<PAGE>

                                                                    EXHIBIT 10.8

WHEREVER CONFIDENTIAL INFORMATION IS OMITTED HEREIN (SUCH OMISSIONS ARE DENOTED
BY AN ASTERISK), SUCH CONFIDENTIAL INFORMATION HAS BEEN SUBMITTED SEPARATELY TO
THE SECURITIES AND EXCHANGE COMMISSION PURSUANT TO A REQUEST FOR CONFIDENTIAL
TREATMENT.

                 AMENDMENT TO MANUFACTURING AND SUPPLY AGREEMENT

      THIS AMENDMENT (the "Amendment") is made and entered into as of December
22, 2003 to the Manufacturing and Supply Agreement entered into as of August 6,
1997 by and between JPI PHARMACEUTICA INTERNATIONAL, a division of Cilag AG
International Zug ("JPI"), JANSSEN PHARMACEUTICA INC. ("JANSSEN US") and
ALKERMES CONTROLLED THERAPEUTICS INC. II ("ACT II"), as amended (the
"Agreement") (any terms used but not defined herein shall have the meaning set
forth in the Agreement).

                                    RECITALS:

       WHEREAS, JPI, JANSSEN US and ACT II have entered into the Agreement; and

       WHEREAS, the parties now wish to enter into this Amendment to clarify the
terms for payment of the Manufacturing Fee as set forth in the Agreement by
amending the terms and conditions of the Agreement as set forth below;

       NOW, THEREFORE, in consideration of the mutual promises contained herein
and other good and valuable consideration, the receipt and sufficiency of which
is hereby acknowledged, the parties agree as follows:

1.     Article 1.3 of the Agreement shall be deleted in its entirety and
replaced with a new Article 1.3 which shall read as follows:

1.3    "Final Product" shall mean a Presentation Form approved and marketed by
       JANSSEN, their Affiliates and licensees, ready for sale to the final
       customer.

2.     Article 1.5 of the Agreement shall be deleted in its entirety and
replaced with Article 1.5(a) and Article 1.5(b) which shall read as follows:

1.5(a) "U.S. Licensed Net Selling Price" shall mean the * offered by JANSSEN,
       its Affiliates or licensees in a given calendar year (or such shorter
       period as may be applicable) to independent third parties for each
       Presentation Form of the Final Product for sale in the United States, its
       territories and possession, less deductions for (i) trade, cash and
       ordinary business discounts allowed; (ii) allowances or credits to
       customers on account of rejection or return of Final Product; and (iii)
       managed care rebates or allowances and mandatory price allowances imposed
       by governments.

       If JANSSEN, its Affiliates or licensees sell any Presentation Form of the
       Final Product in the United States in such a manner that the * of the
       same is not readily identifiable then the * shall be whichever is the
       higher of (i) the fair market value of such Final Product or (ii) the
       proportion of the bundled price attributed to such Final Product by
       JANSSEN, its Affiliates or licensees whenever the Final Product is sold
       as o part of a package of products or services. For the purpose hereof
       "fair market

<PAGE>


       value" shall mean, without limitation, the value of such Final Product
       sold to similar third parties in similar quantities in the United States.
       If the fair market value cannot be determined in the United States, the
       fair market value will be negotiated by the parties in good faith.

1.5(b) "ROW Licensed Net Selling Price" shall mean the * offered by JANSSEN, its
       Affiliates or licensees in a given calendar year (or such shorter period
       as may be applicable) to independent third parties for each Presentation
       Form of the Final Product for sale in the Territory (other than the
       United States, its territories and possessions), less deductions for (i)
       trade, cash and ordinary business discounts allowed; (ii) allowances or
       credits to customers on account of rejection or return of Final Product;
       and (iii) managed care rebates or allowances and mandatory price
       allowances imposed by governments.

