A recent law passed in the USA means that new drugs do not need to be tested on animals before human studies. With support from over 200 organizations, the "FDA Modernization Act 2.0" expands the range of models that can be used to test a compound before clinical trials.1 Under this new law animal testing can still be used, but alternative testing methods now also represent a legitimate option. In this article, we will explore the ethical implications of this new legislation, what it might mean for the use of alternative models, and why some researchers are skeptical about its impact.
What is the FDA Modernization Act 2.0?
The FDA Modernization Act 2.02 is a law that was passed on the 29th of December 2022 as part of the wider Consolidation Appropriations Act.3 In the spirit of "modernizing clinical trials" this law allows drugs to progress to human studies if they have been tested in any of the following models;
- Cell-based assays
- Organ chips and micro-physiological systems (MPS)
- Computer modeling i.e., in silico research
- Other non-human or human biology-based test methods e.g., human tissue testing
- Animal models
Alternative testing methods covered by the FDA Modernization Act 2.0
For years, animal testing was regarded as the gold standard of preclinical testing. The reliance of Pharma on animal testing has however been decreasing due to the development of more translational experimental models. The FDA Modernization Act 2.0. legitimizes four alternative testing methods for advancing a drug to human studies. Some of these have been used in pharmacological research for years (e.g., cell-based assays) while others are much newer (e.g., computer modeling).
Cell-based assays use isolated cells to investigate the effect of drugs on intracellular signaling pathways. Whilst these assays have been used historically, recent technological advances have made them more relevant to human physiology.
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Relevance of cell-based assays
The relevance of cell-based assays varies greatly depending on model complexity and cell type; cheaper methodologies are more widely used because they are easier to scale, but this can result in less clinically translational results.4
It is widely accepted that primary cells are more relevant to human physiology than cell lines. This is because cell lines over- and/or under-express certain genes so that they can be cultured for extended periods. It is also widely accepted that 3D cell-based assays mimic drug behavior more accurately than 2D cell-based assays.4,5 This is because when cells are cultured in 2D they adopt a flattened shape that does not resemble their structure in vivo.
For this reason, there has been an increased focus on the development of 3D models using primary cells in the last decade. These 3D systems more accurately mimic the metabolic environment, diffusion kinetics, and cell polarity that are observed in vivo. Resultantly, cells grown in 3D show improved cell-cell and cell-matrix communication4 achieving more translational data. The below video explains how 3D cell scaffolds, like Alvetex, encourage more natural cell growth and behavior outside of the body compared with monolayer cultures.
Organ chips and micro-physiological systems (MPS)
Organ chips are micro-physiological systems (MPS) designed to recreate the functionality of organs in vitro.4 These chips elevate cell-based modeling by using microfluidics to create an artificial vasculature that perfuses cells with nutrients throughout culture (table 1).4 If established using human cells, organ chips negate the species-species differences observed in animal testing. Researchers have even designed multiple-organ MPS that could act as an animal surrogate in vitro.4,6
Despite their potential to reduce the cost of developing new drugs, quantitative analysis of these models is challenging.4 Therefore, researchers are developing effective qualitative analytical methods, such as imaging, to use with organ chip systems. Increased advances in imaging technology and analysis could therefore improve the relevance of these models and increase their use in drug development.4
|2D cell culture||3D cell culture||Micro-tissues||Organ chips|
|Gradients||Lack of soluble gradients||Soluble factor gradients
||O2, nutrients, proliferation gradients||Controlled O2, nutrients, proliferation gradients|
|Limited forced polarity||𐤕||✓||✓||✓|
|Adhesions in xyz||𐤕||✓||✓||✓|
|Spreading, migration & proliferation controlled by matrix||𐤕||✓||✓||✓|
|Nutrients replaced through continuous perfusion||𐤕||𐤕||𐤕||✓|
|Optimized for multicell culture||𐤕||𐤕||𐤕||✓|
|Applied fluid shear stress||𐤕||𐤕||𐤕||✓|
Table 1: Different features of in vitro models of increasing complexity. Adapted from Peel S et al, 2020.4
Computer modeling and simulation are the newest nonclinical approaches included in the FDA Modernisation Act 2.0. In the same way that social media algorithms can be used to predict human behavior, computer modeling can be used to predict human drug responses in silico.7 Their uses include, but are not limited to;
- target identification
- toxicity testing
- clinical trial design
One disadvantage of these simulations is that their predictions are only as accurate as the input data. Additionally, computer models may take a number of months to develop and test, as explained in the video below. However, once they are designed they provide results rapidly, and can be used to model a variety of outcomes and situations.
