Tyrosine kinase inhibitors (TKIs) result in excellent responses in most Chronic Myeloid Leukemia (CML) patients. However, up to 35% of patients treated with imatinib (IM) exhibit resistance and more recently nilotinib (NIL) and dasatinib (DAS) resistance have also been observed. Mutations in the BCR-ABL kinase domain (KD) are the main cause of secondary TKI resistance. Other mechanisms include overexpression of BCR-ABL, LYN and ABCB1. Predicting patients with susceptibility to mutation development and disease progression is crucial, thus we investigated the kinetics of TKI resistance emergence in vitro and in vivo.

ABCB1 is implicated in TKI efflux hence we postulated that overexpression of ABCB1 leads to reduced intracellular TKI concentrations, resulting in inferior inhibition of Bcr-Abl predisposing cells to resistance development. Accordingly, 3 CML blast crisis (BC) cell lines (K562, K562-Dox, KU812) were cultured in increasing concentrations of IM to 2 μM, NIL to 2 μM and DAS to 200 nM until we observed overt resistance defined as a significant increase in survival in cytotoxicity assays and p-Crkl dependent IC50. Mechanisms of resistance were investigated in cell line intermediates: BCR-ABL, ABCB1 and LYN mRNA expression levels were determined by RT-PCR and KD mutation sequencing was performed.

In our TKI resistant cell lines (Table 1), an increase in ABCB1 mRNA was the initial change observed prior to the development of additional resistance mechanisms (KD mutations, ABCB1 BCR-ABL and LYN overexpression). Interestingly, in 4/6 cells lines ABCB1 mRNA reduced to basal levels or below following establishment of these additional resistance mechanisms.

Table 1

Fold change in ABCB1 mRNA compared to naïve in selected resistant intermediates

Cell Line Intermediate (sequential samples)
#1#2#3
Further Resistance Mechanism
#4
Overt Resistance
K562 IM 2.3↑ p=0.019 2.8 ↑ p=0.029 Lyn overexpression 0.13 ↓ p=0.004 
KU812 IM 5.5 ↑ p=0.003 8.6 ↑ p=0.006 F359C 1.3 ↔ p=0.759 
K562 NIL 4.8 ↑ p<0.001 3.4 ↑ p=0.002 Lyn overexpression 1.1 ↔ p=0.385 
K562-Dox NIL 3.1 ↑ p<0.001 2.7 ↑ p=0.002 Bcr-Abl independent 0.004 ↓ p=0.002 
K562-Dox DAS1 2.7 ↑ p=0.003 4.6 ↑ p=0.036 V299L 2.6 ↑ p=0.039 
K562-Dox DAS2 2.2 ↑ p=0.025 4.4 ↑ p=0.010 BCR-ABL overexpression 2.0 ↑ p=0.040 
Cell Line Intermediate (sequential samples)
#1#2#3
Further Resistance Mechanism
#4
Overt Resistance
K562 IM 2.3↑ p=0.019 2.8 ↑ p=0.029 Lyn overexpression 0.13 ↓ p=0.004 
KU812 IM 5.5 ↑ p=0.003 8.6 ↑ p=0.006 F359C 1.3 ↔ p=0.759 
K562 NIL 4.8 ↑ p<0.001 3.4 ↑ p=0.002 Lyn overexpression 1.1 ↔ p=0.385 
K562-Dox NIL 3.1 ↑ p<0.001 2.7 ↑ p=0.002 Bcr-Abl independent 0.004 ↓ p=0.002 
K562-Dox DAS1 2.7 ↑ p=0.003 4.6 ↑ p=0.036 V299L 2.6 ↑ p=0.039 
K562-Dox DAS2 2.2 ↑ p=0.025 4.4 ↑ p=0.010 BCR-ABL overexpression 2.0 ↑ p=0.040 

p-value: difference in %ABCB1 in untreated cells vs respective intermediate (n>4)

ABCB1 levels were assessed in 37 de novo CML patients treated with IM who achieved major molecular response (MMR) compared with patients who progressed to BC, lost MMR or developed KD mutations. ABCB1 levels were determined in blood at diagnosis and following therapy (selected patients summarized in Table 2). A sustained >2 fold rise in ABCB1 was observed prior to disease progression in 3/3 patients and in 13/16 patients who did not achieve MMR. Importantly, the same was not observed in patients who achieved MMR (1/6 patients). The fold change of ABCB1 mRNA at day 22 vs diagnosis in patients achieving MMR was significantly different to that in patients not achieving MMR (p=0.004). ABCB1 increased by >2 fold post therapy and decreased following mutation development in 3/12 patients, confirming observations made in vitro, while 6/12 patients demonstrated sustained increase in ABCB1 post mutation similar to results observed in progression patients. ABCB1 mRNA did not change during therapy in 3/12 patients with mutations. While we recognize the majority of cells present in patients who achieve MMR are normal rather than leukemic, it is important to note that in patients who do not achieve MMR, ABCB1 expression increases in the remaining leukemic cells.

Table 2

ABCB1 mRNA in selected CML patients on IM therapy

Fold change in ABCB1 mRNA from diagnosis
#Day 2212 monthIM cessation*
MMR 1.4 1.6 NA 
1.2 1.7 NA 
1.3 1.1 NA 
No MMR 12.3 16.5 – 
9.0 – 9.6 
5.4 7.7 – 
Disease progression – 8.6 7.6 
2.0 – 2.9 
3.9 – 7.3 
Fold change in ABCB1 mRNA from diagnosis
#Day 2212 monthIM cessation*
MMR 1.4 1.6 NA 
1.2 1.7 NA 
1.3 1.1 NA 
No MMR 12.3 16.5 – 
9.0 – 9.6 
5.4 7.7 – 
Disease progression – 8.6 7.6 
2.0 – 2.9 
3.9 – 7.3 

– = no sample available

NA = not applicable

*

patient ceased IM for lack of efficacy or progression

We conclude ABCB1 overexpression acts as an initial mediator of resistance, providing a favorable environment for development of further resistance. Sustained increased levels of ABCB1 may contribute to disease progression and lack of response to IM. Additionally, ABCB1 may serve as a prognostic indicator (eg: level at day 22) and potentially assist in development of treatment strategies using TKIs in combination with other medications to enhance intracellular TKI concentration.

Disclosures:

Hughes:Ariad: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; BMS: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; Novartis: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; CSL: Research Funding. White:Novartis: Research Funding; BMS: Research Funding, Speakers Bureau; Ariad: Research Funding; CSL: Research Funding.

Author notes

*

Asterisk with author names denotes non-ASH members.

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