The LLR TAP IcICLLe Trial: Detailed assessment of the response to ibrutinib provides insights into the mode of action and pattern of response in treatment naïve and refractory CLL

INTRODUCTION: Ibrutinib is a Bruton’s tyrosine kinase inhibitor with high response rates in CLL. The quality of response improves with duration of treatment following an initial redistribution of CLL cells from lymph nodes to the peripheral blood (PB).AIM: The IcICLLe trial (ISRCTN 12695354) is a feasibility study to investigate the biological response to ibrutinib, expand on disease response in prognostic subgroups, and identify changes that may impact the design of therapeutic strategies.METHODS: 40 participants with CLL requiring treatment received ibrutinib (420 mg/day) until either achievement of <0.01% residual disease in bone marrow (BM) or disease progression. A large panel of markers were assessed on PB & BM taken at screening, 1 & 6 months with additional PB at baseline, 4 hours, 1, 2, 7, & 14 days and 2, 9, & 12 months (M).RESULTS: PB CLL cell counts increased immediately after the first dose, peaking during the first two weeks of treatment, and then declined during subsequent months, decreasing to below baseline levels in the majority of patients after 2-6 months of Ibrutinib treatment. PB CLL cell numbers continued to decline on treatment with 10/12 (83%) evaluable patients achieving peripheral CLL count below 5 x 109/L after 9 months of treatment.T-cell numbers increased significantly at a very early stage of treatment while NK-cell numbers remained stable; both T and NK-cell subsets then gradually declined in number to below baseline over the next six months.PB CLL Ki67 expression initially increased, peaking at 24 hours after the first dose, and then declined below quantifiable levels (<0.5%) in 7/37 (19%) evaluable patients by week 1 and in more than 90% of patients at all subsequent time points. CLL Ki67 expression was below quantifiable levels in the bone marrow of 29/37 (78%) of patients after 1 month of treatment. At this early stage there was no apparent change in the extent of bone marrow infiltration butthere was clear evidence of normal haematopoietic regeneration with subjective increases in the macrophage compartment. After 6 months of treatment, 13/19 (68%) evaluable patients showed more than 20% reduction in BM CLL cells, with 6/19 (32%) showing less than 30% CLL cells as a percentage of total BM leucocytes.Decreases in proliferation-associated markers (CD5/CD23/CD38/CD49d/IRF4), and changes in molecules involved in CLL cell trafficking and adhesion (increased CXCR4/CD24 & decreased CCR7/CD31/CD11a) followed the same kinetics as cell cycle arrest, i.e. differences emerged after 1-2 weeks of treatment and then stabilisedduring subsequent treatment. Phenotypic changes could not be attributed to redistribution of disease or loss of proliferating cells because they are evident within the non-proliferative (Ki67-) fraction in both PB & BM. CD20 expression was substantially decreased after two weeks of treatment, while BCL2 showed a mild reduction relative to baseline but overall expression was persistently strong.CONCLUSIONS: The redistribution of CLL cells during ibrutinib happens much more rapidly than any changes in proteins associated with proliferation, cell trafficking or adhesion. The majority of changes in CLL cells correlate with the loss of the proliferative fraction, mostly stabilising after one month of treatment. At this stage haematopoietic recovery also becomes apparent.