Model Selection in Toxicology: Principles and Practice

Novel Dimethylacetamide-Containing Formulation Improves Infraorbital Anaesthesia Efficacy in Rats with Periodontitis

Background

To evaluate acute toxicity and local anaesthetic activity of a formulation containing a novel dimethylacetamide derivative, antioxidant, and vasoconstrictor in rats with chronic periodontitis.

Methods

Novel anaesthetic dimethylacetamide-containing formulation LHT-15-32 was studied as 2% water solution. Its acute intravenous and subcutaneous toxicity was determined in mice. Pain sensitivity threshold of the upper second molar was determined in rats with experimental periodontitis. Oxidative stress activity and total antioxidant capacity were determined in rats' gingival mucosa by induced chemiluminescence. Local changes were evaluated in periodontal tissue by morphological examination. Tissue IL-1β, IL-10, and TNF-α concentration was quantitatively assessed by an enzyme-linked immunosorbent assay. LHT-15-31 Na-blocking activity was studied on isolated neurons of Limnaea stagnalis' parapharyngeal ganglion. Isolated sciatic nerve of Rana radibunda was perfused with different concentrations of LHT-15-32 to assess its conductivity. Statistical analysis was used, and continuous variables were presented as mean ± square deviation. The normality of distribution was determined using ANOVA. Newman–Keuls parametric criterion was used for intergroup comparison. LD₅₀ indexes were calculated by probit analysis.

Results

LHT-15-32 acute intravenous and subcutaneous toxicity was lower than that of its active substance. The formulation by infraorbital administration induced deep dental anaesthesia which lasted over 70 min and activated the local antioxidant defense system and decreased IL-1β level in gingival tissue. LHT-15-32 triggered tissue reparation around the impacted upper molar in rats assessed five days after administration. At 10⁻⁶ to 10⁻³ M concentration, LHT-15-32 inhibited sciatic nerve conductivity and blocked Na⁺ channels of isolated neurons in a dose-dependent manner.

Conclusions

The formulation may be considered as an effective and safe approach to anaesthetize upper molars with periodontitis.

Systematic Reviews of Animal Experiments Demonstrate Poor Human Clinical and Toxicological Utility

The assumption that animal models are reasonably predictive of human outcomes provides the basis for their widespread use in toxicity testing and in biomedical research aimed at developing cures for human diseases. To investigate the validity of this assumption, the comprehensive Scopus biomedical bibliographic databases were searched for published systematic reviews of the human clinical or toxicological utility of animal experiments. In 20 reviews in which clinical utility was examined, the authors concluded that animal models were either significantly useful in contributing to the development of clinical interventions, or were substantially consistent with clinical outcomes, in only two cases, one of which was contentious. These included reviews of the clinical utility of experiments expected by ethics committees to lead to medical advances, of highly-cited experiments published in major journals, and of chimpanzee experiments — those involving the species considered most likely to be predictive of human outcomes. Seven additional reviews failed to clearly demonstrate utility in predicting human toxicological outcomes, such as carcinogenicity and teratogenicity. Consequently, animal data may not generally be assumed to be substantially useful for these purposes. Possible causes include interspecies differences, the distortion of outcomes arising from experimental environments and protocols, and the poor methodological quality of many animal experiments, which was evident in at least 11 reviews. No reviews existed in which the majority of animal experiments were of good methodological quality. Whilst the effects of some of these problems might be minimised with concerted effort (given their widespread prevalence), the limitations resulting from interspecies differences are likely to be technically and theoretically impossible to overcome. Non-animal models are generally required to pass formal scientific validation prior to their regulatory acceptance. In contrast, animal models are simply assumed to be predictive of human outcomes. These results demonstrate the invalidity of such assumptions. The consistent application of formal validation studies to all test models is clearly warranted, regardless of their animal, non-animal, historical, contemporary or possible future status. Likely benefits would include, the greater selection of models truly predictive of human outcomes, increased safety of people exposed to chemicals that have passed toxicity tests, increased efficiency during the development of human pharmaceuticals and other therapeutic interventions, and decreased wastage of animal, personnel and financial resources. The poor human clinical and toxicological utility of most animal models for which data exists, in conjunction with their generally substantial animal welfare and economic costs, justify a ban on animal models lacking scientific data clearly establishing their human predictivity or utility.

Early microdose drug studies in human volunteers can minimise animal testing: Proceedings of a workshop organised by Volunteers in Research and Testing

Testing the safety and efficacy of a successful human medicine involves many laboratory animals, which can sometimes be subjected to considerable suffering and distress. Also, it is necessary to extrapolate from the test species to humans. UK and European legislation requires that Replacement, Reduction and Refinement of animal procedures (the Three Rs) are implemented wherever possible. Over the last decade, there has been substantial progress with applying in vitro and in silico methods to both drug efficacy and safety testing. This paper is a report of the discussions and recommendations arising from a workshop on the role that might be played by human volunteer studies in the very early stages of drug development. The workshop was organised in November, 2001 by Volunteers in Research and Testing, a group of individuals in the UK which launched an initiative in 1994 to identify where and how human volunteers can participate safely in biomedical studies to replace laboratory animals. It was considered that conducting pre-Phase I very low dose human studies (sub-toxic and below the dose threshold for measurable pharmacological or clinical activity) could enable drug candidates to be assessed earlier for in vivo human pharmacokinetics and metabolism. Moreover, accelerator mass spectrometry (AMS), nuclear magnetic resonance (NMR) spectroscopy and positron emission tomography (PET) are potentially useful spectrometric and imaging methods that can be used in conjunction with such human studies. Some, limited animal tests would still be required before pre-Phase I microdose studies, to take account of the potential risk posed by completely novel chemicals. The workshop recommended that very early volunteer studies using microdoses should be introduced into the drug development process in a way that does not compromise volunteer safety or the scientific quality of the resulting safety data. This should improve the selection of drug candidates and also reduce the likelihood of later candidate failure, by providing in vivo human ADME data, especially for pharmacokinetics and metabolism, at an earlier stage in drug development than is currently the case.