Exhibit 10.50

AMENDED AND RESTATED SIXTH AMENDMENT TO

SECOND AMENDED AND RESTATED OPERATING AGREEMENT

OF

TPG/CALSTRS, LLC

THIS AMENDED AND RESTATED SIXTH AMENDMENT TO SECOND AMENDED AND RESTATED
OPERATING AGREEMENT OF TPG/CALSTRS, LLC (this “Amendment”), is entered into
effective as of February 19, 2010, by and between CALIFORNIA STATE TEACHERS’
RETIREMENT SYSTEM, a public entity (“Investor”), and THOMAS PROPERTIES GROUP,
L.P., a Maryland limited partnership (“Operator”).

Operator and Investor entered into that certain Sixth Amendment to Second
Amended and Restated Operating Agreement of TPG/CalSTRS, LLC as of February 19,
2010 (the “Original Sixth Amendment”). Operator and Investor now desire change
certain of the terms and conditions thereof and hereby amend and restate and
supersede the Original Sixth Amendment in its entirety as set forth below.
Effective as of the Effective Date, the Original Sixth Amendment shall be null
and void and of no force and effect.

RECITALS

A. Investor and Operator, as the sole members of TPG/CalSTRS, LLC, a Delaware
limited liability company (the “Company”), entered into that certain Second
Amended and Restated Operating Agreement of TPG/CalSTRS, LLC, dated as of
October 13, 2004 (the “Original Agreement”), as amended by that certain First
Amendment to Second Amended and Restated Operating Agreement of TPG/CalSTRS,
LLC, dated as of June 8, 2006, that certain Second Amendment to Second Amended
and Restated Operating Agreement of TPG/CalSTRS, LLC, dated as of May 25, 2007,
that certain Third Amendment to Second Amended and Restated Operating Agreement
of TPG/CalSTRS, LLC, dated as of February 1, 2008, that certain Fourth Amendment
to Second Amended and Restated Operating Agreement dated as of November 5, 2008,
and that certain Fifth Amendment to Second Amended and Restated Operating
Agreement dated as of October 30, 2009 (collectively with the Original
Agreement, the “Agreement”). All capitalized terms not otherwise defined herein
shall have the meanings set forth in the Agreement.

B. One of the Projects owned by the Company is that certain office complex
commonly known as City National Plaza, located at 505 – 555 South Flower Street,
Los Angeles, California (the “City National Plaza Project”). The current
ownership structure of the City National Plaza Project is as set forth on
Exhibit “B” to the Master Agreement (defined below).

C. Effective as of the date hereof, Investor and Operator have entered into that
certain Master Agreement for Debt and Equity Restructure of City National Plaza,
as amended by that certain Amended and Restated Master Agreement for Debt and
Equity Restructure of City National Plaza (as amended, the “Master Agreement”)
with respect to the City National Plaza Project. As described in the Master
Agreement, CNP Investor, LLC, a limited liability company (“CNP Investor”) that
is a wholly owned subsidiary of Investor, has agreed to purchase various
mezzanine loans made to Title Holding Subsidiaries of the Company to finance the
City National Plaza Project (the “Mezzanine Loans”). Pursuant to the Master
Agreement, CNP Investor, Investor, Operator, the Company, and various other
parties, agreed that CNP Investor would distribute the Mezzanine Loans to
CalSTRS, which would then convert the Mezzanine Loans into an increased equity
interest in the Company solely as it relates to the Company’s interest in the
City National Plaza Project upon the satisfaction of certain conditions set
forth therein (the “Conversion”).

D. Upon exercise of the Conversion, the ownership structure of the City National
Plaza Project will be as shown on Exhibit “E” to the Master Agreement. At such
time, Investor will own an increased interest in the City National Plaza Project
through the Company and the parties hereto shall execute that certain Eighth
Amendment to Second Amended and Restated Operating Agreement of TPG/CalSTRS to
reflect the Conversion.

