Exhibit 10.21
AMENDMENT TO EXECUTIVE EMPLOYMENT AGREEMENT

This amendment (this “Amendment”) to that certain Executive Employment
Agreement, dated April 22, 2019 (the “Employment Agreement”), as amended by the
Severance Plan Addendum to Executive Employment Agreement adopted by the Board
of Directors on September 18, 2019 (the “Plan Addendum”) (collectively, the
“Kelley Agreement”) by and between E. Lynne Kelley, MD, FACS (“Employee”) and X4
Pharmaceuticals, Inc. (the “Company”) is entered into as of this 5th day of
March, 2020.

WHEREAS, on February 10, 2020, the Compensation Committee of the Board of
Directors of the Company (the “Board”) voted to adjust certain aspects of
Employee’s compensation terms; and

WHEREAS, the parties wish to amend the Employment Agreement to reflect the
Board’s decision.

In consideration and in furtherance of Employee’s continued at-will employment
with the Company, Employee and the Company agree as follows:

1.The below existing language in Section 2(c)(ii) in the Employment Agreement
shall be entirely replaced by the replacement language beneath it, and, for
avoidance of doubt, all other language in Section 2(c)(ii) shall remain
unchanged:

Existing language: “For the purposes of this Agreement, “Good Reason” shall
mean: (A) a material reduction in Executive’s then-current Base Salary; (B) a
material diminution in Executive’s authority, duties, or responsibilities; (C) a
material change in the geographic location at which the Executive provides
services to Company outside of a fifty (50) mile radius from the then-current
location; or (D) any action or inaction by Company that constitutes a material
breach of this Agreement; provided that “Good Reason” shall not be deemed to
have occurred unless: (1) Executive provides Company with written notice that
Executive intends to terminate Executive’s employment hereunder for one of the
grounds set forth above within thirty (30) days of such ground first occurring,
(2) if such ground is capable of being cured, Company has failed to cure such
ground within a period of thirty (30) days from the date of such written notice,
and (3) Executive terminates Executive’s employment within seventy five (75)
days from the date that Good Reason first occurs. For purposes of clarification,
the above-listed conditions shall apply separately to each occurrence of Good
Reason and failure to adhere to such conditions in the event of Good Reason
shall not disqualify Executive from asserting Good Reason for any subsequent
occurrence of Good Reason.”

Replacement language: “For the purposes of this Agreement, “Good Reason” for
resignation from employment with the Company means that any of the following
actions are taken by the Company without Executive’s prior written consent: (A)
a material reduction in Executive’s base salary, which the parties agree is a
reduction of at least 10% of Executive’s base salary (unless pursuant to a
salary reduction program applicable generally to the Company’s similarly
situated employees); or (B) a material reduction in Executive’s duties, position
or responsibilities; or (C) relocation of Executive’s principal place of
employment to a place that increases Executive’s one-way commute by more than
sixty (60) miles as compared to Executive’s then-current principal place of
employment immediately prior to such relocation. In order to resign for Good
Reason, Executive must provide written notice to the Company’s CEO within 30
days after the first occurrence of the event giving rise to Good Reason setting
forth the basis for Executive’s resignation, allow the Company at least 30 days
from receipt of such written notice to cure such event, and if such event is not
reasonably cured within such period, Executive must resign from all positions
Executive then holds with the Company not later than 90 days after the
expiration of the cure period.”

2. The below existing language in Section 4(b)(i) and 4(b)(ii) of the Employment
Agreement shall be entirely replaced by the replacement language beneath it:

Existing language: “(i) Company shall pay Executive an amount equal to
continuation of Executive’s monthly Base Salary for a six (6) month period, with
such payments to be made in accordance with Company’s normal payroll practices
and schedules, less all customary and required taxes and employment-related
deductions. (ii) Company shall pay Executive a pro-rata portion of Executive’s
at-target Annual Bonus for the calendar year in which the termination occurs
based on the period worked by Executive during such calendar year prior to
termination, with

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

such payment to be made in on one lump sum in accordance with Company’s normal
payroll practices and schedules, less all customary and required taxes and
employment-related deductions.

Replacement language: “(i) Company shall pay Executive an amount equal to
continuation of Executive’s monthly Base Salary for a six (6) month period, with
such payments to be made in accordance with Company’s normal payroll practices
and schedules, less all customary and required taxes and employment-related
deductions; provided, however, that if Executive’s resignation or termination
under this Section occurs within twelve (12) months after a Change of Control
(as defined below), then the Company shall instead pay Executive an amount equal
to continuation of Executive’s monthly Base Salary for a twelve (12) month
period, with such payments to be made in accordance with Company’s normal
payroll practices and schedules, less all customary and required taxes and
employment-related deductions. (ii) Company shall pay Executive a pro-rata
portion of Executive’s at-target Annual Bonus for the calendar year in which the
termination occurs based on the period worked by Executive during such calendar
year prior to termination, with such payment to be made in on one lump sum in
accordance with Company’s normal payroll practices and schedules, less all
customary and required taxes and employment-related deductions; provided,
however, that if Executive’s resignation or termination under this Section
occurs within twelve (12) months after a Change of Control (as defined below),
then the Company shall instead pay Executive an amount equal to Executive’s full
Annual Bonus for the calendar year in which the termination occurs in advance of
such Annual Bonus being earned, with such payment to be made in on one lump sum
in accordance with Company’s normal payroll practices and schedules, less all
customary and required taxes and employment-related deductions.

For avoidance of doubt, this Amendment shall supersede Sections 4.1.1 and 4.1.3
of the Plan Addendum and the parties acknowledge and agree that the amendments
to the Plan Addendum described herein shall only apply to Employee.

This Amendment may be executed in several counterparts, all of which taken
together shall constitute one single agreement between the parties. Except as
amended hereby, all of the terms and conditions of the Employment Agreement and
the Kelley Agreement shall remain and continue in full force and effect.

IN WITNESS WHEREOF, the parties have executed this Amendment as of the date
first written above.

X4 Pharmaceuticals, Inc.E. Lynne Kelley, MD, FACS an individual By: /s/ Paula M.
Ragan/s/ Lynne KelleyName: Paula M. Ragan, Ph.D.E. Lynne Kelley, MD, FACSTitle:
CEO