Document:

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                                                                 EXHIBIT 10.28.5

                               FOURTH AMENDMENT TO
                         THE COVENTRY HEALTH CARE, INC.
                     SUPPLEMENTAL EXECUTIVE RETIREMENT PLAN

           WHEREAS, by resolution dated November 4, 1999, the Board of Directors
of Coventry Health Care, Inc. (the "Company") authorized the amendment of the
Coventry Health Care, Inc. Supplemental Executive Retirement Plan (the "SERP")
to: (i) revise the definition of Compensation to exclude income or proceeds from
the Company's Stock Incentive Plan and relocation payments; (ii) clarify the
amounts that an Eligible Employee may defer to the SERP; (iii) revise the
methodology used to calculate benefit payments to provide for annual
recalculation of the monthly payments; and (iv) specifically delegate full power
and discretionary authority to the 401(k) Plan Investment Committee to
administer the SERP.

           NOW, THEREFORE, the Company hereby amends the SERP as follows:

1. Section 2.5 is amended, effective as of January 1, 1999, to provide as
follows:

                               2.5 Committee. "Committee" means the Company's
                               401(k) Plan Investment Committee.

2. Section 2.6 is amended, effective January 1, 1999, to provide as follows:

                               2.6 Compensation. "Compensation" means the total
                               compensation paid by the Employer to a
                               Participant during the Plan Year, excluding
                               income or proceeds from the Company's Stock
                               Incentive Plan and relocation payments, but
                               including base pay, annual management incentive
                               pay, commissions, and amounts not included in
                               income by reason of a Participant's agreement to
                               defer Compensation under the terms of this Plan
                               or a Participant's election under a cash or
                               deferred arrangement under section 401(k) of the
                               Code or a cafeteria plan described in Section 125
                               of the Code.

3. Section 2.7 is amended, effective January 1, 1999, to provide as follows:

                               2.7 Deferred Compensation. "Deferred
                               Compensation" means the amount of Compensation
                               not yet earned during the Plan Year that the
                               Participant and the Employer mutually agree shall
                               be deferred in accordance with the provisions of
                               Section 3.2(a) of this Plan.

4. Section 3.2(a) is amended, effective January 1, 2000, to provide as follows:

                               (a) Form of Deferral. An Eligible Employee may
                               become a Participant by properly executing a
                               Deferred Compensation Agreement and filing such
                               Agreement with the Committee. An Eligible
                               Employee may elect to defer, in whole
                               percentages, 1% to 15% of his base salary (net of
                               salary deferral contributions to a cash or
                               deferred arrangement under section 401(k) of the
                               Code) per pay period and 1% to 100% of his annual
                               management incentive pay for a Plan Year. The
                               Deferred Compensation Agreement shall be executed
                               before the first day of the Plan Year (pursuant
                               to Section 3.2(b) hereof) for which the Deferred
                               Compensation is otherwise payable and shall be
                               effective as of the first day of the first
                               payroll period beginning in such Plan Year;
                               provided,

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                               however, that in the first year that an Employee
                               becomes an Eligible Employee, such Eligible
                               Employee may make an election within 30 days
                               after becoming an Eligible Employee to defer a
                               portion of his base salary (net of salary
                               deferral contributions to a cash or deferred
                               arrangement under section 401(k) of the Code) and
                               annual management incentive pay for a Plan Year,
                               as otherwise set forth in this paragraph, for
                               services to be performed after he becomes an
                               Eligible Employee and such Eligible Employee's
                               Deferred Compensation Agreement shall be
                               effective as of the first day of the payroll
                               period beginning immediately following the
                               execution of such Agreement, or the effective
                               date of such Agreement, if later.

5. Section 5.6(a) is amended, effective as of January 1, 2000, to provide as
follows:

                               (a) Monthly installments over a period of sixty
                               (60) months paid each month. The monthly payment
                               amount will be determined by dividing the balance
                               in the Participant's Deferred Compensation
                               Account by sixty (60), and adjusted each January
                               1 thereafter to reflect the then remaining number
                               of months in the payment period. The last monthly
                               payment will be equal to the remaining balance in
                               the Participant's Deferred Compensation Account.

6. Section 7.1 is amended, effective as January 1, 1999, to provide as follows:

                               7.1 Committee. This Plan shall be administered by
                               the Committee. Members of the Committee may be
                               Participants under the Plan. The Committee shall
                               have full power and discretionary authority to
                               administer, interpret and construe this Plan, to
                               determine and review claims for benefits under
                               this Plan, to establish rules and regulations, to
                               delegate responsibilities to others to assist it
                               in administering this Plan, to establish
                               investment guidelines, and to perform all other
                               acts it believes reasonable and proper in
                               connection with the administration of this Plan.
                               The Committee shall rely on the records of the
                               Company in determining benefits hereunder. The
                               Plan shall be administered and interpreted
                               consistent with the intention that the Plan shall
                               constitute a plan which, for purposes of ERISA,
                               is unfunded and maintained for a select group of
                               management or highly compensated employees.

