NRH (dihydronicotinamide riboside)

Dihydronicotinamide riboside (NRH), the 1,4-dihydro reduced form of nicotinamide riboside (NR), is a naturally occurring pyridine nucleoside and the most potent small-molecule NAD+ concentration enhancer characterized to date in mammalian systems. First formally investigated as an NAD+ precursor by the Sauve laboratory at Weill Cornell Medical College in 2019, NRH increases intracellular NAD+ concentrations by 2.5- to 10-fold over baseline within one hour of administration to mammalian cells, exceeding the potency of nicotinamide riboside (NR), nicotinamide mononucleotide (NMN), and nicotinamide (NAM) by approximately 30-fold at equivalent concentrations in multiple cell lines. The compound operates through a biosynthetic pathway mechanistically distinct from all previously characterized NAD+ precursors: cellular uptake occurs via equilibrative nucleoside transporters (ENTs), followed by ATP-dependent phosphorylation to dihydronicotinamide mononucleotide (NMNH) catalyzed by adenosine kinase (ADK), adenylation to NADH by nicotinamide mononucleotide adenylyltransferases (NMNATs), and oxidation to NAD+. This pathway is entirely independent of the nicotinamide riboside kinases (NRK1, NRK2) that mediate NR salvage. Two independent laboratories (Sauve at Weill Cornell; Canto at the Nestle Institute of Health Sciences and Ecole Polytechnique Federale de Lausanne) concurrently demonstrated in 2019 that NRH is orally bioavailable in mice, is not degraded in plasma (unlike NR, which is rapidly cleaved to nicotinamide), and produces robust tissue NAD+ elevation in liver (5.4-fold), kidney (3.1-fold), brain (2.2-fold), and adipose tissue (2.7-fold) following intraperitoneal administration. The Giroud-Gerbetant et al. study further demonstrated that concurrent NRH administration prevents cisplatin-induced acute kidney injury in mice, establishing the first preclinical therapeutic proof-of-concept for an NRH-specific tissue-protective application. Subsequent investigations have extended the preclinical pharmacology to aminoglycoside-induced ototoxicity (cochlear hair cell protection via SIRT1 activation), post-traumatic osteoarthritis (inhibition of disease development and associated pain in mice), high-fat diet-induced obesity and glucose intolerance (improvements in glucose tolerance, hepatic lipid catabolism, and fat redistribution), and genotoxic stress resistance (protection against hydrogen peroxide and alkylating agent-induced cell death). The compound has also been characterized as a substrate for NQO2 (NAD(P)H:quinone oxidoreductase 2), the flavoenzyme that preferentially uses NRH rather than NAD(P)H for quinone reduction. Safety characterization indicates that NRH is well tolerated at chronic oral doses of 100 mg/kg and acute doses up to 1000 mg/kg intraperitoneally in mice, with no appreciable cytotoxicity in non-transformed cell lines at concentrations up to 1 mM. However, dose-dependent cytotoxicity has been observed in hepatocellular carcinoma cells (HepG3) through a mechanism involving reactive oxygen species generation, mitochondrial membrane depolarization, and PUMA/BAX-mediated apoptosis. Additionally, NRH activates a pro-inflammatory phenotype in macrophages through IKK-dependent signaling, potentiating lipopolysaccharide-induced cytokine expression. No human clinical trials of NRH have been reported; all pharmacological and safety data derive from cell culture and murine models. This monograph reviews the chemistry, synthesis, and stability of NRH; the adenosine kinase-dependent biosynthetic pathway; the comprehensive preclinical pharmacology across tissue-protective, metabolic, and genotoxic stress applications; sourcing and quality considerations; reconstitution and handling under oxygen-sensitive conditions; stack-interaction implications; the adverse-event and safety signal; and a structured comparative assessment of five NAD+ precursor candidates against NRH on five competency standards.