Minimal Residual Disease and Mutational Status Prior to and after SCT for Patients with Philadelphia-Chromosome-Positive Acute Lymphoblastic Leukemia (Ph+ ALL)
Authors: Pfeifer Heike , Wystub Sylvia , Wassmann Barbara , Brueck Patrick , Goekbuget Nicola , Serve Hubert , Hoelzer Dieter , Ottmann Oliver G 
Published: October 2019

Abstract
BCR-ABL Tyrosine Kinase Domain (TKD) mutations have been identified as a major mechanism of acquired resistance in the majority of Ph+ ALL patients who relapse during treatment with ABL kinase inhibitors such as imatinib (IM). TKD mutations are detectable with low frequency in 30%–40% of elderly, IM-naïve patients with newly diagnosed, Ph+ALL. The proportion of younger Ph+ALL patients who already harbor a TKD mutation prior to IM therapy and the frequency with which these mutations develop during combined treatment with IM and intensive chemotherapy are not known. Moreover, the impact of allogeneic SCT on different types of TKD mutations and its ability to eradicate mutations in patients transplanted in CR1 as opposed to CR2 remain to be established. It was the aim of this analysis to determine the impact of combination therapy with imatinib and chemotherapy followed by allogeneic SCT on MRD levels and the frequency of BCR-ABL mutations in patients with Ph+ALL undergoing SCT as front-line (in CR1) as opposed to second-line (≥ CR2) treatment. Bone marrow (BM) and/or peripheral blood (PB) samples were collected pre-treatment (n=46), during IM-based combination therapy prior to SCT (n=228) and serially after SCT (n=665) from a total of 53 pts. with newly diagnosed Ph+ALL (median age: 45 yrs.) who were enrolled in prospective GMALL trials of IM followed by SCT. Additionally, we examined a total of 536 samples from 27 Ph+ALL pts. (median age: 48 yrs.) who were enrolled in the early phase II trials of IM as salvage therapy and transplanted in PR, CR2 or with progressive disease. MRD and mutational status were analysed by quantitative rtPCR of bcr-abl transcripts and by denaturing high-performance liquid chromatography (D-HPLC) plus cDNA sequencing, respectively. PCR negativity was always confirmed by nested rtPCR. Patients in whom BCR-ABL transcripts were undetectable by both quantitative and nested rtPCR were considered to not harbor a TKD mutation at the time of analysis. In this cohort of younger patients, TKD mutations were detected prior to IM administration in 14 of 47 evaluable patients (29%) with newly-diagnosed Ph+ALL and in 6 of 20 (30%) patients relapsing after chemotherapy. Of the 46 pts. with de novo Ph+ALL who were evaluable within 4 weeks prior to allogeneic SCT, 22 pts. were MRD negative and 24 MRD pos., 8 (33%) of whom had a detectable mutation. Within 6 months after SCT, 41 of 53 transplanted patients had become MRD negative (77%). In 5 of the 16 patients (31%) who displayed a TKD mutation at any time prior to SCT, the mutation was still detectable after alloPBSCT. 3/5 pts. relapsed with the T315I and 4 pts. remained MRD-positive with mutated BCR-ABL. Of the 34 pts. with unmutated BCR-ABL prior to SCT, only 2 pts. displayed a TKD mutation after SCT. One of these converted to PCR negativity, the other patient relapsed with the T315I. In contrast, only 1 of 20 evaluable patients who subsequently underwent SCT as part of their salvage treatment became MRD negative before SCT. Ten of the 19 pts. in whom BCR-ABL transcripts were detectable carried a TKD mutation. In 5 of these 10 pts. with an initial TKD mutation the analysis of MRD and mutational status was possible after alloSCT. 3 pts. converted to MRD-negativity, 2 relapsed with mutated BCR-ABL. BCR-ABL mutations are detected prior to SCT in a clinically significant proportion (30%) of younger adult patients with Ph+ALL. Allogeneic SCT results in long-term elimination of mutant clones in the majority of patients. However, patients remain at risk of relapsing with previously undetectable TKD mutations, some of whom are considered responsive to second generation ABL TK inhibitors.