       If JANSSEN, its Affiliates or licensees sell any Presentation Form of the
       Final Product in a country in the Territory (other than the United
       States, its territories and possessions) in such a manner that the * of
       the same is not readily identifiable then the * for that country shall be
       whichever is the higher of (i) the fair market value of such Final
       Product or (ii) the proportion of the bundled price attributed to such
       Final Product by JANSSEN, its Affiliates or licensees whenever the Final
       Product is sold as part of a package of products or services. For the
       purpose hereof "fair market value" shall mean, without limitation, the
       value of such Final Product sold to similar third parties in similar
       quantities. If the fair market value cannot be determined in any given
       country, the fair market value will be determined by the value of such
       Final Product sold to similar customers in countries with similar pricing
       and reimbursement structures and for similar quantities.

3.     Article 1.7 of the Agreement shall be deleted in its entirety and
replaced with a new Article 1.7 which shall read as follows:

1.7    "Manufacturing Fee" shall mean the fee to be paid by JPI and JANSSEN US
       to ACT II for each Presentation Form of the Product in consideration for
       the Manufacture of Products supplied to each of them in accordance with
       the terms hereof and which fee will be calculated as a percentage of the
       U.S. Licensed Net Selling Price and/or the ROW Licensed Net Selling
       Price, as applicable, for each Presentation Form of the Final Product in
       accordance with the mechanism set forth in Article 6.

4.    A new Article 1.13 shall be added to the Agreement which shall read as
follows:

1.13  "Presentation Form" shall mean a form of the Product or the Final Product
      determined by the amount of the single dose (either 25 mg., 37.5 mg. or 50
      mg.) of the depot formulation of Risperidone contained therein.

<PAGE>

5.    Article 6 of the Agreement shall be deleted in its entirety and replaced
with a new Article 6 which shall read as follows:

6.1   In consideration of the manufacturing activities to be performed by ACT II
      hereunder, JPI and JANSSEN US will pay the Manufacturing Fee for the
      Products supplied to each of them.

      The Manufacturing Fee will be calculated as a certain percentage of the
      U.S. Licensed Net Selling Price and/or ROW Licensed Net Selling Price, as
      applicable. Subject to the terms and conditions set forth in this
      Agreement, the actual percentage that shall apply with respect to a given
      calendar year will be determined in accordance with the mechanisms set
      forth in this Article 6 and Exhibit D attached hereto.

      6.1.1 The Manufacturing Fee for calendar year 2002 has been established
      pursuant to Exhibit I hereto.

      6.1.2 (a) Determination of Manufacturing Fee. Subject to the terms and
      conditions set forth in this Article 6.1.2, the Manufacturing Fees for
      calendar year 2003 and any subsequent calendar year will be calculated on
      the basis of the (i) U.S. Licensed Net Selling Prices and/or ROW Licensed
      Net Selling Prices for such calendar year (expressed in USD at the
      exchange rates then applied by JANSSEN in accordance with its normal
      accounting procedures), as applicable, for each Presentation Form of the
      Final Product and (ii) the total amount of Product expressed in units that
      has been Manufactured and shipped pursuant to Article 4 by ACT II for such
      calendar year.

      (b)   Determination of a Provisional Manufacturing Fee. For the sake of
      administrative ease, by *, JANSSEN and ACT II will agree in good faith on
      the "U.S. Provisional Manufacturing Fee" and the "ROW Provisional
      Manufacturing Fee" for each Presentation Form of the Product for the
      upcoming calendar year. The "U.S. Provisional Manufacturing Fee" for each
      such Presentation Form of the Product shall be calculated by taking the
      forecast submitted by JANSSEN in * for the upcoming calendar year in
      accordance with Exhibit E (the "* Forecast"), to determine an estimated
      total amount of Product to be ordered, then determining the applicable
      percentage from Exhibit D to the Agreement (based on the such estimated
      total amount of Product) (the "Applicable Percentage") and adding * to
      facilitate cash flow (the "Additional Percentage") to such Applicable
      Percentage to determine the "Provisional Manufacturing Fee Percentage."
      The Provisional Manufacturing Fee Percentage shall then be applied to
      JANSSEN's estimated U.S. Licensed Net Selling Price for the upcoming
      calendar year (which shall be determined by JANSSEN in good faith and
      submitted to ACT II by * of the prior calendar year) to determine the U.S.
      Provisional Manufacturing Fee. "ROW Provisional Manufacturing Fee " for
      each such Presentation Form of