           IN WITNESS WHEREOF, the Company has caused this Fourth Amendment to
the Coventry Health Care, Inc. Supplemental Executive Retirement Plan to be
executed by its duly authorized officer this 4th day of November, 1999.

Attest:                                           COVENTRY HEALTH CARE, INC.

/s/ Tim A. Silvera                                /s/ Shirley R. Smith
----------------------                            ----------------------
Vice President                                    Senior Vice President

                                       2<PAGE>   1
                                                                   EXHIBIT 10.21

                              EMPLOYMENT AGREEMENT

            AGREEMENT, dated this 27th day of December, 1999 between Nastech
Pharmaceutical Company Inc., a Delaware corporation ("Employer") with offices at
45 Davids Drive, Hauppauge, NY and Andrew P. Zinzi ("Employee").

                             W I T N E S S E T H :

            WHEREAS, the Employee is currently employed as Chief Financial
Officer ("CFO") for the Employer and the Employer and Employee wish to enter
into an employment and compensation arrangement on the following terms and
conditions:

            1. Employment. Subject to the terms and conditions of this
Agreement, including specifically Section 12, Employer agrees to employ Employee
for a period commencing from the effective date of this Agreement and ending
December 31, 2001 to serve as the Chief Financial Officer ("CFO") and to serve
in such capacitates consistent with his position as CFO as the Employer may from
time to time designate. Employee hereby accepts such employment and agrees to
devote his full time and best efforts to the duties provided herein. If Employee
shall remain in the full time employ of the Company beyond December 31, 2001
without any written agreement between the parties, this Agreement shall be
deemed to continue on a month-to-month basis and either party shall have the
right to terminate this Agreement at the end of any ensuing calendar month on
written notice of at least 30 days.

            2. Compensation. For services rendered to Employer during the term
of this Agreement, Employer shall compensate Employee with a salary, payable in
accordance with Employer's standard payroll practices in effect, from time to
time, of $165,000 per annum. Employee's salary may be adjusted for merit and/or
cost of living increases, as determined by Employer in its sole discretion.

            The Employee shall also be entitled to annual incentive compensation
of up to thirty (30%) of the applicable base salary if the Employer's combined
corporate and department objectives as set forth in the Employer's annual
business plan are achieved. The nature and extent of such compensation shall be
approved by the Compensation Committee of the Company's Board of Directors no
later than sixty (60) days following the end of the Employer's fiscal year.

                                       (1)

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            3. Stock Options. As further compensation, Employee acknowledges the
receipt of 35,000 incentive stock options (subject to allowable limitations set
forth in the Internal Revenue Code of 1986, as amended, hereinafter stock
options) as an incentive to enter into this Agreement. The stock options were
issued at the fair market value of the Employer's common stock as of the date
grant and shall then be vested at 50% per full year of service (and shall not be
vested for interim periods on a pro-rata basis, except as otherwise provided in
the applicable Stock Option Agreement) from the date of grant, over a two year
period, all of the foregoing to be in accordance with the provisions of
Employer's Stock Option Plan, as may be amended from time to time, which is
incorporated by reference herein. The Common Stock to be issued upon the
exercise of said options has been registered under the Securities Act of 1933.
Employee's stock options may be increased during the term of this Agreement, as
determined by Employer in its sole discretion.

            4. Expenses. Employer shall pay or reimburse Employee for all
expenses normally reimbursed by Employer, reasonably incurred by him in
furtherance of his duties hereunder and authorized and approved by the Employer
upon submission by him of vouchers or an itemized list thereof prepared in
compliance with such rules relating thereto as the Employer may, from time to
time, adopt and as may be required in order to permit such payments as proper
deductions to Employer under the Internal Revenue Code of 1986, as amended, and
the rule and regulations adopted pursuant thereto now or hereafter in effect.

            5. Benefits. The Employee shall be entitled to participate in or be
covered by any Health and/or Retirement and Executive Compensation plans adopted
by the Employer from time to time.

            6. Insurance and Indemnity. The Employer shall use its best efforts
to maintain, at its expense, officers and directors fiduciary liability
insurance covering the Employee in an amount not less than $5 million. The
Employer shall also indemnity the Employee, to the fullest extent permitted by
law, from any liability asserted against or incurred by the Employee, including
attorney's costs, in his capacity as an officer of Employer. This indemnity
shall survive termination of the Agreement.

