AVANT IMMUNOTHERAPEUTICS INC
S-3/A, 2000-01-20
IN VITRO & IN VIVO DIAGNOSTIC SUBSTANCES
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<PAGE>

    As filed with the Securities and Exchange Commission on January 20, 2000


                                       REGISTRATION STATEMENT NO. 333-89341
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                             ---------------------


                                AMENDMENT NO. 2
                                       TO


                                    FORM S-3

            REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933
                            ------------------------

                         AVANT IMMUNOTHERAPEUTICS, INC.

             (Exact name of Registrant as specified in its charter)

<TABLE>
<S>                                                 <C>
                     DELAWARE                                           13-3191702
         (State or other jurisdiction of                             (I.R.S. Employer
          incorporation or organization)                           Identification No.)
</TABLE>

                               119 FOURTH AVENUE
                          NEEDHAM, MASSACHUSETTS 02494
                                 (781) 433-0771
   (Address, including zip code, and telephone number, including area code of
                   Registrant's principal executive offices)
                         ------------------------------

           UNA S. RYAN, PH.D., PRESIDENT AND CHIEF EXECUTIVE OFFICER
                         AVANT IMMUNOTHERAPEUTICS, INC.
                               119 FOURTH AVENUE
                          NEEDHAM, MASSACHUSETTS 02494
                                 (781) 433-0771
 (Name, address, including zip code, and telephone number, including area code,
                             of agent for service)

                         ------------------------------

                                    Copy to:

                             STUART M. CABLE, P.C.
                            ETTORE A. SANTUCCI, P.C.
                          GOODWIN, PROCTER & HOAR LLP
                                 EXCHANGE PLACE
                        BOSTON, MASSACHUSETTS 02109-2881
                                 (617) 570-1000
                            ------------------------

APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: From time to
time after the effective date of this Registration Statement.

    If the only securities being registered on this form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box. / /

    If any of the securities being registered on this form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, other than securities offered only in connection with dividend or interest
reinvestment plans, check the following box. /X/

    If this form is used to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering. / /

    If this form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. / /

    If delivery of the Prospectus is expected to be made pursuant to Rule 434,
please check the following box. / /

THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR DATES
AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF
THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(a),
MAY DETERMINE.

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- --------------------------------------------------------------------------------
<PAGE>
The information in this prospectus is not complete and may be changed. These
securities may not be sold until the registration statement filed with the
Securities and Exchange Commission is effective. This prospectus is not an offer
to sell these securities and it is not soliciting an offer to buy these
securities in any state where the offer or sale is not permitted.
<PAGE>

                 SUBJECT TO COMPLETION. DATED JANUARY 20, 2000.


PROSPECTUS

                        5,459,375 SHARES OF COMMON STOCK
                         AVANT IMMUNOTHERAPEUTICS, INC.

                            ------------------------


    The selling stockholders named on page 9 are selling 5,459,375 shares of our
common stock. Our common stock is listed on the Nasdaq National Market System
under the symbol "AVAN." On January 14, 2000, our common stock closed at a price
of $3.875 per share.



    SEE "RISK FACTORS" BEGINNING ON PAGE 1 FOR IMPORTANT FACTORS YOU SHOULD
CONSIDER BEFORE YOU INVEST IN OUR COMMON STOCK.


                            ------------------------

    NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES
COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES, OR DETERMINED IF
THIS PROSPECTUS IS TRUTHFUL OR COMPLETE. IT IS ILLEGAL FOR ANY PERSON TO TELL
YOU OTHERWISE.

                            ------------------------


                The date of this prospectus is January   , 2000.

<PAGE>
                               TABLE OF CONTENTS


<TABLE>
<S>                                                           <C>
PROSPECTUS SUMMARY..........................................      1

RISK FACTORS................................................      1
  Our history of losses and uncertainty of our future
    profitability makes the common stock a highly
    speculative investment..................................      1
  We anticipate needing to sell capital stock in the future
    to raise necessary funds, and, if we do, it will dilute
    your investment in us...................................      1
  Future sales of our common stock in the public securities
    markets, and the perception in the market that these
    sales may occur, could lower the trading price of our
    common stock............................................      1
  If our products do not pass required tests for safety and
    effectiveness, we will not be able to commercialize
    them....................................................      2
  Product testing is critical to the success of our products
    but subject to delay or cancellation if we have
    difficulty enrolling patients...........................      2
  Even if we develop a product that passes all required
    tests, it may cause unanticipated medical problems with
    its users...............................................      2
  If we cannot obtain governmental approvals for our
    products then we cannot commercialize them..............      2
  We rely on third parties to plan, conduct, monitor and
    supply our clinical tests, and their failure to perform
    as required would interfere with our product
    development.............................................      3
  We depend greatly on third party collaborators to license,
    develop and commercialize some of our products, and they
    may not meet our expectations...........................      3
  Our reliance on third parties requires us to share our
    trade secrets, which increases the possibility that a
    competitor will discover them...........................      3
  We license technology from other companies to develop our
    products, and those companies could restrict our use of
    it......................................................      4
  We may face delays, difficulties or unanticipated costs in
    establishing sales and distribution capabilities for our
    commercially ready products.............................      4
  We have many competitors in our field and they may develop
    technologies that make ours obsolete....................      4
  We rely on patents, patent applications and other
    intellectual property protections to protect our
    technology and trade secrets and they may not provide
    sufficient protection...................................      4
  We use a substantial amount of intellectual property in
    our business and therefore face heightened risk of
    intellectual property litigation........................      5
  Our business requires us to use hazardous materials, which
    increases our exposure to dangerous and costly
    accidents...............................................      5
  Many of our products are tested on laboratory animals, and
    social factors and regulatory changes may curtail our
    use of them in testing..................................      5
  We depend greatly on the intellectual capabilities and
    experience of our key executives and scientists and the
    loss of any of them could affect our ability to develop
    our product candidates..................................      6

WHERE YOU CAN FIND MORE INFORMATION.........................      6

INCORPORATION OF DOCUMENTS BY REFERENCE.....................      7

FORWARD-LOOKING STATEMENTS..................................      7

USE OF PROCEEDS.............................................      8

REGISTRATION RIGHTS OF THE SELLING STOCKHOLDERS.............      8

SELLING STOCKHOLDERS........................................      8

PLAN OF DISTRIBUTION........................................     10

EXPERTS.....................................................     11

LEGAL MATTERS...............................................     11
</TABLE>


                                       i
<PAGE>
                               PROSPECTUS SUMMARY

    YOU SHOULD READ THIS ENTIRE PROSPECTUS CAREFULLY BEFORE DECIDING WHETHER TO
INVEST IN OUR COMMON STOCK.
                            ------------------------

                      ABOUT AVANT IMMUNOTHERAPEUTICS, INC.

