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 D - alanyl- 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 3. Inhibit the 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 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 Action of bleomycin hydrolase to promote tumor resistance to bleomycin

SCHEME 7.3 Inactivation of a nitrogen mustard by reaction with glutathione (GSH).

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 Example of resistance resulting from activation of alternative pathways

SCHEME 7.5 O 6 -Alkylation of guanine by an alkylating agent and its reversal by O 6 -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.

5. Use of multiple drugs for same target - about 1 in 10 7 bacteria resistant to a drug; if you use two drugs, then only 1 in is resistant to both 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 pathways- block both biosynthetic routes to the same metabolite 4. Efflux pump inhibitors can be made to prevent efflux of the drug

SCHEME 7.6 Proposed mechanism of inactivation of β-lactamase by clavulanate

RAF inhibitor MEK inhibitor V600E RAF is overactive

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).