The Bruton's Tyrosine Kinase Inhibitor, PCI-32765, Is Well Tolerated and Demonstrates Promising Clinical Activity In Chronic Lymphocytic Leukemia (CLL) and Small Lymphocytic Lymphoma (SLL): An Update on Ongoing Phase 1 Studies

Bruton's tyrosine kinase (Btk) is a downstream mediator of B-cell receptor (BCR) signaling and is not expressed in T-cells or NK-cells. As such, Btk represents an ideal therapeutic target for B-cell malignancies dependent upon BCR signaling. Chronic lymphocytic leukemia (CLL)/small lymphocytic leukemia (SLL) has been reported to have constitutively active BCR signaling. PCI-32765 is a potent, selective, irreversible and orally bioavailable small molecule inhibitor of Btk that has pre-clinical activity in B-cell malignancies (Proc Natl Acad Sci 2010;107(29):13075-80). PCI-32765 was therefore moved forward to a Phase 1 study in B-cell malignancies including patients (pts) with CLL/SLL. A subsequent CLL/SLL-specific Phase 1b study was initiated to further explore safety, pharmacokinetics (PK), pharmacodynamics (PD), and efficacy of PCI-32765. This report includes a composite summary of the CLL/SLL experience in both of these studies.

Pts with CLL/SLL who had relapsed or refractory disease after >1 prior treatment regimens were eligible for treatment in each of the studies whereas the second Phase 1b study also included a cohort of elderly pts (aged ≥ 65 years) with CLL/SLL who required treatment and were “treatment-naive”. Responses were assessed by the investigator using the International Working Group CLL criteria (Hallek et al, Blood 2008 for pts with CLL) and the International Workshop to Standardize Response Criteria for Non-Hodgkin's Lymphomas (Cheson et al, J Clin Oncol 2007 for pts with SLL).

To date, 30 CLL/SLL patients (including 4 treatment-naive) have been enrolled across the 2 studies. Eighty-four percent of subjects are men with an overall median age of 68 (range 44–82) years. Of the subjects with prior therapy for CLL/SLL the median number of prior therapies is 3 (range 1–4). Treatment has been well-tolerated; Grade ≥ 3 toxicities have been infrequent (10/30 pts; 33%). Two study-drug related serious adverse events have been reported: 1 case of viral adenitis (Grade 3) and 1 case of viral infection (Grade 2). Two adverse events have led to discontinuation of study drug: a small bowel obstruction (Grade 3) and exacerbation of chronic obstructive disease (Grade 3); both events were reported as unrelated to study drug. No study-drug related deaths have reported. There has been no change in either NK cell or T cell counts. Target inhibition as measured by a probe of Btk drug occupancy showed inhibition of Btk at PCI-32765 exposure levels of ≥ 245 ng•h/mL. Of the 14 patients currently evaluable for response using the pre-defined criteria, the overall response rate is 64% (1 complete remission [CR], 8 partial remissions [PR], and 4 SD). Both studies are ongoing and open to enrollment. An update on response rate, response duration, safety, and PD information will be presented on enrolled patients based on a November 2010 database cut-off.

PCI-32765 is a novel oral and selective “first-in-human” inhibitor of Btk that induces objective partial and complete responses in a substantial proportion of pts with CLL/SLL and has a favorable safety profile. These data support further studies of both monotherapy and also combination treatment with PCI-32765 in CLL/SLL.

O'Brien:Pharmacyclics, Inc: Honoraria, PI grant. Fowler:Pharmacyclics: Consultancy, Research Funding. Advani:Pharmacyclics, Inc: Honoraria, PI grant. Sharman:Pharmacyclics, Inc: Honoraria, PI grant. Furman:Pharmacyclics, Inc: PI grant. Izumi:Pharmacyclics, Inc: Employment. Buggy:Pharmacyclics, Inc: Employment, Equity Ownership. Loury:Pharmacyclics: Employment, Equity Ownership. Hamdy:Pharmacyclics, Inc: Employment, Equity Ownership.