    We engage in discovering, developing and commercializing, e.g.
manufacturing, marketing and selling, products that harness the human body's
immune system to prevent and treat disease.


    Additional information regarding Avant, including our audited financial
statements and descriptions of our business, is contained in the documents
incorporated by reference in this prospectus. See "Where You Can Find More
Information" on page 6.



                                  RISK FACTORS


    YOU SHOULD CONSIDER CAREFULLY THESE RISK FACTORS TOGETHER WITH ALL OF THE
INFORMATION INCLUDED OR INCORPORATED BY REFERENCE IN THIS PROSPECTUS BEFORE YOU
DECIDE TO PURCHASE SHARES OF OUR COMMON STOCK. THIS SECTION INCLUDES SOME
FORWARD-LOOKING STATEMENTS.


OUR HISTORY OF LOSSES AND UNCERTAINTY OF FUTURE PROFITABILITY MAKE THE COMMON
STOCK A HIGHLY SPECULATIVE INVESTMENT



    We have no commercial revenues to date from sales of our products and cannot
predict when we will. We have accumulated net operating losses since inception
of approximately $129.7 million, as of September 30, 1999. We expect to spend
substantial funds to continue research and product testing and to establish
sales, marketing, quality control, and regulatory and administrative
capabilities. We may lose money over the next several years as the number of our
potential products increases and as we expand our efforts in those areas.



    We cannot predict the impact of any future losses. Before we earn any
profits, we must complete the development of some or all of our products, obtain
necessary regulatory approvals and successfully manufacture and market our
products. We are unlikely to make profits in the near future and are uncertain
whether we will do so at all or on a continuing basis.



WE ANTICIPATE NEEDING TO SELL CAPITAL STOCK IN THE FUTURE TO RAISE NECESSARY
FUNDS, AND, IF WE DO, IT WILL DILUTE YOUR INVESTMENT IN US.



    We will need to raise more money in order to commercialize our products. We
anticipate selling additional capital stock to do so. However, based on our
history of losses, we may have difficulty attracting sufficient investment
interest. Any future sale of stock will dilute your investment in us. We may
also try to obtain funding through research grants and agreements with
commercial collaborators. These types of fundings are uncertain since they are
at the discretion of other organizations and companies which have limited funds
and many companies competing with us for those funds. As a result, we may not
receive any research grants or funds from collaborators. If adequate funding is
not available, we will need to limit our research and development programs.



FUTURE SALES OF OUR COMMON STOCK IN THE PUBLIC SECURITIES MARKETS, AND THE
PERCEPTION IN THE MARKET THAT THESE SALES MAY OCCUR, COULD LOWER THE TRADING
PRICE OF OUR COMMON STOCK



    In the past several years we sold a substantial amount of our common stock
in private placement transactions. The purchasers of this common stock may sell
it in the public securities markets over time under registration rights granted
to them under the Securities Act of 1933.


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Additional sales of common stock reserved under our employee benefit and other
incentive plans, including stock options, may also occur in the public
securities markets.



IF OUR PRODUCTS DO NOT PASS REQUIRED TESTS FOR SAFETY AND EFFECTIVENESS, WE WILL
NOT BE ABLE TO COMMERCIALIZE THEM



    Our success depends on our ability to commercialize our products. We have
not yet commercialized any of them and cannot do so until they pass several
tests relating to safety and effectiveness. Our main products in development are
designed to help people recover from physical difficulties often associated with
heart problems, surgery and some types of diseases and infections. Two of those
products, named TP-20 and Therapore-TM-, are in preliminary tests. Our other
products, including our lead product, named TP-10, are in advanced tests. We may
discover during any stage of testing that a product is not safe or effective.
Moreover, regulatory authorities may decide, contrary to our conclusions, that a
product is not safe or effective. In either situation, we would not be able to
commercialize the product.



PRODUCT TESTING IS CRITICAL TO THE SUCCESS OF OUR PRODUCTS BUT SUBJECT TO DELAY
OR CANCELLATION IF WE HAVE DIFFICULTY ENROLLING PATIENTS



    We use patients rather than laboratory animals in advanced testing. Since
many of our products are in advanced testing, and we expect others to follow, we
will need to enroll additional patients. At times, we experience difficulty
enrolling patients because of uncontrollable factors, including:



    - the nature of the clinical test



    - the size of the patient population



    - the distance between patients and clinical test sites



    - the eligibility criteria for the trial



    If we cannot enroll patients as needed, our costs may increase or we may
delay or terminate testing for a product.



EVEN IF WE DEVELOP A PRODUCT THAT PASSES ALL REQUIRED TESTS, IT MAY CAUSE
UNANTICIPATED MEDICAL PROBLEMS WITH ITS USERS



    Product testing cannot ensure the ultimate safety of our products. Some
people may respond differently to our products than our testing indicated.
Problems with a product could prevent its widespread use, require its withdrawal
from the market or expose us to lawsuits for product liability. Any withdrawal
from the market or product liability lawsuit could result in large monetary
awards and negative publicity against us and could prevent commercialization of
other products. We currently carry product liability insurance with only limited
coverage. We may lose the ability to maintain our current amount of coverage.
Even if we maintain our current coverage or increase it, our liability could
exceed the amount of coverage and require us to pay a substantial monetary
award.



