Lecture 25 - Chapter 12

Drugs, Microbes, Host – TheElements of Chemotherapy

Therapiestocombatcausalagentsofdisease

Suppor6veTherapies

Preven6veTherapies

Supportive Therapies:

• Reduction in Stress• Improvement in Diet• Exercise• Good Hygiene

Preventive Therapies:(fending off, interception/prevention of causal agents)

including antimicrobial chemotherapy

• Prevent transmission of causal agents– Disinfection (inanimate objects)– Eradication of reservoirs– Elimination of vectors– Antisepsis

• Prevent infection– Restriction & localization by enforcing

physical, chemical and biological barriers

Antimicrobial Chemotherapy

• Goal of antimicrobial chemotherapy:administer a drug to an infected person,which destroys the infective agent withoutharming the host’s cells

• Chemotherapeutic agents are described withregard to their origin, range of effectiveness,and whether they are naturally produced orchemically synthesized

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Origins of Antimicrobial Drugs

• Antibioitics are secondary metabolicproducts of aerobic bacteria and fungi– Bacteria: Streptomyces and Bacillus– Molds: Penicillium and Cephalosporium

• Chemists have created new generations ofdrugs by altering the structure of naturallyoccurring antibiotics (semi-synthetic drugs)

• Researchers continue to search for newantimicrobial compounds also in organismsother than bacteria and fungi (bioprospecting)

Interactions between Drug & Microbe

• Goal of antimicrobial and antiviral drugs:– Disrupt cellular processes or structures of bacteria,

fungi, and protozoa– Inhibit virus replication

• Most antimicrobial and antiviral drugs– interfere with the function of enzymes required to

synthesize or assemble macromolecules or– destroy structures already formed in the cell

• Antimicrobial drugs should be selectively toxicin that they destroy or inhibit microbial cellswithout damaging host tissues.

MajorMechanismsofDrugAc6on• Inhibition of synthesis and interference with

structure and function of nucleic acids• Inhibition of synthesis and interference with

structure and function of proteins• Inhibition of cell wall synthesis• Interference with cell membrane structure

or function• Inhibition of specific metabolic pathways

such as folic acid synthesis

Figure 12.1

An6microbialDrugsthataffectNucleicAcidSynthesis

• Blocksynthesisofnucleo0des

• Inhibitreplica0on(DNAsynthesis)

• Inhibit(preventorstop)transcrip0on(RNAsynthesis)

An6microbialDrugsthatblockProteinSynthesis

• Inhibittransla0onbyreac0ngwiththeribosome‐mRNAcomplex

• selec0veeffectispossiblebecause:bacterialribosomesaredifferentfromeukaryo0cribosomes

Figure 12.4

An6microbialDrugsthatAffecttheBacterialCellWall

• Ac0vecellswithacellwallmustconstantlysynthesizenewNAM‐NAGunits,transportthemacrosstheplasmamembranetotheproperplaceandincorporatethemintotheexis0ngpep0doglycanlayerinthecellenvelope

• Penicillinsandcephalosporinsreactwithoneormoreoftheenzymesrequiredtocompletethisprocess(PBPs)

===>Bactericidalan0bio0csFigure 12.3

Figure 12.2

An6microbialDrugsthatDisruptCellMembraneFunc6on

• Damagedcellmembranesinvariablyresultindeathfromelimina0onofgradientsorlysis

• Specificityispossiblebecausepar0cularmicrobialgroupshavedifferencesinthetypesoflipidsintheircellmembranes

An6microbialDrugsthatInhibitFolicAcidSynthesis

• Sulfonamidesandtrimethoprim‐actviacompe66veinhibi6on

• Suppliedtocellsinhighconcentra0onstomakesureenzymeisconstantlyoccupiedwiththemetabolicanaloginhibitorratherthanthetruesubstrate

