Developed by the University of South Australia in collaboration with the Medical School at Stanford University, this world first formulation uniquely combines limonene (a citrus essential oil) with a lipid-based drug delivery system to treat dry mouth (xerostomia), a common side effect of radiotherapy.

The new formula demonstrated 180-fold better solubility than pure limonene in lab experiments and boosted relative bioavailability by over 4000% compared to pure limonene in pre-clinical trials.

Dry mouth is the most reported side effect following radiotherapy for the treatment of head and neck cancer, affecting up to 70% of patients due to salivary gland damage. It can lead to difficulty speaking and swallowing, significantly reducing quality of life.

Limonene has protective effects on saliva production during radiotherapy, but its poor solubility means high doses are needed to take effect, and these cause indigestion, abdominal discomfort and unpleasant ‘citrus burps’.

Lead researcher, Professor Clive Prestidge says UniSA’s new limonene-lipid combination creates a ‘super-solubilising’ treatment that reduces dry mouth at lower dose and without uncomfortable side effects.

“The therapeutic benefits of limonene are well known. It’s used as an anti-inflammatory, antioxidant, and mood-enhancing agent, and can also improve digestion and gut function. But despite its widespread use, its volatility and poor solubility have limited its development as an oral therapy,” Prof Prestidge says.

“As limonene is an oil, it forms a film on the top of the stomach contents, causing significant stomach pain and discomfort.

“Our novel formulation combines limonene with healthy fats and oils — called lipids — to create a super-solubilising compound that the body can easily absorb with reduced uncomfortable side effects.

“This increases the dispersion of limonene in the stomach, boosts absorption, and controls biodistribution — all while increasing a patient’s saliva production and reducing dry mouth.”

Co-researcher Dr Leah Wright says the formulation has the potential to significantly improve the quality of life for cancer patients and others suffering dry mouth conditions.

“Cancer patients undergoing radiotherapy and other medical treatments regularly experience dry mouth, which not only prevents them from comfortably swallowing, but can also have other negative and potentially life-threatening outcomes,” Dr Wright says.

“While limonene can be ingested directly, it’s not well tolerated, especially by those with dry mouth. Plus, its poor absorption prevents it from effectively reaching the salivary glands — the target site.

“This inventive and highly impactful limonene-lipid formulation could provide a simple, effective oral solution for dry mouth, offering cancer patients long-lasting relief and comfort, improved oral health, and a higher quality of life during a difficult time.”

Clinical trials for the new formula are ongoing, with next steps to be announced soon.