Minimal Residual Disease

Minimal (Measurable) Residual Disease (MRD) in Oncology

Definition

  • MRD = the presence of leukemic (or malignant) cells below the threshold of conventional microscopy.
  • Detects 1 cancer cell among 10³–10⁶ normal cells, depending on technique.
  • Considered a strong prognostic biomarker → guides risk stratification, treatment intensity, and transplant decisions.

Methods of MRD Detection

  1. Flow cytometry (MFC, multiparameter flow cytometry)
    • Sensitivity: ~10⁻⁴ (1 cell in 10,000)
    • Uses abnormal immunophenotype markers (“leukemia-associated immunophenotype”).
    • Widely available, rapid.
  2. PCR-based assays
    • Detects clone-specific gene rearrangements (IGH, TCR), fusion genes (BCR-ABL1, RUNX1-RUNX1T1), or mutations (NPM1, FLT3).
    • Sensitivity: ~10⁻⁵.
    • Requires known molecular marker.
  3. Next-generation sequencing (NGS)
    • Deep sequencing of immunoglobulin/TCR rearrangements or leukemic mutations.
    • Sensitivity: up to 10⁻⁶.
    • Becoming the most precise and widely adopted method.

MRD in Specific Malignancies

Acute Lymphoblastic Leukemia (ALL)

  • MRD is the strongest prognostic factor in both pediatric and adult ALL.
  • MRD ≥10⁻⁴ after induction/consolidation → high relapse risk.
  • Guides intensity of therapy (e.g., escalation to transplant in adults with Ph+ ALL if MRD+).
  • Used in response-adapted therapy (e.g., blinatumomab indicated for MRD+ B-ALL).

Acute Myeloid Leukemia (AML)

  • MRD+ after induction or consolidation → predicts relapse even if in morphologic CR.
  • Used to determine need for allogeneic HSCT.
  • Monitored via flow cytometry or molecular markers (e.g., NPM1 mutation, RUNX1-RUNX1T1, CBFB-MYH11, FLT3).

Chronic Lymphocytic Leukemia (CLL)

  • MRD negativity is a key endpoint in modern trials.
  • Achieving MRD- (via flow cytometry/NGS in blood or marrow) predicts durable remissions.
  • Targeted therapies (e.g., venetoclax-based regimens) often aim for MRD negativity.

Multiple Myeloma (MM)

  • MRD negativity is now integrated in IMWG response criteria.
  • Achieving MRD- predicts longer PFS and OS, even more than complete remission (CR).
  • Measured by NGS or flow cytometry, with sensitivity 10⁻⁵–10⁻⁶.

Clinical Applications

  • Risk stratification: Identifies patients at high relapse risk.
  • Therapy adjustment: Escalate (e.g., HSCT in MRD+), de-escalate in MRD-.
  • Regulatory endpoint: MRD is being accepted by FDA/EMA as a surrogate endpoint in trials.
  • Drug approvals: Blinatumomab (B-ALL, MRD+) approved specifically for MRD-positive patients.

Key Oncology Pharmacy Points

  • MRD testing directly impacts treatment decisions (e.g., transplant eligibility, continuation vs switch to immunotherapy).
  • MRD-negative status = strong predictor of long-term remission.
  • MRD-positive status after treatment = highest relapse risk, even if morphology shows CR.
  • Techniques differ in sensitivity, availability, and standardization → pharmacists should know which method is used at their center.