Download Antibiotics III

CLASS: 11:00 – 12:00
DATE: December 8, 2010
PROFESSOR: Smith
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ANTIBIOTICS III
Scribe: Adam Baird
Proof:
Page 1 of 4
PROTONSIL [S44]
a. The first antibacterial was a drug called Salvarsan (an organic arsenical). It was used for treating syphilis, but
it was not well tolerated.
b. Sulfonamides, on the other hand, were the first antibacterial drug that was well tolerated (relatively non-toxic).
c. The problem that eventually rose: resistance.
d. They were introduced in the 1930’s in Germany. It was introduced on the concept of “selective toxicity”
(agents that were selectively toxic to bacteria). What is the reasoning behind selective toxicity? Bacterial cells
(structurally, functionally) are different than human cells.
e. A man named Domagk screened azo dyes containing sulfonamides for antibacterial activity on the hope of
building upon the concept of selective toxicity.
f. Domagk discovered the first well-tolerated, widely effective antibacterial-protonosil.
TARGETS FOR SULFONAMIDES AND TRIMETHOPRIM [S45]
a. Sulfonamides have a sulfur, oxygen, nitrogen group (sulfonamide group).
b. Sulfonamides are analogs of PABA (para-aminobenzoic acid). What is PABA? Study the structure. It has a
benzene ring with amino group on one end and a carboxyl group on another end.
c. Why is it active as a bacterial? It is bacteriostatic. Why? Because PABA is one of the three building blocks of
folic acid (an essential co-factor for enzymes required for the synthesis of the building blocks of nucleic acids
and proteins), so it blocks folic acid synthesis.
FOLIC ACID SYNTHESIS [S46]
BASIS FOR SELECTIVE TOXICITY [S47]
a. Today, when a sulfonamide is used as an antibacterial, it is generally combined with trimethoprim.
b. Trimethoprim blocks the activation of folic acid. The active form of folic acid is FAH 4 (tetra-hydrofolic).
Trimethoprim inhibits the folic acid reductase. Trimethoprim was developed by targeting the bacterial folic
acid reductase; the bacterial folic acid reductase differs (modestly) from human folic acid reductase. This
subtle difference in the bacterial reductase was exploited; trimethoprim is 10,000x more potent of an inhibitor
of the bacterial folic acid reductase than the human folic acid reductase.
c. Yesterday we discussed the selective toxicity of sulfonamide (that humans acquire folic acid from diet,
whereas bacteria must synthesize folic acid). In fact, humans get folic acid from the bacteria in the GI tract.
d. By combining sulfonamide with trimethoprim, sequential steps in the production of the active form of folic acid
(the tetra-hydrofolate form) are blocked. The synthesis of the pre-cursors, the building blocks, of DNA, RNA,
and proteins are shut down (this is the basis for the antibacterial activity).
e. Why does the combination greatly decrease the development of resistance? The chance that the first step of
synthesis (where PABA is used) is about 1 in a 1,000,000 (very small). The change that trimethoprim
becomes resistant is also about 1 in a 1,000,000 (very small). The overall chance that a bacterium will
become resistant to both sulfonamide and trimethoprim is about 1 in 1x10^12 (very, very small). This is why
resistance is not a major problem with the combination of sulfonamides and trimethoprims.
f. The combination is commonly used for treating UTIs (because of the accumulation of the drugs in the urinary
tracts). They can also be used for treating lung infections (in some cases).
g. More information is given on the handout.
ANTIBIOTIC COMBINATION THERAPY [S48]
a. The synergistic killing is a very important feature of the combination of a β-lactam and aminoglycoside or the
combination of a vancomycin and aminoglycoside.
b. Combination therapy is sometimes used for infections that are caused by more than one organism or
bacterium at the site of infection – or for a severe infection with an unknown cause.
c. Prevention of resistance: SMZ (sulfamethoxazole) + TMP (trimethoprim). Antibiotic or antibacterial
antagonism (due to the combination of bactericidal and bateriostatic drug) is a problem, so it is a very special
situation when synergism can be achieved (which is why this first has to be tested in a lab prior to using it in
patients).
