Rodent LD50 Tests: Can We Make Reparation?

Perspective

The rodent LD₅₀ test was devised by Trevan (1927) as a means of assessing the toxicity of drugs, and is still widely used. Rodent LD₅₀ values are still a regulatory requirement in the assessment of the potential acute toxicity of chemicals to humans (Gadaleta et al., 2019). The LD₅₀ test is, however, a cruel test, involving the killing of many animals in a single test, and almost certainly causing much pain (Sass, 2000; Jacques, 2011). Many years ago, Russell and Burch (1959) proposed what are now known as “the three Rs” (reduction, refinement and replacement) to minimise and eventually eliminate animal testing. Over the years, various modifications to the LD₅₀ test have been proposed (Erhirhie et al., 2018), but pressure has built over many years for replacement. In 1969 FRAME (Fund for the Replacement of Animals in Medical Experiments) was set up, and has been in the forefront of the battle against animal experimentation (Balls, 1991; Balls & Fentem, 1994). Progress is certainly being made, for from 2013 the European Union has banned the sale of any cosmetics or cosmetics ingredients that have been tested on animals, and a few other countries have followed suit.

Quite apart from the moral implications of animal tests, numerous studies have shown that correlations between human toxicity and rodent LD₅₀ values are generally rather poor (Ekwall et al., 1998a; Hoffmann et al., 2010; Van Norman, 2019). This limits the value of the LD₅₀ test, and has led to the failure of a number of drugs in clinical trials (Erhirhie et al., 2018).

Over almost a century, millions of rodent LD₅₀ tests have been performed, and it could be argued that the bulk of these provided little or nothing towards human safety. A report from the Humane Society of the United States (HSUS 1983) stated, with respect to the LD₅₀ test, that: “tens of thousands of laboratory animals suffer not for the purpose of safeguarding the public but to provide evidence of safety-testing for any company marketing a new substance in case somebody is injured by that substance and decides to sue”.

Walum (1998) pointed out that LD₅₀ values vary from laboratory to laboratory due to, for example, different protocols, different strains of animals, housing conditions and test-chemical source. Other reasons include the sex, age, weight, health and diet of animals, temperature, time of assessment after dosing, route of administration, and time of day and time of year (Zbinden & Flury-Roversi, 1981; Kar et al., 2016; Dearden, 2019). Such differences make the use of LD₅₀ values from different sources of dubious value. I recently found, for example, that published Ror values for codeine ranged from 64 to 487 mg/kg, and Mip values for ethanol ranged from 528 to 9710 mg/kg (Dearden, 2019). With such wide variability, it is not surprising that animal LD₅₀ values have been found not to correlate well with human acute toxicity.