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Anthelmintic discovery and
registration
Nick Sangster
Faculty of Veterinary Science
University of Sydney
Worms
Objectives
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Be aware of research into new modes of action
Explain screening for the discovery of AH
Explain the development process for AH
Detail the important steps in registration
Understand the needs for trials
Describe Good Clinical Practice
Understand the components in the calculation of
withholding periods
Requirements of anthelmintics
• Effective, safe, non toxic to consumer.
• Narrow spectrum, broad spectrum
• Stock Medicines Act 1989 and vets
– Anyone can give stock medicines to non-food
producing animals (we eat horses!)
– Only vets can vary label directions in food producers
but MUST supply (in writing) species, withholding,
dose rate, frequency of treatment, manner of
administration.
– No one can use an unregistered stock medicine except
under permit or order
Aspects of commercial
anthelmintics
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Establishing a need/market
Research
Screening
Refinement
Development
Marketing
Technical support
Research - pharyngeal pumping
L1 pumping video
5-HT stimulates pharyngeal
pumping in T.colubriformis
Frequency index
2.5
5HT
10-5M
2
1.5
1
0.5
0
0.01
0.1
1
10
100
Concentration (M)
1000
ICC of H. contortus
pharynx
Screening for anthelmintic
activity
• Primary in vitro:
– Screens are muscle contraction, larval death in
vitro, failure to hatch, loss of motility.
– False positives: pineapple juice kills worms in
vitro but is not an anthelmintic
– False negatives: some drugs are activated by
host metabolism so are not active in vitro
Screening for anthelmintic activity
• Secondary:
– Controlled tests in infected lab animals like mice
• Syphacia
• Hymenolepis
• Heligosomoides
• Tertiary:
– Controlled tests in target animal eg. sheep with
Haemonchus
• Field tests:
– Trials with natural infections in target animal in real
environments
Screening for anthelmintic activity
• High throughput screens and combinatorial
chemistry
– Decide on a target eg. 5-HT action
– Proof of concept eg. physiological effects of 5-HT, different
receptor compared with host
– Clone, express receptor and develop assay adapted for HTS
(multiwell plate, robotics, automated readout)
– Screen 1 million compounds
– Take ’hits’ and synthesise better analogues
– Move to primary or secondary screens
Development and
Registration
Pharmaceutical
form
Analytical
techniques
Identified compound
Teratoge
icity
Toxicity
Stability, dose rate, spectrum, safety (rats, target),
Presentation, field studies
Stability
studies
Efficacy
studies
Safety
studies
Why we need Registration
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Ensure quality of product
Ensure safety of product
Employ Good Clinical Practice
Define efficacy requirements
Consider toxicity to the environment
Check for exposure of humans to drugs
Drug registration and VICH
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International harmonisation of registration
Standard requirements for all countries
Overseen by VICH (US, EU, Japan)
Under the OIE
Align technical requirements
Can use trial & testing data from other countries to
reduce costs
• Aim for consistency and safety
Registration of anthelmintics
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Stability testing
Analytical procedures
Bioequivalence
Good Clinical Practice
Specific guidelines available for:
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General requirements
Swine
Equines
Canines
Bovines
Ovines
Caprines
Guidelines for
Bovines
• Controlled tests
– Egg counts
– Worm counts
• Natural or induced infections
– Minimum infection rates
• Claims for different species: adults, larvae
• Trials
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Two dose confirmation studies
At least 6 animals per group
Animals >3 mo. old and groups randomised
Administration as for product
Significant difference between treated and control
Effectiveness >90%
Other guidelines, for example:
• Can register as “aids in control”
• Need 100% efficacy for some parasites like
Echinococcus
• Claims for persistent action
• Claims for control or protection
• Equivalence requires blood level measurements in
blood
• Local trails for food producing animals under
different climatic conditions
• Monitor for adverse reactions.
Some aspects of
“Good Clinical Practice”
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Accuracy, integrity and correctness of data
Standard Operating Procedures for ALL activities
Qualified persons to perform them
Informed consent of owner
Quality control of substances
Independent audit
Record adverse findings
Record keeping and reporting
Withholding
• Withdrawal of animal products from human
consumption after treatment
• Important for registration and food export
• Calculated on the persistence of chemicals
in animal organs after treatment
Profile of a drug
Log [A]
Partitioning
(depot)
metabolism
Efficacy driven by
Cmax and time
above threshold
absorption
Dose rate driven
by potency,
bioavailability
elimination
Time
Calculation of withholding period
• Only applies to food producing animals.
• Period before which it is unsafe (illegal) to
slaughter for meat or use milk or eggs for human
consumption because of chemical residues.
• For some compounds, there is NO withholding
period, meaning the levels in the product never
reach threshold levels.
• The withholding period is calculated by
considering the drug dose rate, the expected toxic
level for humans and the drug elimination kinetics
from edible organs.
MRL
• Maximum Residue Level is the highest
allowable level in food products.
• Chemicals are eliminated from the body of
the animal over time, so it is a matter of
waiting long enough.
• There is general harmonisation of levels
‘agreed’ to by Governments, EPA, Food
Standards, WHO
Calculation of MRL for drug A
for humans eating sheep meat
• Say the dose rate of A in sheep is 0.2 mg/kg.
• Let’s say the human ‘no observable effect level’
(NOEL) based on animal studies for A has been
determined as 1.25 mg/kg body weight.
• The safety factor applied is generally 200.
• So, a the adult daily intake (ADI) is 6 g/kg, 200
fold less than the NOEL.
• A 60 kg adult can eat 360 g of A per day.
MRL
• A number of sheep are killed at intervals after
treatment with A and the concentration of A is
measured in various edible tissues.
• By looking at this data you can identify a time (12 +2
days) where eating the following every day you will,
on average, just get your dose (=397 g)
Organ
[A]
kg eaten
Muscle
Fat
Liver
Kidney
10 g/kg
10 g/kg
100 g/kg
10 g/kg
0.3
0.05
0.1
0.05
MRL and elimination profile of A
to determine witholding period
Log [A]
14 days
MRL
Witholding period
Time