics’ financial statements as of and for the period ended December 31, 2019 included in Progenics’ Annual Report on Form10-K, and incorporated by reference inthe joint proxy statement/prospectus, that states that there is substantial doubt about Progenics’ ability to continue as a going concern and that the financial statements do not include any adjustments relating to the recoverability and classification of liabilities that might be necessary should Progenics be unable to continue as a going concern.

On March 15, 2020, Progenics, as borrower, and Lantheus Medical Imaging, as lender, entered into a bridge loan agreement, pursuant to which Lantheus Medical Imaging agreed to provide for a secured short-term loan to Progenics on or after May 1, 2020 in an aggregate principal amount of up to $10 million. The bridge loan matures on the earlier to occur of (a) September 30, 2020 and (b) the date on which Progenics enters into a debt financing or similar arrangements or any amendment to, or replacement of, its existing debt provided by one or more third parties following the termination date of the merger agreement, in either case, having aggregate net cash proceeds that exceed the amount then required to repay all obligations under the bridge loan agreement in full in cash. If such bridge loan is made by Lantheus Medical Imaging, there is no assurance that such borrowings will be repaid in accordance with the terms of the bridge loan agreement, or at all. See “Bridge Loan Agreement” beginning on page 210 ofthe joint proxy statement/prospectus for a more detailed description of the bridge loan agreement.

The third risk factor on page 46 of the joint proxy statement/prospectus is amended and restated as follows (new language and changes underlined):

The unaudited pro forma combined financial information and prospective financial information included inthe joint proxy statement/prospectus are presented for illustrative purposes only and do not represent the actual financial position or results of operations of the combined company following completion of the merger.

The unaudited pro forma combined financial information and prospective financial information contained inthe joint proxy statement/prospectus is presented for illustrative purposes only, contains a variety of adjustments, assumptions and preliminary estimatesmade prior to the time Lantheus Holdings and Progenics entered into the merger agreement and, in some instances, the original merger agreement,and does not represent the actual financial position or results of operations of Lantheus Holdings and Progenicsbetween the date on which the merger agreement was entered into and the date of this supplement, or any other time prior to the merger or that of the combined company following the merger for several reasons. Among other things, the unaudited pro forma combined financial information does not reflect the projected realization of cost savings following completion of the merger. See the sections entitled “Certain Unaudited Pro Forma Condensed Combined Financial Statements,” “Lantheus Holdings Proposal I: Approval of the Stock Issuance and Progenics Proposal I: Adoption of the Merger Agreement—Certain Unaudited Prospective Financial Information” and “Comparative Historical and Unaudited Pro Forma Combined Per Share Data” beginning on pages 59, 157 and 57, respectively, ofthe joint proxy statement/prospectus, and the sections entitled “Certain Unaudited Pro Forma Condensed Combined Financial Statements,” “Update to Comparative Historical and Unaudited Pro Forma Combined Per Share Data” and “Supplement to Lantheus Holdings Proposal I: Approval of the Stock Issuance and Progenics Proposal I: Adoption of the Merger Agreement” beginning on pages 25, 23 and 47 of this supplement.

The actual financial positions and results of operations of Lantheus Holdings and Progenicsbetween the date on which the merger agreement was entered into and the date of this supplement, and any other time prior to the

13

merger and that of the combined company following the merger may not be consistent with, or evident from, and may differ materially and adversely from, the unaudited pro forma combined financial information or prospective financial information included inthe joint proxy statement/prospectus. In addition, the assumptions used in preparing the unaudited pro forma combined financial information and/or the prospective financial information included inthe joint proxy statement/prospectus may not be realized, may not prove to be accurate and may be affected by other factors, such as the potential impact of currentmarket conditions, including the impact of theCOVID-19 pandemic, which could lead to material changes to the combined company’s business that are not reflected in the unaudited pro forma combined financial information.

The first full risk factor on page 47 of the joint proxy statement/prospectus is amended and restated as follows (new language and changes underlined):

The opinions of Progenics’ and Lantheus Holdings’ respective financial advisors do not reflect changes in circumstances that may have occurred or that may occur between the signing of the merger agreement and the completion of the merger.