<PAGE>

      the Product shall be calculated by taking (i) the * Forecast; and
      (ii) JANSSEN's estimated ROW Licensed Net Selling Price for the upcoming
      calendar year (which shall be determined by JANSSEN in good faith and
      submitted to ACT II by * of the prior calendar year) and
      applying the Provisional Manufacturing Fee Percentage to the estimated ROW
      Licensed Selling Price to determine the ROW Provisional Manufacturing
      Fee. Subject to the other provisions of this Article 6.1.2, ACT II shall
      invoice JANSSEN US for all Product to be shipped to JANSSEN US in the
      subsequent calendar year at the U.S. Provisional Manufacturing Fee and
      shall invoice JPI for all Product to be shipped to JPI in the subsequent
      calendar year at the ROW Provisional Manufacturing Fee. Either the "U.S.
      Provisional Manufacturing Fee" and/or the "ROW Provisional Manufacturing
      Fee" may also be referred to as the "Provisional Manufacturing Fee."

      (c)   Recalculation of the Provisional Manufacturing Fee. Within *
      business days after the end of each calendar quarter JANSSEN shall send to
      ACT II a report setting forth for each Presentation Form of the Final
      Product (i) its actual U.S. Licensed Net Selling Price for the calendar
      year to date; (ii) its actual ROW Licensed Net Selling Price for the
      calendar year to date; and (iii) the actual number of units of Product
      ordered by JANSSEN pursuant to Exhibit E for the calendar year to date
      plus the number of units of Product forecast to be ordered by JANSSEN
      during the balance of the calendar year as set forth in the most recent
      forecast submitted in accordance with Exhibit E (specifying such units by
      Presentation Form and by geographical area (US - ROW)) ("Revised Annual
      Total Products"). The parties shall use such Revised Annual Total Products
      to recalculate the Applicable Percentage from Exhibit D by substituting
      such number for the comparable estimated number used to determine the then
      current Provisional Manufacturing Fee Percentage. The parties shall add an
      Additional Percentage to this Applicable Percentage to create a new
      Provisional Manufacturing Fee Percentage. The parties shall also apply the
      Applicable Percentage to the actual year-to-date US Licensed Net Selling
      Price and ROW Licensed Net Selling Price to determine an interim US
      Provisional Manufacturing Fee and an interim ROW Provisional Manufacturing
      Fee to be used for calculation purposes. The parties shall next use such
      newly calculated interim Provisional Manufacturing Fees to recalculate the
      total amount payable for Product shipped to JANSSEN during the current
      calendar year to date to determine if such recalculation would result in
      an underpayment or overpayment for such Product (a "Payment Differential")
      of more than *. The parties shall also calculate the total amount payable
      for Revised Annual Total Products using the newly calculated interim
      Provisional Manufacturing Fees as well as the prior Provisional
      Manufacturing Fees to determine if these amounts represent more than a *
      potential overpayment or underpayment for such Product. In the event that
      there is either a underpayment or overpayment of more than * or * as
      described above, then the party who has overpaid or who has been underpaid
      may request the other party to pay the Payment Differential within * of
      the end of such calendar quarter (such payment a "True Up"). In addition,
      in the event that there is a

<PAGE>

      True Up, the parties shall apply the newly calculated Provisional
      Manufacturing Fee Percentage to the actual year-to-date US Licensed Net
      Selling Price and ROW Licensed Net Selling Price to determine a revised US
      Provisional Manufacturing Fee and ROW Provisional Manufacturing Fee which
      shall become the Provisional Manufacturing Fees to be used during the
      balance of the calendar year, unless replaced by subsequently recalculated
      Provisional Manufacturing Fees in accordance with this paragraph.

