A Phase I Dose Escalation Study of the Btk Inhibitor PCI-32765 in Relapsed and Refractory B Cell Non-Hodgkin Lymphoma and Use of a Novel Fluorescent Probe Pharmacodynamic Assay.
Authors: Pollyea Daniel A , Smith Sonali , Fowler Nathan , Boyd Thomas E , Smith Ashley M , Sirisawad Mint , Honigberg Lee A , Hamdy Ahmed , Advani Ranjana 
Published: October 2019

Abstract
Abstract 3713
Poster Board III-649

Introduction
B-cell receptor (BCR) signaling contributes to the growth of several types of B-cell non-Hodgkin lymphoma (NHL), and recent reports suggest there is clinical utility to targeting this pathway. Bruton's tyrosine kinase (Btk) is an important downstream mediator of BCR signaling as evidenced by genetic mutations in Btk that cause X-linked agammaglobulinemia, a B-cell specific immunodeficiency disease. PCI-32765 is an oral, potent and selective covalent inhibitor of Btk which blocks BCR signaling and inhibits tumor growth in both mouse and spontaneous canine NHL models. Here we report preliminary results of the first-in-human study with this novel agent.

Patients and Methods
In this Phase I dose escalation trial, patients with relapsed or refractory B-cell NHL are being enrolled in cohorts of 6 patients each, with predefined criteria for dose escalation from 1.25 mg/kg/day in cohort 1 to 17.5 mg/kg/day in cohort 6. Dose escalation proceeds if ≤1 patient experiences dose limiting toxicity (DLT). Each cycle includes 28 days of therapy followed by a 7 day observation period. Response assessments occur every 2 cycles, and patients without disease progression or DLT may receive a maximum of 6 cycles. The primary objectives are to determine the safety, pharmacokinetics (PK), and pharmacodynamic (PD) responses. The secondary objective is to evaluate tumor responses. The PD assays include a test for drug occupancy of Btk using a novel fluorescent probe derived from PCI-32765. The probe is added to peripheral blood mononuclear cell (PBMC) lysates ex vivo and probe-labeled Btk is visualized by fluorescent gel scanning. Because the probe binds to Btk at the same site as the drug, occupancy of Btk by PCI-32765 can be detected by the inhibition of probe labeling. The probe assay and other cell signaling assays are used to monitor target inhibition in patient blood samples collected pre-dose on days 1, 2, 8, 15, and 29 and at 4 hours after dosing on days 1 and 8. In the absence of DLTs, dose escalation will proceed to three cohorts above the dose level that achieves >90% occupancy of Btk by PCI-32765.

Results
The study is currently open and enrolling. Seven patients (1 DLBCL, 2 MCL, 4 FL) with a median of 3 prior therapies have been enrolled in the first cohort (one more patient than originally planned). Therapy has been extremely well tolerated with no DLT or greater than grade 2 hematological or non- hematologic toxicity. Six patients have completed at least 1 cycle. PD studies showed the average drug occupancy of Btk in patient PBMCs in this cohort was 89% at 4 hours post-dose and 77% at 24 hours post-dose. Ex vivo stimulation assays showed near complete functional inhibition of the FcεRI signaling pathway in basophils and significant inhibition of the BCR pathway. T-cell responses were not affected, and no significant depletion of peripheral blood B, T or NK cell counts was observed. Two of 7 patients have stable disease (65 days and 135 days), 3 patients are not yet evaluable for response, and 2 have progressed.

Conclusion
PCI-32765 is a novel agent which targets Btk and appears to be well tolerated. The unique PD endpoint suggests high levels of Btk inhibition even at the lowest dosing cohort. Dose escalation to subsequent cohorts is ongoing.

Disclosures:
Smith: Pharmacyclics: Research Funding. Fowler:Pharmacyclics: Research Funding. Boyd:Pharmacyclics: Research Funding. Smith:Pharmacyclics: Employment, Equity Ownership. Sirisawad:Pharmacyclics: Employment, Equity Ownership. Honigberg:Pharmacyclics: Employment, Equity Ownership. Hamdy:Pharmacyclics: Employment, Equity Ownership. Advani:Pharmacyclics: Research Funding.