Other non-human or human biology-based test methods
This new legislation does not specify the meaning of "other non-human or human biology-based test methods". In theory, a range of techniques and models could be included in this group - some of which might not even exist yet.
Bioprinting is a form of 3D printing that uses biomaterials to create living organs and tissues.8 This technology was initially developed for regenerative medicine and has since been used to create transplantable blood vessels, skin, and bone tissues.8 Bioprinting can also be used to design living systems that more accurately predict human drug response. However, like several of the nonclinical methods we have discussed, it relies heavily on technological advances (e.g., improved printing materials) to improve its translatability and accessibility.8
Human tissue testing
Human tissue testing has been used in pharmacology for over 100 years. Like bioprinting, this alternative testing method uses living tissues to predict human drug response. But instead of creating these organs from scratch, human tissue testing uses donated tissues that are not suitable for transplantation. This ex vivo methodology negates species differences and generates drug response data at an early stage in research. Several companies have already used human tissue testing to help them achieve regulatory approval including IPM who tested their dapiverine ring in human uterine tissues.
While a range of pharmacological end-points can be measured using human tissue research (e.g., inflammation, vascular contractility, airway contractility, and oral bioavailability) it can be difficult to access donor tissues for experimentation. Access is not only a logistical challenge, but also requires a range of ethical considerations. Many Pharma companies, therefore, outsource human tissue testing to contract research organizations (CROs) that specialize in this type of work.
Summary table: the advantages and disadvantages of nonclinical models4
|3D co-culture models||Organ chips||Computer modeling||Ex vivo models||Animal testing|
|Relate genetics to clinical response||𐤕||𐤕||✓||✓||𐤕|
|Model whole-body responses||𐤕||𐤕||✓||𐤕||✓|
Why some researchers are skeptical of the FDA Modernization Act 2.0
There is a wealth of data behind animal studies, meaning it is easy to access these techniques and compare data across studies. In addition, alternative testing methods can be expensive or complex to develop.9 It is also not yet possible to study all aspects of biology at the system or whole organism level in alternative models, as can be achieved in vivo, making it difficult to, for example, investigate central nervous system control of the cardiovascular or respiratory system. Therefore, some researchers still believe that animal research is the gold standard of preclinical testing.9,10 One group has even argued for reduced animal testing legislation, claiming that these strict rules are limiting scientific progress.10
Yet the FDA Modernization Act 2.0. does not outlaw animal testing; it simply expands the range of models that can be used to test drugs before human trials. The act will hopefully encourage US researchers and regulators to dedicate more resources to improving alternative testing methods. This will not only reduce animal use, but will eliminate the species differences that are too often responsible for clinical attrition.
Previously, it was a legal requirement for all drugs to be tested in animal models before first-in-man trials. The FDA Modernisation Act 2.0. has expanded the range of nonclinical models that can be used to progress drugs to clinical studies, but also recognizes that animal models are still needed in some cases. The new models included in this legislation are:
- Cell-based assays demonstrate a range of complexity, from simple monolayer cultures to 3D bioengineered tissues.
- Organ chips and Microphysiological Systems (MPS) use microfluidics to recreate organs and multi-organ systems in vitro.
- Computer modeling uses simulations and machine learning (ML) to predict clinical outcomes quickly and accurately.
- Other non-human or human biology-based test methods, including bioprinting and human tissue testing, to test drugs in living tissues or organ systems.
While there has been resistance by some in the scientific community, it is hoped that this new legislation will encourage the use of alternative testing methods, which will improve animal welfare and the translation of nonclinical data to human trials.
- Modernizing Testing. FDA Modernization Act of 2021 Endorsements and Cosponsors. (2023).
- US Congress. FDA Modernisation Act 2.0. (2023).
- US Congress. H.R.2617 - Consolidated Appropriations Act. (2023).
- Peel S et al. Imaging microphsyiological systems: a review. American Journal of Physiology Cell Physiology 320 pp669-680 (2021).
Wang H et al. 3D cell culture models: Drug pharmacokinetics, safety assessment, and regulatory consideration. Clinical Translational Science (2021).
- Sin A et al. The design and fabrication of three-chamber microscale cell culture analog devices with integrated dissolved oxygen sensors. Biotechnology Progress 20:1 (2004).
- Chandrasekaran B et al. Computer-aided Prediction of Pharmacokinetic (ADMET) Properties. Dosage Form Design Parameters (2018).
- Yilmaz B et al. Bioprinting: A review of processes, materials and applications. Bioprinting 23 e00148 (2021).
- Beale A. Animal Research: Reinventing the norm. Drug Discovery World 2:3 pp 14-18 (2022).
- Homberg JR et al. The continued need for animals to advance brain research. Neuron (2021).