Antibody-drug conjugates of NAMPT inhibitors: Discovery, optimization, and preclinical characterization

Nicotinamide phosphoribosyltransferase (NAMPT) regulates the biosynthesis of NAD from nicotinamide via a salvage biosynthetic pathway. Inhibition of NAMPT depletes cellular NAD levels leading to disruption of energy metabolism and cell death. Non-targeted small molecule NAMPT inhibitors have demonstrated poor tolerability in clinical trials and in preclinical models, including cardiac and retinal toxicities in rats. In an effort to improve the therapeutic window of this drug class, we pursued a targeted-delivery approach using antibody-drug conjugates. Through a medicinal chemistry effort, we identified novel NAMPT inhibitors that incorporate chemical functionality in the solvent-exposed terminus to allow construction of enzyme-cleavable drug linkers. Additionally, we applied a pyridinium-based linker strategy that allows for traceless linker attachment through a conserved nicotinamide-mimetic moiety of NAMPT inhibitors. Candidate molecules were evaluated for NAMPT binding affinity and cellular cytotoxicity as free drugs, and for cellular cytotoxicity as ADCs with the alternate linker strategies. Comparisons across inhibitors and linker strategies provide insight into optimal design of cleavable drug linkers for this class of drugs. In vitro, the ADCs deplete NAD and lead to downstream ATP depletion in a time-dependent manner. In vivo evaluation using human tumor xenografts shows translation of the pharmacodynamic effect resulting in tumor regression in models of Hodgkin lymphoma, non-Hodgkin lymphoma, and acute myeloid leukemia. Toxicology studies in Sprague Dawley rats demonstrate excellent tolerability at active doses, with no observable cardiac or retinal toxicities at the highest tested doses in single- and multi-dose regimens. These findings detail the development of a novel payload class and optimized linker strategy for use with antibody-drug conjugates, and demonstrate a preclinical efficacy and safety profile to support continued efforts toward clinical therapeutics. Citation Format: Chris Neumann, Kathleen C. Olivas, Kung Pern Wang, Andrew B. Waight, David W. Meyer, Luke V. Loftus, Margo C. Zaval, Martha E. Anderson, Steven Jin, Julia H. Cochran, Jessica K. Simmons, Paul G. Pittman, Fu Li, Michelle L. Ulrich, Abbie Wong, Weiping Zeng, Robert P. Lyon, Peter D. Senter. Antibody-drug conjugates of NAMPT inhibitors: Discovery, optimization, and preclinical characterization [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 983.