Antibiotics Pharmacology

Why does a cell wall synthesis inhibitor work?

• cell wall needs to be maintained as a required structure for bacterial survival

• new cell wall needs to be synthesized in order for bacterial cells to replicate

• more effective in gram-positive bacteria

B-lactam antibiotics

• penicillins, cephalosporins, monobactams, carbapenems and B-lactamase inhibitors

• four-membered ring = lactam ring

  • needs to be intact to have antibacterial effect

  • can be hydrolyzed by bacterial B-lactamases

B-lactams MOA

• interfere with transpeptidation required for cell wall synthesis

• B-lactam binds to the penicillin-binding protein (PBP) is a a transpeptidase enzyme, takes once AA off peptide and inhibits the formation of a cross-link to the adjacent peptide

• will cause the cell to have a weak or patchy cell wall, and isn’t enough to maintain cellular integrity

• B-lactams bind covalently to the active sites of PBPs

• B-lactams only kill bacterial cells when actively growing and synthesizing cell wall

B-lactams pharmacokinetics

• oral absorption depends on acid stability

• oral absorption impaired by food

  • except amoxicillin

  • needs to be dose 1-2 hours before or after a meal

• primarily excreted by kidneys

• CNS penetration is low

B-lactams adverse effects

• GI upset (NVD)

• secondary opportunistic infections

• skin rash - if aminopenicillin inappropriately prescribed for viral illness

• rare: bone marrow suppression, hepatotoxicity

• serious: hypersensitivity

B-lactams hypersensitivity

• are cross-sensitizing and cross-reacting within class and across classes

• antigens are presented during penicillin metabolism

  • when transiently bound to host protein, results in a hapten and is identified as non-self by the immune system

• non-dose dependent

B-lactams resistance

• primary mechanism is inactivation by B-lactamase

  • several different lactamase enzymes with different spectrums

• secondary mechanism is:

  • alterd PBP

  • impaired penetration

  • drug efflux

B-lactamase inhibitors

• clavulanic acid, sulbactam, tazobactam

• very weak/no antibacterial activity on their own

• very high (irreversible) affinity for catalytic side of B-lactamases

• impairs the ability of bacteria to hydrolyze penicillins

  • increases effective drug concentration at infection site

B-lactams in combo with B-lactamases

• can be used in combination

• addition of B-lactamase inhibitor extends activity

  • covering B-lactamase producing strains S. aureus and some gram-negative

• PK of B-lactams and B-lactamase inhibitors may differ between each other and get into different compartments

  
  

Why are there so many different b-lactams?

• by changing the chemical structure (rings and side chains) can develop antibiotics with:

  • better PK characteristics

  • expanded spectrum of action

  • resistance to B-lactamases

Penicillin

• penicillin G (parenteral)

• penicillin V (oral)

• susceptible to acidic/basic hydrolysis

Stability of penicillins in basic conditions

• the double bonded O becomes negatively charged

• B-lactam ring is hydrolyzed to penicilloic acid

  • which has no activity

  • can cause an allergic reaction

Stability of penicillins in acidic conditions

• depends on the R substituent

  • can be modified to be more stable at the acidic pH

• can undergo intermolecular cyclization and converted to penicilloic acid

Ampicillin/Amoxicillin

• semi-synthetic penicillin

• acid stable

  • due to primary amine is charged at acidic pH

  • becomes an EWG and hinders the intermolecular cyclization because decreases the electron density

• orally active

• sensitive to hydrolysis by B-lactamases and alkaline pH

Antistaphylococcal penicillins

• nafcillin

• methicillin (parenteral)

• cloxacillin (oral)

Cloxacillin

• acid-stable

• pharmacophore: isoxazole

• oral

  • food interferes with absorption

• greater degree of stability towards lactamases

  • from the additional functional groups

• greater hepatic & biliary metabolism/excretion compared to penicillin

Methicillin

• synthetic penicillin

• pharmacophore: dimethoxybenzene

  • IM/IV

• greater antibacterial spectrum versus cloxacillin

• greater degree of stability towards lactamases

  • from the additional functional groups = steric bulk

  
  

