Clinical and biological implications of target occupancy in CLL treated with the BTK inhibitor acalabrutinib

Abstract

Inhibition of the B-cell receptor pathway, and specifically of Bruton tyrosine kinase (BTK), is a leading therapeutic strategy in B-cell malignancies, including chronic lymphocytic leukemia (CLL). Target occupancy is a measure of covalent binding to BTK and has been applied as a pharmacodynamic parameter in clinical studies of BTK inhibitors. However, the kinetics of de novo BTK synthesis, which determines occupancy, and the relationship between occupancy, pathway inhibition and clinical outcomes remain undefined. This randomized phase 2 study investigated the safety, efficacy, and pharmacodynamics of a selective BTK inhibitor acalabrutinib at 100 mg twice daily (BID) or 200 mg once daily (QD) in 48 patients with relapsed/refractory or high-risk treatment-naïve CLL. Acalabrutinib was well tolerated and yielded an overall response rate (ORR) of partial response or better of 95.8% (95% confidence interval [CI], 78.9-99.9) and an estimated progression-free survival (PFS) rate at 24 months of 91.5% (95% CI, 70.0-97.8) with BID dosing and an ORR of 79.2% (95% CI, 57.9-92.9) and an estimated PFS rate at 24 months of 87.2% (95% CI, 57.2-96.7) with QD dosing. BTK resynthesis was faster in patients with CLL than in healthy volunteers. BID dosing maintained higher BTK occupancy and achieved more potent pathway inhibition compared with QD dosing. Small increments in occupancy attained by BID dosing relative to QD dosing compounded over time to augment downstream biological effects. The impact of BTK occupancy on long-term clinical outcomes remains to be determined. This trial was registered at www.clinicaltrials.gov as #NCT02337829.

Graphical abstract

Figure 1.

CONSORT diagram.

Figure 2.

Efficacy of acalabrutinib. (A) The rate of overall response, including complete remission and partial remission, in the BID group and the QD group. (B) Absolute lymphocyte count during treatment. (C) Maximum percent change in the sum of the product of dimensions during treatment. (D) Quantification of disease burden in bone marrow and peripheral blood separated by dosing groups. Line denotes median of each group at the indicated time point. *P < .05. NED, no evidence of disease.

Figure 3.

PFS. (A) PFS in the BID group and the QD group. (B) PFS in TN patients and R/R patients.

Figure 4.

BTK resynthesis rate and occupancy in PBMCs. (A) BTK resynthesis in patients with CLL (n = 48) and healthy volunteers (n = 4), shown as mean ± standard error of the mean (SEM). Linear regression was used to calculate slopes for the return of free BTK. Heathy volunteers received a 100-mg dose. (B) Individual BTK occupancy in the BID group and the QD group over the first 12 months of treatment. The median values are represented by the horizontal lines. *P < .05, **P < .01, ****P < .0001. (C) Individual BTK occupancy in patients before and a median of 3 months (range, 0.5-6) after switching from 200 mg QD to 100 mg BID.

Figure 5.

BTK occupancy in lymph node and marrow. (A) Individual BTK occupancy in the lymph node and marrow. The median values are represented by the horizontal lines. (B) BTK occupancy over time in lymph node during the acalabrutinib withholding period, shown as mean ± SEM. Linear regression was used to calculate slopes for the return of free BTK. (C-D) Correlation of BTK occupancy in paired samples of PBMC and lymph node and marrow. Shapes represent collection time since the last dose of acalabrutinib: 4 hours (●), 12 hours (▪), 24 hours (♦), and 36 hours (▲).

Figure 6.

Transcriptome analysis of circulating tumor cells from patients before and after treatment with acalabrutinib. (A) Principal component analysis of samples collected at baseline (●), after 1 cycle (♦), and after 6 cycles (▪). (B) Heatmap of DE genes (fold-change ≥2, FDR < 0.1) (C) Average ± SEM log₂ fold-change in expression of leading-edge genes of the indicated gene signatures after 1 cycle and after 6 cycles compared with baseline. (D) Average ± SEM log₂ fold-change in leading edge genes from BCR and nuclear factor-κB signatures in the BID group compared with the QD group. *P < .05, **P < .01, ***P < .001.