Venetoclax As a Combination Partner with NAMPT Inhibitor Rpt-1G for T-ALL
Authors: Sanchez John R. , Diaz Rohena Daisy Y. , Williams Charmelle D. , Ciaurro Valerio , Wu Min , Roushar Francis , De Jesús-Díaz Dennise A. , Crimmins Gregory , Puduvalli Vinay , Vega Francisco , Thandapani Palaniraja , Sampath Deepa 
Published: December 2024

T-cell acute lymphoblastic leukemia (T-ALL) is a rare and aggressive malignancy. While most cases of T-ALL occur in children, most deaths (4 out of 5) occur in adults. Though the exact reason for this disparity is not known, it could be related to limited response to chemotherapy seen in adults, as about half of patients in remission will relapse due to the development of resistance, highlighting the need for novel therapeutics.

B-cell lymphoma-2 (BCL-2) family proteins are essential regulators of apoptosis. These proteins have either pro- (BAX, BAK) or anti- (BCL-2, BCL-xL, MCL-1) apoptotic effects and are targets in treating many hematological malignancies. BCL-2 is an anti-apoptotic protein that is overexpressed in ALL. Currently, the BCL-2 inhibitor, venetoclax, is being tested in combination with many other drugs in clinical trials.

Nicotinamide phosphoribosyl-transferase (NAMPT) is a rate-limiting enzyme of the NAD+ salvage pathway and is upregulated in many cancers, including T-ALL. NAMPT works by combining nicotinamide and 5-phosphoribosyl-1-pyrophosphate into nicotinamide mononucleotide which is then used to create NAD+, restoring its levels. NAD+/NADH is a coenzyme that plays a significant role in many cellular processes, such as metabolism and DNA maintenance. While previous NAMPT inhibitors (NAMPTi) had significant anti-cancer effects, they also had major on-target toxicities. This is because complete inhibition of NAMPT disrupts critical biological functions in both normal and cancer cells.

RPT-1G is a 1st-in-class small molecule hyperbolic inhibitor of NAMPT. This novel inhibitor reduces NAMPT enzymatic activity but never completely turns off the enzyme, allowing healthy cells to survive while severely affecting the cancer cells that are addicted to NAD+ for survival. Because of this, we hypothesize that RPT-1G can synergize with venetoclax leading to an increase of efficacy at lower concentrations compared to monotherapy with either drug.

We first characterized BCL-2 dependencies in six T-ALL cell lines. Of these six, three were found to have minimal response to venetoclax when given a range of concentrations for 24 hours. When these same cells were tested with RPT-1G, a similar trend was observed with the venetoclax-insensitive cells. The venetoclax-insensitive cell lines required higher concentrations of RPT-1G to show any response when treated for 24 hours compared to the venetoclax-sensitive cell lines. However, when combined, these two drugs were able to greatly decrease viability compared to monotherapy with either venetoclax or RPT-1G at 24 hours.

In summary, our study defines the efficacy of venetoclax and the novel hyperbolic NAMPTi, RPT-1G, alone and in combination in several T-ALL cell lines. These results suggest that the combination of these two drugs could perform better together than alone and holds significant future implications for patients with venetoclax-resistant T-ALL. Future studies will focus on testing RPT-1G alone and in combination with venetoclax and other FDA-approved therapies on PDX models and primary samples to further explore the utility of RPT-1G in treating T-ALL. Additionally, we aim to characterize the effects of RPT-1G on important biological processes in T-ALL that rely on NAD+. Overall, our findings contribute to a deeper understanding of how venetoclax-insensitivity could be overcome, paving the way for new treatment strategies in T-ALL.