                                       (2)

<PAGE>   3

            7.  Noncompetition.

            A. The Employee agrees that, except in accordance with his duties
under this Agreement on behalf of the Employer, he will not during the term of
this Agreement:

            Participate in, be employed in any capacity by, serve as director,
consultant, agent or representative for, or have any material direct interest in
any enterprise (other than as a passive investor in a publicly traded company)
which is engaged in the business of distributing, selling or otherwise trading
in products which are competitive to any of the Employer's technology or
products distributed, sold or otherwise traded in by the Employer during the
term of the Employee's employment with the Employer, or which are competitive to
any products being actively developed, with the bona fide intent to market same,
by the Employer during the term of the Employee's employment with the Employer;

            B. The Employee hereby agrees that damages and any other remedy
available at law would be inadequate to redress or remedy any loss or damage
suffered by the Employer upon any breach of the terms of this Section 7 by the
Employee, and the Employee therefore agrees that the Employer, in addition to
recovering on any claim for damages or obtaining any other remedy available at
law, also may enforce the terms of this Section 7 by injunction or specific
performance, and may obtain any other appropriate remedy available in equity.

            8. Assignment of Patents. Employee shall disclose fully to the
Employer any and all discoveries of a scientific nature relating to Employer's
business he shall make and any and all ideas, concepts or inventions of a
scientific nature relating to Employer's business which he shall conceive or
make during his period of employment, or during the period of six months after
his employment shall terminate, which are in whole or in part the result of his
work with the Employer. Such disclosure is to be made promptly after each
discovery or conception, and the discovery, idea, concept or invention will
become and remain the property of the Employer, whether or not patent
applications are filed thereon. Upon request and at the expense of the Employer,
the Employee shall make application through the patent solicitors of the
Employer for letters patent of the United States and any and all other countries
at the discretion of the Employer on such discoveries, ideas and inventions, and
to assign all such applications to the Employer, or at its order, forthwith,
without additional payment by the Employer during his period of employment and
for reasonable

                                       (3)

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compensation for time actually spent by the Employee at such work at the request
of the Employer after the termination of the employment. He is to give the
Employer, its attorneys and solicitors, all reasonable assistance in preparing
and prosecuting such applications and, on request of the Employer, to execute
all papers and do all things that may be reasonably necessary to protect the
right of the Employer and vest in it or its assigns the discoveries, ideas or
inventions, applications and letters patent herein contemplated. Said
cooperation shall also include all actions reasonably necessary to aid the
Employer in the defense of its rights in the event of litigation.

            9.  Trade Secrets.

            (a)      In the course of the term of this Agreement, it is
anticipated that the Employee shall have access to secret or confidential
technical and commercial information, records, data, specifications, systems,
formulas, methods, plans, policies, inventions, material and other knowledge
("Confidential Material") owned by the Employer and its subsidiaries. The
Employee recognizes and acknowledges that included within the Confidential
Material are the Employer's confidential commercial information, technology,
methods of manufacture, clinical studies, pre-clinical data and related
materials, all as they may exist from time to time, and that they are valuable
special and unique aspects of the Employer's business. All such Confidential
material shall be and remain the property of the Employer. Except as required by
his duties to the Employer, the Employee shall not, directly or indirectly,
either during the term of his employment or at any time thereafter, disclose or
disseminate to anyone or make use of, for any purpose whatsoever, any
Confidential Material. Upon termination of his employment, the Employee shall
promptly deliver to the Employer all Confidential Material (including all copies
thereof) which are in the possession or under the control of the Employee. The
Employee shall not be deemed to have breached this Section 9 if the Employee
shall be specifically compelled by lawful order of any judicial, legislative, or
administrative authority or body to disclose any confidential material or else
face civil or criminal penalty or sanction.

            (b)      The Employee hereby agrees that damages and any other
remedy available at law would be inadequate to redress or remedy any loss or
damage suffered by the Employer upon any breach of the terms of this Section 9
by the Employee, and the Employee therefore agrees that the Employer, in
addition to recovering on any claim for damages or obtaining any other remedy
available at law, also may enforce the terms of this Section 9 by injunction or
specific performance, and may obtain any other appropriate remedy available in
equity.

                                       (4)

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            10. Vacation. Employee shall be entitled to vacation (with a minimum
of three weeks vacation which shall not accrue or accumulate from year to year,
except as provided by Employer), sick days, personal days and holidays as shall
be governed by Employer's standard personnel policies, which shall be provided
to Employee.

