Tribufos chemical review

The APVMA (then the NRA) reviewed the use of tribufos because of concerns about inadequate toxicological information and possible occupational health and safety risks to the public and workers from short-term and long-term exposure.

In 1994, the APVMA (then the NRA) began a special review of tribufos because of concerns about its toxicity and possible risks to human health, especially worker safety, from its use as a cotton defoliant.

In 1989 the then Drugs and Poisons Schedule Standing Committee had raised concerns about the adequacy of toxicological data for tribufos. At that time, the data were considered deficient by current standards. Tribufos was then rescheduled from schedule 6 to schedule 7 in the Standard for the Uniform Scheduling of Drugs and Poisons (now the Standard for the Uniform Scheduling of Medicines and Poisons) At the same time, the Australian Agricultural and Veterinary Chemicals Council endorsed the deregistration of tribufos products.

In August 1990, the registrant voluntarily withdrew its product containing tribufos (DEF Defoliant) from sale until the toxicological issues were clarified by further studies. Additional toxicological and occupational health and safety studies were provided by the registrant during 1993, 1994, 1995 and 1996.

In the toxicological assessment, a key concern was tribufos’s demonstrated potential to cause so-called ‘organophosphate-induced delayed neuropathy’. The Department of Health and Ageing concluded that, although tribufos was only a relatively weak inhibitor of acetylcholinesterase (an enzyme important in the functioning of the peripheral and central nervous system), it was a potent inhibitor of another enzyme (neuropathic target esterase), the inhibition of which appears to lead to long-term damage of nerves.

The department also noted that there was little justification for keeping a potentially nerve-damaging defoliant on the market when there were a number of much safer alternatives.

In the occupational health and safety assessment, safe use of tribufos could not be demonstrated, even when extensive personal protective equipment was worn and strict engineering controls were in place.

Tribufos has not been sold in Australia since 1990, and on 30 June 1997 the registrant requested that the APVMA cancel the registration of DEF Defoliant.

To consolidate the regulatory activity that had taken place, the APVMA released the Special review of tribufos report in February 1998. The APVMA concluded that the continued use of tribufos would be an undue hazard to the safety of people exposed to it during its handling.

The report recommended:

• cancelling the registration of the product containing tribufos
• that no future applications for registration of products containing tribufos be granted unless new studies addressed the toxicology and worker exposure concerns
• withdrawing the maximum residue limit (MRL) for tribufos.

The APVMA cancelled the registration for DEF Defoliant on 30 June 1999, and in March 2001 withdrew the MRL for tribufos.