Neither the Progenics Board nor the Lantheus Holdings Board has obtained updated opinions from their respective financial advisors as of the date ofthe joint proxy statement/prospectus, nor do either of them expect to receive updated, revised or reaffirmed opinions prior to the completion of the merger. Changes in the operations and prospects of Progenics or Lantheus Holdings, general market and economic conditions and other factors that may be beyond the control of Progenics or Lantheus Holdings,including, without limitation, the impact of theCOVID-19 pandemic on the operations, financial condition and prospects of Lantheus Holdings and Progenics, and on which Progenics’ and Lantheus Holdings’ financial advisors’ opinions were based, may significantly alter the value of Progenics or Lantheus Holdings or the share prices of Progenics common stock or Lantheus Holdings common stock by the time the merger is completed. The opinions do not speak as of the time the merger will be completed or as of any date other than the date of such opinions. Because Progenics’ and Lantheus Holdings’ financial advisors will not be updating their opinions, the opinions will not address the fairness of the merger consideration from a financial point of view at the time the merger is completed. For a description of the opinions that the Progenics Board and the Lantheus Holdings Board received from their respective financial advisors, see “Lantheus Holdings Proposal I: Approval of the Stock Issuance and Progenics Proposal I: Adoption of the Merger Agreement—Opinion of Lantheus Holdings’ Financial Advisor” and “Lantheus Holdings Proposal I: Approval of the Stock Issuance and Progenics Proposal I: Adoption of the Merger Agreement—Opinion of Progenics’ Financial Advisor” beginning on pages 140 and 148, respectively, ofthe joint proxy statement/prospectus.

The third risk factor on page 49 of the joint proxy statement/prospectus is amended and restated as follows (new language and changes underlined):

Progenics stockholders may not receive any payments under the CVRs, which makes it difficult to value the CVRs, and payments under the CVRs are subject to an aggregate payment cap.

Under the merger agreement, in addition to the stock consideration, holders of shares of Progenics common stock have the right to receive one CVR for each share of Progenics common stock held by such person. Each CVR will entitle its holder to receive, subject to the CVR cap, his, her or its pro rata share of aggregate cash payments equal to 40% of U.S. net sales generated by PyL in calendar years 2022 and 2023 in excess of $100 million and $150 million, respectively. The CVRs will not have any voting or dividend rights and will not represent any equity or ownership interest in Lantheus Holdings, Merger Sub, Progenics or any of their respective affiliates. Therefore, Progenics stockholders’ right to receive any future payment with respect to the CVRs will be contingent solely upon the achievement of the foregoing milestones within the specified time periods and other terms and conditions of the CVR agreement, which may or may not occur and may be outside the control of Lantheus Holdings and Progenics. If the milestones are not achieved, no payment will be made under the CVRs and the CVRs will expire valueless. Accordingly, the value, if any, of the CVRs is speculative, and the CVRs may ultimately have no value at all.

14

In addition, payments under the CVRs are subject to an aggregate payment cap. The sum of (i) the aggregate amount of payments paid or payable pursuant to the CVR agreement (including any interest on such amounts paid or payable to the rights agent or any CVR holder) and (ii) the amount of any other cash or the fair market value of any property (other than Lantheus Holdings common stock or the CVRs) paid or payable to Progenics stockholders as consideration pursuant to the merger agreement will not (A) exceed 19.9% of the aggregate amount of consideration paid or payable to Progenics stockholders in the merger or (B) constitute an amount the payment of which, in the opinion of nationally recognized tax counsel, would more likely than not prevent the merger from satisfying the requirement of Section 368(a)(2)(E)(ii) of the Code. For a more detailed description of the CVRs, see the section entitled “Description of the CVRs” beginning on page 225 ofthe joint proxy statement/prospectusand the section entitled “Supplement to the Description of the CVRs” beginning on page 50 of this supplement.

In order to satisfy the requirement of Section 368(a)(2)(E)(ii) of the Code, the aggregate amount of payments paid or payable pursuant to the CVR agreement could be reduced depending on the amount of any other cash or the fair market value of any property (other than Lantheus Holdings common stock or the CVRs) paid or payable to Progenics stockholders as consideration received in the merger, including the amount payable to Progenics stockholders exercising their appraisal rights under Section 262 of the DGCL and the Velan reimbursement amount, in each case to the extent treated as paid by Lantheus Holdings for U.S. federal income tax purposes.

