DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

Resistance Due to Enhanced Excision of Incorporated Drug.
Nucleoside reverse transcriptase inhibitors such as zidovudine are
2′-deoxyribonucleoside analogs that are converted to their 5′-triphosphate
form and compete with natural nucleotides. These drugs are incorporated
into the viral DNA chain and cause chain termination. When
resistance emerges via mutations at a variety of points in the reverse
transcriptase gene, phosphorolytic excisionof the incorporated chainterminating
nucleoside analog is enhanced (Arion et al., 1998).
Hetero-resistance and Viral Quasi Species. Hetero-resistance is
said to be present when a subset of the total microbial population is
resistant, despite the total population being considered susceptible on
testing (Falagas et al., 2008; Rinder, 2001). In a way, this should not
be a surprise given that chromosomal mutations are a stochastic
process and there is a baseline mutation rate for each gene.
Therefore, a subclone that has alterations in genes associated with
drug resistance is expected to reflect the normal mutation rates and
occur at between 10 −6 and 10 −5 colonies. In bacteria, hetero-resistance
has been described especially for vancomycin in S. aureus, vancomycin
in Enterococcus faecium, colistin in Acinetobacter
baumannii-calcoaceticus, rifampin, isoniazid, and streptomycin in
M. tuberculosis, and penicillin in S. pneumoniae (Falagas et al.,
2008; Rinder, 2001). Increased therapeutic failures and mortality
have been reported in patients with hetero-resistant staphylococci
and M. tuberculosis (Falagas et al., 2008; Hofmann-Thiel et al.,
2009). For fungi, hetero-resistance leading to clinical failure has
been described for fluconazole in Cryptococcus neoformans and
Candida albicans (Marr et al., 2001; Mondon et al., 1999).
Viral replication is more error prone than replication in bacteria
and fungi. Viral evolution under drug and immune pressure
occurs relatively easily, commonly resulting in variants or quasi
species that may contain drug-resistant subpopulations. This is not
often termed hetero-resistance, but the principle is the same as
described for bacteria and fungi: A virus may be considered susceptible
to a drug because either phenotypic or genotypic tests reveal
“lack” of resistance, when there is a resistant subpopulation just
below the limit of assay detection. These minority quasi species that
are resistant to antiretroviral agents have been associated with failure
of antiretroviral therapy (Metzner et al., 2009).
EVOLUTIONARY BASIS
OF RESISTANCE EMERGENCE
Development of Resistance via Mutation Selection.
Mutation and antibiotic selection of the resistant mutant
are the molecular basis for resistance for many bacteria,
viruses, and fungi. Mutations may occur in the gene
encoding (1) the target protein, altering its structure so
that it no longer binds the drug; (2) a protein involved in
drug transport; (3) a protein important for drug activation
or inactivation; or (4) in a regulatory gene or promoter
affecting expression of the target, a transport protein, or an
inactivating enzyme. Mutations are not caused by drug
exposure per se. They are random events that confer a survival
advantage when drug is present. Any large population
of drug susceptible bacteria is likely to contain rare
mutants that are only slightly less susceptible than the parent.
However, suboptimal dosing strategies lead to selective
kill of the more susceptible population, which leaves
the resistant isolates to flourish.
In some instances, a single-step mutation results in a high
degree of resistance. Examples include M. tuberculosis katG Ser315
mutations that cause resistance to isoniazid; the M814V mutation in
the reverse transcriptase gene of HIV-1, causing resistance to lamivudine;
a cytochrome-b gene mutation causing resistance to the antimalaria
drug atovaquone; and Candida albicans fks1 Ser645
mutations causing resistance to echinocandins.
In other circumstances, however, it is the sequential acquisition
of more than one mutation that leads to clinically significant
resistance. As an example, the combination of pyrimethamine and
sulfadoxine inhibits Plasmodium falciparum’s folate biosynthetic
pathway via inhibition of dihydrofolate reductase (DHFR) by the
pyrimethamine and inhibition of dihydropteroate synthetase (DHPS)
by sulfadoxine. Clinically meaningful resistance occurs when there
is a single point mutation in the DHPS gene accompanied by at least
a double mutation in the DHFR gene.
Hypermutable Phenotypes. The ability to protect
genetic information from disintegrating and also to be
flexible enough to allow genetic changes that lead to
adaptation to the environment is essential to all living
things. This is accomplished principally by the insertion
of the correct base pair by DNA polymerase III, proofreading
by the polymerase, and postreplicative repair.
The development of a defect in one of these repair
mechanisms leads to a high degree of mutations in many
genes; such isolates are termed mutator (Mut) phenotypes
and may include mutations in genes causing
antibiotic resistance (Giraud et al., 2002). This secondorder
selection of hypermutable (mutator) alleles based
on alterations in DNA repair genes has been implicated
in the emergence of multidrug resistant strains of
M. tuberculosis Beijing genotype (Rad et al., 2003).
Resistance by External Acquisition of Genetic
Elements. Drug resistance may be acquired by mutation
and selection, with passage of the trait vertically to
daughter cells. For mutation and selection to be successful
in generating resistance, the mutation cannot be
lethal and should not appreciably alter virulence. For the
trait to be passed on, the original mutant or its progeny
also must disseminate and replicate; otherwise, the
mutation will be lost until it is “rediscovered” by some
other mutant arising from within a wild-type population.
Drug resistance more commonly is acquired by
horizontal transfer of resistance determinants from a
donor cell, often of another bacterial species, by
transduction, transformation, or conjugation. Resistance
acquired by horizontal transfer can disseminate rapidly
and widely either by clonal spread of the resistant strain
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CHAPTER 48
GENERAL PRINCIPLES OF ANTIMICROBIAL THERAPY