Development of a Pathway-Directed Drug Screen Platform for Cutaneous T Cell Lymphoma Using Patient-Derived Xenograft Models

Cutaneous T cell lymphoma (CTCL) is an orphan disease and represents 3% of non-Hodgkin lymphomas. Mycosis fungoides and Sézary syndrome (MF/SS) are the most common subtypes of CTCL. The development of targeted therapies for orphan diseases is challenging, but particularly so in the case of MF/SS due to the lack of reliable preclinical models for this malignancy, which is limited to the skin at the early stage, but disseminated to lymph nodes and other organs as disease progresses. Here we report a novel patient-derived xenograft (PDX) mouse model of MF/SS that recapitulates the multi-compartmental nature of CTCL and a blood-based genetic biomarker assay for quantitative monitoring of systemic tumor burden in PDX mice. The PDX models were extensively characterized and exhibit cardinal clinical and histologic features of CTCL, including erythematous scaly skin lesions and eventual lymphomatous dissemination to the spleen and other organs, and maintain the molecular characteristics of their clinical counterparts. The malignant T cells harvested from spleen of PDX mice shared identical TCR sequences and immunophenotypes with corresponding MF/SS patient donor, which featured as losing CD7 and CD26 expression in CD4⁺ T cells. We also developed a quantitative assay of tumor burden in PDX mice by determining the amount of human β-actin cell-free DNA (cfDNA) in the plasma. The cfDNA levels in the plasma were increased in a linear fashion and correlated with the tumor growth post-implantation.

To explore the utility of this PDX model for drug testing, we searched for novel agents for CTCL by performing a high-throughput screen of selected available targeted agents and identified phosphatidylinositol 3-kinase (PI3K) as a high value target. A PI3K inhibitor was advanced to our PDX model with favorable results including disease attenuation and survival prolongation. Further experiments using isoform-specific siRNA knockdowns and isoform-selective PI3K inhibitors identified PI3K-δ as the tumor-driving isoform. Additional studies showed synergistic combination of PI3K-δ inhibitors with histone deacetylase (HDAC) inhibitors. The particularly potent combination of copanlisib and panobinostat is proposed for further clinical development.