Patulin is a mycotoxin produced by a variety of molds commonly found in rotting apples, including Aspergillus and Penicillium.[1] Patulin-induced nephropathy and gastrointestinal tract malfunction have been demonstrated in several animal models.[2] The oral LD50 value of patulin ranges between 29 and 55 mg/kg body weight in rodents and 170 mg/kg body weight in poultry.[1] The World Health Organization considers patulin cytotoxic and established a safety level of patulin in apple juice at 50 μM.2 At 100-200 μM, patulin can directly increase intracellular oxidative stress in HEK293 and HL-60 cells.[2] In 1944, this compound was tested in a clinical trial for potential antibiotic properties to treat the common cold.[3]

Reference:
[1]. Puel, O., Galtier, P., and Oswald, I.P. Biosynthesis and toxicological effects of patulin. Toxins 2(4), 613-631 (2010).
[2]. Liu, B.H., Wu, T.S., Yu, F.Y., et al. Induction of oxidative stress response by the mycotoxin patulin in mammalian cells. Toxicological Sciences 95(2), 340-347 (2007).
[3]. Chalmers, I., and Clarke, M. Commentary: The 1994 patulin trial: The first properly controlled multicentre trial conducted under the aegis of the British Medical Research Council. Int.J.Epidemiol. 32, 253-260 (2004).