Interestingly, citrate is used for disinfection and in food preservatives against Clostridium botulinum, an anaerobic Gram-positive pathogen [18]. In addition, citrate shows antibacterial activity against other proliferative gram-positive species, such as Staphylococci, and yeast-like fungi, such as Candida albicans [19,20].
Several years ago, we were the first to demonstrate that citrate, a well-known inhibitor of PFK and the Pasteur effect (i.e., anaerobic fermentation in yeast) [24], inhibits the proliferation of various cancer cells of solid tumors (human mesothelioma, gastric and ovarian cancer cells) at high concentrations (10–20 mM), promoting apoptosis and the sensitization of cells to cisplatin [25,26,27].
In line with our hypothesis that a low citrate level promotes the Warburg effect, it has been shown that the inhibition of paclitaxel-resistant lung cancer cells’ proliferation by dichloroacetate is associated with increasing cytosolic citrate concentration [28]. Moreover, citrate level was found depleted in malignant mesothelioma cells (compared to non-malignant cells) [29], and microRNA-126, which suppresses tumor growth, also restored citrate level through the inhibition of ACLY and the Protein Kinase B (also named Akt), a pathway promoting ACLY activation [29].
Citrate inhibits the growth of several xenograft cancer models in mice, increasing the response to chemotherapy. Indeed, daily intra-peritoneal (i.p.) injection of sodium citrate for 4 weeks (15 to 30 mg/kg/day) reduced tumor development in a gastric cancer model (SGC-7901 cells in nude mice), partly by promoting tumor apoptosis [75]. Similarly, in murine tumor models of human osteosarcoma and fibrosarcoma, i.p. injections (two times per week) of citrate (50 to 100 mg/kg), caffeine (50 to 100 mg/kg), and caffeine citrate (100 to 200 mg/kg) reduced tumor growth (with caffeine citrate showing the stronger effect), and all molecules potentiated the anti-tumoral effect of cisplatin treatment [142]. Oral citrate administration also impacts tumor growth. Indeed, oral gavage of citrate sodium (4 g/kg twice a day) for several weeks (4 to 7 weeks) significantly regressed tumors in various murine models, such as subcutaneously implanted syngeneic pancreatic tumor (Pan02), human lung adenocarcinoma (A549 cells) xenografts in nude mice, Ras-driven lung cancer in genetically engineered mouse (GEM), and breast cancer driven by human epidermal growth factor receptor 2/(Her2/Neu) in GEM [77]. Regression of tumors was frequently associated with differentiation and abundant leukocyte infiltration, predominantly constituted of T lymphocytes. Interestingly, plasma citrate levels of these chronically citrate-treated mice were approximately 3 mM, roughly eight times higher than the ones recorded for non-citrate treated mice [77]. A recent study showed that citrate also suppresses growth of PCa xenograft tumors in mice [145]. Of note, in a pancreatic cancer-xenograft murine model, 14 daily doses (500 mg/kg/day) of oral citrate induced the neutralization of TME acidity and potentiated the therapeutic effect of an oral administration of active 5-fluoro-uracil derivative [146].
Furthermore, sodium citrate is also a basic salt, which similarly to bicarbonate, may buffer acidity in TME. This effect favours the penetration of chemotherapy drugs (such as doxorubicin) in cancer cells, also improving the efficacy of mTORC1 inhibitors (rapamycin), and the response to immunotherapies [152,153,154]. The buffering of extracellular acidity could also counteract cancer cells relying on oxidative metabolism, in particular FAO, which is promoted in cancer cells by chronic acidity [109].
For clinical tests, it should be mentioned that citrate has a very low toxicity (see “citrate” PubChem CID 311, at https://pubchem.ncbi.nlm.nih.gov/, accessed on 17 June 2021), as confirmed also by in vivo studies [145], because it is an endogenous metabolite with a complete and rapid metabolism, and thus a very short half-life [160]. However, if administrated in excess, citrate could cause hypocalcaemia, muscle spasms, convulsions, and also a risk of haemorrhage due to its chelating properties of calcium and other divalent cations. These effects can be treated urgently and at best prevented by administration of calcium chloride. By extrapolating the results of a preclinical model [77], the active dose in man would be likely much lower than the one inducing the adverse effects. Clinical trials should determine the mode and duration of citrate sodium administration, its toxicity, and its efficiency. Knowing that numerous patients worldwide have incurable cancers supported by aerobic glycolysis and key oncogenic drivers (such as IGF-1R, Ras/PI3K/Akt, HER2/neu, WNT/β-catenin, TME acidity and EMT) [145,161,162], all pathways efficiently counteracted by citrate sodium in preclinical studies, we strongly believe that the citrate strategy we have proposed since many years [25] should now be considered for clinical trials. In particular, this strategy could increase both the sensitivity to standard chemotherapy drugs and to targeted therapies, whose resistance is mainly supported by the Warburg effect and its oncogenic drivers.
I am not sure what other options there are, but a large proportion of people who have a blood transfusion also have a citrate transfusion. Smaller amounts of citrate are consumed when people eat citrus fruits.
I have found in the papers you cite justification for the argument that citrate has anti-cancer effects. Obviously if a cell has almost no cytosolic acetyl-CoA it will essentially shut down protein production. However, I don’t think we want to shut down the whole protein production system in all cells.
