AVANT IMMUNOTHERAPEUTICS INC
S-3, 2000-08-07
IN VITRO & IN VIVO DIAGNOSTIC SUBSTANCES
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<PAGE>
     As filed with the Securities and Exchange Commission on August 7, 2000

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

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

                                    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)

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

                                   Copies 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, as amended, 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 of 1933, as amended, 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 of 1933, as amended, 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. / /

                        CALCULATION OF REGISTRATION FEE

<TABLE>
<CAPTION>
                                                                         PROPOSED             PROPOSED
                                                                          MAXIMUM              MAXIMUM
                                                  AMOUNT TO BE      OFFERING PRICE PER        AGGREGATE            AMOUNT OF
      TITLE OF SHARES BEING REGISTERED             REGISTERED            SHARE(1)          OFFERING PRICE      REGISTRATION FEE
<S>                                            <C>                  <C>                  <C>                  <C>
Common Stock, par value $.001................       4,650,953              $6.39             $29,719,590            $7,846
</TABLE>

(1) This estimate is made pursuant to Rule 457(c) under the Securities Act of
    1933, as amended (the "Securities Act"), solely for the purposes of
    determining the registration fee and is based upon the market value of
    outstanding shares of common stock, $.001 par value per share, of AVANT
    Immunotherapeutics, Inc. on July 31, 2000, utilizing the average of the high
    and low sale prices reported on the Nasdaq National Market for that date.
                         ------------------------------

    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 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME EFFECTIVE ON
SUCH DATE AS THE SECURITIES AND EXCHANGE COMMISSION, ACTING PURSUANT TO SAID
SECTION 8(A), MAY DETERMINE.

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
<PAGE>
                 SUBJECT TO COMPLETION. DATED AUGUST   , 2000.
THE INFORMATION IN THIS PROSPECTUS IS NOT COMPLETE AND MAY BE CHANGED. WE MAY
NOT SELL THESE SECURITIES UNTIL THE REGISTRATION STATEMENT FILED WITH THE
SECURITIES AND EXCHANGE COMMISSION IS EFFECTIVE. THIS PROSPECTUS IS NOT AN OFFER
TO SELL THESE SECURITIES AND IS NOT SOLICITING AN OFFER TO BUY THESE SECURITIES
IN ANY STATE WHERE THE OFFER OR SALE IS NOT PERMITTED.
<PAGE>
PROSPECTUS

                          4,650,953 SHARES OF COMMON STOCK

                         AVANT IMMUNOTHERAPEUTICS, INC.

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

    The selling stockholders identified in this prospectus, and any of their
pledgees, donees, transferees or other successors in interest, may offer to sell
up to an aggregate of 4,650,953 shares of common stock of AVANT
Immunotherapeutics, Inc. We are filing the Registration Statement of which this
prospectus is a part at this time to fulfill a contractual obligation to do so,
which we undertook at the time of the original issuance of these shares. We will
not receive any of the proceeds from the sale of the common stock by the selling
stockholders but, in fulfillment of our contractual obligations, we are bearing
the expenses of registration.

    Our common stock is listed on the Nasdaq Stock Market, Inc.'s National
Market System under the symbol "AVAN."

    SEE "RISK FACTORS" BEGINNING ON PAGE 2 FOR CERTAIN 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 August 7, 2000.
<PAGE>
                               TABLE OF CONTENTS

<TABLE>
<S>                                                           <C>
About AVANT.................................................      1
Forward-looking statements..................................      8
Registration rights of the selling stockholders.............      9
Legal matters...............................................     13
Experts.....................................................     13
</TABLE>
<PAGE>
                                  ABOUT AVANT

    We are a biopharmaceutical Company that uses novel applications of
immunology to prevent and treat diseases that arise internally, including
autoimmune diseases, cardiovascular diseases, cancer and inflammation, and by
external conditions, including infectious diseases and organ transplant
rejection. Each of our products address large market opportunities for which
current therapies are inadequate or non-existent.

