Clonal Markers In Relapsed Acute Promyelocytic Leukemia (APL): Clinicopathological Associations and Relation to All-Trans Retinoic Acid (ATRA) Treatment on Intergroup Phase III Trial C9710.

Abstract 1038

The relationships of inherent disease-related characteristics and of treatment to the nature of APL cell clones that emerge at disease relapse are poorly understood. We studied these relationships in 45 patients who relapsed after ATRA/chemotherapy treatment on the non-arsenic trixoxide arm of intergroup phase III trial C9710 (Powell, et al. Blood, Epub). Four variably-expressed APL cell clonal markers were assessed: PML-RARA ligand binding domain mutations (LBD-M), FLT3 internal tandem duplication mutations (ITD), FLT3 receptor tyrosine kinase mutations (D835; RTK-M) and additional chromosome abnormalities (ACA), i.e., in addition to the hallmark t(15;17). The methods for mutation analysis of PML-RARA and FLT3 have been reported and were RNA transcript-initiated supplemented by selected quantitative FLT3 mutation analyses initiated from DNA. Karyotype data were derived by standard cytogenetic methods. Four patients were excluded from analysis because the sample level of PML-RARA, as determined by quantitative RT-PCR, was insufficient to exclude possible false-negative mutation results. In 41 evaluable patients, the marker incidences were: LBD-M, 44%; ITD, 37%; RTK-M, 12%; ACA (22 tested), 59%. At presentation, the corresponding incidences were: LBD-M, 0% (by high-sensitivity testing a minor subclone of the relapse mutation was found in 2/7 patients tested); ITD, 43% (37 tested); RTK-M, 22% (37 tested); ACA, 24% (34 tested). The low frequency of RTK-M impeded further analysis. The other markers, using relapse determinations, were assessed for potential associations between the markers and with the following parameters: age, sex, presenting WBC count, PML-RARA type, time to relapse, relapse on or off ATRA treatment (on = taking or within 30 days of discontinuing) and post-relapse survival. The most essential positive findings are summarized below:

The results confirm reports, which tested pretreatment samples, of a strong association between FLT3ITD mutations present at relapse and high WBC count and S-form PML-RARA. At relapse, there was, also, divergent selection of ITD-harboring vs ACA-harboring clones. LBD-M occurred more frequently in patients with low presenting WBC counts. They, also, occurred in patients who relapsed while on ATRA, consistent with an ATRA selective role for clones harboring LBD mutations. A significant segregation of patients with LBD-M or ITD was observed after relapse off ATRA treatment (p = 0.019), while no segregation was observed in overall relapse patients (p = 0.346) or after relapse on ATRA (p = 0.316). In 2 patients who relapsed on ATRA, quantitative analysis indicated that LBD-M and ITD mutations must be present in the same APL cell clone. There were no significant survival differences related to any clonal marker (post-relapse survival = 66%). Although the small number of cases is cautionary, the overall data suggest that LBD-M may have an overriding, dominant APL clone selection role in relapse that occurs on ATRA therapy. Other inherent clinicopathologic characteristics of APL may predispose to divergent molecular pathways of disease progression and relapse in patients alternatively harboring clones with LBD-M or ITD after the termination of ATRA selection pressure.