--------------------------------------------------------------------------------

E. In connection with the transactions contemplated in the Master Agreement,
Investor and Operator wish to further amend the Agreement as set forth herein.

AGREEMENT

NOW, THEREFORE, in consideration of the mutual covenants and agreements set
forth herein, and other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, Investor and Operator hereby agree
as follows effective as of the date of the acquisition by CNP Investor of the
Mezzanine Loans:

1. A new Section 4.05 is hereby added to the Agreement as follows:

4.05 Capital Contributions for City National Plaza

It is the parties’ intention to refinance the loan secured by the mortgage
encumbering the City National Plaza Project (the “Mortgage Indebtedness”) on or
prior to its maturity date. Anything to the contrary in this Agreement
notwithstanding, Investor shall have the right unilaterally to call for
Additional Contributions to the capital of the Company for the purpose of
repaying upon maturity (whether scheduled or accelerated) the Mortgage
Indebtedness. Such capital call shall not be subject to the requirements of
Sections 2.02(c), 4.01 or 4.03 nor Exhibit D of this Agreement. If Investor
determines to make a capital call in accordance with this Section 4.05, it shall
deliver written notice to Operator specifying the total amount of capital
required and the amount representing the share of each Member. Within ten
(10) business days of receipt of the written notice from Investor of a capital
call in accordance with this Section 4.05, each Member shall contribute its
share of the capital requested. Once contributed, the Capital Contributions
shall be applied to repay the Mortgage Indebtedness. Any Capital Contributions
funded pursuant to this Section 4.05 shall be excluded from the calculation of
the maximum capital commitments set forth in Section 4.01(a) and (b) of this
Agreement and the additional contribution caps set forth in Section 4.01(c) of
this Agreement. If Operator fails to fund its portion of the Additional
Contribution called for in this Section 4.05, then Investor shall elect one of
the following two options: (1) Investor shall receive a refund from the Company
of its Additional Contribution made pursuant to this Section, or (2) Investor
shall fund the entire amount required to repay the Mortgage Indebtedness in
full. If Investor elects the latter option, then Investor shall be treated as
having an independent equity interest in the Project commensurate on a
dollar-for-dollar basis with the outstanding principal, interest and other
payments due under the Mortgage Loan at the time of the capital call provided
for herein, in a manner similar to, and on substantially the same terms as, the
Conversion.

2. The following new definitions are hereby added to Exhibit A to the Agreement
in their appropriate alphabetical order:

“City National Plaza Project” means that certain office complex commonly known
as City National Plaza, located at
505 – 555 South Flower Street, Los Angeles, California.

“CNP Investor” means CNP Investor, LLC, a Delaware limited liability company.

“COD” is defined in Exhibit “C”.

--------------------------------------------------------------------------------

“Mortgage Indebtedness” is defined in Section 4.05.

“Operator’s Deferred COD Income Amount” is defined in Exhibit “C”.

“Section 108(i) Election” is defined in Exhibit “C”.

“Sixth Amendment” is defined in Exhibit “C”.

“TPG Plaza Investments” means TPG Plaza Investments, LLC, a Delaware limited
liability company, which is a Title Holding Subsidiary of the Company.

