keyboard - ALT+left-arrow & ALT+right-arrow not working in tmux (zsh) on Mac OSX

I've configured my zsh to use ALT and left/right arrows to word left/right using the following in my ~/.zshrc file, however when I open a tmux session it doesn't work and just outputs 'D' or 'C'…

bindkey -e
bindkey '^[^[[C' forward-word
bindkey '^[^[[D' backward-word

Can anyone help my get this working?