High Efficiency Of the PDPK1-Inhibitor, BX912, In MCL
Authors: Hutter Grit , Zimmermann Yvonne , Giller Vladimir , Zoellner Anna-Katharina , Weigert Oliver , Hiddemann Wolfgang , Dreyling Martin 
Published: October 2019

Abstract

Background
Mantle cell lymphoma (MCL) is an aggressive lymphoid malignancy with a median survival of 3-5 years. New strategies including proteasome inhibitors, immune modulatory drugs (IMiDs) and mTOR inhibitors achieve high response rate. Emerging data support the clinical efficiency of inhibitors of the B-cell receptor pathway. PDPK1 (3-phosphoinositide dependent protein kinase-1) is an important downstream target of this crucial pathway.

Methods
MCL cell lines (Granta 519, Jeko-1, Rec-1) were exposed to various PDK1-Inhibitors (OSU-03012, BX912; BX517; GSK2334470; 0,0625-1µM) and cell proliferation was analysed by WST assay. The effect of BX912 was tested on 6 MCL. Cell proliferation (trypanblue staining), induction of apoptosis (Annexin V PE/7-AAD staining) and cell cycle (FACS) were investigated. In MCL cell lines protein expression of the PI3K/Akt/mTOR pathway candidates (Akt, mTOR, eIF4E, PDK) was analysed after 24h BX912 exposure. Combined approaches were evaluated by cell proliferation analysis (WST-assay, trypan blue staining). In an alternative approach PDPK1 expression was downregulated by siRNA and consequently investigated in detail.

Results
BX912 appeared to be the most potent PDPK1-Inhibitor in the MCL cell lines tested. Sensitivity to BX912 was detected in 4 out of 6 MCL- (IC50: 0,25 -0,75µM). In addition sensitive MCL cells showed strong G2 arrest. In contrast healthy donor lymphocytes did not respond to PDPK1-inhibition. In MCL cell lines response to BX912 correlated with the PDPK1 protein expression status. Treatment with BX912 led to downregulation of PDPK1 protein expression and dephosphorylation of Rictor, Raptor, RSK and eIF4E proteins in the most sensitive to the inhibitor MCL cell line, Z138, suggesting a mode of action of BX912 mainly through the mTOR pathway. Combination of the PDPK1-inhibitors (BX912; OSU-03012, GSK2334470) with each other revealed synergism especially in combinations with GSK2334470 (BX912 and GSK2334470: CI 0,89; OSU-03012 and GSK2334470: CI 0,484; BX912 and OSU-03012: CI: 1,3), substantiating the therapeutic benefit of comprehensive PDPK1 – inhibition in MCL. Combination experiments of BX912 with inhibitors of the B-cell receptor pathway (PI3K, mTOR, PKCß) and the JAK/STAT-pathway (PIM1, JAK1/2) exhibited BX912 and the PI3K-inhibitor, CAL101, as the most potent combination (CI 0,7 -0,91) in MCL cell lines. To uncover the molecular mode of action of this combination, results of protein expression analysis will be shown.

Conclusions
The PDPK1 inhibitor BX912 shows high efficiency in MCL cells. Our data let suggest PDPK1 inhibition as a new targeted approach in MCL. However further exploration of the underlying molecular mechanisms is warranted to guide the future development of combined treatment approaches.

Disclosures:
Dreyling: Janssen: Support of IITs, Scientiffic advisory board, Speakers honoraria Other; Hoffmann-La Roche: Support of IITs, Speakers honoraria, Support of IITs, Speakers honoraria Other.