Antipseudomonal penicillins

• ticarcillin

• piperacillin

Ticarcillin

• semi-synthetic penicillin

• not stable in the acidic pH of the stomach

  • in acid will do through decarboxylation and loses activity

• pharmacophore: thiophene ring

  • diminished gram-positive activity

• sensitive to hydrolysis by B-lactamases

Piperacillin

• semi-synthetic penicillin

• not stable in the acidic pH of the stomach

• pharmacophore: piperazine

  • superior activity to pseudomonas versus ticarcillin

• sensitive to hydrolysis by B-lactamases

Penicillin: SAR summary

• need B-lactam ring

• fused to 5-thiazolidine or 6-membered ring with nitrogen and sulfur atoms

  • sulfur at C1 necessary

  • dimethyl at C2 necessary

  • carboxylic acid (or esters for prodrugs) at C3 necessary

  • tertiary nitrogen at N4 critical

• can alter substituents at C-6 to improve spectra, b-lactamase stability, in vivo stability

• has H bonds, hydrostatic interactions and VDW forces

Cephalosporins

• same MOA as penicillins

• generally, broader spectrum of activity compared to penicillins

• can be hydrolyzed by some b-lactamases

Cephalosporins pharmacokinetics

• can be prodrug esters

• eliminated renally

• decreased allergic reaction

• generally more resistant to hydrolysis by B-lactamases

• generations based of antibacterial spectrum

  • spectrum and PK sensitive to changes in C7 amino group and C3

Cephalosporins toxicity

• similar to penicillins

• are sensitizing

• cross-sensitivity between penicillins and cephalosporins is 1-5%

• serious: nephrotoxicity

• rare: may cause disulfiram-like reactions by inhibiting alcohol dehydrogenase

Cefotaxime

• not stable at acidic pH

• broad spec activity

• resistance to B-lactamases

• pharmacophore: imino-methoxy amino-thiazole

  • provides steric bulk to protect against B-lactamase

• parenteral

Ceftriaxone

• not stable at acidic pH

  • also has a thiotriazinedione (TTD)

• broad spec activity

• resistance to B-lactamases

• pharmacophore: imino-methoxy amino-thiazole

  • provides steric bulk to protect against B-lactamase

• parenteral

Ceftazidime

• pharmacophore: imino-methoxy amino-thiazole and pyridine

  • provides steric bulk to protect against B-lactamase

• has a positive charge that is highly polar

  • increased ability to penetrate gram-negative

  • poor oral absorption

• parenteral

Cefixime

• pharmacophore: imino-methoxy amino-thiazole

  • provides steric bulk to protect against B-lactamase

• has double bonded carbons

  • increases lipophilicity

• oral

Cefuroxime axetil

• prodrug (ester)

• pharmacophore: imino-methoxy furan

  • provides steric bulk to exhibit resistance to B-lactamase

• oral

Cefepime

• pharmacophore: imino-methoxy amino-thiazole and pyrrolidine

  • imino = steric bulk to exhibit resistance to B-lactamase

  • Pyrrolidine = increase activity against gram-neg

• parenteral

Carbapenems

• imipenem

  • co-administered with cilastatin (inhibitor of dipeptidase enzyme)

• meropenem

  • not sensitive to dipeptidase enzymes

• ertapenem

• all have B-lactam MOA

Carbapenems toxicity

• NV

• rare: seizures with imipenem

  • less with meropenem

• up to 10% cross-reactivity with penicillin allergy

Imipenem

• synthetic penicillin derivative w/ B-lactam

• broad spectrum

• resistance to B-lactamases

  • due to double bond in 5 member ring

• short in vivo half-life due to hydrolysis

• parenteral

  • not stable at stomach pH because no bulk substituents

Meropenem

• synthetic penicillin derivative w/ B-lactam

• broad spectrum

• resistance to B-lactamases

• pharmacophore: pyrrolidine

• parenteral

  • not stable at stomach pH because no bulk substituents

Ertapenem

• synthetic penicillin derivative w/ B-lactam

• broad spectrum

• has longer in vivo half-life versus imipenem and meropenem

  • due to benzoic acid ring (also helps increase activity)

