Topic 15
Chemotherapy –
Controlling Microbial Growth in
the Body
1
Antimicrobial Agents
• Chemicals that affect physiology in any
manner
• Chemotherapeutic agents – drugs that
act against diseases
• Antimicrobial agents – drugs that treat
infections
2
History
• Paul Ehrlich
– “Magic bullets”
– Arsenic compound that killed trypanosomes
and another that worked against treponemes
• Alexander Fleming
– Penicillin released from Penicillium
– Antibiotics – antimicrobial agents produced
naturally by organisms
3
1
Fateful Poor Housekeeping…
[INSERT FIGURE 10.1]
4
More History
• Gerhard Domagk
– Discovered sulfanilamide in 1932
• First antimicrobial agent used to treat wide array of
infections
• Semi-synthetics
– Chemically altered antibiotics that are more
effective than naturally occurring ones
• Synthetics
– Antimicrobials that are completely
synthesized in a lab
5
Some Sources of Antimicrobial
Agents
[INSERT TABLE 10.1]
6
2
How does it Work?
• Some mechanisms include:
– Inhibit Cell Wall Synthesis/Maintenance
– Inhibit Protein Synthesis
– Disrupt Membranes
– Inhibit metabolic pathways
– Inhibit NA Synthesis
7
Mechanisms of Action
• Key is selective toxicity
• Antibacterial drugs = largest number and
diversity of antimicrobial agents
• Fewer drugs to treat eukaryotic infections
• Even fewer antiviral drugs
• Becoming scarce
8
The Cartoon- Some Mechanisms
of Action
[INSERT FIGURE 10.2]
9
3
How Cells Make New Cell Wall
[INSERT FIGURE 10.3a-b]
10
Inhibit Cell Wall Synthesis
• Inhibition of synthesis of bacterial walls
• Most common agents act by preventing crosslinkage of NAM subunits
• Beta-lactams are most prominent in this group;
functional groups are beta-lactam rings
• Beta-lactams bind to enzymes that cross-link NAM
subunits
• Bacteria have weakened cell walls and eventually
lyse
11
How to Inhibit Synthesis
[INSERT FIGURE 10.3c-e]
12
4
About Beta-Lactams
• Semi-synthetic derivatives of beta-lactams
–
–
–
–
More stable in acidic environments
More readily absorbed
Less susceptible to deactivation
More active against more types of bacteria
• Simplest beta-lactams – effective only
against aerobic Gram-negatives
13
More Synthesis Blockers
• Vancomycin and cycloserine - interfere
with alanine-alanine bridges that link NAM
subunits in many Gram-positives
• Bacitracin blocks secretion of NAG and
NAM from cytoplasm
• Isoniazid and ethambutol disrupt formation
of arabinogalactan-mycolic acid in
mycobacterial species
14
Effecting Peptidoglycan
• Prevent bacteria from increasing amount of
peptidoglycan
• No effect on existing peptidoglycan layer
• Effective only for growing cells
• No effect on plant or animal cells because
no peptidoglycan
15
5
Inhibit Protein Synthesis
• Prokaryotic ribosomes are 70S (30S and
50S)
• Eukaryotic ribosomes are 80S (40S and
60S)
• Drugs can selectively target translation
• Because mitochondria of animals and
humans have 70S ribosomes; can be
harmful
16
Inhibiting Protein Synthesis
[INSERT FIGURE 10.4]
17
Disrupt Cytoplasmic Membranes
• Some drugs can incorporate into
cytoplasmic membrane and damage its
integrity
• Amphotericin B attaches to ergosterol in
fungal membranes
• Humans somewhat susceptible because
cholesterol similar to ergosterol
• Bacteria lack sterols; not susceptible
18
6
Amphotericin B Creates a Pore!
[INSERT FIGURE 10.5]
19
More Ways to Disrupt
Cytoplasmic Membranes
• Azoles and allyamines inhibit ergosterol
synthesis
• Polymyxin disrupts cytoplasmic
membranes of Gram-negatives; toxic to
human kidneys
• Some parasitic drugs act against
cytoplasmic membranes
20
Inhibit Metabolic Pathways
• When differences exist between metabolic processes
of pathogen and host, anti-metabolic agents can be
effective
• Quinolones interfere with the metabolism of malaria
parasites
• Heavy metals inactivate enzymes
• Agents that disrupt tubulin polymerization and glucose
uptake by many protozoa and parasitic worms
• Drugs block activation of viruses
• Metabolic antagonists
21
7
How to Inhibit a Pathway
[INSERT FIGURE 10.6]
22
More About Inhibition
• Trimethoprim binds to enzyme involved in conversion of
dihydrofolic acid to THF
• Because humans get folic acid from diet; metabolism
unaffected
• Antiviral agents can target unique aspects of viral
metabolism:
– Amantadine, rimantadine, and weak organic bases
neutralize acidity of phagolysosome and prevent viral
uncoating
• Protease inhibitors interfere with an enzyme (protease)
that HIV needs in its replication cycle
23
Inhibit Nucleic Acid Synthesis
– Some drugs function by blocking DNA
replication or mRNA transcription
– Only slight differences between prokaryotic
and eukaryotic DNA; drugs often affect both
types of cells
– Not normally used to treat infections; used in
research and perhaps to slow cancer cell
replication
24
8
Mechanisms of Action
• Inhibition of Nucleic Acid Synthesis
– Compounds can interfere with function of nucleic acids
(nucleotide analogs)
– Nucleotide Analogs can distort shapes of nucleic acid
molecules and prevent further replication, transcription, or
translation
– Most often used against viruses; viral DNA polymerases more
likely to incorporate and viral nucleic acid synthesis more
rapid than that in host cells
– Also effective against rapidly dividing cancer cells
25
Nucleoside Metabolic Pathway
Example
[INSERT FIGURE 10.7]
26
More Ways to Inhibit NA
Synthesis
