<PAGE>
 
                             AGREEMENT OF SUBLEASE


                                    between


                INTERNEURON PHARMACEUTICALS, INC., Sublandlord


                                      and


                            GENTA, INC., Subtenant




                            Dated:  March 31, 1999




PREMISES:

PORTIONS OF 200 LEVEL
LEDGEMONT DEVELOPMENT CENTER
128 SPRING STREET
LEXINGTON, MASSACHUSETTS
<PAGE>
 
                               SUBLEASE BETWEEN
                INTERNEURON PHARMACEUTICALS, INC., SUBLANDLORD
                                      AND
                            GENTA, INC., SUBTENANT


          SUBLEASE made as of the 31/st/ day of March, 1999 by and between
INTERNEURON PHARMACEUTICALS, INC., a Delaware corporation, having an office at
99 Hayden Avenue, Lexington, Massachusetts 02173 (hereinafter called
"Sublandlord"), and GENTA, INC., a Delaware corporation, having an office at  99
Hayden Avenue, Lexington, Massachusetts 02173 (hereinafter called "Subtenant").

                              W I T N E S S E T H
                              - - - - - - - - - -

          WHEREAS:

          A.   By lease dated February 5, 1997, as amended by that certain Side
Agreement regarding indoor air quality dated January 31, 1997 and that certain
First Amendment to Lease dated February 12, 1997 (the "First Amendment") (which
lease, as the same has been and may hereafter be further amended, is hereinafter
referred to as the "Overlease"), Ledgemont Realty Trust u/d/t dated December 12,
1984 (hereinafter called "Overlandlord") leased to Sublandlord certain space
(hereinafter called the "Leased Space") in the building known as Ledgemont
Development Center located at 128 Spring Street, Lexington, Massachusetts
<PAGE>
 
(hereinafter called the "Building") in accordance with the terms of the
Overlease.  A copy of the Overlease is annexed hereto as Exhibit A.

          B.   Sublandlord and Subtenant desire to consummate a subleasing of a
portion of the Leased Space on terms and conditions contained in this agreement
(hereinafter called the "Sublease").

          NOW, THEREFORE, in consideration of the mutual covenants and
agreements hereinafter contained, it is hereby agreed as follows:

          l.

          1.1. Sublandlord hereby leases to Subtenant and Subtenant hereby
hires from Sublandlord the portion of the 200 level of the East Wing of the
Building (comprising a portion of the Leased Space) shown hatched on Exhibit B
annexed hereto and made a part hereof (hereinafter referred to as the
"Premises"), for a term (the "Sublease Term") to commence on the date that
Sublandlord delivers possession of the Premises to Subtenant (hereinafter called
the "Sublease Commencement Date"), and to end on March 31, 2001 (hereinafter
called the "Sublease Expiration Date"), or until such term shall sooner cease
and terminate as herein provided.

          1.2. The annual fixed rent (hereinafter called "Fixed Rent") payable
by Subtenant hereunder during the Sublease Term shall be paid to Sublandlord at
the rate of FORTY-THREE THOUSAND FOUR HUNDRED FIFTY FOUR AND 26/100 ($43,454.26)
DOLLARS per annum during the period commencing on the Sublease Commencement Date
and ending on the Sublease Expiration Date.  The Fixed Rent is to be paid by
Subtenant to Sublandlord at Sublandlord's office (or such other location as
Sublandlord shall designate) by 

                                       2
<PAGE>
 
check drawn on a Massachusetts bank in equal monthly installments in advance, on
the first day of each month during the Sublease Term without any set-off, off-
set, abatement or reduction whatsoever. The security deposit payable with
respect to Article 19 hereof shall be paid upon the execution of this Sublease
by Subtenant.

          1.3. Simultaneous with the execution of this Sublease, Subtenant is
paying to Sublandlord the sum of $6,174.77 which shall be applied by Sublandlord
to the first (1st) monthly installment of Fixed Rent due hereunder.