• resistance to B-lactamases

• parenteral

Vancomycin

• cell-wall targeting agent

  • will NOT penetrate outer membrane of gram-negative bacteria

• not a B-lactam

• binds alanine-alanine terminus of peptidoglycan pentapeptide

• cyclic glycopeptide

• fermented from Amycolatopsis orientalis

Vancomycin pharmacokinetics

• not orally bioavailable

  • will not absorb, so only use orally for GI infections

  • not stable in the stomach

• short in vivo half-life

• parenteral

• 90% unchanged in urine

• distribution:

  • soft tissue

  • bone distribution

  • some CNS

Vancomycin toxicity

• hypersensitivity reactions

• injection site pain

• flushing, tachycardia, hypotension from rapid IV admin

• skin reactions

• serious:

  • ototoxicity - not correlated well to dose/concentration

  • aminoglycoside nephrotoxicity

Vancomycin resistance

• from mutation of ala-ala binding site

  • changes to ala-lactate or ala-serine

• described as Van A-type through Van E-type - based on specific genes

• other types detected but mechanism unknown

Fosfomycin

• non-B-lactam cell wall agent

• acts as a “PEP” analog which inhibits MurA enzymes that work on cell wall

• decreases ability of bacteria to interact with urinary epithelium

• very little cross-resistance with others b/c unique MOA

Fosfomycin pharmacokinetics

• oral and IV

  • very water soluble

• good for UTIs because naturally accumulates there

• prokinetic GI agents can reduce absorption

Fosfomycin toxicities

• ND and headache

• rare: neutropenia, angioedema

Protein Synthesis Inhibitors

• bacterial ribosome composed of 30S and 50S vs. mammalian 40S and 60S

• differences between bacterial and mammalian translation are enough for agent to be relatively-selective for only bacterial protein synthesis

• aminoglycosides

• tetracyclines

• macrolides

• clindamycin

• linezolid

Tetracyclines

• inhibit bacterial synthesis by binding 30S subunit, blocks binding of any incoming tRNA

  • translation may stop cause cannot add anymore AA’s and get incomplete protein

  • could get wrong AA put on the protein chain

• pharmacophore: napthacene

• tetracycline

• doxycycline

• tigecycline

• minocycline

Tetracycline pharmacokinetics

• excreted in urine and bile with some enterohepatic recycling

• tigecycline excreted unchanged

• minocycline metabolized extensively

Tetracycline toxicities

• NVD

• anorexia

• heartburn

• photosensitization

• rare:

  • liver toxicity large doses

  • renal toxicity with expired (Fanconi syndrome) - from breakdown product

  • vestibular reactions with minocycline

Tetracycline metallic chelation adverse effects

• calcium chelation, agent binds to and damages growing bones and teeth

• can cause discolouration and bone deformity if used in pregnancy and early childhood (<5y)

  • greatest risk between second trimester to 6 months

• related to dose (body weight NOT duration)

• except doxycycline because lower affinity to calcium

• also to any di or trivalent metal ion

  • cause decreased solubility and absorption

  • avoid antacids, mineral supplements and dairy

Tetracycline resistance

• Primary: drug efflux

• secondary:

  • impaired influx

  • increased expression of binding sites that are NOT active

  • enzymatic inactivation

Tetracycline Resistance - Efflux pumps

• Tet(AE): in gram-negative - newer agents are not substrates

• Tet(K): staphylococci confers resistance tetracycline

• Tet(M): in gram-positive resistance against tetracycline, doxycycline and minocycline, NOT tigecycline (bulky side chain confers resistance - but NOT in Proteus or Pseudomonas aeruginosa)