Figure 12.5

PABA: para-aminobenzoic acid

MajorAn6microbialDrugGroups

• About260differentan0microbialdrugs

• Classifiedin20drugfamilies

• Largestnumberofan0microbialdrugsareusedtocombatbacterialinfec6ons

An6bacterialDrugsTarge6ngtheCellWall

• Penicillingroup– Mostendinthesuffix–cillin

– Canobtainnaturalpenicillinthroughmicrobialfermenta0on

– Allconsistofthreeparts:athiazolidinering,abeta‐lactamring,andavariablesidechain

R

Figure 12.6

SubgroupsandUsesofPenicillins TheCephalosporinGroupofDrugs

• Newerthanpenicillins

• Currentlyaccountforthemajorityofallan0bio0csadministered

Figure 12.7

Penicillins

SubgroupsandUsesofCephalosporins

• Broad‐spectrum

• Resistanttomostpenicillinases(PBPs)

• Causefewerallergicreac6onsthanpenicillins

• Fourgenera0onsofcephalosporinsexistbasedontheiran0bacterialac0vity

OtherBeta‐LactamAn0bio0cs

• Imipenem

(ac0vityagainstPseudomonasaeruginosaandtheEnterococcusspecies.

Itisnotac0veagainstmethicillin‐resistantStaphylococcusaureus,MRSA)

• Aztreonam(strongac0vityagainstsuscep0bleGram‐nega0vebacteria,includingPseudomonasaeruginosa.Ithasnousefulac0vityagainstGram‐posi0vebacteriaoranaerobicbacteria)

OtherDrugsTarge0ngtheCellWall

• Bacitracin - interferes with bactoprenol, which exports “pre-fabricated” NAM-NAG units across the bacterial plasma membrane

• Isoniazid - It is activated by KatG to form isonicotinic acyl radical;react with a NADH to form isonicotinic acyl-NADH complex. This complexwill inhibit the synthesis of mycolic acid in the mycobacterial cell wall.

• Vancomycin - for the treatment of serious, life-threateninginfections by Gram-positive bacteria which are unresponsive to other lesstoxic antibiotics.

• Fosfomycin trimethamine - treatment of urinary tractinfections, where it is usually administered as a single dose

An6bacterialDrugsTarge6ngProteinSynthesis

• AminoglycosideDrugs– Productsofvariousspeciesofsoilac0nomycetesinthegeneraStreptomycesandMicromonospora

– Rela0velybroadspectrumbecausetheyinhibitproteinsynthesis

– MostusedtocombataerobicGram‐nega0verodsandcertaingram‐posi0vebacteria:

• Streptomycin:Bubonicplagueandtularemiaandgoodan0tuberculosisagent

• Gentamicin:Lesstoxicandusedforgram‐nega0verods

TetracyclineAn0bio0cs

• Bindtoribosomesandblockproteinsynthesis• Broad‐spectrum• UsedtocombataerobicandanerobicGram‐posi0veandGram‐nega0verodsandcocci– Mycoplasmas,rickeKsias,andspirochetes– STDs,RockyMountainspo]edfever,Lymedisease,typhus,Mycoplasmapneumonia,cholera,leptospirosis,acne(Doxycyclineandminocycline)

– someprotozoa

Chloramphenicol

• Uniquenitrobenzenestructure

• En0relychemosynthe0c

• Blockspep0debondforma0onandproteinsynthesis==>Broad‐spectrum

• Verytoxictohumancellssoitsusesarerestricted

ErythromycinandClindamycin

• Erythromycin– Largelactonerinigwithsugarsa]ached– Rela0velybroad‐spectrum– Fairlylowtoxicity– Blocksproteinsynthesisbya]achingtotheribosome– Mycoplasmapneumonia,legionellosis,Chlamydiainfec6ons,pertussis,diphtheria

• Clindamycin(derivedfromlincomycin)– Broad‐spectrum– Causesadversereac0onsintheGItract,soapplica0onsarelimited.O_enusedagainstoralcavityinfec6ons.