      (d)   Annual Reconciliation. Within * days after the end of each calendar
      year, JANSSEN shall send to ACT II a report stating for each Presentation
      Form of the Final Product (i) its U.S. Licensed Net Selling Price for such
      calendar year; (ii) its ROW Licensed Net Selling Price for such calendar
      year; and (iii) the total number of units of Product shipped to JANSSEN
      during such calendar year (on a country-by-country basis). Any payment
      required by JANSSEN to ACT II or vice versa to compensate for any
      difference between the U.S. Provisional Manufacturing Fee and/or the ROW
      Provisional Manufacturing Fee (calculated in accordance with paragraphs
      (b) and (c) above) and the applicable Manufacturing Fees as determined by
      such actual prices and total number of units of Product shipped shall be
      made to the appropriate Party no later than * of the year in which the
      report is delivered and in accordance with the provisions of Article 6.4.

      (e)   Calculation of Units During Calendar Years *. In calculating the
      total number of units of Product shipped to JANSSEN for each of calendar
      years *, whether or not such units of Product are actually shipped, the
      parties will deem as shipped (i) all Product ordered by JANSSEN prior to
      the end of * of each such calendar year and (ii) all Product ordered
      by JANSSEN in the * forecast for each such calendar year (the "*
      Forecast") submitted in accordance with the ordering procedures set forth
      in Exhibit E. Any Product ordered by JANSSEN other than as set forth
      above, even if such Product is shipped, will not, however, be deemed to be
      shipped when making such calculation, and will also not be deemed to be
      shipped when calculating the total number of units of Product shipped
      during any subsequent calendar year. Any units of Product ordered by
      JANSSEN as set forth above, but not shipped, shall be invoiced to JANSSEN
      when shipped at the Manufacturing Fee for the applicable calendar year,
      rather than at the then current Provisional Manufacturing Fee. From the
      calendar year * onwards, the Manufacturing Fee will be calculated based on
      the total number of units of Product actually shipped to JANSSEN for such
      calendar year.

      (f)   *
<PAGE>

            (g)   *

            (h)   Illustrative Examples. Exhibit J hereto provides examples for
            illustrative purposes only of calculations made pursuant to
            paragraphs (b), (c), (d) and (e) hereof. In the event of any
            conflict between Exhibit J and this Agreement, the provisions of
            this Agreement shall control.

      6.2   JANSSEN shall keep or cause to be kept accurate records in
            sufficient detail to enable the Manufacturing Fees for Products sold
            hereunder to be determined. JANSSEN, upon the written request
            (including reasonable notice) and at the expense of ACT II, and in
            any event not more frequently than once in any calendar year, shall
            permit an independent public accountant of national prominence
            selected by ACT II, and approved by JANSSEN (with approval not
            unreasonably to be withheld), to have access during normal business
            hours to those records as may be reasonably necessary to verify the
            accuracy of the Manufacturing Fees for Products sold for any
            calendar year ending not more man three (3) years prior to the date
            of the aforementioned written request. If such accountant determines
            that the Manufacturing Fees have been overstated or understated,
            then one party shall make a payment to the other party as necessary
            to correct the amount of the Manufacturing Fees paid for Product
            supplied hereunder, which payment shall be based on the difference
            between the actual and the misstated Manufacturing Fees. In
            addition, if such accountant reasonably determines that the
            Manufacturing Fees have been understated for the audited period by
            more than *, then JANSSEN shall pay the reasonable costs of such
            audit. ACT II agrees that all information subject to review under
            this Article 6.2 shall be deemed JANSSEN's Confidential Information
            subject to the terms and conditions of Article 7. ACT II shall
            retain and cause its independent accountant to retain all such
            information in confidence in accordance with Article 7 and such
            information may only be used for purposes germane to this Article
            6.2.