Doxycycline

• semisynthetic tetracycline - with increased stability

• lipophilic compound with long half-life

• both nitrogen substituents are important for activity

• methyl group on C6 is important for oral absorption - more stable in acidic pH

Tigecycline

• new class of tetracycline called glycylcyclines

• contains 9-t-butyl-glycoamido substituent

  • more stable towards bacterial enzymes due to steric bulk

• fairly lipophilic

• parenteral

  • due to rapid phase II conjugation by glucuronic acid

Macrolides

• binds reversibly to 50S ribosomal subunit

• inhibits transpeptidation and translocation in protein formation

  • transpeptidation is not the same as cell wall but in creating peptide chain

  • blocks adding new AA to growing chain

• isolated from Streptomyces species

• made of lactone (cyclic ester) ring and amino sugars linked by glycoside bonds

• erythromycin

• clarithromycin

• azithromycin

Erythromycin

• acid-labile (easily destroyed in acid environment)

  • must be admin with enteric coating

• food interferes with absorption

• shortest half-life of the macrolides

• extensively hepatically metabolized by CYP3A4

  • also inhibits P450 enzymes, mainly CYP3A4

  • lots of drug interaction

Clarithromycin

• best oral absorbed on the macrolides

• longer half-life than erythromycin (3-4h) and less than azithromycin

• 14-member cyclic ring

• extensively hepatically metabolized by CYP3A4

  • also inhibits P450 enzymes, mainly CYP3A4

  • lots of drug interaction

Azithromycin

• best on empty stomach

• 15-member cyclic ring

• any increase in pH (antacid) delays absorption

• longest half-life (68h)

  • NOT substrate for CYP3A4

• tertiary amine in lactone ring increases acid stability

  • will become positively charged so increases gram-negative activity

Macrolide toxicity

• anorexia, NVD

  • clarithromycin and azithromycin are better tolerated

• hypersensitivity reactions

• fever

• eosinophilia

• rashes

• hepatotoxicity due to cholestatic hepatitis

  • mainly erythromycin

Macrolide adverse effect - QT prolongation

• greatest risk erythromycin > clarithromycin > azithromycin

• if gets too long it can trigger Torsades de Pointes

• risk factors:

  • previous diagnosis

  • HR <60bpm

  • changes in configuration of heart

  • low K+ or Mg+

  • certain drugs

Macrolide resistance

• reduced permeability of cell membrane in gram-negatives

• active efflux in gram-positives

• bacterial esterases that hydrolyze macrolides

• altered ribosomal binding site

• increased expression of enzymes that methylate the ribosome

Clindamycin

• chemically distance from macrolides, but same MOA

  • binds 50S ribosomal subunit, identical to macrolides

• sulfur-containing N-methylpyrrolidylcarboxylic acid

• synthetic derivative of lincomycin

• cross-resistance may occur

• may compete for binding

• also can be given as a phosphate ester (prodrug)

  • given parenterally

Clindamycin pharmacokinetics

• fully absorbed, taken with food

• wide distribution

  • NOT the CNS

• extensively metabolized

• may accumulate if hepatic dysfunction is present

Clindamycin toxicity

• ND

• skin rashes

• impaired liver function/tests

• neutropenia

Clindamycin resistance

• ribosomal access

  • very poor porin permeability in gram-negatives

  • NOT a substrate for gram-positive efflux pumps, unlike macrolides

• decreased ribosomal binding

  • altered binding target

  • shared resistance with macrolides

Sulfonamides

• bind to dihydropteroate synthase and inhibit bacterial protein synthesis

• can also act as a false metabolite and get incorporated into DNA biosynthesis

• “sulfa allergy” or allergic skin reactions (rashes and hives) due to presence of -SO2NH2 functional group

Prontosil/sulfanilamide

• prontosil is prodrug

• sulfanilamide is active metabolite

Sulfamethoxazole

• benzenesulfonamide is essential for antibacterial activity

• isoxazole provides optimum activity

  • acts as an EWG which increases

• Hydrogen is important off chain N

  • without loss of activity

• oral

Trimethoprin

• diaminopyrimadine derivative

• inhibits enzyme dihydrofolate reductase (DHFR)

  • highly selective to bacterial DHFR (40,000x)

