YAOUNDE (Reuters) -Following are details of the main opposition challengers to Cameroon's 92-year-old President Paul Biya, the world's oldest head of state, seeking to win an October 12 election that could keep him in office until he is nearly 100. A former government spokesperson who most recently served as employment minister, Tchiroma announced in June he was defecting in response to mounting public frustration after more than four decades under 92-year-old Biya. That raised the possibility of a realignment in Cameroon's northern regions - Tchiroma's home base - which has traditionally delivered strong support for Biya's Cameroon People's Democratic Movement (CPDM).