Which antibiotics inhibit nucleic acid synthesis?
Which antibiotics are included in the first generation of Quinolones?
- 1) Nalidixic Acid
- 2) Cinoxacin
Which antibiotics are included in the second generation of Quinolones?
, which include:
- 1) Ciprofloxacin
- 2) Enoxacin
- 3) Lomefloxacin
- 4) Norfloxacin
- 5) …etc
General Characteristics of Quinolones
Quinolones are broad spectrum antibiotics
Quinolones are bactericidal at the clinical dose
Compared to the First Generation quinolones
, the Fluoroquinolones
exhibit enhanced potency
, extended spectrum of activity
, and improved distribution properties
(they are widely distributed
in body tissues).
Quinolones Mechanism of Action
- Gram Negative Bacteria
- DNA Gyrase is the primary quinolone target in Gram-negative bacteria. Quinolones inhibit DNA synthesis by inhibiting bacterial DNA Gyrase, an enzyme responsible for introducing negative supercoils into circular duplex DNA.
separate the two strands of DNA and allow replication
Gram Postive Bacteria
Bacterial Topoisomerase IV
, which separates interlinked daughter DNA molecules
that are the product of DNA replication, is the primary quinolone target
for inhibition in Gram-positive
Mammalian cells contain a type II DNA Topoisomerase enzyme
similar to the bacterial DNA Gyrase
. Quinolone serum concentrations 100- to 1000-fold higher than the bactericidal plasma concentration
are required to inhibit the mammalian topoisomerase
. Therefore, Quinolones are selective
at the clinical dose.
Mechanisms of Microbial Resistance to the Quinolones
Target Modification Expression of an altered DNA Gyrase or Topoisomerase IV with low binding affinity to the drug as a result of mutation in the genes responsible for encoding these two enzymes.
Active efflux (energy-dependent) of the drug out of bacterial cells.
Contraindications of Quinolones
Quinolones bind to polyvalent cations. As a result, antacids, iron supplements, multivitamins/minerals supplements, calcium supplements, and dairy products can decrease the oral bioavailability of quinolones by as much as 90%.
In addition, it has been shown that drugs which contain divalent or trivalent cations, such as Sucralfate (contains aluminum ions) and Didanosine tablets (contain buffers with aluminum and magnesium ions), reduce oral bioavailability of quinolones during concomitant administration.
Quinolones are inhibitors of cytochrome P450 enzymes in the liver and have been reported to increase blood levels of Theophylline, Digoxin, Caffeine, Warfarin, and Cyclosporine.
Toxicity of Quinolones
Quinolones can cause cartilage malformations
in immature laboratory animals at doses several-fold greater than the clinical dose. As a result, quinolones have the potential to cause Arthropathy
, a chronic degenerative disease of the joints
, and are not recommended for use in pregnant women and children under the age of 16
or tendon rupture may occur rarely with systemic use of any fluoroquinolone
, either while the drug is being taken or for up to several months afterwards. The risk is higher for patients > 60 years old
and for those taking corticosteroids
- Phototoxicity is more common with Sparfloxacin
- and Lomefloxacin than with other fluoroquinolones. Patients should avoid sunlight and artificial UV-light.
- Trovafloxacin, excreted primarily in the bile,
- causes serious liver injury. It should only be used to treat life-threatening infections (its use has been restricted by the FDA). [Note: these restrictions are only with this specific Quinolone.]
- In Summary, the five general properties of Quinolones, a very important class are:
- 1) They chelate polycations (just like tetracyclines), so avoid Calcium, dairy, antacids, etc.
- 2) phototoxic (just like tetracyclines), so avoid sun!
- 3) They inhibit cartilage, so they are for an "adult audience only."
- 4) They can cause tendonitis (rupture of tendons).
- 5) They are inhibitors of cytochrome P450 enzymes.
Which antibiotics are disruptors of cell membrane permeability?
