Haploidentical Allogeneic HSCT

Haploidentical Allogeneic HSCT (Haplo-HSCT)

Definition

• Allogeneic HSCT where the donor is a half HLA match (50%), usually a parent, child, or sibling.
• Expands donor availability: >90% of patients will have a haplo donor, compared to only ~25–30% chance of having a matched sibling.
• Once considered too risky due to severe GVHD and graft rejection, but now feasible due to modern graft manipulation and post-transplant cyclophosphamide (PTCy).

Indications

• Used when no matched sibling donor (MSD) or matched unrelated donor (MUD) is available.
• Increasingly used in:
  • Acute leukemias (AML, ALL)
  • MDS, myelofibrosis
  • Aplastic anemia
  • Lymphomas
  • Inherited disorders (thalassemia, sickle cell disease, immunodeficiencies)

Conditioning Regimens

• Myeloablative (MAC): busulfan + cyclophosphamide, or TBI + cyclophosphamide
• Reduced-intensity (RIC): fludarabine + low-dose busulfan or melphalan + TBI
• Choice depends on patient age, comorbidities, and disease status.

GVHD & Graft Rejection Prevention

• Post-transplant Cyclophosphamide (PTCy):
  • Cyclophosphamide given day +3 and +4 after HSCT
  • Selectively kills proliferating alloreactive T-cells while sparing regulatory T-cells and hematopoietic stem cells
  • Dramatically reduces acute/chronic GVHD and graft failure
• Usually combined with:
  • Tacrolimus or Cyclosporine + Mycophenolate Mofetil (MMF)

Advantages

• Almost every patient has a potential donor (parent/child/sibling).
• Faster donor identification vs unrelated donor registry search.
• Comparable overall survival (OS) to MUD transplants in many studies when PTCy is used.
• Option for urgent transplants.

Challenges / Risks

• Historically higher GVHD, graft failure, and infections, though much improved with PTCy.
• Delayed immune reconstitution → ↑ risk of viral infections (CMV, EBV, adenovirus, BK virus).
• Engraftment delay compared to MSD or MUD.
• Relapse rates may be slightly higher in some settings (disease-specific).

Complications & Supportive Care (Pharmacist Role)

1. GVHD prevention
   • PTCy + tacrolimus/cyclosporine + MMF
   • Monitor drug levels, renal function, drug interactions (azoles, antivirals).
2. Infection prophylaxis
   • Antiviral: acyclovir, letermovir (for CMV high-risk)
   • Antifungal: fluconazole or mold-active azoles (posaconazole)
   • PCP: TMP-SMX ≥ 6 months
   • Consider IVIG if severe hypogammaglobulinemia
3. Engraftment monitoring
   • Chimerism testing (donor vs recipient cells)
   • Growth factors (filgrastim) in delayed engraftment
4. Organ toxicity monitoring
   • Cyclophosphamide → hemorrhagic cystitis (MESNA, hydration)
   • Busulfan → veno-occlusive disease (monitor PK, consider ursodiol/defibrotide)
   • Calcineurin inhibitors → nephrotoxicity, hypertension, electrolyte imbalance

Clinical Outcomes

• With PTCy-based protocols, haplo-HSCT outcomes now approach those of MSD and MUD HSCT.
• Survival is disease- and risk-dependent, but many centers consider haplo as equal second choice to MSD.

Summary for pharmacists:

Haploidentical HSCT has transformed from a high-risk procedure into a mainstream curative option due to post-transplant cyclophosphamide. Pharmacists play a key role in GVHD prophylaxis, infection prophylaxis, drug monitoring, and supportive care.