The International Consortium on Acute Promyelocytic Leukemia’s (ICL-APL) evaluations on MRD monitoring of APL in economically developing countries

The International Consortium on Acute Promyelocytic Leukemia (IC-APL) aims to improve the outcomes of Acute Promyelocytic Leukemia (APL) patients in economically developing countries by adapting European and American diagnostic, supportive and therapeutic guidelines. The adaptation of logistical conditions for monitoring Minimal Residual Disease (MRD) in impoverished countries has been a major drawback for the IC-APL.

Detection of the Promyelocytic Leukemia Retinoic Acid Receptor Alpha (PML-RARA) fusion gene was originally performed by Endpoint Reverse Transcription Polymerase Chain Reaction (endpoint PCR) in economically developing countries. However, the Europe Against Cancer (EAC) suggests that Real-time Quantitative Polymerase Chain Reaction (RT-qPCR) offers a more reliable detection and provides more information regarding sample quality (J.Gabert et al 2003). Moreover, there is a paucity of data comparing the use of the two assays for longitudinal MRD monitoring in APL patients and hence the rationale for this investigation.

In this study by the IC-APL, 103 APL patients (median age = 36) were diagnosed, treated and monitored. The monitoring of MRD obtained from endpoint PCR and RT-qPCR were compared.

The results of the study were published ahead of print in the British Journal of Haematology on the 26^th of December 2016.

The key results are:

• Normalised Copy Number (NCN) at diagnosis of PML-RARA/10⁴ copies of ABL1 for patients with Breakpoint Cluster Region (BCR) protein gene isoform 1 (BCR1, n = 57) and 3 (BCR3, n = 40) was 0.5391 and 0.4382 respectively  
• After induction, RT-qPCR and endpoint PCR detected PML-RARA transcripts in 45/73 (62%) and 40/73 (55%) samples respectively
• After induction, median NCN for BCR1 (n = 47) and BCR3 (n = 25) were 0.00043 and 0.00026 respectively
• Complete Hematological Response rate was 91% (94/103 patients)
• During induction, mortality occurred in 9 of 103 patients (9%)
• After third consolidation, 96.3% (80/83) patients achieved Complete Molecular Remission (CRm) according to endpoint PCR assay 
• Of 96.3% patients that achieved CRm according to endpoint PCR, RT-qPCR showed NCN of 0.0021, 0.0046 and 0.0011 in 3 patients who did not achieve CRm and 4 patients were considered not to have reached CRm.
• 84.4% (430/509) and 94.8% (483/509) of samples were negative for PML-RARA according to RT-qPCR and endpoint PCR assays respectively during maintenance
• Relapse occurred in seven patients with 2/7 patients with concomitant molecular and Central Nervous System (CNS) relapse
• RT-qPCR assay detected PML-RARA transcripts before molecular relapse in 6 of the 7 relapsed patients
• Three cases and two cases of molecular relapse and hematological relapse occurred respectively
• 1/3 patients that had molecular relapse detected by RT-qPCR tested negative for relapse by endpoint PCR
• RT-qPCR detected PML-RARA transcript in 2/2 patients before hematological relapse occurred but not endpoint PCR
• RT-qPCR detected PML-RARA transcript 84days before the first positive result obtained by endpoint PCR in one patient that had molecular relapse diagnosed in the CNS
• In one patient diagnosed with CNS, RT-qPCR predicted relapse 10 months before the first positive result obtained by endpoint PCR

In summary, the findings of this study confirm the absence of primary resistance in APL and supports the prognostic importance of molecular analysis after consolidation. The ICL-APL study concluded that the detection and quantification of PML-RARA transcripts after consolidation therapy is associated with hematological relapse after a short period.

However, in economically developing countries where resources can be scarce the major hindrances in monitoring molecular relapse APL patients were cost and appropriate sample collection.  In light of these circumstances the ICL-APL highlighted the importance of continued medical education and recommended the use of RT-qPCR as a standard assay for MRD monitoring in APL patients owing to the greater precision and consistency observed with regards to sample quality assessment.