Inclusion Criteria:

   1. Participants must be 2 years of age or older, at the time of signing the informed
   consent.

   2. Participants who are allogeneic hematopoietic stem cell transplantation (HSCT)
   recipients with active cGVHD requiring systemic immune suppression. Active cGVHD is
   defined as the presence of signs and symptoms of cGVHD per 2014 NIH Consensus
   Development Project on Criteria for Clinical trials in cGVHD.

   3. Participants with refractory or recurrent active cGVHD despite at least 2 lines of
   systemic therapy.

      - Refractory disease defined as meeting any of the following criteria:

         - The development of 1 or more new sites of disease while being treated for
         cGVHD.

         - Progression of existing sites of disease despite at least 1 month of
         standard or investigation therapy for cGVHD.

         - Participants who have not achieved a response within 3 months on their prior
         therapy for cGVHD and for whom the treating physician believes a new
         systemic therapy is required.

      - Recurrent cGVHD is active, symptomatic disease (after an initial response to
      prior therapy) as defined, based on the NIH 2014 consensus criteria, by
      organ-specific or global assessment or for which the physician believes that a
      new line of systemic therapy is required.

   4. Participants may have persistent, active acute and cGVHD manifestations (overlap
   syndrome), as defined by 2014 NIH Consensus Development Project on Criteria for
   Clinical trials in cGVHD.

   5. Karnofsky Performance Scale of ≥60 (if aged 16 years or older); Lansky Performance
   Score of ≥60 (if aged <16 years)

   6. Adequate organ and bone marrow functions evaluated during the 14 days prior to
   randomization.

   7. Creatinine clearance (CrCl) ≥30 milliliter/minute based on the Cockcroft-Gault formula
   in adult participants and Schwartz formula in pediatric participants.

   8. Contraceptive use by men or women should be consistent with local regulations
   regarding the methods of contraception for those participating in clinical studies.

   9. Concomitant use a of systemic corticosteroid is allowed but not required. Topical and
   inhaled corticosteroid agents are allowed. If a participant is taking corticosteroids
   at study randomization, they must be on a stable dose of corticosteroids for at least
   2 weeks prior to Cycle 1 Day 1.

10. Concomitant use of CNI or mammalian target of repamycin (mTOR) inhibitors (sirolimus
   or everolimus) is allowed but not required.

11. Capable of giving signed informed consent which includes compliance with the
   requirements and restrictions listed in the informed consent form (ICF) and protocol.
   A parent/guardian should provide consent for pediatric participants unable to provide
   consent themselves; in addition, where applicable pediatric participants should sign
   their own assent form.

Exclusion Criteria:

Participants are excluded from the study if any of the following criteria apply:

   1. Has acute GVHD without manifestations of cGVHD.

   2. Any evidence (histologic, cytogenetic, molecular, hematologic, or mixed) of relapse of
   the underlying cancer or post-transplant lymphoproliferative disease at the time of
   screening.

   3. History of acute or chronic pancreatitis.

   4. History of myositis.

   5. History or other evidence of severe illness, uncontrolled infection or any other
   conditions that would make the participant, in the opinion of the Investigator,
   unsuitable for the study.

   6. Participants with acquired immune deficiency syndrome (AIDS).

   7. Hepatitis B (defined as hepatitis B virus [HBV] surface antigen positive and HBV core
   antibody positive, with positive HBV deoxyribonucleic acid [DNA], or HBV positive core
   antibody alone with positive HBV DNA. Hepatitis C (defined as positive hepatitis C
   [HCV] antibody with positive HCV ribonucleic acid [RNA]).

   8. Diagnosed with another malignancy (other than malignancy for which transplant was
   performed) within 3 years of randomization, unless previously treated with curative
   intent and approved by Sponsor's Medical Monitor (for example, completely resected
   basal cell or squamous cell carcinoma of the skin, resected in situ cervical
   malignancy, resected breast ductal carcinoma in situ, or low-risk prostate cancer
   after curative resection).

   9. Female participant who is pregnant or breastfeeding.

10. Previous exposure to CSF1-R targeted therapies.

11. Taking agents for treatment of cGVHD other than corticosteroids or either a CNI or
   mTOR inhibitor is prohibited.

12. For approved or commonly used agents, other than corticosteroids, CNI and mTOR
   inhibitor, a washout of 2 weeks or 5 half-lives, whichever is shorter, is required at
   study enrollment.

13. Receiving another investigational treatment within 28 days of randomization.

14. Participants should not be participating in any other interventional study. Pediatric
   participants are encouraged to also participate in the ongoing developmental studies
   of the Pediatric cGVHD Symptom Scale (PCSS).