Transcript Slide 1

Chapter 7
Drug Resistance and Drug
Synergism
When a formerly effective drug dose is no longer
effective.
Arises mainly from natural selection - replication
of a naturally resistant strain after the drug has
killed all of the susceptible strains.
On average, 1 in 10 million organisms in a colony
has one or more mutations that makes it resistant.
Resistance is different from tolerance - this is
when the body adapts to a particular drug and
requires more of the drug to attain the same initial
effect - lowers the therapeutic index.
It is also possible to develop tolerance to
undesirable effects of drugs, such as sedation by
phenobarbitol - raises the therapeutic index.
1. Altered drug uptake - exclusion of drug from
site of action by blocking uptake of drug - altered
membrane with more + or - charges
2. Overproduction of the target enzyme - gene
expression
3. Altered target enzyme (mutation of amino acid
residues at the active site) - drug binds poorly to
altered form of the enzyme
4. Production of a drug-destroying enzyme - a new
enzyme is formed that destroys the drug
5. Deletion of a prodrug-activating enzyme - the
enzyme needed to activate a prodrug is missing
6. Overproduction of the substrate for the target
enzyme - blocks inhibitor binding
7. New metabolic pathway for formation of the
product of the target enzyme - bypass effect of
inhibiting the enzyme
8. Efflux pump - protein that transports molecules
out of the cell
M184V and M184I mutants of reverse transcriptase are
produced by HIV when exposed to these drugs
If your drug has a structure similar to the
substrate, mutations will lower binding of the
substrate as well as the inhibitor.
FIGURE 7.1 Structure of a peptidoglycan segment prior to cross-linking with another peptidoglycan
fragment catalyzed by peptidoglycan transpeptidase. This structure is an alternative depiction of the
transpeptidase substrate shown on the left in Scheme 4.13, graphic A.
FIGURE 7.2 Complex between vancomycin and the terminal D-alanyl-D-alanine of the peptidoglycan
FIGURE 7.4 Biosynthesis of D-alanyl-D-lactate and incorporation into the peptidoglycan
of vancomycin-resistant bacteria
FIGURE 7.5 Complex between vancomycin and the peptidoglycan with terminal Dalanyl-D-lactate instead of D-alanyl-D-alanine in vancomycin-resistant bacteria
The resistance results from mutations in the
HIV protease target enzyme.
Lopinavir was made from Ritonavir to avoid Cyp450 inhibition, but it is
metabolized very fast. Ritonavir inhibits susceptible HIV and
helps reduce metabolism of Lopinavir, which inhibits the mutant HIV strains.
Mutations at many locations in Bcr-Abl result
in weak inhibition by Imatinib.
H396, E255, and T315 are mutation sites.
But not T315I!
FIGURE 7.6
Evolution of 7.11 optimization for inhibition of Bcr-Abl (T315I)
FIGURE 7.7 Image based on X-ray crystal structure of DC-2036 complexed to Bcr-Abl (T315I). Note
the position of the I315 residue and the hydrogen bonds to Met318; Met318 is analogous to Met793 in
Figures 5.2 and 5.3.
The T790M mutation
in EGFR kinase
affects gefitinib and
Erlotinib binding
L1196M of ALK give resistance
of lung cancer to crizotinib
Mutations of lanosterol 14α-demethylase
can cause resistance.
Overproduction of proteasome subunits causes
resistance to bortezomib
FIGURE 7.8 Image based on X-ray crystal structure of bortezomib complexed to 20S
proteasome
Overproduction of p-aminobenzoate can give
resistance to sulfa drugs
SCHEME 5.3
Biosynthesis of bacterial dihydrofolic acid
1. Make an analog that binds poorly to this new enzyme
2. Alter structure of drug so it is not modified by the new
enzyme, such as tobramycin (5.14), which lacks the OH group
of kanamycins (5.12) that is phosphorylated by resistant
organisms.
resistant organisms
phosphorylate here
3. Inhibit the new enzyme
no OH
group
SCHEME 7.1
An approach to avoid resistance to kanamycin A
These compounds inhibit the phosphorylation as
well as still bind to the ribosome
SCHEME 7.2
bleomycin
Action of bleomycin hydrolase to promote tumor resistance to
SCHEME 7.3
(GSH).
Inactivation of a nitrogen mustard by reaction with glutathione
6-Mercaptopurine is activated by hypoxanthine-guanine
ribosyltransferase
SCHEME 7.4 Conversion of prodrugs fludarabine and cladrabine to
their active form in cells catalyzed by cytidine kinase.
FIGURE 7.9
pathways
Example of resistance resulting from activation of alternative
SCHEME 7.5 O6-Alkylation of guanine by an alkylating agent and
its reversal by O6-alkylguanine-DNA alkyltransferase
Arises when the therapeutic effect of two or more
drugs used in combination is greater than the sum
of the effect of the drugs individually.
1. Inhibition of a drug-destroying enzyme protects the
drug from destruction
2. Sequential blocking - inhibition of two or more
consecutive steps in a metabolic pathway - overcoming
difficulty of getting 100% enzyme inhibition
3. Inhibition of enzymes in different metabolic pathwaysblock both biosynthetic routes to the same metabolite
4. Efflux pump inhibitors can be made to prevent efflux
of the drug
5. Use of multiple drugs for same target - about 1 in 107
bacteria resistant to a drug; if you use two drugs, then
only 1 in 1014 is resistant to both
SCHEME 7.6 Proposed mechanism of inactivation of β-lactamase
by clavulanate
V600E RAF is
overactive
RAF inhibitor
MEK inhibitor
EGFR kinase inhibitor
MET inhibitor
MET and VEGFR inhibitor
SCHEME 7.7 Mechanistic steps for conversion if inosine 5′monophosphate (IMP) to xanthosine 5′-monophosphate (XMP)
catalyzed by the enzyme IMPDH.
FIGURE 7.10 Schematic drawing showing how the complex (B) of
mizoribine monophosphate to IMPDH is believed to mimic the
tetrahedral intermediate (A) for E-XMP hydrolysis (compare Scheme
7.7).