Many of the situations in which inhaled particles or substances can cause toxicity involve repeated exposures to the substance – e.g., cigarette smoking, use of inhaled medications, or on-the-job exposure to agricultural or industrial sprays and vapors.

To test for the effects of repeated inhalation exposures without using animals, scientists are developing increasingly sophisticated in vitro models of the human respiratory system. These in vitro alternatives use cultured respiratory cells and tissues that are kept alive in conditions designed to imitate the environment of the human airway. For repeat-exposure tests, it is necessary to keep these “reconstructed” cells and tissues alive and functional for extended periods of time.

Rats in inhalation exposure chamber

Rats in inhalation exposure chamber – a standard apparatus for testing inhalation toxicity on animals

Scientists with British American Tobacco (BAT) recently demonstrated that a commercially-available reconstructed human airway tissue called MucilAir™ would remain fully responsive for at least six months in their experimental conditions, making it one of a growing number of viable and more human-relevant alternatives to testing inhalation toxicity on animals.

(Image credit: British American Tobacco)

(Image credit: British American Tobacco; click to enlarge)

The scientists monitored such regular lung tissue activities as cilia beat frequency, mucous secretion, enzyme activity and gene expression, and found that the MucilAir tissue showed normal responses throughout the test period.

According to the company press release, BAT will use the tissue model to “compare the toxicological effect of repeated exposures to aerosols generated from conventional and next-generation tobacco and nicotine products.”

image of iPSC-derived branching mini-lung

Branching mini-lung. Credit: Nick Hannan, University of Cambridge

Cultured and engineered respiratory tissues with extended lifespans can also be used to test drugs intended to treat chronic lung diseases such as cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD), and asthma. Scientists at Cambridge University recently created a “mini-lung” model of cystic fibrosis by reprogramming skin cells from patients whose CF was caused by a particular genetic mutation (one that affects 3 out of every 4 CF patients). The skin cells were reprogrammed into an induced pluripotent state, from which they can develop into any kind of cell in the body. The researchers then guided the cells to develop into the distal airway lung tissue most affected by diseases such as CF, certain lung cancers, and emphysema, and demonstrated that the tissue – which the team refers to as “mini-lungs” – was functional. Said lead author Nick Hannan: “We’re confident this process could be scaled up to enable us to screen tens of thousands of compounds and develop mini-lungs with other diseases such as lung cancer and idiopathic pulmonary fibrosis… This is far more practical, should provide more reliable data and is also more ethical than using large numbers of mice for such research.”