IF WE CANNOT OBTAIN GOVERNMENTAL APPROVALS FOR OUR PRODUCTS THEN WE CANNOT
COMMERCIALIZE THEM



    Our products and our research and development and testing programs are
extensively regulated by governmental authorities in the United States and other
countries. In the United States, we must sponsor and file a new drug application
with the Food and Drug Administration for each proposed product. Since we
develop products with foreign companies, including Novartis


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<PAGE>

Pharma AG, Yamanouchi Pharmaceutical and Pasteur Merieux Connaught, and will
attempt to commercialize our products abroad, we must comply with foreign
regulatory rules as well. The regulatory process, in the United States and
abroad, is expensive and often takes many years. Data obtained from product
tests are susceptible to varying interpretations which could delay, limit or
prevent regulatory approval. We may not obtain necessary regulatory approvals
within a reasonable period of time, if at all. Even if we receive regulatory
approval for a product, the approval may require limitations on use, which would
restrict the size of the potential market for the product.



    After regulatory approval is obtained, a product and its manufacturer are
subject to continuing review and periodic inspections of manufacturing
facilities. Subsequent discovery of previously unknown problems with a product
or its manufacturing process may result in restrictions on the product or the
manufacturer, including withdrawal of the product from the market. Failure to
comply with the applicable regulatory requirements can result in fines,
suspensions of regulatory approvals, product recalls, operating restrictions and
criminal prosecution.



WE RELY ON THIRD PARTIES TO PLAN, CONDUCT, MONITOR AND SUPPLY OUR CLINICAL
TESTS, AND THEIR FAILURE TO PERFORM AS REQUIRED WOULD INTERFERE WITH OUR PRODUCT
DEVELOPMENT



    We rely on third parties, including Duke University Medical Center, The
Chicago Center for Clinical Research and SmithKline Beecham to conduct our
clinical tests. If any one of those third parties fails to perform as we expect
or if their work fails to meet regulatory standards, our testing could be
delayed, cancelled or rendered ineffective. We also depend on third party
suppliers, including Walter Reed Army Institute of Research, Marathon
Biopharmaceuticals, Inc., and Multiple Peptide Systems, to provide us with
suitable quantities of materials necessary for clinical tests. If these
materials are not available in suitable quantities of appropriate quality, in a
timely manner, and at a feasible cost, our clinical tests will face delays.
Delays in our clinical tests will also delay the commercialization of our
products.



WE DEPEND GREATLY ON THIRD PARTY COLLABORATORS TO LICENSE, DEVELOP AND
COMMERCIALIZE SOME OF OUR PRODUCTS, AND THEY MAY NOT MEET OUR EXPECTATIONS



    We have agreements with other companies, including Heska Corporation,
Innogenetics, Inc., Novartis, Pasteur Merieux, SmithKline Beecham, and
Yamanouchi, for the licensing, development and commercialization of our
products. Some of those agreements give substantial responsibility over the
products to the collaborator. Some collaborators may be unable or unwilling to
devote sufficient resources to develop our products as their agreements require.
They often face business risks similar to ours, which could interfere with their
efforts. Also, collaborators may choose to devote their resources to products
that compete with ours. If a collaborator does not successfully develop any one
of our products, we will need to find another collaborator to do so. Our search
for a new collaborator will depend on our legal right to do so at the time and
whether the product remains commercially viable.



    We lack manufacturing capabilities and will depend on other companies to
produce our products in adequate quantities, in compliance with regulatory
requirements, and at a competitive cost. We may not find third parties capable
of meeting those manufacturing needs. If we cannot, we will face delay and
additional costs while we develop internal manufacturing capabilities.



OUR RELIANCE ON THIRD PARTIES REQUIRES US TO SHARE OUR TRADE SECRETS, WHICH
INCREASES THE POSSIBILITY THAT A COMPETITOR WILL DISCOVER THEM



    Because we rely on third parties to develop our products, we must share
trade secrets with them. We seek to protect our proprietary technology in part
by confidentiality agreements and, if applicable, inventor's rights agreements
with our collaborators, advisors, employees and


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<PAGE>

consultants. If these agreements are breached, our competitors may discover our
trade secrets. A competitor's discovery of our trade secrets would impair our
competitive position. Moreover, we conduct a significant amount of research
through academic advisors and collaborators prohibited from entering into
confidentiality or inventor's rights agreements by their academic institutions.



WE LICENSE TECHNOLOGY FROM OTHER COMPANIES TO DEVELOP OUR PRODUCTS, AND THOSE
COMPANIES COULD RESTRICT OUR USE OF IT



    In order to use licensed technology, the companies that license it to us may
require us to achieve milestones or devote minimum amounts of resources to
develop our products. They may also require us to make significant royalty and
milestone payments, including a percentage of any sublicensing income, as well
as payments to reimburse them for patent costs. If we fail to perform our
obligations fully, the licensor can terminate the licenses or permit our
competitors to use the technology. Moreover, we may lose our right to market and
sell any products based on the licensed technology. We cannot predict our
ability to meet our obligations under our licenses. Furthermore, we may need to
obtain licenses to additional technologies for some of our products under
development and, if we cannot obtain any required licenses, we may lose our
ability to commercialize the products.



WE MAY FACE DELAYS, DIFFICULTIES OR UNANTICIPATED COSTS IN ESTABLISHING SALES
AND DISTRIBUTION CAPABILITIES FOR OUR COMMERCIALLY READY PRODUCTS



    We currently lack sales, distribution and marketing capabilities for our
products in development. To market any of our products directly, we must develop
a substantial marketing and sales force with technical expertise and a
supporting distribution capability. We have little expertise in this area, and
we may not succeed. We may find it necessary to enter into strategic
partnerships on uncertain but potentially unfavorable terms to sell, market and
distribute our products.


