W2_P2 ANTIBIOTICS

Beta-lactam Antibiotics

prevents final cross-linking reaction catalyzed by transpeptidase enzymes (which are inhibited by the beta-lactams) → leading to death

Vancomycin

• bactericidal (narrow spectrum)

  • Streptomyces orientalis

• prevents transglycosidation step by acting as artificial receptor

• ultimately causing cell lysis (via preventing NAG, NAM, and peptide chain from joining together to form the bacterial cell wall)

• peptide has a rigid conformation (“capping” the building block and shielding it from transglycosidation enzyme)

• weaken cell membrane + potentiate effect of aminoglycosides when combined

Resistance to Vancomycin

• VRSA and VRE resistance → caused by mutation in pentapeptide chain (of cell wall building block)

• D-Alanine → replaced by D-lactate (changes peptide to ester bond)

  • loss of important H+ that made H-bond w/ Vancomycin = weaken the binding

  • Vancomycin can still bind but LACK of H+ weakened the binding!

• to ALL cell wall inhibitors: resistance caused by upregulation/activation of cell wall stimulus genes

Stimulus Genes

• cluster of genes in bacterial genome (cell wall synthesis)

  • e.g. bacterial sensors (wa1K, graR, rpoB, and vraS)

• activates different signaling cascades → stimulates cell wall synthesis

Resistance Modifying Agents

inhibits bacterial cell wall sensors

Aminoglycosides

• bind to ribosomal 30S

  • prevent ribosomes from moving across the mRNA

• bactericidal

  • Streptomyces griseus

• carbohydrate structure + basic amine groups

• @ physiological pH → acquire a + charge as the amines ionize

  • increases absorption through outer membrane of G-ve bacteria

Resistance to Aminoglycosides

• high selective toxicity =

  • drug is very specific for the pathogen’s structures or processes that humans don’t have (or are very different in humans) →

    • this makes the drug effective against microbes while being less toxic to human cells

• amine + hydroxide groups make key H-bonding interactions with ribosomal RNA

  • aminoglycoside modif. enzymes via 3 ways:

    1. acetylation of amino groups

    2. phosphorylation of hydroxide groups

    3. addition of ADP to hydroxide groups

• METHYLATION of key nucleotides PREVENT H-bond formation

• activate efflux pumps (in resistant bacteria)

• increase in membrane protease expression (break down faulty proteins produced due to aminoglycosides disrupting protein elongation)

Tetracyclines

• make multiple H-bonds with ribosomal 30S subunit

• bacteriostatic (broad spectrum)

  • Streptomyces aureofaciens

  • first agent: Chlortetracycline

• bind to bacterial ribosome to prevent tRNA binding

Tetracyclines SAR

• stereochemistry at all other positions is ESSENTIAL

  • NOT 6 and 7 substitutions

  • NOT substitutions in rings C and D

• tetracycline core decorated by polar groups

• lower face of rings is hydrophilic (binds with H-bonds)

• upper face is hydrophobic

Resistance to Tetracyclines

• due to farming

• introducing amino groups @ 7 and/or 9 of D ring = produce active tetracyclines to combat resistant strains to overcome bacterial efflux pumps

• ***selective toxicity arises (bacterial cells have a much higher rate of tetracyclines being concentrated inside)

Chloramphenicol

• binds to ribosomal 50S subunit → inhibits peptide chain transfer

  • Streptomyces venezuelae

• share same binding region with macrolides and lincosamides (are NOT used together in combo therapy)

• R, R-isomer is active

• chloramphenicol acetyltransferase = resistant to this drug because the enzyme acetylates important primary alcohol group

• nitro substitution on benzyl ring = chloramphenicol becomes highly toxic for humans (KEPT because it’s involved in important target interaction)

Macrolides Part I

• binds to ribosomal 50S subunit via hydrophobic interactions (hydroxide group forms a KEY interaction w/ adenine base)

  • 14 membered macrocyclic lactone ring attached to sugar and amino sugar

  • hydroxide and tertiary amine on amino sugar = ESSENTIAL for activity

  • MUTATIONS in adenine base causes erythromycin resistance

    • Streptomyces erythreus

    • first agent: Erythromycin

• ONE OF THE SAFEST!!!

