Document:

Exhibit
4.1

AMENDMENT
TO RIGHTS AGREEMENT

THIS AMENDMENT (the “Amendment”),
dated as of the 29th day of January, 2007, to the Rights Agreement
(the “Rights Agreement”), dated May
6, 2003, as amended March 4, 2005, between ALLOS
THERAPEUTICS, INC., a Delaware corporation (the “Company”) and MELLON INVESTOR SERVICES LLC (the “Rights
Agent”), is being executed at the direction of the Company and
shall be effective immediately prior to the Effective Time (as defined
below).  For purposes of this Amendment,
the “Effective Time” shall mean the
execution of that certain Purchase Agreement to be entered into on or about the
date hereof by and between the Company and Merrill Lynch & Co., Merrill
Lynch, Pierce, Fenner & Smith Incorporated and each of the underwriters
named in Schedule A thereto; provided, however, if (i) the Purchase Agreement is not executed
within 30 days from the date of this Amendment, of which the Company will
notify the Rights Agent in writing, or (ii) the Purchase Agreement terminates
or is terminated prior to the payment for and delivery of the shares for sale
thereunder, of which the Company will notify the Rights Agent in writing, this
Amendment shall terminate immediately upon the Rights Agent’s receipt of such
notice and be of no further force and effect.  Capitalized terms used in this
Amendment and not otherwise defined herein shall have the meanings given them
in the Rights Agreement.

WHEREAS,
Section 27 of the Rights Agreement provides that the Company and the Rights
Agent shall, if the Company so directs, supplement or amend any provision of
the Rights Agreement without the approval of any holders of the Rights, any
such supplement or amendment to be evidenced by a writing signed by the Company
and the Rights Agent; and

WHEREAS,
pursuant to Section 27 of the Rights Agreement, the Company has delivered to
the Rights Agent a certificate signed by an authorized officer of the Company
certifying that the proposed amendment of the Rights Agreement is in compliance
with the terms of Section 27 of the Rights Agreement.

NOW, THEREFORE,
in consideration of the foregoing and the agreements, provisions and covenants
herein contained, the parties agree as follows:

1.             Section 1(h)
shall be amended and restated in its entirety as follows:

“Excluded
Stockholder” shall include each of (i) Warburg Pincus Private
Equity VIII, L.P. (including its Affiliates and Associates) (“Warburg”); provided, however, that Warburg shall not be an Excluded
Stockholder if, without the prior approval of the Board of Directors, Warburg
becomes the Beneficial Owner of more than 44% of (x) the outstanding Common
Shares, plus (y) the Common Shares issuable upon the exchange of the Company’s
outstanding Series A Exchangeable Preferred Stock (including any quarterly
accruing dividends thereon) (the “Exchange Shares”), calculated as if such
Exchange Shares had been issued pursuant to an exchange as of immediately
following the original issuance of each such share of outstanding Series A
Exchangeable Preferred Stock and (ii) Baker Brothers Life Sciences, L.P.
(including its Affiliates and Associates) (“Baker”); provided,
however, that Baker shall not be an Excluded Stockholder if,

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without the prior
approval of the Board of Directors, Baker becomes the Beneficial Owner of more
than 20% of the outstanding Common Shares.

2.             The
Rights Agreement shall not otherwise be supplemented or amended by virtue of
this Amendment, but shall remain in full force and effect.  This Amendment may be executed in one or more
counterparts, all of which shall be considered one and the same amendment and
each of which shall be deemed an original.

[Remainder
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IN WITNESS WHEREOF, the
parties hereto have caused this Amendment to be duly executed and attested, all
as of the day and year first above written.

	
  Allos Therapeutics,
  Inc.

  	
   

  	
  Mellon Investor
  Services LLC, as Rights Agent

  
	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  
	
  By:

  	
   

  	
  /s/ Marc Graboyes

  	
   

  	
  By:

  	
  /s/ Lisa Porter

  
	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  
	
  Name: Marc Graboyes

  	
   

  	
  Name: Lisa Porter

  
	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  	
   

  
	
  Title: Vice President, General Counsel &
  Secretary

  	
   

  	
  Title: Officer

  

 

 3Exhibit
10.1

BAKER BROS. INVESTMENTS, L.P.