          1.4. Sublandlord and Subtenant agree to the following with respect to
Subtenant's use of services within and access to the Premises:

          (a)  Subtenant may use: (i) the kitchen/lounge, (ii) the exercise
facilities, provided and on condition that each of Subtenant's employees
utilizing such exercise facilities furnishes Sublandlord with a written waiver
of liability, satisfactory to Sublandlord, and (iii) either of the front
training rooms (A or B), provided that Subtenant's employees shall not be
entitled to use both front training rooms at the same time and provided further
that any such use shall be subject to unavailability, including, without
limitation, unavailability resulting from Sublandlord's employees' priority
status with respect to access to and use of the same.

          (b)  Subtenant may utilize the administrative services of Richard
Hector or any employee hereafter substituted by Sublandlord for Richard Hector,
to the extent available and provided that all use of such administrative
services shall first be approved by Sublandlord's office manager.  Subtenant
shall be billed from time to time for all such administrative services in excess
of one hour per month at the hourly rate of $40.00, which Subtenant shall pay as
additional rent within ten (10) days after being billed therefor.

                                       3
<PAGE>
 
          2.

          2.1. Subtenant shall not (a) assign this Sublease, nor (b) permit
this Sublease to be assigned by operation of law or otherwise, nor (c) underlet
all or any part of the Premises, nor (d) permit the Premises or any desk space
therein to be occupied by any person(s) other than Subtenant, nor (e) pledge or
encumber this Sublease, the term and estate hereby granted or the rentals
hereunder.

          3.

          3.1. Except as herein otherwise expressly provided and except for the
obligation to pay rent and additional rent under the Overlease, all of the
terms, covenants, conditions and provisions in the Overlease are hereby
incorporated in, and made a part of this Sublease, and such rights and
obligations as are contained in the Overlease are hereby imposed upon the
respective parties hereto; the Sublandlord herein being substituted for the
Landlord in the Overlease, and the Subtenant herein being substituted for the
Tenant named in the Overlease; provided, however, that the Sublandlord herein
shall not be liable for any defaults by Overlandlord.  If the Overlease shall be
terminated for any reason during the term hereof, then and in that event this
Sublease shall thereupon automatically terminate and Sublandlord shall have no
liability to Subtenant by reason thereof.  Upon the termination of this
Sublease, whether by forfeiture, lapse of time or otherwise, or upon the
termination of Subtenant's right to possession, Subtenant will at once surrender
and deliver up the Premises in good condition and repair, reasonable wear and
tear excepted.  Notwithstanding any language to the contrary contained in this
Sublease, Subtenant agrees that Sublandlord may at any time after the date
hereof surrender the Overlease and the premises demised thereunder to
Overlandlord, provided 

                                       4
<PAGE>
 
Overlandlord shall deliver a written agreement to Subtenant providing that
notwithstanding such surrender Overlandlord shall not disturb Subtenant's
occupancy of the Premises so long as Subtenant is not in default hereunder if
Subtenant shall at Overlandlord's election either (i) attorn to Overlandlord as
if Overlandlord were the sublandlord hereunder or (ii) enter into a lease with
Overlandlord for the remaining term of the Sublease on the same terms and
conditions contained herein.

          3.2. For purposes of this Sublease, the second paragraph of Section
2.1, Sections 2.3, 2.5, 2.6, 2.7, 2.8, 3.1, 4.1.1, 4.1.2, the second sentence of
Section 5.1.9, Sections 5.1.11, 5.2.3, 8.1, 9.1, Appendix A, Appendix E, and
Paragraphs 1 and 2 of the First Amendment, and all references in the Overlease
to the aforesaid Articles, Sections or Appendices of the Overlease between
Overlandlord and Sublandlord shall not be incorporated in or made a part hereof.
In addition, for the purposes of this Sublease, Article 1 of the Overlease is
incorporated only to the extent the definitions contained therein are consistent
with the terms hereof (i.e., references to Premises, Landlord, Tenant, Initial
Term, Commencement Date, Rent Commencement Date, Ending Date, Annual Fixed Rent-
Initial Term, Extension Term, Annual Fixed Rent-Extension Term, Tenant
Improvement Allowance, Parking Spaces, Tenant's Operating Percentage Share,
Tenant's Tax Percentage Share and Broker are not applicable to this Sublease),
and Sections 4.3.1, 4.3.2, 4.4.1, and 4.4.2 of the Overlease are incorporated
herein only for the purposes of Article 11 of this Sublease.