    We are developing our products using a broad set of technologies that work
together to regulate the body's complement system, regulate T and B cell
activity, and enable us and others to create and deliver vaccines that prevent
and treat some diseases. We are using these technologies to develop both
vaccines and immunotherapeutics that prevent or treat disease caused by
infectious organisms and drugs and vaccines that modify undesirable activity of
the body's own proteins or cells. All of our products are in various stages of
research and development.

    Our common stock has been quoted on the Nasdaq National Market under the
symbol "AVAN" since August 24, 1998.

    Prior to that, our common stock traded on the Nasdaq National Market under
the symbol "TCS".

    Our executive offices are located at 119 Fourth Avenue, Needham,
Massachusetts 02494-2725 and our telephone number is (781) 433-0771. Additional
information regarding our Company, 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
7 and "Incorporation of Documents by Reference" on page 7.

                                  THE OFFERING

    This prospectus relates to up to 4,650,953 shares of our common stock that
may be offered for sale by the selling stockholders. We originally issued these
shares to the selling stockholders in a private placement on July 17, 2000. In
connection with this private placement, we entered into a registration rights
agreement with the selling stockholders. We are registering the common stock
covered by this prospectus in order to fulfill our contractual obligations under
the registration rights agreement. Registration of the common stock does not
necessarily mean that all or any portion of the common stock will be offered for
sale by the selling stockholders.

    We have agreed to bear the expenses of the registration of the common stock
under federal and state securities laws, but we will not receive any proceeds
from the sale of any common stock offered under this prospectus.

                                       1
<PAGE>
                                  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 $38 million, as of June 30, 2000. 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 I/II
TP10               pediatric cardiac surgery   clinical phase I/II
TP10               heart attacks               clinical phase I
TP20               stroke                      pre-clinical
CETi-1 Vaccine     atherosclerosis             clinical phase I
Rotavirus          rotavirus infection
Vaccine                                        clinical phase II
Cholera Vaccine    cholera infection           clinical phase II
                   respiratory synctial
Adjumer-Registered Trademark- virus            clinical phase II
Adjumer-Registered Trademark- lyme disease     pre-clinical
Adjumer-Registered Trademark- influenza        pre-clinical
Therapore-TM-      hepatitis                   pre-clinical
Therapore-TM-      HIV                         pre-clinical
Therapore-TM-      cancer                      pre-clinical
TCAR               multiple sclerosis          clinical phase II
</TABLE>

    The product development and regulatory approval process can be generally
described as follows. Pre-clinical tests are performed at an early stage of a
product's development and provide information about a product's effectiveness in
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 more than a year to receive FDA approval.

    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.

                                       2
<PAGE>
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 who 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 July 2000, we sold 4,650,953 shares of our common stock in a private
placement at $7.85 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, 5,459,375 shares we sold in a September 1999 private placement at
$1.92 per share, 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 3,124,008 shares that employees may
purchase under stock options at prices ranging from $0.30 to $14.69 per share,
can be resold in the public securities markets without restriction. These shares
in total account for approximately 33% of our total common stock outstanding as
of June 30, 2000, and approximately 30% 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. TP10 has also undergone phase I
clinical testing for use in treating heart attacks and phase II clinical testing
for organ transplant. Other products in our vaccine programs are in various
stages of preclinical and clinical testing. Preclinical tests are performed at
an early stage of a product's development and provide information about a
product's effectiveness on laboratory animals. Preclinical tests can last years.
If a product passes its preclinical tests satisfactorily, we file an
investigational new drug application for the product with the FDA, and if the
FDA gives its approval we begin phase I clinical tests. Phase I testing
generally lasts between 6 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 more than a year to receive FDA approval. 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

                                       3
<PAGE>
companies, including Novartis Pharma AG, Smithkline Beecham and Aventis Pasteur,
who 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 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
Cleveland Clinic, PPD International, Pharmaceuticals Research Associates, 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 and manufacturers, including Walter Reed Army Institute of Research,
Marathon Biopharmaceuticals, Inc., Lonza

                                       4
<PAGE>
Biologies plc 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.

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 Novartis Pharma AG,
Aventis Pasteur and SmithKline Beecham, 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, and this 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, AND THIS
RELIANCE 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. Our competitors may
discover our trade secrets either through breach of these agreements or through
independent development. 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.