WE HAVE MANY COMPETITORS IN OUR FIELD AND THEY MAY DEVELOP TECHNOLOGIES THAT
MAKE OURS OBSOLETE


    Biotechnology, pharmaceuticals and therapeutics are rapidly evolving fields
in which scientific and technological developments are expected to continue at a
rapid pace. We have many competitors in the United States and abroad, including
Alexion Pharmaceutical, Bayer, Merck, Pfizer, Immune Response and Wyeth-Lederle.
Our success depends upon our ability to develop and maintain a competitive
position in the product categories and technologies on which we focus. Many of
our competitors have greater capabilities, experience and financial resources
than we do. Competition is intense and is expected to increase as new products
enter the market and new technologies become available. Our competitors may:



    - develop technologies and products that are more effective than ours,
      making ours obsolete or otherwise noncompetitive



    - obtain regulatory approval for products more rapidly or effectively than
      us


    - obtain patent protection or other intellectual property rights that would
      block our ability to develop competitive products


WE RELY ON PATENTS, PATENT APPLICATIONS AND OTHER INTELLECTUAL PROPERTY
PROTECTIONS TO PROTECT OUR TECHNOLOGY AND TRADE SECRETS AND THEY MAY NOT PROVIDE
SUFFICIENT PROTECTION



    Our success depends in part on our ability to obtain and maintain patent
protection for technologies that we use. Biotechnology patents involve complex
legal, scientific and factual questions and are highly uncertain. To date, there
is no consistent policy regarding the breadth of


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<PAGE>

claims allowed in biotechnology patents, particularly in regard to patents for
technologies for human uses like those we use in our business. We cannot predict
whether the patents we seek will issue. If they do issue, a competitor may
challenge them and limit their scope. Moreover, our patents may not afford
effective protection against competitors with similar technology. A successful
challenge to any one of our patents could result in a third party's ability to
use the technology covered by the patent. We also face the risk that others will
infringe, avoid or circumvent our patents. Technology that we license from
others is subject to similar risks, which could harm our ability to use that
technology.



    If we, or a company that licenses technology to us, were not the first
creator of an invention that we use, our ability to use the underlying product
or technology will face restrictions. At a minimum, we would need to obtain
licenses to the technology on potentially unfavorable terms. If those licenses
from third parties are necessary, but are unattainable, a delay in, or the
prevention of, the commercialization of our affected products will result.



WE USE A SUBSTANTIAL AMOUNT OF INTELLECTUAL PROPERTY IN OUR BUSINESS AND
THEREFORE FACE HEIGHTENED RISK OF INTELLECTUAL PROPERTY LITIGATION



    If we must defend against suits brought against us or prosecute suits
against others involving intellectual property rights, we will incur substantial
costs. In addition to any potential liability for significant monetary damages,
a decision against us may require us to obtain licenses to patents or other
intellectual property rights of others. In that event, our ability to obtain
those licenses on acceptable terms is uncertain. If we cannot acquire necessary
licenses, we would attempt to design around third-party patents, which is
expensive and may ultimately be impracticable.



OUR BUSINESS REQUIRES US TO USE HAZARDOUS MATERIALS, WHICH INCREASES OUR
EXPOSURE TO DANGEROUS AND COSTLY ACCIDENTS



    Our research and development activities involve the use of hazardous
chemicals, biological materials and radioactive compounds. Although we believe
that our safety procedures for handling and disposing hazardous materials comply
with the standards prescribed by applicable laws and regulations, we cannot
completely eliminate the risk of accidental contamination or injury from these
materials. In the event of an accident, an injured party will likely sue us for
any resulting damages with potentially significant liability. The ongoing cost
of complying with environmental laws and regulations is significant and may
increase in the future. In addition, in connection with our merger with Virus
Research Institute, Inc. in 1998, we assumed the real property lease at Virus
Research Institute, Inc.'s former site. We understand that this property has a
low level of oil-based and other hazardous material contamination. We believe
that the risks posed by this contamination are low, but we cannot predict
whether additional hazardous contamination exists at this site, or that changes
in applicable law will not require us to clean up the current contamination of
the property.



MANY OF OUR PRODUCTS ARE TESTED ON LABORATORY ANIMALS, AND SOCIAL FACTORS AND
REGULATORY CHANGES MAY CURTAIL OUR USE OF THEM IN TESTING



    Our research and development efforts often involve the controlled use of
laboratory animals. Activist groups such as People for the Ethical Treatment of
Animals oppose this type of testing and may interfere with it directly or by
generating negative publicity about us. In addition, physical force and
demonstrations may occur against us or our collaborative partners which could
delay the testing of our products. In addition, lobbying efforts against the use
of animals in testing is ongoing, and may result in changes in laws, regulations
or accepted clinical procedures that would restrict the use of animals in
testing our products. These changes would delay our product development until we
found other methods for effective testing.


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<PAGE>

WE DEPEND GREATLY ON THE INTELLECTUAL CAPABILITIES AND EXPERIENCE OF OUR KEY
EXECUTIVES AND SCIENTISTS AND THE LOSS OF ANY OF THEM COULD AFFECT OUR ABILITY
TO DEVELOP OUR PRODUCTS



    The loss of Dr. Una S. Ryan, our president and chief executive officer, or
other key members of our staff could harm us. We have employment agreements with
Dr. Ryan and one other senior executive officer. We do not have any key-person
insurance coverage. We also depend on our scientific collaborators and advisors,
all of whom have outside commitments that may limit their availability to us. In
addition, we believe that our future success will depend in large part upon our
ability to attract and retain highly skilled scientific, managerial and
marketing personnel, particularly as we expand our activities in clinical
trials, the regulatory approval process and sales and manufacturing. We face
significant competition for this type of personnel from other companies,
research and academic institutions, government entities and other organizations.
We cannot predict our success in hiring or retaining the personnel we require
for continued growth.


                      WHERE YOU CAN FIND MORE INFORMATION

    We must comply with the informational requirements of the Securities
Exchange Act of 1934 and we are required to file reports and proxy statements
and other information with the Securities and Exchange Commission. You may read
and copy these reports, proxy statements and information at the public reference
facilities maintained by the Securities and Exchange Commission at Judiciary
Plaza, 450 Fifth Street, N.W., Room 1024, Washington, D.C. 20549, and at the
Securities and Exchange Commission's Regional Offices at 7 World Trade Center,
13th Floor, New York, New York 10048, and Citicorp Center, 500 W. Madison
Street, Suite 1400, Chicago, Illinois 60661-2511. You may also obtain copies at
the prescribed rates from the Public Reference Section of the Securities and
Exchange Commission at its principal office in Washington, D.C. You may call the
Securities and Exchange Commission at 1-800-SEC-0330 for further information
about the public reference rooms. The Securities and Exchange Commission
maintains a web site that contains reports, proxy and information statements and
other information regarding registrants, including, Avant, that file
electronically with the Securities and Exchange Commission. You may access the
Securities and Exchange Commission's web site at http://www.sec.gov.