- Cyclic Lipopeptides
Which antibiotics are part of the Polypeptides?
- 1) Polymixins
- 2) Gramicidin
General Properties of the Polypeptides
They are bactericidal at the clinical dose.
Large molecules (mostly cyclic).
They frequently contain D-amino acids and/or ‘unnatural’ amino acids not found in higher plants or animals.
They can be acidic, basic, zwitterionic, or neutral depending on the number of free carboxylic acid and amino or guanidino groups in their structures (polymyxins are basic; gramicidins are neutral).
They are not absorbed from the GI Tract. Therefore, must be administered parenterally...yet...
Nephrotoxic and neurotoxic when used systemically.
Which drugs are included in the Polymixins?
- 1) Polymyxin B
- 2) Polymyxin E (Colistin)
General Properties of the Polymixins
- Active mainly against Gram-negative bacilli, including Pseudomonas aeruginosa, E. coli, H.
- influenzae, and Klebsiella pneumoniae.
They are not active
cocci (N. gonorrhea
, N. meningitides
), and fungi
They are not absorbed from the GI Tract
. They do not penetrate into the CSF, synovial fluid, aqueous humor of the eye, or across the placenta.
- Because they have a very high affinity for
- cell membranes (both bacterial and mammalian), there is little systemic absorption of the polymyxins following topical administration (even when applied to open wounds).
They are administered parenterally
, or Intrathecal
) to treat severe systemic infections
caused by susceptible Gram-negative
They are nephrotoxic
and neurotoxic when administered parenterally
. They should not be used with other nephrotoxic or neurotoxic agents.
They are rarely if ever used systemically due to toxicity
and the availability of less toxic antibiotics.
The bottom line is Polymixins are used as a LAST RESORT when literally all else has failed
. This is because of their extreme toxicity
What is the difference between the parenteral and systemic route of administration?
Polymixins Mechanism of Action
Polymyxins are relatively simple, highly basic peptides
with molecular weights of about 1000 daltons.
- They are amphipathic, surface-active
- agents (containing both lipophilic and hydrophilic groups within the molecule) that act as ‘cationic detergents’.
In other words, they have a hydrophobic head
(which serves as the FUEL of the molecule
because it has Carbons
molecules) and the hydrophilic tail
, which is very polar
and water soluble
because of the many amino groups on it. Therefore, this hydrophobic tail
allows the molecule to be positively charged
in a physiologic pH, and therefore the molecule can act like soap
, a cationic detergent
They are capable of penetrating the outer membrane (cell wall)
bacteria through porin channels
to act on the inner cell membrane (The much thicker cell wall of Gram-positive
bacteria is an effective barrier to penetration by the polymyxins). So Polymixins are active against Gram Negative
and NOT Gram Positive
They bind ionically to phospholipids
(recall that on the phospholipid bilayer you have negatively charged phosphate groups on the outside that will attract the positively charged Polymixin molecule
) and penetrate into the structure of cell membranes
. As a result, they disrupt cell membrane permeability
They also bind to phospholipids of the outer membrane
bacteria, which contributes to the overall sensitivity of these organisms to the polymyxins
In vivo activity of the polymyxins is decreased by the presence of divalent cations
- The cell wall of certain resistant Gram-negative species (Proteus and Serratia) may prevent access of the polymyxins to the cell membrane.
- Cell membrane permeability changes immediately on contact with the polymyxins, resulting in the loss of intracellular components that are essential to the survival of the cell.
such as calcium, which is believed to interfere with the binding of the drug to the cell membrane
General Properties of Cyclic Lipopeptide Antibiotics: Daptomycin (Cubicin®)
Daptomycin is a bactericidal lipopeptidolactone antibiotic.
It is indicated for the treatment of serious Gram-positive bacterial infections.
It is administered as an IV infusion.
