Chemotherapy - drug resistance

Question 1

What causes treatment failure?

Answer 1

Incorrect diagnosis
Non-compliance
Fake drug/poor activity
Drug levels in blood do not reach theraputic levels
Competative inhibition/interaction due to diet/disease
Parasites resistant to drug

Question 2

Poor diagnosis

Answer 2

Diagnosis from symptoms is not an exact science, especially in the 3rd world due to poor facilities.
Viral infections difficult to diagnose.
Patient expectation of drug treatment may lead to GP prescribing drug even though they know it will not work.
Wrong drug, won’t cure anything.

Question 3

Poor compliance

Answer 3

Patients get fed up (increased side effects lead to decreased compliance)
Long courses and complex regimens (such as HAART - highly active anti-retroviral therapy)
Unpalatable drugs - quinine is very bitter
Recurrence of disease due to non-compliance is jot the same as drug resistance.

Question 4

Fake/poorly active drug

Answer 4

Counterfeit drugs - no active ingredients
Poorly produced drugs - active ingredient at incorrect levels
Out of date drugs
Poorly stored drugs (cold chain, humidity)

Question 5

Inadequate drug levels in blood

Answer 5

Poor absorption (diet) - digestive remedies decrease tetracycline absorption, griseofulvin must be taken with milk or fat as it is fat soluble, pharmacists and doctors must advise patients correctly.
Fast/slow metabolisers
Poor conversion of pro-drugs due to cyp mutations
Wrong dose given (clinician/pharmacist/patient mistake)

Question 6

Competative inhibition/interactions

Answer 6

Competative inhibition by diet (sulphadoxine outcompeted by high folate intake)
Drug-drug interactions
Interactions with other diseases (diarrhoeal diseases affect absorption)

Question 7

Drug resistance

Answer 7

The ability of a pathogen to survive and/or multiply despite the administration and absorption of a drug given in doses equal to or higher than those usually recommended, but within the tolerance of the subject.
Treatment failure is not always due to drug resistance.

Question 8

What causes drug resistance?

Answer 8

Reduced drug uptake into organism (in pathogens where drug is taken up by active transport)
Increased efflux of the drug (pathogen actively transports drug out of itself)
Increased breakdown of the drug
Alterations in drug target (very common) -
Decreased binding
Decreased inhibition
Increased production of target (mops up drug, drug ineffective)

Question 9

How do pathogens aquire resistance?

Answer 9

Inherent resistance
Mutation
DNA transfer from resistant organisms -
1. Plasmid acquisition (or chromosomal DNA)
2. Bacteriophage transfer (transduction)
3. Recombination

Question 10

Inherent resistance

Answer 10

Some species may be resistant to a particular drug without requirjng any genetic change.
Use of that drug will therefore select the entire species.
Example - entercocci are unaffected by fluoroquinolones and cephalosporins.

Question 11

Mutations

Answer 11

Mutations occur in all organisms as a result of errors during DNA replication.
A base substitution occurs approx every 10^8 - 10^10 base pairs of the genome (regardless of organism)
Exception of retroviruses as RT is very error prone.
Copying errors can also lead to deletions or insertions of DNA bases.
Mutations occuring in genes can bring about a change in the amino acid encoded by the triplet codon. If this amino acid is in the drug binding site of the target drug, binding may be reduced or prevented and the pathogen becomes resistant.
Mutations must allow natural substrate to still fit or organism will die.
If the altered target is no longer functional or is less effective than before there is a cost to the parasite. This is a fitness cost in the abscence of a drug but a fitness advantage in the presence of a drug.
Mutations in silent DNA do not normally change amino acids or cause resistance. An exception is when the mutation occurs in the promoter region which can result in over production of drug target.

Question 12

DNA transfer from resistant organims

Answer 12

Via -
Plasmid acquisition
Mating and recombinant DNA

Question 13

Generation of multiple drug resistance

Answer 13

Mutation of multiple genes within the same pathogen (most common in viruses but also works in bacteria and parasites).
Acquisition of multiple plasmids, each braring a single resistance gene (only bacteria).
Acquisition of a single plasmid containing multiple resistance genes (bacteria only).
Recombination between pathogens with single (different) resistance genes (mostly parasites).

Question 14

Evolution of resistance

Answer 14

Acquisition of resistant pathogens from cross-infection.
Acquisition of resistant plasmids.
Development and/or selection of resistance during treatment.

Question 15

Initial evolution of resistance

Answer 15

For a pathogen with a mutation rate of 10^-8 and a genome size 10^8bp a mutation will occur in the genome once everytime it replicates.
If population size of the infected host is 10^8, then inknly a few replication cycles, pathogens will exists within the host with basepair changes at all possible sites in the genome.
After one replication cycles 2x10^8 New pathogens wil exists, each with a mutation at one base pair in the genome. Each mutation is present in two pathogens.

Question 16

Factors favouring evolution of resistance

Answer 16

High pathogen burden.
Small genome size.
High mutation frequency (retrovirus)
High pathogen replication rate
Single mutation conferring resistance
Evolution of resistant pathogens is very likely is total pathogen burden exceeds mutational frequency to resistance.

Question 17

How does drug resistance spread in populations

Answer 17

In the abscence of drug pressure, the New mutant is unlikely to survive to be transmitted, and will probably not outgrow the non-mutant pathogens as it is present j such a minority.
If the infected person is treated with adequate drugs, all sensitive pathogens will be killed, only mutants will survive and replicate.
One of the most important factors for spread of resistance is the amount of drug being used.
The more people taking the drug, or with the drug in their body, the greater the chance that the organism will come into contact with the drug, and the greater the level of selection of resistant organisms.

Question 18

Fitness costs

Answer 18

The fitness cost of resistance is also important in untreated hosts (no drug).
If the fitness cost is large, the mutant form cannot compete with the wildtype.
If the fitness cost is small the mutant may be able to compete with the wildtype and will be maintained at the same frequency.
If the fitness cost is negative ie the mutant is better than the wildtype, mutant form will outcompete the wildtype.
Although drug resistant pathogens usually show an initial loss of fitness, this reduces over time so that the drug resistant pathogens becomes as fit or in some cases more fit than its drug sensitive parent (initial loss in fitness = adaptive valley).

Question 19

Optimum conditions for rapid spread of resistant pathogens

Answer 19

High drug usage

Higher fitness of mutant pathogens compared to wildtype.

Question 20

Minimising evolution of drug resistance

Answer 20

Use antipathogen drugs only when necessary - proper diagnosis, correct dosage, compliance.
Do not always use same drug - drug cycling
Use combination drugs - resistance must arise to each drug in one pathogen, evolution is slowed by using multiple drugs.
Minimise people taking drug - so selection pressure in New hosts is low.
Minimise spread from populations with resistance - prevent movement of people from regions of high resistance to low resistance if infected.
Minimise spread from people withnidentified resistant pathogens - isolation nursing, hygiene in hospitals, antitransmissional drugs, behavioral changes (STDs)