• Quinolones and fluoroquinolones act against prokaryotic
DNA gyrase; little effect on eukaryotes or viruses
• Other drugs, including rifampin, bind to and inhibit action
of RNA polymerase during transcription
• Reverse transcriptase inhibitors act against the enzyme,
reverse transcriptase, an enzyme HIV uses in its
replication cycle
– Inhibitor does not harm people because humans lack
reverse transcriptase
27
9
Prevent Virus Attachment
• Attachment can be blocked by peptide
and sugar analogs of attachment or
receptor proteins (attachment
antagonists)
• New area of antimicrobial drug
development
28
Clinical Considerations When Prescribing Antimicrobial
Drugs:
• Ideal Antimicrobial Agent
– Readily available
– Inexpensive
– Chemically stable
– Easily administered
– Nontoxic and nonallergenic
– Selectively toxic against wide range of pathogens
29
Clinical Considerations – Spectrum
• Broad-spectrum antimicrobials may allow
for secondary or superinfections to
develop
• Killing of normal flora reduces microbial
antagonism
30
10
Example Spectrum of Effect
[INSERT FIGURE 10.8]
31
Clinical Considerations - Efficacy
• Ascertained by
– Diffusion susceptibility tests
– Minimum inhibitory concentration test
– Minimum bactericidal concentration test
32
Clinical Considerations – Zones of Inhibition
[INSERT FIGURE 10.9]
33
11
Clinical Considerations – Effective Concentration
[INSERT FIGURE 10.10]
34
Clinical Considerations – Gradient Concentrations
[INSERT FIGURE 10.11]
35
Clinical Considerations – Routes of Administration
• Topical application of drug if infection is external
• Oral – simplest; lower drug concentrations; no reliance
on health care provider; patients do not always follow
prescribing information
• Intramuscular – requires needle; concentration never
as high as IV administration
• Intravenous – requires needle or catheter; drug
concentration diminishes as liver and kidneys remove
drug from circulation
• Must know how antimicrobial agent will be distributed
to infected tissues
36
12
Clinical Considerations – Comparison of Administration
Route
37
Clinical Considerations – Safety & Side
Effects
• Toxicity
– Exact cause of many adverse reactions poorly
understood
– Drugs can be toxic to kidneys, liver, or nerves
– Special attention when prescribing drugs to pregnant
women
• Allergies
– Although allergic reactions are rare, they may be life
threatening
– Anaphylactic shock
38
Example Adverse Drug Reactions
39
13
Clinical Considerations – Disrupt
Normal Flora
– May result in secondary infections
– Overgrowth of normal flora – superinfections
– Of greatest concern for hospitalized patients
40
Resistance to Antimicrobial
Drugs
– Some are naturally partially or completely
resistant
– Resistance by bacteria acquired in two ways
• New mutations of chromosomal genes
• Acquisition of R-plasmids via transformation,
transduction, and conjugation
41
Example Resistance to
Antimicrobial Drugs
[INSERT FIGURE 10.15]
42
14
Mechanisms of Resistance to
Antimicrobial Drugs
• At least six mechanisms of resistance:
• Resistant cells may produce an enzyme that destroys or
deactivates the drug
• Microbes may slow or prevent the entry of the drug into
the cell
• Alter the target of the drug so it cannot attach or binds less
effectively
• Alter their metabolic chemistry
• Pump the antimicrobial out of the cell before it can act
• Mycobacterium tuberculosis produces MfpA protein, that
binds to DNA gyrase preventing the binding of
fluoroquinolone drugs
43
Breaking the Beta-lactam Ring!
[INSERT FIGURE 10.16]
44
Multiple & Cross Resistance to
Antimicrobial Drugs
– Pathogen can acquire resistance to more than
one drug at a time
– Common when R-plasmids exchanged
– Develop in hospitals and nursing homes;
constant use of drugs eliminates sensitive cells
– Superbugs
– Cross resistance
45
15
Retarding Resistance to
Antimicrobial Drugs
– High concentration of drug maintained in
patient long enough to kill all sensitive cells
and inhibit others so immune system can
destroy
– Use antimicrobial agents in combination;
synergism vs. antagonism
46
Overlap Drug Example
[INSERT FIGURE 10.17]
47
More Retarding Resistance
– Limit use of antimicrobials to necessary cases
– Develop new variations of existing drugs
• Second-generation drugs
• Third-generation drugs
– Search for new antibiotics, semi-synthetics, and synthetics
• Bacteriocins
• Design drugs complementary to the shape of microbial
proteins to inhibit them
48
16
Extra Slides:
Drug Resistance Tables from
Book
(just FYI)
49
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.2]
50
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.2]
51
17
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.2]
52
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.2]
53
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.2]
54
18
Resistance to Antimicrobial
Drugs
[INSERT TABLE 10.3]
55
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.3]
56
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.3]
57
19
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.4]
58
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.5]
59
Resistance to Antimicrobial
Drugs
[INSERT TABLE 10.5]
60
20
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.6]
61
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.6]
62
Resistance to Antimicrobial Drugs
[INSERT TABLE 10.6]
63
21