Its mechanism of action is distinct from any other antibiotic; because of its unique mode of action, daptomycin retains potency against Gram-positive bacteria that are resistant to other antibiotics including methicillin, vancomycin, and linezolid.
Cyclic Lipopeptide Antibiotics: Daptomycin (Cubicin®) Mechanism of Action
Daptomycin inhibits the biosynthesis of the lipoteichoic acid component of the cell wall of Gram-positive bacteria.
Daptomycin, in complex with calcium ions, disrupts cell membrane permeability through a membrane-seeking, surface-active behavior (similar to the mode of action of the polymyxins).
In addition, daptomycin binds to bacterial cell membranes (like the polymixins) and causes a rapid depolarization of membrane potential (unlike the polymixins); this leads to inhibition of bacterial protein, DNA, and RNA synthesis.
Toxicity of Cyclic Lipopeptide Antibiotics: Daptomycin (Cubicin®)
Major toxicities of daptomycin include superinfection (e.g., pseudomembranous colitis, Candida infections) and peripheral or cranial neuropathy.
Which antibiotics are part of the Antifolates?
- 1) Sulfonamides
- 2) Trimethoprim
Which antibiotics are Sulfonamides?
- 1) Sulfanilimide
- 2) Sulfadiazine
- 3) Sulfamethoxazole
- 4) Sulfisoxazole
General Characteristics of Sulfonamides
They are bacteriostatic at the clinical dose
- They inhibit:
- 1) dihydrofolic acid
- 2) tetrahydrofolic acid
is what both human cells and bacterial cells
use to make purines and DNA
When you inhibit Tetrahydrofolic acid
, you will inhibit the biosynthesis of DNA
- Sulfonamides are readily absorbed from the GI Tract and well distributed throughout the
- body (including the CSF). They pass through the placenta and reach the fetal circulation.
The usefulness of the sulfonamides in treating systemic infections, however, has diminished
considerably due to resistance
To overcome resistance
, sulfonamides are rarely used as single agents
to treat infections.
What is an important adverse effect of the Sulfonamides?
The sulfonamides may cause crystalluria, which is crystallization of a sulfonamide in the urine.
In other words, the drug molecule will come out of solution in the urine to become a solid particle. This causes problems because the solid causes friction (by rubbing against the tissues) and can cause kidney damage.
Ionization of the Sulfonamides
Sulfonamides are acidic molecules
, so they have positive protons
in their chemistry. These protons can be lost at certain pH levels
(such as in the pH of urine, which is between 5 or 6). This would then cause crystalluria
Crystalluria of the Sulfonamides
Recall the pKa is a measure of how strong or weak an acid
is, and therefore, how likely a molecule is to give up a proton
. Because of their chemistry, sulfonamides are likely to give up a proton
, and therefore crystallize into a solid in the pH of urine
How do we minimize the risk of crystalluria from sulfonamide therapy?
- 1) Use Sulfanilamides with lower pKa values (a pKa value closer to the pH of urine). All sulfonamides that are used clinically today have pKa
- values between 5 and 8.
2) Increase the pH of urine
by administering oral sodium bicarbonate
- 3) Increase urine flow by drinking more fluids.
- Water is the best think to drink. Avoid acidic drinks such as soda or fruit juices.
4) Avoid any decrease in the pH of urine
. As you decrease the pH of urine, you bring the pKa value further from the the pKa of solubility
Sulfonamides Mechanism of Action
They are bacteriostatic
Sulfonamides are structural analogs
and competitive antagonists
of p-aminobenzoic acid (PABA)
, thus preventing normal bacterial utilization of PABA for the synthesis of folic acid
They are competitive inhibitors
of Dihydropteroate Synthase
, the bacterial enzyme responsible for the incorporation of PABA
into dihydropteroic acid
, the immediate precursor of folic acid
Sulfonamides do not affect mammalian cells
by this mechanism. Mammalian cells utilize preformed folic acid from the diet
(they do not synthesize folic acid from PABA
Mechanisms of Microbial Resistance to the Sulfonamides
Resistance against the sulfonamides originates by random mutation and selection or by transfer of R-Factors.