The third risk factor on page 52 of the joint proxy statement/prospectus is amended and restated as follows (new language and changes underlined):

A pandemic, epidemic or outbreak of an infectious disease, such asCOVID-19, or coronavirus, may materially and adversely affect Lantheus Holdings or Progenics and their respective financial results.

The recent outbreak ofCOVID-19 originated in Wuhan, China, in December 2019 and has since spread throughout the world including the United States and Europe. The spread ofCOVID-19 has already affected the global economy and may adversely affect Lantheus Holdings’ or Progenics’ operations and businesses by causing a period of business disruption, including the potential interruption of global supply chains, manufacturing activities, logistics, radiopharmacies and other distribution channels as well as commercial and clinical development activities. Also, this outbreak may impact the operations of hospitals and clinics at which Lantheus Holdings’ or Progenics’ commercial products are used or product candidates are tested, and it may affect the number of medical procedures in which those products are used or product candidates are tested, thereby potentially reducing demand for commercial products and delaying clinical development of product candidates. In addition, there could be a potential effect ofCOVID-19 on FDA or other health or regulatory authorities, which could result in delays of reviews and approvals, including with respect to Lantheus Holdings’ or Progenics’ product candidates.

Recent actions by U.S. federal, state and foreign governments to address theCOVID-19 pandemic, including thestay-at-home mandates and related safety measures such as the delay or cancellation of elective medical procedures and/or treatments or the inability of some patients to visit hospitals or clinics, may also have a significant adverse effect on the markets in which Lantheus Holdings and Progenics conduct their businesses. The impact to date also has included periods of significant volatility in various markets and industries. This volatility could have an adverse impact on Lantheus Holdings or Progenics’ customers and on the companies’ business, financial condition and results of operations. The extent of impacts resulting from theCOVID-19 pandemic and other events beyond the control of Lantheus Holdings, Progenics and the combined company will depend on future developments, which are highly uncertain and cannot be predicted, including new information which may emerge concerning the severity of theCOVID-19 outbreak and actions taken to containCOVID-19 or its impact, among others.

Neither Lantheus Holdings nor Progenics can presently predict the scope and severity of any potential business shutdowns or disruptions, but if Lantheus Holdings or Progenics or any of the third parties with whom Lantheus Holdings or Progenics engages were to experience shutdowns or other business disruptions, Lantheus Holdings’

15

or Progenics’ ability to conduct business in the manner and on the timelines presently planned could be materially and negatively impacted, which could have a material adverse effect on Lantheus Holdings’ or Progenics’ business and their results of operation and financial condition.

The following risk factor is added as a new risk factor after the first full paragraph on page 53 of the joint proxy statement/prospectus:

TheCOVID-19 pandemic could have a material impact on the business, results of operations and financial condition, operating results, cash flows and prospects of the combined company.

In December 2019, a novel strain of coronavirus(COVID-19) emerged in Wuhan China. Less than four months later, in March 2020, the World Health Organization declaredCOVID-19 a pandemic. While the outbreak initially was largely concentrated in China and caused significant disruptions in its economy, the virus has now spread to many other countries and regions, and every state within the United States, including Massachusetts, where the primary offices and manufacturing facility of the combined company will be located, as well as New York, New Jersey, Puerto Rico and Canada, where other offices and manufacturing facilities of the combined company will be located.