            11.  Termination.

            (a)      Employee's employment with Employer shall be at will.
Either Employer or the Employee may terminate this Agreement and Employee's
employment at any time, with or without Cause or Good Reason (as such terms are
defined below), in its or his sole discretion, upon thirty (30) days prior
written notice of termination.

            (b)      Without limiting the foregoing Section 12(a), (i) the
Employee may terminate his employment with the Employer at any time for Good
Reason, or (ii) the Employer may terminate his employment at any time for Cause.
Good Reason shall mean death, Disability (as defined below) or a termination of
employment as a result of a substantial diminution in the Employee's
responsibilities, or a reduction in base salary or a demotion in title as CFO or
demotion in position as CFO of a publicly held company. Cause shall mean (i) the
Employee's willful, repeated or neglectful failure to perform his duties
hereunder or to comply with any reasonable or proper direction given by or on
behalf of the Employer's Chief Executive Officer and approved by the Board of
Directors following five (5) days written notice to such effect, other than any
Employee's refusal to execute any document or statement that contains erroneous
or misleading financial information; (ii) the Employee being guilty of serious
misconduct on the Company's premises or elsewhere, whether during the
performance of his duties or not, which may cause damage to the reputation of
the Employer or render it difficult for the Employee to satisfactorily continue
to perform his duties; (iii) the Employee being found guilty in a criminal court
of any offense of a nature likely to affect the reputation of the Employer or to
prejudice its interests if the Employee were to continue to be employed by the
Employer; (iv) the Employee's commission of any act of fraud, theft or
dishonesty, or any intentional tort against the Company; or (v) the Employee's
violation of any of the material terms, covenants, representations or warranties
contained in this Agreement.

                                       (5)

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            (c)      "Disability" shall mean that the Employee, in the good
faith determination of the Board of Directors of the Employer, is unable to
render services of the character contemplated hereby and that such inability (i)
may be expected to be permanent, or (ii) may be expected to continue for a
period of at least three (3) consecutive months (or for shorter periods totaling
more than six (6) months during any period of twelve consecutive months).
Termination resulting from Disability may only be effected after at least thirty
(30) days written notice by the Employer of its intention to terminate the
Employee's employment.

            (d)      "Termination Date" shall mean (i) if this Agreement is
terminated on account of death, the date of death; (ii) if this Agreement is
terminated for Disability, the date established by the Company pursuant to
Section 11(c) hereof; (iii) if this Agreement is terminated by the Employer, the
date on which a notice of termination is given to the Employee; (iv) if the
Agreement is terminated by the Employee, the date the Employee ceases work.

            12.         Severance.

            (a)      If (i) the Employer terminates the employment of the
Employee against his will and without Cause, or (ii) the Employee terminates his
employment for Good Reason, the Employee shall be entitled to receive salary,
incentive compensation and vacation accrued through the Termination Date plus
six month's salary payable in one lump-sum at the Termination Date and all stock
options shall become exercisable and shall remain exercisable for a period of
one year after such Termination Date. Notwithstanding the foregoing, the
Employer shall not be required to pay any severance pay for any period following
the Termination Date if the Employer violates the provisions of Section 8,
Section 9 or Section 10 of this Agreement. In such event, the Employer shall
provide written notice to the Employee detailing such violation.

            (b)      If (i) the Employee voluntarily terminates his employment
other than for Good Reason, or (ii) the Employee is terminated by the Employer
for Cause, then the Employee shall be entitled to receive salary, incentive
compensation and accrued vacation through the Termination Date plus all stock
options that are exercisable shall remain exercisable for a period of one year
after such Termination Date.

                                       (6)

<PAGE>   7

            (c)      In addition to the provisions of Section 13(a) and 13(b)
hereof, to the extent COBRA shall be applicable to the Employer or as provided
by law, the Employee shall be entitled to continuation of group health plan
benefits for a period of one (1) year following the Termination Date if the
Employee makes the appropriate conversion and payments.

            (d)      The Employee acknowledges that, upon termination of his
employment, he is entitled to no other compensation, severance or other benefits
other than those specifically set forth in this Agreement or the Stock Option
Agreement.

13.         Payment and Other Provisions After Change of Control

            (a)      In the event Employee's employment with the Employer is
terminated following the occurrence of a Change of Control (other than as a
consequence of death or disability) either (x) by the Employer for any reason
other than for Cause, or (y) by Employee for Good Reason, then Employee shall be
entitled to receive from the Employer, in lieu of the severance payment
otherwise payable pursuant to Section 12, the following:

                     (i)         Base Salary: Employee's annual base salary as
in effect at the date of termination shall be paid on the date of termination;

                     (ii)        Incentive Compensation: The amount of the
Employee's target incentive compensation under the applicable Incentive
Compensation Plan for the fiscal year in which the date of termination occurs,
shall be paid on the date of termination; and

                     (iii)       Other Benefits: Notwithstanding the vesting
period provided for in the Employer's Stock Option Plan and any related stock
option agreements between the Employer and the Employee for stock options
("options") granted Employee by the Employer all of options shall be fully
vested and exercisable for a period of two years from the Termination Date.

