Animated Film

Animated films of "Old Possum's Book of Practical Cats" have been considered many times over the years, including adaptions of the musical. As yet, none have come to fruition.

Disney
TS Eliot specifically did not want his Jellicle Cats turned into cutesy cartoon cats by Disney. Instead of making an animated feature of "Old Possum's", Disney instead produced "The Aristocats".

Amblimation 1991
Information from Hans Bacher's blog

Under the direction of Steven Spielberg, who decided to set the show during the Blitz in London (1940-1941)

Luc Desmarchelier also blogs about the development

LA mid 1990s
Hans Bacher mentions that a studio in LA was working on the Cats project, but was finally cancelled in 1997, and the 1998 Cats Movie as we know it went into production.