AVANT IMMUNOTHERAPEUTICS INC
S-3/A, 2000-02-11
IN VITRO & IN VIVO DIAGNOSTIC SUBSTANCES
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   As filed with the Securities and Exchange Commission on February 11, 2000


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

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


                                AMENDMENT NO. 4
                                       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>

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 under the
symbol "AVAN." On February 10, 2000, our common stock closed at a price of $9.00
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 February 11, 2000.

<PAGE>
                               TABLE OF CONTENTS


<TABLE>
<S>                                                           <C>
ABOUT AVANT IMMUNOTHERAPEUTICS, INC.........................      1

RISK FACTORS................................................      1
  Our history of losses and uncertainty of our future
    profitability makes the common stock a highly
    speculative investment..................................      1
  If we cannot sell capital stock to raise necessary funds,
    it may force us to limit our research, development and
    testing programs........................................      2
  If selling stockholders choose to sell shares in large
    volume, the trading price of our common stock could
    suffer..................................................      2
  If our products do not pass required tests for safety and
    effectiveness, we will not be able to derive commercial
    revenue from them.......................................      2
  Product testing is critical to the success of our products
    but subject to delay or cancellation if we have
    difficulty enrolling patients...........................      3
  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............................................      3
  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...........................      4
  We may face delays, difficulties or unanticipated costs in
    establishing sales, distribution and manufacturing
    capabilities for our commercially ready products........      4
  Our reliance on third parties requires us to share our
    trade secrets, which increases the possibility that a
    competitor will discover them...........................      4
  We license technology from other companies to develop our
    products, and those companies could restrict our use of
    it......................................................      4
  We have many competitors in our field and they may develop
    technologies that make ours obsolete....................      5
  We rely on patents, patent applications and other
    intellectual property protections to protect our
    technology and trade secrets; they are expensive and may
    not provide sufficient protection.......................      5
  Our business requires us to use hazardous materials, which
    increases our exposure to dangerous and costly
    accidents...............................................      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>
                      ABOUT AVANT IMMUNOTHERAPEUTICS, INC.

    Our business is the discovery and development of products that harness the
human body's immune system to prevent and treat disease. We also plan to market
and sell our products if and when they receive the necessary regulatory
approvals.

    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 and "Incorporation of Documents by Reference" on page 7.

                                  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 had 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 of the
following products we have in the pre-clinical and clinical testing stages of
development:

<TABLE>
<CAPTION>
    PRODUCT                   USE                    STAGE
    -------                   ---                    -----
<S>                <C>                         <C>
TP10               organ transplantation       clinical phase II
TP10               pediatric cardiac surgery   clinical phase II
TP10               heart attacks               clinical phase I
TP20               stroke                      pre-clinical
Rotavirus          rotavirus infection
Vaccine                                        clinical phase II
Cholera Vaccine    cholera infection           clinical phase II
Adjumer-Registered Trademark- influenza
Vaccine                                        clinical phase II
Adjumer-Registered Trademark- respiratory synctial
Vaccine            virus                       clinical phase II
Adjumer-Registered Trademark- lyme disease
Vaccine                                        pre-clinical
Therapore-TM-      hepatitis                   pre-clinical
Therapore-TM-      HIV                         pre-clinical
Therapore-TM-      cancer                      pre-clinical
CETP Vaccine       atherosclerosis             clinical phase I
TCAR               multiple sclerosis          clinical phase II
</TABLE>


    Pre-clinical tests are performed at an early stage of a product's
development and provide information about a product's effectiveness on
laboratory animals. Pre-clinical tests can last years. If a product passes its
pre-clinical tests satisfactorily, we file an investigational new drug
application for the product with the Food and Drug Administration, and if the
FDA gives its approval we begin phase I clinical tests. Phase I testing
generally lasts between six and 12 months. If phase I test results are
satisfactory and the FDA gives its approval, we can begin phase II clinical
tests. Phase II testing generally lasts between six and 18 months. If phase II
test results are satisfactory and the FDA gives its approval, we can begin phase
III pivotal studies. Phase III studies generally last between 12 and 36 months.
Once clinical testing is completed and a new drug application is filed with the
FDA, it may take several more years to receive FDA approval.