                                       5
<PAGE>
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 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
Pharmaceuticals, 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, and
this 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 could 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

                                       6
<PAGE>
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. This would cause higher development costs and delays, and
may ultimately prove impracticable.

OUR BUSINESS REQUIRES US TO USE HAZARDOUS MATERIALS, AND THIS 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.

                 CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS

    Mr. Sears, a Director of the Company since May 1999, purchased 50,000 shares
of common stock of the Company at $1.92 per share, having an aggregate value of
$96,000, in connection with the Company's private placement of stock in
September 1999 and purchased 12,739 shares of common stock of the Company at
$7.85 per share, having an aggregate value of $100,001, in connection with the
Company's private placement of stock in July 2000.

                      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. You may
access the Securities and Exchange Commission's web site at http://www.sec.gov.

                    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 them. 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

                                       7
<PAGE>
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, 1999
      (as amended on Form10-K/A filed on July 25, 2000)

    - our Current Report on Form 8-K filed on July 19, 2000

    - our Quarterly Reports on Form 10-Q for the quarters ended March 31, 2000
      and June 30, 2000

    - the definitive Proxy Statement for our annual meeting of stockholders
      filed on March 28, 2000

    - 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, including any beneficial
owner, to whom this prospectus is delivered, upon written or oral 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 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 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.

                                       8
<PAGE>
                REGISTRATION RIGHTS OF THE SELLING STOCKHOLDERS

    The following is a summary of the material terms and provisions of the
registration rights agreement 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
registration rights agreement.

    Under the registration rights agreement, we must file by August 6, 2000 a
Registration Statement covering the sale by the selling stockholders of the
common stock that they purchased on July 17, 2000. We must use our best efforts
to cause the Registration Statement to be declared effective by the Securities
and Exchange Commission no later than November 14, 2000 and to keep the
Registration Statement continuously effective until the earliest to occur of:

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

    - such time as all the common stock held by the selling stockholders could
      be sold under Rule 144 of the Securities Act, during any 90 day period
      without restriction (including without limitation as to volume); or

    - the date which is two years after the date the Registration Statement of
      which this prospectus forms a part was originally declared effective by
      the Securities and Exchange Commission.

    Any common stock sold by the selling stockholders pursuant to this
prospectus will no longer be entitled to the benefits of the registration rights
provisions of the registration rights agreement.

    The registration rights agreement requires that we bear all expenses of
registering the common stock with the exception of any underwriting commission
or brokerage fees and taxes of any kind and any legal, accounting and other
expenses incurred by the selling stockholders. We agreed to indemnify the
selling stockholders and any person who controls the selling stockholders
against all losses, claims, damages, actions, liabilities, costs and expenses
arising under the securities laws in connection with an untrue statement or
omission in the Registration Statement or this prospectus, subject to
limitations specified in the registration rights agreement. In addition, the
selling stockholders agreed to indemnify us and our directors, officers and any
person who controls our Company, subject to limitations specified in the
registration rights agreement, against all losses, claims, damages, actions,
liabilities, costs and expenses arising under the securities laws if they result
from an untrue statement or omission contained in any written information
furnished to us by the selling stockholders expressly for use in the
Registration Statement or this prospectus or any amendments to the Registration
Statement or any prospectus supplements.

                           [INTENTIONALLY LEFT BLANK]

                                       9
<PAGE>
                              SELLING STOCKHOLDERS

    The following table provides the name and number of shares of common stock
owned by each selling stockholder as of July 17, 2000, 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. The selling stockholders
will receive all of the proceeds from the sale of shares of common stock
pursuant to this prospectus.