      6.3   ACT II shall invoice JPI or JANSSEN US for the Provisional
            Manufacturing Fee due with respect to each batch of Product supplied
            to each of them or their

<PAGE>
            respective designee when shipped pursuant to Article 4. JPI and
            JANSSEN US shall pay such, invoice within * after the date of the
            invoice.

      6.4   All payments required to be paid hereunder shall be made in United
            States Dollars by wire transfer of immediately available funds to
            the financial institution, account number, account party's name and
            wire transfer information designated in writing by ACT II to JPI and
            JANSSEN US as the place of payment.

      6.5   No party shall have the right to reduce, by set off, counterclaim,
            adjustment or otherwise, any amount owed by it to the other party
            pursuant to this Agreement, unless explicitly provided for
            otherwise.

      6.6   JANSSEN shall bear all applicable national, federal, provincial,
            municipal and other governmental taxes (such as sale, use or similar
            taxes), duties, or import charges, except for any tax on profits or
            income of ACT II, that ACT II may be required to pay or collect as a
            result of the payments of the Manufacturing Fee or the Provisional
            Manufacturing Fee.

      6.7   Within * after the end of each month, JANSSEN shall deliver to ACT
            II a report setting forth the dollar amount and the units of each
            Presentation Form of the Final Product sold during the prior month
            on a country-by-country basis. In addition, each month JANSSEN shall
            provide to ACT II the foreign currency exchange rates used to
            calculate the Final Product sales for each country.

      6.    Exhibit D shall be amended by deleting the words "Licensed Net
      Selling Price" and substituting the words "U.S. Licensed Net Selling Price
      and/or ROW Licensed Net Selling Price."

      7.    This Amendment and the Agreement shall be governed by and construed
      in accordance with the laws of the State of New York without regard to its
      conflict of law provisions.

      8.    Except as expressly provided in this Amendment, all other terms,
      conditions and provisions of the Agreement shall continue in full force
      and effect as provided therein. This Amendment and the Agreement
      constitute the entire agreement between the parties hereto relating to the
      subject matter hereof and thereof and supersede all prior and
      contemporaneous negotiations, agreements, representations, understandings
      and commitments with respect thereto.

                            [signature page follows]

<PAGE>

      IN WITNESS WHEREOF, JPI, JANSSEN US and ACT II have executed and
delivered this Amendment effective as of the date first set forth above.

      ALKERMES CONTROLLED                      JPI PHARMACEUTICA
       THERAPEUTICS INC. II              INTERNATIONAL represented by
                                          CILAG AG INTERNATIONAL ZUG

By:    /s/ Michael Landine              By:  /s/  Erik Rombouts
       ---------------------                 ----------------------------------
Name:  Michael Landine                  Name: ERIK ROMBOUTS
Title: Vice President                   Title: VP ALLIANCE MGMT

                                        By:   /s/ Heinz Schmid
                                             ----------------------------------
                                        Name:  HEINZ SCHMID
                                        Title: GENERAL MANAGER

                                        JANSSEN PHARMACEUTICA INC.

                                        By: /s/ Peter Miller
                                            ------------------------------------
                                        Name: Peter Miller
                                        Title: President
<PAGE>

                                   EXHIBIT I

                                     [***]

THIS EXHIBIT HAS BEEN REDACTED AND IS THE SUBJECT OF A CONFIDENTIAL TREATMENT
REQUEST. REDACTED MATERIAL IS MARKED WITH "*" AND HAS BEEN FILED SEPARATELY
WITH THE SECURITIES AND EXCHANGE COMMISSION.
<PAGE>

                                   EXHIBIT J

                                     [***]

THIS EXHIBIT HAS BEEN REDACTED AND IS THE SUBJECT OF A CONFIDENTIAL TREATMENT
REQUEST. REDACTED MATERIAL IS MARKED WITH "*" AND HAS BEEN FILED SEPARATELY
WITH THE SECURITIES AND EXCHANGE COMMISSION.