<PAGE>   1
                                                                EXHIBIT 10.22(b)

                                    ADDENDUM
                                    --------

This Addendum, when signed by both parties, constitutes a valid and enforceable
addition to the agreement between the parties dated January 15, 1993, relating
to the licensing of CDP-Choline (hereinafter referred to as the "Agreement").

FIRST
-----

Following execution of the Agreement, business relations between the parties
have been duly developed for their mutual benefit.

SECOND
------

Notwithstanding the above, as consequence of the INTERNEURON U.S. Patent
applications US Prior. 08/399.262, US Prior. 08/603.102 and US Prior.
08/609.448, and corresponding (WO)PCT 96/27.380 (hereinafter jointly referred to
as the "New Patent"), the parties have upheld different positions interpreting
whether the New Patent should be included under the stipulations of Article 5.3
of the Agreement.

THIRD
-----

The parties have reached the following agreement for the use of the New Patent,
while in force and also as long as pending, in those countries other than the
United States of America, Canada, and Puerto Rico (referred to as the
"Territory" in the Agreement and hereinafter):

a)       Pursuant to Article 5.3 of the Agreement, FERRER, its associates, its
         licensees, and its distributors have an exclusive right to freely use
         and dispose of the New Patent in all the countries other than the
         Territory.

b)       Notwithstanding the above, and as compensation of the aforementioned
         exclusive right for the license or distribution agreements granted by
         FERRER to third parties following execution of this Addendum which
         relate to the Compound and/or the Preparation, FERRER shall grant to
         INTERNEURON the right to receive, separately for each one of the
         countries other than the Territory, the following payments, when
         applicable:

             i)   twenty percent (20%) of the royalties that FERRER would
                  receive from the relevant country, or

            ii)   two percent (2%) of the Compound exports done by FERRER to the
                  relevant country, or

           iii)   one and a half percent (1.5%) of FERRER exports if done as
                  finished pharmaceutical speciality or in its bulk form to the
                  relevant country.






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         In the given case that FERRER undertakes with respect to any country
         outside the Territory more than one of the activities described above
         in paragraphs (i), (iii) and (iii), then FERRER shall pay INTERNEURON
         the relevant corresponding amounts as set forth above as applicable to
         payments received by FERRER in respect of such activities.

c)       FERRER shall give notice of any infringement or claim by a third party
         related to the New Patent of which FERRER has knowledge of in those
         countries other than the Territory.

d)       Pursuant to Article 4 of the Agreement, it is understood and agreed
         that FERRER shall communicate and provide to INTERNEURON all
         experimental and clinical data relating to the Compound and the
         Preparation which FERRER and its sublicensees may generate after the
         date of execution of this Addendum and which FERRER is legally entitled
         to disclose.

FOURTH
------

4.1      It is provided that the terms set forth in provision THIRD b) are not
         applicable to all those agreements already subscribed up to the date of
         execution of this Addendum by and between FERRER and third parties but
         shall be applicable to (i) all sublicense, distributor or other similar
         agreements relating to the Preparation which FERRER shall grant after
         the date of this Addendum in which the New Patent is included or (ii)
         any amendments made after the date of execution of this Addendum to the
         pre-existing agreements, improving for FERRER the economic terms of
         such agreements, resulting from the inclusion in such agreements of the
         New Patent; in such case only the new economic terms will be notified
         to INTERNEURON, and the terms set forth in provision THIRD b) will be
         applied based exclusively on the corresponding and resulting economic
         increases. To such effect, FERRER shall inform INTERNEURON of the
         signature of future agreements with third parties as well as of the
         economic terms contained therein that make reference to the terms and
         conditions set forth in this Addendum.

4.2      In the given case of FERRER granting license over the Preparation
         and/or the Compound to Takeda Chemical Industries Ltd. (hereinafter
         referred to as "Takeda") and in consideration for the negotiating
         efforts produced by INTERNEURON, the compensation set forth in
         provision THIRD b) would exceptionally be substituted by fifty percent
         (50%) of the royalties that FERRER would receive relating to such
         license from Takeda, Takeda's licensees or its distributors in each one
         of the relevant countries. If in the aforementioned case FERRER should
         not receive royalties but does receive other payments as provided in
         provision THIRD b), then FERRER will apply to such payments the terms
         set forth in such provision.

4.3      In the event that FERRER shall grant license over the Preparation
         and/or the Compound to Synthelabo or licensees of Synthelabo (or a
         replacing third party, in the given case that an agreement with
         Synthelabo should not be achieved), FERRER will exceptionally apply the
         compensation set forth in Section THIRD b) to such license even in the
         case of the New Patent being cancelled, rejected or withdrawn in a
         country included in such license. Such



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         exceptional application of the compensation shall follow the terms set
         forth in Section THIRD b) and will have a duration of three (3) years,
         commencing such term on the date of cancellation, rejection or
         withdrawal of the New Patent in the corresponding country.

FIFTH
-----

The terms and conditions set forth in this Addendum in respect of the New Patent
are exceptional, so, therefore, the terms and conditions set forth in this
Addendum in respect of the New Patent shall not be understood as a binding
precedent between the parties, so all other terms and conditions of the
Agreement are hereby retained in full force and effect, and are confirmed as of
the date hereof.

In witness whereof, the parties, through their authorized representatives, have
executed this Addendum, whereby they evidence their intent to be legally bound.


Barcelona and Lexington, _________________________.


FERRER INTERNACIONAL S.A.                   INTERNEURON PHARMACEUTICALS, INC.


   /s/ Rafael Foguet                        By:    /s/ Mark S. Butler
--------------------------                  --------------------------
By:  Rafael Foguet                                   Mark S. Butler

Title:  Chief Executive Officer             Title:  Executive Vice President
               26-05-98                                      6/1/98




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                                SECOND AMENDMENT
                                ----------------

This Second Amendment, when signed by both of us, constitutes a valid and
enforceable amendment to the agreement between us dated January 15, 1993,
relating to the licensing of CDP Choline ("the Agreement").

The Agreement is hereby amended as follows:

FIRST
-----

(i)      Section 8.2.a. is hereby amended and restated in its entirety to read
         as follows:

         "a. if INTERNEURON fails to obtain a Registration of the Preparation in
         the Territory by January 31, 2002 and/or".

(ii)     The last paragraph of Article 8 is hereby amended and restated in its
         entirety to read as follows:

         "The above-mentioned period (by January 31, 2002) shall be
         automatically extended by successive six (6) month periods up to a
         maximum two (2) year extension if INTERNEURON provides information to
         FERRER which tends to establish that INTERNEURON has carried out the
         steps for obtaining the Registration of the Preparation and that the
         latter has not been obtained for reasons beyond the control of
         INTERNEURON."

SECOND
------

All other terms and conditions of the Agreement are hereby retained in full
force and effect, and are confirmed as of the date hereof.

In witness whereof, the parties, through their authorized representatives, have
executed this Amendment, whereby they evidence their intent to be legally bound.


Barcelona and Lexington, _________________________.


FERRER INTERNACIONAL S.A.                   INTERNEURON PHARMACEUTICALS, INC.


   /s/ Rafael Foguet                        By:    /s/ Mark S. Butler
--------------------------                  --------------------------
By:  Rafael Foguet                                 Mark S. Butler

Title:  Chief Executive Officer             Title:  Executive Vice President
               26-05-98                                                6/1/98