Decreased nicotinamide adenine dinucleotide (oxidized form) (NAD ⁺ ) levels are reportedly associated with several aging-related disorders. Thus, supplementation with NAD ⁺ precursors, such as nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR), exhibits beneficial effects against these disorders. However, the in vivo metabolic pathways of NMN and NR remain to be elucidated. In this study, we comprehensively analyzed the fate of orally and intravenously administered NMN and NR in mice using NAD ⁺ metabolomics. We found that only a small portion of orally administered NMN and NR was directly absorbed from the small intestine and that most of them underwent gut microbiota–mediated deamidation and conversion to nicotinic acid (NA). Moreover, intravenously administered NMN and NR were rapidly degraded into nicotinamide and secreted to bile followed by deamidation to NA by gut microbiota. Thus, enterohepatic circulated NA is preferentially used in the liver. These findings showed that NMN and NR are indirectly converted to NAD ⁺ via unexpected metabolic pathways.

Glioblastoma multiforme (GBM) is an incurable brain cancer with an average survival of approximately 15 months. Temozolomide (TMZ) is a DNA alkylating agent for the treatment of GBM. However, at least 50% of the patients treated with TMZ show poor response, primarily due to elevated expression of the repair protein O⁶-methylguanine-DNA methyltransferase (MGMT) or due to defects in the mismatch repair (MMR) pathway. These resistance mechanisms are either somatic or arise in response to treatment, highlighting the need to uncover treatments to overcome resistance. We found that administration of the NAD⁺ precursor dihydronicotinamide riboside (NRH) to raise cellular NAD⁺ levels combined with PARG inhibition (PARGi) triggers hyperaccumulation of poly(ADP-ribose) (PAR), resulting from both DNA damage-induced and replication-stress-induced PARP1 activation. Here, we show that the NRH/PARGi combination enhances the cytotoxicity of TMZ. Specifically, NRH rapidly increases NAD⁺ levels in both TMZ-sensitive and TMZ-resistant GBM-derived cells and enhances the accumulation of PAR following TMZ treatment. Furthermore, NRH promotes hyperaccumulation of PAR in the presence of TMZ and PARGi. This combination strongly suppresses the cell growth of GBM cells depleted of MSH6 or cells expressing MGMT, suggesting that this regimen may improve the efficacy of TMZ to overcome treatment resistance in GBM.

Through evolution, eukaryote organisms have developed the ability to use different molecules as independent precursors to generate nicotinamide adenine dinucleotide (NAD⁺), an essential molecule for life. However, whether these different precursors act in an additive or complementary manner is not truly well understood. Here, we have evaluated how combinations of different NAD⁺ precursors influence intracellular NAD⁺ levels. We identified dihydronicotinic acid riboside (NARH) as a new NAD⁺ precursor in hepatic cells. Second, we demonstrate how NARH, but not any other NAD⁺ precursor, can act synergistically with nicotinamide riboside (NR) to increase NAD⁺ levels in cultured cells and in mice. Finally, we demonstrate that the large increase in NAD⁺ prompted by the combination of these two precursors is due to their chemical interaction and conversion to dihydronicotinamide riboside (NRH). Altogether, this work demonstrates for the first time that NARH can act as a NAD⁺ precursor in mammalian cells and how different NAD⁺ precursors can interact and influence each other when co-administered.

We report the synthesis of vitamin B1, B2, and B3 derived nucleotides and dinucleotides generated either through mechanochemical or solution phase chemistry. Under the explored conditions, adenosine and thiamine proved to be particularly amenable to milling conditions. Following optimization of the chemistry related to the formation pyrophosphate bonds, mixed dinucleotides of adenine and thiamine (vitamin B1), riboflavin (vitamin B2), nicotinamide riboside and 3-carboxamide 4-pyridone riboside (both vitamin B3 derivatives) were generated in good yields. Furthermore, we report an efficient synthesis of the MW+4 isotopologue of NAD⁺ for which deuterium incorporation is present on either side of the dinucleotidic linkage, poised for isotopic tracing experiments by mass spectrometry. Many of these mixed species are novel and present unexplored possibilities to simultaneously enhance or modulate cofactor transporters and enzymes of independent biosynthetic pathways.

Dihydronicotinamide rioside (NRH), the reduced form of nicotinamide riboside (NR), is a recently identified, naturally occurring precursor of arguably the most crucial cofactor for cellular function, nicotinamide adenine dinucleotide (NAD+). Recent investigation suggests that NRH is more adept at increasing NAD+ stores than traditional NAD+ precursors, and such extreme NAD+ boosting via NRH supplementation induces cytotoxicity in certain cellular contexts. It has also been shown that the lack of functional BRCA protein in epithelial ovarian cancer (EOC) directly impacts intracellular NAD+ levels. Given that altered cellular metabolism and DNA repair mechanisms are central alterations in EOC, and these processes are functionally dependent on NAD+, we sought to assess whether NRH supplementation in EOC cell lines enhanced cellular cytotoxicity alone and in combination with standard therapeutic agents. Significant cytotoxicity was noted in NRH treated cells (~40%) with minimal cell death in the nicotinic acid (NA)-treated lines. Levels of NAD(P)H were confirmed to have increased with NRH supplementation, albeit at different levels among the different cell lines. Overall, the cytotoxicity associated with NRH supplementation appears to be independent of ROS generation. Strikingly, NRH supplementation enhanced cytotoxicity of carboplatin in OVCAR8, but not ES2 or SKOV3. Paclitaxel cytotoxicity was also enhanced by the addition of NRH in OVCAR8, but not ES2 or SKOV3 cell lines. NA supplementation had no effect on baseline treatment-induced cytotoxicity. PARP inhibition by olaparib requires NAD+. Interestingly, NRH supplementation enhanced olaparib cytotoxicity in SKOV3 and OVCAR8, but not ES2 cells. NRH in combination with olaparib completely altered mitochondrial respiration, thereby shutting down energy consumption, which would lead to cell death. Coupled together with expression data of key enzymes required for NRH/NAD metabolism, this could be key in understanding mechanisms of cell death with NRH supplementation. Here, we showed that in the context of EOC, exploitation of the NAD+ bioenergetic phenotype through NRH supplementation is a biologically feasible strategy to enhance the response of traditional therapy with potentially minimal toxicity. These data suggest several potential mechanisms by which cellular NAD+ availability impacts treatment efficacy and resistance and highlights the potential utility of NAD+ metabolomics as a biomarker to guide treatment decisions.