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                                                                   EXHIBIT 10.18

                   [SIGNAL PHARMACEUTICALS, INC. LETTERHEAD]

December 8, 1993

Alan Lewis Ph.D.
5 Shara Lane
Pennington, NJ 08534

Dear Alan:

Signal Pharmaceuticals, Inc. ("Signal" or the "Company") is pleased to offer
you the position of President and Director. Your employment with Signal will
commence on February 1, 1994. We would expect that you would spend three to
four days at Signal in the intervening period and attend the Board of Directors
Meeting on January 19, 1994.

Your base salary will be $225,000 annually, less all required withholding for
taxes and social security. A bonus of 25% of salary will be awarded based upon
performance against measurable, mutually agreed upon goals. Individual goals
will be assigned specific percent weighting. Thus if all goals were met a bonus
of up to 150% (of the 25%) would exist. For calendar year 1994 Signal will
guarantee a minimum bonus of $25,000.

Signal will offer you a signing bonus of $50,000. The bonus must be returned to
Signal if you leave voluntarily before the end of the first year (February 1,
1995).

In addition, under Signal's 1993 Long-Term Incentive Plan (the "Plan"), the
Company will offer you an opportunity to option or purchase outright 450,000
(four hundred fifty thousand) shares of SPI Common Stock at the current fair
market value of $0.10/share. Under the Plan, one-tenth (10%) of the shares will
vest on your first day of employment, one-tenth (10%) will vest on the first
day of your thirteenth month of employment. The remaining shares will vest in
equal quarterly installments over a four year period. Note that the initial
grant of 10% of the option or purchase must be returned to the Company if you
leave voluntarily before the end of the first year (February 1, 1995).

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Alan Lewis, Ph.D.
December 08, 1993
page 2

An additional bonus option of 150,000 (one hundred fifty thousand) shares at
$0.10/share will be granted and vest as described above. This bonus option is
contingent upon you being appointed the Chief Executive Officer of Signal. The
criteria for appointment to the CEO position would be successful performance
against 1994 and 1995 incentive bonus goals mutually agreed upon with the Board
of Directors.

All shares in Signal are subject to our standard purchase agreement. Signal
shares will not be marketable until such time as an initial public offering has
been successfully completed, and the "lock-up" period has elapsed. While the
company is privately held, the company maintains a first right of refusal on
any potential sale of shares which you own.

Signal will reimburse you for relocation expenses involved in moving your
family to the San Diego area. Signal will also reimburse you for a house
hunting trip for you and your family.

You will be eligible for the standard Company benefits. Signal reserves the
right to modify compensation and benefits from time to time as it deems
necessary.

Signal will reimburse you for the rental cost of a furnished townhouse or
equivalent until such time as your family has relocated and you have permanent
housing. Signal will pay this rental cost until September 1, 1994 if necessary.

Signal will reimburse you for one round trip air fare per month to visit your
family in New Jersey prior to their move to San Diego.

Signal will assist you with the financing of a home purchase. The specifics will
depend upon your investigation of the local real estate market. For example,
Signal will loan you up to $300,000 for the purchase of a family residence in
the San Diego area. This loan would be interest bearing only for the first five
years at the lowest interest rate permissible by the IRS. The loan could be
extended for an additional five years and fully amortized over those five years
at the lowest interest rate permissible by the IRS.

If you visit Signal during the end of December, Signal will reimburse you for
round-trip travel expenses to San Diego for you and your family.

You employment agreement is for one year. Your employment is renewable on an
annual basis.
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Alan Lewis, Ph.D.
December 08, 1993
page 3

You will be asked to sign an Employee's Proprietary Information and Inventions
Agreement. In addition, you will be expected to abide by the Company rules and
regulations as described in the Company handbook. The employment terms in this
letter supersede any other agreements or promises made to you. As required by
law, this offer is subject to satisfactory proof of your right to wok in the
United States.

This offer will expire on Saturday December 11, 1993.

Please indicate your acceptance of this offer by signing one of the originals
and returning it to me. Please fax a signed copy to me at (619)-456-4041.

Brook, Luke, Pat and I look forward to working with you and helping build
Signal into a successful public pharmaceutical company.

Sincerely,

/s/ HARRY F. HIXSON, JR.
--------------------------
Harry F. Hixson, Jr.
Director

ACCEPTED BY:

/s/ ALAN J. LEWIS
--------------------------

1-4-94
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Date:<PAGE>   1
                                                            EXHIBIT 10.19
[SIGNAL PHARMACEUTICALS, INC. LETTERHEAD]

March 4, 1994

David W. Anderson, Ph.D.
14446 Trailwind Drive
Poway, CA 92064

Dear David:

Signal Pharmaceuticals, Inc. ("Signal" or the "Company") is pleased to offer you
the position of Vice President, Drug Discovery and Preclinical Development
reporting to Alan J. Lewis (President). Your employment with Signal will
commence on March 24.