BAKER BROS. INVESTMENTS II, L.P.

BAKER/TISCH INVESTMENTS, L.P.

BAKER BIOTECH FUND I, L.P.

14159, L.P.

BAKER BROTHERS LIFE SCIENCES, L.P.

667 Madison Avenue, 17th Floor

New York,
New York 10021

January 28, 2007

Allos Therapeutics, Inc.

11080 CirclePoint Road

Westminster, CO 80020

Attention: Chief
Executive Officer

Gentlemen:

In connection with the
proposed acquisition of shares of Common Stock, par value $0.001 per share (the
“Common Stock”), of Allos
Therapeutics, Inc., a Delaware corporation (the “Company”),
in connection with the registered offering of Common Stock pursuant to a
Registration Statement on Form S-3 (File No. 333-134965) (the “Offering”) by each of Baker Brothers
Life Sciences, L.P., Baker/Tisch Investments, L.P., Baker Biotech Fund I, L.P.,
and 14159, L.P. (collectively, the “Purchaser Group”),
the Company and the Purchaser Group agree as follows:

1.             Definitions.  For purposes of this letter
agreement, the following terms have the respective meanings set forth below:

(a)           “Affiliates” shall mean any fund,
whether existing now or in the future, that (i) directly, or indirectly through
one or more intermediaries, controls or is controlled by, or is under common
control with, any member of the Purchaser Group, or (ii) is otherwise an
affiliate (as defined in Rule 405 under the Securities Act of 1933, as amended)
of any member of the Purchaser Group.

(b)           “Beneficially Owns” (including the
terms “Beneficial Ownership”, “Beneficially Owned” or “Beneficially Owning”) shall mean
beneficial ownership within the meaning of Rule 13d-3 under the Exchange Act.

(c)           “Board” shall mean the Board of
Directors of the Company.

(d)           “Effective Time” shall mean the time
and date of sale and delivery of the shares of Common Stock by the Company
pursuant to Section 2(b) of the Purchase Agreement.

 

(e)           “Independent Directors”
shall mean those directors that the Board has determined to be independent
within the meaning of NASD Marketplace Rule 4200(15) (or any successor rule).

(f)            “Purchase Agreement” shall mean that
certain Purchase Agreement to be entered into on or about the date hereof by
and between the Company and Merrill Lynch & Co., Merrill Lynch, Pierce,
Fenner & Smith Incorporated and each of the Underwriters named in Schedule
A thereto, with respect to the issue and sale by the Company and the purchase
by the Underwriters, acting severally and not jointly, of the respective
principal amounts set forth in Schedule A thereto of an aggregate of 9,000,000
shares of the Company’s Common Stock.

2.             Standstill.

(a)           For
a period of four years from the Effective Time (the “Standstill
Period”), no member of the Purchaser Group or any of their
respective Affiliates shall, without the prior written consent of a majority of
the Independent Directors who are not affiliated with the Purchaser Group, in
any manner acquire, agree or seek to acquire, or make any proposal or offer
(other than to a member of the Board or senior management of the Company by
means that would not cause public dissemination thereof) to acquire, whether
directly or indirectly:

(i)            any
material assets of the Company; or

(ii)           Beneficial
Ownership of any shares of Common Stock, voting equity securities of the
Company or any securities convertible or exchangeable into or exercisable for
any such shares of Common Stock or other securities (including derivatives), in
excess of twenty (20)% of the outstanding Common Stock (the “Permitted Shares”).