                                       6
<PAGE>
                    INCORPORATION OF DOCUMENTS BY REFERENCE

    The Securities and Exchange Commission allows us to incorporate by reference
in this prospectus the information that we file with it. Incorporation by
reference means that we can disclose important information to you by referring
you to other documents that are legally considered to be part of this
prospectus, and later information that we file with the Securities and Exchange
Commission will automatically update and supersede the information in this
prospectus, any supplement and the documents listed below. We incorporate by
reference the specific documents listed below and any future filings made with
the Securities and Exchange Commission under Section 13(a), 13(c), 14, or 15(d)
of the Securities Exchange Act of 1934 until all of the shares are sold:

    - our Annual Report on Form 10-K for the fiscal year ended December 31, 1998
      (as amended on Form10-K/A filed on November 8, 1999)

    - our Quarterly Reports on Form 10-Q for the quarters ended March 31, 1999,
      June 30, 1999 (as amended on Form 10-Q/A filed on November 8, 1999), and
      September 30, 1999

    - our Current Report on Form 8-K filed on September 23, 1999

    - the definitive Proxy Statement for our annual meeting of stockholders held
      May 6, 1999

    - the description of our common stock contained in our registration
      statement on Form 8-A, filed on September 22, 1986, including all
      amendments and reports updating that description

    We will furnish without charge to each person to whom this prospectus is
delivered, upon request, a copy of any documents incorporated by reference other
than exhibits to those documents. Requests should be addressed to: 119 Fourth
Avenue, Needham, Massachusetts 02494, Attention: Corporate Secretary (telephone
number (781) 433-0771).

    You should rely only on the information incorporated by reference or
provided in this prospectus. We have not authorized anyone to provide you with
different information. You should not assume that the information in this
prospectus or the documents incorporated by reference is accurate as of any date
other than the date on the front of this prospectus or those documents.

                           FORWARD-LOOKING STATEMENTS

    Some statements incorporated by reference or made under the caption "Risk
Factors" and elsewhere in this prospectus are "forward-looking statements"
within the meaning of Section 27A of the Securities Act of 1933 and Section 21E
of the Securities Exchange Act of 1934. When we use the words "anticipate,"
"assume," "believe," "estimate," "expect," "intend" and other similar
expressions, they generally identify forward-looking statements. Forward-looking
statements include, for example, statements relating to development activities,
business strategy and prospects, future capital expenditures, sources and
availability of capital, governmental regulations and their effect on us and
competition.

    You should exercise caution in interpreting and relying on forward-looking
statements since they involve known and unknown risks, uncertainties and other
factors which are, in some cases, beyond our control and could materially affect
our actual results, performance or achievements. Some of the factors that could
cause our actual results, performance or achievements to differ materially from
those expressed or implied by forward-looking statements include, but are not
limited to, the matters discussed under the caption "Risk Factors."

    We caution you that, while forward looking statements reflect our good faith
beliefs, they are not guarantees of future performance. In addition, we disclaim
any obligation to publicly update or

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<PAGE>
revise any forward-looking statement, whether as a result of new information,
future events or otherwise.

                                USE OF PROCEEDS

    Avant will not receive any proceeds from the sale of the shares by the
selling stockholders.

                REGISTRATION RIGHTS OF THE SELLING STOCKHOLDERS

    The following is a summary of the material terms and provisions of the
securities purchase agreement with the selling stockholders relating to the
registration of the common stock covered by this prospectus. It may not contain
all the information that is important to you. You can access complete
information by referring to the securities purchase agreement.

    Under the securities purchase agreement, we must file a registration
statement covering the sale by the selling stockholders of the common stock that
they purchased on September 22, 1999. We must use our best efforts to cause the
registration statement to be declared effective by the Securities and Exchange
Commission no later than February 18, 2000 and to keep the registration
statement continuously effective until the earlier of:

    - the date on which the selling stockholders no longer hold any of the
      purchased common stock or

    - September 21, 2001.

    Any common stock sold by the selling stockholders pursuant to this
prospectus will not have the benefits of the registration rights provisions of
the securities purchase agreement.

    We must pay all expenses of registering the common stock. We will not pay
brokerage and underwriting commissions and taxes of any kind and any legal,
accounting and other expenses incurred by the selling stockholders. We also
agreed to indemnify the selling stockholders and their officers, directors and
other affiliated persons and any person who controls any selling stockholder
against all losses, claims, damages, actions, liabilities, costs and expenses
arising under the securities laws in connection with the registration statement
or this prospectus, subject to limitations specified in the securities purchase
agreement. In addition, the selling stockholders agreed to indemnify us and our
directors, officers and any person who controls Avant against all losses,
claims, damages, actions, liabilities, costs and expenses arising under the
securities laws if they result from:

    - written information furnished to us by the selling stockholders for use in
      the registration statement or this prospectus or any amendments to the
      registration statement or any prospectus supplements or

    - breaches or alleged breaches by selling stockholders of any of their
      representations, warranties, covenants, agreements or obligations under
      the terms of the securities purchase agreement.

                              SELLING STOCKHOLDERS

    The following table provides the name and number of shares of common stock
owned by each selling stockholder as of September 30, 1999, the number of shares
of common stock covered by this prospectus and the total number of shares of
common stock which the selling stockholders will beneficially own upon
completion of this offering. The shares offered by this prospectus may be
offered from time to time by the selling stockholders named below, or by any of
their pledgees, donees, transferees or other successors in interest.

                                       8
<PAGE>
    Because the selling stockholders may sell all, some or none of the shares,
we have assumed that the selling stockholders will sell all of the shares in
determining the number and percentage of shares of common stock that each
selling stockholder will own upon completion of the offering to which this
prospectus relates. The amounts set forth below are based upon information
provided by the selling stockholders and are accurate to the best of our
knowledge. It is possible, however, that the selling stockholders may acquire or
dispose of additional shares of common stock or units from time to time after
the date of this prospectus.

