Philadelphia’s Largest Office Complex Gets Downgraded: Commercial mortgage-backed securities bonds tied to a loan on 1500 Market St., a two-building, 1.8 million-square-foot property in downtown Philadelphia — the city’s largest office complex have been downgraded by Moody's Investors Service. Moody's said it lowered the ratings due to the loan’s delinquent status after the borrower was unable to pay off a $368 million final payment in December.