                                       1
<PAGE>

    If and when any of these products receive FDA approval, we will need to make
substantial investments to establish sales, marketing, quality control, and
regulatory compliance capabilities. We cannot predict how quickly our lead
products will progress through the regulatory approval process. As a result, we
may continue to lose money for several years. We will disclose the progress each
product is making through pre-clinical and clinical testing, and the
preparations we are making for products that are nearing approval for sale in
our periodic reports under the Securities Exchange Act of 1934.


IF WE CANNOT SELL CAPITAL STOCK TO RAISE NECESSARY FUNDS, IT MAY FORCE US TO
LIMIT OUR RESEARCH, DEVELOPMENT AND TESTING PROGRAMS


    We will need to raise more capital from investors to advance our lead
products through the clinical testing and to fund our operations until we
receive final FDA approval and our products begin to generate revenues for us.
However, based on our history of losses, we may have difficulty attracting
sufficient investment interest. We may also try to obtain funding through
research grants and agreements with commercial collaborators. This kind of
funding is at the discretion of other organizations and companies which have
limited funds and many companies compete with us for those funds. As a result,
we may not receive any research grants or funds from collaborators. We will
provide specific information about the sources and adequacy of funding for our
active research and development programs in our periodic reports under the
Securities Exchange Act of 1934.


IF SELLING STOCKHOLDERS CHOOSE TO SELL SHARES IN LARGE VOLUME, THE TRADING PRICE
OF OUR COMMON STOCK COULD SUFFER

    In September 1999, we sold 5,459,375 shares of our common stock in a private
placement at $1.92 per share. This was the latest of several private placements
of our common stock. Those shares, which are covered by this prospectus, plus
among others, 2,043,494 shares we sold in a March 1998 private placement at
$1.90 per share, 1,433,750 shares we issued in June 1998 in settlement of a
contract dispute with a landlord, and 3,138,559 shares that employees may
purchase under stock options at prices ranging from $0.30 to $7.81 per share,
can be resold in the public securities markets without restriction. These shares
in total account for approximately 25.1% of our total common stock outstanding
as of December 31, 1999, and approximately 22.7% of our common stock on a fully
diluted basis. If large numbers of shares are sold over a short period of time,
the price of our stock may decline rapidly or fluctuate widely.

IF OUR PRODUCTS DO NOT PASS REQUIRED TESTS FOR SAFETY AND EFFECTIVENESS, WE WILL
NOT BE ABLE TO DERIVE COMMERCIAL REVENUE FROM THEM


    For AVANT to succeed, we will need to derive commercial revenue from the
products we have under development. The FDA has not approved any of our lead
products for sale to date. Our lead drug, TP10, is undergoing phase II clinical
testing for use in pediatric cardiac surgery and organ transplantation. TP10 is
also undergoing phase I clinical testing for use in treating heart attacks.
Other products in our vaccine programs are in various stages of pre-clinical and
clinical testing. We will disclose the progress of our ongoing tests and any FDA
action on our products in our periodic reports under the Securities Exchange Act
of 1934.


    In all cases we must show that a pharmaceutical product is both safe and
effective before the FDA, or drug approval agencies of other countries where we
intend to sell the product, will approve it for sale. Our research and testing
programs must comply with drug approval requirements both in the United States
and in other countries, since we are developing our lead products with
companies, including Novartis Pharma AG, Yamanouchi Pharmaceutical and Aventis
Pasteur, which intend to commercialize them both in the U.S. and abroad. A
product may fail for safety or effectiveness at any stage of the testing
process. The key risk we face is that none of our products

                                       2
<PAGE>
under development will come through the testing process to final approval for
sale, with the result that we cannot derive any commercial revenue from them
after investing significant amounts of capital in multiple stages of
pre-clinical and clinical testing.

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

    As our portfolio of potential products moves from pre-clinical testing to
clinical testing, and then through progressively larger and more complex
clinical trials, we will need to enroll an increasing number of patients with
the appropriate characteristics. At times we have experienced difficulty
enrolling patients and we may experience more difficulty as the scale of our
clinical testing program increases. The factors that affect our ability to
enroll patients are largely uncontrollable and include principally the
following:

    - 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

    As clinical tests currently in progress continue and new tests begin, we
will disclose in our periodic reports under the Securities Exchange Act of 1934
our progress in enrolling sufficient patients to keep our various programs
moving forward, including any specific difficulties we face from time to time
and their expected consequences on the affected program. If we cannot enroll
patients as needed, our costs may increase or it could force us to delay or
terminate testing for a product.

