Acetaminophen modification in an attempt to lessen
the liver damage while retaining drug function
Callista Jerman
Cluster 8
COSMOS, UC Davis, 2009
Abstract
This study compared acetaminophen to nine variants that might be a possible
substitute in clinical use. Acetaminophen is a commonly used painkiller, but
when metabolized in the liver becomes NAPQI (n-acetyl-p-benzoquinone
imine), which attaches to proteins in the liver and causes liver damage.
All of the variants (numbered 1-9) were created by leaving the benzene ring
and amide portions of acetaminophen and modifying the alcohol and
hydrogen groups. The basic electronegativity and size of the molecules
remained roughly constant.
All ten structures were passed through a computerized filter to test for
toxicity, solubility and other factors. Only one passed the chosen filter
(variant 4) and since it is known that acetaminophen is already an acceptable
drug, the results of this filter were judged to be inaccurate and thus all ten of
the molecules were docked into the PPAR gamma active sites.
PPAR (peroxisome proliferators-activated receptor) is a protein in the
cannabinoid system of the central nervous system. It is known that
acetaminophen influences the cannabinoid system, especially binding to
proteins that are responsive to heat and low pH, registering pain.
After docking, it was determined that variant number 4 was closest to
acetaminophen in docking strength and solubility, while variant 8 was the
strongest overall, making it a good choice for a stronger version of the drug.
Introduction
Acetaminophen is one of the most commonly taken drugs in the world,
usually under the brand name of Tylenol. However, it can cause liver
damage in the case of overdose or an unknown combination of two drugs
that both contain the chemical. This study was in an attempt to lessen the
potential toxicity of the drug.
Acetaminophen is metabolized by the enzyme P-450 into NAPQI (n-acetaylp-benzoquinone imine). Normally NAPQI then binds to glutathione and is
excreted from the body, causing no harm. However, in the case of an
extreme overdose or a lack of glutathione (usually caused by an excess of
alcohol), NAPQI binds instead to microsomal proteins in the liver and
causes cell death. (Gahlin, Miwa, Lu, Nelson, 1984) The path for
metabolism of acetaminophen is illustrated below. Because of how NAPQI
bonds to the liver protein, we can predict that if modifications are made to
the functional groups around the benzene ring, either NAPQI will be unable
to form or, if it does, it will be unable to bind to the protein.
Photo from [Weblog] The Dangers of acetaminophen and alcohol. Live Happy. Live Healthy. Retrieved
June 26, 2009, from http://livehappylivehealthy.com/2008/09/acetaminophen-and-alcohol/
Nine variants of acetaminophen were tested as well as the original drug,
which acted as a control. All variants include the benzene ring as the
backbone, and all contain the amide functional group. Also, all contained a
heteroatom in the para position, usually in the form of a carbonyl or an
alcohol. This maintained the basic electronegativity surface and shape of the
molecules, increasing the likelihood that they would have a similar effect in
the body.
Acetaminophen was long thought to work in a similar method to aspirin and
ibuprofen. However, it is now being shown that acetaminophen actually
affects the cannabinoid system in the brain, the same that is influenced with
the use of marijuana. The proteins in the cannabinoid system react to low
pH and high temperatures, sending signals that register as pain. Because of
this, the ability of acetaminophen to influence this system is not surprising,
as it would explain both the fever reducing and analgesic nature of the drug
(Ottani, Leone, Sandrini, Ferrari, Bertolini, 2005). Because of this, all of the
acetaminophen variants were tested on PPAR gamma, a membrane protein
in the brain involved in the cannabinoid receptor system.
Method
The nine chemical structures shown below were used in this study. The
leftmost is acetaminophen itself, while the others are variants.
ChemDraw. CambridgeSoft Corporation, 2007
All are relatively the same size, and all contain the benzene-amide formation
of the original molecule. Electronegativity is maintained, except for slight
deviations caused by replacing hydrogen atoms with fluorides.
The computer programs used were Chem3D Pro, CambridgeSoft
Corporation, 2007, VIDA 3.0 OpenEye Scientific Software Inc., 2007, and
FRED RECEPTOR 2.2.5, OpenEye Scientific Software Inc., 2007.