          4.

          4.1. Subtenant has examined the Premises, is aware of the physical
condition thereof, and agrees to take the same "as is," with the understanding
that there shall be no 

                                       5
<PAGE>
 
obligation on the part of Sublandlord to perform any work, supply any materials
or incur any expense whatsoever in connection with the preparation of the
Premises for Subtenant's occupancy thereof.

          4.2. Subtenant shall pay to Sublandlord upon demand therefor as
additional rent all costs incurred by Sublandlord in making the Premises
available to the Subtenant, including, but not limited to costs to move
Sublandlord's furniture, fixtures, computer and communications connections from
the Premises and relocate and reinstall the same elsewhere.  Upon termination of
the Sublease, Subtenant shall pay to Sublandlord upon demand therefor all costs
incurred by Sublandlord to restore the Premises to the condition as the same
were prior to Subtenant's occupation thereof pursuant to this Sublease and all
costs incurred by Sublandlord to move its furniture, fixtures, computer and
communications connections back to and reinstall the same in the Premises to the
extent they existed prior to this Sublease.

          5.

          5.1. Subtenant agrees that the Premises shall be occupied only as
executive, administrative and general offices for Subtenant's business.

          6.

          6.1. This Sublease is conditioned upon the consent by Overlandlord to
this Sublease which consent shall be evidenced by Overlandlord's signature
appended hereto or a separate consent in the form utilized by Overlandlord for
such purposes.

          6.2. Subtenant stipulates that it is familiar with the provisions of
Section 5.1.11 of the Overlease.  In the event that Overlandlord shall exercise
any of its options pursuant to Section 5.1.11 of the Overlease with respect to
the Premises upon Sublandlord's request for 

                                       6
<PAGE>
 
Overlandlord's consent to this Sublease, Sublandlord will so notify Subtenant
and, upon receipt of such notification by Sublandlord, this Sublease shall be
deemed to be null and void and without force or effect, and Sublandlord and
Subtenant shall have no further obligations or liabilities to the other with
respect to this Sublease.

          6.3. In the event Overlandlord shall not exercise any of its options
pursuant to Section 5.1.11 of the Overlease with respect to the Premises,
Sublandlord makes no representation with respect to obtaining Overlandlord's
approval of this Sublease and, in the event that Overlandlord notifies
Sublandlord that Overlandlord will not give such approval, Sublandlord will so
notify Subtenant and, upon receipt of such notification by Sublandlord of the
disapproval by Overlandlord, this Sublease shall be deemed to be null and void
and without force or effect, and Sublandlord and Subtenant shall have no further
obligations or liabilities to the other with respect to this Sublease.

          6.4. Except as otherwise specifically provided herein, wherever in
this Sublease Subtenant is required to obtain Sublandlord's consent or approval,
Subtenant understands that Sublandlord may be required to first obtain the
consent or approval of Overlandlord.  If Overlandlord should refuse such consent
or approval, Sublandlord shall be released of any obligation to grant its
consent or approval whether or not Overlandlord's refusal, in Subtenant's
opinion, is arbitrary or unreasonable.  Subtenant agrees that Sublandlord shall
not be required to dispute any determinations or other assertions or claims of
Overlandlord regarding the obligations of Sublandlord under the Overlease for
which Subtenant is or may be responsible under the terms of this Sublease.

          7.

