A molecular circuit linking the BCR to the NAD biosynthetic enzyme NAMPT is an actionable target in Richter’s syndrome

This works defines for the first time a molecular circuit connecting NAMPT activity to the B cell receptor (BCR) pathway. Using four distinct Richter’s syndrome patient-derived xenograft models (RS-PDX), we show that B cell receptor cross-linking results in transcriptional activation of the NAD-biosynthetic enzyme nicotinamide mononucleoside phosphoribosyl transferase (NAMPT), with increased protein expression, in turn positively affecting global cellular NAD levels and sirtuins activity. NAMPT blockade, by using the novel OT-82 inhibitor in combination with either BTK or PI3K inhibitors (BTKi or PI3Ki), induces rapid and potent apoptotic responses in all four models, independently of their mutational profile and of the expression of the other NAD biosynthetic enzymes, including nicotinate phosphoribosyltransferase (NAPRT). The connecting link in the circuit is represented by AKT, which is both tyrosine- and serine-phosphorylated by PI3K and deacetylated by sirtuin 1 and 2 to obtain full kinase activation. Acetylation (i.e., inhibition) of AKT following OT-82 administration was shown by 2D gel electrophoresis and immunoprecipitation. Consistently, pharmacological inhibition or silencing of sirtuin 1 and 2 impairs AKT activation and induces apoptosis of RS cells in combination with PI3Ki or BTKi. Lastly, treatment of RS-PDX mice with the combination of PI3Ki and OT-82 results in significant inhibition of tumor growth, with evidence of in vivo activation of apoptosis. Collectively, these data highlight a novel application for NAMPT inhibitors in combination with BTKi or PI3Ki in aggressive lymphomas.