3. The following new Section 2.8 is hereby added to Exhibit C to the Agreement:

Section 2.8 COD Income Arising from Purchase of Mezzanine Loans; Election Under
Section 108(i) of the Code. The Members recognize and agree that the purchase of
the Mezzanine Loans described in Recital C of the Sixth Amendment to Second
Amended and Restated Operating Agreement of TPG/CalSTRS, LLC (the “Sixth
Amendment”) will result in the recognition of cancellation of indebtedness
(“COD”) income by TPG Plaza Investments and an allocation of such COD income to
the Company from TPG Plaza Investments. This Agreement does not specify how such
COD income is to be allocated. Instead the Tax Matters Partner shall cause the
Company’s accountants to allocate the COD income in accordance with applicable
income tax laws and regulations. The Operator has requested that TPG Plaza
Investments make the election described in Section 108(i) of the Code [and
Section 4.01 of Revenue Procedure 2009-37, 2009-36 I.R.B.] (the “Section 108(i)
Election”) with respect to all or such portion, to be determined by Operator in
its sole discretion and communicated to TPG Plaza Investments in a timely
manner, of the COD income that is ultimately to be allocated to Operator as
provided in the immediately preceding sentence (the “Operator’s Deferred COD
Income Amount”). The Operator has further requested that the Company apportion
the entire Operator’s Deferred COD Income Amount to the Operator as permitted in
Section 4.12 of Revenue Procedure 2009-37. The Management Committee has agreed
to accommodate the Operator’s requests by (i) causing the Company, as the sole
member of the manager of TPG Plaza Investments, to make the Section 108(i)
Election with respect to Operator’s Deferred COD Income Amount as described
above, and (ii) causing the Company to allocate the entire Operator’s Deferred
COD Income Amount as described above. The Operator understands and agrees that
neither TPG Plaza Investments nor the Company will make any election under
Section 108(i), or otherwise allocate any COD income that is subject to
Section 108(i), with respect to all or any portion of the COD income that is not
the Operator’s Deferred COD Income Amount. By executing the Sixth Amendment the
Operator further agrees as follows: (i) that it will indemnify, defend and hold
the Company, TPG Plaza Investments, CNP Investor, and Investor and Investor’s
Constituents harmless from and against any and all tax, economic or other
consequences resulting from the Section 108(i) Election; (ii) that it will
furnish to the Company and/or TPG Plaza Investments in a timely manner all
information requested by the Company and/or TPG Plaza Investments in connection
with making the Section 108(i) Election or complying at any time with
Section 108(i), Revenue Procedure 2009-37 or any other relevant Federal, state
or local income tax law or procedure; (iii) that it will assist the Company
and/or TPG Plaza Investments in complying with all relevant procedures
prescribed by the Internal Revenue Service or any relevant state or local income
tax authority, including the election procedures and the annual

--------------------------------------------------------------------------------

information statements that are described in Sections 4 and 5, respectively, of
Revenue Procedure 2009-37; and (iv) that it will reimburse the Company and/or
TPG Plaza Investments for any and all additional accounting, legal or compliance
costs associated with (x) making the Section 108(i) Election or for maintaining
and keeping the records required in connection with the Section 108(i) Election
so made, (y) filing any future returns, documents, elections or forms in
connection with the Section 108(i) Election so made and (z) dealing with any and
all income tax authorities, audits, contests or litigation in connection with
the Section 108(i) Election so made.

4. Except as expressly provided in this Amendment, all of the terms and
provisions of the Agreement remain unmodified and in full force and effect.

5. Each individual executing this Amendment on behalf of an entity hereby
represents and warrants to the other party or parties to this Amendment that
(a) such individual has been duly and validly authorized to execute and deliver
this Amendment on behalf of such entity; and (b) this Amendment is and will be
duly authorized, executed and delivered by such entity.

6. This Amendment may be executed in any number of counterparts each of which
shall be deemed an original and all of which shall constitute one and the same
agreement with the same effect as if all parties had signed the same signature
page. This Amendment shall be deemed executed and delivered upon each party’s
delivery of executed signature pages of this Amendment, which signature pages
may be delivered electronically or by facsimile with the same effect as delivery
of the originals.

[signatures begin on next page]

--------------------------------------------------------------------------------

IN WITNESS WHEREOF, Investor and Operator have executed this Amendment as of the
day and year first above written.

 

“Investor”  

CALIFORNIA STATE TEACHERS’

RETIREMENT SYSTEM, a public entity

    By:  

/s/ Christopher J. Ailman

      Christopher J. Ailman, Chief Investment Officer “Operator”  

THOMAS PROPERTIES GROUP, L.P., a

Maryland limited partnership

  By:  

THOMAS PROPERTIES GROUP, INC., a

Delaware corporation, General Partner

    By:  

/s/ John R. Sischo

      John R. Sischo, Co-Chief Operating Officer