TETRACYCLINES [S49]
a. Tetracyclines are true antibiotics produces by Streptomyces.
b. They have 4 fused, unsaturated rings (shown in picture).
c. There are several different sulfonamides, but they all have essentially the same antibacterial activity and
antibacterial spectrum.
d. They bind to the smaller (of the two) subunits of the ribosome and block protein synthesis (acting in a
somewhat different way than the aminoglycosides).
e. They are actively accumulated by sensitive bacteria.
f. They are not bactericidal; they are bacteriostatic.
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: December 8, 2010
Proof:
PROFESSOR: Smith
ANTIBIOTICS III
Page 2 of 4
g. Resistance generally occurs by the bacteria acquiring a gene that codes for a “pump” (that pumps out the
tetracycline, preventing it from accumulating to a sufficient concentration inside the bacterium to inhibit the
ribosome). This is the fundamental basis for toxicity.
VII.
TETRACYCLINES [S50]
a. Tetracyclines have a broad spectrum (active against Gram-positive and Gram-negative bacteria). They are
also active against “unusual” bacteria (like rickettsia, mycoplasma, and Chlamydia, for example). Most
bacterial infections are caused by bacteria multiplying at the site of infection, which occurs in the extracellular
space (blood). Rickettsias are different; they go into human cells to grow and multiply and cause infection.
b. Tetracycline is typically well tolerated.
c. They form insoluble chelates (complexes with metals like calcium, magnesium, iron, aluminum etc.). If a
patient takes some form of vitamin that contains one of these metals at the same time as taking tetracycline,
a chelate will form in the GI tract (and will eventually just pass through the body). It won’t hurt, but it will
prevent the tetracycline from being absorbed.
d. They do cause some liver toxicity (potentially, especially if they are overdosed on or during pregnancy). They
are generally not used in young children (because they can be deposited in the bones or teeth, stunting bone
growth or discoloring teeth).
e. They do cause a photosensitivity reaction (may cause skin to turn red).
f. Doxycycline is sometimes used in patients who have the potential to develop kidney damage (because it is
the least likely from to nephrotoxicity).
g. They are widely distributed throughout the body when taken, so they are effective for treating infections.
VIII.
TETRACYCLINES [S51]
a. Tetracycline usage in agriculture is an important public health consideration. If you’re interested in this topic,
you may review (but it will not be covered in this lecture).
IX.
ANTIBIOTICS IN AGRICULTURE [S52]
a. The largest quantities of tetracyclines that are used are in animal feed (not for treating any infection for
livestock though, but because they have been shown to have a growth-promoting effect, potentially
harvesting animals quicker than normal).
X.
ANTIBIOTIC RESISTANCE [S53]
a. Livestock usage of tetracycline is becoming gradually discontinued. It really hasn’t been proven to actually
have any beneficial effects.
XI.
ERYTHROMYCIN [S54]
a. Erythromycin is called a lactone (the large ring structure with the oxygen atom at one end of it).
b. They come from Streptomyces.
XII.
MACROLIDES [S55]
a. There are some newer versions of erythromycin: azithromycin, clarithromycin, etc.
b. They are antibacterials that bind the larger ribosome subunit. They don’t inhibit the human (eukaryotic)
ribosome.
c. They block the elongation of translation (protein synthesis).
d. They are mainly bacteriostatic, although they can be bactericidal under some conditions.
e. They have a broad spectrum
XIII.
ERYTHROMYCIN (EM) [S56]
a. Erythromycin is affective against the organism that causes whooping cough (B. petrussis).
b. They are against mycoplasma. If a patient has pneumonia, one of the microlides may be helpful.
c. Legionella got major news coverage during a veteran convention. Legionella, as it turned out, was growing in
the cooling towers that contained water and was used for air conditioning (meaning that the bacteria spread
throughout the hotel where the veterans convention was). It resulted in a large, centralized outbreak of the
disease.
d. They are active (chiefly) against Gram-positive cocci.
e. A lot of resistance has developed against erythromycin though. This presents a cause for concern.
f. How can a soil bacterium produce an antibiotic (and not be inhibited by it)? Think about it. There is some
evident diversity among bacteria, obviously.
g. Interesting: The resistance to erythromycin (that is encountered clinically by Staphylococcus) is caused by
addition of a methyl group to the RNA of the ribosome; this addition of the methyl group is the cause of the
resistance. The bacteria that are resistant have an enzyme that methylates the 23S (large RNA ribosome).