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 an employment agreement
with Dr. Ryan. 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.

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.

                                       3
<PAGE>
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 Pharma AG, Aventis Pasteur, SmithKline Beecham, and
Yamanouchi Pharmaceutical, for the licensing, development and ultimate
commercialization of most 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 MAY FACE DELAYS, DIFFICULTIES OR UNANTICIPATED COSTS IN ESTABLISHING SALES,
DISTRIBUTION AND MANUFACTURING CAPABILITIES FOR OUR COMMERCIALLY READY PRODUCTS

    We have chosen to retain, rather than license, all rights to some of our
lead products, such as TP10 for pediatric cardiac surgery. If we proceed with
this strategy, we will have full responsibility for commercialization of these
products if and when they are approved for sale. We currently lack the
marketing, sales and distribution capabilities that we will need to carry out
this strategy. 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 when they are approved for sale.

    We do not currently plan to develop internal manufacturing capabilities to
produce any of our products if they are approved for sale. To the extent that we
choose to market and distribute products ourselves, this strategy will make us
dependent 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.

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 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 who are 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

    Companies that license to us technologies we use in our research and
development programs may require us to achieve milestones or devote minimum
amounts of resources to develop products using those technologies. 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. The number and variety of our research and development

                                       4
<PAGE>
programs require us to establish priorities and to allocate available resources
among competing programs. From time to time we may choose to slow down or cease
our efforts on particular products. If in doing so we fail to perform our
obligations under a license 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 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 U.S. 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; THEY ARE EXPENSIVE AND
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 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 use of the underlying product or technology will face restrictions,
including elimination.

    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 on potentially unfavorable terms. If
those licenses from third parties are necessary but we cannot acquire them, we
would attempt to design around the relevant technology, which would cause higher
development costs and delays, and may ultimately prove impracticable.

                                       5
<PAGE>
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.

                      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

                                       7
<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

    We are registering 5,459,375 shares for resale by the selling stockholders
to satisfy our commitment to do so under a contract with them, but the
registration of these shares does not necessarily mean that the selling
stockholders will sell any or all of the shares registered. The selling
stockholders, or their pledgees, donees, transferees or other successors in
interest, may sell the shares from time to time at fixed prices that may be
changed, at market prices prevailing at the time of sale, at prices related to
prevailing market prices or at prices otherwise negotiated. The selling
stockholders may sell shares by one or more of the following methods:

    - block trades in which the broker or dealer so engaged will attempt to sell
      the shares as agent but may position and resell a portion of the block as
      principal to facilitate the transaction

    - purchases by a broker or dealer as principal and resale by the broker or
      dealer for its own account pursuant to this prospectus

    - sales on a stock exchange or automated interdealer quotation system on
      which our shares are listed on in the over-the-counter market

    - ordinary brokerage transactions and transactions in which the broker
      solicits purchases

    - privately negotiated transactions

    - short sales

    - one or more underwritten offerings on a firm commitment or best efforts
      basis

    - through the writing of options on the shares, whether or not the options
      are listed on an options exchange

    - through the distribution of the shares by any selling stockholder to its
      partners, members or stockholders

    The selling stockholders may also transfer the shares by gift. We do not
know of any specific arrangements by the selling stockholders for the sale of
any of the shares at the present time.

    The selling stockholders may engage brokers and dealers, and any brokers or
dealers may arrange for other brokers or dealers to participate in effecting
sales of the shares. These brokers, dealers or underwriters may act as
principals, or as an agent of a selling stockholder. Broker-dealers may agree
with a selling stockholder to sell a specified number of the shares at a
stipulated price per security. If the broker-dealer is unable to sell shares
acting as agent for a selling stockholder, it may purchase as principal any
unsold shares at the stipulated price. Broker-dealers who acquire shares as
principals may thereafter resell the shares from time to time in transactions in
any stock exchange or automated interdealer quotation system on which the shares
are then listed, at prices and on terms then prevailing at the time of sale, at
prices related to the then-current market price or in negotiated transactions.
Broker-dealers may also use block transactions and sales to and through
broker-dealers as described above.

