A Murine Phase 2 Proof-of-Concept Study of Polatuzumab Vedotin in Combination with Venetoclax in Experimental Therapy of BCL2-Positive Aggressive Lymphomas

Mantle cell lymphoma (MCL) is a chronically relapsing B-cell lymphoma. Prognosis of R/R MCL after failure of Bruton tyrosine kinase inhibitor ibrutinib remains dismal and calls for novel treatment options. Polatuzumab-vedotin (POLA), an anti-CD79B conjugated to monomethyl-auristatin E (MMAE), revolutionized the therapy of patients with newly diagnosed and relapsed diffuse large B-cell lymphoma (DLBCL). Venetoclax (VEN) is a nanomolar inhibitor of anti-apoptotic BCL2 with promising efficacy in DLBCL and mantle cell lymphoma (MCL). It was previously shown that MMAE induces downregulation of anti-apoptotic MCL1 protein. The combination of POLA and VEN might thus represent an innovative BCL2 and MCL1-targeting strategy.

We implemented a murine phase 2 proof-of-concept efficacy study of POLA, single agent, and in combination with VEN, on a panel of 17 patient-derived xenograft (PDX) models derived from patients with R/R MCL (n=11) and R/R DLBCL (n=6), and on 8 cell line-based xenograft (CDX) models of MCL. POLA-resistant (POLA-R) tumors were derived in vivo by repeated retreatments of mice xenografted with tumors obtained from mice after failure of POLA. Expression of CD79B was quantified by flow cytometry BD QuantiBrite assay in the tumors obtained from the untreated mice (CTRL) and POLA-R tumors. To get further insight into molecular mechanisms associated with POLA resistance, bulk transcriptome analyses were performed on POLA-R tumors compared to respective CTRL tumors in 7 different MCL models (5 PDX models, and 2 CDX models). The cDNA library was sequenced on Illumina platform with PE150 mode. Reads were mapped to hg38 using HISAT2 and differential expression was analyzed using DESeq2.

We demonstrated excellent single agent efficacy of POLA in DLBCL, and MCL bearing mice including ibrutinib-resistant cases. In most of the tested PDX models, the combination of POLA and VEN induced anti-tumor synergy without measurable toxicity (e.g., wasting, diarrhea, etc.). While lack of expression of CD79B correlated with POLA resistance, the level of CD79B positivity did not correlate with the extent of POLA efficacy. After failure of POLA, CD79B downregulation was observed in several (but not all) of the tested models suggesting additional mechanisms of acquired resistance to POLA. The bulk transcriptome analysis revealed a shared transcriptome signature of acquired POLA resistance across the 7 tested MCL models. Among the most significantly (p<0.000001) upregulated genes (in the POLA-R compared to CTRL tumors) were exportin (XPO) 5, transcription factors ETV4, ETV5, PRDM1, and a pleckstrin homology-like domain member PHLDA1. Of note, BCL2 was also significantly (p<0.001) upregulated suggesting its potential contributive role in mediating POLA-resistance. Among the most significantly downregulated genes were transmembrane proteins TMEM91, TMEM160, TMEM259, TMEM86B, members of the activator protein-1 (AP-1) transcription complex FOSB, JUNB, JUND, and an array of mitochondrial genes (e.g., MT-ND1/2/4/5, MT-CO1/2/3, MT-ATP8). Upregulated pathways in POLA-R tumors including JAK-STAT, PI3K-AKT, carbohydrate digestion and absorption, or bacterial invasion of epithelial cells. Downregulated pathways comprised oxidative phosphorylation and several pathways associated with neurological diseases clearly reflecting the deregulation of mitochondrial genes. The transcriptome data thus suggest a complex net of adaptive changes that plausibly contribute to acquired POLA resistance including metabolic rewiring, deregulated signaling pathways, apoptosis, or endocytosis.

Our data strongly support investigation of POLA in combination with VEN as a BCL2- and MCL1-targeting therapy in R/R MCL and BCL2-positive R/R DLBCL in the clinical grounds.

Financial Support: Czech Health Research Council grant AZV NU21-03-00386, National Institute for Cancer Research, EXCELES- LX22NPO5102, and the Leukemia & Lymphoma Society (LLS), grant i.d. MCL 7005-24.