<PAGE>   1
                                                                   EXHIBIT 10.68


                                December 1, 1999


Dr. Kenneth G. Kasses
41353 N. 106 Street
Scottsdale Arizona 85262


Dear Ken:

         This letter agreement confirms the terms of the termination of your
employment with Genta Incorporated ("Genta" or the "Company").

         1. As of the Termination Date (as defined below), all existing
employment agreements between you and Genta whether oral or written (including
but not limited to, your letter agreement entered into with Genta on September
4, 1997 (the "Agreement")), are hereby terminated.

         2. Your employment as Chief Executive Officer, President and Chairman
of the Board of Directors of Genta (the "Board of Directors") shall terminate
effective December 1, 1999 (the "Termination Date"). You represent that you do
not have any claim, action, or pending proceeding against Genta and that there
is no existing basis for any claim action or proceeding against Genta.

         3. Except as necessary to enforce the terms of this letter agreement,
and in exchange for and in consideration of the promises, covenants and
agreements set forth herein, you hereby release Genta from any and all manner of
claims, demands, causes of action, obligations, damages or liabilities
whatsoever of every kind and nature, at law or in equity, known or unknown, and
whether or not discoverable, which you have or may have for any period prior to
and including the date of the execution of this letter agreement, including, but
not limited to, any claim of defamation, wrongful discharge, breach of contract,
claims for unpaid wages, claims under the Agreement and claims of discrimination
under the Age Discrimination in Employment Act of 1967 or any other federal,
state or local laws, and any claim for attorneys' fees or costs.

         4. In full consideration for your agreements set forth in this letter
agreement:

                  (a) Genta will continue to pay your base salary through
November 30, 2000. Such payments shall be made in accordance with Genta's normal
payroll practices, and all applicable withholdings for federal, state and local
income taxes, Social Security and all other customary withholdings will be made.

                  (b) Genta will enter into an amendment to your stock option
agreement in the form attached hereto as Exhibit A.

                  (c) Except as provided in the letter agreement entered into
between you and Genta dated September 4, 1997, to the extent permitted by the
applicable insurance plans and by law, Genta will continue to provide you with
medical insurance, dental insurance,

<PAGE>   2

life insurance and long-term disability insurance through December 31, 2000 in
accordance with its benefit plans or programs. Genta reserves its right, in its
sole discretion, to amend or terminate its benefit plans or programs at any
time. To the extent Genta is unable to keep you enrolled in these insurance
plans, Genta will reimburse you through December 31, 2000 for your COBRA
payments for continued medical insurance and will, to the extent practicable,
provide you with dental insurance, life insurance and long-term disability
insurance through December 31, 2000 substantially comparable to what you would
have had if you had continued in these plans.

                  (d) Genta will continue to provide you with voicemail and
electronic mail access through February 29, 2000.

                  (e) Except as provided in this paragraph 4, you shall not be
entitled to any sum of money or benefits from Genta.

         5. Ownership of Proprietary Information. (a) You confirm and agree that
all information that has been, either prior to the Termination Date or at any
time thereafter, created, discovered or developed by the Company, or any of its
subsidiaries, affiliates, licensors, licensees, successors or assigns (each, an
"Affiliate" and, collectively, the "Affiliates") (including, without limitation,
information relating to the Company's business created, discovered, developed or
made known to the Company or an Affiliate by you during your employment with the
Company and information relating to the Company's customers, clients, suppliers,
vendors, consultants, licensors and licensees) or information in which
proprietary rights have been assigned, licensed or otherwise conveyed to the
Company or any Affiliate, has been, is and shall be the sole property of the
Company or such Affiliate, as applicable, and the Company or the Affiliate, as
the case may be, has been, is and shall be the sole owner of all patents, patent
applications, copyrights, copyright applications and other rights in connection
therewith, including but not limited to, the right to make application for
statutory protection of any kind in any country. All of the aforementioned
information is hereinafter called "Proprietary Information." By way of
illustration, but not limitation, Proprietary Information includes trade
secrets, processes, discoveries, structures, designs, ideas, works of
authorship, copyrightable works, trademarks, copyrights, formulas, data, data
structures, know-how, show-how, improvements, Intellectual Property (as defined
in Section 7), product concepts, specifications, techniques, information or
statistics contained in, or relating to, marketing plans, strategies, forecasts,
blueprints, sketches, records, notes, devices, drawings, customer lists, patent
applications, copyright and trademark applications of any kind and information
about the Company's or any Affiliate's employees and/or consultants (including,
without limitation, the compensation, job responsibility and job performance of
such employees and/or consultants).