This is what Streptomyces does (as it naturally produces erythromycin) to protect itself from the poison that it
makes – and so this is where some of the resistance might originate.
h. Erythromycin is very well tolerated.
XIV.
CLINDAMYCIN PHOSPHATE [S57]
a. Clindamycin is an antibiotic produced by Streptomyces.
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: December 8, 2010
Proof:
PROFESSOR: Smith
ANTIBIOTICS III
Page 3 of 4
b. It is important in clinical use. It is used to treat anaerobic infection (from Gram-positive bacteria). They occur
(sometimes) in the oral cavity; so Gram-positive cocci and certain anaerobes are sensitive to clindamycin.
c. It blocks the bacterial ribosome.
d. It is orally effective.
e. It penetrates most bone and tissue (except the CNS).
f. The methylation of the rRNA is the cause of resistance (as it is for the microlides, like erythromycin).
g. It is metabolized in the liver; excreted in the urine.
h. Major side effect: severe colitis (which we talked about in the previous lecture).
XV.
CLINDAMYCIN [S58]
XVI.
CHLORAMPHENICOL [S59]
a. May be used topically to treat eye infections.
b. It has an unusual structure.
c. It is a true antibiotic; it is from soil; it is from Streptomyces.
d. It is a very simple molecule too. It is small. It has a molecule weight of about 300 (similar to glucose).
e. It is widely distributed throughout the body (including the CNS).
f. It’s functional group seems to be the amide bond (shown by the arrow in the picture), which is the same bond
that links two amino acids together in the protein. So, it blocks protein synthesis at the ribosome. The amide
bond, then, is essential for the bacterial action for chloramphenicol.
g. Notice the circle around the nitro group.
XVII. CHLORAMPHENICOL [S60]
a. Chloramphenicol has a broad antibacterial spectrum (can be used against Gram-positives, Gram-negatives,
anaerobes, rickettsia, etc.). It is not active against Chlamydia though!
b. It is orally effective. It can penetrate the CNS. It is reserved for severe, life-threatening incidences though.
Why? Because it causes a low incidence (1 in 25,000 or less) of aplastic anemia (lack of blood cells), which
wipes out all blood cells (and it can be irreversible). It is thought to be not “dose-related”; however, there
should be caution when using it. Someone who takes low doses may not get aplastic anemia, as the chances
of getting it are increased with higher dosages. In fact, it can even occur after doses have been stopped. The
suspected cause of aplastic anemia: the nitro group (because it can become chemically reactive if reduced,
damaging DNA).
c. It can cause reversible bone marrow suppression too.
XVIII. CHLORAMPHENICOL [S61]
a. Chloramphenicol binds to the large subunit of the ribosome (50s) and blocks elongation, ultimately interfering
with erythromycin (EM) and clindamycin (CL) site. It preferentially inhibits the bacterial ribosome (which is a
little smaller and a little simpler than eukaryotic ribosomes, found in humans, for example).
b. It is bacteriostatic. It binds reversibly to the ribosome (it doesn’t chemically react with the ribosome).
c. Resistance is caused by a chemical modification of the chloramphenicol by a bacterial enzyme called CAT
(chloramphenicol acetyl transferase), which transfers an acetyl group to chloramphenicol and inactivates it.
XIX.