<TABLE>
<CAPTION>
                                                                                         Shares of
                                                                                        Common Stock
                                               Shares of             Shares of             Owned
                                              Common Stock             Common        After the Offering
                                           Beneficially Owned          Stock        --------------------
Selling Stockholder                     as of September 30, 1999   Offered Hereby   Number(1)   Percent
- -------------------                     ------------------------   --------------   ---------   --------
<S>                                     <C>                        <C>              <C>         <C>
Nomura International plc(2)...........         2,604,167              2,604,167             0       0
Kleinwort Benson Holdings, Inc(3).....           520,833                520,833             0       0
Pictet & Cie(4).......................           520,833                520,833             0       0
Lombard Odier & Cie(5)................         2,151,506                546,875     1,604,631     3.2
International BM Biomedicine Holdings
  AG(6)...............................           975,625                390,625       585,000     1.2
Apollo Medical Partners, L.P.(7)......           260,417                260,417             0       0
Brandon Fradd(7)......................            15,625                 15,625             0       0
Bank of New Nominees Limited(8).......           150,000                150,000             0     1.3
Curran Capital Partners, L.P.(9)......           768,700                150,000       618,700       0
Clarion Capital Corporation(10).......           100,000                100,000             0       0
Clarion Partners, L.P.(10)............            39,000                 39,000             0       0
Clarion Offshore Fund LTD(10).........            11,000                 11,000             0       0
Catalyst Partners, L.P.(11)...........         1,175,000                100,000     1,075,000     2.2
Peter Sears(12).......................            50,000                 50,000             0       0
</TABLE>

- ------------------------

 (1) Assumes that all shares hereby offered by the selling stockholders are
    sold.

 (2) The selling stockholder's address is Nomura House, 1 St. Martin's-le-Grand,
    London ECIA 4NP UK.

 (3) The selling stockholder's address is 75 Wall Street, New York, NY 10005,
    USA.

 (4) The selling stockholder's address is 29, Blvd. Georges-Favon, Geneva
    CH-1211, Switzerland.

 (5) The selling stockholder's address is 11 Rue de la Corrateria, Geneva
    CH-1204, Switzerland. Includes warrants to purchase 40,000 shares of common
    stock.

 (6) The selling stockholder's address is Aeschen Platz 7, PO Box 136, Basel
    CH-4010 Switzerland.

 (7) The selling stockholder's address is 68 Jane Street, Suite 2E New York, New
    York 10014, USA. Mr. Fradd is a managing director of Apollo Medical
    Partners, L.P. and he may be deemed to beneficially own the 260,417 shares
    of common stock held by Apollo Medical Partners, L.P.

 (8) The selling stockholder's address is 30 Cannon Street London EC4M 6XH, UK.

 (9) The selling stockholder's address is 237 Park Avenue, 9th Floor, New York,
    New York 10017, USA.

(10) The selling stockholder's address is 1801 East 9th Street, Suite 1120,
    Cleveland, OH 44104, USA.

(11) The selling stockholder's address is 350 Park Avenue, New York, New York
    10022, USA.

(12) The selling stockholder's address is 8 Paul Road, St. David, PA 19087, USA.
    Mr. Sears has been a director of Avant since May, 1999.

                                       9
<PAGE>
                              PLAN OF DISTRIBUTION

    This prospectus relates to the possible sale from time to time of up to an
aggregate of 5,459,375 shares of common stock by the selling stockholders, or
any of their pledgees, donees, transferees or other successors in interest. We
are registering the shares due to our contractual obligation to do so under the
registration rights provisions of the securities purchase agreement, but the
registration of the shares does not necessarily mean that any of the shares will
be offered or sold by the selling stockholders.

    The distribution of the shares may be in one or more underwritten
transactions at a fixed price or prices, which may be changed, or at market
prices prevailing at the time of sale, at prices related to prevailing market
prices or at negotiated prices. Any underwritten offering may be on a "best
efforts" or a "firm commitment" basis. In connection with any underwritten
offering, underwriters or agents may receive compensation in the form of
discounts, concessions or commissions from the selling stockholders.
Underwriters may sell the shares to or through dealers, and the dealers may
receive compensation in the form of discounts, concessions or commissions from
the underwriters and commissions from the purchasers for whom they may act as
agents.

    The selling stockholders and any underwriters, dealers or agents that
participate in the distribution of the shares may be deemed to be underwriters
under the Securities Act of 1933, and any profit on the sale of the shares by
them and any discounts, commissions or concessions received by any underwriters,
dealers or agents might be deemed to be underwriting discounts and commissions
under the Securities Act. At any time a particular offer of shares is made by
the selling stockholders, a prospectus supplement, if required, will be
distributed that will, where applicable:

    - identify any underwriter, dealer or agent

    - describe any compensation in the form of discounts, concessions,
      commissions or otherwise received by each underwriter, dealer or agent and
      in the aggregate to all underwriters, dealers and agents

    - identify the amounts underwritten

    - identify the nature of the underwriter's obligation to take the shares

    - provide any other required information

    Selling stockholders may sell the shares by selling them directly to
purchasers or to or through broker-dealers. In connection with any sale, any
broker-dealer may act as agent for the selling stockholders or may purchase from
the selling stockholders all or a portion of the shares as principal, and may be
made any of the methods described below. Sales may be made on the Nasdaq
National Market System or other exchanges on which our common stock is then
traded, in the over-the-counter market, in negotiated transactions or otherwise
at prices and at terms then prevailing or at prices related to the then-current
market prices or at prices otherwise negotiated.

    The shares may also be sold in one or more of the following transactions:

    - block transactions in which a broker-dealer may sell all or a portion of
      the shares as agent but may position and resell all or a portion of the
      block as principal to facilitate the transaction

    - purchases by any broker-dealer as principal and resale by any
      broker-dealer for its own account under any supplement to this prospectus

    - a special offering, an exchange distribution or a secondary distribution
      in accordance with applicable Nasdaq National Market or other stock
      exchange rules

                                       10
<PAGE>
    - ordinary brokerage transactions and transactions in which any
      broker-dealer solicits purchasers

    - sales "at the market" to or through a market maker or into an existing
      trading market, on an exchange or otherwise, for the shares

    - sales in other ways not involving market makers or established trading
      markets, including direct sales to purchasers

    Broker-dealers engaged by the selling stockholders to carry out sales may
arrange for other broker-dealers to participate. Broker-dealers will receive
commissions or other compensation from the selling stockholders in amounts to be
negotiated immediately prior to the sale that will not exceed those customary in
the types of transactions involved. Broker-dealers may also receive compensation
from purchasers of the shares which is not expected to exceed that customary in
the types of transactions involved.

