Anthelmintic discovery and

registration

Nick Sangster

Faculty of Veterinary Science

University of Sydney

Worms

Objectives

• 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

• 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

5HT 10-5M0

0.5

1

1.5

2

2.5

0.01 0.1 1 10 100 1000Concentration (M)

Fre

qu

ency

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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

Identified compound

Pharmaceutical form

Analyticaltechniques

Teratogeicity

Toxicity

Stability, dose rate, spectrum, safety (rats, target), Presentation, field studies

Efficacy studies

Safety studies

Stability studies

Why we need Registration

• 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

• 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

• Stability testing• Analytical procedures• Bioequivalence • Good Clinical Practice• Specific guidelines available for:

– 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

– 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”

• 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]

Timeabsorption

Partitioning (depot)

metabolism

eliminationEfficacy driven by Cmax and time above threshold

Dose rate driven by potency, bioavailability

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 10 g/kg 0.3Fat 10 g/kg 0.05Liver 100 g/kg 0.1Kidney 10 g/kg 0.05

MRL and elimination profile of A to determine witholding period

Log [A]

Time

MRL

Witholding period

14 days