Tetracycline Antibiotics
How to increase stability of tetracycline?
SO removal of the C6 OH will INCREASE stability □ Removes the main site for degradation in both acids and bases □ Ex. Seen in minocycline and doxycycline Do NOT have an OH at position 6 (R1) ◊ Instead, have just an H More stable AND more potent Can be given much less often ◊ The ones with an OH group need to be given 4 times a day ◊ These more stable forms need only be given twice a day
which drugs are most unstable in GI?
-OH on the R1 position makes for the most unstable drug in GI and thus must be given more frequently (i.e. QID):: Tetracycline | Demeclocycline | Oxytetracycline à isotetracycline
How can tetracycline get degraded?
In both acidic and basic pH
where does tetracyline chelation happens?
etracyclines chelate with Mg inside the bacterial cells o ↑ [Mg] inside the cell
which drug can undergo CYP demethylation?
minocyclin (tertiary amine that can undergo cyp demethylation)
which drugs are used BID?
minocycline and doxycycline more potent and stable than the rest
• More recent development:
o Tigecyclin, a glycine-derived new tetracycline is more potent in vitro and has been developed for treating infections caused by bacteria strains that are resistant to commonly used tetracuclines (tetracycline, minocycline, doxycycline), methicillin and vancomycin has the 4 rings of other tetracyclines but with an additional glycine side chain attached to the tetracycline through an amide bond creates an additional binding site to ribosome and increases potency. Commonly used for skin infections
Tetracyclines Selective Toxicity
○ Human ribosomes are different in structure than bacteria Tetracyclines can't bind our ribosomes as well --> selectivity Binds the 30s MUCH better than the 40s we have Will only affect mitochondrial ribosomes (much more similar to bacteria) in very high concentrations □ Would have to cross the mitochondrial membrane a TON □ BUT you can't keep increasing the dose to treat very resistant strains - would eventually harm mitochondria ○ Concentrations inside bacterial cells will be WAY higher than in humans Accumulates inside --> much more potent in bacterial cells Due to the absorption via a transporter □ This specific transporter is not found in human cells The only way to get into human cells is via diffusion (much less common)
At acidic pH, what happens to tetracyclin?
At acidic pH: FIRST WAY! Second ring □ OH at the 6 position (R1) is removed with H (H20 eliminated) □ Ketone can also be enolized --> OH Undergoes keto-enol tautomerization Becomes Anhydrotetracycline (inactivated form) One or both of the above happening inactivates it SECOND WAY: 4th ring dimethyl amine on the fourth ring rearrange ->change stereochemisty --> form epitetracycline, ->then mean time the ring with will undergo the same rxn as above where the second ring tautomerizes and the OH leaves, forming a new double bond in the second ring, this time producing epianhydrotetracyclinee ***the OH group increases the change of this elimination process in tetracycline
what happens at basic pH? 2 ways
At basic pH: ( First WAY: second ring □OH group at the 6 position (R1) can be deprotonated can attack the ketone to form isotetracycline (opening of the ring structure). And this reaction is faster when pH is high. Second WAy: dimethylamine is in the neighborhood of an alpha -beta unsaturated ketone, and the proton (on the same carbon as the amine group) can leave, and that brings the ring to the middle flat composition, and then the H can come attack (re-protonate) and when this happens it can either regenerate the original chirality or the new one. This is why this can result epi-merization of the stereochemistry at the 4-postion. (inactive)
who should not take tetracycline? How to avoid chelation?
Tetracycline/Ca2+ complexes deposit in the teeth and bones and cause discoloration. Tetracycline interacts with photons of light which causes further degradation and discoloration of the teeth. □ NOT used in children (still building bone structure) □ Should NOT be given with antacids Will chelate in GI tract before they can be absorbed (like many others)
MORE NOTES ON TETRACYCLINE
- Tetracyclines can be given orally but the OH group at R1creates many stability issues as discussed in previous slide. When given orally, these agents have low bioavailability. They are still stable enough to be given orally but the agents with the R1 OH group are unstable in the GI tract and must be given QID - Minocycline and doxycycline don't have the R1 OH group so they are more potent and stable when given orally. Within this family, these 2 agents are more potent and more stable than the rest. They are given BID. Minocycline has a dimethyl amine group which is the site for CYP metabolism - Demeclocycline has a Cl attached to the aromatic ring at the X position. This makes the compound more risky for phototoxicity. Similar to quinolones which are polyunsaturated ring systems, the tetracyclines also have phototoxicity risks. When tetracyclines form complexes with Ca and deposit in the teeth, they can react with photons to cause discoloration. This is especially prevalent in the tetracyclines with halide groups. Cl group makes this agent even more reactive with photons. Polyunsaturated ring systems with chlorides have higher phototoxicity and are more reactive with photons.
