PF-4878691

The toll‐like receptor (TLR) 7 agonist 852A, a small‐molecule imidazoquinoline, stimulates plasmacytoid dendritic cells to produce multiple cytokines. We conducted a Phase II study of 852A in patients with recurrent hematologic malignancies. The primary objective was assessing the activity of 852A administered subcutaneously twice weekly for 12 weeks. Secondary objectives were assessing the safety of 852A and its ability to activate the immune system with prolonged dosing. Patients with relapsed hematologic malignancies of any age with adequate organ function were eligible. Patients initiated dosing at 0.6 mg/m2 twice weekly and escalated by 0.2 mg/m2 after every two doses as tolerated to a target dose of 1.2 mg/m2. Patients with responses or stable disease were eligible for additional cycles. Seventeen patients (15 males) entered the study: 6 with AML, 5 ALL, 4 NHL, 1 Hodgkin's lymphoma, and 1 multiple myeloma. The mean age was 41 years (12–71 years). The median number of prior chemotherapy regimens was 5 (range = 1–14). Thirteen patients completed all 24 injections. Grade 3‐4 toxicities included nausea, dyspnea, fever, myalgia, malaise, and cough. Responses included one complete response (ALL), one partial response (AML), two stable disease (AML and NHL), and 9 progressive disease. This is the first in‐human hematologic malignancy trial of a subcutaneously (SC) delivered TLR7 agonist using a prolonged dosing schedule. 852A was safely administered up to 1.2 mg/m2 twice weekly with evidence of sustained tolerability and clinical activity in hematologic malignancies. Systemic TLR agonists for the treatment of hematologic malignancies warrant further study. Am. J. Hematol. 2012. © 2012 Wiley Periodicals, Inc.

Hepatitis C virus (HCV) infection is an issue of global concern, and studies are ongoing to identify new therapies that are both effective and safe. PF-4878691 is a Toll-like receptor 7 (TLR7) agonist modeled so as to dissociate its antiviral activities from its inflammatory activities. In a proof-of-mechanism study in healthy volunteers who received doses of 3, 6, and 9 mg of PF-4878691 twice a week for 2 weeks, PF-4878691 induced biomarkers of the immune and interferon (IFN) responses in a dose-dependent and dose-frequency-related manner. A novel finding was induction of TLR7 expression in vivo in response to PF-4878691, leading to an amplified biomarker response. A nonresponder at the 9-mg dose had a polymorphism in the IFN-α receptor 1 subunit (Val168Leu). Two subjects who had received 9-mg doses experienced serious adverse events (SAEs), characterized by flu-like symptoms, hypotension, and lymphopenia, leading to early termination of the study. TLR7 stimulation results in a pharmacologic response at levels commensurate with predicted antiviral efficacy, but these doses are associated with SAEs, raising concerns about the therapeutic window of this class of compounds for the treatment of HCV infection.