    From time to time, the selling stockholders may pledge, hypothecate or grant
a security interest in some or all of the shares owned by them. The pledges,
secured parties or persons to whom the shares have been hypothecated will, upon
foreclosure in the event of default, be deemed to be selling stockholders. The
number of a selling stockholder's shares offered under this prospectus will
decrease as and when it takes such actions. The plan of distribution for that
selling stockholder's shares will otherwise remain unchanged. In addition, a
selling stockholder may, from time to time, sell the shares short, and, in those
instances, this prospectus may be delivered in connection with the short sales
and the shares offered under this prospectus may be used to cover short sales.

                                       10
<PAGE>
    To the extent required under the Securities Act of 1933, the aggregate
amount of selling stockholders' shares being offered and the terms of the
offering, the names of any agents, brokers, dealers or underwriters and any
applicable commission with respect to a particular offer will be disclosed in a
prospectus supplement. Any underwriters, dealers, brokers or agents
participating in the distribution of the shares may receive compensation in the
form of underwriting discounts, concessions, commissions or fees from a selling
stockholder and/or purchasers of shares for whom they may act (which
compensation as to a particular broker-dealer might be in excess of customary
commissions).

    The selling stockholders and any underwriters, brokers, dealers or agents
that participate in the distribution of the shares may be deemed to be
"underwriters" within the meaning of the Securities Act of 1933, and any
discounts, concessions, commissions or fees received by them and any profit on
the resale of the shares sold by them may be deemed to be underwriting discounts
and commissions.

    A selling stockholder may enter into hedging transactions with
broker-dealers and the broker-dealers may engage in short sales of the shares in
the course of hedging the positions they assume with that selling stockholder,
which may include distributions of the shares by those broker-dealers. A selling
stockholder may enter into option or other transactions with broker-dealers that
involve the delivery of shares to the broker-dealers, who may then resell or
otherwise transfer those shares. A selling stockholder may also loan or pledge
shares to a broker-dealer and the broker-dealer may sell those shares.

    The selling stockholders and other persons participating in the sale or
distribution of the shares will be subject to applicable provisions of the
Securities Exchange Act of 1934 and the rules and regulations of the Securities
and Exchange Commission, including Regulation M. This regulation may limit the
timing of purchases and sales of any of the shares by the selling stockholders
and any other person. The anti-manipulation rules under the Securities Exchange
Act of 1934 may apply to sales of securities in the market and to the activities
of the selling stockholders and their affiliates. Furthermore, Regulation M may
restrict the ability of any person engaged in the distribution of the shares to
engage in market-making activities with respect to the particular shares being
distributed for a period of up to five business days before the distribution.
These restrictions may affect the marketability of the shares and the ability of
any person or entity to engage in market-making activities with respect to the
shares.

    We have agreed to indemnify the selling stockholders and any brokers,
dealers and agents who may be deemed to be underwriters against liabilities,
including liabilities under the Securities Act of 1933 arising from disclosures
we make or fail to make. The selling stockholders have agreed to indemnify us
against liabilities, under the Securities Act of 1933 arising from information
supplied to us by them. We will pay all expenses relating to the offering and
sale of shares by the selling stockholders, other than the commissions,
discounts and fees of underwriters, broker-dealers or the selling stockholders'
separate counsel or advisors.

                                    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

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


                               FEBRUARY 11, 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
February 10, 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       February 10, 2000
Una S. Ryan, Ph.D.                                       Officer)

*                                                      Chairman
- -------------------------------------------                                         February 10, 2000
J. Barrie Ward, Ph.D.

                                                       Chief Financial Officer
/s/ AVERY W. CATLIN                                      (Principal Financial
- -------------------------------------------              Officer and Principal      February 10, 2000
Avery W. Catlin                                          Accounting Officer)

*                                                      Director
- -------------------------------------------                                         February 10, 2000
Harry H. Penner, Jr.

*                                                      Director
- -------------------------------------------                                         February 10, 2000
Peter Sears, Esq.

*                                                      Director
- -------------------------------------------                                         February 10, 2000
Thomas R. Ostermueller

*                                                      Director
- -------------------------------------------                                         February 10, 2000
John L. Littlechild

*                                                      Director
- -------------------------------------------                                         February 10, 2000
Frederick W. Kyle
</TABLE>


                                      II-4
<PAGE>

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

                                      II-5
<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
February 8, 2000


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