(b)           For
so long as the Purchaser Group and their Affiliates Beneficially Own more than
ten percent (10%) of the Company’s outstanding Common Stock, no member of the
Purchaser Group or any of their respective Affiliates shall, without the prior
written consent of a majority of the Independent Directors who are not
affiliated with the Purchaser Group:

(i)            propose
to any person (other than to a member of the Board or senior management of the
Company by means that would not cause public dissemination thereof) or effect,
seek to effect or enter into, whether alone or in concert with others, any
merger, tender offer, consolidation, acquisition, scheme, business combination
or other extraordinary transaction in which the Company or any of its
subsidiaries is a constituent corporation or party (a “Business
Combination”);

(ii)           solicit
proxies or stockholder consents or participate in any such solicitation for any
purpose relating to the election or removal of directors of the Company;

(iii)         support,
solicit proxies or stockholder consents or participate in any such solicitation
or vote in favor of any Business Combination, or propose to any person or
effect, seek to effect or enter into, whether alone or in concert with others,
any Business Combination, in which the Purchaser Group receives or would be
entitled to receive consideration on a per share basis which is greater than
the consideration to be received on a per share basis by the other holders of
Common Stock;

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(iv)          form,
join, encourage, influence, advise or participate in a “group” (as defined in
Section 13(d)(3) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”)) in connection with
any of the foregoing;

(v)            make,
or take any action (including a request to waive or amend any provision of this
agreement) that would cause the Company to make, a public announcement regarding
any intention of the Purchaser Group or any of their respective Affiliates to
take an action which would be prohibited by any of the foregoing.

3.             Waiver of Section 203.  The
Company represents and warrants to the Purchaser Group that the Board has taken
all action necessary to approve the acquisition by the Purchaser Group of the
Common Stock in the Offering for the purposes of the provisions of Section 203
of the General Corporation Law of the State of Delaware (“Section
203”) solely as it relates to the acquisition by the Purchaser
Group of beneficial ownership of the Common Stock (the “Waiver”);
provided, however,
such Waiver provides that, to the fullest extent permitted by law, it shall no
longer be applicable if, subsequent to becoming an “interested stockholder” (as
defined in Section 203), the Purchaser Group no longer has Beneficial Ownership
of fifteen percent (15%) or more of the Common Stock as a result of any sale or
disposition of Beneficial Ownership of Common Stock by the Purchaser Group.

4.             Amendments to Rights Agreement.  The
Company represents and warrants to the Purchaser Group that the Rights
Agreement, dated May 6, 2003, amended March 4, 2005, by and between the Company
and Mellon Investor Services LLC, as rights agent (the “Rights
Agreement”), shall be duly amended, effective immediately prior
to the Effective Time, to exclude the Purchaser Group from the definition of
the term “Acquiring Person” as such term may relate to the acquisition by the
Purchaser Group (including by “affiliates” and “associates”, as such terms are
defined in Rule 12b-2 under the Exchange Act, of the Purchaser Group) of
Beneficial Ownership of the Permitted Shares described in Section 2(a)(ii)
hereof.  During the Standstill Period, so
long as the Purchaser Group is not in material breach of its obligations under
this Agreement, the Company shall not amend or modify the definition of “Acquiring
Person” in the Rights Agreement, if, as a result of such amendment or
modification, the Purchaser Group would be deemed to be an “Acquiring Person”
thereunder.  During the Standstill
Period, so long as the Purchaser Group is not in material breach of its
obligations under this Agreement, the Company shall not adopt a new rights
agreement or an agreement having substantially the same effect of the Rights
Agreement if the Purchaser Group would be considered an “Acquiring Person” (or
would have the same or substantially similar effect of an “Acquiring Person”
under the Rights Agreement).

5.             Representations.  Each
party represents to the other that: (a) this letter agreement has been duly
authorized by all necessary corporate or partnership action, as the case may
be; and (b) this letter agreement is a valid and binding agreement of such
party, enforceable against it in accordance with its terms.

