Friday, 5 August 2016

revisiting quercetin

Found this on another site.  In light of earlier observations about quercetin, maybe some reevaluation is warranted.   Thoughts Stephen


Here is what I have found out about the Temodar resistance of MGMT unmethylated:
"Our results demonstrated that U87/TR was MGMT negative, which indicated that MGMT made no contribution for TMZ-resistance of U87/TR.
It indicated that activation of Akt and Wnt/β-catenin pathways may be response for the chemo-resistance and increased invasion of U87/TR cells, and the phosphorylation of PRAS40 and inactivated mTOR may be related to cell cycle arrest in U87/TR cells."
http://www.ncbi.nlm.nih.gov/pubmed/27423571

And here they see that Quercetin could reverse that:

The cell survival rate was less than 50% in U87/TMZ group that was pretreated by quercetin combined with TMZ at the concentration of 50 μmol·L-1.Conclusion The drug-resistant U87/TR cell line was established. Combination of quercetin with TMZ could reverse drug resistance of U87/TR cells.  It suggests that quercetin combined with TMZ maybe enhances chemotherapeutic effect on glioma.

3 comments:

  1. The devil is in the details. This part about quercetin is from a different study in a Chinese journal I can't access.

    http://en.cnki.com.cn/Article_en/CJFDTotal-YYDB201306007.htm

    "pretreated by quercetin combinated with TMZ at the concentration of 50 μmol·L-1"

    Was it the quercetin or the TMZ that was diluted to 50 uM? I wouldn't be surprised if they had used quercetin at that concentration, it would be typical.

    The first thing one needs to ask about any in vitro study is: is this drug concentration achievable?

    50 uM of quercetin (if that's what was used) is a joke frankly, as far as in vivo achievability, unless quercetin were to be injected directly into the tumor. After humans ingested 500 mg quercetin 3 times daily for 7 days maximum plasma levels of quercetin were 1.5 uM including all conjugated forms, but unconjugated (free) quercetin was only found at 0.05 uM. So the quercetin concentration of 50 uM is 33 to 1000 times higher than levels you would get in the plasma with ordinary oral dosing. This doesn't even consider the blood-brain barrier. When rats were injected intraperitoneally with quercetin (50 mg/kg), brain levels of quercetin in the rats were only 0.53 uM (almost 100 times lower than 50 uM).

    http://www.ncbi.nlm.nih.gov/pubmed/24252772

    The authors of this same rat study linked directly above had previously found an in vitro effect of quercetin at 25 uM. They later did the rat study to validate the in vitro study and found the opposite of what they were expecting: an increase in tumor volume rather than a decrease. The difference between the in vitro concentration of 25 uM and what was actually achieved in the rat brain (0.53 uM) was a 47-fold difference.

    This does not necessarily mean quercetin is always counterproductive, though it was in this particular tumor model at that particular dose, with that particular species of rat.

    In a different model (C6 rat glioma, Sprague Dawley rats) intravenous quercetin led to slightly smaller tumor volumes, and the combination of quercetin + chloroquine (both intravenous) was even more effective.

    http://www.ncbi.nlm.nih.gov/pubmed/26454746
    "Inhibition of autophagy induced by quercetin at a late stage enhances cytotoxic effects on glioma cells."

    However, it's highly likely that far higher levels of both drugs were achieved in the plasma and tumors of these rats by this intravenous administration than would be achieved by oral administration.

    My view is that compelling evidence for oral use of quercetin for glioma is lacking, but as the saying goes, absence of evidence is not evidence of absence (of efficacy), and there's a chance it could help, but the finding in rats mentioned above makes me especially cautious about quercetin.

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    Replies
    1. Hi Stephen,
      That's an excellent explanation on quercetin! Regarding the other flavonoids, are the IRL achievable concentration also about 0.5 - 1 uM?

      One thing I've noticed in most in vitro studies is that they often employ TMZ in concentrations of 100 uM, and even then it is often only moderately cytotoxic. But IRL it seems to be far more effective than that? Are the metabolites of it more toxic?

      How should in vitro studies showing a marked hormetic response be considered as opposed to in vivo studies, like the one on Quercetin above? I found this one on Honokiol which states "While EGCG was relatively consistent in producing sigmoidal dose–response curves for cell death, the dose–response kinetics for HNK varied between cell types. In addition, HNK produced marked hormetic responses (stimulation of cell growth at low doses) on cell viability in all cell lines, which were not apparent with EGCG."

      http://www.ncbi.nlm.nih.gov/pubmed/23807168

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    2. Hi Peter,
      In general, the total plasma concentrations of nutraceutical polyphenols tend to be low (1 or 2) micromolar at best, but only considering free, unbound (to plasma proteins) and unconjugated forms, the concentrations are much smaller (ie nanomolar). It's far from certain how much activity the glucuronidated or sulfonated conjugates have, if any, relative to the free compound. This sort of conjugation takes place to increase the water solubility of the compound to prepare it for elimination from the body in the urine etc. (see phase II detoxification). My working assumption is that only free, unconjugated forms of a compound have full activity. The free, unbound forms are what get tested in the lab in vitro.

      That's a great question about TMZ. Achievable concentration of TMZ is about 5 uM. However, TMZ is a prodrug that is spontaneously converted to its active metabolite in the body, (without requirement for enzymatic metabolism by the liver). My theory is that this conversion takes place more slowly in in vitro laboratory conditions than it does in the body, so that higher concentrations are required in vitro than are effective in vivo. It's also a possibility that there is some immunogenic cell death happening with TMZ treatment that would not be a factor in vitro, though this remains to be proven.

      I feel I need to consider your last question further before replying. All very good questions!

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