Droplet digital PCR pioneered for minimal residual disease detection in leukaemia

Droplet digital polymerase chain reaction (ddPCR) has been pioneered for more precise detection of minimal residual disease (MRD) to assess treatment response and prognosis of leukaemia patients at the Hong Kong Sanatorium & Hospital (HKSH).

Introduction of MRD monitoring has transformed the management of haematological cancers. Now an international standard for evaluating therapeutic efficacy, detecting post-treatment recurrence risk and guiding regimen selection, MRD monitoring is usually performed by cellular analysis (eg, flow cytometry) or molecular analysis (eg, next-generation sequencing [NGS], real-time quantitative PCR [RQ-PCR] and ddPCR). [Cancers (Basel) 2021;13:4582; Int J Mol Sci 2020;21:1054]

The emerging ddPCR technique has shown promise compared with conventional methods. [Biosci Rep 2018;38:BSR20181170]  “The sensitivity and resolution of ddPCR is 100 times higher than conventional PCR methods, allowing physicians to obtain a more detailed picture of MRD status,” said Dr Edmond Ma, Director of Clinical Pathology and Molecular Pathology Division, HKSH. “The meticulously designed ddPCR assays aim to guide individualized therapy for each patient by accurately tracking and targeting the rare mutated genes in these patients.”

As the first private medical institution in Hong Kong to use ddPCR for MRD detection, HKSH has applied the technology in 118 patients in the past 4 years as of 6 October 2022. Of these patients, 113 had acute myeloid leukaemia (AML), three had acute lymphoid leukaemia (ALL), one had chronic myeloid leukaemia (CML), and one had lymphoma. Out of 79 genetic variants targeted, 85 percent were mutations (including 67 types of genetic mutations) and 15 percent were gene fusions (including 12 fusion genes).

“Given the presence of rare genetic variants or fusion genes in some patients, treatment for these cases becomes increasingly complex. Close monitoring of MRD level is needed even after these patients have achieved remission, to ensure continued treatment efficacy,” explained Dr Raymond Liang, Director of Comprehensive Oncology Centre, HKSH.

“Accurate detection of MRD helps us ‘catch’ the very small amount of residual tumour cells and intervene in their growth as soon as possible. ddPCR can provide us with highly precise quantitative data to tell whether the patient is in full remission or not. We can then predict the patient’s prognosis and risk of relapse, and determine whether a bone marrow transplant is needed,” Liang continued.

“MRD information provided by ddPCR also provides guidance for clinical decisions such as dosage of chemotherapy and targeted therapy, as well as the risk and timing of transplantation,” Liang added.

“[ddPCR] is particularly pertinent to patients with rare genetic translocations or mutations, which may not be effectively tracked with conventional cellular or molecular analysis,” added Ma.

The valuable clinical data provided by ddPCR is anticipated to lead to wider adoption of ddPCR assays in clinical practice, according to Liang.

Experience in ddPCR accumulated in the past few years has enabled Ma and Liang to design the most suitable treatment strategies for patients with haematological malignancies.  In the future, their team will investigate the use of ddPCR in haematological cancers as well as solid tumours (eg, liver, breast and prostate cancers) under the auspices of liquid biopsy.