    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 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 July 17, 2000   Offered Hereby   Number(1)   Percent
-------------------                           -------------------   --------------   ---------   --------
<S>                                           <C>                   <C>              <C>         <C>
Narragansett I, LP(2).......................           54,777             54,777         0          0
Narragansett Offshore, LTD(2)...............           72,611             72,611         0          0
BayStar Capital L.P.(3).....................          891,720            891,720         0          0
Pictet Global Sector Fund-Biotech(4)........        1,146,497          1,146,497         0          0
Pictet Asset Management for Pictet Biotech
  Fund(5)...................................          127,388            127,388         0          0
Dresdner RCM Biotechnology Fund(6)..........          509,554            509,554         0          0
Framlington Health Fund(7)..................          275,000            275,000         0          0
Munder Framlington Healthcare Fund(8).......          325,000            325,000         0          0
JALAA Equities L.P.(9)......................          127,388            127,388         0          0
Halifax Fund, L.P.(10)......................          254,777            254,777         0          0
Clarion Capital Corporation(11).............           63,694             63,694         0          0
Clarion Partners, L.P.(11)..................           43,312             43,312         0          0
Clarion Offshore Fund LTD(11)...............           20,382             20,382         0          0
Catalyst Partners, L.P.(12).................          445,860            445,860         0          0
Catalyst International, LTD(12).............           63,694             63,694         0          0
Peter Sears(13).............................           12,739             12,739         0          *
Finsbury Technology Trust (14)..............          127,388            127,388         0          0
Consulta Technology Fund (14)...............           66,404             66,404         0          0
Pulsar Technology Fund (14).................           17,076             17,076         0          0
FGI Biotechnology Fund(14)..................            5,692              5,692         0          0
                                                    ---------          ---------         --         --
TOTAL.......................................        4,650,953          4,650,953         0          0
                                                    =========          =========         ==         ==
</TABLE>

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

*   denotes less than 1%

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

 (2) The selling stockholder's address is 375 Park Avenue, Suite 1404, New York,
    NY 10152.

 (3) The selling stockholder's address is 50 California Street, 33rd Floor, San
    Francisco, CA 94111.

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

                                       10
<PAGE>
 (5) The selling stockholder's address is Tower 42, Level 37, 25 Old Broad
    Street, London, England EC2N 1HQ.

 (6) The selling stockholder's address is Four Embarcadero Center Suite 3000,
    San Francisco, CA 94111.

 (7) The selling stockholder's address is 155 Bishopsgate, London, England EC ZM
    3XJ.

 (8) The selling stockholders' address is State Street Bank, 225 Franklin
    Street, Boston, MA 02101.

 (9) The selling stockholder's address is 34 Sumner Road, Greenwich, CT 06831.

(10) The selling stockholder's address is 195 Maplewood Avenue, Maplewood, NJ
    07040.

(11) The selling stockholders' address is 1801 East 9th Street, Suite 1120,
    Cleveland, OH 44114.

(12) The selling stockholder's address is 350 Park Avenue, 11th Floor, New York,
    NY 10022.

(13) The selling stockholder's address is 8 Paul Road, St. Davids, PA 19087. Mr.
    Sears has been a director of AVANT since May, 1999.

(14) The selling stockholder's address is 12 Appold Street, London, England EC2A
    2AW.

                              PLAN OF DISTRIBUTION

    We are registering 4,650,953 shares of common stock 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 transactions in which a broker-dealer will attempt to sell all or a
      portion of such shares as agent but may position and resell all or a
      portion of the block as principal to facilitate the transaction

    - purchases by any such broker-dealer as principal and resale by such
      broker-dealer for its own account pursuant to any supplement 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 any such
      broker-dealer solicits purchasers

    - 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

                                       11
<PAGE>
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 pledgees,
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.

    To the extent required under the Securities Act, 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, 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 recall 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 arising from disclosures we make
or fail to make. The selling stockholders have agreed to indemnify us against
liabilities, under the Securities Act arising from information supplied to us by

                                       12
<PAGE>
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.

                                 LEGAL MATTERS

    The validity of the common stock being offered by the selling shareholders
will be passed upon for us by Goodwin, Procter & Hoar LLP, Boston,
Massachusetts.

                                    EXPERTS

    The consolidated financial statements incorporated in this prospectus by
reference to the Annual Report on Form 10-K for the year ended December 31,
1999, as amended by our Form 10-K/A filed on July 25, 2000, 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.

                                       13
<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.

                                4,650,953 SHARES

                                     AVANT
                              IMMUNOTHERAPEUTICS,
                                      INC.

