Buy-out firms Apollo and TPG have written off around USD 600m (GBP 321m) of their investment in American casino business Harrah's, in the biggest ever known private equity markdown. The writedowns equate to more than 20pc of the value of the private equity houses' investment in Harrah's. Apollo and TPG, the private equity house that walked away from Bradford & Bingley last month, paid USD 17bn for Harrah's in January, but since then economic woes have hit US gaming companies hard.