The present invention provides a method of treatment and/or prophylaxis of arthritis in a vertebrate comprising administering to said vertebrate in need of said treatment and/or prophylaxis a therapeutically effective amount of a compound of Formula (I) or a pharmaceutically acceptable salt thereof, optionally together with a pharmaceutically acceptable carrier, diluent or excipient, wherein said compound of formula (I) is defined as: A--(L--Y).sub.p, wherein: A comprises at least one substantially cell-membrane impermeable pendant group; L comprises any suitable linker and/or spacer group; Y comprises at least one arsenoxide or arsenoxide equivalent; p is an integer from 1 to 10; and the sum total of carbon atoms in A and L together, is greater than 6.