<PAGE>
 
                              CONSENT TO SUBLEASE

     THIS CONSENT TO SUBLEASE (this "Consent") is dated as of March 31, 1999, is
made with reference to that certain sublease (the "Sublease") dated March 31,
1999, by and between Interneuron Pharmaceuticals, Inc. with an address at 99
Hayden Avenue, Lexington, Massachusetts 02173 ("Tenant") and Genta, Inc., with
an address at 99 Hayden Avenue, Lexington, Massachusetts 02173 ("Sublessee"),
and is entered into by and among Ledgemont Realty Trust, with an address at 177
Milk Street, Boston, Massachusetts 02109 ("Landlord"), Tenant and Sublessee,
with reference to the following facts:

     (A)  Landlord and Tenant are the parties to that certain Lease dated as of
February 5, 1997, as amended ("Master Lease");

     (B)  Tenant and Sublessee wish to enter into the Sublease;

     (C)  The Master Lease provides, inter alia, that Tenant may not enter into
any sublease without Landlord's prior written approval;

     (D)  Tenant and Sublessee have herewith presented the fully-executed
Sublease (a true copy of which is attached hereto) to Landlord for Landlord's
approval, upon all of the terms and conditions hereinafter appearing.

  NOW, THEREFORE, for good and valuable consideration, the parties agree as
follows:

     Landlord hereby consents to the Sublease upon the terms and conditions set
forth in the General Conditions of Consent to Sublease attached hereto and made
an integral part hereof.

     EXECUTED under seal as of the date first written above.

LANDLORD:                     TENANT:

LEDGEMONT REALTY TRUST        INTERNEURON PHARMACEUTICALS, INC.


By: /s/ Robert L. Beal        By:/s/ G. L. Cooper
   ---------------------         ----------------
   Trustee                       Duly Authorized

                              SUBLESSEE:

                              GENTA, INC.


                              By: /s/ G. M. Schimmoeller
                                 -----------------------
                                 Duly Authorized

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<PAGE>
 
                   GENERAL CONDITIONS OF CONSENT TO SUBLEASE

     The following terms and conditions are an integral part of the foregoing
Consent to Sublease.

     1.   Neither the Master Lease, the Sublease nor this Consent shall be
deemed, nor are they intended, to grant to Sublessee any rights whatsoever
against Landlord. Sublessee hereby acknowledges and agrees that its sole remedy
for any alleged or actual breach of its rights in connection with the Sublease
shall be solely against Tenant. Sublessee acknowledges and agrees that it is not
a third party beneficiary under the Master Lease, and is not entitled to assert
any of Tenant's rights thereunder against Landlord, whether in its own right or
on behalf of Tenant.

     2.   This Consent shall not release Tenant from any existing or future
duty, obligation or liability to Landlord pursuant to the Master Lease, nor
shall this Consent change, modify or amend the Master Lease in any manner,
except insofar as it constitutes Landlord's consent to the sublease.
Notwithstanding the generality of the foregoing, (i) this Consent expressly
shall not absolve Tenant from the requirement set forth in the Master Lease that
Tenant obtain Landlords' prior written approval of any additional subleases,
assignments or other dispositions of its interest in the Master Lease or the
Premises (as defined in the Master Lease) and (ii) this Consent shall not
constitute Landlord's consent to any alteration of the premises leased under the
Master Lease.

     3.   In the event Tenant is in default under any of the terms and
provisions of the Master Lease, Landlord may elect to receive directly from
Sublessee all sums due or payable to Tenant by Sublessee pursuant to the
Sublease, and upon receipt of Landlord's notice, Sublessee shall thereafter pay
to Landlord any and all sums becoming due or payable under the Sublease and
Tenant shall receive from Landlord a corresponding credit for such sums actually
received by written notice nor the receipt of such direct payments shall cause
Landlord to assume any of Tenant's duties, obligations and/or liabilities under
the Sublease, nor shall such event impose upon Landlord the duty or obligation
to honor the Sublease, nor subsequently to accept any purported attornment by
Sublessee.

     4.   Sublessee hereby acknowledges that it has read and has knowledge of
all of the terms, provisions, rules and regulations of the Master Lease and
agrees not to do or omit to do anything which would cause Tenant to be in breach
of the Master Lease. Any such act or omission shall also constitute a breach of
the Master Lease and this Consent and shall entitle Landlord to recover any
damage, loss, cost, or expense which it thereby suffers, from Tenant and/or
Sublessee.

     5.   In the event of any litigation between the parties hereto with respect
to the subject matter hereof, the unsuccessful party agrees to pay to the
successful party all reasonable costs, expenses and attorneys' fees incurred
therein by the successful party, which may be included as a part of a judgment
rendered therein.

      6.  The parties acknowledge that the Sublease constitutes the entire
agreement between Tenant and Sublessee with respect to the subject matter
thereof insofar as Landlord may be concerned, and that no amendment,
termination, modification or change therein will be binding upon Landlord unless
Landlord shall have given its prior written consent thereto.

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<PAGE>
 
     7.   This Consent shall be binding upon and shall inure to the benefit of
the parties' respective successors in interest and assigns, subject at all
times, nevertheless, to all agreements and restrictions contained in the Master
Lease, the Sublease, and herein, with respect to subleasing, assignment, or
other transfer. The agreements contacted herein constitute the entire
understanding between the parties with respect to the subject matter hereof, and
supersede all prior agreements, written or oral, inconsistent herewith. Tenant
and Sublessee warrant and agree that neither Landlord nor any of its agents or
other representatives have made any representations concerning the Premises,
their condition, the Sublease or the Master Lease.

     8.   Notice required or desired to be given hereunder shall be effective
either upon personal delivery or three (3) days after deposit in the United
States mail, by registered or certified mail, return receipt requested,
addressed to the Landlord at the address set forth in the Master Lease for the
payment of rent, or to Tenant or Sublessee at the address of the Premises or of
the subleased Premises, respectively. Any party may change in address for notice
by giving notice in the manner hereinabove provided.

     9.   Notwithstanding anything to the contrary set forth herein or
elsewhere, if the Master Lease was guaranteed at the time of execution or at any
time prior hereto by any guarantor, then Tenant shall deliver to Landlord with
this Consent a counterpart of this Consent indicating the approval thereof by
any and all such guarantor(s).

     10.  Tenant and Sublessee agree to indemnify and hold Landlord harmless
from and against any loss, cost, expense, damage or liability, including
reasonable attorneys' fees, incurred as a result of a claim by any person or
entity (i) that is entitled to a commission, finder's fee or like payment in
connection with the sublease or (ii) relating to or arising out of the Sublease
or any related agreements or dealings.

                                     -35-