                                       7
<PAGE>
 
          7.1. Subtenant acknowledges that all services, repairs, restorations,
equipment and access to and for the Premises and any insurance coverage of the
Building will in fact be provided by Overlandlord, and Sublandlord shall have no
obligation during the term of this Sublease to provide any such services,
repairs, restorations, equipment, access or insurance.  Subtenant agrees to look
solely to Overlandlord for the furnishing of such services, repairs,
restorations, equipment, access and insurance.  Sublandlord shall in no event be
liable to Subtenant nor shall the obligations of Subtenant hereunder be impaired
or the performance thereof excused because of any failure or delay on
Overlandlord's part in furnishing such services, repairs, restorations,
equipment, access or insurance.  If Overlandlord shall default in any of its
obligations to Sublandlord with respect to the Premises, Subtenant shall be
entitled to participate with Sublandlord in the enforcement of Sublandlord's
rights against Overlandlord, but Sublandlord shall have no obligation to bring
any action or proceeding or to take any steps to enforce Sublandlord's rights
against Overlandlord.  If, after written request from Subtenant, Sublandlord
shall fail or refuse to take appropriate action for the enforcement of
Sublandlord's rights against Overlandlord with respect to the Premises within a
reasonable period of time considering the nature of Overlandlord's default,
Subtenant shall have the right to take such action in its own name, and for that
purpose and only to such extent, all of the rights of Sublandlord under the
Overlease hereby are conferred upon and assigned to Subtenant and Subtenant
hereby is subrogated to such rights to the extent that the same shall apply to
the Premises.  If any such action against Overlandlord in Subtenant's name shall
be barred by reason of lack of privity, nonassignability or otherwise, Subtenant
may take such action in Sublandlord's 

                                       8
<PAGE>
 
name provided Subtenant has obtained the prior written consent of Sublandlord,
which consent shall not be unreasonably withheld or delayed, provided, and
Subtenant hereby agrees, that Subtenant shall indemnify and hold Sublandlord
harmless from and against all liability, loss, damage or expense, including,
without limitation, reasonable attorney's fees, which Sublandlord shall suffer
or incur by reason of such action.

          7.2. Anything contained in any provisions of this Sublease to the
contrary notwithstanding, Subtenant agrees, with respect to the Premises, to
comply with and remedy any default claimed by Overlandlord and caused by
Subtenant, within the period allowed to Sublandlord as tenant under the
Overlease, even if such time period is shorter than the period otherwise allowed
in the Overlease, due to the fact that notice of default from Sublandlord to
Subtenant is given after the corresponding notice of default from Overlandlord.
Sublandlord agrees to forward to Subtenant, upon receipt thereof by Sublandlord,
a copy of each notice of default received by Sublandlord in its capacity as
tenant under the Overlease.  Subtenant agrees to forward to Sublandlord, upon
receipt thereof, copies of any notices received by Subtenant with respect to the
Premises from Overlandlord or from any governmental authorities.

          8.

          8.1. Sublandlord represents (a) that it is the holder of the interest
of the tenant under the Overlease and (b) that the Overlease is in full force
and effect.

          9.

          9.1. This Sublease is subject to, and Subtenant accepts this Sublease
subject to, any amendments and supplements to the Overlease hereafter made
between Overlandlord and Sublandlord, provided that any such amendment or
supplement to the Overlease will not prevent 

                                       9
<PAGE>
 
or adversely affect the use by Subtenant of the Premises in accordance with the
terms of this Sublease, increase the obligations of Subtenant or decrease its
rights under the Sublease or in any other way materially adversely affect
Subtenant.

          9.2.  This Sublease is subject and subordinate to the Overlease and to
all ground or underlying leases and to all mortgages which may now or hereafter
affect such leases or the real property of which the Premises are a part and all
renewals, modifications, replacements and extensions of any of the foregoing.
This Section 9.2 shall be self-operative and no further instrument of
subordination shall be required.  To confirm such subordination, Subtenant shall
execute promptly any certificate that Sublandlord may request.