DNA GYRASE INHIBITORS: FLUOROQUINOLONES [S62]
a. Fluoroquinolones are the most used.
b. It has a fluorine atom
c. It is a large improvement over the original quinolones.
d. They are based on the structure of nalidicix acid.
e. Fluoroquinolones have increased potency and a broader range of activity against bacterial DNA gyrase
(which, recall, humans do not have; humans have other enzymes, topoisomerases, to uncoil DNA). The
topoisomerase drugs that block topoisomerase are used to treat tumors.
f. DNA gyrase is the target of fluoroquinolone (and naladicix acid).
g. They are used for treating patients who have been exposed to anthrax (bacteria that produce spores, which
resist hydration). This is when ciprofloxacin was in the news (because it can be used to prevent anthrax or for
treating anthrax). Ciprofloxacin inhibits the P450 of human cells and inhibits theophylline and warfarin
metabolism.
h. They are well tolerated.
i. They have a broad antibacterial spectrum. They are orally effective. They can be used for long-term therapy.
Obviously, it is a very important class.
j. They are also effective for simpler infections, like UTIs, diarrhea, etc.
k. They are effective in treating infections of soft tissues, bones/joint infections (by Gram-negative organisms
that are resistant against other drugs).
XX.
FLUORINATED QUINOLONES: NALADIXIC ACID DERIVATIVES [S63]
XXI.
MEMBRANE ACTIVE ANTIBIOTICS [S64]
a. Polymixin is used topically. It has a “detergent-like” action. Polymixin is one of the components of Neosporin.
CLASS: 11:00 – 12:00
Scribe: Adam Baird
DATE: December 8, 2010
Proof:
PROFESSOR: Smith
ANTIBIOTICS III
Page 4 of 4
b. They are extremely nephrotoxic (which is why they are strictly used topically, particularly against Gramnegative bacteria). Gram-positive are resistant (due to the very think PG of the cell wall).
c. They are used for treating eye/ear infections (topically).
XXII. POLYMYXINS [S65]
XXIII. METRONIDAZOLE [S66]
a. Metronidazole has a nitro group (an essential group). Metronidazole was oringially developed as an antiprotozoal agent (not as an antibacterial). But it was later determined how effective it was against Gramnegative bacteria, for example.
b. Metronidazole is also active against anaerobes.
c. It is used for treating protozoal infections.
XXIV. METRONIDAZOLE: NITRO GROUP [S67]
a. It works because the anaerobes reduce the nitro group and it becomes mutagenic (and hopefully only
mutagenic to bacterial DNA, and not human’s DNA, because humans are not very efficient as the anaerobic
bacteria). There is also some risk of damage to human’s DNA due to chloramphenicol or metronidazole.
These are the only two antibacterials that have a nitro group that have DNA mutagen potential.
b. It should be noted that metronidazole is carcinogenic (as indicated in the picture).
XXV. METRONIDAZOLE: NO2 GROUP [S68]
a. It is contraindicated during pregnancy.
b. It is well absorbed and distributed.
c. It is metabolized in the liver.
d. It may have some mild GI disturbance, but it is generally well tolerated.
e. It potentiates the anticoagulant effect of warfarin and other coumarin anticoagulants.
XXVI. NO TITLE [S69]
XXVII. “ANTIBIOTICS ARE EVERYWHERE…” [S70]
XXVIII. ANTIBACTERIALS SUMMARY [S71]
a. Antibacterials aren’t just in soil, for example. They are everywhere (our skin, frog’s skin, etc.). We can find
bacterials wherever we look! We just have to find the right ones that will be effective.
b. To produce a tumor requires a change in about 3 genes in 1 of our cells (to cause unregulated, uncontrolled
growth while maintaining growth of the tumor). We now have the ability to identify the critical genes that are
responsible for specific tumors (by developing drugs that target these particular genes). We delete about 1
trillion cells a day (via apotosis). We have to do this. We do it naturally.
c. Study the mechanisms of action and resistance. Anytime a new drug comes into clinical use, this foundation
of knowledge will help you determine whether it is a good drug or not.
d. No matter how hard we try, infectious disease will not be eliminated. A lot of research is being done, yet there
is still more to learn, know, and understand.
e. Review the summary. Study this slide and the indicated information.
[End 50:00 mins]