    In states where necessary to comply with applicable state securities laws,
the shares will be sold, only through registered or licensed brokers or dealers.
In addition, the shares may not be sold in some states unless they have been
registered or qualified for sale in the state or an exemption from the
registration or qualification requirement is available and is complied with.

    All expenses relating to the offering and sale of the shares, other than
commissions, discounts and fees of underwriters, broker-dealers or agents, will
be paid by us.

                                    EXPERTS

    The consolidated financial statements incorporated in this prospectus by
reference to the Annual Report on Form 10-K/A for the year ended December 31,
1998, have been so incorporated in reliance upon the report of
PricewaterhouseCoopers LLP, independent accountants, given on the authority of
said firm as experts in auditing and accounting.

                                 LEGAL MATTERS

    The validity of the common stock we are offering will be passed upon for us
by Goodwin, Procter & Hoar LLP, Boston, Massachusetts.

                                       11
<PAGE>
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------

YOU SHOULD RELY ONLY ON THE INFORMATION CONTAINED IN THIS PROSPECTUS,
INCORPORATED HEREIN BY REFERENCE OR CONTAINED IN A PROSPECTUS SUPPLEMENT.
NEITHER WE NOR THE SELLING STOCKHOLDERS HAVE AUTHORIZED ANYONE ELSE TO PROVIDE
YOU WITH DIFFERENT OR ADDITIONAL INFORMATION. THE SELLING STOCKHOLDERS ARE NOT
MAKING AN OFFER OF THESE SECURITIES IN ANY STATE WHERE THE OFFER IS NOT
PERMITTED. YOU SHOULD NOT ASSUME THAT THE INFORMATION IN THIS PROSPECTUS, OR
INCORPORATED HEREIN BY REFERENCE, OR IN ANY PROSPECTUS SUPPLEMENT IS ACCURATE AS
OF ANY DATE OTHER THAN THE DATE ON THE FRONT OF THOSE DOCUMENTS

                                5,459,375 SHARES

                                     AVANT
                              IMMUNOTHERAPEUTICS,
                                      INC.

                                  COMMON STOCK
                             ---------------------

                                   PROSPECTUS

                             ---------------------


                                JANUARY   , 2000


- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
<PAGE>
                                    PART II
                     INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION. (1)

    The following are the estimated expenses of the distribution of the shares
registered hereunder on Form S-3:

<TABLE>
<S>                                                           <C>
Registration Fee--Securities and Exchange Commission........  $    2,944.35
Accountants Fees and Expenses...............................       2,500
Blue Sky Fees and Expenses..................................       1,500
Legal Fees and Expenses.....................................      15,000
Printing Expenses...........................................       5,000
Miscellaneous...............................................       1,500
                                                              ----------
    Total...................................................  $   28,444.35
                                                              ==========
</TABLE>

- ------------------------

(1) The amounts set forth above, except for the SEC Registration Fee, are
    estimated.

ITEM 15. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

    The Company is a Delaware corporation. Reference is made to Section 145 of
the Delaware General Corporation Law (the "DGCL"), which enables a corporation
to eliminate or limit the personal liability of a director for monetary damages
for violations of the director's fiduciary duty, except for liability (i) for
any breach of the director's duty of loyalty to the company, (ii) for acts or
omissions not in good faith or which involve intentional misconduct or a knowing
violation of law, (iii) under to Section 145 or (iv) for any transaction from
which a director derived an improper personal benefit. The Company has adopted
such provisions in the company's Amended and Restated Bylaws (the "Bylaws").

    The DGCL permits, but does not require, a corporation to indemnify its
directors, officers, employees or agents and expressly provides that the
indemnification provided for under the DGCL shall not be deemed exclusive of any
indemnification right under any bylaw, vote of stockholders or disinterested
directors, or otherwise. The DGCL permits indemnification against expenses and
certain other liabilities arising out of legal actions brought or threatened
against such persons for their conduct on behalf of the corporation, provided
that each such person acted in good faith and in a manner that he or she
reasonably believed was in or not opposed to the corporation's best interests
and in the case of a criminal proceeding, had no reasonable cause to believe his
or her conduct was unlawful. The DGCL does not allow indemnification of
directors in the case of an action by or in the right of the corporation
(including stockholder derivative suits) unless the directors successfully
defend the action or indemnification is ordered by the court. The Bylaws of the
company provide for indemnification to the fullest extent authorized by the DGCL
and, therefore, these statutory indemnification rights are available to the
directors, officers, employees and agents of the Companies. Insofar as
indemnification for liabilities arising under the Securities Act of 1933, as
amended (the "Securities Act") may be permitted to directors and officers of the
company pursuant to the foregoing provision or otherwise, the company has been
advised that, in the opinion of the Commission, such indemnification is against
public policy as expressed in the Securities Exchange Act of 1934, as amended
(the "Exchange Act"), and is therefore, unenforceable.

    The Company currently carries a directors' and officers' liability insurance
policy which provides for payment of expenses of the company's directors and
officers in connection with threatened,

                                      II-1
<PAGE>
pending or completed actions, suits or proceedings against them in their
capacities as directors and officers, in accordance with the Bylaws and the
DGCL.

ITEM 16. EXHIBITS.

<TABLE>
<CAPTION>
       EXHIBIT
         NO.            DESCRIPTION
- ---------------------   -----------
<C>                     <S>
         *5.1           Opinion of Goodwin, Procter & Hoar LLP

         23.1           Consent of PricewaterhouseCoopers LLP

        *23.2           Consent of Goodwin, Procter & Hoar LLP (included in Exhibit
                        5.1)

        *24.1           Power of Attorney (included on signature pages)

        *99.1           Securities Purchase Agreement dated as of September 17,
                        1999, between the Company and the Selling Stockholders
</TABLE>

- ------------------------

*   previously filed

ITEM 17. UNDERTAKINGS.