                  (b) You further confirm and agree that at all times, both
prior to the Termination Date and thereafter, you have kept and will keep in
strictest confidence and trust all Proprietary Information, and you have not
used or disclosed and will not use or disclose any Proprietary Information or
anything directly relating to it without the prior written consent of the
Company or the Affiliates, as appropriate, except as may have been necessary
prior to the Termination Date in the ordinary course of performing your duties
as Chief Executive Officer, President and Chairman of the Board of Directors, or
as may be necessary after the Termination Date in performing your duties as a
member of the Board of Directors. You acknowledge that


                                      -2-
<PAGE>   3

the Proprietary Information constitutes a unique and valuable asset of the
Company and, as applicable, each Affiliate, acquired at great time and expense,
which is secret and confidential and which has been or will be communicated to
you, if at all, in confidence in the course of the performance of your duties,
and that any disclosure or other use of the Proprietary Information other than
for the sole benefit of the Company would be wrongful and could cause
irreparable harm to the Company or an Affiliate.

                  (c) Notwithstanding the foregoing, you are free to use
information which would otherwise be Proprietary Information, if such
information is in the public domain not as a result of a breach of this letter
agreement or the breach of any other duty owed to the Company by any third
party.

                  (d) You recognize that the Company has received and in the
future will receive confidential or proprietary information from third parties
subject to a duty on the Company's part to maintain the confidentiality of such
information and, in some cases, to use it only for certain limited purposes. You
confirm and agree that you have owed and will continue owe the Company and such
third parties, both during the term of your employment and thereafter, a duty to
hold all such confidential or proprietary information in the strictest
confidence and not to disclose it to any person, firm, partnership, joint
venture, corporation or other business entity (collectively, "Person") (except
in a manner that is consistent with the Company's agreement with the third
party) or use it for the benefit of anyone other than the Company or such third
party (consistent with the Company's agreement with the third party), unless
expressly authorized to act otherwise by the Board of Directors.

         6. Confidential Information. (a) You confirm and agree that, both prior
to the Termination Date and thereafter, you have kept and will continue to keep
in strictest confidence and trust and have not disclosed or made accessible, and
will not use, disclose or make accessible, to any Person without the prior
written consent of the Company, the Company's or any Affiliate's products,
services and technology, promotion and marketing programs, lists, trade secrets
and other confidential and proprietary business information of the Company or
any Affiliate or any of its or their clients, consultants, suppliers, customers,
vendors, licensors, licensees and other third parties including, without
limitation, Proprietary Information (all the foregoing is referred to herein as
the "Confidential Information"), except as required in connection with the
performance of your duties to the Company. You confirm and agree that, both
during your employment with the Company prior to the Termination Date and
thereafter, (i) you have not used and will not use any such Confidential
Information for yourself or any third parties; and (ii) you have not taken and
will not take any material or reproductions embodying any Confidential
Information from the Company's facilities (or any of the Affiliates' facilities)
at any time, except as required in connection with the performance of your
duties to the Company.

         Notwithstanding the foregoing, the parties agree that you are free to
use information which would otherwise be Confidential Information, if such
information is in the public domain not as a result of a breach of this letter
agreement or the breach of any other duty owed to the Company by any Person.

                  (b) You confirm and agree that, except with prior written
authorization by the Company, you have not disclosed or published and will not
disclose or publish any of the


                                      -3-
<PAGE>   4

Confidential Information or any confidential, technical or business information
of any other party to whom the Company or any of its Affiliates has owed or owes
an obligation of confidence, at any time prior to the Termination Date and
thereafter.

                  (c) Upon written notice by the Company, if you are no longer
serving on the Company's Board of Directors, you shall promptly deliver to the
Company, or, if requested by the Company, promptly destroy all written
Confidential Information and any other written material containing Confidential
Information (whether prepared by the Company, you or a third party). You agree
not to retain any copies, extracts, summaries or other reproductions in whole or
in part of such written Confidential Information (and you shall, upon request of
the Company, certify such delivery or destruction to the Company in a written
instrument reasonably acceptable to the Company and its counsel).

                  (d) In the event that you are requested or required (by oral
questions, deposition, interrogatories, requests for information or documents,
subpoena, civil investigative demand or any other process) to disclose all or
any part of any Confidential Information, you will provide the Company with
prompt notice of such request or requirement, as well as notice of the terms and
circumstances surrounding such request or requirement, so that the Company, or,
as applicable, one or more of its Affiliates, may seek an appropriate protective
order or waive compliance with the provisions of this letter agreement. In such
case, the parties will consult with each other on the advisability of pursuing
any such order or other legal action or available steps to resist or narrow such
request or requirement. If, failing the entry of a protective order or the
receipt of a waiver hereunder, you are, in the opinion of counsel reasonably
acceptable to the Company, legally compelled to disclose Confidential
Information, you may disclose that portion of such information which counsel
advises you that you are legally compelled to disclose. In any event, you will
use your best efforts to obtain, and will not oppose action by the Company (or,
as applicable, one or more of its Affiliates) to obtain, an appropriate
protective order or other reliable assurance that confidential treatment will be
accorded the disclosure of any Confidential Information.