Macrolides Part II

• erythromycin = unstable in stomach pH (given ORALLY as enteric tablet)

  • instability because of ketal ring formation (rearrangement of keto and 2 hydroxide groups in acidic pH)

• TO INCREASE STABILITY and IMPROVE ORAL BIOAVAILABILITY

  • protect hydroxide group

  • remove ketone group

Lincosamides

• make multiple H-bonds with ribosomal 30S subunit

  • Streptomyces lincolnensis

  • first agent: Lincomycin

• propyl group is ESSENTIAL = interfering with tRNA binding to ribosomal RNA

• propyl group binds to same ribosomal RNA base that interacts with nitro benzyl group of chloramphenicol

• resistance occur to both if mutation happens to this specific base)

Oxazolidinones

• binds to 50S subunit and PREVENTS it from combining with 30S

• this inhibits protein synthesis before it starts

  • first agent: Linezolid

• binds to ribosome via van der Waals and pi stacking (AND one H-bond through the acetamide group

• stronger agents developed by adding extra functional groups = forms more interactions w/ the binding site

Sulfonamides & Trimethoprim MOA

• bacteriostatic

• target dihydropteroate synthetase DHPS (not present in human cells) = selective toxicity

  • DHPS catalyzes reaction between PABA + pteridine diphosphate → dihydropteroic acid

  • important for biosynthesis of dihydrofolate (DHF) → converted to tetrahydrofolate (THF) cofactor*

  • THF is important for biosynthesis of pyrimidine and DNA → cell growth and division

• sulfonamides = mimic PABA enzyme substrate that bind to the active site and block access to PABA

  • competitive (reversible) enzyme inhibitors

• trimethoprim = inhibit *dihydrofolate reductase (DHFR)

  • converts DHF to THF

• sequential blocking = two enzymes (DHPS & DHPR) in one biosynthetic route are inhibited

Bacterial vs. Human Cell

• dihydropteroate synthetase DHPS (ONLY in bacterial cells) = selective toxicity

• human cells have transport protein for externally ingested folic acid used to synthesis DHF

Sulfonamide SAR

• ESSENTIAL:

  • para-Amino group (R1-H)

  • para-Amido group (R1-acyl)

  • aromatic ring (aromatic and hetero-aromatic rings are allowed)

  • para-Substitution (NOT ortho/meta)

  • Sulfonamide

• sulfonamide nitrogen (CAN ONLY BE primary/secondary)

• R2 can be varied:

  • affects solubility

  • affects plasma protein binding; determines blood levels and drug lifetime

Sulfonamide Notes + Analogues

• drugs are eliminated renally (proper for UTI)

• less soluble in acidic urine (cause crystalluria)

• to prevent crystalluria:

  • alkalize urine

  • drink 2-3 liters of water per day

• sulfa allergy:

  • DOES NOT increase likelihood of allergy to sulfur powder, sulfite preservatives, sulfate salts

  • non-antibiotic sulfonamide drugs: glipizide, furosemide, HCTZ, topiramate, celecoxib, sotalol

  • DO NOT cross react sulfonamide antibiotics

• sulfonamide analogues:

  • sulfamethoxazole + trimethoprim

  • silver sulfadiazene

Quinolone MOA

• bactericidal

• inhibit DNA replication & transcription via stabilizing the complex formed between DNA and topoisomerase enzymes:

  • inhibition → inaccessible DNA → cell death

Quinolone SAR + Analogues

• lead compound (Nalixidic acid)

• carboxylic acid and ketone (involved in binding)

• reduction of the 2,3-double bond or 4-keto group = inactivates molecule

• substitution at C-2 interferes

Ciprofloxacin

• fluoro @ C-6 (Fluoroquinolones) → Activity

• increased lipophilicity = increased penetration

• increased topoisomerase inhibitory action

Oxofloxacin and Levofloxacin

Oxofloxacin:

• racemic mixture

Levofloxacin:

• twice active