6.             Specific Enforcement; Legal Effect.  The
parties hereto agree that any breach of this letter agreement would result in
irreparable injury to other party and that money damages would not be an
adequate remedy for such breach.  Accordingly,
without prejudice to the rights and remedies otherwise available under
applicable law, either party shall be entitled to specific performance and
equitable relief by way of injunction or otherwise if the other party breaches
or threatens to breach any of the provisions of this letter agreement.  It is further understood and agreed that no
failure or delay by

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either party in exercising any right, power or privilege hereunder
shall operate as a waiver thereof, nor shall any single or partial exercise
thereof preclude any other or further exercise thereof or the exercise of any
right, power or privilege hereunder.  If
any term, provision, covenant or restriction in this letter agreement is held
by a court of competent jurisdiction to be invalid, void or unenforceable, the
remainder of the terms, provisions, covenants and restrictions of this letter
agreement shall remain in full force and effect and shall in no way be
affected, impaired or invalidated, provided that the parties hereto shall
negotiate in good faith to attempt to place the parties in the same position as
they would have been in had such provision not been held to be invalid, void or
unenforceable.  This letter agreement
contains the entire agreement between the parties hereto concerning the matters
addressed herein.  No modification of
this letter agreement or waiver of the terms and conditions hereof shall be
binding upon either party hereto, unless approved in writing by each such
party; provided, however,
that no waiver or amendment shall be effective as against the Company unless
such waiver or amendment is approved in writing by the vote a majority of the
independent members of the Board who are not affiliated with the Purchaser
Group.  This Agreement shall be governed
by and construed in accordance with the law of the State of Delaware without
regard to principles of conflicts of law that would cause the application of
the laws of any jurisdiction other than the State of Delaware.

7.             Termination.  This agreement shall continue
in full force and effect from the Effective Time until such time as the
Purchaser Group and their Affiliates Beneficially Own less than ten percent
(10%) of the Company’s outstanding Common Stock; provided,
however, that if (i) the Purchase Agreement is not executed
within thirty (30) days from the date of this agreement, or (ii) the Purchase
Agreement terminates or is terminated prior to the payment for and delivery of
the shares for sale thereunder, this agreement shall terminate immediately upon
such date and be of no further force and effect.

8.             Counterparts.  This letter agreement may be
executed in counterpart (including by facsimile), each of which shall be deemed
an original.

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If you are in
agreement with the terms set forth above, please sign this letter agreement in
the space provided below and return an executed copy to the undersigned.

Very truly yours,

BAKER
BROS. INVESTMENTS, L.P.

By:  Baker Bros. Capital, L.P.,

its general partner

By:  Baker Bros. Capital (GP), LLC,

its general partner

By:          /s/ Julian Baker                     

Name:
Julian Baker

Title:   Managing Member

BAKER
BROS. INVESTMENTS II, L.P.

By:  Baker Bros. Capital, L.P.,

its general partner

By:  Baker Bros. Capital (GP), LLC,

its general partner

By:          /s/ Julian Baker                     

Name:
Julian Baker

Title:   Managing Member

BAKER /
TISCH INVESTMENTS, L.P.

By:  Baker / Tisch Capital, L.P.,

its general partner

By:  Baker / Tisch Capital (GP), LLC,

its general partner

By:          /s/ Julian Baker                     

Name:
Julian Baker

Title:  Managing Member

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BAKER
BIOTECH FUND I, L.P.

By:  Baker Biotech Capital, L.P.,

its general partner

By:  Baker Biotech Capital (GP), LLC,

its general partner

By:          /s/ Julian Baker                     

Name:
Julian Baker

Title:   Managing Member

14159,
L.P.

By:  14159 Capital, L.P.,

its general partner

By:  14159 Capital (GP), LLC,

its general partner

By:          /s/ Julian Baker                     

Name:  Julian Baker

Title:   Managing Member

BAKER
BROTHERS LIFE SCIENCES, L.P.

By:  Baker Brothers Life Sciences Capital, L.P.,

its general partner

By:  Baker Brothers Life Sciences Capital (GP),

LLC, its general partner

By:          /s/ Julian Baker                     

Name:  Julian Baker

Title:   Managing Member

 

Confirmed and Agreed:

ALLOS THERAPEUTICS, INC.

By:   /s/
Marc H. Graboyes                

Name:     Marc H.
Graboyes

Title:       Vice President, General Counsel

 

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