tetracyline vs. aminoglycoside
-Tetracycline -bacterialstatic (does not kill enzyme, only slow down rate and protein synthesis) -competitive inhibition /reversible-decreases chance of tRNA binding -binds at different binding site compared to aminoglycosides -binding of tetracycline inhibits protein synthesis all together does not cause synthesis of truncated proteins aminoglycoside -bacteriacidal -Most tetracyclines do NOT bind the ribosomes as tightly as aminoglycosides
which mechanism of resistance are major? which is minor?
Major Loss of accumulation of tetracyline inside bacteria Induction fo ribosome protection proteins Minor • Induction of an enzyme that inactivates the tetracyclines by oxidative metabolism. This mechanism is rare and of minor clinical significance.
what 2 antibiotics can chelate with metals?
Tetracyclines can chelate with Ca, Mg most often Chelates between the 2 middle rings At the conjugated carboxylic acid group Same structure is also seen in quinolones □ Acidic H is lost --> deprotonated --> attracted to positive metal
Spectrum of Activity of Tetracyclines
• In general, tetracyclines have the broadest spectrum of activity but are less potent. Therefore: o Penicillins are preferred for serious infections against gram (+) infections o Aminoglycosides and cephalosporins are preferred for serious infections by gram (--) bacteria o Tetracyclines are not typically used in hospital setting because they are not potent enough
Spectrum of Activity of Tetracyclines
• Broad spectrum of activity against different bacterial strains • Problem with tetracyclines is that they have poor potency • Used against rare infections if there is nothing else active against that organism • Other agents are usually more popular for serious infections
Mechanisms of Tetracycline Resistance
• Loss of accumulation (**major**) o Mutations in the uptake transporters that are responsible for accumulation of tetracyclines (i.e. OmpF) leads to a lower concentration of tetracyclines inside the bacterial cells and greater likelihood of bacteria developing resistance o Induction of efflux pumps (Tet efflux proteins such as TetA, TetK) o Cross resistance in the class is common but not absolute • Induction of ribosome protection proteins (**major**) o Tetracyclines are competitive inhibitors of the A site so the bacteria may product protective proteins that prevent tetracycline binding o These proteins bind to the 30S subunits in such a manner that it blocks tetracycline binding but does NOT block amino acyl tRNA binding translation is thus restored. o Cross resistance to all tetracyclines occurs
Commonly Used Tetracyclines: which one has the highest photosensitivity? which will have group that is protonated at physiological pH?
• Structure Specifics ○ Commonly Used Tetracyclines: 1. Tetracycline Has methyl group at R2 2. Demecloclycine Loss of methyl group at R2 Cl added onto 1st ring Has the highest phototoxicity due to the Cl group on the 1st ring Makes a poly-unsaturated system Can produce free radicals 3. Minocycline Loss of methyl group at R2 Amino group is added onto 1st ring A tertiary amine Will be protonated at physiological pH 4. Sancycline - ALL H's 5. Oxytetracycline Has an additional OH group at R3 6. Methacycline Has a methyl at R2 Has an OH at R3 Does not have ANYTHING for R1 (because double bond is added in) 7. Doxycline Does not have an OH group at R1 (why it is called de-oxy)
MOA of Tetracyclines . how does it enter cells? what form of tetracycline binds target?
• Tetracyclines enter bacterial cells by active transport across the cell membrane (inner membrane for gram (--)) •Binds on A site of 30s subunits->prevents tRNAs from binding --> protein chain can not be made --> inhibited protein synthesis Must be chelated to Mg or Ca to bind the target site o Tetracyclines chelate with Mg inside the bacterial cells o ↑ [Mg] inside the cell o The chelated compound binds to the A site on 30S unit • The binding site of tetracyclines on the 30S subunit is different from that of aminoglycosides or other antibacterial agents that inhibit protein synthesis
What is the basic structure of tetracyclines? its action?
○ Based on 4 rings (why it is called tetracyclines) Tetracyclines are protein synthesis inhibitors