          10.

          10.1. Subtenant covenants, represents and warrants that Subtenant has
had no dealings or communications with any broker or agent in connection with
the consummation of this Sublease, and Subtenant covenants and agrees to pay,
hold harmless and indemnify Sublandlord from and against any and all cost,
expense (including reasonable attorneys' fees) or liability for any
compensation, commissions or charges claimed by any broker or agent with respect
to this Sublease or the negotiation thereof.

          11.

          11.1. Subtenant stipulates that it is familiar with the provisions of
Sections 4.3.1. and 4.3.2. of the Overlease.  In the event of any payment of
additional rent by Sublandlord to Overlandlord during the term of this Sublease
which is attributable to the provisions of Sections 4.3.1. and 4.3.2. of the
Overlease (such additional rent payable by Sublandlord pursuant to Sections
4.3.1. and 4.3.2. of the Overlease being hereinafter called "Section 4.3 Rent"),
then 

                                       10
<PAGE>
 
Subtenant shall pay as additional rent pursuant to this Sublease an amount equal
to 1.248% of the Section 4.3 Rent. For purposes of this Section 11.1 of this
Sublease, the Premises shall be deemed to contain approximately 2,276 rentable
square feet and the Leased Space shall be deemed to contain 182,317 square feet.
At such time as the Section 4.3 Rent payable by Sublandlord is adjusted by
reason of any change in the rentable area of the Leased Space, the percentage
thereof payable by Subtenant to Sublandlord shall be similarly adjusted. At any
time after payment by Sublandlord to Overlandlord of any Section 4.3 Rent,
Sublandlord may deliver to Subtenant a statement with respect to the payment of
the Section 4.3 Rent and, within ten (10) days after delivery of such statement,
Subtenant shall pay to Sublandlord additional rent determined as aforesaid in
this Section 11.1. Additional rent payable pursuant to this Section 11.1 shall
be based solely upon actual payments made by Sublandlord pursuant to the
provisions of Sections 4.3.1. and 4.3.2. of the Overlease. Subtenant shall not
have the right to question the propriety of or the basis for any such payment
and Sublandlord shall be under no obligation to contest any such payment.
Sublandlord shall, however, at the written request of Subtenant, furnish to
Subtenant evidence of such payment.

          11.2.  Subtenant stipulates that it is familiar with the provisions of
Sections 4.4.1. and 4.4.2. of the Overlease.  In the event of any payment of
additional rent by Sublandlord to Overlandlord during the term of this Sublease
which is attributable to the provisions of Sections 4.4.1. and 4.4.2. of the
Overlease (such additional rent payable by Sublandlord pursuant to Sections
4.4.1. and 4.4.2. of the Overlease being hereinafter called "Section 4.4 Rent"),
then Subtenant shall pay as additional rent pursuant to this Sublease an amount
equal to 4.473% of the Section 4.4 Rent.  For purposes of this Section 11.2 of
this Sublease, the Premises shall be 

                                       11
<PAGE>
 
deemed to contain approximately 2,276 rentable square feet and the Leased Space
shall be deemed to contain 50,883 square feet. At such time as the Section 4.4
Rent payable by Sublandlord is adjusted by reason of any change in the rentable
area of the Leased Space, the percentage thereof payable by Subtenant to
Sublandlord shall be similarly adjusted. At any time after payment by
Sublandlord to Overlandlord of any Section 4.4 Rent, Sublandlord may deliver to
Subtenant a statement with respect to the payment of the Section 4.4 Rent and,
within ten (10) days after delivery of such statement, Subtenant shall pay to
Sublandlord additional rent determined as aforesaid in this Section 11.2.
Additional rent payable pursuant to this Section 11.2 shall be based solely upon
actual payments made by Sublandlord pursuant to the provisions of Sections
4.4.1. and 4.4.2. of the Overlease. Subtenant shall not have the right to
question the propriety of or the basis for any such payment and Sublandlord
shall be under no obligation to contest any such payment. Sublandlord shall,
however, at the written request of Subtenant, furnish to Subtenant evidence of
such payment.

