Generation of a Patient Derived T-Cell Prolymphocytic Leukemia Xenograft Model to Evaluate Pre-Clinical Treatment with Small Molecule Inhibitors <i>In Vivo</i>

T-cell prolymphocytic leukemia (T-PLL) is a rare and deadly mature leukemia with few treatment options and adverse prognosis. Currently there is no effective treatment therapy for patients with relapse disease, with an overall survival of less than 6 months. Our group demonstrated a strong T-PLL dependency on the BCL2 family of antiapoptotic proteins using BH3 profiling (2023, ASH Annual Meeting, #4192). We generated an in vivo patient derived T-PLL xenograft (PDX) model by engrafting a relapsed/refractory T-PLL patient sample into NSG (NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ) mice. This model reproduces key human clinicopathological features of T-PLL. Mice develop enlarged spleens, immunohistochemical and flow cytometry studies show engrafted T-PLL tumor cells are CD3+, CD4+ and can be detected in liver, spleen, and peripheral blood (PB). Tumor cells have TCL1 chromosomal rearrangements detected by FISH. The model was expanded and utilized in a pre-clinical trial to evaluate a novel therapeutic combination using Fadraciclib (a CDK2/9 inhibitor) and Venetoclax (BCL2 inhibitor). In vitro evidence from our team has demonstrated an additive/synergistic effect of Venetoclax in combination with Fadraciclib to cause mitochondrial dysfunction and cell death of T-PLL. These drugs inhibit separate, but partially overlapping signaling pathways covering BCL2, and MCL1. In our first set of experiments, we evaluated the effect of Venetoclax and Fadraciclib independently and in combination on the overall survival of T-PLL PDX mice. Mice were enrolled for treatment once human/Total CD45 positive cells reached 5% in PB. Fadraciclib (10mg/kg) and Venetoclax (25mg/kg) administered via gavage once daily for 5 days, 2 days of rest, and repeat for a total of 2 weeks was well tolerated. We demonstrate that therapy with Fadraciclib plus Venetoclax significantly prolonged survival compared with vehicle (p=0.0267, Log-rank (Mantel-Cox) test), and Fadraciclib alone (p=0.0103). We also evaluated the effects of these drugs on the percentage of circulating T-PLL cells in peripheral blood (PB; hCD45+). The Venetoclax and Fadraciclib combination maintained a low percentage of circulating T-PLL cells in PB, (maximum average 4.2%), Fadraciclib (20.8%), Venetoclax (17.9%), and Vehicle (42.2%) during treatment. In conclusion, we have developed a T-PLL PDX model that can be used to reproducibly evaluate the effect of new drugs and combinations for T-PLL in a preclinical setting. Combination therapy with the CDK9 inhibitor Fadraciclib and the BCL2 inhibitor Venetoclax was well tolerated, controlled the burden of circulating T-PLL cells in PB better than independent drugs and vehicle, and improved survival of T-PLL mice that were treated with the combination. Such data can provide the rationale for a clinical trial exploring this combination in patients with relapsed or refractory T-PLL.