LP-118, a Selective Bcl-2 Inhibitor with Tuned Bcl-Xl Activity, Causes Myeloid Differentiation and Cell Death in Acute Myeloid Leukemia (AML)

Introduction

Targeted therapy against bcl-2 has proven efficacious in AML, notably with bcl-2 inhibitor venetoclax. Bcl-2 is one of several proteins in the bcl-2 protein family of anti-apoptotic proteins. Primary and adaptive resistance to venetoclax-induced apoptosis is common and can be due to increased levels of other bcl-2 family proteins besides bcl-2, notably bcl-xL and mcl-1. LP-118, a next generation oral, highly potent, selective bcl-2 inhibitor with tuned bcl-xL activity, has demonstrated preliminary efficacy in venetoclax relapsed/refractory (R/R) acute lymphocytic leukemia cell lines.

Methods

Cell-based viability assays (MTS) were performed in primary AML cells and AML cell lines. Colony forming unit (CFU) assays were performed on primary AML cells and CD34+ stem cells from healthy donor umbilical cord blood. An in vivo study was conducted in a MOLM-13 AML mouse model (Charles River Laboratories; 8-week-old male NGC mice were injected with AML cells day 1, started treatment day 4). Immunoblot studies were performed to measure on-target activity of LP-118 and bcl-2 family proteins. Flow cytometry and cytospins were performed on primary AML cells and AML cell lines to test for maturation.

Results

A 24-hour (hr) continuous exposure of LP-118 against AML cell lines demonstrated growth inhibition in all except OCI-AML3 and U937 (also resistant to venetoclax), with an IC₅₀ in the nM range of 2.2 in MOLM-13 (venetoclax IC₅₀ 9), 3.5 in MV4-11, 0.75 in HL-60 (venetoclax IC₅₀ 2), 86.4 in K562, 7.8 in KG-1 (venetoclax IC₅₀ 30), and 4.3 in Kasumi-1 (venetoclax IC₅₀ 20). A 72-hr continuous exposure of LP-118 against primary AML cells demonstrated growth inhibition with an IC₅₀ of 12.9uM. Venetoclax R/R primary AML cells were sensitive to LP-118 between 5 and 100uM after 72-hr continuous exposure. Primary AML cells also exhibited a 63% decrease in CFUs at 10uM dose from vehicle, while CD34+ stem cells maintained colony forming capability by CFU assay. Induction of apoptosis was observed with caspase-3 and PARP cleavage on immunoblot in AML cell lines and primary AML cells.

Relative baseline levels of bcl-2 family proteins by immunoblot did not correlate with AML cell line sensitivity or resistance to LP-118. Following 4- and 24-hr exposure to LP-118 in MOLM-13, MV4-11, and Kasumi-1 cell lines, BCL-2, BCL-xL, MCL-1, and BCL-W protein levels were not increased relative to vehicle by immunoblot, but BCL2A1 levels increased 3-fold. Similar results were seen following a 24-hour exposure to LP-118 in primary AML cells.

Myeloid differentiation was demonstrated in primary AML cells (n=3) and AML cell lines MOLM-13 (n=2) and Kasumi-1 (n=6) by flow cytometric detection of CD11b expression after 48- and 72-hr continuous exposure to LP-118, as well as, morphologically on cytospin images. CD11b was also detected after 48-hr continuous venetoclax exposure on Kasumi-1 cells. Limiting-cell RNA-sequencing is ongoing to identify differentially expressed genes following LP-118-induced myeloid differentiation.

In a MOLM-13 mouse model, single-agent LP-118 (50mg/kg PO daily) had a statistically significant median overall survival (mOS) of 24 days compared to control (20 days), but venetoclax (100mg/kg daily for 3 days then weekly) did not (mOS 21 days). There was no significant difference in OS between LP-118+azacitidine (5mg/kg IP days 1-5 of 21-day cycle) and venetoclax+azacitidine. After 60 days, the median OS of LP-118+gilteritinib (15.24 mg/kg PO daily) was not reached and showed increased OS compared to azactidine+gilteritinib and venetoclax+azacitidine. By splenic histopathology, blasts are notably reduced in mice treated with azacitidine+gilteritnib, but spleens are effaced by neoplastic blasts in all other treatment groups.

Conclusions

LP-118 shows effective cell killing in multiple AML cell lines and primary samples with various molecular and cytogenetic profiles, including a patient sample with a PTPN11 mutation, against which venetoclax is known to be ineffective. Compared to venetoclax, LP-118 is effective at lower IC₅₀ values in AML cell lines and is active in venetoclax R/R primary AML samples. LP-118 also proves efficacious in vivo, exhibiting improved OS and decreased disease burden. Finally, LP-118 induces myeloid differentiation, to our knowledge a novel mechanism of action of bcl-2/bcl-xL inhibition in AML. These data support the recently initiated AML clinical trial (NCT04771572).