SynercidandOxazolidones• Synercid

– Combinedan0bio0cfromthestreptogramingroup– Effec0veagainstStaphylococcusandEnterococcusspeciesandagainstresistantstrainsofStreptococcus

– Bindstositesonthe50Sribosome,inhibi0ngtransla0on• Oxazolidones

– Inhibittheini0a0onofproteinsynthesis– Notfoundinnature==>Hopethatdrugresistanceamongbacteriawillbeslowtodevelop

– Usedtotreatinfec0onscausedbytwoofthemostdifficultclinicalpathogens:methicillin‐resistantStaphylococcusaureus(MRSA)andvancomycin‐resistantEnterococcus(VRE)

An6bacterialDrugsTarge6ngFolicAcidSynthesis

• Sulfonamides,Trimethoprim,andSulfones– Sulfonamides(“Sulfadrugs”)

• Firstmodernsynthe0can0microbialdrug• Usedtocombatshigellosis,acuteUTIs,certainprotozoaninfec6ons

– Trimethoprim• Inhibitstheenzyma0cstepimmediatelyfollowingthestepinhibitedbysulfonamidesinthesynthesisoffolicacid

• O_engivenincombina0onwithsulfamethoxazole• OneoftheprimarytreatmentsforpneumoniacausedinAIDSpa0entsbyPneumocys6scarinii(andP.jiroveci),calledPCP

– Sulfones• Chemicallyrelatedtosulfonamides• Lacktheirbroad‐spectrumeffects• Keydrugsintrea0ngleprosy(Hansen’sdisease)

An0bacterialDrugsTarge0ngDNAorRNAsynthesis

• Fluoroquinolones

• Highpotencyandbroadspectrum

• InhibitawidevarietyofGram‐posi0veandGram‐nega0vebacteriaevenatlowconcentra0onsofbacteria

NorfloxacinandCiprofloxacin

• UTIs,STDs,gastrointes6nalinfec6ons,osteomyeli0s,respiratoryinfec0ons(anthrax!),so_0ssueinfec0ons

SparfloxacinandLevofloxacin

•Pneumonia,bronchi0ssinusi0s

Rifampi(ci)n

• ProductofthegenusStreptomyces

• Limitedinspectrum

• Mainlyforinfec0onsbyseveralGram‐posi0verodsandcocci(afewGram‐nega0vebacteria)

• Inpar0cular,usedagainstmycobacterialinfec6onssuchastuberculosisandleprosy

• Usuallygivenincombina6onwithotherdrugs

An6bacterialDrugsTarge6ngCellMembranes

• Polymyxins:narrow‐spectrumpep0dean0bio0cs– FromBacilluspolymyxa– Limitedbytheirtoxicitytothekidney– Somecanbeusedtotreatdrug‐resistantPseudomonasaeruginosa

• Daptomycin– Lipopep0demadebyStreptomyces– Mostac0veagainstGram‐posi6vebacteria

AgentstoTreatFungalInfec0ons

• Fungalcellsareeukaryo0c,sopresentspecialproblems– Majorityofchemotherapeu0cdrugsaredesignedtoactonbacteriaandareineffec6veforfungalinfec6ons

– Similari0esbetweenfungalandhumancells‐toxicitytohumans

• Fourmaingroups– Macrolidepolyenean0bio0cs,Griseofulvin,Synthe0cazoles,Flucystosine

An0parasi0cChemotherapy• An6malarialDrugs:Quinineanditsderiva0ves

– Quinine:extractedfromthebarkofthecinchonatree– Replacedbysynthesizedquinolines(chloroquineandprimaquine),whichhavelesstoxicitytohumans

• ChemotherapyforOtherProtozoanInfec0ons– Metronidazole(Flagyl)‐Amoebicide

• trea0ngintes0nalinfec0onsbyEntamoebahistoly>ca• Orallyappliedalsosuitedtocombatinfec0onsbyGiardialambliaandTrichomonasvaginalis

– Quinicrine,sulfonamides,tetracyclines

An0helminthicDrugTherapy

• Flukes,tapeworms,androundwormshavegreatersimilari0estohumanphysiology– Usingdrugstoblocktheirreproduc0onisusuallynotsuccessfulineradica0ngadultworms

