The use of animal testing has significant benefits in medical research; however, increased costs, extremely high failure rates, and delays in drug approval have led many to re-evaluate its value in drug development. 

The concordance between animal and human trials

A high rate of withdrawal of new drugs during or even after clinical trials is due to lack of efficacy or adverse side-effects, which were not identified during expensive and time-consuming preclinical testing, mainly animal testing.

Animal models are traditionally utilized to find targets for disease; however, there are significant challenges in terms of predictability or reliability on these models and transferability of the results to humans. Animal models are unable to replicate the human pathophysiology entirely and hence cannot predict mechanisms involved in human diseases accurately.

Drugs that survive clinical trials and attain market approval may still be recalled later due to toxicity identified only after months or years of in-human use. As per WITHDRAWN (2016 data), a public resource for withdrawn and discontinued drugs, of the 578 discontinued and withdrawn drugs in Europe and the United States, almost one-half were withdrawn or discontinued in post-approval actions due to toxicity. A study by Van Meer et al. found that of 93 post-marketing serious adverse outcomes, only 19% were identified in preclinical animal studies. The most common toxicity types associated with drug withdrawals in the United States and Europe are hepatic (21%), cardiovascular (16%), hematological (11%), neurological (9%), and carcinogenicity (8%).

When animal tests falsely identify a safe chemical as "toxic”

Probability of false-positive toxicity findings with animal studies is also a matter of concern. When animal tests falsely identify a safe chemical as "toxic," the almost inevitable outcome is the abandonment of further development. Undoubtedly many potentially beneficial drugs have failed animal testing and been lost to patients, even though they would have been both safe and effective. Because a drug that shows toxicity in animal models is unlikely to ever undergo human testing, the proportion of this type of "error" is unknown. Many highly beneficial drugs would have failed animal testing and would have never reached the market, however; they were developed before animal testing was required. A well-known example is penicillin, which is fatal to guinea pigs and paracetamol, which is toxic in dogs and cats.

Reproducibility and interspecies reliability of animal tests

Reproducibility of animal studies within species, even when carried out under rigorous protocols, is questionable. Results for a single chemical often differed with animal model, strain, dose, and delivery route. Using a database of more than 800,000 animal toxicity studies performed for 350 chemicals under stringent guidelines, a study found toxicity to be repeatable just 70% of the time in the same species.

Alternatives to Animal Testing

The past few decades have witnessed a dramatic increase in alternative methods for preclinical testing of drugs. Alternate mode of testing that predicts potential dropouts during drug development can reduce expenses and allow resources to be redirected toward agents more likely to pass clinical trials. Alternative testing methods that provide more consistent, rapid and translatable results can also increase human safety. 

Some alternatives to animal testing include: 

• In vitro tests using cell lines

• Tissue samples

• Use of alternative organisms such as bacteria

• 3-dimensional modeling and bioprinting

• In silico tests

• Organ-on-chip technologies such as 3-dimensional organoids

• Computer modeling, and phase 0 in-human microdosing trials 

In silico modeling

In silico testing is an innovative tool for preclinical evaluation. In silico design involves virtual investigation with computer models to complement and accelerate in vivo and in vitro practices.

In silico method is a modeling approach using known characteristics of the drug and details regarding the fundamental chemical or biological system in which it will be used. In silico methods also utilize the available preclinical and clinical data to predict a given drug molecule's system-level behaviors.

Today, practically all toxicological research includes in silico elements. Computational methods do not yet provide a complete replacement of animal testing; however, if utilized efficiently, it can tremendously increase the scale and speed of preclinical drug development, thus reducing expenses in animal testing phases. 

Another important advantage of in silico testing is its potential to identify areas for rescue or repurposing of existing drugs. In rescue or repurposing of existing drugs, preclinical phases and animal testing can often be bypassed altogether. If human trials are required, they may be abbreviated to late-phase studies for the new indication, saving years of clinical testing.