         7. Disclosure and Ownership of Intellectual Property. (a) You confirm
and agree that, both prior to the Termination Date and thereafter, you have
promptly disclosed and will promptly disclose to the Company, or any persons
designated by the Company, all improvements, inventions (whether patentable or
not), designs, ideas, works of authorship (whether copyrightable or not)
copyrightable works, discoveries, trademarks, copyrights, trade secrets,
formulas, processes, structures, product concepts, marketing plans, strategies,
customer lists, information about employees and/or consultants (including,
without limitation, job performance of such employees and/or consultants),
techniques, blueprints, sketches, records, notes, devices, drawings,
specifications, know-how, data, data structures, patent applications, copyright
applications and other applications for statutory protection of any kind in any
country relating to the current or reasonably foreseeable business of the
Company, made or conceived or reduced to practice or learned by you, either
alone or jointly with others, during the course of your employment prior to the
Termination Date or thereafter in the performance of your duties as a director
of the Company (collectively hereinafter referred to as the "Intellectual
Property").

                  (b) You confirm and agree that all Intellectual Property above
has been and shall be the sole property of the Company and to the extent
permitted by law shall be


                                      -4-
<PAGE>   5

"works made for hire" as that term is defined in the United States Copyright Act
(17 USCA, Section 101). The Company has been and shall be the sole owner of all
patents, copyrights, trade secret rights and other Intellectual Property and all
other rights in connection therewith. You hereby assign to the Company all
right, title and interest you have or acquire in all Intellectual Property. You
further agree to assist the Company in every proper way (but at the Company's
expense) to obtain and confirm and from time to time enforce patents, copyrights
or other rights in the Intellectual Property in any and all countries, and to
that end you will execute all documents necessary:

         (i)      to apply for, obtain and vest in the name of the Company alone
                  (unless the Company otherwise directs) letters patent,
                  copyrights or other analogous protection in any country
                  throughout the world and when so obtained or vested to renew
                  and restore the same on behalf of the Company; and

         (ii)     to defend any opposition proceedings in respect of such
                  applications and any opposition proceedings or petitions or
                  applications for revocation of such letters patent, copyright
                  or other analogous protection.

                  (c) Your obligation to assist the Company in obtaining,
confirming and enforcing patents and copyrights for the Intellectual Property
above in any and all countries shall continue beyond the Termination Date. The
Company agrees to compensate you at a reasonable rate after the Termination Date
for time actually spent by you at the Company's request on such assistance.

         8. Enforcement. You agree that the remedy at law for any breach or
threatened breach of any covenant contained in Sections 5, 6 or 7 of this letter
agreement would be inadequate and cause irreparable damage to the Company. In
the event that you breach or threaten to breach any provisions of Sections 5, 6
or 7, in addition to any other rights which the Company may have at law or in
equity, the Company shall be entitled, without the posting of a bond or other
security, to injunctive relief to enforce the restrictions contained in this
letter agreement. In the event that an actual proceeding is brought in equity to
enforce any of the provisions of Sections 5, 6 or 7, you shall not assert as a
defense that there is an adequate remedy at law nor shall the Company be
prevented from seeking any other remedies, including without limitation monetary
damages, which may be available to it.

         9. In executing this letter agreement, neither you nor Genta admits any
liability or wrongdoing, and the considerations exchanged herein do not
constitute an admission of any liability, error, contract violation, or
violation of any federal, state, or local law or regulation.

         10. This letter agreement shall be binding upon and inure to the
benefit of the parties hereto and their respective successors and permitted
assigns.


                                      -5-
<PAGE>   6

         11. The unenforceability or invalidity of any provision or provisions
of this letter agreement shall not render any other provision or provisions
hereof unenforceable or invalid.

         12. This letter agreement constitutes the entire agreement between you
and Genta and cannot be altered except in a writing signed by both you and
Genta. You acknowledge that you entered into this letter agreement voluntarily,
that you fully understand all of its provisions, and that no representations
were made to induce execution of this letter agreement that are not expressly
contained herein.

         13. The parties agree that any disputes concerning the interpretation
or application of this letter agreement shall be governed by New York law,
without regard to principles of conflict of law or where the parties are located
at the time a dispute arises.

         14. You acknowledge that you have been afforded an opportunity to take
at least (21) twenty-one days to consider this letter agreement and have been
advised to consult with the attorneys of your choice prior to executing this
letter agreement. You acknowledge that you have had an adequate opportunity to
review this letter agreement before its execution. You further acknowledge that
you will have a period of seven (7) calendar days following the execution and
delivery of this letter agreement in which to revoke your consent (by delivering
such written notice of revocation to Monica Lord, Esq. of Kramer Levin Naftalis
& Frankel LLP, 919 Third Avenue, New York, New York 10022 within such seven
calendar days), and that this letter agreement will not become effective until
the revocation period has expired.


                                      -6-
<PAGE>   7

         Please indicate your acceptance of the terms of this letter agreement
by countersigning below and returning this letter agreement to me.


                                       Very truly yours,

                                       GENTA INCORPORATED


                                       By: ___________________________________
                                                 Mark C. Rogers, M.D.


Agreed and accepted as of the date first set forth above:


__________________________________
Dr. Kenneth G. Kasses


                                      -7-