          11.3.  Subtenant shall also pay to Sublandlord any "Tenant Surcharges"
(as that term is hereinafter defined).  "Tenant Surcharges" shall mean any and
all amounts other than Fixed Rent, Section 4.3 Rent and Section 4.4 Rent which,
by the terms of the Overlease, become due and payable by Sublandlord to
Overlandlord as additional rent or otherwise and which would not have become due
and payable but for the acts, requests for services, and/or failures to act of
Subtenant, its agents, officers, representatives, employees, servants,
contractors, invitees, licensees or visitors under this Sublease, including, but
not limited to: (i) any increases in Overlandlord's fire, rent or other
insurance premiums, as provided in Article 6 of the Overlease, resulting from
any act or omission of Subtenant, (ii) any additional charges to Sublandlord on

                                       12
<PAGE>
 
account of Subtenant's use of heating, ventilation or air conditioning after
hours, (iii) any charges which may be imposed on Sublandlord pursuant to the
Overlease, to the extent that such charges are attributable to the Premises or
the use thereof or services or utilities provided thereto, and (iv) any
additional charges to Subtenant on account of Subtenant's use of cleaning and
elevator services after hours or in excess of normal usage.  Within a reasonable
time after receipt by Sublandlord of any statement or written demand from
Overlandlord, including any Tenant Surcharges, Sublandlord will furnish
Subtenant with a copy of such statement or demand, together with Sublandlord's
statement of the amount of any such Tenant Surcharges, and Subtenant shall pay
to Sublandlord the amount of such Tenant Surcharges within five (5) days after
Subtenant's receipt of such statement or demand; provided, however, that in any
instance in which Subtenant shall receive any such statement or demand directly
from Overlandlord, Subtenant may pay the amount of the same directly to
Overlandlord.  Payments shall be made pursuant to this Section 11.3
notwithstanding the fact that the statement to be provided by Sublandlord is
furnished to Subtenant after the expiration of the term of this Sublease and
notwithstanding the fact that by its terms this Sublease shall have expired or
have been cancelled or terminated.

          12.

          12.1.  Any notice, demand or communication which, under the terms of
this Sublease or under any statute or municipal regulation must or may be given
or made by the parties hereto, shall be in writing and given or made by mailing
the same by registered or certified mail, return receipt requested, addressed to
the party for whom intended at its address as aforesaid, except that, after the
Sublease Commencement Date, Subtenant's address shall be 

                                       13
<PAGE>
 
deemed to be the Building unless Subtenant shall give notice to the contrary.
Either party, however, may designate such new or other address to which such
notices, demands or communications thereafter shall be given, made or mailed by
notice given in the manner prescribed herein. Any such notice, demand or
communication shall be deemed given or served, as the case may be, on the date
of the posting thereof.

          13.

          13.1.  Subtenant shall pay to Sublandlord 4.473% of the charges
payable under Section 5.2.2 of the Overlease for electricity.

          13.2.  Subtenant's use of electric current in the Premises shall not
at any time exceed the capacity of any of the electrical conductors and
equipment in or otherwise serving the Premises.

          13.3.  Sublandlord shall not be liable in any way to Subtenant for any
failure or defect in the supply or character of electric energy furnished to the
Premises by reason of any requirement, act or omission of the public utility
serving the Building with electricity or for any other reason not attributable
to Sublandlord.

          14.

          14.1.  Subtenant may make no changes, alterations, additions,
improvements or decorations in, to or about the Premises without Overlandlord's
and Sublandlord's prior written consent.

          15.