Towards the end of the first quarter of 2020, each of Lantheus Holdings and Progenics began to experience, and through the date of this filing, each are continuing to experience impacts to their respective businesses and operations related to theCOVID-19 pandemic, including the impact ofstay-at-home mandates and related safety measures such as the delay of elective medical procedures and/or treatments, resulting in a decline in the volume of procedures and/or treatments using Lantheus Holdings and Progenics products. In response to the pandemic, healthcare providers have had to, and may need to further, reallocate resources, such as physicians, staff and facilities, as they prioritize limited resources and personnel capacity to focus on the treatment of patients withCOVID-19 and implement limitations on access to hospitals and other medical institutions due to concerns about the potential spread ofCOVID-19 in such settings. These actions have significantly delayed the provision of other medical care including elective and diagnostic procedures and/or treatments involving Lantheus Holdings and Progenics products, having an adverse effect on each company’s revenue. These measures and challenges may continue for the duration of theCOVID-19 pandemic, which is uncertain, and may significantly reduce revenue and cash flows of the combined company while the pandemic continues and thereafter until the combined company and its customers are able to resume normal business operations. Lantheus Holdings and Progenics anticipate that in the second quarter of 2020, the impact of theCOVID-19 pandemic on each company’s business will be more significant than either company experienced in the first quarter as pandemic precautions continue to limit demand for products of each company. Although Lantheus Holdings and Progenics cannot predict the magnitude or duration of the pandemic’s impact on the combined company’s business, each believes the pandemic will result in a material reduction of its net revenues in the second quarter of 2020 and that the timing and speed of any potential revenue recovery cannot be predicted.

In connection with theCOVID-19 pandemic, the following risks could have a material effect on the business, financial condition, results of operations and prospects of the combined company:

16

17

The extent to which theCOVID-19 pandemic impacts the combined company’s business and its results of operations and financial condition will depend on future developments that are highly uncertain and cannot be predicted, including new information that may emerge in connection with the severity of the virus, the ability to treat and ultimately prevent it, its potential recurrence, and actions that may be taken to contain its impact.

18

UPDATE TO SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA OF LANTHEUS HOLDINGS

The following tables present selected historical consolidated financial data of Lantheus Holdings. The selected historical consolidated financial data of Lantheus Holdings for the fiscal years ended December 31, 2019, 2018 and 2017 and as of December 31, 2019 and 2018 are derived from Lantheus Holdings’ audited consolidated financial statements and related notes contained in its Annual Report on Form10-K for the fiscal year ended December 31, 2019, which is incorporated by reference into this supplement and the joint proxy statement/prospectus. The selected historical consolidated financial data of Lantheus Holdings for the fiscal years ended December 31, 2016 and 2015, and as of December 31, 2017, 2016 and 2015, have been derived from Lantheus Holdings’ audited consolidated financial statements for such years, which have not been incorporated by reference into this supplement or the joint proxy statement/prospectus.

The selected historical consolidated financial data of Lantheus Holdings for the three months ended March 31, 2020 and 2019 and as of March 31, 2020 are derived from Lantheus Holdings’ unaudited condensed consolidated financial statements and related notes contained in its Quarterly Report on Form10-Q for the quarterly period ended March 31, 2020, which is incorporated by reference into this supplement and the joint proxy statement/prospectus. The selected historical consolidated financial data of Lantheus Holdings as of March 31, 2019 are derived from Lantheus Holdings’ unaudited condensed consolidated financial statements and related notes contained in its Quarterly Report on Form10-Q for the quarterly period ended March 31, 2019, which has not been incorporated by reference into this supplement or the joint proxy statement/prospectus. Lantheus Holdings’ unaudited condensed consolidated financial statements have been prepared on a basis consistent with its audited financial statements and include all normal and recurring adjustments necessary for a fair presentation of the results for each interim period.

19

The following selected historical consolidated financial data of Lantheus Holdings set forth below is only a summary and is not necessarily indicative of future results. You should read the following information in conjunction with Lantheus Holdings’ audited consolidated financial statements contained in its Annual Report on Form10-K for the year ended December 31, 2019 and unaudited condensed consolidated financial statements contained in its Quarterly Report on Form10-Q for the quarterly period ended March 31, 2020, including “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and the notes to Lantheus Holdings’ consolidated financial statements for significant events affecting the comparability of results as well as material uncertainties regarding Lantheus Holdings’ future financial condition and results of operations in its entirety. See the section entitled “Where You Can Find More Information” beginning on page 260 of the joint proxy statement/prospectus and on page 58 of this supplement.

