Favorable Therapeutic Responses in Newly Diagnosed CML-CP Patients Induced by Dasatinib Are Reflected At the CD34+CD38+ Progenitor Cell but Not At the CD34+CD38− Stem Cell Level: Results From Randomized NordCML006 Study
Authors: Mustjoki Satu , Richter Johan , Barbany Gisela , Ehrencrona Hans , Dybedal Ingunn , Fioretos Thoas , Gedde-Dahl Tobias , Gjertsen Bjorn , Hovland Randi , Jalkanen Sari E , Josefsen Dag , Koskenvesa Perttu , Lassen Carin , Latvala Kirsi , Majeed Waleed , Malm Claes , Markevärn Berit , Mosfegh Ali , Ohm Lotta , Olofsson Tor , Olsson-Strömberg Ulla , Rapakko Katrin , Remes Kari , Stentoft Jesper , Stenke Leif , Suominen Merja , Thunberg Sara , Bjerrum Ole Weiss , Simonsson Bengt , Porkka Kimmo , Hjorth-Hansen Henrik 
Published: October 2019

Abstract
Abstract 784

Background:
In vitro studies have suggested that CML stem cells are resistant to tyrosine kinase inhibitors (TKIs), but in vivo effects in patients have not been prospectively assessed. Furthermore, the inter-individual variation of the leukemic stem cell pool at diagnosis and its possible prognostic value is unknown.

Patients:
46 newly diagnosed CML-CP patients were randomized 1:1 to receive either dasatinib 100 mg or imatinib 400 mg QD. The primary endpoint was a comparison of the proportion of Ph+ cells in CD34+CD38− and CD34+CD38+ compartment at 6 months between the study arms. Key secondary endpoints were the fraction of Ph+ cells in the stem cell compartments at 1 and 3 months, and molecular and cytogenetic responses at 3, 6, 12 and 18 months. Experimental endpoints included the percentage of Ph+ cells in the stem cell compartment at diagnosis and its correlation with therapeutic response.

Results:
One patient in the imatinib arm and none in the dasatinib arm progressed to blast crisis within first 12 months. 4/22 of dasatinib patients have discontinued the treatment due to side-effects (mainly pleural effusion) and 1 patient due to insufficient response. 3/24 imatinib patients have discontinued the therapy (1 blast crisis, 1 side-effects, 1 other malignancy).
Early cytogenetic responses were superior in the dasatinib arm: the median percentage of Ph+ cells in the bone marrow was 81% (imatinib) vs. 70% (dasatinib) at 1 month (p=0.15) and 5% vs. 0% at 3 months (p=0.0085). At 12 months all dasatinib (n=20) and 19/20 imatinib patients were in CCyR (results based on patients on treatment at 12 months).
MMR rate was significantly higher in the dasatinib arm already at 6 months (70% vs. 20%, p=0.002) and similarly at 9 (75 vs. 26%, p=0.004) and 12 months (88% vs. 40%, p=0.009). Undetectable BCR-ABL1 transcripts (at least CMR4) were observed in 20% of the dasatinib patients at 6 months compared to none in the imatinib arm (p=0.11) and 44% in the dasatinib arm at 12 months compared to 7% in the imatinib arm (p=0.037).
The median percentage of Ph+ cells, as measured by FISH (1000 cells analyzed), in the CD34+CD38− fraction at diagnosis was 79% (range 1–100%) compared to 96% (range 50–100%) in CD34+CD38+ fraction. The proportion of Ph+ cells in CD34+CD38− fraction correlated with WBC count (r=0.50, p<0.001), splenomegaly (r=0.43, p=0.0055), anemia (r=-0.44, p=0.004) and blood blast percentage (r=0.57, p=0.0001) at diagnosis. There was also a significant correlation between Ph+ cells in CD34+CD38− fraction at diagnosis and cytogenetic response at 1 (r=0.63, p<0.0001), 3 (r=0.48, p=0.0025) and 6 months (r=0.36, p=0.0271). Furthermore, leukemic stem cell burden at diagnosis correlated significantly with BCR-ABL1 transcript levels at 3 (r=0.54, p=0.0005), 6 (r=0.42, p=0.0088) and 9 months (r=0.40, p=0.0123). All patients who were not in MMR at 18 months, had >79% of Ph+ cells in CD34+CD38− fraction at diagnosis.
During TKI therapy, the proportion of Ph+ cells decreased rapidly in the stem cell fractions. At 1 month, the median proportion of Ph+ cells was 14% and 56% in CD34+CD38− and CD34+CD38+ fractions compared to 69% in whole BM (p<0.0001, n=38). At 3 months, the respective numbers were 0.40%, 0.20% and 0.80% (p=0.087, n=33) and at 6 months 0%, 0% and 0.1% (p=0.23, n=41).
Dasatinib-treated patients had significantly lower proportion of Ph+ cells in CD34+CD38+ fraction at 3 months than imatinib patients (0.05% vs. 0.68%, p=0.0318). A similar trend was also observed at 1 month (24% vs. 69%, p=0.05), but no difference existed at 6 months. In CD34+CD38− fraction the proportions of Ph+ cells did not differ significantly at 1 (11 vs. 17%, p=0.91), 3 (0.2 vs. 0.3%, p=0.44) or 6 months (0 vs. 0%, p=0.75).

Conclusions:
In comparison to imatinib, dasatinib induced superior therapeutic responses with remarkably high MMR and CMR rates. This was associated with a faster reduction of Ph+ cells at the progenitor CD34+CD38+ cell level. Surprisingly, both drugs rapidly depleted Ph+ cells from the more primitive CD34+CD38− compartment.
The proportion of Ph+ stem cells at diagnosis varied significantly among individual patients and bore prognostic value. Patients with a low proportion of Ph+ leukemic stem cells at diagnosis achieved faster and better cytogenetic and molecular responses. The leukemic stem cell burden at diagnosis may be a biomarker predicting treatment outcome and reflecting key biological factors in CML.

Disclosures:
Mustjoki: Novartis: Honoraria; Bristol-Myers Squibb: Honoraria. Richter:Novartis: Honoraria; Bristol-Myers Squibb: Honoraria. Dybedal:Novartis: Travel support. Fioretos:Cantargia AB: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Qlucore AB: Equity Ownership, Membership on an entity's Board of Directors or advisory committees. Weiss Bjerrum:Novartis: Consultancy; Bristol-Myers Squibb: Consultancy. Simonsson:Novartis, BMS, Merck, Pfizer: Consultancy, Honoraria. Porkka:Novartis: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding. Hjorth-Hansen:Novartis: Honoraria; Bristol-Myers Squibb: Honoraria.