          15.1.  Subtenant agrees to look solely to Sublandlord's estate and
interest in this Sublease, and the Premises, for the satisfaction of any right
or remedy of Subtenant for the 

                                       14
<PAGE>
 
collection of a judgment (or other judicial process) requiring the payment of
money by Sublandlord, in the event of any liability by Sublandlord, and no other
property or assets of Sublandlord shall be subject to levy, execution,
attachment, or other enforcement procedure for the satisfaction of Subtenant's
remedies under or with respect to this Sublease, the relationship of Sublandlord
and Subtenant hereunder, or Subtenant's use and occupancy of the Premises, or
any other liability of Sublandlord to Subtenant.

          16.

          16.1.  So long as Subtenant pays all of the Fixed Rent and additional
rent due under this Sublease and performs all of Subtenant's other obligations
hereunder, Sublandlord shall not disturb or terminate Subtenant's leasehold
estate hereunder, subject, however, to the terms, provisions and obligations of
this Sublease and the Overlease.

          17.

          17.1.  This Sublease may not be changed orally, but only by an
agreement in writing signed by the party against whom enforcement of any waiver,
change, modification or discharge is sought.

          17.2.  This Sublease shall not be binding upon Sublandlord unless and
until it is signed by Sublandlord and delivered to Subtenant.  This Section 17.2
shall not be deemed to modify the provisions of Article 6 hereof.

          17.3.  This Sublease constitutes the entire agreement between the
parties and all representations and understandings have been merged herein.

          17.4.  This Sublease shall inure to the benefit of all of the parties
hereto, their successors and (subject to the provisions hereof) their assigns.

                                       15
<PAGE>
 
          17.5.  The term "Sublandlord" as used in this Sublease shall mean only
the Sublandlord named herein, so that in the event of any assignment of the
Sublease, the Sublandlord named herein shall be and hereby is entirely freed and
relieved of all future covenants, obligations and liabilities of Sublandlord
hereunder, and it shall be deemed and construed without further agreement
between the parties or their successors in interest that the assignee of the
Sublease has assumed and agreed to carry out any and all such covenants,
obligations and liabilities of Sublandlord hereunder.

          18.

          18.1.  Subject to the Sublease Expiration Date set forth in Section
1.1 hereof, either Sublandlord or Subtenant may elect, at its option, to
terminate this Sublease and the term and estate hereby granted, by written
notice of such termination (hereinafter called the "Sublease Termination
Notice") to the other party, which Sublease Termination Notice shall contain a
surrender date (such date being hereinafter called the "Sublease Surrender
Date"), which Sublease Surrender Date shall be the last day of the month
designated in the Sublease Termination Notice which shall be not less than six
(6) months following the date on which such Sublease Termination Notice is
given.

          18.2.  In the event of the giving of such Sublease Termination Notice
by either Sublandlord or Subtenant, this Sublease and the term and estate hereby
granted (unless the same shall have expired sooner pursuant to any of the
conditions of limitation or other provisions of this Sublease or pursuant to
law) shall terminate on the Sublease Surrender Date with the same effect as if
such date were the date hereinbefore specified for the expiration of the term of
this 

                                       16
<PAGE>
 
Sublease, and the Fixed Rent and other charges payable hereunder shall be
apportioned as of the Sublease Surrender Date.

          19.