            (b)      For purposes of this Agreement, the term "Change in
Control" shall mean:

       (i)    The acquisition, other than from the Employer, by any individual,
              entity or group (within the meaning of Rule 13d-3 promulgated
              under the Exchange Act or any successor provision) of 50% or more
              of either (a) the then outstanding shares of Common Stock of the
              Employer or (b) the combined voting power of the then outstanding
              voting securities of the Employer entitled to vote generally in
              the election of directors (the "Voting

                                       (7)

<PAGE>   8

              Securities").
       (ii)   Individuals who, as of the Effective Date of this Agreement,
              constitute the Board (the "Incumbent Board") cease for any reason
              to constitute at least a majority of the Board.
       (iii)  Approval by the Shareholders of the Employer of a reorganization,
              merger or consolidation (a "Business Combination"), in each case,
              with respect to which all or substantially all holders of the
              outstanding Common Stock or Voting Securities immediately prior to
              such Business Combination do not, following such Business
              Combination, beneficially own, directly or indirectly, more than
              60% of, respectively, the then outstanding shares of common stock
              or the combined voting power of the then Voting Securities
              entitled to vote generally in the election of directors, as the
              case may be, of the Employer resulting from the Business
              Combination, or
       (iv)   (a) a complete liquidation or dissolution of the Employer or (b) a
              sale or other disposition of all or substantially all of the
              assets of the Employer with respect to which, following such sale
              or disposition, more than 60% of, respectively, the then
              outstanding Common Shares or Voting Securities is then owned
              beneficially, directly or indirectly, by all or substantially all
              of the equity holders, respectively, of the Common Shares or
              Voting Securities immediately prior to such sale or disposition in
              substantially the same proportion as their ownership of the Common
              Shares or Voting Securities, as the case may be, immediately prior
              to such sale or disposition.

              (c)    In the event that involuntary termination of Employee as
CFO occurs subsequent to change in senior management or the appointment of a new
Chairman of the Board of Directors, the Company would (i) continue to pay base
salary and health benefits for the remaining term of this Agreement, provided
the Employee remains unemployed in the official capacity as a CFO, and such
payments would not be less than six months salary and benefits, and (ii) all
stock options currently outstanding would become vested and exercisable through
December 31, 2002.

            14. Notices. Any notice required or permitted to be given under this
Agreement shall be sufficient if in writing and if sent by registered or
certified mail, return receipt requested to his residence in the case of the
Employee, or to its principal office in the case of the Employer, or to such
other addresses as they may respectively designate in writing.

            15. Entire Agreement; Waiver. This Agreement contains the entire
understanding of the parties and 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. Waiver of or failure to
exercise any rights provided by this Agreement in any respect shall not be
deemed a waiver of any further or future rights.

                                       (8)

<PAGE>   9

            16. Assignment. Employee's rights hereunder are personal to and
shall not be transferable nor assignable by the Employee. This Agreement shall
bind and inure to the benefit of the Employee and the Employer and their
respective legal representatives, successors and assigns.

            17. Arbitration. Any and all claims, disputes, or controversies
arising out of or related to this Agreement, or the breach thereof, shall be
resolved exclusively by arbitration in Suffolk County, New York, in accordance
with the rules of the American Arbitration Association then in existence. The
determination or award rendered therein shall be binding and conclusive upon the
parties, and judgment may be entered thereon in accordance with applicable law
in any court having jurisdiction. No party shall be entitled to seek or be
awarded punitive damages.

            18. Governing Law. This Agreement shall be governed by and construed
in accordance with the laws of the State of New York.

            19. Counterparts. This Agreement may be executed in counterparts,
each of which shall be deemed an original, but all of which together will
constitute one and the same instrument.

            IN WITNESS WHEREOF, NASTECH PHARMACEUTICAL COMPANY INC. has caused
this instrument to be signed by a duly authorized officer and the Employee has
hereunto set his hand the day and year first above written.

NASTECH PHARMACEUTICAL COMPANY INC.

By /s/ Vincent D. Romeo
   -------------------------------
   VINCENT D. ROMEO

  /s/ Andrew P. Zinzi
----------------------------------
  ANDREW P. ZINZI

                                       (9)

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