20

UPDATE TO SELECTED HISTORICAL CONSOLIDATED FINANCIAL DATA OF PROGENICS

The following tables present selected historical consolidated financial data of Progenics. The selected historical consolidated financial data of Progenics for the fiscal years ended December 31, 2019, 2018 and 2017 and as of December 31, 2019 and 2018 are derived from Progenics’ audited consolidated financial statements and related notes contained in its Annual Report on Form10-K for the year ended December 31, 2019, which is incorporated by reference into this supplement and the joint proxy statement/prospectus. The selected historical consolidated financial data of Progenics for the fiscal years ended December 31, 2016 and 2015 and as of December 31, 2017, 2016 and 2015 are derived from Progenics’ audited consolidated financial statements for such years, which have not been incorporated by reference into this supplement or the joint proxy statement/prospectus.

The selected historical consolidated financial data of Progenics for the three months ended March 31, 2020 and 2019 and as of March 31, 2020 are derived from Progenics’ unaudited condensed consolidated financial statements and related notes contained in its Quarterly Report on Form10-Q for the quarterly period ended March 31, 2020, which is incorporated by reference into this supplement and the joint proxy statement/prospectus. The selected historical consolidated financial data of Progenics as of March 31, 2019 are derived from Progenics’ unaudited condensed consolidated financial statements and related notes contained in its Quarterly Report on Form10-Q for the quarterly period ended March 31, 2019, which has not been incorporated by reference into this supplement or the joint proxy statement/prospectus. Progenics’ unaudited condensed consolidated financial statements have been prepared on a basis consistent with its audited financial statements and include all normal and recurring adjustments necessary for a fair presentation of the results for each interim period.

The following selected historical consolidated financial data of Progenics set forth below is only a summary and is not necessarily indicative of future results. You should read the following information in conjunction with Progenics’ audited consolidated financial statements contained in its Annual Report on Form10-K for the year ended December 31, 2019 and unaudited condensed consolidated financial statements contained in its Quarterly Report on Form10-Q for the quarterly period, ended March 31, 2020, including “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and the notes to Progenics’ consolidated financial statements for significant events affecting the comparability of results as well as material uncertainties regarding Progenics’ future financial condition and results of operations. See the sections entitled “Where You Can Find More Information” beginning on page 260 of the joint proxy statement/prospectus and on page 58 of this supplement.

21

22

UPDATE TO COMPARATIVE HISTORICAL AND UNAUDITED PRO FORMA COMBINED PER SHARE DATA

The following table sets forth selected historical and unaudited pro forma combined per share information for Lantheus Holdings and Progenics.

Historical Per Share of Common Stock Information of Lantheus Holdings and Progenics. The historical per share of common stock information of each of Lantheus Holdings and Progenics below is derived from the audited consolidated financial statements of each of Lantheus Holdings and Progenics as of and for the year ended December 31, 2019 and the unaudited condensed consolidated financial statements of each of Lantheus Holdings and Progenics as of and for the three months ended March 31, 2020.

Unaudited Pro Forma Combined Per Lantheus Holdings Share of Common Stock Data. The Lantheus Holdings unaudited pro forma combined per share of common stock data set forth below gives effect to the merger under the acquisition method of accounting as if the merger had been effective on January 1, 2019, the first day of Lantheus Holdings’ fiscal year ended December 31, 2019. The unaudited pro forma combined book value per share of Lantheus Holdings common stock data set forth below gives effect to the merger under the acquisition method of accounting as if the merger had been effective March 31, 2020, assuming that each outstanding share of Progenics common stock had been converted into shares of Lantheus Holdings common stock based on the exchange ratio of 0.31.

The acquisition method of accounting is based on ASC 805, and uses the fair value concepts defined in Accounting Standards Codification 820,Fair Value Measurements (“ASC 820”). Acquisition accounting requires, among other things, that certain assets acquired and liabilities assumed be recognized at their fair values as of the acquisition date. The pro forma adjustments reflect Lantheus Holdings’ provisional estimates of fair value for certain assets and liabilities of Progenics. Differences between these preliminary estimates and the final fair values determined as part of acquisition accounting upon closing of the transaction will occur, and these differences could have a material impact on the unaudited pro forma combined per share information set forth in the following table.