• primary amines important for activity

• pharmacophore: pyrimidine

• oral

DNA-targeted antibiotics

• target bacterial DNA synthesis and replication

  • different enough from human to get relative-selectivity

• fluoroquinolones

• sulfonamides

• trimethoprim

Fluoroquinolones

• bactericidal

• pharmacophore: quinolone

• ciprofloxacin

• norfloxacin

• ofloxacin

• sub-class called respiratory fluoroquinolones

  • levofloxacin

  • gemifloxacin

  • moxifloxacin

Fluoroquinolones MOA

• block DNA synthesis by inhibiting 2 bacterial enzymes:

  • topoisomerase II (aka DNA gyrase) - supercoiled DNA cannot relax

  • topoisomerase IV replicated chromosomal DNA does not separate into daughter cells during cell division

• relatively-selective 100-1000x bacterial > mammalian

Fluoroquinolones pharmacokinetics

• oral

• food delays peak concentration, but not bioavailability

• chelation with divalent cations

• wide distribution into bones and joints

• cross the placenta

• renal excretion

Fluoroquinolones toxicity

• VND

• photosensitivity

• headache, dizziness, drowsiness

• skin rash

• abnormal liver function

• QT prolongation

• hyper/hypo- glycemia

Fluoroquinolones toxicity - cartilage damage

• joint and tendon toxicity in preclinical

• not to be used in patients still in process of developing tendons and bones

  • <18y

  • >60y - tendonitis, tendon rupture

  • corticosteroids

  • immunosuppressants

  • pregnancy (relative)

Fluoroquinolones resistance

• mutation in binding region of target enzyme

  • specifically gyrA subunit of DNA gyrase

• change in permeability

• expression of proteins that protect DNA gyrase or inhibit fluoroquinolones

Ciprofloxacin

• contains piperazine ring at C7

  • increases antibacterial spectrum and PK

• contains cyclopropyl off N in ring

  • provides broad spectrum activirty

• contains fluorine off C6

  • aids in cell wall penetration

• can cause reactive oxygen species

• pharmaceutical drug interaction with calcium (and divalent/trivalent cations) due to double bonded O and COOH

  • decreases absorption and therapeutic effect

Levofloxacin

• contains morpholine ring

  • broad spectrum activity compared to ciprofloxacin

• is the S-enantiomer of ofloxacin

• more lipophilic than ciprofloxacin

• oral and parenteral

Moxifloxacin

• contains pyrrolopyridine at C7 and cyclopropyl at N1

  • increases gram-positive activity versus ciprofloxacin

• contains methoxy at C8

  • reduces degree of phototoxicity

• oral, parenteral and topical

Fidaxomicin

• 18-member macrocyclic lactone

• binds to sigma subunit of RNA polymerase

• can undergo metabolism through esterase

  • metabolite has similar activity to parent

• Chlorines on benzene rings increase activity

• less stable in acidic pH

• lots of double bonds - need to be protected from light

• oral

  • for local effect only

  • polar and will accumulate in local site (colon)

Metronidazole

• is a prodrug

  • converted to many metabolites

  • by bacterial enzymes or liver

• pharmacophore: imidazole

• NO2 needs to be at C5 for activity

• oral or parenteral

• side effects is because of the reactive metabolites

  • can react with proteins causing toxicity

Metronidazole MOA

• nitro group in chemically reduced in anaerobic bacteria/protozoa

  • leads to nitro radical anion that reacts with DNA (positively charged groups in cell)

• activated by the ferrodoxins and other electron transporters NOT found in aerobic bacteria

• after radical nitro anion reacts with target, oxidized metronidazole reformed

• O2 inhibits metronidazole by competing for electrons

Metronidazole pharmacokinetics

• oral, suppository, and IV

• widely distributed with CNS

• metabolized in liver

  • can accumulate in patients with liver dysfunction

Metronidazole toxicity

• headache

• NVD

• dry mouth, metallic taste

• disulfiram-like effects

• can increase prothrombin time

• rare: more serious CNS (ataxia, paresthesia or numbness in extremities)

  • associated with duration

Metronidazole resistance

• any changes that impair production of nitro anion

• decreased oxygen-scavenging ability

  • leads to increased O2 in the cells

  • not enough to kill bacteria, but enough to inactivate metronidazole

• lowered levels of ferredoxin and other electron transporters so less activation