Your starting salary will be $165,000 annually, less all required withholding
for taxes and social security. In addition Signal is offering a one time signing
bonus of $25,000 to help you quickly make the transition to join us. You will be
eligible for the standard Company benefits. Signal reserves the right to modify
compensation and benefits from time to time as it deems necessary,

In addition, under Signal's 1993 Long-Term Incentive Plan (the "Plan"), the
Company will offer you an opportunity to purchase outright 200,000 shares of SPI
Common Stock at the current fair market value, subject to approval by the
Compensation Committee of the Board of Directors. Under the Plan, one-fifth
(20%) of the shares will vest on the first day of your thirteenth month of
employment. The remaining shares will vest in equal quarterly installments over
a four year period. The invested shares will be subject to repurchase by the
company at their cost to you, should you leave the company before all shares
have vested. In addition, all shares issued under the Plan are subject to
certain limitations on the sale and use of the stock,

Your employment is for an indefinite term. In other words, the employment
relationship is "at will," and either you or SPI has the right to terminate that
employment relationship at any time.

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David W. Anderson,Ph.D.
March 4, 1994
page 2

You will be required to sign an Employee's Proprietary Information and
Inventions Agreement before your employment begins. In addition, you will be
expected to abide by the Company rules and regulations as described in the
Company handbook.

The employment terms in this letter supersede any other agreements or promises
made to you. As required by law, this offer is subject to satisfactory proof of
your right to work in the United States.

Please indicate your acceptance of this offer by signing one of the originals
and returning it to me by the close of business on Wednesday, March 8, 1994, at
which time this offer will lapse.

We have enjoyed our meetings with you and we look forward to your joining the
Signal team. We feel that your experience and enthusiasm can make a substantial
contribution to the success of Signal.

Sincerely,

/s/ ALAN LEWIS (LJC)

Alan J. Lewis, Ph.D.
President

ACCEPTED BY:

/s/ DAVID W. ANDERSON
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David W. Anderson

3/8/94
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Date:<PAGE>   1
[SIGNAL PHARMACEUTICALS, INC. LETTERHEAD]                          EXHIBIT 10.20

August 18, 1994

Bradley B. Gordon
P.O. Box 9173
Rancho Santa Fe, CA 92067

Dear Brad:

Signal Pharmaceuticals, Inc. ("Signal" or the "Company") is pleased to offer you
the position of Vice President Finance and Chief Financial Officer, reporting to
the President. Your employment with Signal will commence on August 29, 1994.

Your base salary will be $130,000 annually, less all required withholding for
taxes, social security and other required deductions. A bonus of up to 20% of
salary will be awarded based upon performance against measurable, mutually
agreed upon goals. Individual goals will be assigned specific percent weighting.

In addition, under Signal's 1993 Long-Term Incentive Plan (the "Plan"), the
Company will offer you an option to purchase 150,000 (one hundred and fifty
thousand) shares of SPI Common Stock at fair market value (currently 10 cents a
share). Under the Plan, one-fifth (20%) of the shares will vest on the first day
of your thirteenth month of employment. The remaining shares will vest 5% per
quarter over the next four years.

All shares in Signal are subject to our standard purchase agreement. Signal
shares will not be marketable until such time as an initial public offering has
been successfully completed, and the "lock-up" period has elapsed. While the
company is privately held, the company maintains a first right of refusal on any
potential sale of shares which you own.

You will be eligible for the standard Company benefits. Signal reserves the
right to modify compensation and benefits from time to time as it deems
necessary.

Your employment is "at will," however to cover the unlikely event that your
employment is terminated in the first year without cause, we will offer you

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Brad Gordon
August 18, 1994
page 2

continued salary for a maximum of 6 months or until you find a new job, if
sooner. Under this circumstance you will vest shares according to the number of
months worked at the company. After the first year, the maximum salary
continuance will be 3 months. If you choose to leave at your own will or you are
terminated for cause there will be no salary continuation.

You will be asked to sign an Employee's Proprietary Information and Inventions
Agreement. In addition, you will be expected to abide by the Company rules and
regulations as described in the Company handbook. The employment terms in this
letter supersede any other agreements or promises made to you. As required by
law, this offer is subject to satisfactory proof of your right to work in the
United States.

Please indicate your acceptance of this offer by signing one of the originals
and returning it to me. Please fax a signed copy to me at (619)558-7513

Due to the importance of this position we must receive a speedy acceptance to
this offer. If we have not received a signed acceptance from you prior to 5pm
California time on Monday, August 22, 1994, this offer will expire and we will
have to move forward with the other candidates.

Brad we are very excited about the prospect of your joining the Signal team and
hope you are pleased with this attractive offer. Please call me if you have any
additional questions.

Sincerely,

/s/ ALAN J. LEWIS

Alan J. Lewis
President

                                        ACCEPTED BY:

                                        /s/ BRADLEY B. GORDON
                                        ----------------------------------------

                                        8-22-94
                                        ----------------------------------------
                                        Date:

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