– Mosteffec0vedrugsimmobilize,disintegrate,orinhibitthemetabolismofallstagesofthelifecycle

An6viralChemotherapeu0cAgents

• Selec0vetoxicityisalmostimpossibletoachievebecausethensamemetabolicsystemisresponsibleforthewell‐beingofbothvirus&host

• Severalan0viraldrugshavebeendevelopedthattargetspecificpointsintheinfec0ouscycleofviruses

• Threemajormodesofac0on:– Prevent attachment & penetration of the virus into the host cell– Blocking the transcription & translation of viral macromolecules– Preventing the maturation of viral particles

Interferon(IFN):AnAlterna0vetoAr0ficialDrugs

• Glycoprotein produced by fibroblasts and leukocytes inresponse to various immune stimuli

• Produced also using recombinant DNA technologies• Known therapeutic benefits:

– Reducing the time of healing and some of the complications incertain infections

– Preventing or reducing some symptoms of cold and HPV– Slowing the progress of certain cancers– Treating hairy-cell leukemia (a rare cancer), hepatitis C, genital

warts, and Kaposi’s sarcoma in AIDS patients• Often results in serious side effects

Interactions Between Microbes and Drugs:The Acquisition of Drug Resistance

• Drugresistance:anadap0veresponseinwhichmicroorganismsbegintotolerateanamountofdrugthatwouldordinarilybeinhibitory

• Canbeintrinsicoracquired• Microbesbecomenewlyresistanttoadruga_er

– Spontaneousmuta0onsincri0calchromosomalgenes– Acquisi0onofnewgenesorsetsofgenesviatransferfromanotherspecies(i.e.,viaplasmids)calledresistancefactors

Figure 12.13

NewApproachestoAn0microbialTherapy

• O_enresearcherstrytofindnewtargetsinthebacterialcellandcustom‐designdrugsthataimforprocessessuchas:– iron‐scavengingcapabili0esofbacteria

– gene0ccontrolmechanismsinbacteriareferredtoasriboswitches

• Probio6csandprebio6cs,Lan6bio6cs

ProblemswithChemotherapy:

ToxicitytoOrgans

• Liver,kidneys,gastrointes0naltract,cardiovascularsystemandblood‐forming0ssue,nervoussystem,respiratorytract,skin,bones,andteeth,etc.

AllergicResponsestoDrugs

• Allergy:heightenedsensi0vity

• Thedrugactsasanan0genands0mulatesanallergicresponse

• Reac0onssuchasskinrash,respiratoryinflamma0on,andrarelyanaphylaxis

Superinfec6on

• Whencertainbeneficialspecies(naturalflora)aredestroyed,microbesthatwereoncekeptincheckcanbegintoovergrowandcausedisease,calledthenasuperinfec6on– Usingabroad‐spectrumcephalosporinforUTIdestroyslactobacilliinthevagina;withoutthelactobacilli,Candidaalbicanscanproliferateandcauseayeastinfec0on

– Oraltherapywithtetracyclines,clindamycin,andbroad‐spectrumpenicillinsandcephalosporinsisassociatedwithan0bio0c‐associatedcoli0s

Selecting an Antimicrobial Drug

• Three factors must be known– The nature of the microorganism causing the infection– The degree of the microorganism’s susceptibility to

various drugs– The overall medical condition of the patient

• Identifying the Agent– Direct examination of body fluids, sputum, or stool is a

rapid initial method– The choice of drug will be based on experience with

drugs that are known to be effective against the microbe:the “informed best guess”

• Testing for the Drug Susceptibility of Microorganisms

TheMICandTherapeu0cIndex

• MIC‐minimuminhibitoryconcentra6on:thesmallestconcentra0on(highestdilu0on)ofdrugthatvisiblyinhibitsbacterialgrowth

• Oncetherapyhasbegun,itisimportanttoobservethepa0ent’sclinicalresponse