The unaudited pro forma combined per share of Lantheus Holdings common stock data is presented for illustrative purposes only and does not purport to represent the actual financial position or results of operations that Lantheus Holdings would have achieved had the companies been combined during these periods or to project the future financial position or results of operations that Lantheus Holdings may achieve after completion of the merger.

Unaudited Pro Forma Combined Per Progenics Equivalent Share Data. The Progenics unaudited pro forma combined per Progenics equivalent share data set forth below shows the effect of the merger from the perspective of an owner of shares of Progenics common stock. The information was calculated by multiplying the unaudited pro forma combined per share of Lantheus Holdings common stock amounts by the exchange ratio of 0.31.

General Information. You should read the below information in conjunction with the selected historical consolidated financial data included elsewhere in this supplement and the historical consolidated financial statements of Lantheus Holdings and Progenics and related notes that have been filed with the SEC, certain of which are incorporated by reference into this supplement. See “Update to Selected Historical Consolidated Financial Data of Lantheus Holdings,” “Update to Selected Historical Consolidated Financial Data of Progenics” beginning on pages 19 and 21, respectively, of this supplement and “Where You Can Find More Information” beginning on page 260 of the joint proxy statement/prospectus and on page 58 of this supplement. The unaudited pro forma combined per share of Lantheus Holdings common stock data and the unaudited pro forma combined per Progenics equivalent share data is derived from, and should be read in conjunction with, the Lantheus Holdings and Progenics unaudited pro forma condensed combined financial statements and related notes

23

included in this supplement. See “Certain Unaudited Pro Forma Condensed Combined Financial Statements” beginning on page 25 of this supplement.

24

CERTAIN UNAUDITED PRO FORMA CONDENSED

COMBINED FINANCIAL STATEMENTS

The following unaudited pro forma condensed combined financial statements are presented to illustrate the estimated effects of the merger.

The unaudited pro forma condensed combined statements of operations for the year ended December 31, 2019 and for the three months ended March 31, 2020 combine the historical consolidated statements of operations of Lantheus Holdings and Progenics, giving effect to the merger as if it occurred on January 1, 2019. The unaudited pro forma condensed combined balance sheet as of March 31, 2020 combines the historical consolidated balance sheets of Lantheus Holdings and Progenics, giving effect to the merger as if the merger had occurred on March 31, 2020. The historical consolidated financial statements have been adjusted in the unaudited pro forma condensed combined financial statements to give effect to pro forma adjustments that are (1) directly attributable to the merger, (2) factually supportable and (3) with respect to the unaudited pro forma condensed combined statements of operations, expected to have a continuing impact on the combined results.

The unaudited pro forma condensed combined financial statements should be read in conjunction with the accompanying notes to the unaudited pro forma condensed combined financial statements. In addition, the unaudited pro forma condensed combined financial statements have been derived from and should be read in conjunction with the following historical consolidated financial statements and notes incorporated by reference into this supplement and the joint proxy statement/prospectus: (a) the audited consolidated financial statements of Lantheus Holdings contained in its Annual Report on Form10-K for the year ended December 31, 2019; (b) the audited consolidated financial statements of Progenics contained in its Annual Report on Form10-K for the year ended December 31, 2019; (c) the unaudited condensed consolidated financial statements of Lantheus Holdings contained in its Quarterly Report on Form10-Q for the quarterly period ended March 31, 2020; and (d) the unaudited condensed consolidated financial statements of Progenics contained in its Quarterly Report on Form10-Q for the quarterly period ended March 31, 2020. Refer to the section titled “Where You Can Find More Information” of the joint proxy statement/prospectus and this supplement for instructions on how to obtain these documents.

The unaudited pro forma condensed combined financial statements have been prepared by management in accordance with Article 11, Pro Forma Financial Information, under RegulationS-X of the Exchange Act, and are for illustrative and informational purposes only. The unaudited pro forma condensed combined financial statements are not necessarily indicative of what the combined company’s financial position or results of operations actually would have been had the merger been completed as of the dates indicated. In addition, the unaudited pro forma condensed combined financial statements do not purport to project the future financial position or operating results of the combined company. There were no material transactions between Lantheus Holdings and Progenics during the periods presented in the unaudited pro forma condensed combined financial statements that would need to be eliminated.

