Trainer Alex Wong Siu-tan was hit with a hefty $150,000 fine yesterday at the end of the world's first positive test for Isosorbide, a diuretic drug. Wong was stunned by the size of the fine - the largest ever handed out in Hong Kong for an offence under rule 140(1) - after pleading guilty to failing to present Winmark for racing free of any prohibited substance. The trainer left the hearing at the Happy Valley racecourse inquiry room in a hurry. He said he would consider an appeal but admitted it was unlikely. Wong told the Jockey Club's raceday stewards that he did not know how the 'significant' amount of the substance came to be in his horse. Jockey Club director of racing Winfried Engelbrecht-Bresges said: 'This is the first finding of this drug anywhere in the racing world, and it has not been tested so we could not say for certain what the effect on the horse would have been in the race. 'While it is a heavy fine, we did have to take into account that Alex Wong Siu-tan did have a prior offence of this kind. But he was fortunate in one aspect - very few laboratories would have the capability to detect this substance. The saving factor was that the horse did not run or we would have been looking at an even more serious situation.' In May 1986, Wong had been fined $40,000 after presenting My Treasure to race with a prohibited hormone in his system. Isosorbide was the prohibited substance identified in the routine pre-race urine test of Winmark when he was a sensational withdrawal at Happy Valley on December 7, just hours before he was due to run as favourite for the Shum Shui Po Handicap. The horse was tagged 'potentially positive' by the Jockey Club's senior racing chemist, Dr Terrence Wan, and scratched by the racing stewards. Subsequent blood and urine tests identified the substance as Isosorbide, and independent analysis at the Australian Racing Forensic Laboratory in Sydney confirmed that finding this week. Evidence was taken yesterday from the trainer, his assistant Chris Cheung Ting-pong, Dr Wan and the Jockey Club's senior veterinary surgeon, Dr Keith Watkins. Dr Watkins gave evidence that Isosorbide is not contained in any substance used by the Jockey Club's veterinary department. Engelbrecht-Bresges said: 'This was not a contamination or some accident. The senior chemist has told the inquiry that this was a significant amount of the drug and we have been told that this kind of thing needs to be applied. 'The trainer doesn't know how the substance came to be there and there is nothing on our security tapes of his stable, although there is no special camera in the feed room. 'Any suggestion as to how this happened is speculation. However, this inquiry did not have to deal with that, only the fact that it was there.' Winmark has not run since the December 7 withdrawal but is entered for a race on Wednesday night. 'There is no action taken against the horse,' Engelbrecht-Bresges said. 'This kind of drug is out of a horse's system very quickly and has no long-standing effect.'