          19.1.  Subtenant has deposited with Sublandlord on the date hereof an
amount equal to $6,174.77 as security for the faithful performance and
observance by Subtenant of the terms, provisions, covenants and conditions of
this Sublease.  It is agreed that in the event Subtenant defaults in respect to
any of the terms, provisions, covenants and conditions of this Sublease,
including, but not limited to, the payment of Fixed Rent and additional rent,
Sublandlord may use, apply or retain the whole or any part of the security so
deposited to the extent required for the payment of any Fixed Rent and
additional rent or any other sum as to which Subtenant is in default or for any
sum which Sublandlord may expend or may be required to expend by reason of
Subtenant's default in respect of any of the terms, provisions, covenants and
conditions of this Sublease, including, but not limited to, any damages or
deficiency accrued before or after summary proceedings or other re-entry by
Sublandlord.  In the event that Subtenant shall fully and faithfully comply with
all of the terms, provisions, covenants and conditions of this Sublease, the
security shall be returned to Subtenant after the date fixed as the end of the
Sublease and after delivery of entire possession of the Premises to Sublandlord.
In the event of an assignment of the Sublease, Sublandlord shall have the right
to transfer the security to the assignee and Sublandlord shall ipso facto be
released by Subtenant from all liability for the return of such security; and
Subtenant agrees to look solely to the new sublandlord for the return of said
security; and it is agreed that the provisions hereof shall apply to every
transfer or assignment made of the security to a new sublandlord.  Subtenant
further covenants that it will 

                                       17
<PAGE>
 
not assign or encumber or attempt to assign or encumber the monies deposited
herein as security and that neither Sublandlord nor its successors or assigns
shall be bound by any such assignment, encumbrance, attempted assignment or
attempted encumbrance. In the event Sublandlord applies or retains any portion
or all of the security deposited, Subtenant shall restore the amount so applied
or retained within 3 days after the application thereof so that at all times the
amount deposited hereunder shall be equal to the amount set forth above.

          19.2.  Sublandlord shall deposit the security in an interest-bearing
account at Fleet Bank, and the interest earned thereon, less an administrative
fee of one (1%) percent per annum of principal, shall be credited to Subtenant
annually.

          20.

          20.1.  Sublandlord represents that to the best of its knowledge there
are no Hazardous Materials (as hereinafter defined) within the Premises.
Sublandlord agrees to indemnify and hold Subtenant harmless from any expense
paid or incurred in connection with the investigation, removal, containment,
replacement, enclosure, encapsulation, abatement, remediation or other treatment
or repairs or cleaning of areas (herein collectively called "remediation") in
connection with the presence of Hazardous Materials in the Premises to the
extent that such remediation is made necessary by Sublandlord's use or manner of
use of the Premises and required under Legal Requirements existing as of the
date hereof.  For purposes hereof, the term "Hazardous Materials" shall mean any
flammable explosives, radioactive materials, hazardous wastes, hazardous and
toxic substances, or related materials, asbestos or any material containing
asbestos, or any other substance or material, as defined by any federal, state
or local environmental law, ordinance, rule or regulation, including, without
limitation, the 

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<PAGE>
 
Comprehensive Environmental Response Compensation and Liability Act of 1980, as
amended, the Hazardous Materials Transportation Act, as amended, the Resource
Conservation and Recovery Act, as amended, and in the regulations adopted and
publications promulgated pursuant to each of the foregoing. Hazardous Materials
shall not include normal cleaning and/or normal office substances or materials.

          IN WITNESS WHEREOF, the parties have hereunto set their hands and
seals as of the day and year first above written.


                         INTERNEURON PHARMACEUTICALS, INC.,
                     SUBLANDLORD


                         BY:  /s/ G. L. Cooper
                             ----------------------------------
                               NAME: G. L. Cooper
                               TITLE: President and CEO


                         GENTA, INC., SUBTENANT


                         BY:  /s/ G. M. Schimmoeller
                             ----------------------------------
                               NAME: G. M. Schimmoeller
                               TITLE: Vice President and 
                                      Chief Financial Officer

                                       19
<PAGE>
 
STATE OF MASSACHUSETTS   )
                         :  ss.:
COUNTY OF MIDDLESEX      )


          On the 9th day of April, 1999, before me personally came G. M.
Schimmoeller, to me known, who, being by me duly sworn, did depose and say that
he resides in No. Andover, MA; that he is the VP amd CFO of GENTA, INC., the
corporation described in and which executed the above instrument; and that he
signed his name thereto by order of the board of directors of said corporation.

                                     /s/ Carolyn A. Grasso
                                    ------------------------------------
                                               Notary Public

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