The unaudited pro forma condensed combined financial statements have been prepared using the acquisition method of accounting under ASC 805, with Lantheus Holdings being the accounting acquirer in the proposed merger with Progenics. As of the date of this supplement, Lantheus Holdings has not completed the detailed valuation studies necessary to arrive at the final estimates of the fair market value of certain Progenics assets to be acquired and liabilities to be assumed and the related allocations of purchase price, nor has it identified all adjustments necessary to conform Progenics to Lantheus Holdings’ accounting policies,except for the ones described in the accompanying notes. The acquisition method of accounting is dependent upon certain valuations that are provisional in nature and subject to change. Accordingly, the pro forma adjustments in the unaudited pro forma condensed combined financial statements are preliminary, based upon available information and made solely for the purpose of preparing these unaudited pro forma condensed combined financial statements. Actual results will differ from the unaudited pro forma condensed combined financial statements once the final acquisition accounting by Lantheus Holdings has been completed, Lantheus Holdings has determined the final

25

purchase price for Progenics, the valuation studies necessary to finalize the required purchase price allocations have been completed and if Lantheus Holdings identifies any additional conforming accounting policy changes outside of the ones provided in the accompanying notes. There can be no assurance that such finalization will not result in material changes.

The assumptions used in preparing the unaudited pro forma combined financial information included in this supplement may not be realized, may not prove to be accurate and may be affected by other factors, such as the potential impact of current market conditions, including the impact of theCOVID-19 pandemic, which could lead to material changes to the combined company’s business that are not reflected in the unaudited pro forma combined financial information. See “Cautionary Statement Regarding Forward-Looking Statements” and “Supplement to Lantheus Holdings Proposal I: Approval of the Stock Issuance and Progenics Proposal I: Adoption of the Merger Agreement” beginning on pages 42 and 47, respectively, of this supplement for further information.

The unaudited pro forma condensed combined financial statements do not reflect any expected cost savings, operating synergies or revenue enhancements that the combined company may achieve as a result of the merger, any termination, restructuring or other costs to integrate the operations of Lantheus Holdings and Progenics or the costs necessary to achieve any such cost savings, operating synergies or revenue enhancements.

26

Lantheus Holdings, Inc.

Unaudited Pro Forma Condensed Combined Balance Sheet

As of March 31, 2020

(in thousands)

The accompanying notes are an integral part of these unaudited pro forma condensed combined financial statements.

27

Lantheus Holdings, Inc.

Unaudited Pro Forma Condensed Combined Statement of Operations

For the Three Months Ended March 31, 2020

(in thousands, except per share data)

The accompanying notes are an integral part of these unaudited pro forma condensed combined financial statements.

28

Lantheus Holdings, Inc.

Unaudited Pro Forma Condensed Combined Statement of Operations

For the Year Ended December 31, 2019

(in thousands, except per share data)

The accompanying notes are an integral part of these unaudited pro forma condensed combined financial statements.

29

NOTES TO THE UNAUDITED PRO FORMA CONDENSED

COMBINED FINANCIAL STATEMENTS

On October 1, 2019, Lantheus Holdings, Merger Sub and Progenics entered into the original merger agreement. On February 20, 2020, the parties entered into the merger agreement that amended and restated the original merger agreement in its entirety. Subject to the terms and conditions of the merger agreement and in accordance with applicable law, Merger Sub will be merged with and into Progenics, with Progenics continuing as the surviving corporation and a wholly-owned subsidiary of Lantheus Holdings. Each share of Progenics common stock issued and outstanding will be converted into 0.31 shares of Lantheus Holdings common stock and one CVR per share of Progenics common stock representing the right to receive, subject to the CVR cap as discussed in Note 5, such stockholder’s pro rata share of aggregate cash payments equal to 40% of U.S. net sales generated by PyL in calendar years 2022 and 2023 in excess of $100.0 million and $150.0 million, respectively, as set forth in the CVR agreement. In addition, Lantheus Holdings will issue replacement awards, subject to the exchange ratio of 0.31, for eachin-the-money vested and unvested Progenics stock option outstanding and unexercised immediately before the closing of the merger. Holders ofin-the-money Progenics stock options as of the closing date are also entitled to receive one CVR for each share of Progenics common stock underlying suchin-the-money options. Holders ofout-of-the-money Progenics stock options will receive replacement awards converted on an exchange ratio adjusted based on actual trading prices of common stock of Progenics and Lantheus Holdings prior to the effective time of the merger. See “The Merger Agreement—Treatment of Progenics Stock Options” beginning on page 183 of the joint proxy statement/prospectus for a more detailed explanation.

