TGR-1202 Suppresses AML and ALL Cells Via Selective Inhibition of PI3Kδ Kinase.
Authors: Vakkalanka Swaroop , Viswanadha Srikant , Niecestro Robert , Sportelli Peter , Savona Michael 
Published: October 2019

Abstract
Abstract 2610

Background:
Acute leukemia, characterized by the presence clonal hematopoietic cells in peripheral blood and bone marrow, comprises approximately 40% of newly diagnosed leukemias. First line treatment for acute leukemias with multi-agent cytotoxic chemotherapy is usually associated with significant toxicity. Advances in therapy have been slow, and nearly all effective therapies lead to prolonged marrow suppression and toxicities associated with subsequent cytopenias. Herein, we describe the biological and pharmacokinetic properties of TGR-1202, a novel small molecule PI3Kδ inhibitor with scope to be developed as a safe and effective therapy for acute myeloid (AML) and lymphoblastic (ALL) leukemia.

Material & Methods:
Activity of TGR-1202 against individual isoforms of the PI3K enzyme was determined via enzyme, cellular, and whole blood based assays. Potency of the compound was confirmed via leukemic cell viability and Annexin V/PI staining besides testing for inhibition of pAkt, a downstream kinase regulating cell survival and growth. These assays were conducted with cell lines (CCRF-CEM, HL-60, and MOLT-4) and patient derived cells. Anti-tumor efficacy of the compound was studied in vivo with the subcutaneous MOLT-4 xenograft model. Lastly, ADME and pharmacokinetic properties of the molecule were determined.

Results:
TGR-1202 demonstrated significant potency against PI3Kδ (22.2 nM) with several fold selectivity over the α (>10000), β (>50), and γ (>48) isoforms. Additionally, the compound inhibited B-cell proliferation (24.3 nM) and FcεR1 induced CD63 expression in human whole blood basophils (68.2 nM) indicating specificity towards the delta isoform. Viability testing demonstrated that the compound caused a dose-dependent inhibition in growth of immortalized as well as patient-derived AML and ALL cells. Reduction in viability was accompanied by a reduction in pAKT (>50% @ 0.3–1 μM) along with a significant induction in apoptosis in both cell lines (CCRF-CEM, HL-60, and MOLT-4) and patient samples. In tumor xenografts, oral administration of 150 mg/kg RP5264 salt over a 25-day period resulted in significant inhibition (>50%) of MOLT-4 tumor growth in mice. Pharmacokinetic studies across species indicated good oral absorption (>40% bioavailability for mice, rat, and dog) with favorable plasma concentrations (3–10 μM @ 20 mg/kg for mice, rat, and dog) relevant for efficacy. In addition, early toxicological evaluation of the molecule indicated a MTD > 500 mg/kg over a 14-day treatment period in Balb/c mice.

Conclusions:
TGR-1202, primarily, through its activity at the δ isoform of PI3K, has activity in both myeloid and lymphoid acute leukemia cell lines and primary patient tumors. Further evaluation of this molecule in the treatment of AML and ALL is justified, and current testing of TGR-1202 in various leukemia cell lines and within a variety of primary leukemias is ongoing.

Disclosures:
Vakkalanka: Rhizen Pharmaceuticals S A: Employment, Equity Ownership. Viswanadha:Incozen Therapeutics: Employment. Niecestro:TG Therapeutics, Inc.: Consultancy, Equity Ownership. Sportelli:TG Therapeutics, Inc.: Employment, Equity Ownership.