Known resistance mechanisms include
Target Modification via expression of an altered dihydropteroate synthase that has lower affinity for the sulfonamides
Decreased bacterial permeability or active efflux of the drug
Developing an alternative metabolic pathway for the synthesis of THF
(e.g., the use of preformed folic acid from the host to make THF).
Increased production of PABA
(e.g., some resistant Staph. strains may synthesize as much as 70 times the amount of PABA
produced by the sensitive parent strains). Notice in the picture below the very similar chemistry between a Sulfanilamide and PABA. Thus, there is COMPETITION and inhibition is CONCENTRATION DEPENDENT. Therefore, some bacterial cells resist by increasing PABA concentration
Is there cross resistance among the sulfonamides?
Cross-resistance among the sulfonamides is 100%.
Toxicity of the Sulfonamides
Hemolytic anemia (in individuals with a genetic deficiency in glucose-6-phosphate dehydrogenase enzyme)
Aplastic anemia (bone marrow does not produce sufficient amount of red blood cells)
Agranulocytosis diminshed numbers of neutrophils
Crystalluria crystallization of drug molecule that damages kidneys by friction
General Properties of the Sulfamethoxazole - Trimethoprim Combination (Bactrim® or Septra®)
The Sulfamethoxazole-Trimethoprimcombination is bactericidal
For the greatest number of bacterial species, the most synergistic concentration
ratio of the two drugs in the combination is 20:1
- Selective toxicity against bacterial cells is achieved
- as follows:
- 1) Mammalian cells do not synthesize folic acid from PABA. They utilize preformed folic
- acid from the diet.
- 2) Trimethoprim is a highly selective inhibitor
- of bacterial DHFR. Its affinity to the bacterial DHFR is 100,000-fold higher than its affinity to the mammalian DHFR.
The Sulfamethoxazole - Trimethoprim Combination (Bactrim® or Septra®) Mechanism of Action
Trimethoprim is a potent and selective competitive inhibitor of microbial Dihydrofolate Reductase (DHFR)
, the enzyme that reduces dihydrofolic acid to tetrahydrofolic acid (THF)
Combining Trimethoprim and a Sulfonamide (sulfamethoxazole) introduces sequential blocks in the pathway
by which microorganisms synthesize THF from precursor molecules.
Since the two drugs act on sequential steps
in an obligate enzymatic pathway in bacteria, their combination is Synergistic
. In vitro and in vivo studies have confirmed the synergistic nature of the combination.
Mechanisms of Microbial Resistance to the Sulfamethoxazole - Trimethoprim Combination (Bactrim® or Septra®)
Bacterial resistance to trimethoprim sulfamethoxazole combination is a rapidly increasing problem, although resistance is lower than it is to either of the agents alone.
Target Modification Resistance is often due to the acquisition of a plasmid that encodes for an altered DHFR.
Toxicity of the Sulfamethoxazole - Trimethoprim Combination (Bactrim® or Septra®)
Trimethoprim (in Bactrim®) can cause leukopenia, megaloblastosis, and thrombocytopenia in patients with folate deficiency due to inhibition of human DHFR.
Must Antituberculosis agents be used in combination?
Yes! Anti-TB agents must be used in combination therapy.
Typically a patient would receive 3 or 4 drugs for long term treatment (at least 6 months...sometimes greater than a year)
If microbial resistance is detected during treatment, the patient is switched to the second-line of therapy.
What are the primary (first-line for treatment) Antituberculosis Agents?
What are the retreatment (second-line for treatment) Antituberculosis Agents?
Mechanism of Action for Antituberculosis Agents
): inhibits folic acid synthesis
): inhibits cell wall synthesis
): inhibits cell wall synthesis
): inhibits bacterial protein synthesis