Based on the closing price of Lantheus Holdings common stock of $13.05 on April 30, 2020, the common stock component of the purchase price is $350.3 million. The total estimated purchase price inclusive of the fair value of the portion of the replacement awards attributable to precombination services and the CVRs is $359.3 million.

The transaction is subject to approval by Lantheus Holdings and Progenics stockholders and the satisfaction of customary closing conditions and regulatory approvals. Lantheus Holdings and Progenics expect to complete the transaction late in the second quarter of 2020.

The unaudited pro forma condensed combined financial statements were prepared using the acquisition method of accounting and were based on the historical financial statements of Lantheus Holdings and Progenics. Certain reclassifications have been made to the historical financial statements of Progenics to conform to Lantheus Holdings’ presentation, which are discussed in more detail in “Note 4. Historical Progenics.”

The acquisition method of accounting is based on ASC 805 and utilizes the fair value concepts in ASC 820.

ASC 805 requires, among other things, that certain assets acquired and liabilities assumed be recognized at their fair values as of the acquisition date. In addition, ASC 805 establishes that the consideration transferred be measured at the closing date of the merger at fair value based upon the then-current market price of Lantheus Holdings common stock; this particular requirement will likely result in a per share equity component that is different from the amount assumed in these unaudited pro forma condensed combined financial statements, and that difference may be material.

ASC 820 defines the term “fair value” and sets forth the valuation requirements for any asset or liability measured at fair value. Fair value is defined in ASC 820 as “the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.” This is an exit price concept for the valuation of the asset or liability. In addition, market participants are assumed to be buyers and sellers in the principal (or the most advantageous) market for the asset or liability. Fair value

30

measurements for an asset assume the highest and best use by these market participants. As a result of these standards, Lantheus Holdings may be required to record assets which are not intended to be used or sold and/or to value assets at fair value measures that do not reflect Lantheus Holdings’ intended use of those assets. Many of these fair value measurements can be highly subjective and it is also possible that other professionals, applying reasonable judgment to the same facts and circumstances, could develop and support a range of alternative estimated amounts.

Under ASC 805, acquisition-related transaction costs are not included as a component of consideration transferred but are accounted for as expenses in the period in which the costs are incurred. Total transaction costs in connection with the merger are estimated to be approximately $31.0 million, which does not include the impacts of any revenue, cost or other operating synergies that may result from the merger or any related restructuring costs that may be contemplated.

The anticipated merger related transaction costs expected to be incurred after March 31, 2020 are reflected in the unaudited pro forma condensed combined balance sheet as an increase to accrued expenses and other liabilities and an increase to accumulated deficit. All transaction expenses that were incurred by Lantheus Holdings and Progenics have been eliminated from the unaudited pro forma condensed combined statements of operations for the three months ended March 31, 2020 and for the year ended December 31, 2019 as these costs do not have a continuing impact.

The unaudited pro forma condensed combined financial statements do not assume any differences in accounting policies, except as described below, as Lantheus Holdings is not aware of any differences that would have a material impact on the unaudited pro forma condensed combined financial statements. Further review of Progenics’ detailed accounting policies following the completion of the merger may result in the identification of additional differences between the accounting policies of the two companies that, when conformed, could have a material impact on the financial statements of the combined company. Certain reclassifications have been made to the historical financial statements of Progenics to conform to Lantheus Holdings’ presentation, which are discussed in more detail in “Note 4. Historical Progenics.”

Certain reclassifications have been made to the historical financial statements of Progenics to conform to Lantheus Holdings’ presentation as follows:

Unaudited pro forma condensed combined balance sheet as of March 31, 2020

31

Unaudited pro forma condensed combined statement of operations for the three months ended March 31, 2020