Phoenix:Mike Tyson pleaded not guilty to charges of drug possession and driving under the influence of drugs in Phoenix, Arizona, USA on Monday. The former undisputed heavyweight champion was pulled over by police after he left a Scottsdale nightclub on December 29. Officers allege they found bags of cocaine in his back pocket and in cigarette packets in his car and, if found guilty, Tyson faces up to seven-and-a-half years in prison. Maricopa County Attorney Andrew Thomas has said he would pursue a prison sentence for Tyson, who spent three years in jail after being convicted of rape in 1992 and pleaded no contest to assault charges in Maryland in 1999. Tyson's lawyers said he was seeking professional help for his drug addiction and other problems and that their client's rehabilitation would not benefit from imprisonment. Downhill This is the latest in a catalogue of incidents which have marked the decline of one of the world's greatest ever boxers. After a troubled childhood, Tyson became the youngest heavyweight champion in history in 1986 when, at the age of 20, he knocked out the late Trevor Berbick. He unified the three title belts by beating Tony Tucker the following year but he lost his focus in 1990 and suffered his first defeat to James 'Buster' Douglas in Tokyo. Two years later, he was found guilty of the rape of a beauty pageant winner and served three years in prison. He won back the WBC title from Frank Bruno in March 1996 and added the WBA title in September of that year by knocking out Bruce Seldon in 93 seconds. Huge debt But his powers were on the wane and after losing to Evander Holyfield, he shocked the world when he bit off a chunk of his rival's right ear during their rematch in June 1997. In 1999, he served nine months behind bars after pleading no contest to charges that he assaulted two people following a car crash. He returned to the ring but his reputation was all but destroyed when champion Lennox Lewis handed him an eight-round boxing lesson in June 2002. Tyson finally quit the ring in June 2005 but spiralling debts have seen him embark on a wide range of money-making schemes from appearing in Japan's PRIDE fighting championships to sparring in front of tourists in Las Vegas. His cocaine use ended his involvement in PRIDE fighting and he later announced that he would return to the ring for a series of four-round exhibition fights around the world. Tyson is due back in court on February 26.