1. First, all molecules were created in Chem 3D Pro.
2. All ten molecules were minimized, but due to time constraints,
molecular dynamics were not run
3. All molecules were opened in VIDA and placed in a single file for
quicker filtering.
4. All molecules were run through a filter called filter_drug2, which
tested for Lipinski’s Rules, solubility, molecular size and toxicity.
5. Since only one molecule passed the filter, and it was not the
acetaminophen control (which, as a working drug should pass), all
files were combined to dock
6. The active site of PPAR gamma was defined in Fred
7. All ten molecules were docked (placed in the active site to see how
they interact)
Results
Variant 1:
Solubility- high
Total docking score of -54.3
Steric docking score of -41.3
Variant 2
Solubility- soluble
Total docking score of -52.9
Steric docking score of -49.4
Variant 3
Solubility- moderate
Total docking score of -49.6
Steric docking score of -49.4
Variant 4
Solubility- high
Total docking score of -51.6
Steric docking score of -50.7
Variant 5
Solubility- insoluble
Total docking score of -48.3
Steric docking score of -50.1
Variant 6
Solubility-poor
Total docking score of -48.6
Steric docking score of -38.2
Variant 7
Solubility- high
Total docking score of -47.6
Steric docking score of -43.8
Variant 8
Solubility- high
Total docking score of -60.1
Steric docking score of -52.3
Variant 9
Solubility-high
Total docking score of -45.7
Steric docking score of -45.3
Acetaminophen
Solubility-high
Total docking score of -49.3
Steric docking score of -44.4
Discussion
Although all of the analogs used are visually similar, all had very different
scores in the filter and protein docking. Total docking varied from -48.3 to 60.1 (where greater negativity indicates stronger docking), with an average
of -50.8. Steric docking varied from -50.7 to -38.2, with an average of -46.2.
Variant 3 had the closest scores to that of acetaminophen, but exhibited only
moderate solubility and therefore would be unable to reach the necessary
receptors in the body. Variant 4 is a closer match, both in solubility and
docking strength; therefore it is the most likely replacement for
acetaminophen that would not metabolize into NAPQI. Variant 8 had high
solubility and a far stronger docking strength than acetaminophen, making it
a likely possibility for a more potent non-opiod analgesic.
There are several limitations in this study. One of the largest is the fact that
it is still not known entirely how acetaminophen works in the body.
Although recently it was discovered that the molecule influences the
cannabinoid system, researchers on the topic are still not entirely sure how it
does this. There is some debate about its influence on the TRPV1 proteins
in the brain. The second problem is related- although the PPAR protein is
definitely an important and relevant part of the cannabinoid system in the
brain; it is not by any means the only protein in that category. As PPAR is
well known and has been fully diagrammed and modeled, it is used to
represent the others in this study, but under no conditions does this mean
that because a molecule docked well into PPAR, it will be able to bind to
others in the cannabinoid system. This is especially true because the main
proteins that acetaminophen is known to affect (cannabinoid CB1 receptors)
have not been fully modeled and are not available for study. There is simply
not enough knowledge available to run these models at the greatest possible
accuracy.
Also, due to time constraints, the molecules were minimized in energy but
molecular dynamics were not run. This makes it more likely that the
structures are simply in local minima instead of absolute ones, but there is
no way of knowing for sure.
In future, many of these molecules should be tested in other cannabinoid
proteins and, if possible, the complete CB1 proteins, so we can better
understand how they might function as drugs. Also, more research should
be done on acetaminophen itself.
Acknowledgements
I would like to thank Professors Annaliese Franz, Toby Allen and Dean
Tantillo for their help and support, and especially for giving up four weeks
of their lives to teach high school students.
I would also like to thank Rebecca Davis and Phil Painter for teaching us the
software and programming necessary for our projects.
Also, thanks to all the other graduate students who helped us- Josh Hansen,
Ngon Tran, and Kaleb Jentzsch.
Of course, I must also thank the UC COSMOS program for giving all of us
this opportunity.
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