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

                                   PROSPECTUS

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

                                 AUGUST 7, 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........  $ 7,846
Accountants Fees and Expenses...............................  $ 3,000
Blue Sky Fees and Expenses..................................  $ 1,000
Legal Fees and Expenses.....................................  $25,000
Printing Expenses...........................................  $ 1,000
Transfer Agent Expenses.....................................  $ 2,500
Miscellaneous...............................................  $
Nasdaq Listing Fee..........................................  $
                                                              -------
    Total...................................................  $
                                                              =======
</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 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, 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            Powers of Attorney (included on the signature page hereto)

        99.3            Securities Purchase Agreement dated as of July 13, 2000,
                        between the Company and the Selling Stockholders

        99.4            Registration Rights Agreement dated as of July 13, 2000,
                        between the Company and the Selling Stockholders
</TABLE>

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;

        (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
information required to be included in a post-effective amendment by those
paragraphs is contained in periodic reports filed with or furnished to the
Securities and Exchange Commission by the Registrant pursuant to Section 13 or
15(d) of the Securities Exchange Act of 1934 that are incorporated by reference
in this 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.

    (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
    Securities Exchange Act of 1934, as amended (and, where applicable, each
    filing of an employee benefit plan's annual report pursuant to Section 15(d)
    of the Securities

                                      II-2
<PAGE>
    Exchange Act of 1934, as amended) 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 Securities and Exchange 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 August 7,
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>

                               POWER OF ATTORNEY

    Pursuant to the requirements of the Securities Act of 1933, as amended, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated, each of whom also constitutes and
appoints Una S. Ryan and Avery W. Catlin, and each of them singly, his true and
lawful attorney-in-fact and agent, for him, with full power of substitution and
resubstitution, for him and in his name, place and stead, in any and all
capacities, to sign any and all amendments to this Registration Statement, and
to file the same and all exhibits thereto and any other documents in connection
therewith with the Securities and Exchange Commission, granting unto each
attorney-in-fact and agent full power and authority to do and perform each and
every act and thing requisite and necessary to be done, as fully to all intents
and purposes as he might or could do in person, hereby ratifying and confirming
all that each attorney-in-fact and agent or his substitute or substitutes may
lawfully do or cause to be done by virtue hereof.

<TABLE>
<CAPTION>
SIGNATURE                                         TITLE                                DATE
---------                                         -----                                ----
<S>                                               <C>                                  <C>
/s/ UNA S. RYAN PH.D                              President, Chief Executive Officer
--------------------------------------              and Director (Principal Executive  August 7, 2000
Una S. Ryan Ph.D                                    Officer)

/s/ J. BARRIE WARD, PH.D.                         Chairman
--------------------------------------                                                 August 7, 2000
J. Barrie Ward, Ph.D.

                                                  Senior Vice President, Chief
/s/ AVERY W. CATLIN                                 Financial Officer and Treasurer
--------------------------------------              (Principal Financial Officer and   August 7, 2000
Avery W. Catlin                                     Principal Accounting Officer)

/s/ HARRY H. PENNER, JR.                          Director
--------------------------------------                                                 August 7, 2000
Harry H. Penner, Jr.

/s/ PETER A. SEARS, ESQ.                          Director
--------------------------------------                                                 August 7, 2000
Peter A. Sears, Esq.

/s/ THOMAS R. OSTERMUELLER                        Director
--------------------------------------                                                 August 7, 2000
Thomas R. Ostermueller
</TABLE>

                                      II-4
<PAGE>

<TABLE>
<CAPTION>
SIGNATURE                                         TITLE                                DATE
---------                                         -----                                ----
<S>                                               <C>                                  <C>
/s/ JOHN L. LITTLECHILD                           Director
--------------------------------------                                                 August 7, 2000
John L. Littlechild

/s/ FREDERICK W. KYLE                             Director
--------------------------------------                                                 August 7, 2000
Frederick W. Kyle
</TABLE>

                                      II-5
<PAGE>
                                 EXHIBIT INDEX

<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            Powers of Attorney (included on the signature page hereto)

        99.3            Securities Purchase Agreement dated as of July 13, 2000,
                        between the Company and the Selling Stockholders

        99.4            Registration Rights Agreement dated as of July 13, 2000,
                        between the Company and the Selling Stockholders
</TABLE>

                                      II-6


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