(a) The undersigned Registrant hereby undertakes:

    (1) To file, during any period in which offers or sales are being made, a
       post-effective amendment to this Registration Statement:

         (i) To include any prospectus required by Section 10(a)(3) of the
             Securities Act;

        (ii) To reflect in the prospectus any facts or events arising after the
             effective date of the Registration Statement (or the most recent
             post-effective amendment thereof) which, individually or in the
             aggregate, represent a fundamental change in the information set
             forth in the Registration Statement. Notwithstanding the foregoing,
             any increase or decrease in volume of securities offered (if the
             total dollar value of securities offered would not exceed that
             which was registered) and any deviation from the low or high and of
             the estimated maximum offering range may be reflected in the form
             of prospectus filed with the Commission pursuant to Rule 424(b) if,
             in the aggregate, the changes in volume and price represent no more
             than a 20 percent change in the maximum aggregate offering price
             set forth in the "Calculation of Registration Fee" table in the
             effective registration statement.

        (iii) To include any material information with respect to the plan of
              distribution not previously disclosed in the Registration
              Statement or any material change to such information in the
              registration statement.

PROVIDED, HOWEVER, that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply if the
Registration Statement is on Form S-3, Form S-8 or Form F-3, and the information
required to be included in a post-effective amendment by those paragraphs is
contained in periodic reports filed with or furnished to the Commission by the
Registrant pursuant to Section 13 or 15(d) of the Exchange Act that are
incorporated by reference in the Registration Statement.

    (2) That, for the purpose of determining any liability under the Securities
       Act, each such post-effective amendment shall be deemed to be a new
       registration statement relating to the securities offered therein, and
       the offering of such securities at that time to be the initial BONA FIDE
       offering thereof.

                                      II-2
<PAGE>
    (3) To remove from registration by means of a post-effective amendment any
       of the securities being registered which remain unsold at the termination
       of the offering.

(b) The undersigned Registrant hereby undertakes that, for purposes of
    determining any liability under the Securities Act, each filing of the
    Registrant's annual report pursuant to Section 13(a) or 15(d) of the
    Exchange Act (and, where applicable, each filing of an employee benefit
    plan's annual report pursuant to Section 15(d) of the Exchange Act) that is
    incorporated by reference in the Registration Statement shall be deemed to
    be a new registration statement relating to the securities offered therein,
    and the offering of such securities at that time shall be deemed to be the
    initial BONA FIDE offering thereof.

(c) Insofar as indemnification for liabilities arising under the Securities Act
    may be permitted to directors, officers or persons controlling the
    Registrant pursuant to the foregoing provisions, the Registrant has been
    informed that in the opinion of the Commission such indemnification is
    against public policy as expressed in the Securities Act and is, therefore,
    unenforceable. In the event that a claim for indemnification against such
    liabilities (other than the payment by the Registrant of expenses incurred
    or paid by a director, officer or controlling person of the Registrant in
    the successful defense of any action, suit or proceeding) is asserted by
    such director, officer or controlling person in connection with the
    securities being registered, the Registrant will, unless in the opinion of
    its counsel the matter has been settled by controlling precedent, submit to
    a court of appropriate jurisdiction the question whether such
    indemnification by it is against public policy as expressed in the
    Securities Act and will be governed by the final adjudication of such issue.

                                      II-3
<PAGE>
                                   SIGNATURES


    Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this registration
statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Needham, Commonwealth of Massachusetts, on
January 20, 2000.



<TABLE>
<S>                                                    <C>  <C>
                                                       AVANT IMMUNOTHERAPEUTICS, INC.

                                                       BY:  /S/ UNA S. RYAN, PH.D.
                                                            -----------------------------------------
                                                            Una S. Ryan, Ph.D.
                                                            President, Chief Executive Officer and
                                                            Director
</TABLE>


    Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated.


<TABLE>
<CAPTION>
SIGNATURE                                              TITLE                         DATE
- ---------                                              -----                         ----
<S>                                                    <C>                           <C>
                                                       President, Chief Executive
/s/ UNA S. RYAN, PH.D.                                   Officer and Director
- -------------------------------------------              (Principal Executive          January 20, 2000
Una S. Ryan, Ph.D.                                       Officer)

*                                                      Chairman
- -------------------------------------------                                            January 20, 2000
J. Barrie Ward, Ph.D.

                                                       Chief Financial Officer
/s/ AVERY W. CATLIN                                      (Principal Financial
- -------------------------------------------              Officer and Principal         January 20, 2000
Avery W. Catlin                                          Accounting Officer)

*                                                      Director
- -------------------------------------------                                            January 20, 2000
Harry H. Penner, Jr.

*                                                      Director
- -------------------------------------------                                            January 20, 2000
Peter Sears, Esq.

*                                                      Director
- -------------------------------------------                                            January 20, 2000
Thomas R. Ostermueller

*                                                      Director
- -------------------------------------------                                            January 20, 2000
John L. Littlechild

*                                                      Director
- -------------------------------------------                                            January 20, 2000
Frederick W. Kyle
</TABLE>


<TABLE>
<S>   <C>                                                    <C>                        <C>
*By:  /s/ UNA S. RYAN, PH.D.
      --------------------------------------
      Una S. Ryan, Ph.D.
      Attorney-in-Fact
</TABLE>

                                      II-4
<PAGE>
                                 EXHIBIT INDEX

<TABLE>
<CAPTION>
       Exhibit
       Number                                   Description
       -------                                  -----------
<S>                     <C>
 23.1                   --  Consent of PricewaterhouseCoopers LLP
</TABLE>

                                      II-5

<PAGE>
                                                                    EXHIBIT 23.1

                         CONSENT OF INDEPENDENT ACCOUNTANTS

    We hereby consent to the incorporation by reference in this Registration
Statement on Form S-3 of our report dated March 1, 1999 relating to the
financial statements which appears in Avant Immunotherapeutics, Inc.'s Annual
Report on Form 10-K/A for the year ended December 31, 1998. We also consent to
the reference to us under the heading "Experts" in such registration statement.

/s/ PricewaterhouseCoopers LLP

PricewaterhouseCoopers LLP
Boston, Massachusetts
January 18, 2000


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