Though advantageous, there are several challenges with in-silico methods. Just like animal testing, lack of specificity could prompt unnecessary testing during drug development or halt a safe and efficacious drug candidate from progressing to further stages of drug development because of false-positive toxicity findings. Additionally, the lack of proper knowledge and understanding of computational model creates a "black box effect" and limits trust and acceptance of in-silico data. Another challenge to computational modeling is achieving the computational power necessary to simulate complex mechanical and physiological systems sufficiently.

Tissue Engineering

Tissue engineering is a novel approach to drug development. Utilizing tissue engineering, models mimicking human corneal epithelium and stroma, urothelium, and human oral and vaginal mucosa have been developed. Using functioning human tissue to help identify potential drug candidates could speed up development and pave the way to personalized medicine while reducing the dependence on animal studies and saving expenses.

Unlike cell suspensions and tissue culture, tissue engineering closely mimic the considerable influence that 3D cell-to-cell and cell-to-matrix interactions have over cell behavior in actual tissue and organ systems. Also, tissue engineering can allow the creation of 3D tissue structures utilizing human cells and the exact therapeutic target; which likely increase the probability that activity in the engineered human tissue will more accurately reflect or predict the outcomes in human patients.

Organ-on-chip 

Organ-on-chip refers to a physiological organ biomimetic system built on microfabrication and microfluid technology. Organ-on-chip consists of microchannels lined with cultured human cells and microsensor capabilities. Microchannels permit microfluid flow that mimics breathing motions, muscle contractions, and other physiologic processes. Next, the chips are placed in a research system like a computer, which help predict response to various stimuli, including drug responses. Organ-on-chip has also been combined into multiorgan chip interactions to mimic whole-body responses. 

A revolution in thinking and practice is needed 

A revolution in thinking and practice is the need of the hour - It's time to reduce reliance on animal tests and focus on novel alternative methods for drug testing. Several sophisticated alternative methods capable of providing answers to specific questions are currently available; however, no one approach will completely replace animal testing. The wise decision would be to use the alternative methods selectively and, in combination and stepwise schemes with animal tests. Strategies should be tailored for specific purposes and situations while considering the molecular and mechanistic basis of the target diseases, desirable drug actions, and undesired adverse effects.

References

1.     Balls M, Bailey J, Combes RD. How viable are alternatives to animal testing in determining the toxicities of therapeutic drugs?. Expert Opin Drug Metab Toxicol. 2019;15(12):985-987.

2.     Hackam DG, Redelmeier DA. Translation of research evidence from animals to humans. JAMA. 2006;296(14):1731-1732.

3.     Wang B, Gray G. Concordance of Noncarcinogenic Endpoints in Rodent Chemical Bioassays. Risk Anal. 2015;35(6):1154-1166.

4.     Van Norman GA. Limitations of Animal Studies for Predicting Toxicity in Clinical Trials: Is it Time to Rethink Our Current Approach?. JACC Basic Transl Sci. 2019;4(7):845-854.

5.     Siramshetty VB, Nickel J, Omieczynski C, Gohlke BO, Drwal MN, Preissner R. WITHDRAWN--a resource for withdrawn and discontinued drugs. Nucleic Acids Res. 2016;44(D1):D1080-D1086.

6.     van Meer PJ, Kooijman M, Gispen-de Wied CC, Moors EH, Schellekens H. The ability of animal studies to detect serious post marketing adverse events is limited. Regul Toxicol Pharmacol. 2012;64(3):345-349.

Tissue Engineering and Regenerative Medicine. Available at: https://www.nibib.nih.gov/science-education/science-topics/tissue-engineering-and-regenerative-medicine. Accessed on: 31 August 2020.

About the Author:

Neeta Ratanghayra is a freelance medical writer, who creates quality medical content for Pharma and healthcare industries. A Master’s degree in Pharmacy and a strong passion for writing made her venture into the world of medical writing. She believes that effective content forms the media through which innovations and developments in pharma/healthcare can be communicated to the world."