I think it is important to discuss the weight loss with a vet, to get an idea of ideal weight, a safe rate of weight loss, and to determine the cat's current state of health...as a "diet" is a good idea for long term health, but done wrong it can be harmful for the cat. So if you haven't already discussed it with your vet, I would suggest that.
The vet *may* suggest a dry "lite" food...which I do not agree with. The thinking on that is shifting some, but there are some vets who still subscribe to that way of thinking. Newer research is suggesting that a "catkins" diet is more effective. A good high quality canned food, high in protein and low in carbs would be my personal suggestion. Dry foods are high in carbs...and those tend to add pounds to the cat...whereas they utilize protein and fat much more efficiently.
This link has some good info...http://catinfo.org/
And this one ... http://www.littlebigcat.com/index.ph...needcannedfood